X-ray selected normal galaxies

A key development from recent Chandra and XMM-Newton surveys is the identification of normal galaxies outside the local Universe, at cosmologically interesting redshifts (z~1). The X-ray emission in these sources is from X-ray binaries and/or hot gas, i.e. stellar processes, providing the opportunity to study, for the first time, how these components evolve with redshift. The importance of this development is that the X-ray wavelengths are the only tool available to directly probe the X-ray binary population and hot gas in galaxies, providing information on the evolution of these systems that is complementary to that inferred from other wavelengths

Moreover, the deepest X-ray surveys currently available suggest that these systems will outnumber the traditional X-ray population of AGNs at fluxes below f_X=10^-17erg/s/cm², making them a significant component of the X-ray Universe at these faint limits (Hornschemeier et al. 2003).

On the left we show the optical image, with the X-ray contours overlaid, and the optical spectrum (both from the SDSS) of an emission-line galaxy at z=0.005. This system is detected in the Needles in the Haystack Survey (NHS). This ongoing project aims to compile the first complete sample of X-ray selected `normal' galaxies at z~0.1 for comparison with higher-z surveys. Combining low with high-z samples is essential to provide a wide redshift baseline to constrain the evolution of these systems.