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

Abstract

We 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 $K\sim 17.5$. We present a near-infrared imaging survey, conducted at the 3.5m 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 $\sim 0.27$deg${}^2$ to a depth of $K\sim 19$ ($3\ddot{I}ƒ$,Vega). 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. We find that the overall shape of the number counts can be grouped into three regimes: the classic Euclidean slope regime ($d\log N/dm\sim 0.6$) at bright magnitudes; a transition regime at intermediate magnitudes, dominated by $M^{\ast }$ galaxies at the redshift that maximizes the product $\ddot{I}{•}^{\ast }\frac{d{V}_c}{d\hat{I}\text{c◯}}$; and an $\hat{I}\pm$ dominated regime at faint magnitudes, where the slope asymptotically approaches -0.4($\hat{I}\pm$+1) controlled by post-$M^{\ast }$ galaxies. The slope of the $K$-band number counts presents an averaged decrement of $\sim 50$ in the range $15.5<K<18.5$ ($d\log N/dm\sim 0.6-0.30$). 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 $\ddot{I}{•}_{K,obs}^{\ast }$ ($\sim 60$ from $z=0.5$ to $z=1.5$) and an almost flat evolution of $M_{K,obs}^{\ast }$ (1$\ddot{I}ƒ$ compatible with $M_{K,obs}^{\ast }=-22.89\pm 0.25$ in the same redshift range).

18 pages, 22 figures, Accepted by Astronomy & Astrophysics