With the goal of investigating the degree to which the mid-infrared emission traces the star formation rate (SFR), we analyze Spitzer 8 µm and 24 µm

We present a sample of low-resolution 5-38 µm Spitzer IRS spectra of the inner few square kiloparsecs of 59 nearby galaxies spanning a large range of star formation properties. A robust method for decomposing mid-infrared galaxy spectra is described, and used to explore the behavior of PAH emission and the prevalence of silicate dust extinction. Evidence for silicate extinction is found in ∼1/8 of the sample, at strengths which indicate most normal galaxies undergo A V 3 magnitudes averaged over their centers. The contribution of PAH emission to the total infrared power is found to peak near 10% and extend up to ∼20%, and is suppressed at metallicities Z Z ⊙ /4, as well as in low-luminosity AGN environments. Strong inter-band PAH feature strength variations (2-5×) are observed, with the presence of a weak AGN and, to a lesser degree, increasing metallicity shifting power to the longer wavelength bands. A peculiar PAH emission spectrum with markedly diminished 5-8 µm features arises among the sample solely in systems with relatively hard radiation fields harboring low-luminosity AGN. The AGN may modify the emitting grain distribution and provide the direct excitation source of the unusual PAH emission, which cautions against using absolute PAH strength to estimate star formation rates in systems harboring active nuclei. Alternatively, the low star formation intensity often associated with weak AGN may affect the spectrum. The effect of variations in the mid-infrared spectrum on broadband infrared surveys is modeled, and points to more than a factor of two uncertainty in results which assume a fixed PAH emission spectrum, for redshifts z = 0 − 2.5.

The Cluster Lensing And Supernova survey with Hubble (CLASH) is a 524-orbit multi-cycle treasury program to use the gravitational lensing properties of 25 galaxy clusters to accurately constrain their mass distributions. The survey, described in detail in this paper, will definitively establish the degree of concentration of dark matter in the cluster cores, a key prediction of structure formation models. The CLASH cluster sample is larger and less biased than current samples of space-based imaging studies of clusters to similar depth, as we have minimized lensing-based selection that favors systems with overly dense cores. Specifically, twenty CLASH clusters are solely X-ray selected. The X-ray selected clusters are massive (kT > 5 keV) and, in most cases, dynamically relaxed. Five additional clusters are included for their lensing strength (θ Ein > 35 at z s = 2) to optimize the likelihood of finding highly magnified high-z (z > 7) galaxies. A total of 16 broadband filters, spanning the near-UV to near-IR, are employed for each 20-orbit campaign on each cluster. These data are used to measure precise (σ z ∼ 0.02(1+z)) photometric redshifts for newly discovered arcs. Observations of each cluster are spread over 8 epochs to enable a search for Type Ia supernovae at z > 1 to improve constraints on the time dependence of the dark energy equation of state and the evolution of supernovae. We present newly re-derived X-ray luminosities, temperatures, and Fe abundances for the CLASH clusters as well as a representative source list for MACS1149.6+2223 (z = 0.544).

We measure the evolution of the stellar mass function (SMF) from z = 0 − 1 using multi-wavelength imaging and spectroscopic redshifts from the PRism MUlti-object Survey (PRIMUS) and the Sloan Digital Sky Survey (SDSS). From PRIMUS we construct an i < 23 flux-limited sample of ∼ 40, 000 galaxies at z = 0.2 − 1.0 over five fields totaling ≈ 5.5 deg 2 , and from the SDSS we select ∼ 170, 000 galaxies at z = 0.01−0.2 that we analyze consistently with respect to PRIMUS to minimize systematic errors in our evolutionary measurements. We find that the SMF of all galaxies evolves relatively little since z = 1, although we do find evidence for mass assembly downsizing; we measure a ≈ 30% increase in the number density of ∼ 10 10 M ⊙ galaxies since z ≈ 0.6, and a 10% change in the number density of all 10 11 M ⊙ galaxies since z ≈ 1. Dividing the sample into star-forming and quiescent using an evolving cut in specific star-formation rate, we find that the number density of ∼ 10 10 M ⊙ star-forming galaxies stays relatively constant since z ≈ 0.6, whereas the space-density of 10 11 M ⊙ star-forming galaxies decreases by ≈ 50% between z ≈ 1 and z ≈ 0. Meanwhile, the number density of ∼ 10 10 M ⊙ quiescent galaxies increases steeply towards low redshift, by a factor of ∼ 2 − 3 since z ≈ 0.6, while the number of massive quiescent galaxies remains approximately constant since z ≈ 1. These results suggest that the rate at which star-forming galaxies are quenched increases with decreasing stellar mass, but that the bulk of the stellar mass buildup within the quiescent population occurs around ∼ 10 10.8 M ⊙ . In addition, we conclude that mergers do not appear to be a dominant channel for the stellar mass buildup of galaxies at z < 1, even among massive ( 10 11 M ⊙ ) quiescent galaxies.

We combine Hα emission-line and infrared continuum measurements of two samples of nearby galaxies to derive dust attenuation-corrected star formation rates (SFRs). We use a simple energy balance based method that has been applied previously to HII regions in the Spitzer Infrared Nearby Galaxies Survey (SINGS), and extend the methodology to integrated measurements of galaxies. We find that our composite Hα + IR based SFRs are in excellent agreement with attenuation-corrected SFRs derived from integrated spectrophotometry, over the full range of SFRs (0.01 -80 M ⊙ yr −1 ) and attenuations (0 -2.5 mag) studied. We find that the combination of Hα and total infrared luminosities provides the most robust SFR measurements, but combinations of Hα measurements with monochromatic luminosities at 24 µm and 8 µm perform nearly as well. The calibrations differ significantly from those obtained for HII regions (Calzetti et al. 2007), with the difference attributable to a more evolved population of stars heating the dust. Our results are consistent with a significant component of diffuse dust (the 'IR cirrus' component) that is heated by a non-star-forming -2population. The same methodology can be applied to [O II]λ3727 emissionline measurements, and the radio continuum fluxes of galaxies can be applied in place of IR fluxes when the latter are not available. We assess the precision and systematic reliability of all of these composite methods.-4 -The advent of large sets of multi-wavelength observations of nearby galaxies now provides us with the opportunity to derive attenuation-corrected Hα and UV continuum luminosities of galaxies by combining these fluxes with various components of the IR emission. Moreover the availability of integrated optical spectra (and in some cases Pa α maps) of the same galaxies allows us to derive additional attenuation estimates, and test the precision and systematic reliability of the respective attenuation-corrected SFR measurements.The Spitzer Infrared Nearby Galaxies Survey (SINGS; Kennicutt et al. 2003) offers an ideal dataset for testing and calibrating such multi-wavelength SFR estimators. The survey includes imaging of a diverse sample of 75 galaxies within 30 Mpc, with wavelength coverage extending from the UV to the radio, including ultraviolet imaging at 150 and 230 nm, Hα, and 7 infrared wavelengths over 3.6 -160 µm. In addition, drift-scanned spectra over the wavelength range 3600 -6900Å are available, which complement matching infrared spectra over the range 10 -40 µm. We first applied these data to calibrate and test the combined use of Hα and 24 µm infrared fluxes of individual HII regions to derive attenuation-corrected emission-line fluxes (Calzetti et al. 2007, hereafter denoted C07;Kennicutt et al. 2007;Prescott et al. 2007). These studies revealed that the Spitzer 24 µm sources were highly correlated in position and flux with those of optical HII region counterparts. Kennicutt et al. (2007) and C07 found that the ratio of 24 µm to Hα fluxes yielded attenuation-corrected Hα lum...

Physical dust models are presented for 65 galaxies in SINGS that are strongly detected in the four IRAC bands and three MIPS bands. For each galaxy we estimate (1) the total dust mass, (2) the fraction of the dust mass contributed by PAHs, and (3) the intensity of the starlight heating the dust grains. We find that spiral galaxies have dust properties resembling the dust in the local region of the Milky Way, with similar dust-to-gas ratio and similar PAH abundance. The observed SEDs, including galaxies with SCUBA photometry, can be reproduced by dust models that do not require ''cold'' (T P 10 K) dust. The dust-to-gas ratio is observed to be dependent on metallicity. In the interstellar media of galaxies with A O 12 þ log 10 (O/H ) > 8:1, grains contain a substantial fraction of interstellar Mg, Si, and Fe. Galaxies with A O < 8:1 and extended H i envelopes in some cases appear to have global dust-to-gas ratios that are low for their measured oxygen abundance, but the dust-to-gas ratio in the regions where infrared emission is detected generally appears to be consistent with a substantial fraction of interstellar Mg, Si, and Fe being contained in dust. The PAH index q PAH , the fraction of the dust mass in the form of PAHs, correlates with metallicity. The nine galaxies in our sample with A O < 8:1 have a median q PAH ¼ 1:0%, whereas galaxies with A O > 8:1 have a median q PAH ¼ 3:55%. The derived dust masses favor a value X CO % 4 ; 10 20 cm À2 (K km s À1 ) À1 for the CO-to-H 2 conversion factor. Except for some starbursting systems (Mrk 33, Tol 89, NGC 3049), dust in the diffuse ISM dominates the IR power.

We present intermediate-resolution optical spectrophotometry of 65 galaxies obtained in support of the Spitzer Infrared Nearby Galaxies Survey (SINGS). For each galaxy we obtain a nuclear, circumnuclear, and semi-integrated optical spectrum designed to coincide spatially with mid-and far-infrared spectroscopy from the Spitzer Space Telescope. We make the reduced, spectrophotometrically calibrated one-dimensional spectra, as well as measurements of the fluxes and equivalent widths of the strong nebular emission lines, publically available. We use optical emission-line ratios measured on all three spatial scales to classify the sample into star-forming, active galactic nuclei (AGN), and galaxies with a mixture of star formation and nuclear activity. We find that the relative fraction of the sample classified as star-forming versus AGN is a strong function of the integrated light enclosed by the spectroscopic aperture. We supplement our observations with a large database of nebular emission-line measurements of individual H ii regions in the SINGS galaxies culled from the literature. We use these ancillary data to conduct a detailed analysis of the radial abundance gradients and average H iiregion abundances of a large fraction of the sample. We combine these results with our new integrated spectra to estimate the central and characteristic (globally-averaged) gas-phase oxygen abundances of all 75 SINGS galaxies. We conclude with an in-depth discussion of the absolute uncertainty in the nebular oxygen abundance scale.

We present new calibrations of far-ultraviolet (FUV) attenuation as derived from the total infrared to FUV luminosity ratio (IRX) and the FUV-NUV color. We find that the IRX-corrected FUV luminosities are tightly and linearly correlated with the attenuation-corrected Hα luminosities (as measured from the Balmer decrement), with a rms scatter of ±0.09 dex. The ratios of these attenuation-corrected FUV to Hα luminosities are consistent with evolutionary synthesis model predictions, assuming a constant star formation rate over 100 Myr, solar metallicity and either a Salpeter or a Kroupa IMF with lower and upper mass limits of 0.1 and 100M ⊙ . The IRX-corrected FUV to Balmer-corrected Hα luminosity ratios do not show any trend with other galactic properties over the ranges covered by our sample objects. In contrast, FUV attenuation derived from the FUV-NUV color (UV spectral slope) show much larger random and systematic uncertainties. When compared to either Balmercorrected Hα luminosities or IRX-corrected FUV luminosities the color-corrected FUV luminosities show ∼ 2.5 times larger rms scatter, and systematic nonlinear deviations as functions of luminosity and other parameters. Linear combinations of 25µm and 1.4GHz radio continuum luminosities with the observed FUV luminosities are also well correlated with the Balmer-corrected Hα luminosities. These results provide useful prescriptions for deriving attenuation-corrected star