Based on the photometry of 10 near-ultraviolet, optical, and near-infrared bands of the Chandra Deep FieldSouth, we estimate the photometric redshifts for 342 X-ray sources, which constitute $99% of all the detected X-ray sources in the field. The models of spectral energy distribution are based on galaxies and a combination of power-law continuum and emission lines. Color information is useful for source classifications: type I active galactic nuclei (AGNs) show nonthermal spectral features that are distinct from galaxies and type II AGNs. The hardness ratio in X-ray and the X-ray-to-optical flux ratio are also useful discriminators. Using rudimentary color separation techniques, we are able to further refine our photometric redshift estimations. Among these sources, 173 have reliable spectroscopic redshifts, which we use to verify the accuracy of photometric redshifts and to modify the model inputs. The average relative dispersion in redshift distribution is $8%, among the most accurate for photometric surveys. The high reliability of our results is attributable to the high quality and broad coverage of data as well as the applications of several independent methods and a careful evaluation of every source. We apply our redshift estimations to study the effect of redshift on broadband colors and to study the redshift distribution of AGNs. Our results show that both the hardness ratio and U À K color decline with redshift, which may be the result of a K-correction. The number of type II AGNs declines significantly at z > 2 and that of galaxies declines at z > 1. However, the distribution of type I AGNs exhibits less redshift dependence. As well, we observe a significant peak in the redshift distribution at z ¼ 0:6. We demonstrate that our photometric redshift estimation produces a reliable database for the study of X-ray luminosity of galaxies and AGNs.

We introduce the Galactic Bulge Survey (GBS) and we provide the Chandra source list for the region that has been observed to date. Among the goals of the GBS are constraining the neutron star equation of state and the black hole mass distribution via the identification of eclipsing neutron star and black hole low-mass X-ray binaries. The latter goal will, in addition, be obtained by significantly enlarging the number of black hole systems for which a black hole mass can be derived. Further goals include constraining X-ray binary formation scenarios, in particular the common envelope phase and the occurrence of kicks, via source-type number counts and an investigation of the spatial distribution of X-ray binaries, respectively. The GBS targets two strips of 6 • × 1 • (12 square degrees in total), one above (1 • < b < 2 • ) and one below (−2 • < b < −1 • ) the Galactic plane in the direction of the Galactic Center at both X-ray and optical wavelengths. By avoiding the Galactic plane (−1 • < b < 1 • ) we limit the influence of extinction on the X-ray and optical emission but still sample relatively large number densities of sources. The survey is designed such that a large fraction of the X-ray sources can be identified from their optical spectra. The X-ray survey, by design, covers a large area on the sky while the depth is shallow using 2 ks per Chandra pointing. In this way we maximize the predicted number ratio of (quiescent) low-mass X-ray binaries to Cataclysmic Variables. The survey is approximately homogeneous in depth to an 0.5-10 keV flux of 7.7×10 −14 erg cm −2 s −1 . So far, we have covered about two-thirds (8.3 square degrees) of the projected survey area with Chandra providing over 1200 unique X-ray sources. We discuss the characteristics and the variability of the brightest of these sources.

We investigate the relationship of the H II region luminosity function (H II LF) to the H II region size distribution and density wave triggering in grand-design spiral galaxies. We suggest that the differential nebular size distribution is described by a power law of slope ∼ −4, with flattening at radii below ∼ 130 pc. This contrasts with the conventional exponential description, but it is physically and quantitatively consistent with the typical observed value of −2 for the H II LF slope.We have developed an interactive code that computes spatial isochrones for the evolving loci of spiral density waves in disk galaxies. This allows comparison of the nebular spatial distribution with the spatial isochrones for simple rotation curve parameters. Our comparisons for four grand-design galaxies suggest that the corotation radius r co coincides with the outer ends of the star-forming arms. This value for r co yields the best spatial correspondence between the H II regions and the isochrones, and also appears to yield a coincidence between the Inner Lindblad Resonance with the radial onset of star formation in the arms. Thus, we suggest that isochrones offer a new, simple, and effective technique for determining r co , and thus the spiral pattern speed. However, application of the isochrones also demonstrates that evolution of the nebular population is difficult to spatially isolate in these galaxies.