Galaxy counts and recent measurements of the luminosity density in the near-infrared have indicated the possibility that the local universe may be under-dense on scales of several hundred megaparsecs. The presence of a large-scale under-density in the local universe could introduce significant biases into the interpretation of cosmological observables, and, in particular, into the inferred effects of dark energy on the expansion rate. Here we measure the K−band luminosity density as a function of redshift to test for such a local under-density. For our primary sample in this study, we select galaxies from the UKIDSS Large Area Survey and use spectroscopy from the SDSS, 2DFGRS, GAMA, and other redshift surveys to generate a K−selected catalog of ∼ 35, 000 galaxies that is ∼ 95% spectroscopically complete at K AB < 16.3 (K AB < 17 in the GAMA fields). To complement this sample at low redshifts, we also analyze a K−selected sample from the 2M++ catalog, which combines 2MASS photometry with redshifts from the 2MASS redshift survey, the 6DFGRS, and the SDSS. The combination of these samples allows for a detailed measurement of the K−band luminosity density as a function of distance over the redshift range 0.01 < z < 0.2 (radial distances D ∼ 50 − 800 h −1 70 Mpc). We find that the overall shape of the z = 0 rest-frame K−band luminosity function (M * = −22.15 ± 0.04 and α = −1.02 ± 0.03) appears to be relatively constant as a function of environment and distance from us. We find a local (z < 0.07, D < 300 h −1 70 Mpc) luminosity density that is in good agreement with previous studies. Beyond z ∼ 0.07, we detect a rising luminosity density that reaches a value of roughly ∼ 1.5 times higher than that measured locally at z > 0.1. This suggests that the stellar mass density as a function of distance follows a similar trend. Assuming that luminous matter traces the underlying dark matter distribution, this implies that the local mass density of the universe may be lower than the global mass density on a scale and amplitude sufficient to introduce significant biases into the determination of basic cosmological observables. An under-density of roughly this scale and amplitude could resolve the apparent tension between direct measurements of the Hubble constant and those inferred by Planck.

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We present a current best estimate of the integrated near-infrared (NIR) extragalactic background light (EBL) attributable to resolved galaxies (Integrated Galaxy Light, IGL) in J, H, and K s . Our results for measurements of νI ν in units of nW m −2 sr −1 are 11.7 +5.6 −2.6 in J, 11.5 +4.5 −1.5 in H, and 10.0We derive these new limits by combining our deep wide-field NIR photometry from five widely separated fields with other studies from the literature to create a galaxy counts sample that is highly complete and has good counting statistics out to JHK s ∼ 27-28. As part of this effort we present new ultra-deep K s -band galaxy counts from 22 hr of observations with the Multi Object Infrared Camera and Spectrograph (MOIRCS) instrument on the Subaru Telescope. We use this MOIRCS K s -band mosaic to estimate the total missing flux from sources beyond our detection limits. Our new limits to the NIR EBL are in basic agreement with, but 10%-20% higher than, previous estimates, bringing them into better agreement with estimates of the total NIR EBL (resolved + unresolved sources) obtained from TeV γ -ray opacity measurements and recent direct measurements of the total NIR EBL as well as the modeled NIR IGL. We examine field-to-field variations in our photometry to show that the integrated light from galaxies is isotropic to within uncertainties, consistent with the expected large-scale isotropy of the EBL. Our data also allow for a robust estimate of the NIR light from Galactic stars, which we find to be 14.7 ± 2.4 in J, 10.1 ± 1.9 in H, and 7.6 ± 1.8 in K s in units of nW m −2 sr −1 .

We present a study of the largest available sample of near-infrared selected (i.e., stellar mass selected) dynamically close pairs of galaxies at low redshifts (z < 0.3). We combine this sample with new estimates of the major-merger pair fraction for stellar mass selected galaxies at z < 0.8, from the Red Sequence Cluster Survey (RCS1). We construct our low-redshift K−band selected sample using photometry from the UKIRT Infrared Deep Sky Survey (UKIDSS) and the Two Micron All Sky Survey (2MASS) in the K−band (∼ 2.2 µm). Combined with all available spectroscopy, our K−band selected sample contains ∼ 250, 000 galaxies and is > 90% spectroscopically complete. The depth and large volume of this sample allow us to investigate the low-redshift pair fraction and merger rate of galaxies over a wide range in K−band luminosity. We find the major-merger pair fraction to be flat at ∼ 2% as a function of K−band luminosity for galaxies in the range 10 8 − 10 12 L ⊙ , in contrast to recent results from studies in the local group that find a substantially higher low-mass pair fraction. This low-redshift major-merger pair fraction is ∼ 40 − 50% higher than previous estimates drawn from K−band samples, which were based on 2MASS photometry alone. Combining with the RCS1 sample we find a much flatter evolution (m = 0.7 ± 0.1), in the relation f pair ∝ (1 + z) m , than indicated in many previous studies. These results indicate that a typical L ∼ L * galaxy has undergone ∼ 0.2 − 0.8 major mergers since z = 1 (depending on the assumptions of merger timescale and percentage of pairs that actually merge).

Recent cosmological modeling efforts have shown that a local underdensity on scales of a few hundred Mpc (out to z ∼ 0.1) could produce the apparent acceleration of the expansion of the universe observed via Type Ia supernovae. Several studies of galaxy counts in the near-infrared (NIR) have found that the local universe appears underdense by ∼25%-50% compared with regions a few hundred Mpc distant. Galaxy counts at low redshifts sample primarily L ∼ L * galaxies. Thus, if the local universe is underdense, then the normalization of the NIR galaxy luminosity function (LF) at z > 0.1 should be higher than that measured for z < 0.1. Here we present a highly complete (>90%) spectroscopic sample of 1436 galaxies selected in the H band (1.6 μm) to study the normalization of the NIR LF at 0.1 < z < 0.3 and address the question of whether or not we reside in a large local underdensity. Our survey sample consists of all galaxies brighter than 18th magnitude in the H band drawn from six widely separated fields at high Galactic latitudes, which cover a total of ∼2 deg 2 on the sky. We find that for the combination of our six fields, the product φ * L * at 0.1 < z < 0.3 is ∼30% higher than that measured at lower redshifts. While our statistical errors in this measurement are on the ∼10% level, we find the systematics due to cosmic variance may be larger still. We investigate the effects of cosmic variance on our measurement using the COSMOS cone mock catalogs from the Millennium Simulation and recent empirical estimates of cosmic variance. We find that our survey is subject to systematic uncertainties due to cosmic variance at the 15% level (1σ ), representing an improvement by a factor of ∼2 over previous studies in this redshift range. We conclude that observations cannot yet rule out the possibility that the local universe is underdense at z < 0.1. The fields studied in this work have a large amount of publicly available ancillary data and we make available the images and catalogs used here.