The XMM-Newton/2dF survey-I. X-ray properties of normal galaxies

This paper explores the X-ray properties of ‘normal’ galaxies using a shallow (2–10 ks) XMM–Newton survey covering an area of ≈1.5 deg2. The X-ray survey overlaps with the 2dF Galaxy Redshift Survey. Compared with previous studies this has the advantage that high-quality spectra and spectral classifications (early-, late-type) exist for all galaxies to bj= 19.4. Moreover, sources with optical spectra revealing powerful active galactic nuclei (AGN) can easily be discarded from the normal galaxy sample used here. In particular, we present stacking analysis results for about 200 galaxies from the 2dF Galaxy Redshift Survey at a mean redshift of z≈ 0.1. We detect a strong signal for the whole sample (≈6σ) in the soft 0.5–2 keV band corresponding to a flux of ≈7 × 10−16 erg s−1 cm−2 and a luminosity of ≈2 × 1040 erg s−1. A statistically significant signal is also detected for both the early and late galaxy subsamples with X-ray luminosities of ≈3 × 1040 and ≈5 × 1039 erg s−1, respectively. In contrast, no signal is detected in the hard 2–8 keV band for any of the above samples. The mean LX/LB ratio of the spiral galaxy sample is found to be consistent with both local (<100 Mpc) and distant (z≈ 1) samples (after accounting for differences in LB), suggesting little or no evolution of the X-ray emission mechanisms relative to the optical. The 0.5–2 keV X-ray background (XRB) contribution of the spiral galaxy subsample at z≈ 0.1 is estimated to be 0.4 per cent, in broad agreement with the XRB fractions estimated in previous studies. Assuming that star-forming galaxies evolve with redshift as (1 +z)k, the present data combined with previous studies suggest k < 3. The k values are constrained by the relatively low fraction of the soft X-ray background that remains unresolved by deep surveys (6–26 per cent). Higher k values will result in an overproduction of the soft X-ray background, given the fraction already attributed to AGN, groups and clusters. The mean X-ray emissivity of spiral galaxies at z≈ 0.1 is also estimated and is found to be consistent within the uncertainties with that of local H II galaxy samples. Using the mean X-ray emissivity of the spiral galaxy subsample we estimate a global star formation density of 0.009 ± 0.007 M⊙ yr−1 Mpc−3 at z≈ 0.1. Although the uncertainty is large, this is lower than previous results based on galaxy samples selected at different wavelengths. Nevertheless, given the large uncertainties involved in converting X-ray luminosity to star formation rate the agreement is surprising.