Infrared Spectral Energy Distributions of Nearby Galaxies

TLDR

The Spitzer Infrared Nearby Galaxies Survey (SINGS) is conducting a comprehensive multiwavelength survey of 75 nearby galaxies. The authors present 1–850 µm spectral energy distributions constructed from Spitzer, 2MASS, ISO, IRAS, and SCUBA broadband imaging, and use Spitzer’s sensitivity and resolution to examine SEDs at sub‑galactic scales in NGC 3031, NGC 5194, and NGC 7331. Infrared colors largely agree with existing star‑forming galaxy models but show significant deviations; within each galaxy a wide range of spectral shapes is observed, with strong correlations between local star‑formation rate and the 70 µm/160 µm and 24 µm/160 µm colors, and variations in 24 µm emission driving 8 µm/24 µm color changes, implying that extrapolating total infrared luminosities from 24 µm flux can be uncertain by up to a factor of five (and up to 10–20 for redshifted 8 µm), warranting caution.

Abstract

The Spitzer Infrared Nearby Galaxies Survey (SINGS) is carrying out a comprehensive multiwavelength survey on a sample of 75 nearby galaxies. The 1-850 μm spectral energy distributions (SEDs) are presented using broadband imaging data from Spitzer, 2MASS, ISO, IRAS, and SCUBA. The infrared colors derived from the globally integrated Spitzer data are generally consistent with the previous generation of models that were developed using global data for normal star-forming galaxies, although significant deviations are observed. Spitzer's excellent sensitivity and resolution also allow a detailed investigation of the infrared SEDs for various locations within the three large, nearby galaxies NGC 3031 (M81), NGC 5194 (M51), and NGC 7331. A wide variety of spectral shapes is found within each galaxy, especially for NGC 3031, the closest of the three targets and thus the galaxy for which the smallest spatial scales can be explored. Strong correlations exist between the local star formation rate and the infrared colors fν(70 μm)/fν(160 μm) and fν(24 μm)/fν(160 μm), suggesting that the 24 and 70 μm emission are useful tracers of the local star formation activity level. Preliminary evidence indicates that variations in the 24 μm emission, and not variations in the emission from polycyclic aromatic hydrocarbons at 8 μm, drive the variations in the fν(8.0 μm)/fν(24 μm) colors within NGC 3031, NGC 5194, and NGC 7331. If the galaxy-to-galaxy variations in SEDs seen in our sample are representative of the range present at high redshift, then extrapolations of total infrared luminosities and star formation rates from the observed 24 μm flux will be uncertain at the factor of 5 level (total range). The corresponding uncertainties using the redshifted 8.0 μm flux (e.g., observed 24 μm flux for a z = 2 source) are factors of 10-20. Considerable caution should be used when interpreting such extrapolated infrared luminosities.