90 research outputs found

    Estimating the Correlation Length r_0 from the Number of Galaxy Pairs with Similar Redshifts

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    We discuss methods that can be used to estimate the spatial correlation length r_0 of galaxy samples from the observed number of pairs with similar redshifts. The standard method is unnecessarily noisy and can be compromised by errors in the assumed selection function. We present three alternatives, one less noisy, one that responds differently to systematic errors, the third insensitive to the selection function, and quantify their performance by applying them to a cosmological N-body simulation and to the Lyman-break survey of galaxies at redshift z~3. Researchers adopting the standard method could easily conclude that the Lyman-break galaxy comoving correlation length was r_0 \~ 11 Mpc/h, several times larger than the correct value. The use of our proposed methods would make this error impossible, except in the small sample limit. When N_gal<~20, major errors in estimates of r_0 occur alarmingly often.Comment: 19 pages, accepted for publication in the Ap

    The Kinematics of Morphologically Selected z~2 Galaxies in the GOODS-N Field

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    We present near-IR spectra of H-alpha emission from 13 galaxies at z~2 in the GOODS-N field. The galaxies were selected primarily because they appear to have elongated morphologies, and slits were aligned with the major axes (as determined from the rest-frame UV emission) of 11 of the 13. If the galaxies are elongated because they are highly inclined, alignment of the slit and major axis should maximize the observed velocity and reveal velocity shear, if present. In spite of this alignment, we see spatially resolved velocity shear in only two galaxies. We show that the seeing makes a large difference in the observed velocity spread of a tilted emission line, and use this information to place limits on the velocity spread of the ionized gas of the galaxies in the sample: we find that all 13 have v_{0.5} < 110 km/s, where v_{0.5} is the velocity shear (half of the velocity range of a tilted emission line) that would be observed under our best seeing conditions of ~0.5". When combined with previous work, our data also indicate that aligning the slit along the major axis does not increase the probability of observing a tilted emission line. We then focus on the one-dimensional velocity dispersion \sigma, which is much less affected by the seeing, and see that the elongated subsample exhibits a significantly lower velocity dispersion than galaxies selected at random from our total H-alpha sample, not higher as one might have expected. We also see some evidence that the elongated galaxies are less reddened than those randomly selected using only UV colors. Both of these results are counter to what would be expected if the elongated galaxies were highly inclined disks. It is at least as likely that the galaxies' elongated morphologies are due to merging subunits.Comment: 9 pages, 5 figures. Accepted for publication in Ap

    The Direct Detection of Lyman Continuum Emission from Star-forming Galaxies at z~3

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    We present the results of rest-frame UV spectroscopic observations of a sample of 14 z ~ 3 star-forming galaxies in the SSA 22a field. These spectra are characterized by unprecedented depth in the Lyman continuum region. For the first time, we have detected escaping ionizing radiation from individual galaxies at high redshift, with 2 of the 14 objects showing significant emission below the Lyman limit. We also measured the ratio of emergent flux density at 1500 Å to that in the Lyman continuum region, for the individual detections (C49 and D3) and the sample average. If a correction for the average IGM opacity is applied to the spectra of the objects C49 and D3, we find f_(1500)/f_(900,corr,C49) = 4.5 and f_(1500)/f_(900,corr,D3) = 2.9. The average emergent flux density ratio in our sample is = 22, implying an escape fraction ~4.5 times lower than inferred from the composite spectrum from Steidel and coworkers. If this new estimate is representative of LBGs, their contribution to the metagalactic ionizing radiation field is J_ν(900) ~ 2.6 × 10^(-22) ergs s^(-1) cm^(-2) Hz^(-1) sr^(-1), comparable to the contribution of optically selected quasars at the same redshift. The sum of the contributions from galaxies and quasars is consistent with recent estimates of the level of the ionizing background at z ~ 3, inferred from the H I Lyα forest optical depth. There is significant variance among the emergent far-UV spectra in our sample, yet the factors controlling the detection or nondetection of Lyman continuum emission from galaxies are not well determined. Because we do not yet understand the source of this variance, significantly larger samples will be required to obtain robust constraints on the galaxy contribution to the ionizing background at z ~ 3 and beyond

    Spectroscopy of Lyman Break Galaxies in the Hubble Deep Field

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    We report on the initial results of a spectroscopic investigation of galaxies in the Hubble Deep Field which exhibit spectral discontinuities between the F450W and F300W passbands, indicative of the presence of the Lyman continuum break in the redshift range 2.4 < z < 3.4. We have employed color selection criteria similar to those we have used for selecting high redshift galaxy candidates from ground--based images. We find that, as for the ground--based color selection, the criteria are very successful in selecting high redshift objects. Of the 8 galaxies observed (selected from a list of 23 candidates with magnitudes equivalent to R <= 25.3, five have confirmed redshifts in the range 2.59 <= z <= 3.22, with the remaining 3 being indeterminate because of contamination from nearby brighter objects. As expected, the HST filter system is sensitive to a somewhat broader range of redshifts than our ground--based U_nGR filter system, and therefore the surveyed volume per unit area on the sky is correspondingly larger. The distribution of candidates on the plane of the sky is clearly non--uniform, consistent with the available ground--based data on the high redshift galaxies. Most Lyman break objects in the Hubble Deep Field exhibit a similar range of morphological properties to the z > 3 galaxies we have previously identified in other fields, characterized by very compact cores (some with multiple components) with half--light radii of 0.2-0.3 arcseconds, often surrounded by more diffuse and asymmetric ``halos''. A few of the brighter HDF Lyman break galaxies, however, have particularly unusual morphologies.Comment: Accepted for publication in the Astronomical Journal. 18 pages including 6 postscript figures. Uses AAStex macros; aaspp4.sty include

    A Possible Correlation Between the Luminosities and Lifetimes of Active Galactic Nuclei

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    We use the clustering of galaxies around distant AGN to show with ~90% confidence that fainter AGN are longer lived. Our argument is simple: since the measured galaxy-AGN cross-correlation length r_0 ~ 5h^-1 Mpc does not vary significantly over a 10 magnitude range in AGN optical luminosity, faint and bright AGN must reside in dark matter halos with similar masses. The halos that host bright and faint AGN therefore have similar intrinsic abundances, and the large observed variation in AGN number density with luminosity reflects a change in duty cycle.Comment: 9 emulateapj pages, accepted for publication in the Ap

    Evidence for Solar Metallicities in Massive Star-forming Galaxies at z>~2

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    We present results of near-IR spectroscopic measurements of 7 star-forming galaxies at 2.1<z<2.5. Drawn from a large spectroscopic survey of galaxies photometrically pre-selected by their U_nGR colors to lie at z~2, these galaxies were chosen for their bright rest-frame optical luminosities (K_s<=20.0). Most strikingly, the majority of the sample of 7 galaxies exhibit [NII]/Ha nebular emission line ratios indicative of at least solar HII region metallicities, at a lookback time of 10.5 Gyr. The broadband colors of the K_s-bright sample indicate that most have been forming stars for more than a Gyr at z~2, and have already formed stellar masses in excess of 10^11 Msun. The descendants of these galaxies in the local universe are most likely metal-rich and massive spiral and elliptical galaxies, while plausible progenitors for them can be found among the population of z~3 Lyman Break Galaxies. While the K_s-bright z~2 galaxies appear to be highly evolved systems, their large Ha luminosities and uncorrected Ha star-formation rates of 24-60 Msun/yr indicate that active star formation is still ongoing. The luminous UV-selected objects presented here comprise more than half of the high-redshift (z>1.5) tails of current K-band-selected samples such as the K20 and Gemini Deep Deep surveys.Comment: 15 pages including 5 figures. Accepted for publication in Ap

    Optical Selection of Galaxies at Redshifts 1<z<3

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    Few galaxies have been found between the redshift ranges z < ~1 probed by magnitude-limited surveys and z > ~3 probed by Lyman-break surveys. Comparison of galaxy samples at lower and higher redshift suggests that large numbers of stars were born and the Hubble sequence began to take shape at the intermediate redshifts 1<z<3, but observational challenges have prevented us from observing the process in much detail. We present simple and efficient strategies that can be used to find large numbers of galaxies throughout this important but unexplored redshift range. All the strategies are based on selecting galaxies for spectroscopy on the basis of their colors in ground-based images taken through a small number of optical filters: GRi for redshifts 0.85<z<1.15, GRz for 1<z<1.5, and UGR for 1.4<z<2.1 and 1.9<z<2.7. The performance of our strategies is quantified empirically through spectroscopy of more than 2000 galaxies at 1<z<3.5. We estimate that more than half of the UV-luminosity density at 1<z<3 is produced by galaxies that satisfy our color-selection criteria. Our methodology is described in detail, allowing readers to devise analogous selection criteria for other optical filter sets.Comment: 13 pages including 20 figures. Accepted for publication in the Ap
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