Investigating the far-IR/radio correlation of star-forming Galaxies to z = 3

In order to examine the far-infrared (far-IR)/radio correlation at high redshift, we have studied the Spitzer 70 µm/far-IR properties of sub-mJy radio sources from the 13H XMM-Newton/Chandra Deep Field by redshift and galaxy type: active galactic nucleus (AGN) or star-forming galaxy (SFG). We directly detect 70 µm counterparts (at >3s significance) for 22.5 per cent (92/408) of the radio sources, while for the rest we perform stacking analysis by redshift and galaxy type. For the sources detected at 70 µm, we find that the median and scatter of the observed flux density ratio, q 70, are similar to previous results in the literature, but with a slight decrease in q70 towards higher redshifts. Of the radio sources detected at 70 µm 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 70 µm 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 12-35 µm. The combined observed mean value of q70 for the SFGs (detected and non-detected at 70 µm) 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 ultraluminous infrared galaxies (ULIRGs)] is 2 s 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. © 2009 RAS.