3.3. Galaxy Structures at Medium Redshift 1 < z < 2

Morphological counts of galaxies from early Hubble Space Telescope imaging found a large increase in the number of peculiar/irregular galaxies at fainter magnitudes (e.g., Driver et al. 1995; Glazebrook et al. 1995). It was however unknown during these early Hubble observations what the redshifts, and therefore the characteristics, of these peculiar galaxies were. When redshifts for these faint galaxies became available, it was argued that Hubble types appeared in abundance by z ~ 1, and evolve only slightly down to lower redshifts (van den Bergh et al. 2001; Kajisawa & Yamada 2001).

Ultimately what is desired is a determination of Hubble types as a function of redshift for galaxies of different luminosities and stellar masses. This was performed for bright galaxies in the Hubble Deep Field North and South by Conselice et al. (2004b). The results of this are shown in Figure 3 for galaxies brighter than I = 27. As described in Section 3.1 there is a rapid decline with increasing redshift in the number of normal galaxies between z ~ 1 and z ~ 1.5, such that the co-moving density increases by 8.3 × 10³ Gpc^-3 Gyr^-1 for ellipticals and 5.7 × 10³ Gpc^-3 Gyr^-1 for spirals during this 1.6 Gyr period, roughly a factor of 10 increase in number densities. As discussed briefly in Section 3.5 this change in morphology is not caused by so-called morphological k-corrections in which galaxies appear different at different wavelengths. It can however be partially produced by selection effects, although a strong drop is also found when considering galaxies at a fixed absolute magnitude (Figure 2). Both spirals and ellipticals with M_B < - 20 should be found in the Hubble Deep Fields up to z ~ 2, and at even higher redshifts if passive evolution is considered (e.g., Conselice et al. 2004b).

The redshift range 1 < z < 2 is obviously critical for understanding the final onset and production of the Hubble sequence and the origin of the galaxy structure-redshift relationship. It is also the epoch (during a short 2.5 Gyrs!) where the star formation rate, AGN activity and stellar mass assembly is at its highest. Understanding how the galaxy structure-redshift relationship evolves during this epoch is critical for understanding the causes behind galaxy formation. It is therefore worth spending some time discussing what is found morphologically and structurally in the galaxy population between 1 < z < 2.

Figure 4 shows Advanced Camera for Surveys (ACS) images of the brightest galaxies in the rest-frame optical found within the Chandra Deep Field South Great Observatory Origins Survey (GOODS) imaging. Clearly, there is a rich morphological mix at this redshift, with many galaxies appearing similar to modern ellipticals and spirals, but with important structural differences that make them fundamentally different from modern normal galaxies. Another way to investigate this population is to study systems that have spectral energy distributions that likely place them at 1 < z < 2, such as the extremely red objects, discussed in Section 4.1. The images of these galaxies reveal that some are almost normal, with outer shell like features and what appear to be large star forming complexes. Understanding the physical causes behind these features will help reveal the formation mechanisms of galaxies.

Figure 4. The brightest galaxies in ACS GOODS images whose photometric redshifts place them at 1 < z < 2. These are ordered from brightest to faintest down to MB = -21. The upper number is the MB of each galaxy and the lower number is its redshift. There is a large diversity of properties, from systems that appear very peculiar to those that look similar to normal galaxies. Scale of these images is ~ 2" on each side, corresponding to ~ 17 kpc at these redshifts. See Figure4.gif for a high resolution version.