2. HIGH REDSHIFT GALAXY MORPHOLOGY

The first line of evidence is the result of a quantitative comparison between the observed morphologies of local galaxies and the predicted appearance of their non-evolved high-redshift counterparts. In order to undertake this comparison we have developed a technique for artificially redshifting local galaxy CCD images by assigning separate spectral energy distributions to individual pixels. Spectral energy distributions for each pixel are determined by using optical colors to interpolate between template spectra corresponding to local S0, Sab, Sbc, Scd, Sdm, and starbursting galaxies. Figure 1 illustrates the power of this technique by showing the excellent agreement between "predicted" and observed far-UV morphologies for a typical galaxy in our calibration sample ⁽¹⁾. This figure illustrates the extreme case in which the I-band morphology is extrapolated to the far-UV, corresponding to redshifts z > 4. In fact, at z > 4 the galaxian internal dynamical timescale is a substantial fraction of the age of the Universe, so the existence of morphologically unevolved galaxies is not expected.

Figure 2. The number-magnitude relations for morphologically segregated samples of galaxies from the HDF and MDS (from Abraham et al. 1996a). Open circles indicate counts obtained from automated classifications, closed circles indicate the results from the visual classifications of Ellis, and crosses indicate the results from the visual classifications of van den Bergh. The MDS counts are indicated by the stars on each panel. The no-evolution = 1 curves from Glazebrook et al. (1995), extrapolated to I = 25 mag, are superposed. The dashed line on the E/S0 diagram shows the effect of assuming = 0.1. The dotted line in panel (a) shows the I-band number counts determined by Smail et al. (1995) from two deep fields imaged with the Keck telescope.

With this capability to model the effects of bandshifting on observed morphology, and by assuming an approximate redshift distribution, one is able account for the impact of morphological K-corrections on morphologically segregated deep number-magnitude counts. Because the subjective nature of visual classification makes comparisons between different groups susceptible to large systematic errors [20], the best approach to making these counts is to adopt a quantitative morphological classification system. Several quantitative classification systems have now been developed [1, 3, 21]. Figure 2 shows the number-magnitude counts which result from applying a particularly simple system (based on measurements of central concentration, C, and asymmetry, A) to data from the Hubble Deep Field. Also shown are the no-evolution predictions for ellipticals, spirals, and irregular/peculiar/merger systems, constructed as described in Glazebrook et al. (1995) and Abraham et al. (1996a, b), by adopting Schechter luminosity functions (LFs) with parameters given by Loveday et al. (1992), and a high normalization = 0.03 h³ Mpc^-3. The predicted counts for the elliptical galaxies are based on a flat slope ( = -1) for the faint-end of the LF, rather than the turn-over originally found by Loveday et al. The steep counts for the irregular/peculiar/merger systems continues to the limits of the survey. Beyond I₈₁₄ = 22 mag the spiral counts show a significant excess over the no-evolution predictions. A weaker trend is seen for the spheroidal systems (whose counts are only marginally above the no-evolution prediction) and there is some evidence of a turn-over in the last magnitude interval.