B. Redshift Surveys

The analysis presented in the previous section is fairly sensitive to the assumed redshift distribution of the peculiar galaxy population. However recent evidence from redshift surveys, most notably the Canada-France Redshift Survey (CFRS) suggests that, at least to I < 22 mag (the typical morphologically resolved magnitude limit for deep HST imaging, prior to the Hubble Deep Field), the great majority of objects are at redshifts z < 1.5 [18]. Therefore even quite deep HST WF/PC2 I₈₁₄-band (the band most commonly used to quantify the morphological composition of the distant field) images should generally be compared with B-band or U-band local galaxy data. Since most local surveys of galaxy morphology are based on blue-sensitive photographic plates, the rest-frame B-band is in fact where we are most familiar with the appearance of galaxies, and the effects of bandshifting on these galaxies are likely to be less significant than the effects of limited signal-to-noise or binning. Images of local galaxies in U-band (blueward of the 4000Å break) are often substantially more irregular than B-band data, but they do not yet show the fantastic morphological variations (such as disappearing bulges) seen in images in the far-UV (~ 1500Å), as presented by O'Connell and others at this conference.

This line of reasoning has recently been spectacularly confirmed [4] by Brinchmann et al. (1997), who have applied an objective classification scheme, again calibrated using pixel-by-pixel K-corrections, to a set of ~ 300 HST I₈₁₄-band images of galaxies with known redshifts taken from the CFRS and LDSS [11] surveys. Because the statistical completeness of this sample is very well understood, reliable number-redshift histograms can be constructed for the various morphological types. The morphologically resolved n (z) result obtained by Brinchmann et al. is shown in Figure 3, and confirms that irregular/peculiar/merging systems are already greatly in excess of the predictions of no-evolution and mild-evolution models at redshifts z ~ 1. It is emphasized that: (a) because of the complete redshift information, morphological K-corrections have been accounted for explicitly for each galaxy in this study, and (b) in any case morphological K-correction effects cannot be dominant in this survey, because the peculiar excess is already large by z = 1, at which point one is only just beginning to probe into the ultraviolet.

Figure 3. Morphologically segregated number counts from Brinchmann et al. 1997 (in preparation), based on data from the CFRS/LDSS collaboration. The bins show counts as a function of redshift for irregular/peculiar/merger systems (top), spirals (middle),and ellipticals (bottom). Morphological classifications have been made from WF/PC2 images using the central concentration vs. asymmetry system described in the text and in Abraham et al. 1996a. The shaded region corresponds to the size of the "morphological K-correction" on the classification. Superposed on the histograms are the predictions of no-evolution and 1 mag linear evolution to z = 1 models.