Near – infrared luminosity function in the Coma cluster ?

We present the near–infrared H band luminosity function (hereafter LF) of the Coma cluster of galaxies. It is the deepest ever computed in the near–infrared, for any type of environment , extending over 7 magnitudes, down to ∼ M * H +6. The LF was computed on a near–infrared selected sample of galaxies whose photometry, complete down to the typical dwarf lumi-nosity, is presented in a companion paper. The Coma LF can be described by a Schechter function with intermediate slope (α ∼ −1.3), plus a dip at M H ∼ −22 mag. The shape of the Coma LF in H band is quite similar to the one found in the B band and, with less confidence, to the R band LF as well. The similarity of the LF in the optical and H bands implies that in the central region of Coma there is no new population of galaxies which is too faint to be observed in the optical band (because dust enshrouded, for instance), down to the magnitudes of dwarfs. The exponential cut of the LF at the bright end is in good agreement with the one derived from shallower near–infrared samples of galaxies, both in clusters and in the field. This fact is suggestive of a similarity of the tip of the mass function of galaxies, irrespective of the environment where they are found. The dip at M H ∼ −22 mag is instead unique among all the so far measured near–infrared LF, although several published observations are not deep enough or spanning a suitable wide field to distinctly detect this feature. The faint end of the LF, reaching M H ∼ −19 mag (roughly M B ∼ −15), is steep, but less than previously suggested from shallower near–infrared observations of an adjacent region in the Coma cluster. The differences between our measured LF and that measured previously in other regions suggest a dependency on environment of the faint end of the mass function (below M * + 2.5).