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.

Using free-free emission measured in the Ka-band (26 − 40 GHz) for 10 star-forming regions in the nearby galaxy NGC 6946, including its starbursting nucleus, we compare a number of star formation rate (SFR) diagnostics that are typically considered to be unaffected by interstellar extinction. These diagnostics include non-thermal radio (i.e., 1.4 GHz), total infrared (IR; 8 − 1000 µm), and warm dust (i.e., 24 µm) emission, along with hybrid indicators that attempt to account for obscured and unobscured emission from star-forming regions including Hα + 24 µm and UV + IR measurements.The assumption is made that the 33 GHz free-free emission provides the most accurate measure of the current SFR. Among the extranuclear star-forming regions, the 24 µm, Hα + 24 µm and UV + IR SFR calibrations are in good agreement with the 33 GHz free-free SFRs. However, each of the SFR calibrations relying on some form of dust emission overestimate the nuclear SFR by a factor of ∼2 relative to the 33 GHz free-free SFR. This is more likely the result of excess dust heating through an accumulation of non-ionizing stars associated with an extended episode of star formation in the nucleus rather than increased competition for ionizing photons by dust. SFR calibrations using the non-thermal radio continuum yield values which only agree with the 33 GHz free-free SFRs for the nucleus, and underestimate the SFRs from the extranuclear star-forming regions by an average factor of ∼2 and ∼4−5 before and after subtracting local background emission, respectively. This result likely arises from the CR electrons decaying within the starburst region with negligible escape, whereas the transient nature of star formation in the young extranuclear star-forming complexes allows for CR electrons to diffuse significantly further than dust heating photons, resulting in an underestimate of the true SFR. Finally, we find that the SFRs estimated using the total 33 GHz flux density appear to agree well with those from using the free-free emission due to the large thermal fractions present at these frequencies even when local diffuse backgrounds are not removed. Thus, rest-frame 33 GHz observations may act as a reliable method to measure the SFRs of galaxies at increasingly high redshift without the need of ancillary radio data to account for the non-thermal emission.

We present the deepest 100 to 500 μm far-infrared observations obtained with the Herschel Space Observatory as part of the GOODS-Herschel key program, and examine the infrared (IR) 3-500 μm spectral energy distributions (SEDs) of galaxies at 0 < z < 2.5, supplemented by a local reference sample from IRAS, ISO, Spitzer, and AKARI data. We determine the projected star formation densities of local galaxies from their radio and mid-IR continuum sizes. We find that the ratio of total IR luminosity to rest-frame 8 μm luminosity, IR8 (≡L tot IR /L 8 ), follows a Gaussian distribution centered on IR8 = 4 (σ = 1.6) and defines an IR main sequence for star-forming galaxies independent of redshift and luminosity. Outliers from this main sequence produce a tail skewed toward higher values of IR8. This minority population (<20%) is shown to consist of starbursts with compact projected star formation densities. IR8 can be used to separate galaxies with normal and extended modes of star formation from compact starbursts with high-IR8, high projected IR surface brightness (Σ IR > 3 × 10 10 L kpc −2 ) and a high specific star formation rate (i.e., starbursts). The rest-frame, UV-2700 Å size of these distant starbursts is typically half that of main sequence galaxies, supporting the correlation between star formation density and starburst activity that is measured for the local sample. Locally, luminous and ultraluminous IR galaxies, (U)LIRGs (L tot IR ≥ 10 11 L ), are systematically in the starburst mode, whereas most distant (U)LIRGs form stars in the "normal" main sequence mode. This confusion between two modes of star formation is the cause of the so-called "mid-IR excess" population of galaxies found at z > 1.5 by previous studies. Main sequence galaxies have strong polycyclic aromatic hydrocarbon (PAH) emission line features, a broad far-IR bump resulting from a combination of dust temperatures (T dust ∼ 15-50 K), and an effective T dust ∼ 31 K, as derived from the peak wavelength of their infrared SED. Galaxies in the starburst regime instead exhibit weak PAH equivalent widths and a sharper far-IR bump with an effective T dust ∼ 40 K. Finally, we present evidence that the mid-to-far IR emission of X-ray active galactic nuclei (AGN) is predominantly produced by star formation and that candidate dusty AGNs with a power-law emission in the mid-IR systematically occur in compact, dusty starbursts. After correcting for the effect of starbursts on IR8, we identify new candidates for extremely obscured AGNs.

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.