Friday, 11 April 2008

Evolution of the field galaxy pair fraction

Figure 13 of Hsieh et al.,

This plot shows the average number of galaxy companions as a function of redshift for different pair separations. With increasing separations, the evolution of the pair fraction decreases. Assuming these pairs represent early-stage mergers, this may imply that the infall/merging timescales are changing with redshift: at high redshift it takes longer for a galaxy to finish merging (i.e. go from 20 kpc to 0 kpc) than at low redshift, relative to the inital (r=150 to 50 kpc) infall. The authors suggest that such a change in timescale may be due to dark matter halos at lower redshift being more concentrated: since the density is lower in the outskirts of highly-concentrated halos, the dynamical friction timescale at large radii is longer, and therefore one might expect merging galaxies to spend more time at larger radii (thus increasing the large-separation pair fraction) at lower redshifts.

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