GOODS-Herschel: Separating High Redshift active galactic Nuclei and star
  forming galaxies Using Infrared Color Diagnostics
  We have compiled a large sample of 151 high redshift (z=0.5-4) 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-infrared spectrum into contributions from star formation and activity in
the galactic nuclei. In addition, we have a wealth of photometric data from
Spitzer IRAC/MIPS and Herschel PACS/SPIRE. We explore how effective different
infrared color combinations are at separating our mid-IR spectroscopically
determined active galactic nuclei from our star forming galaxies. We look in
depth at existing IRAC color diagnostics, and we explore new color-color
diagnostics combining mid-IR, far-IR, and near-IR photometry, since these
combinations provide the most detail about the shape of a source's IR spectrum.
An added benefit of using a color that combines far-IR and mid-IR photometry is
that it is indicative of the power source driving the IR luminosity. For our
data set, the optimal color selections are S250/S24 vs. S8.0/S3.6 and S100/S24
vs. S8.0/S3.6; both diagnostics have ~10% contamination rate in the regions
occupied primarily by star forming galaxies and active galactic nuclei,
respectively. Based on the low contamination rate, these two new IR color-color
diagnostics are ideal for estimating both the mid-IR power source of a galaxy
when spectroscopy is unavailable and the dominant power source contributing to
the IR luminosity. In the absence of far-IR data, we present color diagnostics
using the WISE mid-IR bands which can efficiently select out high z (z~2) star
forming galaxies.
