The evolution of the near-infrared galaxy luminosity function and colour bimodality up to z ≃ 2 from the UKIDSS Ultra Deep Survey Early Data Release

Abstract

We present new results on the cosmological evolution of the near‐infrared (near‐IR) galaxy luminosity function (LF), derived from the analysis of a new sample of ∼22 000 KAB≤ 22.5 galaxies selected over an area of 0.6 deg2 from the Early Data Release of the UKIDSS Ultra Deep Survey (UDS). Our study has exploited the multiwavelength coverage of the UDS field provided by the new UKIDSS WFCAM K‐ and J‐band imaging, the Subaru/XMM–Newton Deep Survey and the Spitzer Wide‐Area Infrared Extragalactic survey. The unique combination of large area and depth provided by this new survey minimizes the complicating effect of cosmic variance and has allowed us, for the first time, to trace the evolution of the brightest sources out to z≃ 2 with good statistical accuracy. In agreement with previous studies, we find that the characteristic luminosity of the near‐IR LF brightens by ≃1 mag between z= 0 and z≃ 2, while the total density decreases by a factor of ≃2. Using the rest‐frame (U−B) colour to split the sample into red and blue galaxies, we confirm the classic luminosity‐dependent colour bimodality at z≲ 1. However, the strength of the colour bimodality is found to be a decreasing function of redshift, and seems to disappear by z≳ 1.5. Due to the large size of our sample, we are able to investigate the differing cosmological evolution of the red and blue galaxy populations. It is found that the space density of the brightest red galaxies (MK≤− 23) stays approximately constant with redshift, and that these sources dominate the bright end of the LF at redshifts z≲ 1. In contrast, the brightening of the characteristic luminosity and mild decrease in space density displayed by the blue galaxy population leads them to dominate the bright end of the LF at redshifts z≳ 1.

How was the Hubble sequence 6 Gyr ago?

“…An optimal balance would be provided if we had chosen an [oii]λ3727 EW of about 11Å. This might slightly alter the comparison with Cirasuolo et al (2007) in Fig. 2 (b, c, see also below).…”

“…There is a relatively good correspondence between starforming and quiescent galaxies defined as above with blue and red galaxies, respectively. Figure 3 compares rest-frame (U − B) colors of galaxies in the distant sample with the limit applied by Cirasuolo et al (2007) to separate blue from red galaxies. It is, however, slightly imbalanced as there are more quiescent galaxies in the blue cloud than star-forming ones in the red cloud.…”

“…A high fraction (77% of the sample) have spectra observed with VLT/FORS2 ensuring the best accuracy in measuring [oii]λ3727 EW. Figure 2c shows the corresponding distribution of the J-band absolute magnitudes that is compared to the luminosity function from Cirasuolo et al (2007) for red galaxies in the redshift range 0.25 < z < 0.75. The J − K magnitude correction was done in the same way as in the previous subsample.…”

“…Figure 2b shows the distribution of the J-band absolute magnitudes of the distant starforming galaxy subsample. To further verify its representativeness, we over-plotted in this histogram the K-band luminosity function obtained by Cirasuolo et al (2007) for blue galaxies in the redshift range 0.25 < z < 0.75. The correction from K-band to J-band is very small in the AB system, according to the available values calculated in the IMAGES survey.…”

GMASS ultradeep spectroscopy of galaxies at z ~ 2

“…The latter also showed that the blue and red galaxy populations most probably had different evolutionary histories. Using a much larger sample, including ∼22 000 galaxies selected over an area of 0.6 deg 2 from the Early Data Release of the UKIDSS Ultra Deep Survey, Cirasuolo et al (2007) found that the colour bimodality disappears at z 1.5. With a large K-selected sample of more than 30 000 galaxies, Williams et al (2009) showed, that the bimodal distribution of star-forming and quiescent galaxies is still seen in a subsample of galaxies in the redshift range 1.5 < z < 2, but not above z = 2.…”