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 (gt;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 × 10⁴⁰ 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-mly 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.