Prof Chris Lintott

Abstract:

We present a statistical observational study of the tidal dwarf (TD) population in the nearby Universe by exploiting a large, homogeneous catalogue of galaxy mergers compiled from the Sloan Digital Sky Survey. 95percent of TD-producing mergers involve two spiral progenitors (typically both in the blue cloud), while most remaining systems have at least one spiral progenitor. The fraction of TD-producing mergers where both parents are early-type galaxies is less than 2percent, suggesting that TDs are unlikely to form in such mergers. The bulk of TD-producing mergers inhabit a field environment and have mass ratios greater than ~1:7 (the median value is 1:2.5). TDs forming at the tidal-tail tips are ~4 times more massive than those forming at the base of the tails. TD stellar masses are less than 10percent of the stellar masses of their parents (the median is 0.6percent) and lie within 15 optical half-light radii of their parent galaxies. The TD population is typically bluer than the parents, with a median offset of ~0.3mag in the (g-r) colour and the TD colours are not affected by the presence of active galactic nuclei (AGN) activity in their parents. An analysis of their star formation histories indicates that TDs contain both newly formed stars (with a median age of ~30Myr) and old stars drawn from the parent discs, each component probably contributing roughly equally to the stellar mass of the object. Thus TDs are not formed purely through gas condensation in tidal tails but host a significant component of old stars from the parent discs. Finally, an analysis of the TD contribution to the observed dwarf-to-massive galaxy ratio in the local Universe indicates that ~6percent of dwarfs in nearby clusters may have a tidal origin, if TD production rates in nearby mergers are representative of those in the high-redshift Universe. Even if TD production rates at high redshift were several factors higher, it seems unlikely that the entire dwarf galaxy population today is a result of merger activity over the lifetime of the Universe. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Abstract:

Galaxy merging is a fundamental aspect of the standard hierarchical galaxy formation paradigm. Recently, the Galaxy Zoo project has compiled a large, homogeneous catalogue of 3373 mergers, through direct visual inspection of the entire SDSS spectro- scopic sample. We explore a subset of galaxies from this catalogue that are spheroidal 'post-mergers' (SPMs) - where a single remnant is in the final stages of relaxation after the merger and shows evidence for a dominant bulge, making them plausible progenitors of early-type galaxies. Our results indicate that the SPMs have bluer colours than the general early-type galaxy population possibly due to merger-induced star formation. An analysis using optical emission line ratios indicates that 20 of our SPMs exhibit LINER or Seyfert-like activity (68%), while the remaining 10 galaxies are classified as either star forming (16%) or quiescent (16%). A comparison to the emission line activity in the ongoing mergers from Darg et al. indicates that the AGN fraction rises in the post-mergers, suggesting that the AGN phase probably becomes dominant only in the very final stages of the merger process. The optical colours of the SPMs and the plausible mass ratios for their progenitors indicate that, while a minority are consistent with major mergers between two early-type galaxies, the vast majority are remnants of major mergers where at least one progenitor is a late-type galaxy.