ABSTRACT

Using deep HST/WFC images, originally taken to study faint radio galaxies, we find 81 clear serendipitous galaxy images, of which 34 are pair members. Based on nearby magnitude-limited samples, this is an excess of more than 4σ above the expected number of pair members. We take this result as strong evidence that the galaxy merger rate was higher in the past, and has declined over time.

INTRODUCTION: GALAXY MERGERS AND EVOLUTION

Galaxy interactions and mergers have been implicated as driving galaxy evolution in several ways:

Triggering starbursts, thus making the star-forming history episodic Driving global winds from starbursts, sweeping merger remnants free of gas and dust Transforming galaxy morphology through mergers and tidal impulses Triggering nuclear activity

Counts of local pairs and mergers, plus N-body modelling of orbital decays, suggest that many (perhaps most) present galaxies underwent mergers during cosmic history. This means that the merger rate was probably higher in the past. We are using galaxy and pair counts from deep HST serendipitous fields to constrain the merger rate.

We cannot uniformly trace mergers themselves to large redshifts, because (1) cosmological (1 + z)4 surface-brightness dimming makes the characteristic tidal features too faint for detection and (2) at large redshifts, the disturbed structures can be too small for detection given surface-brightness constraints. We therefore trace the merger rate by studying the evolution of galaxy pairs some of which are the immediate precursors of mergers.