Uncovering the near-IR dwarf galaxy population of the coma cluster with Spitzer IRAC

We present the first results of a Spitzer IRAC ( Infrared Array Camera) wide-field survey of two regions of the Coma Cluster. The observations cover two fields; the first is a 0.733 deg(2) region in the core of the cluster ( Coma 1), the second a 0.555 deg(2) off-center region located similar to 57 ' (1.7 Mpc) southwest from the core ( Coma 3). The IRAC observations, although short 70-90 s exposures, are very sensitive; we detect similar to 29,200 sources at 3.6 mu m over the total similar to 1.3 deg(2) survey area. We construct 3.6 mu m galaxy luminosity functions (LFs) for each field using selection functions based on spectroscopic redshifts. At the bright end, the LFs are well modeled by a traditional Schechter function; < M*(3.6) mu m, alpha(1)> <-25.17, -1.18 > and <-24.69, -1.30 > for Coma 1 and Coma 3 respectively. However, at the faint end (M-3:6 mu m > -20.5) there is a steep increase in the LF slope in both fields indicative of large numbers of red dwarf galaxies. The reality of this population is examined using SDSS optical counterparts with optical color filtering ( g - r < 1: 3). The steep increase can be modeled with a power-law function, with slopes of alpha(2) = -2.18 (Coma 1) and alpha(2) = -2.60 (Coma 3), the difference likely indicating a change in environmental effects between the two fields. A qualitative comparison with optical (B- and R- band) LFs shows that we are likely to be observing a larger population of dwarf galaxies in the near-IR, which may be a low surface brightness (LSB) population that IRAC is particularly sensitive to, or a population too red to be detected in the existing optical surveys down to R similar to 20.