The impact of evolving infrared spectral energy distributions of
  galaxies on star formation rate estimates
  We combine Herschel-PACS data from the PEP program with Spitzer 24 um and 16
um photometry and ultra deep 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 around the main sequence (the redshift-dependent relation of
star formation rate and stellar mass). Our deep data confirm from individual
far-infrared detections that z~2 star formation rates are overestimated if
based on 24 um fluxes and SED templates that are calibrated via local trends
with luminosity. Galaxies with similar ratios of rest-frame nuLnu(8) to 8-1000
um 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 nuLnu(8)/LIR regardless of LIR
and redshift, up to z~2.5, and nuLnu(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 um as a function of 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 PAH to LIR rather than continua of hidden
AGN. Alternatively, we discuss the dependence of nuLnu(8)/LIR on LIR. Same
nuLnu(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~1 and
z~2. Corresponding SED template calibrations are provided for use if no stellar
masses are in hand. For most of those z~2 star forming galaxies that also host
an AGN, the mid-infrared is dominated by the star forming component.
