Chandra X-ray Sources in the LALA Cetus Field

The 174 ks Chandra Advanced CCD Imaging Spectrometer exposure of the Large Area Lyman Alpha Survey (LALA) Cetus field is the second of the two deep Chandra images on LALA fields. In this paper we present the Chandra X-ray sources detected in the Cetus field, along with an analysis of X-ray source counts, stacked X-ray spectrum, and optical identifications. A total of 188 X-ray sources were detected: 174 in the 0.5-7.0 keV band, 154 in the 0.5-2.0 keV band, and 113 in the 2.0-7.0 keV band. The X-ray source counts were derived and compared with LALA Bootes field (172 ks exposure). Interestingly, we find consistent hard band X-ray source density, but 36+-12% higher soft band X-ray source density in Cetus field. The weighted stacked spectrum of the detected X-ray sources can be fitted by a powerlaw with photon index Gamma = 1.55. Based on the weighted stacked spectrum, we find that the resolved fraction of the X-ray background drops from 72+-1% at 0.5-1.0 keV to 63+-4% at 6.0-8.0 keV. The unresolved spectrum can be fitted by a powerlaw over the range 0.5-7 keV, with a photon index Gamma = 1.22. We also present optical counterparts for 154 of the X-ray sources, down to a limiting magnitude of r' = 25.9 (Vega), using a deep r' band image obtained with the MMT.Comment: 21 pages, including 6 figures, 1 table, ApJ accepte

Effects of Dust Geometry in Lyman Alpha Galaxies at z = 4.4

Equivalent widths (EWs) observed in high-redshift Lyman alpha galaxies could be stronger than the EW intrinsic to the stellar population if dust is present residing in clumps in the inter-stellar medium (ISM). In this scenario, continuum photons could be extinguished while the Lyman alpha photons would be resonantly scattered by the clumps, eventually escaping the galaxy. We investigate this radiative transfer scenario with a new sample of six Lyman alpha galaxy candidates in the GOODS CDF-S, selected at z = 4.4 with ground-based narrow-band imaging obtained at CTIO. Grism spectra from the HST PEARS survey confirm that three objects are at z = 4.4, and that another object contains an active galactic nuclei (AGN). If we assume the other five (non-AGN) objects are at z = 4.4, they have rest-frame EWs from 47 -- 190 A. We present results of stellar population studies of these objects, constraining their rest-frame UV with HST and their rest-frame optical with Spitzer. Out of the four objects which we analyzed, three objects were best-fit to contain stellar populations with ages on the order of 1 Myr and stellar masses from 3 - 10 x 10^8 solar masses, with dust in the amount of A_1200 = 0.9 - 1.8 residing in a quasi-homogeneous distribution. However, one object (with a rest EW ~ 150 A) was best fit by an 800 Myr, 6.6 x 10^9 solar mass stellar population with a smaller amount of dust (A_1200 = 0.4) attenuating the continuum only. In this object, the EW was enhanced ~ 50% due to this dust. This suggests that large EW Lyman alpha galaxies are a diverse population. Preferential extinction of the continuum in a clumpy ISM deserves further investigation as a possible cause of the overabundance of large-EW objects that have been seen in narrow-band surveys in recent years.Comment: Submitted to the Astrophysical Journal. 35 pages, 7 figures and 4 table

Lyman Alpha Galaxies: Primitive, Dusty or Evolved Galaxies?

We present stellar population modeling results for 10 newly discovered Lyman alpha emitting galaxies (LAEs), as well as four previously known LAEs at z ~ 4.5 in the Chandra Deep Field - South. We fit stellar population models to these objects in order to learn specifically if there exists more than one class of LAE. Past observational and theoretical evidence has shown that while many LAEs appear to be young, they may be much older, with Lyman alpha EWs enhanced due to resonant scattering of Lyman alpha photons in a clumpy interstellar medium (ISM). Our results show a large range of stellar population age (3 - 500 Myr), stellar mass (1.6 x 10^8 - 5.0 x 10^10 Msol) and dust extinction (A_1200 = 0.3 - 4.5 mag), broadly consistent with previous studies. With such a large number of individually analyzed objects, we have looked at the distribution of stellar population ages in LAEs for the first time, and we find a very interesting bimodality, in that our objects are either very young ( 450 Myr). This bimodality may be caused by dust, and it could explain the Lyman alpha duty cycle which has been proposed in the literature. We find that eight of the young objects are best fit with a clumpy ISM. We find that dust geometry appears to play a large role in shaping the SEDs that we observe, and that it may be a major factor in the observed Lyman alpha equivalent width distribution in high redshift Lyman alpha galaxies, although other factors (i.e. outflows) may be in play. We conclude that 12 out of our 14 LAEs are dusty star-forming galaxies, with the other two LAEs being evolved galaxies.Comment: Replaced with ApJ accepted versionl. 20 pages, 10 figures, four table

Ultraviolet Luminosity Density of the Universe During the Epoch of Reionization

The spatial fluctuations of the extragalactic background light trace the total emission from all stars and galaxies in the Universe. A multi-wavelength study can be used to measure the integrated emission from first galaxies during reionization when the Universe was about 500 million years old. Here we report arcminute-scale spatial fluctuations in one of the deepest sky surveys with the Hubble Space Telescope in five wavebands between 0.6 and 1.6 $\mu$m. We model-fit the angular power spectra of intensity fluctuation measurements to find the ultraviolet luminosity density of galaxies at $z$ > 8 to be $\log \rho_{\rm UV} = 27.4^{+0.2}_{-1.2}$ erg s$^{-1}$ Hz$^{-1}$ Mpc$^{-3}$ $(1\sigma)$. This level of integrated light emission allows for a significant surface density of fainter primeval galaxies that are below the point source detection level in current surveys.Comment: The official typeset version is available from the Nature Communications website at http://www.nature.com/ncomms/2015/150907/ncomms8945/full/ncomms8945.html The data used in this work can be found at http://herschel.uci.edu/CANDELS

Hubble Space Telescope Imaging of Lyman Alpha Emission at z=4.4

We present the highest redshift detections of resolved Lyman alpha emission, using Hubble Space Telescope/ACS F658N narrowband-imaging data taken in parallel with the Wide Field Camera 3 Early Release Science program in the GOODS CDF-S. We detect Lyman alpha emission from three spectroscopically confirmed z = 4.4 Lyman alpha emitting galaxies (LAEs), more than doubling the sample of LAEs with resolved Lyman alpha emission. Comparing the light distribution between the rest-frame ultraviolet continuum and narrowband images, we investigate the escape of Lyman alpha photons at high redshift. While our data do not support a positional offset between the Lyman alpha and rest-frame ultraviolet (UV) continuum emission, the half-light radii in two out of the three galaxies are significantly larger in Lyman alpha than in the rest-frame UV continuum. This result is confirmed when comparing object sizes in a stack of all objects in both bands. Additionally, the narrowband flux detected with HST is significantly less than observed in similar filters from the ground. These results together imply that the Lyman alpha emission is not strictly confined to its indigenous star-forming regions. Rather, the Lyman alpha emission is more extended, with the missing HST flux likely existing in a diffuse outer halo. This suggests that the radiative transfer of Lyman alpha photons in high-redshift LAEs is complicated, with the interstellar-medium geometry and/or outflows playing a significant role in galaxies at these redshifts.Comment: Submitted to the Astrophysical Journal. 11 pages, 10 figure

CANDELS: The Contribution of the Observed Galaxy Population to Cosmic Reionization

We present measurements of the specific ultraviolet luminosity density from a sample of 483 galaxies at 6<z<8. These galaxies were selected from new deep near-infrared HST imaging from the CANDELS, HUDF09 and ERS programs. In contrast to the majority of previous analyses, which assume that the distribution of galaxy ultraviolet (UV) luminosities follows a Schechter distribution, and that the distribution continues to luminosities far below our observable limit, we investigate the contribution to reionization from galaxies which we can observe, free from these assumptions. We find that the observable population of galaxies can sustain a fully reionized IGM at z=6, if the average ionizing photon escape fraction (f_esc) is ~30%. A number of previous studies have measured UV luminosity densities at these redshifts that vary by 5X, with many concluding that galaxies could not complete reionization by z=6 unless a large population of galaxies fainter than the detection limit were invoked, or extremely high values of f_esc were present. The observed UV luminosity density from our observed galaxy samples at z=7-8 is not sufficient to maintain a fully reionized IGM unless f_esc>50%. Combining our observations with constraints on the emission rate of ionizing photons from Ly-alpha forest observations at z=6, we can constrain f_esc<34% (2-sigma) if the observed galaxies are the only contributors to reionization, or <13% (2-sigma) if the luminosity function extends to M_UV = -13. These escape fractions are sufficient to complete reionization by z=6. These constraints imply that the volume ionized fraction of the IGM becomes less than unity at z>7, consistent with a number of complementary reionization probes. If faint galaxies dominate reionization, future JWST observations will probe deep enough to see them, providing an indirect constraint on the ionizing photon escape fraction [abridged].Comment: 16 pages, 7 figures, Submitted to the Astrophysical Journa

Spectrophotometric Redshifts In The Faint Infrared Grism Survey: Finding Overdensities Of Faint Galaxies

We improve the accuracy of photometric redshifts by including low-resolution spectral data from the G102 grism on the Hubble Space Telescope, which assists in redshift determination by further constraining the shape of the broadband Spectral Energy Disribution (SED) and identifying spectral features. The photometry used in the redshift fits includes near-IR photometry from FIGS+CANDELS, as well as optical data from ground-based surveys and HST ACS, and mid-IR data from Spitzer. We calculated the redshifts through the comparison of measured photometry with template galaxy models, using the EAZY photometric redshift code. For objects with F105W $< 26.5$ AB mag with a redshift range of $0 < z < 6$, we find a typical error of $\Delta z = 0.03 * (1+z)$ for the purely photometric redshifts; with the addition of FIGS spectra, these become $\Delta z = 0.02 * (1+z)$, an improvement of 50\%. Addition of grism data also reduces the outlier rate from 8\% to 7\% across all fields. With the more-accurate spectrophotometric redshifts (SPZs), we searched the FIGS fields for galaxy overdensities. We identified 24 overdensities across the 4 fields. The strongest overdensity, matching a spectroscopically identified cluster at $z=0.85$, has 28 potential member galaxies, of which 8 have previous spectroscopic confirmation, and features a corresponding X-ray signal. Another corresponding to a cluster at $z=1.84$ has 22 members, 18 of which are spectroscopically confirmed. Additionally, we find 4 overdensities that are detected at an equal or higher significance in at least one metric to the two confirmed clusters.Comment: 17 pages, 13 figures. To appear in Ap