In this paper we use a 50 ks XMM- Newton pointing overlapping with the Phoenix Deep Survey , a homogeneous radio survey reaching \mu Jy sensitivities , to explore the X-ray properties and the evolution of star-forming galaxies .
Multiwavelength UV , 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 ( total of 82 ) .
Faint radio galaxies with R < 21.5 mag and spiral galaxy SEDs ( total of 34 ) are then segregated into two redshift bins with a median of z = 0.240 ( total of 19 ) and 0.455 ( total of 15 ) respectively .
Stacking analysis for both the 0.5–2 keV and 2–8 keV bands is performed on the two subsamples .
A high confidence level signal ( > 3.5 \sigma ) is detected in the 0.5–2 keV band corresponding to a mean flux of \approx 3 \times 10 ^ { -16 } erg s ^ { -1 } cm ^ { -2 } for both subsamples .
This flux translates to mean luminosities of \approx 5 \times 10 ^ { 40 } and \approx 1.5 \times 10 ^ { 41 } erg s ^ { -1 } for the z = 0.240 and 0.455 subsamples respectively .
Only a marginally significant signal ( 2.6 \sigma ) 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 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 \approx 0.3 is estimated and is found to be elevated compared to local H II galaxies .
The observed increase is consistent with X-ray luminosity evolution of the form \approx ( 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 \approx 0.3 of 0.029 \pm 0.007 M _ { \odot } yr ^ { -1 } Mpc ^ { -3 } .
This is found to be in fair agreement with previous results based on galaxy samples selected at different wavelengths .