We present nearly simultaneous Chandra and NuSTAR observations of two actively star-forming galaxies within 50 Mpc: NGC 3256 and NGC 3310. Both galaxies are detected by both Chandra and NuSTAR, which together provide the first-ever spectra of these t
wo galaxies spanning 0.3-30 keV. The X-ray emission from both galaxies is spatially resolved by Chandra; we find that hot gas dominates the E < 1-3 keV emission while ultraluminous X-ray sources (ULXs) dominate at E > 1-3 keV. The NuSTAR galaxy-wide spectra of both galaxies follow steep power-law distributions with Gamma ~ 2.6 at E > 5-7 keV, similar to the spectra of bright individual ULXs and other galaxies that have been studied by NuSTAR. We find that both NGC 3256 and NGC 3310 have X-ray detected sources coincident with nuclear regions; however, the steep NuSTAR spectra of both galaxies restricts these sources to be either low luminosity AGN or non-AGN in nature (e.g., ULXs or crowded X-ray sources that reach L2-10 keV ~ 10^40 erg/s cannot be ruled out). Combining our constraints on the 0.3-30 keV spectra of NGC 3256 and NGC 3310 with equivalent measurements for nearby star-forming galaxies M83 and NGC 253, we analyze the SFR-normalized spectra of these starburst galaxies. The spectra of all four galaxies show sharply declining power-law slopes above 3-6 keV due to ULX populations. Our observations therefore constrain the average spectra of luminous accreting binaries (i.e., ULXs). This result is similar to the super-Eddington accreting ULXs that have been studied individually in a targeted NuSTAR ULX program. We also find that NGC 3310 exhibits a factor of ~3-10 elevation of X-ray emission over the other star-forming galaxies. We argue that the excess is most likely explained by the relatively low metallicity of the young stellar population in NGC 3310.
We present the first results of a Spitzer IRAC (Infrared Array Camera) wide-field survey of the Coma cluster. The observations cover two fields of different galaxy densities; 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 ~57 arcmin (1.7 Mpc) south-west from the core (Coma 3). The observations, although short 70-90 s exposures, are very sensitive; we detect ~29,200 sources at 3.6 micron over the total ~1.3 deg^2 survey area. We construct 3.6 micron 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^star (3.6 micron), 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 micron) > -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 optical counterparts in SDSS data 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). 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 existing optical surveys down to R~20.
