We report early results on galaxies at z ∼ 6, selected from Hubble Space Telescope imaging for the Great Observatories Origins Deep Survey. Spectroscopy of one object with the Advanced Camera for Surveys grism and from the Keck and VLT observatories a shows a strong continuum break and asymmetric line emission, identified as Lyα at z = 5.83. We detect only five spatially extended, z ∼ 6 candidates with signal-to-noise ratios > 10, two of which have spectroscopic confirmation. This is many fewer than would be expected if galaxies at z = 6 had the same luminosity function as those at z = 3. There are many fainter candidates, but we expect substantial contamination from foreground interlopers and spurious detections. Our best estimates favor a z = 6 galaxy population 1 Based on observations taken with the NASA/ESA Hubble Space Telescope, which is operated by AURA, Inc., under NASA contract NAS5-26555; from the W. M. Keck Observatories; and from the Very Large Telescope (VLT) at Cerro Paranal, Chile, operated by the European Southern Observatory, under programs 170.A-0788 and 168.A-0485.

We present Keck Low Resolution Imaging Spectrometer optical spectra of 17 Ly-emitting galaxies and one Lyman break galaxy at z % 4:5 discovered in the Large Area Lyman Alpha survey. The survey has identified a sample of $350 candidate Ly-emitting galaxies at z % 4:5 in a search volume of 1:5 ; 10 6 comoving Mpc 3 . We targeted 25 candidates for spectroscopy; hence, the 18 confirmations presented herein suggest a selection reliability of 72%. The large equivalent widths (median W rest k % 80 8) but narrow physical widths (Áv < 500 km s À1 ) of the Ly emission lines, along with the lack of accompanying high-ionization state emission lines, suggest that these galaxies are young systems powered by star formation rather than by active galactic nucleus activity. Theoretical models of galaxy formation in the primordial universe suggest that a small fraction of Ly-emitting galaxies at z % 4:5 may still be nascent, metal-free objects. Indeed, we find with 90% confidence that three to five of the confirmed sources show W rest k > 240 8, exceeding the maximum Ly equivalent width predicted for normal stellar populations. Nonetheless, we find no evidence for He ii k1640 emission in either individual or composite spectra, indicating that although these galaxies are young, they are not truly primitive, Population III objects.

We present a catalog of 59 z % 4:5 Ly -emitting galaxies spectroscopically confirmed in a campaign of Keck DEIMOS follow-up observations to candidates selected in the Large Area Ly (LALA) narrowband imaging survey. We targeted 97 candidates for spectroscopic follow-up; by accounting for the variety of conditions under which we performed spectroscopy, we estimate a selection reliability of $76%. Together with our previous sample of Keck LRIS confirmations, the 59 sources confirmed herein bring the total catalog to 73 spectroscopically confirmed z % 4:5 Lyemitting galaxies in the %0.7 deg 2 covered by the LALA imaging. As with the Keck LRIS sample, we find that a nonnegligible fraction of the confirmed Ly lines have rest-frame equivalent widths (W rest k ) that exceed the maximum predicted for normal stellar populations: 17%Y31% (93% confidence) of the detected galaxies show W rest k > 190 8, and 12%Y27% (90% confidence) show W rest k > 240 8. We construct a luminosity function of z % 4:5 Ly emission lines for comparison to Ly luminosity functions spanning 3:1 < z < 6:6. We find no significant evidence for Ly luminosity function evolution from z % 3 to z % 6. This result supports the conclusion that the intergalactic medium remains largely reionized from the local universe out to z % 6:5. It is somewhat at odds with the pronounced drop in the cosmic star formation rate density recently measured between z $ 3 and z $ 6 in continuum-selected Lyman-break galaxies, and therefore potentially sheds light on the relationship between the two populations.

1 Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the CaliforniaWe report on observations of a Type II quasar at redshift z = 3.288, identified as a hard X-ray source in a 185 ks observation with the Chandra X-ray Observatory and as a high-redshift photometric candidate from deep, multiband optical imaging. CXO J084837.9+445352 (hereinafter CXO52) shows an unusually hard X-ray spectrum from which we infer an absorbing column density N H = (4.8 ± 2.1) × 10 23 cm −2 (90% confidence) and an implied unabsorbed 2−10 keV rest-frame luminosity of L 2−10 = 3.3 × 10 44 ergs s −1 , well within the quasar regime. Hubble Space Telescope imaging shows CXO52 to be elongated with slight morphological differences between the WFPC2 F814W and NICMOS F160W bands. Optical and near-infrared spectroscopy of CXO52 show high-ionization emission lines with velocity widths ∼ 1000 km s −1 and flux ratios similar to a Seyfert 2 galaxy or radio galaxy. The latter are the only class of high-redshift Type II luminous AGN which have been extensively studied to date. Unlike radio galaxies, however, CXO52 is radio quiet, remaining undetected at radio wavelengths to fairly deep limits, f 4.8GHz < 40µJy. High-redshift Type II quasars, expected from unification models of active galaxies and long-thought necessary to explain the X-ray background, are poorly constrained observationally with few such systems known. We discuss recent observations of similar Type II quasars and detail search techniques for such systems: namely (1) X-ray selection, (2) radio selection, (3) multi-color imaging selection, and (4) narrow-band imaging selection. Such studies are likely to begin identifying luminous, high-redshift Type II systems in large numbers. We discuss the prospects for these studies and their implications to our understanding of the X-ray background.

We present a catalog of 74 galaxies detected serendipitously during a campaign of spectroscopic observations of the Hubble Deep Field North (HDF) and its environs. Among the identified objects are five candidate Ly-alpha emitters at z > 5, a galaxy cluster at z = 0.85, and a Chandra source with a heretofore undetermined redshift of z = 2.011. We report redshifts for 25 galaxies in the central HDF, 13 of which had no prior published spectroscopic redshift. Of the remaining 49 galaxies, 30 are located in the single-orbit HDF Flanking Fields. We discuss the redshift distribution of the serendipitous sample, which contains galaxies in the range 0.10 < z < 5.77 with a median redshift of z = 0.85, and we present strong evidence for redshift clustering. By comparing our spectroscopic redshifts to optical/IR photometric studies of the HDF, we find that photometric redshifts are in most cases capable of producing reasonable predictions of galaxy redshifts. Finally, we estimate the line-of-sight velocity dispersion and the corresponding mass and expected X-ray luminosity of the galaxy cluster, we present strong arguments for interpreting the Chandra source as an obscured AGN, and we discuss in detail the spectrum of one of the candidate z > 5 Ly-alpha emitters.Comment: 18 pages, 9 figures, accepted for publication in the Astronomical Journa

No abstract

We present a redshift z = 6.535 galaxy discovered by its Lyman-α emission in a 9180Å narrowband image from the Large Area Lyman Alpha (LALA) survey. The Lyman-α line luminosity (1.1×10 43 erg s −1 ) is among the largest known for star forming galaxies at z ≈ 6.5. The line shows the distinct asymmetry that is characteristic of high-redshift Lyman-α. The 2σ lower bound on the observer-frame equivalent width is > 530Å. This is hard to reconcile with a neutral intergalactic medium unless the Lyman-α line is intrinsically strong and is emitted from its host galaxy with an intrinsic Doppler shift of several hundred km s −1 . If the IGM is ionized, it corresponds to a rest frame equivalent width > 40Å after correcting for Lyman-α forest absorption. We also present complete spectroscopic followup of the remaining candidates with line flux > 2×10 −17 erg cm −2 s −1 in our 1200✷ ′ narrowband image. These include another galaxy with a strong emission line at 9136Å and no detected continuum flux, which however is most likely an [O III] λ5007 source at z = 0.824 based on a weak detection of the [O III] λ4959 line.

Narrow-band searches for Lyman-α emission are an efficient way of identifying starforming galaxies at high redshifts. We present Keck telescope spectra confirming redshifts z ≈ 5.7 for three objects discovered in the Large Area Lyman Alpha (LALA) survey at Kitt Peak National Observatory. All three spectra show strong, narrow emission lines with the asymmetric profile that is characteristically produced in high redshift Lyman-α emitters by preferential HI absorption in the blue wing of the line. These objects are undetected in deep B W , V , R, and λ ≈ 6600Å narrow-band images from the NOAO Deep Wide-Field Survey and from LALA, as expected from Lyman break and Lyman-α forest absorption at redshift z ≈ 5.7. All three objects show large equivalent widths ( 150Å in the rest-frame), suggesting at least one of the following: a top-heavy initial mass function, very low stellar metallicity, or the presence of an active nucleus. We consider the case for an active nucleus to be weak in all three objects due to the limited width of the Lyman-α emission line (< 500 km s −1 ) and the absence of any other indicator of quasar activity. The three confirmed high redshift objects were among four spectroscopically observed targets drawn from the sample of 18 candidates presented by Rhoads & Malhotra (2001). Thus, these spectra support the Lyman-α emitter population statistics from our earlier photometric study, which imply little evolution in number density from z = 5.7 to z = 4.5 and provide strong evidence that the reionization redshift z r > 5.7.