3.2. Galaxy Structures at Low Redshift z < 1

At redshifts z < 1 most of the bright (M_B < - 20) and massive galaxies (M_* > 10¹⁰) are normal galaxies, that is ellipticals and spirals (Table 1; Figure 2). This relative fraction remains largely similar out to z ~ 1, with some important exceptions. In general, the co-moving density of elliptical and disk galaxies remains constant, to within a factor of 2, out to z ~ 1 with a slight decline (Figure 3; Brinchmann & Ellis 2000; Conselice et al. 2004b).

There is a more pronounced change in other features of normal galaxies from z ~ 1 to z ~ 0. These properties include co-moving B-band luminosity densities (_LB), and stellar mass densities (_*). While the number density evolution of Hubble types is the physical manifestation of the galaxy structure-redshift relationship, the evolution of other properties can reveal important clues for how this relationship is put into place, and why it might be evolving. The rest-frame B-band luminosity luminosity density evolution for galaxies of known morphology is shown in Figure 3. There is a clear decline with cosmic time in luminosity densities at z < 1 for all galaxies, including ellipticals and spirals. This peak in the B-band luminosity density at z ~ 1 is produced in normal galaxies, and must be due to recent star formation, as the stellar mass density for normal galaxies grows with time (Figure 3). The stellar mass density for ellipticals is half of its modern value at z ~ 1 in the Hubble Deep Field North (HDF-N). There is perhaps an over density of ellipticals at z ~ 1 in the HDF-N, and cosmic variance is an issue. Although a lower density of early types would only enhance the evolution in stellar mass for these systems. This effect is also seen in studies considering galaxies on the `red sequence', defined by the tight correlation between magnitude and color for early types. The stellar mass in red sequence galaxies increases by a factor of two from z = 1 to z = 0 (Bell et al. 2004), exactly the increase found when considering morphologically selected early types. Because of the large amount of co-moving luminosity in normal galaxies at z ~ 1, star formation must be occurring in early type galaxies during this time (see also Stanford et al. 2004). Luminosity and stellar mass functions suggest that this evolution is occurring in lower mass and lower luminosity systems (Conselice et al. 2004b), while the higher mass systems are perhaps largely formed by z ~ 1, or even earlier (e.g., Glazebrook et al. 2004).

Figure 3. The relative co-moving number (N), rest-frame B-band luminosity (LB), and stellar mass density (*) of galaxies as a function of redshift from deep NICMOS imaging of the Hubble Deep Field North (Conselice et al. 2004b). Points at redshifts z < 0.5 are taken from Brinchmann & Ellis (2000), Fukugita et al. (1998) and the 2dF/2MASS surveys (Cole et al. 2001).