Next Contents Previous

3.4. Galaxy Structures at High Redshift z > 2

The structures and morphologies of galaxies at z > 2 are just now being studied in detail. Early work in this area suggested that galaxies selected by the Lyman-break technique have compact structures with outer light envelopes (Giavalisco et al. 1996). These compact structures have half-light radii a few kpc in size, similar to the bulges of modern spirals or moderate luminosity spheroids. Many of these compact galaxies have steep light profiles and asymmetrically distributed outer nebulosity. The depth of this early imaging was quickly superseded by the Hubble Deep Field North which showed a rich diversity of galaxy structures (Figure 5; Ferguson, Dickinson & Williams 2000). The morphologies of these galaxies is still however a largely unexplored area of parameter space.

Figure 5

Figure 5. The morphologies of bright galaxies, mostly at z > 2, in the Hubble Deep Field North showing the peculiar and non-compact structures of these galaxies, many with several bright central regions or knots.

One of the reasons the morphologies of z > 2 galaxies have not been studied in detail is that describing their structures is not a trivial problem, as very few galaxies at high redshift can be identified as objects that would fit on the Hubble Sequence (e.g., Giavalisco et al. 1996; Conselice et al. 2003a; Lotz et al. 2003). One way to approach this problem is to use a purely descriptive approach described above, while another is to use quantitative techniques to characterize these structures. The has been done in Conselice et al. (2003a) and Conselice et al. (2004b) for galaxies at z > 2. The CAS systems shows that at the highest redshifts there is a real dichotomy in the galaxy population such that the most luminous and most massive galaxies are consistent with undergoing a major merger based on CAS indices, particularly the asymmetry index (Conselice et al. 2003a). What is found is that half of all bright, MB < - 21 or massive M* > 1011 Modot galaxies are actively undergoing a major merger. The fainter and lower mass galaxies have peak merger fractions at z ~ 2.5 that are only 20%, or lower. The relative fraction of mergers declines at lower redshifts very quickly for these massive and luminous galaxies as power law alpha (1 + z)3-5 (Conselice et al. 2003). A comparison to hierarchical assembly models of galaxies is shown in Figure 6 using GALFORM simulations (e.g., Benson et al. 2002). These models over-predict the number of major mergers occurring for the brightest galaxies at z < 1, consistent with the fact that there are too many bright K-band selected galaxies at z ~ 1 - 1.5 than predicted in Cold Dark Matter based models (Somerville et al. 2004; see also Section 4.3).

Figure 6

Figure 6. Left panel: major merger fractions to z ~ 3 at magnitude limits MB = - 21 and -19. Semi-analytical model predictions are also shown. Right panel: Stellar mass accretion history from major mergers as a function of initial mass (see Conselice et al. 2003a).

Next Contents Previous