New H-band galaxy number counts: A large local hole in the galaxy distribution

Abstract

We examine H-band number counts determined using new photometry over two fields with a combined solid angle of 0.30 deg2 to H ≈ 19, as well as bright data (H ≤ 14) from the two-Micron All-Sky Survey (2MASS). First, we examine the bright number counts from 2MASS extracted for the ≈4000-deg2 Automated Plate Measuring machine (APM) survey area situated around the southern Galactic pole. We find a deficiency of ≈ 25 per cent at H = 13 with respect to homogeneous predictions, in line with previous results in the B and Ks bands. In addition, we examine the bright counts extracted for |b| > 20° (covering ≈ 27 000 deg2); we find a relatively constant deficit in the counts of ≈15-20 per cent to H = 14. We investigate various possible causes for these results; namely, errors in the model normalization, unexpected luminosity evolution (at low and high redshifts), errors in the photometry, incompleteness and large-scale structure. In order to address the issue of the model normalization, we examine the number counts determined for the new faint photometry presented in this work and also for faint data (H ≲ 20) covering 0.39 deg2 from the Las Campanas Infrared Survey (LCIRS). In each case a zero-point is chosen to match that of the 2MASS photometry at bright magnitudes using several hundred matched point sources in each case. We find a large offset between 2MASS and the LCIRS data of 0.28 ± 0.01 mag. Applying a consistent zero-point, the faint data, covering a combined solid angle of 0.69 deg2, is in good agreement with the homogeneous prediction used previously, with a best-fitting normalization a factor of 1.095-0.034 higher. Addressing the contribution from large-scale structure, we estimate the cosmic variance in the bright counts over the APM survey area and for |b| > 20° expected in a Lambda cold dark matter (ACDM) cosmology using 27 mock 2MASS catalogues constructed from the ACDM Hubble Volume simulation. The APM survey area bright counts are in line with a rare fluctuation in the local galaxy distribution of ≈2-3σ. The |b| > 20° counts represent a 2.5-4σ fluctuation and would imply a local hole which extends over the entire local galaxy distribution, which may start to be at odds with ACDM. The increase in faint near-infrared data from the UK Infrared Deep Sky Survey should help to resolve this issue. © 2006 RAS.