Satellites and Tidal Streams
ING-IAC Joint Conference
La Palma, Canary Islands, Spain — May 26-30, 2003

List of abstracts

Dynamical properties of external systems of satellite galaxies
Marco Azzaro, ING, S.C. de La Palma, Spain definite poster
We are analyzing a sample of closeby galaxy systems, each comprising a bright isolated spiral and its satellites.
These systems have been selected from the Sloan DSS Data Release~1. We show here some preliminary results concerning the orbital parameters of the satellite population.
Coupling between satellite dwarfs and the Milky Way warp
Jeremy Bailin, Steward Observatory, Tucson, AZ, USA definite talk
The Milky Way's disk is warped, as seen both in neutral hydrogen and in the stellar distribution. The perturbations of satellite galaxies, in particular the Large Magellanic Cloud (LMC), have been repeatedly proposed and discounted as the cause. Most recently, Tsuchiya~(2002) performed simulations that suggested that in a sufficiently massive halo, the wake of the LMC through the halo could excite a warp of the observed magnitude (as discussed by Weinerg~1998). However, as noted by Garc\'{i}a Ruiz~et~al.~(2000), a warp due to a satellite should have its line of nodes in the plane of the satellite's orbit (due to the transfer of angular momentum from the satellite to the disk), while the Galactic warp line of nodes is perpendicular to the LMC orbit. The Sgr dSph galaxy has an appropriately-oriented orbit; however its direct tidal torque, according to the formula of Hunter~\&~Toomre~(1969), should not be sufficient to produce the Milky Way's warp. If the perturber is not the satellite itself but the wake it sets up in the halo, the closer orbit of Sgr combined with a centrally concentrated halo results in a tidal effect 10--50 times stronger than that of the LMC, perhaps sufficient to create the warp.

If angular momentum is being transferred between these systems, their angular momenta may be correlated. I have calculated the orbital angular momentum of Sgr, and also the warped component of the disk angular momentum. I find that they have the same magnitude of (2--$8)\times 10^{12}~M_{\odot}~\mathrm{kpc~km~s^{-1}}$ (where the errors in both cases are dominated by uncertainty in the mass) and are anti-aligned. I have computed the orbital angular momentum of the other satellite dwarfs with measured proper motions, and find they span more than three orders of magnitude, and cover a wide range of directions. Therefore, the coincidence of the angular momenta of the warp and of Sgr suggests they are coupled, i.e. that Sgr may be the perturbation responsible for the warp.

In order to further evaluate this possibility, I have run numerical N-body simulations aimed specifically at modelling the Sgr-MW system with sufficient particles in the halo and disk to resolve the wake that Weinberg~(1998) and Tsuchiya~(2002) discuss. I will talk about the preliminary results of these simulations, and what they imply about the origin of the Galactic warp.

Globular clusters in the Sgr dwarf galaxy and in the Sgr stream
Michele Bellazzini, INAF - Osservatorio Astronomico di Bologna, Bologna, ITALY definite talk
It has been recently demonstrated (Bellazzini, Ferraro & Ibata 2003; BFI03) that the orbital path of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) is a preferential subset of phase-space for the Galactic globular clusters having $10\le R_{GC}\le 40$ kpc. This means that some of these globulars is physically associated with the Sgr Stream. We report on the finding of Sgr Stream stars (selected from the 2MASS catalogue) around some of the clusters indicated by BFI03 as the most likely candidates for association with the Sgr Stream.
Heating of Galactic Disks by Dark Satellites
Andrew Benson, California Institute of Technology, Pasadena, CA, USA definite talk
I will present calculations of the heating of galactic disks by the population of dark satellites predicted by cold dark matter models. These will be used to place constraints on the degree of substructure present in CDM halos.
Parameters of Minor Merger
Michael Bertschik, Max-Planck-Institute fuer Astronomie, Heidelberg, Germany definite talk
We present here a study of numerical simulations of minor mergers in a Lambda CDM universe. In particular, we analyzed the orbital/kinematical parameters of the merging satellites, the mass spectrum of these satellites and the abundance of minor merger in dependency of z.
PDF of Density Fluctuations and the complete Zeldovich Approximation
Juan Betancort-Rijo, Instituto de Astrofísica de Canarias, Spain definite talk
We describe the complete Zeldovich approximation and use it to develope an analytic procedure for obtaining the probability distribution function of density fluctuations in the non-linear regime.
Dark Matter in Dwarf Galaxies
Leo Blitz, Astronomy Department University of California, USA definite talk
We present the derived radial dark matter distribution in two galaxies based on high resolution, multiwavelength, two dimensional spectral line data for NGC 4605 and NGC 2976. We also show the HI Images of a candidate Dark Galaxy, a dwarf galaxy apparently associated with LGS3
with no stars. If there is time, we also show the results of HI observations probing the dark matter potentials in 4 Local Group dwarf irregulars indicating near constant velocity dispersions to distances in excess from .5 - 5 kpc from their centers.
Tidal Streams and Low Mass Companions of M31
Robert Braun, ASTRON, Dwingeloo, The Netherlands definite talk
Several major observational initiatives have recently been undertaken to study the immediate and extended environment of M31 in HI emission. A sensitive mosaic made with the WSRT array now covers the central 85x45 kpc at 50 parsec resolution, providing the most detailed image ever made of the neutral ISM of a galaxy. Deep GBT imaging has been obtained of the central 85x85 kpc region to an rms sensitivity of 3x10^17 cm^-2 with a 2 kpc beam. A wide-field total power survey using the telare detected in the lower resolution data. Preliminary results of these three surveys will be presented.
Plotting Galactic Halo Stars in Phase Space: A Hint of Satellite Accretion?
Chris Brook, Swinburne University of Technology, Hawthorn, Victoria, Australia definite talk
The present day chemical and dynamical properties of the Milky Way bear the imprint of the Galaxy's formation and evolutionary history. One of the most enduring and critical debates surrounding Galactic evolution is that regarding the competition between "satellite accretion'' and "monolithic collapse''; the apparent strong correlation between orbital eccentricity and metallicity of halo stars was originally used as supporting evidence for the latter. While modern-day unbiased samples no longer support the claims for a significant correlation, recent evidence has been presented by Chiba & Beers (2000,AJ,119,2843) for the existence of a minor population of high-eccentricity metal-deficient halo stars. It has been suggested that these stars represent the signature of a rapid (if minor) collapse phase in the Galaxy's history. Employing velocity- and integrals of motion-phase space projections of these stars, coupled with a series of N body/Smoothed Particle Hydrodynamic (SPH) chemodynamical simulations, we suggest an alternative mechanism for creating such stars may be the recent accretion of a polar orbit dwarf galaxy.
Dwarf Satellites and the Residue of Hierarchical Cosmology
James Bullock, Harvard CfA definite talk
I present results on the merger histories and substructure content of hierachically-formed (LCDM) dark matter halos and discuss how fossil evidence of this process may be observable in the Local Group. I specifically compare standard $n=1$ LCDM models to models with redued small-scale power. I also discuss the expectation that a number of very low-surface brightness dwarfs (undergoing tidal disruption) may yet to be discovered within 100 kpc of the Milky Way.
Morphologycal evolution of dwarf galaxies in the Local Group
Giovanni Carraro, Universita' di Padova, Italy definite poster
We present Nbody/Hydro simulations of dwarf galaxy tranformation under the tidal field of the Milky Way
The Effect of Isolated Halos on Lensing Searches for Substructure
Jacqueline Chen, University of Chicago, CfCP, Dept. of Astron. & Astrophys, Chicago, IL, USA definite oral
Anomalous flux ratios in multiply-imaged quasar lenses can be used to constrain the substructure mass fraction in galaxy-sized dark matter halos. The flux ratios, however, can be affected by both the substructure in the lens halo and by isolated small-mass halos along the entire line-of-sight to the lensed source. Here we estimate the potential contribution of isolated clumps to the substructure lensing signal using a simple model motivated by cosmological simulations. We find that the contribution of isolated clumps to the total lensing optical depth ranges from a few to tens percent, and, therefore, the contribution of isolated clumps should not be neglected in detailed analyses of substructure lensing. The prediction, however, is highly sensitive to the spatial distribution of substructure halos in the innermost regions of the lens halo, which is still very uncertain. Constraints on the properties of the substructure population or accurate cosmological constraints are difficult if not impossible to derive at this point.
Substructure in the Fornax dSph Galaxy
Matthew Coleman, Mount Stromlo Observatory, Research School of Astronomy and Astrophysics, Weston Creek, ACT, Australia definite talk
We present the initial results of a photometric survey of the Fornax dwarf spheroidal (dSph) galaxy using the Wide-Field Imager on the Siding Spring Observatory 1 metre telescope. Our data include a small overdensity of stars located approximately $15'$ from the centre of Fornax. Using colour-magnitude data in both $V-$ and $I-$bands down to
$V = 21$, we have calculated the density of this feature is approximately $20\%$ higher than the surrounding region, and has a surface brightness of $25.8$ mag/arcsec${}^2$ in $V$. The overdensity has a well defined boundary, with the approximate dimensions $55 \times 110$ arsec${}^2$. We examine the colour-magnitude properties of this feature using deep photometry previously obtained by Stetson (1998), and discuss possible formation scenarios of this object within the dynamical history of the Fornax dSph galaxy.
Dwarf Dark Matter Halos
Pedro Colin, Instituto de Astronomia, Campus Morelia, Morelia, Michoacan, Mexico definite talk
The structure of dwarf dark matter halos, some of which resolved with hundreds of thousands of particles, with maximum circular velocities around 20 km/s is studied using a high-resolution numerical simulation. Density profiles of halos at $z \sim 3$ that are close to equilibrium are well fitted by a NFW profile, contradicting Ricotti's results. At this redshift, the mean concentration $C_{vir}$ is around 6 which extrapolated to $z = 0$ (with the 1+z law, Bullock et al) gives $\sim 24$. We have also studied the halo mass function and the substructure population.
Planetary nebulae as tracers of stellar populations at large
galactocentric distances
Romano L.M. Corradi, Isaac Newton Group, Santa Cruz de la Palma, Spain
(R. Corradi, C. Davenport et al.) definite poster
Planetary nebulae are excellent tracers of stellar populations in large volumes with a relatively low density of stars, whose integrated stellar light is low and hardly detectable, like the intergalactic and intracluster space and in the outermost regions of any type of galaxies. We present here some preliminary results of our ongoing observational programme aimed at searching for PNe in nearby galaxies and in their surroundings. Candidate planetary nebulae have been found at large distances from the centre of the Local Group galaxies IC 10 and NGC 6822, as well as in the intergalactic region between M 81 and M82, suggesting the existence of stellar populations with very low surface brightness, likely belonging to extended haloes, tidal tails or streams.
Probing Omega Cen's tidal tails with 2dF
Gary Da Costa, Australian National University, Research School of Astronomy & Astrophysics, Mt Stromlo Observatory, Weston, ACT, Australia definite poster
Using the Wide-Field CCD mosaic imager on the Siding Spring Observatory 1m telescope, we have surveyed a 3.5 $\times$ 2 degree field centered on "tidal tails'' identified from star counts by Leon et al. (2000, A&A, 359, 907). The cluster colour-magnitude diagram from the central field was used to define a colour and magnitude limited `box' covering the cluster subgiant branch. A large sample of stars from this box (corrected for reddening variations across the field) covering the full survey area was then spectroscopically observed at the beginning of May using the Anglo Australian Telescope $2dF$ multi-fibre instrument . This instrument permits up to 400 stars to be observed simultaneously with a velocity precision of 10 -15 kms-1. This precision and the high radial velocity of $\omega$ Cen allows a clean separation between field and cluster stars. Preliminary results from this survey, which will ultimately reveal the distribution of extra-tidal cluster stars and provide an estimate of their kinematics, will be presented.
Probing substructure with gravitational lensing
Neal Dalal, Institute for Advanced Study, School of Natural Sciences, Princeton, New Jersey, USA definite talk
Strong gravitational lensing provides one of the only probes of the dark substructure predicted to exist in galaxy halos. I will discuss how lensing may be used to study substructure and how to distinguish the effects of substructure from other effects. I will present results from an analysis from an existing sample of radio lens systems and discuss future prospects for strong lensing as a probe of substructure.
Poster with R. Corradi
Colin Davenport, ING Sea Level Office, Santa Cruz de La Palma, Spain definite poster
Dwarf galaxy populations in varying environments
Jonathan Davies, Cardiff University, Dept Physics and Astronomy, UK definite talk
We have carried out a survey for dwarf galaxies in a wide range of different environments sampling the nearby Virgo cluster, the Virgo Southern Extention and the general field. The survey extends to very low surface brightness levels and utilises an automated detection and measurement method. A comparision of the dwarf-to-giant ratio for these various fields indicates a large variation. The field data is consistent with recent determinations of the field galaxy luminosity function by SLOAN and 2dF - there is not a population of faint low surface brightness field galaxies that has been missed by these surveys. In the Virgo cluster there is a substantial population of faint low surface brightness dwarf galaxies consistent with typical standard CDM simulations. If the explanation of the relatively flat field galaxy luminosity function is some form of feedback mechanism then this mechanism has been turned off in the cluster environment. Alternately the dwarf population has been created in the cluster, possibly by interactions with or as part of the formation of the brighter galaxies.
Galaxy Formation in CDM Cosmology: Crises and Possible Solutions
Avishai Dekel, The Hebrew University of Jerusalem, Racah Institute of Physics, Israel possible talk
Galaxy formation in the framework of CDM cosmology is facing three major problems: (a) the cusp/core problem, where dark-matter simulations predict a steep inner cusp while at least some galaxies show flat cores; (b) the angular-momentum catastrophe, where simulations including gas produce disks significantly smaller than the galactic disks observed, and with a different internal distribution of angular momentum; and (c) the missing dwarfs problem, where the simulations predict many more dwarf galaxies than are actually observed in the Local Group. These problems will be addressed in simple physical terms, focusing on the buildup of galaxies by a hierarchy of mergers and the associated tidal effects. The proposed process for resolving the discrepancies is the inevitable feedback into the intergalactic medium, e.g., by supernova-driven galactic outflows and by photo-ionization.
Cooling and Thermal Conduction in Clusters of Galaxies
Nelson Falcon, Univ. de Carabobo (Venezuela) - IAC (España) definite poster
Clusters and groups of galaxies contain enormous quantities of diffuse gas.
The intraclustergas acts as a fluid fully ionized where the heat propagation
is dominated by electrons. The usual models to explain the X-ray emission
consider the thermal propagation in the diffusion approach. As the matter
it is degenerate, the thermal history of the means should be incorporate through the Cattaneo-law for heat propagation, facilitating the cooling by
means of thermal waves.We present a simple model of hot gas in galaxy
clusters, assuming the causal propagation for thermal conduction and hydrostatic equilibrium. The results show that the cooling timescale could
be different to those recently calculated by Zakamska and Naraya (2003).
Implications in X-ray observations and interferometry is discussed.
Indirect searches for SUSY Dark Matter with the next generation of Cherenkov Telescopes
Josep Flix Molina, Institut de Fisica d'Altes Energies (IFAE), Universitat Autonoma de Barcelona, Spain definite talk
Neutralinos are the natural well-motivated candidates to provide much or all the non-baryonic dark matter of the universes which may produce detectable signals through their annihilation into neutrinos, photons or positrons. Due to the high Flux Sensitivities and low Energy Thresholds, the next generation of Cherenkov Telescopes could potentially detect the neutralino annihilation high energy photons products. By working in the framework of minimal supergravity, the parameter space is scanned in different benchmarck scenarios defined after accelerator constraints. Different neutralino density profiles are also studied to infrerred the most optimum observation regions for the detection of neutralino photon signatures with the next Cherenkov Telescopes.
The MAGIC Cherenkov Telescope
Josep Flix Molina, Institut de Fisica d'Altes Energies (IFAE), Universitat Autonoma de Barcelona, Spain definite poster
The Major Atmospheric Gamma-ray Imaging Cherenkov Telescope (MAGIC) is in comissioning phase and will start to become operational in mid-2003. The Telescope is located at "El Roque de los Muchachos" in La Palma (Canary Islands, Spain) and it has the biggest reflector area (17m diameter) of all the existing Cherenkov Telescopes around the world. As a experimental challenge, new technologies have been used to reduce the Energy Threshold for gamma detection to about 30 GeV. Due to the high sensitivity measuring fluxes and low Energy Threshold, the catalog of Very High Energy sources will considerably increase with MAGIC observations, anticipating exciting results for the near future. The MAGIC Telescope project and a highlight of the scientific research is explained in this poster.
Star Formation in Satellite Galaxies
Jose Funes, Vatican Observatory, Tucson, AZ, USA definite poster
The study of satellite galaxies can provide information on the merging and aggregation processes which, according to the hierarchical clustering models, form the larger spiral galaxies we observe. With the aim of testing hierarchical models of galaxy formation, we have conducted an observational program which comprises broadband photometry in the optical and in the H$\alpha$ narrow-band for both the parent and the satellite galaxies, taken from the compilation by Zaritsky et al. (1997) that
contains 115 galaxies orbiting 69 primary isolated spiral galaxies. The
broband photometry comprises 60 galaxies with morphological type ranging from E to Irr and M$_B$ from -15.5 to -19.6 (Guti\'errez et al. 2002). For the H$\alpha$ imaging we have observed a subsample of 28 spiral and irregular galaxies. The aim of this study is to determine star formation properties of the sample galaxies. In this work we present the preliminary results of the H$\alpha$ survey that we have carried out with the 1.8-m Vatican Telescope (VATT).
Satellite galaxies within the potential of massive host galaxies
Stuart Gill, Swinburne Universtiy of Technology, Hawthorn, Victoria, Australia definite poster
In this poster preliminary results from a series of high-resolution Nbody simulations that focus on 8 dark matter halos each of order a million particles within $r_{vir}$ are presented. We follow the time evolution of thousands of individually tracked satellite galaxies and relate their physical properties to the differing halo environmental conditions. Unlike previous work our results are performed in a fully self-consistent cosmological context. The preliminary results demonstrate although environment may vary considerably with respects to formation time and density of substructure, the satellites evolve in quite similar ways and showing universal scaling relations, respectively.
The shape of the Milky Way Halo and the Satellite Tidal Streams
Maria Angeles Gomez-Flechoso, Universidad Europea de Madrid, Villaviciosa de Odon (Madrid), Spain definite talk

The dwarf galaxies orbiting a main galaxy suffer strong tidal forces produced by the dark halo of the main galaxy. As a consequence, substructures and tidal tails could appear in the dwarf satellites. These
features are more prominent in less massive and low density satellites. Therefore, the observation of these structures in dwarf galaxies could give us information about the dark matter content of the main and the dwarf galaxies. The Milky Way satellites, because of their proximity, are a good sample to study the effects of the tidal forces. The shape of the potential of the Milky Way could be infer from the observational data of the tidal tails of the Milky Way satellites. The Sagittarius dwarf galaxy is one of the most interesting satellites as it presents a long tidal tail that covers a wide angle on the sky with a large variation of the heliocentric distance along the stream.

Satellites and Streams in the Galactic Halo: Galactic Structure with the Sloan Digital Sky Survey
Eva K. Grebel, Max-Planck Institute for Astronomy, Heidelberg, Germany definite talk
Satellite galaxies and tidal streams are excellent probes of the mass distribution and of the potential of the massive galaxy they are orbiting. Furthermore, their properties impose stringent constraints on hierarchical structure formation in standard CDM scenarios. The Milky Way and its immediate surroundings offer the unique possibility of conducting "near-field cosmology'' at highest resolution and in in the greatest detail. Observationally, this requires large-area coverage. The Sloan Digital Sky Survey (SDSS) is an ideal tool for such Galactic structure studies owing to its homogeneity, field coverage, depth, resolution, and multi-wavelength information. In my talk I will review the contributions of the SDSS to this area: (1) Structural studies of Galactic satellites, and search for extratidal stars; (2) halo substructure studies such as of the known (Sagittarius) and new (e.g., Monoceros) satellite streams, and (3) constraints from streams from dissolving globular clusters. This will be placed into the context of other ground-based surveys (e.g., the RAVE project, and our other ongoing radial velocity surveys at the AAT and at ESO) and the future astrometric space missions SIM and GAIA.
The Tidal Perturbation of the Low-mass Global Cluster NGC 5466
Michael Odenkirchen & Eva K. Grebel, Max-Planck Institute for Astronomy, Heidelberg, Germany definite poster

NGC5466 is a relatively sparse metal-poor globular cluster in the halo of our Galaxy, with an estimated mass of $6\times10^4 M_\odot$. Orbital data suggest that NGC 5466 recently crossed the Galactic disk at a distance of about 8\,kpc from the Galactic center. This presumably caused a tidal shock and may have triggered mass loss.
We will describe a study of the spatial distribution of cluster member candidates in the outer region of the cluster. We find the shape of the distribution to be non-spherical.
This provides evidence for a possible tidal perturbation and eventual tidal mass loss. We compare the observations with a simulation of the spread of tidal debris along the cluster's orbit.

Globular Cluster Tidal Tails in the 2MASS Point Source Catalog
Carl J. Grillmair, Infrared Processing & Analysis Center, Stanford University, USA definite poster

We examine the recently released 2-Micron All Sky Survey Point Source Catalog for evidence of tidal tails associated with the globular clusters NGC 288 and NGC 5897. Based on their sizes and predicted disruption times, we expect these clusters to be good candidates for the detection of extended tidal tails. The 2MASS point source catalog's magnitude limit is rather too bright to explore cluster stellar populations much below the horizontal branch for even the nearest clusters. Nonetheless, we find evidence for extended tidal tails which are consistent with the measured proper motions for these

Stellar population and HI properties of the newly discovered gas-rich dwarf galaxies in the CenA group
Marco Grossi, Dep. of Physics and Astronomy, Cardiff University, Cardiff, Wales definite talk
We present HST/WFPC2 observations (F555W, F814W) and ATCA high resolution HI maps of three gas-rich dwarf galaxies in the Centaurus A group which were discovered in the HI Parkes All-Sky Survey (HIPASS) and a deep 21-cm blind survey in a 4 by 8 degree region in Centaurus (HIDEEP). The conversion of gas into stars has been somehow inefficient in these galaxies as indicated by their high HI mass-to-light ratios ($>$ 2), faint luminosities ($M_B \sim -11$) and low surface brightness ($\mu_B > 24$ mag arcsec$^{-2}$). We have constructed Color Magnitude Diagrams that show at best the upper two magnitudes under the tip of the red giant branch (RGB) and the younger populations at bluer colours. We compare their varied individual properties and discuss their star formation history. Although we can poorly constrain the age of the oldest population from the diagrams, the presence of an extended population of red giant stars in all the three objects suggests that these systems were not formed recently. From the I magnitude of the tip of the RGB, the distances of the dwarfs seem consistent with membership in Centaurus A . For one of them, HIPASS1321-31, we have obtained a distance of 5.2 Mpc which places it at the very edge of the group.
On the nature of the ring-like structure in the outer Galactic disk
Amina Helmi, Astronomical Institute Utrecht, TA Utrecht, Netherlands definite talk
The formation of ring-like stellar structures similar to that recently discovered in the outer disk of the Milky Way may be a natural by-product of the hierarchical formation of disk galaxies. These tidal features can arise from the disruption of satellite galaxies in orbits roughly coplanar with the disk. In this context two interpretations appear plausible. One is that the ring is a transitory, localized radial density enhancement reflecting the apocenter of particles stripped from a satellite during a recent pericentric passage (a " tidal arc'' reminiscent of the tidal arms seen in disk galaxy mergers). In the second scenario, the ring is analogous to the
"shells'' found around some elliptical galaxies, and would result from a minor merger that took place several Gyr ago. Both scenarios are able to reproduce currently available data, but they could easily be distinguished from one another with more detailed observations.
Newly Discovered Globular Clusters in the Outer Halo of M31
Avon Huxor, University of Hertfordshire, Hatfield, United Kingdom definite poster
We present nine newly discovered globular clusters in the outer halo of M31, found by a semi-automated procedure from the INT Wide Field Survey of the region. Initial candidates were found from the survey database, based on magnitude, colour and ellipticity.
These were visually inspected to identify final candidates, out to a projected radius of about 55 kpc. The search was motivated by the need for additional probes of the dynamics and chemistry of the outer halo, to address issues of galaxy formation.
CDM cosmological models have had considerable success in describing hierarchical structure formation and galaxy evolution. However, important questions still remain. For example, where are the predicted large number of low mass dark halos? What are the profiles of the dark halos and stellar halos? One way to address such issues is by detailed study of low redshift galaxies, attempting to deduce evolutionary history from present day galaxy structure. The members of Local Group are ideal for such investigations because they are near enough for detailed study of their resolved stellar populations.
Recent modelling by Evans et al. (2000) suggests that, contrary to accepted wisdom, the mass of the halo of M31 is comparable to, or less than, that of the Milky Way. However, the uncertainties are significant, and Evans and Wilkinson (2000) note the need to have good dynamical test particles at large galactocentric radii.
Streams in the halos of the Milky Way and Andromeda
Rodrigo Ibata, Observatoire de Strasbourg, Strasbourg, France definite talk
Galaxy formation models, particularly the popular models based on Cold Dark Matter (CDM), have focussed their attention on galaxy halos, since these are overwhelmingly the most important components of galaxies in terms of their mass. Observationally, however, very little is known about these dark regions of galaxies, and even less is known About the "outer halo'' regions, where structures can survive the disruptive Galactic tidal forces for more than a Hubble time. The advent of powerful wide-field cameras and wide-field multi-object spectrographs, now brings the study of these low-density outer regions of galaxy halos within reach. If CDM theory is correct, there should be hundreds of halo substructures in this volume. I will review the searches for substructures in the halos of the Milky Way and Andromeda galaxies, the two best targets for this study, and discuss how the detected structures can be used to illuminate the dark and immensely massive region they inhabit.
A Wide Field Survey of M31
Mike Irwin, Institute of Astronomy, Madingley Road, Cambridge, England definite talk
Our recent wide field CCD surveys of M31 have revealed a wealth of complex structure ranging from halo stellar streams to signficant stellar overdensities just beyond the outer disk. This talk will give an overview of the survey and the results obtained to date.
Zeljko Ivezic, Princeton University, Dept. of Astrophysical Sciences, Princeton, NJ, USA definite talk
I will discuss a new sample of several thousand candidate RR Lyrae stars in the outer halo. These candidates are selected using variability, colors, and spectroscopy from a region 30 times larger than that used in previous published SDSS analyses. I will summarize the properties of the outer halo, including its clumpiness, overall shape, size, and radial stellar density distribution, inferred from the analysis of this sample.
Telling Tails About Galaxies
Kathryn Johnston, Wesleyan University, Van Vleck Observatory, Middletown, CT, USA definite talk
This talk will be an overview of theoretical work on the formation and evolution of tidal debris. The enphasis will be on the interpretation of observational data as a consequence of this work.
XMM observations of Hot Gas in Low Mass Dwarf Galaxies
Michael Kappes, Radioastronomisches Institut der Universität Bonn, Germany definite talk
We present XMM--Newton observations of the nearby low--mass dwarf galaxies IC~2574, Holmberg~I and Sextans~A. These galaxies are part of our ensemble observed within a multi--frequency campaign to study the physical conditions of the interstellar medium (ISM) in dwarf galaxies in general. The selected ensemble of low--mass dwarf galaxies was chosen to show prominent supergiant HI holes in their ISM which are considered to be tracers of past massive star formation. Using very sensitive XMM Newton data we search for X--ray emission emerging from coronal gas inside these supergiant HI holes. We present our search for hot gas that may have been vent into the halo of these low--mass systems. Provided that the XMM--Newton data are corrected for soft proton events originating from the Sun, we developed a new algorithm to calculate high fidelity exposure maps from deep X--ray pointings in order to disclose the diffuse X--ray emission of the aforementioned galaxies.
Galaxies and overmerging: what does it take to destroy a satellite galaxy?
Stelios Kazantzidis, University of Zürich, Institute for Theoretical Physics, Zürich, Switzerland definite talk
The Ultra Compact Dwarf Galaxies (UCDs) recently discovered in the Fornax and Virgo clusters exhibit structural similarity to the nuclei of dwarf ellipticals suggesting that remainder of the galaxy and its halo have been gravitationally disrupted. We use high resolution N-body simulations with up to ten million particles to investigate in detail the disruption of dwarf galaxy halos that orbit within a cluster potential. Complete disruption of satellites only occurs for those models with shallow cusp slopes and with very low concentrations, whereas satellites with steep central density cusps are resilient to tidal disruption. The result remains valid for cluster potentials spanning the entire range of possible concentrations. We are unable to reproduce the UCDs within a Cold Dark Matter model and this problem is even more exacerbated when we consider dwarf models having a stellar disk, as this increases even further the central density.
Dark Matter in the Ursa Minor Draco
Jan Kleyna, University of Cambridge, Institute of Astronomy definite talk
We present recent measurements of stellar velocities in the Draco and Ursa Minor dSphs, and discuss the implications for the nature of their dark matter haloes.
Dark matter in galaxies
Anatoly Klypin, New Mexico State University, Department of Astronomy, NM, USA definite talk
I will review observational and theoretical status on constraints on the dark matter in galaxies: 1) rotation curves of dwarf and LSB galaxies: central subKpc region: effects of inclanation, bars, bulges, test of mass modeling; 2) Bars and status of disruption of cusps by bars or satellites; 3) dark matter on intermediate (10kpc) scales; 4) dark matter on 100kpc scales; 5) MOND and rotation curves.
The luminosity function of the globular cluster Palomar 5 and its tidal tails
Andreas Koch, Max-Planck Institut fuer Astronomie, Heidelberg, Germany definite poster
We present the stelar main sequence luminosity function (LF) of the tidally disrupted globular cluster (GC) Palomar 5 and that of a portion of its tidal tails. These tails have been discovered (Odenkirchen et al. 2001) to emanate from the cluster as a result of its tidal interaction with the Milky Way, increasingly during disk-passages. Our results indicate that the LF of the cluster's main body exhibits a significant degree of flattening compared to other GCs, due to its advanced dynamical evolution. The LF of the tails is, in turn, enhanced with faint, low-mass stars, which we interpret as evidence of mass segregation due to energy equipartition.
Parametric Tidal Excitation of Dwarf Spheroidal Galaxies
Jeff Kuhn, Institute for Astronomy, Honolulu, HI, USA definite talk
Even mildly elliptical Dwarf Spheroidal (dS) orbits around the Milky Way (MW) can cause significant dynamical effects within a dS stellar system over the MW lifetime. We describe (analytically and numerically) a parametric oscillation mechanism which is likely to have inflated the velocity dispersions of many of the MW dS. New deep wide-field observations of Ursa Minor and Draco support our conclusion that these are not dark matter-dominated stellar systems.
New Deep Wide-field Observations of Draco and Ursa Minor
Jeff Kuhn, Institute for Astronomy, Honolulu, HI, USA definite poster
Wide-field CFHT photometric observations of Ursa Minor and Draco have been obtained in order to study the color-magnitude (c-m) and statistical properties of the dS stellar, and background star and galaxy object distributions. These new data reach 25 magnitude in V and I and cover approximately one square degree. The data provide particularly clean c-m information and interesting constraints on the ``clumpiness'' of the dS stars.
Modeling the Tidal Tails of the Sagittarius Dwarf Galaxy
David Law, University of Virginia, Department of Astronomy Charlottesville, VA, USA definite talk
N-body simulations are used to model the tidal disruption of the Sagittarius (Sgr) dwarf galaxy. Critical to this endeavor are constraints set by the positions and velocities of M giants in the Sgr tidal arms recently revealed by the Two Micron All-Sky Survey (2MASS), supplemented by other data from the literature. The simulated Sgr dwarf is placed on a variety of orbits within a Johnston-Hernquist Milky Way potential parameterized by variable circular velocities, halo-to-disk ratios, solar Galactocentric distances, halo flattenings and radial profiles. 200 separate test particle orbits have been run to explore a wide range of model Milky Way potentials and dwarf galaxy characteristics. This family of models is delimited by the data to a relatively narrow allowed range of parameters. We present our best-fitting model, and discuss the orbital period, apoGalacticon distance, current space velocity, mass to light ratio, and other characteristics of the Sgr dwarf. In addition, we discuss the implications of this model for the rotation curve of the Milky Way and the flattening of the Galactic halo.
Tidal dwarfs in Wolf-Rayet galaxies
Ángel R. López-Sánchez, Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain definite talk
Wolf-Rayet (WR) galaxies are ideal objects to investigate the triggering mechanisms of the star formation. We think that interactions between dwarf galaxies could explain the triggering in many cases. The ages of these systems showing the WR feature are well estimated, therefore, if an external companion object has induced the star formation, it must be near the burst and their interrelation probably very evident. We are performing a morphological and spectroscopic analysis of a sample of Wolf-Rayet galaxies looking for these interaction features. We have found several interesting cases of mergers, tidal tails and dwarf objects surrounding the main WR galaxies. We present in this contribution our preliminary conclusions about these systems, remarking the cases of Hickson Compact Group 31, Mrk 1087 and IRAS 08208+2816. We will emphasise the nature and probably evolution of the tidal dwarfs found in these particular WR galaxies.
Ghostly Streams from the past still present
Donald Lynden Bell, Cambridge Institute of Astronomy, Cambridge, United Kingdom definite talk
A brief history of stellar and gaseous streams within the Galaxy and around galaxies lays the foundation for the far more accurate and definitive results that the future promises. The possibility that the Galaxy's dark halo may still have invisible clumps of dark matter similar to those that harbour the dwarf spheroidals is explored and one possible consequence suggests that a first detection may be awaiting interpretation.
New Surveys of Companions to E and S0 Galaxies
Barry Madore, Carnegie Observatories, Pasadena, CA, USA definite talk
We have investigated the number of physical companion galaxies for a sample of relatively isolated elliptical galaxies. The {\it NASA/IPAC Extragalactic Database} (NED) has been used to reinvestigate the incidence of satellite galaxies for a sample of 34 elliptical galaxies, first investigated by Bothun \& Sullivan (1977) using a visual inspection of Palomar Sky Survey prints out to a projected search radius of 75 kpc. We have repeated their original investigation using data cataloged data in NED. Nine of these ellipticals appear to be members of galaxy clusters: the remaining sample of 25 galaxies reveals an average of +1.0 $\pm $ 0.5 apparent companions per galaxy within a projected search radius of 75~kpc, in excess of two equal-area comparison regions displaced by 150-300~kpc. This is nearly an order of magnitude larger than the +0.12 $\pm$ 0.42 companions/galaxy found by Bothun \& Sullivan for the identical sample. Making use of published radial velocities, mostly available since the completion of the Bothun-Sullivan study, identifies the physical companions and gives a somewhat lower estimate of +0.4 companions per elliptical. This is still a factor of 3$\times$ larger than the original statistical study, but given the incomplete and heterogeneous nature of the survey redshifts in NED, it still yields a firm lower limit on the number (and identity) of physical companions. An expansion of the search radius out to 300~kpc, again restricted to sampling only those objects with known redshifts in NED, gives another lower limit of 4.3 physical companions per galaxy. (Excluding five elliptical galaxies in the Fornax cluster this average drops to 3.5 companions per elliptical.) These physical companions are individually identified and listed, and the ensemble-averaged radial density distribution of these associated galaxies is presented. For the ensemble, the radial density distribution is found to have a fall-off consistent with $\rho \propto R^{-0.5}$ out to approximately 150~kpc. For non-Fornax cluster companions the fall-off continues out to the 300-kpc limit of the survey. The velocity dispersion of these companions is found to be constant with projected radial distance from the central elliptical, holding at a value of approximately $\sigma \sim \pm 300-350$~km/sec overall.
We will also report on a survey for companions surrounding all S0 (lenticular) galaxies listed in the Revised Shapley-Ames Catalogue, and introduce the concept od a "morphological redshift.''
Substructure in the Galactic Halo
Steven Majewski, University of Virginia, Dept. of Astronomy, Charlottesville, VA, USA definite talk
The notion that the Galactic halo should be networked with streams from the disruption of satellite systems has a long theoretical history. At long last, observations are catching up to bear out theoretical models of a highly substructured Milky Way halo. Along with a brief history of the subject, I will review the present state of our (at least my!) knowledge about the accreted halo, with emphasis on the observational picture. This will include discussion of various programs to map the Sagittarius and other halo star streams as well as observations of potential stream progenitors: dwarf spheroidal galaxies and star clusters.
Tidal disruption of the Milky Way dwarf satellites
David Martinez Delgado, Instituto de Astrofisica de Canarias, La Laguna, Tenerife, Spain definite oral
We are carrying on a long-term project to study the tidal disruption of the dwarf companions of our Galaxy. Our searching strategy is based in wide field, deep color-magnitude diagrams (CMDs) reaching the old main sequence turn-off of the galaxy's stellar population. This technique has already been sucessful in detecting the tidal tail of the Ursa Minor dSph, the tidal stream of the Sgr stream or the remnants of the Sgr around the young globular cluster Pal 12. Our observations are compared with N-body simulations of the tidal disruption of the satellites, to ultimately constrain the role of the satellite destruction in the building of the Galactic halo, the dark matter of the satellite and the potential of the Milky Way. In this talk, I report some of our last results of this project, including; i) new observations of the Sgr northern stream, including an updated theoretical model of its tidal disruption;
ii) the association of some globular cluster to the Sgr dwarf galaxy;
iii) new insights on the tidal disruption of the Ursa Minor dSph and its dark matter content.
The origin of the Magellanic Stream and constraints on the halo gas density
Chiara Mastropietro, University of Zürich Institute for Theoretical Physics, Switzerland definite talk
We use high resolution N-Body/SPH simulations to study the interaction between the Magellanic Clouds and the Milky Way, the oldest studied interacting "galaxy-stream" system whose origin still proves controversial. For the first time we self-consistently follow the hydro-dynamical stripping processes and gravitational tidal effects modeling the dark and hot extended halo components as well as the stellar/gaseous disk of the Milky Way and Magellanic Clouds. We compare our simulations with the latest observational data on the kinematics and properties of the Stream and internal structure of the Magellanic Clouds. The velocity structure along the Stream is very sensitive to the halo gas density which can be accurately constrained out to 100 kpc: we do not find evidence for missing baryonic material in the form of diffuse gas.
Dwarf spheroidals in a CDM universe
Lucio Mayer, Institute of Theoretical Physics, University of Zurich, Switzerland definite talk
Small disk galaxies embedded in large dark matter halos undergo progressive tidal disruption and/or morphological transformations into dwarf spheroidals as they orbit within the massive halos of spiral galaxies like the Milky Way. In the process they produce tidal tails of dark and baryonic material. The degree of tidal disruption, and thus the prominence of the tails, will depend on the depth of their potential wells and on their orbits. We use high-resolution N-Body/SPH simulations and semi analytical models to explore the disruption process with galaxy models motivated by cold dark matter scenarios. We find that complete disruption is difficult to achieve for galaxies with halos as concentrated as expected in cold dark matter models at scales of $50$ km/s ($c > 12$). These systems would hardly produce extended stellar tails as those possibly detected around Carina. Instead, gas can be removed more easily than stars because the effects of ram pressure and photoevaporation by the cosmic UV background add to that of tidal stripping.
Determining the Structure of M31's Giant Tidal Stream.
Alan McConnachie, Cambridge University, Institute of Astronomy, United Kingdom
(A. McConnachie, M. Irwin, A. Fergusson, N. Tanvir, R. Ibata and G. Lewis) definite poster
The wide-field CCD camera at the CFH telescope was used to survey the giant stellar stream in the Andromeda galaxy, resolving stars down the red giant branch in M31 to $I \simeq 25$, a magnitude deeper than our previous INT survey of this galaxy and extending $1^{\circ}$ further out. The stream is seen to extend out to the south-east of M31 as far as we have surveyed (some $4.5^{\circ}$, corresponding to a projected distance $\sim60$ kpc). It is a linear structure in projection, and the eastern edge of the stream presents a sharp boundary in star counts suggesting that it remains a coherent structure. By analysing the luminosity function of the metal rich component of the stream we find that, at the furthest extent of our survey, the stream is $100$kpc further away along the line of sight than M31. It can then be traced to a point on the north-western side of the galaxy where it is some 30 kpc in front of M31, at which point the
stream turns away from our survey area.
The Stellar Populations in the Sagittarius Dwarf spheroidal
Lorenzo Monaco, INAF - Osservatorio Astronomico di Bologna, Bologna, Italy definite poster
We present a series of new results coming from a V,I photometry of a wide field 1 deg x 1 deg around M54, which is considered the nucleus of the Sagittarius Dwarf spheroidal galaxy (SGR). In particular we show evidences for: 1) an extended blue HB (which suggest the presence of an old metal poor population) 2) a complex RGB structure, where at least three main component (at different metallicities) can be distinguished.
Satellite galaxies and dark matter substructure
Ben Moore, University of Zurich, Institute for Theoretical Physics, Zurich, Switzerland definite talk
I will review what we can learn about structure formation and dark matter from satellite galaxies.
Milky Way halo substructure from the Sloan Digital Sky Survey
Heidi Newberg, Rensselaer Polytechnic Institute, Physics Dept., Troy, NY, USA definite talk
We have used thousands of square degrees of Sloan Digital Sky Survey (SDSS) data to discover and study spatial substructure in the halo of the Milky Way, including the tidal stream of the Sagittarius dwarf galaxy and a ring of stars 18-20 kpc from the Galactic Center. Stellar populations in the streams are studied through color-magnitude diagrams; kinematics of the structures are studied with thousands of spectra taken with the SDSS spectrograph and elsewhere.
Statistical Studies of Dwarf Galaxies Using The 2dF Galaxy Redshift Survey
Noelia Noel, Instituto de Astrofisica de Canarias, Tenerife, Spain definite poster
We present results from the statistical studies of satellites galaxies around their host galaxies to redshift 0.03
Proper Motions of the Carina, Fornax, Sculptor and Ursa Minor Dwarf Spheroidals
Slawomir Piatek, Rutgers Univ. & NJIT, Dept of Physics & Astronomy, Piscataway, NJ, USA definite poster
We present proper motions, and the space velocities and Galactic orbits derived from them, for four dwarf spheroidal galaxies (dSphs): Carina, Fornax, Sculptor, and Ursa Minor. The data consist of images of 2--3 distinct fields in each dSph taken at three epochs spaced over 2--5~yrs with the Hubble Space Telescope and either the Planetary Camera 2 or the Space Telescope Imaging Spectrograph. Every field is approximately centered on a known quasi-stellar object, which serves as a reference point. We briefly discuss the implications of a dSph's orbit on: 1.- the star formation history of the dSph; 2.-the importance of the Galactic tidal force on the structure of the dSph; and 3.- the membership of the dSph in proposed "streams.''
Tidal Streams around External Galaxies
Michael Pohlen, Instituto de Astrofísica de Canarias (IAC), La Laguna, Tenerife, Spain definite poster
In the current hierarchical clustering scenarios of galaxy formation minor mergers and accretion events have played a significant role building up present day disk galaxies. This view is supported by recent discoveries of tidal features such as ring-like structures around the Milky Way (eg.~the Sagittarius dwarf galaxy) and M31. We present several cases of tidal streams around other external spiral galaxies outside dense clusters to provide more typical examples how such features look like. We provide a compilation of serendipitously detected streams plus the presentation of a systematic survey to characterise the structure and number of stellar streams up to a limiting brightness by means of a deep, wide field survey. Simulations will be used to interpret these as debris from satellite disruption.
Observing the Dark Matter Density Profile of Isolated Galaxies from SDSS Satellite Dynamics
Francisco Prada, IAC and ING, Spain definite oral
Using the Sloan Digital Sky Survey (SDSS), we probe the halo mass distribution by studying the velocities of satellites orbiting isolated galaxies. In a subsample that covers 2500 sq. degrees on the sky, we detect about 3000 satellites with absolute blue magnitudes going down to $M_B = -14$; most of the satellites have $M_B=$-16 to -18, comparable to the magnitudes of M32 and the Magellanic Clouds. After a careful, model-independent removal of interlopers, we find that the line-of-sight velocity dispersion of satellites declines with distance to the primary. For an $L_*$ galaxy the r.m.s. line-of-sight velocity changes from $\sim 120$ km/s at 20 kpc to $\sim 60$ km/s at 350 kpc. This decline agrees remarkably well with theoretical expectations, as all modern cosmological models predict that the density of dark matter in the peripheral parts of galaxies declines as $\rho_{DM} \propto r^{-3}$. Thus, for the first time we find direct observational evidence of the density decline predicted by cosmological models; we also note that this result contradicts alternative theories of gravity such as MOND. We also find that the velocity dispersion of satellites within 100 kpc scales with the absolute magnitude of the central galaxy as $\sigma \propto L^{0.3}$; this is very close to the Tully-Fisher relation for normal spiral galaxies.
Galaxy Formation and the Faint End of the Luminosity Function
Joel Primack, University of California, Santa Cruz, USA definite talk
I review recent work on galaxy formation, both semi-analytic modeling and hydrodynamic simulations, with special attention to the faint end of the LF. Cold dark matter predicts that the abundance of dark halos increases rapidly with decreasing halo mass or circular velocity, while astrophysical processes such as photoionization and feedback can decrease star formation in an increasing fraction of these halos at decreasing halo circular velocity. The question is whether this basic idea works in detail and adequately accounts for the observed abundance of dwarf galaxies in various environments. Other data that might also be relevant will also be discussed briefly, including properties of damped Lyman alpha systems.
Mass substructure from gravitational lensing
Peter Schneider, Inst. f. Astrophysik, Bonn University Germany definite talk
If galaxies contain subhalos as predicted by CDM models, but not observed in the form of satellite galaxies, they will have an impact on the flux ratios of multiply-imaged sources in gravitational lens systems. I shall explain this effect, demonstrate that nearly all quadruply-imaged lens systems have anomolous flux ratios, and that these are most naturally explained in terms of the `missing satellites'. Hence, the difficulties to explain observed flux ratios is in fact an asset to test one of the robust predictions of the standard cosmological model.
Is the Draco dwarf spheroidal galaxy tidally disrupted?
Mathieu Segall, University Strasbourg, Strasbourg, France definite poster
We present a new survey of the Draco dwarf spheroidal galaxy taken with the Wide Field Camera of the Isaac Newton Telescope. These new data, which cover approximately 4 sq deg around this galaxy probe significantly deeper than previous wide field surveys. The goal of this study is to try to find substructures in the halo of the Draco dwarf spheroidal galaxy, and in particular to search for evidence of tidal disruption.
A triple nucleus in the brightest cluster galaxy in Abell 193
Marc Seigar, UK Infrared Telescope, Joint Astronomy Centre, Hilo, USA possible poster
We present infrared images of the brightest cluster galaxy (BCG) in Abell 193. These images reveal a triple nucleus structure. Spectra of the three nuclei, also suggest that they are at the same redshift and are not chance projecttions of foreground or background galaxies. This suggests that a merger of 3 galaxies took place in order to form what we are currently seeing. Furthermore, the optical-infrared colors of the nuclei are also indicative of a merger in the recent history of this galaxy.
Dark Matter in Dwarf Galaxies: Latest Density Profile Results and Tests for Systematics
Joshua Simon, Astronomy Department University of California, USA definite talk
We present high-resolution two-dimensional velocity fields in H$\alpha$ and CO of the nearby dwarf galaxy NGC 2976. Our observations were made at both higher spatial resolution ($\sim75$ pc) and higher velocity resolution (13 km/s in H$\alpha$ and 2 km/s in CO) than most previous studies. We show that NGC 2976 has a very shallow dark matter density profile, with $\rho(r)$ lying between $\rho \propto r^{-0.3}$ and $\rho \propto r^{0}$. We carefully test the effects of systematic uncertainties on our results, and demonstrate that well-resolved, two-dimensional velocity data can eliminate many of the systematic problems that beset longslit observations. We also present a preliminary analysis of the velocity field of NGC 5963, which may have a nearly NFW density profile.
Satellites of Elliptical Galaxies
Rodney Smith, Cardiff University definite talk
Using well-defined selection criteria applied to the LEDA galaxy catalogue we have derived a sample of elliptical galaxies that can be classified as isolated. From this we have investigated the neighbourhood of these galaxies to determine the frequency and radial distribution of faint faint galaxies around them and derive an estimate of their surrounding satellite population. These are compared and contrasted to the satellite population around isolated spiral galaxies.
The Satellite-substructure Connection
James Taylor, University of Oxford, United Kingdom definite talk
The study of small satellite galaxies is of unprecedented importance in the current era of precision cosmology. Many important and unconstrained parameters, such as the tilt of the primordial power spectrum or the interaction and annihilation cross-sections of the dark matter particle, can be tested most sensitively on these scales. I describe some recent attempts to model halo substructure down to very small masses, using a semi-analytic model of halo formation. While comparing the predicted properties of dark matter substructure to the observed properties of satellite galaxies remains problematic, I show how some basic properties of halo substructure can be used to constrain models of dwarf galaxy formation. I then discuss how observations of satellites and halo substructure can, in turn, constrain models of dark matter and the shape of the dark matter power spectrum.
Is there evidence for cores in the halos of disk galaxies?: The Case of NGC-3109
Octavio Valenzuela, Department of Astronomy, New Mexico State University, Las Cruces, NM, USA definite talk
We present a fully self-consistent barred model for the Local Group Magellanic type galaxy NGC3109. Our model is analyzed in a similar way as a real galaxy is observed. The model fits reasonably well the observed rotation curve, the one dimensional disk surface brightness profile as well as the two dimensional photometry and the bar orientation. The required stellar mass to light ratio is consistent with the observed (B-R) color and the detected amount of neutral hydrogen. We conclude that non-circular motions can actually explain the apparent mismatch between the observed rotation curve and the circular velocity of a cosmologically motivated model of NGC-3109. We do not find the need of a core in the dark matter halo or a modification in the Newtonian Gravity. This might well be the case for several other dwarfs and LSB galaxies.
Velocities of RR Lyrae Stars in the Sagittarius Tidal Stream
Anna Katherina Vivas, Centro de Investigaciones de Astronomia (CIDA), Merida, Venezuela definite talk
The QUEST RR Lyrae survey has discovered several substructures in the halo of the Milky Way which are likely related with tidal debris from disrupting dwarf spheroidal galaxies or globular clusters. All RR Lyrae stars in these groups have a very small range in mean magnitudes, indicating that they are located at the same distance. In order to understand better the origin of these clumps it is necessary to obtain kinematic information and confirm that the structures are coherent also in velocity space. We present here results of recent VLT spectroscopy of RR Lyrae stars associated with the tidal debris from the Sagittarius dwarf spheroidal galaxy.
Effelsberg HI Survey of Compact High-Velocity Clouds
Tobias Westmeier, Radioastronomisches Institut der Universitaet Bonn, Germany definite poster
Compact high-velocity clouds (CHVCs) are observed across the entire sky in the 21-cm line of neutral hydrogen. They are thought to represent the missing gaseous dark-matter satellites left over from the formation of the Local Group galaxies. CHVCs are characterised by small angular sizes and high radial velocities in excess of any Galactic rotation model. Neither stars nor molecular gas in CHVCs have been detected, so far.
We have investigated a selected sample of 12 CHVCs in the 21-cm line emission with the 100-m telescope in Effelsberg. We obtain an average peak column density of $N_{\rm HI} \sim 5 \cdot 10^{19} \; {\rm cm^{-2}}$ and typical angular sizes of about $30' \ldots 60'$ with respect to the $5 \cdot 10^{18} \; {\rm cm^{-2}}$ column density level. Our observations reveal a complex and inhomogeneous distribution of warm and cold neutral gas within most of these clouds, suggesting that they are not in equilibrium but disturbed and interacting. Head-tail structures and bow-shock shapes are quite common among our CHVC sample. In a few cases, clear signs for the warm envelope being stripped off the cold core have been found, indicating ram-pressure interaction of these CHVCs with a surrounding medium. Therefore, CHVCs appear to be embedded within an ambient medium. But it is not yet clear whether this ambient medium is associated with the Milky Way halo gas or the intergalactic medium of the Local Group.
Our results emphasise the importance of high-resolution, single-dish observations of CHVCs. Only single-dish telescopes can reveal both, the diffuse warm medium and the embedded cold clumps. Our results demonstrate that the study of both gas phases is necessary to extract the physical conditions within CHVCs and their surroundings.
On the possible merger origin of dwarf galaxies
Markus Wetzstein, Max-Planck-Institut fuer Astronomie, Heidelberg, Germany definite talk
We present merger simulations of disk galaxies with special emphasize on evolution of structures in the tidal tails. This is achieved by careful refinement of those regions in the progenitor galaxies which later form the tidal tails. Bound stellar systems (hereafter Bound Tidal Objects, BTO) form within the tails and are analyzed. Their masses lie in the range of several 10^5 to a few 10^9 solar masses. The massive BTOs are resolved with several 10^4 particles, allowing for a detailed photometric and kinematical analysis. Preliminary results are that the systems have exponential surface brightness profiles, do not rotate and are supported by anisotropic velocity dispersions. We find no indication for the presence of dark matter, while gas contributes a large fraction to the total mass of those objects. In isolated systems the BTOs can escape or remain bound to their host, in which case they can become bound satellites which might eventually be destroyed in the tidal field of the merger remnant. In a cluster environment, even tightly bound BTOs can escape from their host and might contribute to the population of cluster dwarf galaxies. We compare our results to observations of dwarf galaxies in the field and in clusters.
Satellites and CDM models
Simon White, Max Planck Institute for Astrophysics, Garching, Germany definite talk
I will discuss which aspects of the observed properties of satellite galaxies, in particular of Local Group satellites, are/are not surprising within the standard LCDM model.
The Tidal Tails of the Globular Cluster Omega~Cen.
Simone Zaggia, INAF - Oss. Astr. Trieste, ITALY definite talk
We will present some new results on the northern tidal tail of Omega~Cen based on wide field CCD deep photometry of the cluster.
Halo Substructure in the QUEST RR Lyrae Survey
Robert Zinn, Yale University, Department of Astronomy, New Haven, Connecticut, USA definite talk
The QUEST survey has discovered 480 RR Lyrae variables in 380 sq. deg. of the sky, down to a limiting magnitude of $V\approx19.7$ ($\approx60$ kpc from the Sun). We will briefly discuss the techniques used to discover the variables and to determine their periods and light curves and the completeness of the survey. The spatial distribution of the stars reveals several significant density enhancements. One of these features is part of the tidal stream from the Sgr dSph galaxy, and another is related to the globular cluster Pal 5. The other enhancements have not yet been associated with known objects in the halo. We will also discuss the first results from a spectroscopic follow-up project to measure the radial velocities and metal abundances of the stars in the density enhancements.