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

Authors: Laird, E. S.; Nandra, K.1; Adelberger, K. L.2; Steidel, C. C.3; Reddy, N. A.3

Source: Monthly Notices of the Royal Astronomical Society, Volume 359, Number 1, May 2005 , pp. 47-56(10)

Publisher: Blackwell Publishing

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Abstract:

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 (14.9σ and 3.2σ, respectively). The derived mean 2–10 keV rest-frame luminosity is 2.97 ± 0.26 × 1040 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 ∼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, LX∝LUV , 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 LX/LUV 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 ∼100 for the UV-selected z∼ 1 galaxies in this sample.

Keywords: galaxies: high-redshift; galaxies: starburst; X-rays: galaxies

Document Type: Research article

DOI: 10.1111/j.1365-2966.2005.08986.x

Affiliations: 1: Astrophysics Group, Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ 2: Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA 3: California Institute of Technology, MS 105-24, Pasadena, CA 91125, USA

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