A Lyman Alpha Galaxy at Redshift z=6.944 in the COSMOS Field

Lyman-alpha emitting galaxies can be used to study cosmological reionization, because a neutral intergalactic medium scatters Lyman-alpha photons into diffuse halos whose surface brightness falls below typical survey detection limits. Here we present the Lyman-alpha emitting galaxy LAE J095950.99+021219.1, identified at redshift z=6.944 in the COSMOS field using narrowband imaging and followup spectroscopy with the IMACS instrument on the Magellan I Baade telescope. With a single object spectroscopically confirmed so far, our survey remains consistent with a wide range of IGM neutral fraction at redshift seven, but further observations are planned and will help clarify the situation. Meantime, the object we present here is only the third Lyman-alpha selected galaxy to be spectroscopically confirmed at redshift seven, and is 2--3 times fainter than the previously confirmed redshift seven Lyman alpha galaxies.Comment: 15 pages including 3 figures. Accepted for publication in The Astrophysical Journal Letter

The DEEP2 Galaxy Redshift Survey: Redshift Identification of Single-Line Emission Galaxies

We present two methods for determining spectroscopic redshifts of galaxies in the DEEP2 survey which display only one identifiable feature, an emission line, in the observed spectrum ("single-line galaxies"). First, we assume each single line is one of the four brightest lines accessible to DEEP2: Halpha, [OIII] 5007, Hbeta, or [OII] 3727. Then, we supplement spectral information with BRI photometry. The first method, parameter space proximity (PSP), calculates the distance of a single-line galaxy to galaxies of known redshift in (B-R), (R-I), R, observed wavelength parameter space. The second method is an artificial neural network (ANN). Prior information, such as allowable line widths and ratios, rules out one or more of the four lines for some galaxies in both methods. Based on analyses of evaluation sets, both methods are nearly perfect at identifying blended [OII] doublets. Of the lines identified as Halpha in the PSP and ANN methods, 91.4% and 94.2% respectively are accurate. Although the methods are not this accurate at discriminating between [OIII] and Hbeta, they can identify a single line as one of the two, and the ANN method in particular unambiguously identifies many [OIII] lines. From a sample of 640 single-line spectra, the methods determine the identities of 401 (62.7%) and 472 (73.8%) single lines, respectively, at accuracies similar to those found in the evaluation sets.Comment: 11 pages, 6 figures, accepted to Ap

Kiloparsec-scale Spatial Offsets in Double-peaked Narrow-line Active Galactic Nuclei. I. Markers for Selection of Compelling Dual Active Galactic Nucleus Candidates

Merger-remnant galaxies with kpc-scale separation dual active galactic nuclei (AGNs) should be widespread as a consequence of galaxy mergers and triggered gas accretion onto supermassive black holes, yet very few dual AGNs have been observed. Galaxies with double-peaked narrow AGN emission lines in the Sloan Digital Sky Survey are plausible dual AGN candidates, but their double-peaked profiles could also be the result of gas kinematics or AGN-driven outflows and jets on small or large scales. To help distinguish between these scenarios, we have obtained spatial profiles of the AGN emission via follow-up long-slit spectroscopy of 81 double-peaked narrow-line AGNs in SDSS at 0.03 < z < 0.36 using Lick, Palomar, and MMT Observatories. We find that all 81 systems exhibit double AGN emission components with ~kpc projected spatial separations on the sky, which suggests that they are produced by kpc-scale dual AGNs or kpc-scale outflows, jets, or rotating gaseous disks. In addition, we find that the subsample (58%) of the objects with spatially compact emission components may be preferentially produced by dual AGNs, while the subsample (42%) with spatially extended emission components may be preferentially produced by AGN outflows. We also find that for 32% of the sample the two AGN emission components are preferentially aligned with the host galaxy major axis, as expected for dual AGNs orbiting in the host galaxy potential. Our results both narrow the list of possible physical mechanisms producing the double AGN components, and suggest several observational criteria for selecting the most promising dual AGN candidates from the full sample of double-peaked narrow-line AGNs. Using these criteria, we determine the 17 most compelling dual AGN candidates in our sample.Comment: 12 pages, 8 figures, published in ApJ. Modified from original version to reflect referee's comment

Characterizing Circumgalactic Gas around Massive Ellipticals at z~0.4 - II. Physical Properties and Elemental Abundances

We present a systematic investigation of the circumgalactic medium (CGM) within projected distances d<160 kpc of luminous red galaxies (LRGs). The sample comprises 16 intermediate-redshift (z=0.21-0.55) LRGs of stellar mass M_star>1e11 M_sun. Combining far-ultraviolet Cosmic Origin Spectrograph spectra from the Hubble Space Telescope and optical echelle spectra from the ground enables a detailed ionization analysis based on resolved component structures of a suite of absorption transitions, including the full HI Lyman series and various ionic metal transitions. By comparing the relative abundances of different ions in individually-matched components, we show that cool gas (T~1e4 K) density and metallicity can vary by more than a factor of ten in in an LRG halo. Specifically, metal-poor absorbing components with <1/10 solar metallicity are seen in 50% of the LRG halos, while gas with solar and super-solar metallicity is also common. These results indicate a complex multiphase structure and poor chemical mixing in these quiescent halos. We calculate the total surface mass density of cool gas, \Sigma_cool, by applying the estimated ionization fraction corrections to the observed HI column densities. The radial profile of \Sigma_cool is best-described by a projected Einasto profile of slope \alpha=1 and scale radius r_s=48 kpc. We find that typical LRGs at z~0.4 contain cool gas mass of M_cool= (1-2) x1e10 M_sun at d<160 kpc (or as much as 4x1e10 M_sun at d<500 kpc), comparable to the cool CGM mass of star-forming galaxies. Furthermore, we show that high-ionization OVI and low-ionization absorption species exhibit distinct velocity profiles, highlighting their different physical origins. We discuss the implications of our findings for the origin and fate of cool gas in LRG halos.Comment: Accepted for publication in MNRAS after a minor revision. 23 pages, 14 figures, and a 29-page Appendix with 27 additional figure

The DEEP2 Redshift Survey: Lyman Alpha Emitters in the Spectroscopic Database

We present the first results of a search for Lyman-alpha emitters (LAEs) in the DEEP2 spectroscopic database that uses a search technique that is different from but complementary to traditional narrowband imaging surveys. We have visually inspected ~20% of the available DEEP2 spectroscopic data and have found nine high-quality LAEs with clearly asymmetric line profiles and an additional ten objects of lower quality, some of which may also be LAEs. Our survey is most sensitive to LAEs at z=4.4-4.9 and that is indeed where all but one of our high-quality objects are found. We find the number density of our spectroscopically-discovered LAEs to be consistent with those found in narrowband imaging searches. The combined, averaged spectrum of our nine high-quality objects is well fit by a two-component model, with a second, lower-amplitude component redshifted by ~420 km/s with respect to the primary Lyman-alpha line, consistent with large-scale outflows from these objects. We conclude by discussing the advantages and future prospects of blank-sky spectroscopic surveys for high-z LAEs.Comment: Accepted for publication in Ap

The DEEP2 Galaxy Redshift Survey: Mean Ages and Metallicities of Red Field Galaxies at z ~ 0.9 from Stacked Keck/DEIMOS Spectra

As part of the DEEP2 galaxy redshift survey, we analyze absorption line strengths in stacked Keck/DEIMOS spectra of red field galaxies with weak to no emission lines, at redshifts 0.7 <= z <= 1. Comparison with models of stellar population synthesis shows that red galaxies at z ~ 0.9 have mean luminosity-weighted ages of the order of only 1 Gyr and at least solar metallicities. This result cannot be reconciled with a scenario where all stars evolved passively after forming at very high z. Rather, a significant fraction of stars can be no more than 1 Gyr old, which means that star formation continued to at least z ~ 1.2. Furthermore, a comparison of these distant galaxies with a local SDSS sample, using stellar populations synthesis models, shows that the drop in the equivalent width of Hdelta from z ~ 0.9 to 0.1 is less than predicted by passively evolving models. This admits of two interpretations: either each individual galaxy experiences continuing low-level star formation, or the red-sequence galaxy population from z ~ 0.9 to 0.1 is continually being added to by new galaxies with younger stars.Comment: A few typos were corrected and numbers in Table 1 were revise

The Clustering of Extremely Red Objects

We measure the clustering of Extremely Red Objects (EROs) in ~8 deg^2 of the NOAO Deep Wide Field Survey Bo\"otes field in order to establish robust links between ERO z~1.2 and local galaxy z<0.1 populations. Three different color selection criteria from the literature are analyzed to assess the consequences of using different criteria for selecting EROs. Specifically, our samples are (R-K_s)>5.0 (28,724 galaxies), (I-K_s)>4.0 (22,451 galaxies) and (I-[3.6])>5.0 (64,370 galaxies). Magnitude-limited samples show the correlation length (r_0) to increase for more luminous EROs, implying a correlation with stellar mass. We can separate star-forming and passive ERO populations using the (K_s-[24]) and ([3.6]-[24]) colors to K_s=18.4 and [3.6]=17.5, respectively. Star-forming and passive EROs in magnitude limited samples have different clustering properties and host dark halo masses, and cannot be simply understood as a single population. Based on the clustering, we find that bright passive EROs are the likely progenitors of >4L^* elliptical galaxies. Bright EROs with ongoing star formation were found to occupy denser environments than star-forming galaxies in the local Universe, making these the likely progenitors of >L^* local ellipticals. This suggests that the progenitors of massive >4L^* local ellipticals had stopped forming stars by z>1.2, but that the progenitors of less massive ellipticals (down to L^*) can still show significant star formation at this epoch.Comment: 19 pages, 16 figures, 4 tables, Accepted to ApJ 27th November 201