X-ray properties of UV-selected star-forming galaxies at z∼1 in the Hubble Deep Field North

We present an analysis of the X-ray emission from a large sample of ultraviolet (UV) selected, star-forming galaxies with 0.74 < z < 1.32 in the Hubble Deep Field North (HDF-N) region. By excluding all sources with significant detected X-ray emission in the 2-Ms Chandra observation we are able to examine the properties of galaxies for which the dominant emission in both UV and X-ray is expected to be predominantly the result of star formation. Stacking the X-ray flux from 216 galaxies in the soft and hard bands produces significant detections (14.9 sigma and 3.2 sigma, respectively). The derived mean 2 - 10 keV rest-frame luminosity is 2.97 +/- 0.26 x 10(40) erg s(-1), corresponding to an X-ray derived star formation rate (SFR) of 6.0 +/- 0.6 M-. yr(-1). Comparing the X-ray value with the mean UV derived SFR, uncorrected for attenuation, we find that the average UV attenuation correction factor is similar to 3. By binning the galaxy sample according to UV magnitude and colour, and stacking the observed frame soft band X-ray flux in each bin, correlations between UV and X-ray emission are examined. We find a strong positive correlation between X-ray emission and rest-frame UV emission, consistent with a strict linear relationship, L-X proportional to L-UV, at the 90 per cent level. A correlation between the ratio of X-ray-to-UV emission and UV colour is also seen, such that L-X/L-UV increases for redder galaxies. We find no direct relation between X-ray flux and UV colour. Given that X-ray emission offers a view of star formation regions that is relatively unaffected by extinction, results such as these can be used to evaluate the effects of dust on the UV emission from high-z galaxies. For instance, using the observed correlation between UV colour excess and the ratio of X-ray-to-UV emission ( a measure of UV obscuration), we derive a relationship for estimating UV attenuation corrections as a function of colour excess. The observed relation is inconsistent with the Calzetti et al. reddening law, which overpredicts the range in UV attenuation corrections by a factor of similar to 100 for the UV-selected z similar to 1 galaxies in this sample.