A Surprisingly High Pair Fraction for Extremely Massive Galaxies at z ~ 3 in the GOODS NICMOS Survey

Abstract

We calculate the major pair fraction and derive the major merger fraction and rate for 82 massive ($M_{\ast }>10^{11}{M}_{\odot }$) galaxies at $1.7<z<3.0$ utilising deep HST NICMOS data taken in the GOODS North and South fields. For the first time, our NICMOS data provides imaging with sufficient angular resolution and depth to collate a sufficiently large sample of massive galaxies at z $>$ 1.5 to reliably measure their pair fraction history. We find strong evidence that the pair fraction of massive galaxies evolves with redshift. We calculate a pair fraction of $f_m$ = 0.29 +/- 0.06 for our whole sample at $1.7<z<3.0$. Specifically, we fit a power law function of the form $f_m=f_0(1+z{)}^m$ to a combined sample of low redshift data from Conselice et al. (2007) and recently acquired high redshift data from the GOODS NICMOS Survey. We find a best fit to the free parameters of $f_0$ = 0.008 +/- 0.003 and $m$ = 3.0 +/- 0.4. We go on to fit a theoretically motivated Press-Schechter curve to this data. This Press-Schechter fit, and the data, show no sign of levelling off or turning over, implying that the merger fraction of massive galaxies continues to rise with redshift out to z $\sim$ 3. Since previous work has established that the merger fraction for lower mass galaxies turns over at z $\sim$ 1.5 - 2.0, this is evidence that higher mass galaxies experience more mergers earlier than their lower mass counterparts, i.e. a galaxy assembly downsizing. Finally, we calculate a merger rate at z = 2.6 of $\Re$ $<$ 5 $\times$ 10${}^5$ Gpc${}^{-3}$ Gyr${}^{-1}$, which experiences no significant change to $\Re$ $<$ 1.2 $\times$ 10${}^5$ Gpc${}^{-3}$ Gyr${}^{-1}$ at z = 0.5. This corresponds to an average $M_{\ast }>10^{11}{M}_{\odot }$ galaxy experiencing 1.7 +/- 0.5 mergers between z = 3 and z = 0.

5 pages, 3 figures, accepted to MNRAS

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