New results from the X-ray and optical survey of the Chandra Deep Field-South: The 300 kilosecond exposure. II.

We present results from 300 ks of X-ray observations of the Chandra Deep Field-South. The field of the four combined exposures is now 0.1035 deg(2), and we reach a flux limit of 10(-16) ergs s(-1) cm(-2) in the 0.5-2 keV soft band and 10(-15) ergs s(-1) cm(-2) in the 2-10 keV hard band, i.e., a factor of 2 fainter than the previous 120 ks exposure. The total catalog is composed of 197 sources including 22 sources detected only in the hard band, 51 only in the soft band, and 124 detected in both bands. We now have the optical spectra for 86 optical counterparts. The log N-log S relationship of the whole sample confirms the flattening with respect to the ASCA hard counts and the ROSAT soft counts. The average logarithmic slopes of the number counts are alpha = 0.66 +/- 0.06 and alpha = 0.92 +/- 0.12 in the soft and hard bands, respectively. Double power-law fits to the differential counts show evidence of further flattening at the very faint end to slopes of 0.5 +/- 0.1 and 0.6 +/- 0.2 in the soft and hard bands, respectively. We compute the total contribution to the X-ray background (XRB) in the 2-10 keV band, which now amounts to (1.45 +/- 0.15) x 10(-11) ergs cm(-2) s(-1) deg(-2) (after the inclusion of the ASCA sources to account for the bright end) to a lower flux limit of 10(-15) ergs s(-1) cm(-2). This corresponds to 60%-90% of the unresolved hard XRB, given the uncertainties on its actual value. We confirm previous findings on the average spectrum of the sources, which is well described by a power law with Gamma = 1.44 +/- 0.03, and the progressive hardening of the sources at lower fluxes. In particular, we find that the average spectral slope of the sources is flatter than the average for fluxes lower than 9 x 10(-15) ergs s(-1) cm(-2) in the hard band. The hardening of the spectra is consistent with an increasing fraction of absorbed objects (N-H > 10(22) cm(-2)) at low fluxes. From 86 redshifts available at present, we find that hard sources have on average lower redshifts (z less than or equal to 1) than soft sources. Their typical luminosities and optical spectra show that most of these sources are obscured active galactic nuclei (AGNs), as expected by AGN population synthesis models of the XRB. We are still in the process of finding hard sources that constitute the remaining fraction of the total XRB. Most of the sources detected only in the soft band appear to be optically normal galaxies with luminosities L-x similar or equal to 10(40) -10(42) ergs s(-1). This population appears to be a mix of normal galaxies, possibly with enhanced star formation, and galaxies with low-level nuclear activity.