Color bimodality: Implications for galaxy evolution

We use a sample of 69726 galaxies from the SDSS to study the variation of the bimodal color-magnitude (CM) distribution with environment. Dividing the galaxy population by environment (Sigma_5) and luminosity (-23<M_r<-17), the u-r color functions are modeled using double-Gaussian functions. This enables a deconvolution of the CM distributions into two populations: red and blue sequences. The changes with increasing environmental density can be separated into two effects: a large increase in the fraction of galaxies in the red distribution, and a small color shift in the CM relations of each distribution. The average color shifts are 0.05+-0.01 and 0.11+-0.02 for the red and blue distributions, respectively, over a factor of 100 in projected neighbor density. The red fraction varies between about 0% and 70% for low-luminosity galaxies and between about 50% and 90% for high-luminosity galaxies. This difference is also shown by the variation of the luminosity functions with environment. We demonstrate that the effects of environment and luminosity can be unified. A combined quantity, Sigma_mod = Sigma_5/Mpc^{-2} + L_r/L_{-20.2}, predicts the fraction of red galaxies, which may be related to the probability of transformation events. Our results are consistent with major interactions (mergers and/or harassment) causing galaxies to transform from the blue to the red distribution. We discuss this and other implications for galaxy evolution from earlier results and model the effect of slow transformations on the color functions.Comment: 14 pages, 8 figures, in AIP Conf. Proc., The New Cosmology, eds. R. E. Allen et al. (aka. The Mitchell Symposium), see http://proceedings.aip.org/proceedings/confproceed/743.jsp ; v2: replaced Figure 5 which was incomplete in original submissio

The Star Formation History of the Hubble Sequence: Spatially Resolved Colour Distributions of Intermediate Redshift Galaxies in the Hubble Deep Field

We analyse the spatially resolved colours of distant galaxies of known redshift in the Hubble Deep Field, using a new technique based on matching resolved four-band internal colour data to the predictions of evolutionary synthesis models. We quantify the relative age, dispersion in age, ongoing star-formation rate, star-formation history, and dust content of these galaxies. To demonstrate the potential of the method, we study the near-complete sample of 32 I ~ 0.5 studied by Bouwens et al (1997). The dispersion of the internal colours of a sample of 0.4<z<1 early-type field galaxies in the HDF indicates that ~40% [4/11] show evidence of star formation which must have occurred within the past third of their ages at the epoch of observation. For a sample of well-defined spirals, we similarly exploit the dispersion in colour to analyse the relative histories of bulge and disc stars, in order to resolve the current controversy regarding the ages of galactic bulges. Dust and metallicity gradients are ruled out as major contributors to the colour dispersions we observe in these systems. The median ages of bulge stars are found to be signicantly older than those in galactic discs, and exhibit markedly different star-formation histories. This result is inconsistent with a secular growth of bulges from disc instabilities, but consistent with gradual disc formation by accretion of gas onto bulges, as predicted by hierarchical theories. We extend our technique in order to discuss the star formation history of the entire Bouwens et al sample in the context of earlier studies concerned with global star formation histories.Comment: 8 colour postscript figures plus LaTeX source; submitted to MNRAS. Uses the mnras.sty LaTeX style fil

A faint galaxy redshift survey to B=24

Using the multislit LDSS-2 spectrograph on the {\it William Herschel Telescope} we have completed a redshift survey in the magnitude range $22.5<B< 24$ which has produced 73 redshifts representing a 73\% complete sample uniformly-selected from four deep fields at high Galactic latitude. The survey extends out to $z>1$ and includes the highest redshift galaxy ($z=1.108$) yet discovered in a field sample. The median redshift, \zmed=0.46, and form of the redshift distribution constitute compelling evidence against simple luminosity evolution as an explanation of the large excess of faint galaxies ($\simeq\times$2--4 no-evolution) seen in this magnitude range. Rather we identify the excess population as blue objects with $z\sim 0.4$ and $B$\, luminosities similar to local $L^*$ galaxies indicating a dramatic decrease in the density of such objects over the last Hubble time, confirming the trends found in brighter redshift surveys. We also find a marked absence of {\it very} low redshift galaxies ($z<$0.1) at faint limits, severely constraining any significant steepening of the local field galaxy luminosity function at low luminosities.Comment: uuencoded compressed postscript. The preprint are also available at URL http://www.ast.cam.ac.uk/preprint/PrePrint.htm

Galaxy Morphology from NICMOS Parallel Imaging

We present high resolution NICMOS images of random fields obtained in parallel to other HST observations. We present galaxy number counts reaching H=24. The H-band galaxy counts show good agreement with the deepest I- and K-band counts obtained from ground-based data. We present the distribution of galaxies with morphological type to H<23. We find relatively fewer irregular galaxies compared to an I-band sample from the Hubble Deep Field, which we attribute to their blue color, rather than to morphological K-corrections. We conclude that the irregulars are intrinsically faint blue galaxies at z<1.Comment: 13 pages, including 4 figures. Accepted for publication in ApJ Letter

The connection between the peaks in velocity dispersion and star-forming clumps of turbulent galaxies

We present Keck/OSIRIS adaptive optics observations with 150-400 pc spatial sampling of 7 turbulent, clumpy disc galaxies from the DYNAMO sample ($0.07<z<0.2$). DYNAMO galaxies have previously been shown to be well matched in properties to main sequence galaxies at $z\sim1.5$. Integral field spectroscopy observations using adaptive optics are subject to a number of systematics including a variable PSF and spatial sampling, which we account for in our analysis. We present gas velocity dispersion maps corrected for these effects, and confirm that DYNAMO galaxies do have high gas velocity dispersion ($\sigma=40-80$\kms), even at high spatial sampling. We find statistically significant structure in 6 out of 7 galaxies. The most common distance between the peaks in velocity dispersion and emission line peaks is $\sim0.5$~kpc, we note this is very similar to the average size of a clump measured with HST H$\alpha$ maps. This could suggest that the peaks in velocity dispersion in clumpy galaxies likely arise due to some interaction between the clump and the surrounding ISM of the galaxy, though our observations cannot distinguish between outflows, inflows or velocity shear. Observations covering a wider area of the galaxies will be needed to confirm this result.Comment: Accepted for publication in MNRA

The Mass Assembly Histories of Galaxies of Various Morphologies in the GOODS Fields

We present an analysis of the growth of stellar mass with cosmic time partitioned according to galaxy morphology. Using a well-defined catalog of 2150 galaxies based, in part, on archival data in the GOODS fields, we assign morphological types in three broad classes (Ellipticals, Spirals, Peculiar/Irregulars) to a limit of z_AB=22.5 and make the resulting catalog publicly available. We combine redshift information, optical photometry from the GOODS catalog and deep K-band imaging to assign stellar masses. We find little evolution in the form of the galaxy stellar mass function from z~1 to z=0, especially at the high mass end where our results are most robust. Although the population of massive galaxies is relatively well established at z~1, its morphological mix continues to change, with an increasing proportion of early-type galaxies at later times. By constructing type-dependent stellar mass functions, we show that in each of three redshift intervals, E/S0's dominate the higher mass population, while spirals are favored at lower masses. This transition occurs at a stellar mass of 2--3 times 10^{10} Msun at z~0.3 (similar to local studies) but there is evidence that the relevant mass scale moves to higher mass at earlier epochs. Such evolution may represent the morphological extension of the ``downsizing'' phenomenon, in which the most massive galaxies stop forming stars first, with lower mass galaxies becoming quiescent later. We infer that more massive galaxies evolve into spheroidal systems at earlier times, and that this morphological transformation may only be completed 1--2 Gyr after the galaxies emerge from their active star forming phase. We discuss several lines of evidence suggesting that merging may play a key role in generating this pattern of evolution.Comment: 24 pages, 1 table, 8 figures, accepted for publication in Ap

Galaxy bimodality versus stellar mass and environment

We analyse a z<0.1 galaxy sample from the Sloan Digital Sky Survey focusing on the variation of the galaxy colour bimodality with stellar mass and projected neighbour density Sigma, and on measurements of the galaxy stellar mass functions. The characteristic mass increases with environmental density from about 10^10.6 Msun to 10^10.9 Msun (Kroupa IMF, H_0=70) for Sigma in the range 0.1--10 per Mpc^2. The galaxy population naturally divides into a red and blue sequence with the locus of the sequences in colour-mass and colour-concentration index not varying strongly with environment. The fraction of galaxies on the red sequence is determined in bins of 0.2 in log Sigma and log mass (12 x 13 bins). The red fraction f_r generally increases continuously in both Sigma and mass such that there is a unified relation: f_r = F(Sigma,mass). Two simple functions are proposed which provide good fits to the data. These data are compared with analogous quantities in semi-analytical models based on the Millennium N-body simulation: the Bower et al. (2006) and Croton et al. (2006) models that incorporate AGN feedback. Both models predict a strong dependence of the red fraction on stellar mass and environment that is qualitatively similar to the observations. However, a quantitative comparison shows that the Bower et al. model is a significantly better match; this appears to be due to the different treatment of feedback in central galaxies.Comment: 19 pages, 17 figures; accepted by MNRAS, minor change