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Register a new userConstraints to the evolution of Ly-Alpha bright galaxies between z=3 and z=6
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Galaxies at high redshift with a strong Ly-Alpha emission line trace massive star formation in the absence of dust, and can therefore be regarded as a prime signature of the first major starburst in galaxies. We report results of the Ly-Alpha search within the Calar Alto Deep Imaging Survey (CADIS). With imaging Fabry-Perot interferometer CADIS can detect emission lines in three waveband windows free of night-sky emission lines at 700nm, 820nm, and 920nm. The typical flux detection limit for Ly-Alpha emission redshifted into these windows, Flim > 3X10^(-20) Wm^(-2), corresponds to (unobscured) star formation rates of >10Msun/yr at z=6. Candidate Ly-Alpha-emitting galaxies are selected from the total emission line sample, which contains more than 97% of objects at z<1.2, by the absence of flux below the Lyman limit (B-band dropouts), and the non-detection of secondary emission lines in narrow band filters. We have detected 5 bright Ly-Alpha-emitting galaxy candidates at z ~ 4.8, and 11 candidates at z ~ 5.7. For two of four observed Ly-Alpha candidates, one candidate at z ~ 4.8, and the other at z ~ 5.7, the emission line detected with the Fabry-Perot has been verified spectroscopically at the VLT. When compared to Ly-Alpha surveys at z<3.5 even the upper limits set by our list of candidates show that bright Ly-Alpha galaxies are significantly rarer at z>5 than the assumption of a non-evolving population would predict. Therefore we conclude that the Ly-Alpha bright phase of primeval star formation episodes reached its peak at redshifts between 3 and 6.
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We describe the results of a new, wide-field survey for z=3.1 Ly-alpha emission-line galaxies (LAEs) in the Extended Chandra Deep Field South (ECDF-S). By using a nearly top-hat 5010 Angstrom filter and complementary broadband photometry from the MUSYC survey, we identify a complete sample of 141 objects with monochromatic fluxes brighter than 2.4E-17 ergs/cm^2/s and observers-frame equivalent widths greater than ~ 80 Angstroms (i.e., 20 Angstroms in the rest-frame of Ly-alpha). The bright-end of this dataset is dominated by x-ray sources and foreground objects with GALEX detections, but when these interlopers are removed, we are still left with a sample of 130 LAE candidates, 39 of which have spectroscopic confirmations. This sample overlaps the set of objects found in an earlier ECDF-S survey, but due to our filters redder bandpass, it also includes 68 previously uncataloged sources. We confirm earlier measurements of the z=3.1 LAE emission-line luminosity function, and show that an apparent anti-correlation between equivalent width and continuum brightness is likely due to the effect of correlated errors in our heteroskedastic dataset. Finally, we compare the properties of z=3.1 LAEs to LAEs found at z=2.1. We show that in the ~1 Gyr after z~3, the LAE luminosity function evolved significantly, with L* fading by ~0.4 mag, the number density of sources with L > 1.5E42 ergs/s declining by ~50%, and the equivalent width scale-length contracting from 70^{+7}_{-5} Angstroms to 50^{+9}_{-6} Angstroms. When combined with literature results, our observations demonstrate that over the redshift range z~0 to z~4, LAEs contain less than ~10% of the star-formation rate density of the universe.
We report on deep spectroscopy using LRIS on Keck I and FORS2 on the VLT of a sample of 22 candidate z~6 Lyman Break galaxies (LBGs) selected by the i-z> 1.3 dropout criterion. Redshifts could be measured for eight objects. These redshifts are all in the range z = 5.5 - 6.1, confirming the efficiency of the i-z color selection technique. Six of the confirmed galaxies show Ly-alpha emission. Assuming that the 14 objects without redshifts are z~6 LBGs, but lack detectable Ly-alpha emission lines, we infer that the fraction of Ly-alpha emitting LBGs with Ly-alpha equivalent widths greater than 20 Angstroms among z~6 LBGs is ~30%, similar to that found at z~3. Every Ly-alpha emitting object in our sample is compact with r <= 0.14. Furthermore, all the Ly-alpha emitting objects in our sample are more compact than average relative to the observed size-magnitude relation of a large i-dropout sample (332 candidate z~6 objects). We can reject the hypothesis that the Ly-alpha emitting population is a subset of the rest of the z~6 LBG population at >97% confidence. We speculate the small sizes of Ly-alpha emitting LBGs are due to these objects being less massive than other LBGs at z~6.
We present Ly$alpha$ and ultraviolet-continuum (UV) luminosity functions (LFs) of galaxies and active galactic nuclei (AGN) at $z=2.0-3.5$ determined by the un-targetted optical spectroscopic survey of the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). We combine deep Subaru imaging with HETDEX spectra resulting in $11.4$ deg$^2$ of fiber-spectra sky coverage, obtaining $18320$ galaxies spectroscopically identified with Ly$alpha$ emission, $2126$ of which host type 1 AGN showing broad (FWHM$~>1000$ km s$^{-1}$) Ly$alpha$ emission lines. We derive the Ly$alpha$ (UV) LF over 2 orders of magnitude covering bright galaxies and AGN in $log L_mathrm{Lyalpha}/mathrm{[erg~s^{-1}]}=43.3-45.5$ ($-27<M_mathrm{UV}<-20$) by the $1/V_mathrm{max}$ estimator. Our results reveal the bright-end hump of the Ly$alpha$ LF is composed of type 1 AGN. In conjunction with previous spectroscopic results at the faint end, we measure a slope of the best-fit Schechter function to be $alpha_mathrm{Sch}=-1.70^{+0.13}_{-0.14}$, which indicates $alpha_mathrm{Sch}$ steepens from $z=2-3$ towards high redshift. Our UV LF agrees well with previous AGN UV LFs, and extends to faint-AGN and bright-galaxy regimes. The number fraction of Ly$alpha$-emitting objects ($X_mathrm{LAE}$) increases from $M_mathrm{UV}^*sim-21$ to bright magnitude due to the contribution of type 1 AGN, while previous studies claim that $X_mathrm{Lyalpha}$ decreases from faint magnitude to $M_mathrm{UV}^*$, suggesting a valley in the $X_mathrm{Lyalpha}-$magnitude relation at $M_mathrm{UV}^*$. Comparing our UV LF of type 1 AGN at $z=2-3$ with those at $z=0$, we find that the number density of faint ($M_mathrm{UV}>-21$) type 1 AGN increases from $zsim2$ to $zsim0$ as opposed to the evolution of bright ($M_mathrm{UV}<-21$) type 1 AGN, suggesting the AGN downsizing in the rest-frame UV luminosity.
Spectroscopic observations of massive $z>7$ galaxies selected to have extremely large [OIII]+H$beta$ equivalent width (EW $sim1500$ r{A}) have recently revealed large Ly$alpha$ detection rates, in contrast to the weak emission seen in the general population. Why these systems are uniquely visible in Ly$alpha$ at redshifts where the IGM is likely significantly neutral is not clear. With the goal of better understanding these results, we have begun a campaign with MMT and Magellan to measure Ly$alpha$ in galaxies with similar [OIII]+H$beta$ EWs at $zsimeq2-3$. At these redshifts, the IGM is highly ionized, allowing us to clearly disentangle how the Ly$alpha$ properties depend on the [OIII]+H$beta$ EW. Here we present Ly$alpha$ EWs of $49$ galaxies at $z=2.2-3.7$ with intense [OIII]+H$beta$ line emission (EW $=300-3000$ r{A}). Our results demonstrate that strong Ly$alpha$ emission (EW $>20$ r{A}) becomes more common in galaxies with larger [OIII]+H$beta$ EW, reflecting a combination of increasingly efficient ionizing photon production and enhanced transmission of Ly$alpha$. Among the galaxies with the most extreme [OIII]+H$beta$ emission (EW $sim1500$ r{A}), we find that strong Ly$alpha$ emission is not ubiquitous, with only $50$ per cent of our population showing Ly$alpha$ EW $>20$ r{A}. Our data suggest that the range of Ly$alpha$ strengths is related to the observed ellipticity, with those systems that appear edge-on or elongated having weaker Ly$alpha$ emission. We use these results to interpret the anomalous Ly$alpha$ properties seen in $z>7$ galaxies with extreme [OIII]+H$beta$ emission and discuss implications for the escape of ionizing radiation from these extreme line emitting galaxies.
Spectroscopic confirmation of galaxies at z~7 and above has been extremely difficult, owing to a drop in intensity of Ly-alpha emission in comparison with samples at z~6. This crucial finding could potentially signal the ending of cosmic reionization. However it is based on small datasets, often incomplete and heterogeneous in nature. We introduce a flexible Bayesian framework, useful to interpret such evidence. Within this framework, we implement two simple phenomenological models: a smooth one, where the distribution of Ly-alpha is attenuated by a factor es with respect to z~6; a patchy one where a fraction ep is absorbed/non-emitted while the rest is unabsorbed. From a compilation of 39 observed z~7 galaxies we find es=0.69+-0.12 and ep=0.66+-0.16. The models can be used to compute fractions of emitters above any equivalent width W. For W>25AA, we find X^{25}_{z=7}=0.37+-0.11 (0.14+-0.06) for galaxies fainter (brighter) than M_{UV}=-20.25 for the patchy model, consistent with previous work, but with smaller uncertainties by virtue of our full use of the data. At z~8 we combine new deep (5-sigma flux limit 10^{-17}ergs^{-1}cm^{-2}) Keck-NIRSPEC observations of a bright Y-dropout identified by our BoRG Survey, with those of three objects from the literature and find that the inference is inconclusive. We compute predictions for future near-infrared spectroscopic surveys and show that it is challenging but feasible to constrain the distribution of Ly-alpha emitters at z~8 and distinguish between models.
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