Deep [ITAL]Hubble[/ITAL] [ITAL]Space[/ITAL] [ITAL]T[/ITAL][ITAL]elescope[/ITAL] Galaxy and Pair Counts as Tests of Merger History

Abstract

We consider the use of counts of faint galaxy pairs as tests of the role of merging in galaxy evolution, using both simulations and Hubble Space Telescope (HST) data in comparison with models incorporating a range in the evolution of merger rate. Artificial data, including simulation of the surface-brightness selection effects implicit in HST imagery, indicate that simple pairing statistics are robust enough for this test. However, the differences between pair fractions for various plausible scenarios of merger-rate evolution are small enough to be easily hidden by statistical errors or environmental differences. With a Poisson-based algorithm, we identify pairs with physical distances between components 60 h-1 kpc. Deep HST WFPC2 images of the field around the radio galaxy 53W002 at z = 2.4 in F814W (I) and F606W (V) show that 12%?20% (~10 pairs) of galaxies at mI 24 or mV 24.6 are paired. This supports a mild enhancement of pair fraction toward high redshifts (1?2), as concluded in other recent studies of faint galaxy pairs. The magnitude limits eliminate most possible members of the grouping associated with 53W002 itself. We compare the counts of galaxies and galaxy pairs in the 53W002 field and other recent studies to a detailed model incorporating overall traditional galaxy spectral evolution and various merger rates, in conjunction with our previous evaluation of the local merger rate. The data support evolution of the merger rate, such that the comoving density of merging systems varies as (1 + z)?, with 0 ? ? ? 2. This is consistent with the constraint ? < 3, beyond which local elliptical galaxies (E/S0 types) are overproduced from merging, and includes the case ? ? 2 required by our model to support the merger hypothesis, whereby almost all elliptical galaxies are merger remnants. The ? ? 2 case also expects a large fraction of early galaxies undergoing merging. Therefore, we support the merger-rate evolution required to support the merger hypothesis and the expectation that early merging is important in the overall evolution of galaxies. Assuming merger remnants to be elliptical galaxies subsequently undergoing passive evolution, our model can reproduce at once counts of faint galaxies and galaxy pairs, as well as the present-day luminosity function of ellipticals, for the Hubble constant H0 ? 50 km s-1 Mpc-1 and a cosmological constant ? = 0. These lines of evidence do not appear to offer compelling arguments for more exotic cosmologies or star formation histories. A preliminary study shows the fraction of paired galaxies in the 53W002 field to be much larger than that in the Hubble Deep Field (5%?6% at most in the HDF), suggesting physical field-to-field fluctuations in pair statistics. Further progress to constrain merging history requires not only larger samples, preferably coupled to at least partial redshift information, but better indications as to which faint objects in multiple systems should be considered as independent galaxies and which are parts of single dynamical units.