3.4. X-ray Emission from Distant Starburst and Normal Galaxies

While the majority of the 0.5-10 keV CXRB has now been resolved, most of the extragalactic X-ray sources throughout the Universe have yet to be detected. These are distant starburst and normal galaxies, where most of the X-ray emission arises from X-ray binaries, ultraluminous X-ray sources, supernova remnants, starburst-driven outflows, and hot gas. Accreting nuclear SMBH (i.e., low-luminosity and heavily obscured AGN) are certainly also present in some cases (e.g., see Section 2.4), although they probably contribute 1/3 of these galaxies' X-ray emission at low energies. Some starburst and normal galaxies at cosmological distances (z 0.1-1.5) are now being detected at 0.5-2 keV fluxes below 5 × 10^-16 erg cm^-2 s^-1. They constitute a rising fraction of the total number of X-ray sources toward fainter fluxes (up to 30-40%; see Figure 3), and this trend will continue until they become the numerically dominant source population at 0.5-2 keV fluxes of 5 × 10^-18 erg cm^-2 s^-1 (e.g., Hornschemeier et al. 2002, 2003; Miyaji & Griffiths 2002; Ranalli, Comastri & Setti 2003; Bauer et al. 2004). This transition from AGN dominance to galaxy dominance of the X-ray number counts is broadly analogous to that observed in the radio band below a few millijanskys (e.g., Windhorst 2003 and references therein).

Distant starburst and normal galaxies in X-ray deep fields can be identified based upon their X-ray, optical, infrared, and radio properties. In the X-ray band, these galaxies typically have luminosities below 10⁴² erg s^-1, spectra with effective power-law photon indices of 1.7-2.2, and X-ray-to-optical flux ratios of log(f_X / f_I) < - 1 (see Figure 4). Their observed X-ray luminosity function has a lognormal form, as is also observed for galaxies at infrared and radio wavelengths (Norman et al. 2004). Optically, the distant galaxies detected in X-rays have relatively high [O II] 3727 luminosities and morphologies consistent with those of field galaxies (e.g., Cohen 2003; Hornschemeier et al. 2003). Off-nuclear X-ray sources appear relatively important, as they are observed in 20% of these galaxies at z 0.2. These have X-ray luminosities of 10³⁹ to a few times 10⁴⁰ erg s^-1 and are probably luminous black-hole binaries or groups of X-ray binaries (e.g., Hornschemeier et al. 2004).

The X-ray emission from starburst and normal galaxies can provide an independent measure of their SFRs that is relatively immune to extinction effects (e.g., Bauer et al. 2002b; Cohen 2003; Grimm, Gilfanov & Sunyaev 2003; Ranalli, Comastri & Setti 2003; Persic et al. 2004). X-ray derived SFRs agree respectably with SFRs from optical, infrared, and radio measurements, at least when the X-ray emission from high-mass X-ray binaries dominates or can be isolated. Many of the distant galaxies detected in X-ray deep fields have remarkably high estimated SFRs of 10-300 M yr^-1. These galaxies are members of the strongly evolving, dusty starburst population responsible for creating much of the infrared background (e.g., Alexander et al. 2002).

The stellar sources of X-ray emission in galaxies should show substantial evolution with redshift in response to the factor of 10-100 increase in cosmic SFR out to z 1-3. The high-mass X-ray binary population is expected to track the cosmic SFR closely, peaking at z 1-3, while low-mass X-ray binaries should track it with a lag of a few Gyr due to their longer evolutionary timescales, therefore peaking at z 0.5-1 (e.g., Ghosh & White 2001). Source-stacking analyses have allowed the average X-ray properties of z 0.1-4 galaxies to be measured, complementing the individual X-ray detections of galaxies (e.g., Brandt et al. 2001a, c; Hornschemeier et al. 2002; Nandra et al. 2002; Seibert, Heckman & Meuer 2002; Georgakakis et al. 2003; Reddy & Steidel 2004; Lehmer et al. 2005). These analyses indicate that the ratio of X-ray to B-band luminosity for galaxies rises from z 0-1, such that galaxies at z 1 are 2-5 times as X-ray luminous (per unit B-band luminosity) as their local counterparts. Lyman break galaxies at z 2-4 have typical X-ray properties similar to those of the most X-ray luminous local starbursts, and X-ray estimates of SFRs in Lyman break galaxies are in respectable agreement with those derived using rest-frame UV data (after appropriate UV reddening corrections are made).