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

Abstract

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 13^H 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 >3σ 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 q_(70) 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 q_(70) 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 q_(70) broadly agree at low redshift. However, at z ~ 1, the observed q_(70) [for ultraluminous infrared galaxies (ULIRGs)] is 2 σ 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 q_(70) do not allow us to determine if such a difference persists.

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