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Register a new userSpectroscopy of z~6 i-dropout Galaxies: Frequency of Ly-alpha Emission and the Sizes of Ly-alpha Emitting Galaxies
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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.
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We study the average Ly$alpha$ emission associated with high-$z$ strong (log $N$(H I) $ge$ 21) damped Ly$alpha$ systems (DLAs). We report Ly$alpha$ luminosities ($L_{rm Lyalpha}$) for the full as well as various sub-samples based on $N$(H I), $z$, $(r-i)$ colours of QSOs and rest equivalent width of Si II$lambda$1526 line (i.e., $W_{1526}$). For the full sample, we find $L_{rm Lyalpha}$$< 10^{41} (3sigma) rm erg s^{-1}$ with a $2.8sigma$ level detection of Ly$alpha$ emission in the red part of the DLA trough. The $L_{rm Lyalpha}$ is found to be higher for systems with higher $W_{1526}$ with its peak, detected at $geq 3sigma$, redshifted by about 300-400 $rm km s^{-1}$ with respect to the systemic absorption redshift, as seen in Lyman Break Galaxies (LBGs) and Ly$alpha$ emitters. A clear signature of a double-hump Ly$alpha$ profile is seen when we consider $W_{1526} ge 0.4$ AA and $(r-i) < 0.05$. Based on the known correlation between metallicity and $W_{1526}$, we interpret our results in terms of star formation rate (SFR) being higher in high metallicity (mass) galaxies with high velocity fields that facilitates easy Ly$alpha$ escape. The measured Ly$alpha$ surface brightness requires local ionizing radiation that is 4 to 10 times stronger than the metagalactic UV background at these redshifts. The relationship between the SFR and surface mass density of atomic gas seen in DLAs is similar to that of local dwarf and metal poor galaxies. We show that the low luminosity galaxies will contribute appreciably to the stacked spectrum if the size-luminosity relation seen for H I at low-$z$ is also present at high-$z$. Alternatively, large Ly$alpha$ halos seen around LBGs could also explain our measurements.
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.
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.
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.
We describe a method for obtaining a flux-limited sample of Ly-alpha emitters from GALEX grism data. We show that the multiple GALEX grism images can be converted into a three-dimensional (two spatial axes and one wavelength axis) data cube. The wavelength slices may then be treated as narrowband images and searched for emission-line galaxies. For the GALEX NUV grism data, the method provides a Ly-alpha flux-limited sample over the redshift range z=0.67-1.16. We test the method on the Chandra Deep Field South field, where we find 28 Ly-alpha emitters with faint continuum magnitudes (NUV>22) that are not present in the GALEX pipeline sample. We measure the completeness by adding artificial emitters and measuring the fraction recovered. We find that we have an 80% completeness above a Ly-alpha flux of 10^-15 erg/cm^2/s. We use the UV spectra and the available X-ray data and optical spectra to estimate the fraction of active galactic nuclei in the selection. We report the first detection of a giant Ly-alpha blob at z<1, though we find that these objects are much less common at z=1 than at z=3. Finally, we compute limits on the z~1 Ly-alpha luminosity function and confirm that there is a dramatic evolution in the luminosity function over the redshift range z=0-1.
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