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We present the first results of a Spitzer IRAC (Infrared Array Camera)wide-field survey of the Coma cluster. The observations cover two fields ofdifferent galaxy densities; the first is a 0.733 deg^2 region in the core ofthe cluster (Coma 1), the second a 0.555 deg^2 off-center region located ~57arcmin (1.7 Mpc) south-west from the core (Coma 3). The observations, althoughshort 70-90 s exposures, are very sensitive; we detect ~29,200 sources at 3.6micron over the total ~1.3 deg^2 survey area. We construct 3.6 micron galaxyluminosity functions (LFs) for each field using selection functions based onspectroscopic redshifts. At the bright end, the LFs are well modeled by atraditional Schechter function; [M^star (3.6 micron), alpha_1] = [-25.17,-1.18] and [-24.69, -1.30] for Coma 1 and Coma 3 respectively. However, at thefaint end (M(3.6 micron) > -20.5), there is a steep increase in the LF slope inboth fields indicative of large numbers of red dwarf galaxies. The reality ofthis population is examined using optical counterparts in SDSS data withoptical color filtering (g-r<1.3). The steep increase can be modeled with apower-law function, with slopes of alpha_2 = -2.18 (Coma 1) and alpha_2 = -2.60(Coma 3). A qualitative comparison with optical (B- and R-band) LFs shows thatwe are likely to be observing a larger population of dwarf galaxies in thenear-IR, which may be a low-surface-brightness (LSB) population that IRAC isparticularly sensitive to, or a population too red to be detected in existingoptical surveys down to R~20.

Uncovering the Near-IR Dwarf Galaxy Population of the Coma Cluster with Spitzer IRAC