Investigating the Far-IR/Radio Correlation of Star Forming Galaxies to
  z=3
  In order to examine the far-IR/radio correlation at high redshift we have
studied the Spitzer 70um/far-infrared (far-IR) properties of sub-mJy radio
sources from the 13^H XMM-Newton/Chandra Deep Field by redshift and galaxy
type: active galactic nucleus (AGN) or star forming galaxy (SFG). We directly
detect 70um counterparts (at >3sigma significance) for 22.5% (92/408) of the
radio sources, while for the rest we perform stacking analysis by redshift and
galaxy type. For the sources detected at 70um we find that the median and
scatter of the observed flux density ratio, q70, are similar to previous
results in the literature, but with a slight decrease in q70 towards higher
redshifts. Of the radio sources detected at 70um 8/92 were already classified
as AGN, but two of which maybe SFGs. For the stacked sources we obtain a
detection for the SFGs at every redshift bin which implies they have mean flux
densities a factor ~5 below the original 70um detection limit. For the stacked
AGN we obtain a detection only in our highest redshift bin (1<~z<~5) where we
may be sampling hot dust associated with the AGN at rest-frame 12-35um. The
combined observed mean value of q70 for the SFGs (detected and non-detected at
70um) decreases gradually with redshift, consistent with tracks derived from
empirical spectral energy distributions (SEDs) of local SFGs. Upon closer
inspection and when comparing with tracks of appropriate luminosity, the values
of q70 broadly agree at low redshift. However, at z~1, the observed q70 (for
ULIRGs) is 2sigma below the value seen for local ULIRGs tracks, implying a
difference in the SED between local and z~1 ULIRGs. At higher redshifts, the
convergence of the tracks and the higher uncertainties in q70 do not allow us
to determine if such a difference persists.
