We present the results of a new search for bright star-forming galaxies at redshift z ≃ 7 within the UltraVISTA second data release (DR2) and UKIDSS (UKIRT Infrared Deep Sky Survey) UDS (Ultra Deep Survey) DR10 data, which together provide 1.65 deg2 of near-infrared imaging with overlapping optical and Spitzer data.
Using a full photometric redshift analysis, to identify high-redshift galaxies and reject contaminants, we have selected a sample of 34 luminous (−22.7<MUV<−21.2) galaxies with 6.5 < z < 7.5. Crucially, the deeper imaging provided by UltraVISTA DR2 confirms all of the robust objects previously uncovered by Bowler et al., validating our selection technique. Our new expanded galaxy sample includes the most massive galaxies known at z ≃ 7, with M* ≃ 1010 M⊙, and the majority are resolved, consistent with larger sizes (r1/2 ≃ 1–1.5 kpc) than displayed by less massive galaxies.
The z = 7 UV (∼1500 Å) LF showing a scaled CDM halo MF as described in the text (solid black line). The results from our sample of galaxies from the UltraVISTA DR2 and UDS fields are shown as the red filled circles. Data points from other studies are as described in the caption for Fig. 7. The best-fitting Schechter function at z = 7 from McLure et al. (2013) is plotted as the dotted black line, and the best-fitting DPL to our data points and those fromMcLure et al. (2013) is shown as the dashed line. The 1σ confidence limit on the Schechter function parameters (M, φ and α) is shown as the grey shaded region. We highlight the quenching mass derived by Peng et al. (2010) with an arrow, after converting to a UV luminosity using the typical mass-to-light ratio displayed by our sample.
From our final robust sample, we determine the form of the bright end of the rest-frame UV galaxy luminosity function (LF) at z ≃ 7, providing strong evidence that it does not decline as steeply as predicted by the Schechter-function fit to fainter data. We exclude the possibility that this is due to either gravitational lensing, or significant contamination of our galaxy sample by active galactic nuclei (AGN). Rather, our results favour a double power-law form for the galaxy LF at high redshift, or, more interestingly, an LF which simply follows the form of the dark matter halo mass function at bright magnitudes. This suggests that the physical mechanism which inhibits star formation activity in massive galaxies (i.e. AGN feedback or some other form of ‘mass quenching’) has yet to impact on the observable galaxy LF at z ≃ 7, a conclusion supported by the estimated masses of our brightest galaxies which have only just reached a mass comparable to the critical ‘quenching mass’ of M* ≃ 1010.2 M⊙ derived from studies of the mass function of star-forming galaxies at lower redshift.
The full paper, including technical details about the determination of the LF, can be found in Bowler et al. 2014, MNRAS, 440, 2810.