THE CLUSTERING OF BLUE AND RED GALAXIES AT B-SIMILAR-TO-25.5 MAG

Deep CCD imaging at the Isaac Newton Telescope provided a sample of si milar to 7000 faint galaxies, with B - R colours, to faint limits of B similar or equal to 25.5 mag and R similar or equal to 24.5 mag. The number counts of these galaxies are consistent with the previous galax y number counts of Metcalfe et al. and Tyson, showing at faint magnitu des an excess above non-evolving predictions, consisting of galaxies w ith moderately blue colours of 0.0 less than or equal to B - R less th an or equal to 1.5. The number of redder (B - R > 1.5) galaxies did no t exceed the non-evolving prediction. We calculate the angular correla tion function, omega(theta), of these faint galaxies, and compare with the predictions of new models which take into account the observed de pendence of galaxy clustering properties on morphology and luminosity. In agreement with previous studies (e.g., by Roche et al.), the omega (theta) amplitude of faint galaxies in our data set is found to fall w ell below the predictions of models in which galaxy clustering remains stable in proper coordinates and the redshift distribution N(z) has a non-evolving form, suggesting that the faint galaxy N(z) is more exte nded than this and includes a high proportion of z > 1 galaxies. We fi nd that a pure luminosity evolution model, with a steep luminosity fun ction for late-type galaxies, a significant increase with redshift in the star-forming activity of Sab and Sbc spirals, and mild dust-redden ing, gives a good fit to both the galaxy number counts and the omega(t heta) results, as obtained from this data set and from other published CCD surveys. However, the peak in the faint galaxy colour distributio n is closer to B - R similar or equal to 0.8 than to the bluer colour of B - R similar or equal to 0.4 predicted by our model (the reason fo r this discrepancy remains unclear). The omega(theta) amplitude of the redder (B - R > 1.5) galaxies in our sample appears to be higher than that of the bluer (B - R < 1.5) galaxies, in agreement with Roche et al. and Neuschaefer et al. We also find very little cross-correlation between red and blue galaxies. The lower omega(theta) amplitude of the bluer galaxies suggests that the steep fall in galaxy omega(theta) am plitudes at B > 23 is caused by the same blue galaxies that produce th e excess in the number counts at these magnitudes. The colour dependen ce of omega(theta) is well fitted by our PLE model, in which, at B sim ilar to 25, almost all of the redder galaxies would lie at z < 1, whil e the bluer galaxy subsample would consist of both dwarf late-type gal axies at low/moderate redshifts and evolving L similar to L galaxies widely distributed in redshift from z similar to 0.5 out to z similar to 3, giving a much more extended N(z), and consequently a lower omega (theta) amplitude, for bluer galaxies. The stronger clustering of the redder galaxies suggests that galaxy clustering is approximately stabl e (epsilon = 0) out to at least z similar to 0.6, and therefore that t he low omega(theta) amplitude of the full sample does not result simpl y from very rapid evolution of the clustering of ail galaxies. These r esults may argue against models, such as merging-dominated models, in which the excess blue galaxies are confined to lower redshifts. The ex cess blue galaxies could only lie within a no-evolution N(z) if they b elong to a separate population of dwarf starburst galaxies which is in trinsically very weakly clustered. However, there is evidence (e.g., C ole et al.) that dwarf starburst galaxies are normally clustered. This would leave significant L evolution, with a redshift distribution ex tending to z similar to 3 at B similar to 25, as the most plausible ex planation of the low omega(theta) amplitude of B similar to 25 galaxie s.