We present the results of spectroscopic studies with the LRIS spectrograph on Keck of two of the Hawaii deep survey fields. The 393 objects observed cover a 26.2 arcmin 2 area and constitute a nearly complete sample down to K = 20, I = 23, and B = 24.5. The rest-frame K -band luminosity function and its evolution with redshift are described. Comparisons are made with other optically selected (B and I ) samples in the literature, and the corresponding rest-frame Bband luminosity function evolution is presented. The B -band counts at B ∼ 24 are shown to be a mixture of normal galaxies at modest redshifts and galaxies undergoing rapid star formation, which have a wide range of masses and which are spread over the redshift interval from z = 0.2 to beyond z = 1.7.The luminosity functions, number counts, and color distributions at optical and IR wavelengths are discussed in terms of a consistent picture of the starforming history of the galaxy sample. [O II] emission-line diagnostics or restframe ultra-violet-infrared color information are used in combination with restframe absolute K magnitudes to construct a "fundamental plane" in which the evolution of the global star-formation rate with redshift can be shown, and we find that the maximum rest-frame K luminosity of galaxies undergoing rapid star formation has been declining smoothly with decreasing redshift from a value near L * at z > 1. This smooth decrease in the characteristic luminosity of galaxies dominated by star formation can simultaneously account for the high B -band galaxy counts at faint magnitudes and the redshift distribution at z < 1 in both the B -and K -selected samples. Finally, the overall K -band light density evolution is discussed as a tracer of the baryonic mass in stars and compared with the overall rates of star formation inferred from the rest-frame ultraviolet light density as a function of redshift.

We use highly spectroscopically complete deep and wide-area Chandra surveys to determine the cosmic evolution of hard X-ray-selected active galactic nuclei (AGNs). For the deep fields, we supplement the spectroscopic redshifts with photometric redshifts to assess where the unidentified sources are likely to lie. We find that the median redshifts are fairly constant with X-ray flux at z $ 1.We classify the optical spectra and measure the FWHM line widths. Most of the broad-line AGNs show essentially no visible absorption in X-rays, whereas the sources without broad lines (FWHM < 2000 km s À1 ; ''optically narrow'' AGNs) show a wide range of absorbing column densities. We determine hard X-ray luminosity functions for all spectral types with L X ! 10 42 ergs s À1 and for broad-line AGNs alone. At z < 1:2, both are well described by pure luminosity evolution, with L * evolving as (1 þ z) 3:2AE0:8 for all spectral types and as (1 þ z) 3:0AE1:0 for broad-line AGNs alone. Thus, all AGNs drop in luminosity by almost an order of magnitude over this redshift range. We show that this observed drop is due to AGN downsizing rather than to an evolution in the accretion rates onto the supermassive black holes.We directly compare our broad-line AGN hard X-ray luminosity functions with the optical QSO luminosity functions and find that at the bright end they agree extremely well at all redshifts. However, the optical QSO luminosity functions do not probe faint enough to see the downturn in the broad-line AGN hard X-ray luminosity functions and even appear to be missing some sources at the lowest luminosities they probe.We find that broad-line AGNs dominate the number densities at the higher X-ray luminosities, while optically narrow AGNs dominate at the lower X-ray luminosities. We rule out galaxy dilution as a partial explanation for this effect by measuring the nuclear UV/optical properties of the Chandra sources using the Hubble Space Telescope Advanced Camera for Surveys GOODS-North data. The UV/optical nuclei of the optically narrow AGNs are much weaker than expected if the optically narrow AGNs were similar to the broad-line AGNs. We therefore postulate the need for a luminosity-dependent unified model. An alternative possibility is that the broad-line AGNs and the optically narrow AGNs are intrinsically different source populations. We cover both interpretations by constructing composite spectral energy distributions-including long-wavelength data from the mid-infrared to the submillimeterby spectral type and by X-ray luminosity. We use these spectral energy distributions to infer the bolometric corrections (from hard X-ray luminosities to bolometric luminosities) needed to map the accretion history.We determine the accreted supermassive black hole mass density for all spectral types and for broad-line AGNs alone, using the observed evolution of the hard X-ray energy density production rate and our inferred bolometric corrections. We find that only about one-half to one-quarter of the supermassive black hole mass ...

The global star formation rate in high redshift galaxies, based on optical surveys, shows a strong peak at a redshift of z ∼ 1.5, which implies that we have already seen most of the formation. High redshift galaxies may, however, emit most of their energy at submillimeter wavelengths if they contain substantial amounts of dust. The dust would absorb the starlight and reradiate it as far-infrared light, which would be redshifted to the submillimeter range. Here we report a deep survey of two blank regions of sky performed at submillimeter wavelengths (450 and 850µm). If the sources we detect in the 850µm band are powered by star formation, then each must be converting more than 100 solar masses of gas per year into stars, which is larger than the maximum star formation rates inferred for most optically-selected galaxies. The total amount of high redshift star formation is essentially fixed by the level of background light, but where the peak occurs in redshift for the submillimeter is not yet established. However, the background light contribution from only the sources detected at 850µm is already comparable to that from the optically-selected sources. Establishing the main epoch of star formation will therefore require a combination of optical and submillimeter studies.In recent years high redshift optical galaxy searches have become increasingly successful at uncovering significant populations of galaxies that are likely to be in early phases of evolution. However, the global star formation rate (SFR) inferred 1, 3, 4 omits the many fainter sources that are now being detected. 5,6 Furthermore, the effects of dust can cause the SFRs in the detected UV-bright objects to be grossly underestimated (see, e.g., ref. 7), and many rapid star forming galaxies may even be omitted from the optical samples.Nearby star forming galaxies emit a large fraction of their bolometric luminosity in the far infrared waveband, which for distant sources is redshifted into the submillimeter waveband. Because the spectra of these star forming galaxies are very steep, if they are at large redshifts their flux density decreases much less rapidly with increasing redshift

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We present point-source catalogs for the %2 Ms exposure of the Chandra Deep Field North, currently the deepest X-ray observation of the universe in the 0.5-8.0 keV band. Five hundred and three (503) X-ray sources are detected over an %448 arcmin 2 area in up to seven X-ray bands. Twenty (20) of these X-ray sources lie in the central %5.3 arcmin 2 Hubble Deep Field North (13; 600 þ3800 À3000 sources deg À2 ). The on-axis sensitivity limits are %2.5 Â 10 À17 ergs cm À2 s À1 (0.5-2.0 keV) and %1.4 Â 10 À16 ergs cm À2 s À1 (2-8 keV). Source positions are determined using matched-filter and centroiding techniques; the median positional uncertainty is %0>3. The X-ray colors of the detected sources indicate a broad variety of source types, although absorbed AGNs (including a small number of possible Compton-thick sources) are clearly the dominant type. We also match lower significance X-ray sources to optical counterparts and provide a list of 79 optically bright (R d 23) lower significance Chandra sources. The majority of these sources appear to be starburst and normal galaxies. The average backgrounds in the 0.5-2.0 keV and 2-8 keV bands are 0.056 and 0.135 counts Ms À1 pixel À1 , respectively. The background count distributions are very similar to Poisson distributions. We show that this %2 Ms exposure is approximately photon limited in all seven X-ray bands for regions close to the aim point, and we predict that exposures up to %25 Ms (0.5-2.0 keV) and %4 Ms (2-8 keV) should remain nearly photon limited. We demonstrate that this observation does not suffer from source confusion within %6 0 of the aim point, and future observations are unlikely to be source-confusion limited within %3 0 of the aim point even for source densities exceeding 100,000 deg À2 . These analyses directly show that Chandra can achieve significantly higher sensitivities in an efficient, nearly photon-limited manner and be largely free of source confusion. To allow consistent comparisons, we have also produced pointsource catalogs for the %1 Ms Chandra Deep Field South (CDF-S). Three hundred and twenty-six (326) X-ray sources are included in the main Chandra catalog, and an additional 42 optically bright X-ray sources are included in a lower significance Chandra catalog. We find good agreement with the photometry of the previously published CDF-S catalogs; however, we provide significantly improved positional accuracy.

The origin of the hard (2-10 keV) X-ray background has been a mystery for over 35 years. Most of the soft X-ray background has been resolved into individual sources (mainly quasars), but these sources do not have the spectral energy distribution required to match the spectrum of the X-ray background as a whole. Here we report the results of a deep survey, using the Chandra satellite, in which the detected hard X-ray sources account for at least 75 per cent of the hard X-ray background. The mean X-ray spectral energy distribution of these sources is in good agreement with that of the background. Moreover, most of those hard X-ray sources are associated unambiguously with either the nuclei of otherwise normal bright galaxies, or with optically faint sources. The latter could be active nuclei in dust-enshrouded galaxies or a population of quasars at extremely high redshift.

The text of this paper has today been replaced with a version which contains all the figures and tables, including the galaxy redshifts.Unfortunately, I and my group, being small (because I have received no federal funding for this research for several years) have not finished two final pieces of analysis: (1) a paper on the evolution in the galaxy luminosity function and (2) a paper on the relationships between broadband colors, morphologies, and narrow spectral features (lines and breaks). I therefore request that the redshifts not be used for projects which are substantially similar to either of these two projects until I have submitted such papers to astro-ph. I expect this to happen by 2000 October. Thank you very much.

The final chapter in the long-standing mystery of the gamma-ray bursts (GRBs) centres on the origin of the short-hard class of bursts, which are suspected on theoretical grounds to result from the coalescence of neutron-star or black-hole binary systems. Numerous searches for the afterglows of short-hard bursts have been made, galvanized by the revolution in our understanding of long-duration GRBs that followed the discovery in 1997 of their broadband (X-ray, optical and radio) afterglow emission. Here we present the discovery of the X-ray afterglow of a short-hard burst, GRB 050709, whose accurate position allows us to associate it unambiguously with a star-forming galaxy at redshift z = 0.160, and whose optical lightcurve definitively excludes a supernova association. Together with results from three other recent short-hard bursts, this suggests that short-hard bursts release much less energy than the long-duration GRBs. Models requiring young stellar populations, such as magnetars and collapsars, are ruled out, while coalescing degenerate binaries remain the most promising progenitor candidates.