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 rest-frame ultraviolet properties of galaxies during the first three billion years of cosmic time (redshift z > 4) indicate a rapid evolution in the dust obscuration of such galaxies. This evolution implies a change in the average properties of the interstellar medium, but the measurements are systematically uncertain owing to untested assumptions and the inability to detect heavily obscured regions of the galaxies. Previous attempts to measure the interstellar medium directly in normal galaxies at these redshifts have failed for a number of reasons, with two notable exceptions. Here we report measurements of the forbidden C ii emission (that is, [C II]) from gas, and the far-infrared emission from dust, in nine typical star-forming galaxies about one billion years after the Big Bang (z ≈ 5-6). We find that these galaxies have thermal emission that is less than 1/12 that of similar systems about two billion years later, and enhanced [C II] emission relative to the far-infrared continuum, confirming a strong evolution in the properties of the interstellar medium in the early Universe. The gas is distributed over scales of one to eight kiloparsecs, and shows diverse dynamics within the sample. These results are consistent with early galaxies having significantly less dust than typical galaxies seen at z < 3 and being comparable in dust content to local low-metallicity systems.

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.

ALMA observations of the long wavelength dust continuum are used to estimate the interstellar medium (ISM) masses in a sample of 708 galaxies at z=0.3 to 4.5 in the COSMOS field. The galaxy sample has known farinfrared luminosities and, hence, star formation rates (SFRs) and stellar masses ( * M ) from the optical-infrared spectrum fitting. The galaxies sample SFRs from the main sequence (MS) to 50 times above the MS. The derived ISM masses are used to determine the dependence of gas mass on redshift, * M , and specific SFR (sSFR) relative to the MS. The ISM masses increase approximately with the 0.63 power of the rate of increase in SFRs with redshift and the 0.32 power of the sSFR/sSFR MS . The SF efficiencies also increase as the 0.36 power of the SFR redshift evolution and the 0.7 power of the elevation above the MS; thus the increased activities at early epochs are driven by both increased ISM masses and SF efficiency. Using the derived ISM mass function, we estimate the accretion rates of gas required to maintain continuity of the MS evolution (>  M 100 yr −1 at z > 2.5). Simple power-law dependencies are similarly derived for the gas accretion rates. We argue that the overall evolution of galaxies is driven by the rates of gas accretion. The cosmic evolution of total ISM mass is estimated and linked to the evolution of SF and active galactic nucleus activity at early epochs.