The Chandra deep field north survey. XII. The link between faint X-ray and radio source populations

We investigate the relationship between faint X-ray and 1.4 GHz radio source populations detected within 3′ of the Hubble Deep Field (North) using the 1 Ms Chandra and 40 μJy VLA surveys. Within this region, we find that ≈42% of the 62 X-ray sources have radio counterparts and ≈71% of the 28 radio sources have X-ray counterparts; thus, a 40 μJy VLA survey at 1.4 GHz appears to be well matched to a 1 Ms Chandra observation. Among the different source populations sampled, we find that the majority of the 18 X-ray-detected emission-line galaxies (ELGs) have radio and mid-infrared ISOCAM counterparts and appear to be luminous star-forming galaxies at z = 0.3-1.3. Importantly, the radio-detected ELGs make up ≈35% of the X-ray source population at 0.5-8.0 keV X-ray fluxes between ≈1 × 10^-16 and ≈5 × 10^-16 ergs cm^-2 s^-1 and signal the emergence of the luminous, high-z starburst galaxy population in the X-ray band. We find that the locally determined correlation between X-ray luminosities and 1.4 GHz radio luminosity densities of the late-type galaxies can easily be extended to include the luminous intermediate-redshift ELGs, suggesting that the X-ray and radio emission processes are generally associated in star-forming galaxies. This result implies that the X-ray emission can be used as an indicator of star formation rate for star-forming galaxies. Finally, we show that there appear to be two statistically distinct types of ISOCAM-detected star-forming galaxies: those with detectable radio and X-ray emission and those without. The latter type may have stronger mid-infrared emission-line features that increase their detectability at mid-infrared wavelengths.