On the nature of the extragalactic number counts in the K-band

Free access article

Issue		A&A Volume 494, Number 1, January IV 2009


Page(s)		63 - 79
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:200810633
Published online		20 November 2008

A&A 494, 63-79 (2009)
DOI: 10.1051/0004-6361:200810633

On the nature of the extragalactic number counts in the K-band

G. Barro¹, J. Gallego¹, P. G. Pérez-González¹, C. Eliche-Moral¹, M. Balcells², V. Villar¹, N. Cardiel¹, D. Cristobal-Hornillos³, A. Gil de Paz¹, R. Guzmán⁴, R. Pelló⁵, M. Prieto², and J. Zamorano¹

¹ Departamento de Astrofísica, Universidad Complutense de Madrid, 28040 Madrid, Spain
e-mail: gbc@astrax.fis.ucm.es
² Instituto de Astrofísica de Canarias, vía Lactea, 38200 La Laguna, Canary Islands, Spain
³ Instituto de Astrofísica de Andalucía, 18080 Granada, Spain
⁴ Department of Astronomy, 477 Bryant Space Center, University of Florida, Gainesville, FL, USA
⁵ Laboratoire d'Astrophysique de Toulouse-Tarbes, CNRS, Université de Toulouse, 14 Avenue Edouard Belin, 31400 Toulouse, France

Received 18 July 2008 / Accepted 27 October 2008

Abstract
Context. The galaxy number counts has been traditionally used to test models of galaxy evolution. However, the origin of significant differences in the shape of number counts at different wavelengths is still unclear. By relating the most remarkable features in the number counts with the underlying galaxy population it is possible to introduce further constraints on galaxy evolution.
Aims. We aim to investigate the causes of the different shape of the K-band number counts when compared to other bands, analyzing in detail the presence of a change in the slope around $K \sim 17.5$ .
Methods. We present a near-infrared imaging survey, conducted at the 3.5 m telescope of the Calar Alto Spanish-German Astronomical Center (CAHA), covering two separated fields centered on the HFDN and the Groth field, with a total combined area of ~0.27 deg² to a depth of $K \sim 19$ (3 $\sigma$ , Vega). By combining our data with public deep K-band images in the CDFS (GOODS/ISAAC) and high quality imaging in multiple bands, we extract K-selected catalogs characterized with highly reliable photometric redshift estimates. We derive redshift binned number counts, comparing the results in our three fields to sample the effects of cosmic variance. We derive luminosity functions from the observed K-band in the redshift range [0.25–1.25], that are combined with data from the references in multiple bands and redshifts, to build up the K-band number count distribution.
Results. The overall shape of the number counts can be grouped into three regimes: the classic Euclidean slope regime ( ${\rm d} \log N/{\rm d}m \sim 0.6$ ) at bright magnitudes; a transition regime at intermediate magnitudes, dominated by $M^{\ast}$ galaxies at the redshift that maximizes the product $\phi^{\ast}\frac{{\rm d}V_{\rm c}}{{\rm d}\Omega}$ ; and an $\alpha$ dominated regime at faint magnitudes, where the slope asymptotically approaches –0.4( $\alpha+1$ ) controlled by post- $M^{\ast}$ galaxies. The slope of the K-band number counts presents an averaged decrement of ~50% in the range 15.5 < K < 18.5 ( ${\rm d} \log N/{\rm d}m \sim 0.6$ –0.30). The rate of change in the slope is highly sensitive to cosmic variance effects. The decreasing trend is the consequence of a prominent decrease of the characteristic density $\phi^{\ast}_{K,{\rm obs}}$ (~60% from z=0.5 to z=1.5) and an almost flat evolution of $M^{\ast}_{K,{\rm obs}}$ (1 $\sigma$ compatible with $M^{\ast}_{K,{\rm obs}}=-22.89 \pm 0.25$ in the same redshift range).

Key words: galaxies: evolution -- galaxies: high-redshift -- infrared: galaxies

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