The summary figure for understanding the galaxy structure-redshift relationship is Figure 2. This relationship can be summarized simply as: At higher redshifts (early times) the fraction of bright galaxies that are peculiar in structure and morphology increases gradually at the expense of both spirals and ellipticals.
The final state of galaxy evolution surrounds us, and the modern universe is dominated by galaxies that can be classified on the Hubble sequence. A large fraction of all modern massive and bright galaxies are either ellipticals or spirals; only roughly 1-2% of all bright galaxies with MB < - 20 can be classified as peculiars (e.g., Marzke et al. 1998) (see Table 1). This changes gradually with redshift up to z ~ 1 and then more rapidly between 1 < z < 2; at z ~ 1 most galaxies have relaxed morphologies while at z ~ 2 most galaxies are peculiar (Conselice et al. 2004b; Table 1).
Redshift | Ellipticals | Spirals | Peculiars |
0.0 | 6.41 ± 0.01 | 6.98 ± 0.01 | 5.34 ± 0.03 |
0.5 | 6.3 ± 0.2 | 6.6 ± 0.1 | 5.7 ± 0.2 |
0.8 | 6.3 ± 0.2 | 6.2 ± 0.2 | 5.4 ± 0.3 |
1.0 | 6.4 ± 0.2 | 6.5 ± 0.2 | 6.1 ± 0.2 |
1.2 | 6.2 ± 0.2 | 6.1 ± 0.2 | 5.7 ± 0.3 |
1.3 | 6.0 ± 0.2 | 5.9 ± 0.2 | 6.3 ± 0.2 |
1.5 | 5.7 ± 0.2 | ... | 5.4 ± 0.3 |
1.6 | ... | 5.4 ± 0.3 | 5.7 ± 0.3 |
1.7 | 5.4 ± 0.3 | 5.7 ± 0.2 | 6.1 ± 0.2 |
1.9 | 5.7 ± 0.2 | 5.4 ± 0.4 | 5.7 ± 0.3 |
2.0 | 5.9 ± 0.2 | 6.1 ± 0.2 | 6.4 ± 0.1 |
a Galaxies at z = 0 are taken from the "Third Reference Catalogue of Bright Galaxies" (de Vaucouleurs et al. 1991). The other galaxy densities at z > 0 are an average between the densities found for each type in the Hubble Deep Fields North and South. |