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Register a new userBulges of disk galaxies at intermediate redshifts.II. Nuclear, disk and global colours in the Groth Strip
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Added by
Lilian Dom\\'inguez Palmero
Publication date
2008
fields
Physics
and research's language is
English
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We analyse colours of the nuclear regions of intermediate redshift disk galaxies, with the aim of obtaining empirical information of relative ages of bulges and disks at 0.1 < z < 1.3. We work with an apparent-diameter limited parent sample of 248 galaxies from the HST Groth Strip Survey. We apply a conservative criterion to identify bulges and potential precursors of present-day bulges based on nuclear surface brightness excess above the exponential profile of the outer parts. We measure bulge colours on wedge profiles opening on the semi-minor axis, and compare them to disk, and global galaxy colours. For 60% of galaxies with bulges, the rest-frame nuclear colour distribution shows a red sequence that is well fit by passive evolution models of various ages, while the remainder 40% scatters towards bluer colours. In contrast, galaxies without central brightness excess show typical colours of star forming population and lack a red sequence. We also see that, as in the local Universe, most of the minor axis colour profiles are negative (bluer outward), and fairly gentle, indicating that nuclear colours are not distinctly different from disk colours. This is corroborated when comparing nuclear, global and disk colours: these show strong correlations, for any value of the central brightness prominence of the bulge. Comparison with synthetic models of red sequence bulge colours suggests that such red bulges have stopped forming stars at an epoch earlier than ~ 1 Gyr before the observation. The correlation between nuclear and disk colours and the small colour gradients hints at an intertwined star formation history for bulges and disks: probably, most of our red bulges formed in a process in which truncation of star formation in the bulge did not destroy the disk.
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We aim to define a sample of intermediate-z disk galaxies harbouring central bulges, and a complementary sample of disk galaxies without measurable bulges. We intend to provide colour profiles for both samples, as well as measurements of nuclear, disk, and global colours, which may be used to constrain the relative ages of bulges and disks. We select a diameter-limited sample of galaxies in images from the HST/WFPC2 Groth Strip survey, which is divided into two subsamples of higher and lower inclination to assess the role of dust in the measures quantities. Mergers are visually identified and excluded. We take special care to control the pollution by ellipticals. The bulge sample is defined with a criterion based on nuclear surface brightness excess over the inward extrapolation of the exponential law fitted to the outer regions of the galaxies. We extract colour profiles on the semi-minor axis least affected by dust in the disk, and measure nuclear colours at 0.85 kpc from the centre over those profiles. Disk colours are measured on major axis profiles; global colours are obtained from 2.6-diameter apertures. We obtain a parent sample containing 248 galaxies with known redshifts, spectroscopic or photometric, spanning 0.1 < z < 1.2. The bulge subsample comprises 54 galaxies (21.8% of the total), while the subsample with no measureable bulges is 55.2% of the total (137 galaxies). The remainder (23%) is composed of mergers. We list nuclear, disk, and global colours (observed and restframe) and magnitudes (apparent and absolute), as well as galaxy colour gradients for the samples with and without bulges. We also provide images, colour maps, plots of spectral energy distributions, major-axis surface brightness profiles, and minor-axis colour profiles for both samples.
By combining surface brightness profiles from images taken in the HST/NICMOS F160W and ground-based (GB) $K$ bands, we have obtained NIR profiles for a well studied sample of inclined disk galaxies, spanning radial ranges from 20 pc to a few kpc. We fit PSF-convolved Sersic-plus-exponential laws to the profiles, and compare the results with the fits to the ground-based data alone. HST profiles show light excesses over the best-fit Sersic law in the inner ~1 arcsec. This is often as a result of inner power-law cusps similar to the inner profiles of intermediate-luminosity elliptical galaxies.
ABRIDGED: We use HSTACS and NICMOS imaging to study the structure and colors of a sample of nine late-type spirals. We find: (1) A correlation between bulge and disks scale-lengths, and a correlation between the colors of the bulges and those of the inner disks. Our data show a trend for bulges to be more metal-enriched than their surrounding disks, but otherwise no simple age-metallicity connection between these systems; (2) A large range in bulge stellar population properties, and, in particular, in stellar ages. Specifically, in about a half of the late-type bulges in our sample the bulk of the stellar mass was produced recently. Thus, in a substantial fraction of the z=0 disk-dominated bulged galaxies, bulge formation occurs after the formation/accretion of the disk; (3) In about a half of the late-type bulges in our sample, however, the bulk of the stellar mass was produced at early epochs; (4) Even these old late-type bulges host a significant fraction of stellar mass in a young(er) c component; (5) A correlation for bulges between stellar age and stellar mass, in the sense that more massive late-type bulges are older than less massive late-type bulges. Since the overall galaxy luminosity (mass) also correlates with the bulge luminosity (mass), it appears that the galaxy mass regulates not only what fraction of itself ends up in the bulge component, but also when bulge formation takes place. We show that dynamical friction of massive clumps in gas-rich disks is a plausible disk-driven mode for the formation of old late-type bulges. If disk evolutionary processes are responsible for the formation of the entire family of late-type bulges, CDM simulations need to produce a similar number of initially bulgeless disks in addition to the disk galaxies that are observed to be bulgeless at z=0.
We present high resolution absorption-line spectroscopy of 3 face-on galaxies, NGC 98, NGC 600, and NGC 1703 with the aim of searching for box/peanut (B/P)-shaped bulges. These observations test and confirm the prediction of Debattista et al. (2005) that face-on B/P-shaped bulges can be recognized by a double minimum in the profile of the fourth-order Gauss-Hermite moment h_4. In NGC 1703, which is an unbarred control galaxy, we found no evidence of a B/P bulge. In NGC 98, a clear double minimum in h_4 is present along the major axis of the bar and before the end of the bar, as predicted. In contrast, in NGC 600, which is also a barred galaxy but lacks a substantial bulge, we do not find a significant B/P shape.
We present a sample of over 50 luminous field bulges (including ellipticals) found in the Groth Strip Survey (GSS), with 0.73< z < 1.04 and with bulge magnitudes I <= 23. The exponential disk light is removed via decomposition of HST images using GIM2D. We find that 85% of these bulges are nearly as red as local E/S0s and have a shallow slope and a small color dispersion in the color-luminosity relation, suggesting roughly coeval formation. The surface brightnesses of these bulges are about 1 mag higher than local bulges. These results are explained adopting a drizzling scenario where a metal-rich early formation is later polluted by small amounts of additional star formation. Almost all disks have the same or bluer colors than their accompanying bulges, regardless of the bulge-disk ratio and bulge luminosity, as expected from semi-analytic hierarchical galaxy formation models. We present evidence that the few blue bulge candidates are not likely to be genuine blue ellipticals or bulges. Our deeper, more extensive, and less disk-contaminated observations challenge prior claims that 30% to 50% of field bulges or ellipticals are in a blue, star-forming phase at z < 1. We conclude that field bulges and ellipticals at z ~ 1, like luminous early- type cluster galaxies at the same redshift, are already dominated by metal-rich, old stellar populations that have been fading from a formation epoch earlier than z ~ 1.5. (abridged)
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