THE IMPACT OF EVOLVING INFRARED SPECTRAL ENERGY DISTRIBUTIONS OF GALAXIES ON STAR FORMATION RATE ESTIMATES

We combine Herschel-Photodetector Array Camera and Spectrometer (PACS) data from the PACS Evolutionary Probe (PEP) program with Spitzer 24 mu m and 16 mu m photometry and ultra deep Infrared Spectrograph (IRS) mid-infrared spectra to measure the mid-to far-infrared spectral energy distribution (SED) of 0.7 < z < 2.5 normal star-forming galaxies (SFGs) around the main sequence (the redshift-dependent relation of star formation rate (SFR) and stellar mass). Our very deep data confirm from individual far-infrared detections that z similar to 2 SFRs are overestimated if based on 24 mu m fluxes and SED templates that are calibrated via local trends with luminosity. Galaxies with similar ratios of rest-frame nu L-nu (8) to 8-1000 mu m infrared luminosity (LIR) tend to lie along lines of constant offset from the main sequence. We explore the relation between SED shape and offset in specific star formation rate (SSFR) from the redshift-dependent main sequence. Main-sequence galaxies tend to have a similar nu L-nu(8)/LIR regardless of LIR and redshift, up to z similar to 2.5, and nu L-nu(8)/LIR decreases with increasing offset above the main sequence in a consistent way at the studied redshifts. We provide a redshift-independent calibration of SED templates in the range of 8-60 mu m as a function of Delta log(SSFR) offset from the main sequence. Redshift dependency enters only through the evolution of the main sequence with time. Ultra deep IRS spectra match these SED trends well and verify that they are mostly due to a change in ratio of polycyclic aromatic hydrocarbon (PAH) to LIR rather than continua of hidden active galactic nuclei (AGNs). Alternatively, we discuss the dependence of nu L-nu(8)/LIR on LIR. The same nu L-nu(8)/LIR is reached at increasingly higher LIR at higher redshift, with shifts relative to local by 0.5 and 0.8 dex in log(LIR) at redshifts z similar to 1 and z similar to 2. Corresponding SED template calibrations are provided for use if no stellar masses are on hand. For most of those z similar to 2 SFGs that also host an AGN, the mid-infrared is dominated by the star-forming component.