Abstract details

The chronology of bulge and disk formation is a major unsolved issue in galaxy formation, which impacts on our global understanding of the Hubble sequence. We present colors of the nuclear regions of intermediate-redshift disk galaxies, with the aim of obtaining empirical information of ages of bulges at $0.1 < z < 1.3$. We work with 248 (123 inclined + 125 face-on) galaxies from the HST Groth Strip Survey and 404 (214 inclined + 190 face-on) from the HST GOODS-N field, covering redshifts $0.1 < z < 1.3$. We selected all objects with apparent diameter $R > 1.4''$, and with inclination $50\deg < i < 70\deg$, for the inclined samples, and $i < 50\deg$, for the face-on samples. We find that, as in the Local Universe, the minor axis color profiles are negative (bluer outward), and fairly gentle, indicating that bulge colors are not distinctly different from disk colors. We apply a conservative criterion to identify bulges and potential precursors of present-day bulges, based on nuclear excess surface brightness above the exponential profile of the outer parts. For galaxies with central brightness excesses, rest-frame color distributions show a red sequence. In contrast, galaxies without central brightness excesses show colors typical of star-forming populations. Clearly, something had truncated star formation in many high-density cores, already at $z=1$. The truncation epoch is uncertain, $1.5 < z < 10$. The color-magnitude distribution of intermediate-z bulges shows more color dispersion than that of bulges in the Local Universe. About $50\%$ of bulges are as red as local bulges, while the remainder are significantly bluer, a possible sign of late bulge formation.