The Phoenix Deep Survey: X‐ray properties of faint radio sources

In this paper, we use a 50‐ks XMM–Newton pointing overlapping with the Phoenix Deep Survey, a homogeneous radio survey reaching μJy sensitivities, to explore the X‐ray properties and the evolution of star‐forming galaxies. Multiwavelength ultraviolet, optical and near‐infrared photometric data are available for this field and are used to estimate photometric redshifts and spectral types for all radio sources brighter than R = 21.5 mag (a total of 82). Faint radio galaxies with R < 21.5 mag and spiral galaxy spectral energy distributions (a total of 34) are then segregated into two redshift bins with a median of z = 0.240 (a total of 19) and 0.455 (a total of 15), respectively. A stacking analysis for both the 0.5–2 and 2–8 keV bands is performed on the two subsamples. A high confidence level signal (>3.5σ) is detected in the 0.5–2 keV band, corresponding to a mean flux of ≈ 3 × 10 −16 erg s −1 cm −2 for both subsamples. This flux translates to mean luminosities of ≈ 5 × 10 40 and ≈ 1.5 × 10 41 erg s −1 for the z = 0.240 and 0.455 subsamples, respectively. Only a marginally significant signal (2.6σ) is detected in the 2–8 keV band for the z = 0.455 subsample. This may indicate hardening of the mean X‐ray properties of sub‐mJy sources at higher redshifts and/or higher luminosities. Alternatively, this may be due to contamination of the z = 0.455 subsample by a small number of obscured active galactic nuclei (AGNs). On the basis of the observed optical and X‐ray properties of the faint radio sample, we argue that the stacked signal above is dominated by star formation, with the AGN contamination being minimal. The mean X‐ray‐to‐optical flux ratio and the mean X‐ray luminosity of the two subsamples are found to be higher than optically selected spirals and similar to starbursts. We also find that the mean X‐ray and radio luminosities of the faint radio sources studied here are consistent with the L X – L 1.4 correlation of local star‐forming galaxies. Moreover, the X‐ray emissivity of sub‐mJy sources to z ≈ 0.3 is estimated and is found to be elevated compared with local H ii galaxies. The observed increase is consistent with X‐ray luminosity evolution of the form ≈ (1 + z ) 3 . Assuming that our sample is indeed dominated by star‐forming galaxies, this is direct evidence for evolution of such systems at X‐ray wavelengths. Using an empirical X‐ray luminosity to star formation rate (SFR) conversion factor, we estimate a global SFR density at z ≈ 0.3 of 0.029 ± 0.007 M ⊙ yr −1 Mpc −3 . This is found to be in fair agreement with previous results based on galaxy samples selected at different wavelengths.