GOODS-Herschel: Impact of Active Galactic Nuclei and Star Formation
  Activity on Infrared Spectral Energy Distributions at High Redshift
  We explore the effects of active galactic nuclei (AGN) and star formation
activity on the infrared (0.3-1000 microns) spectral energy distributions of
luminous infrared galaxies from z = 0.5 to 4.0. We have compiled a large sample
of 151 galaxies selected at 24 microns (S24 > 100 uJy) in the GOODS-N and ECDFS
fields for which we have deep Spitzer IRS spectroscopy, allowing us to
decompose the mid-IR spectrum into contributions from star formation and AGN
activity. A significant portion (~25%) of our sample is dominated by an AGN in
the mid-IR. Based on the mid-IR classification, we divide our full sample into
four sub-samples: z~1 star-forming (SF) sources; z~2 SF sources; AGN with clear
9.7 micron silicate absorption; and AGN with featureless mid-IR spectra. From
our large spectroscopic sample and wealth of multi-wavelength data, including
deep Herschel imaging at 100, 160, 250, 350, and 500 microns, we use 95
galaxies with complete spectral coverage to create a composite spectral energy
distribution (SED) for each sub-sample. We then fit a two-temperature component
modified blackbody to the SEDs. We find that the IR SEDs have similar cold dust
temperatures, regardless of the mid-IR power source, but display a marked
difference in the warmer dust temperatures. We calculate the average effective
temperature of the dust in each sub-sample and find a significant (~20 K)
difference between the SF and AGN systems. We compare our composite SEDs to
local templates and find that local templates do not accurately reproduce the
mid-IR features and dust temperatures of our high redshift systems. High
redshift IR luminous galaxies contain significantly more cool dust than their
local counterparts. We find that a full suite of photometry spanning the IR
peak is necessary to accurately account for the dominant dust temperature
components in high redshift IR luminous galaxies.
