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Register a new userA Hard Ionizing Spectrum in z=3-4 Ly-alpha Emitters with Intense [OIII] Emission: Analogs of Galaxies in the Reionization Era?
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We present Keck/MOSFIRE spectra of the diagnostic nebular emission lines [OIII]5007,4959, [OII]3727, and H-beta for a sample of 15 redshift z=3.1-3.7 Ly-alpha emitters (LAEs) and Lyman break galaxies (LBGs). In conjunction with spectra from other surveys, we confirm earlier indications that LAEs have a much higher [OIII]/[OII] line ratio than is seen in similar redshift LBGs. By comparing their distributions on a [OIII]/[OII] versus R23 diagram, we demonstrate that this difference cannot arise solely because of their lower metallicities but most likely is due to a harder ionizing spectrum. Using measures of H-beta and recombination theory, we demonstrate, for a subset of our LAEs, that xi_ion - the number of Lyman continuum photons per UV luminosity - is indeed 0.2-0.5 dex larger than for typical LBGs at similar redshifts. Using photoionization models we estimate the effect this would have on both [OIII]/[OII] and R23 and conclude such a hard spectrum can only partially explain such intense line emission. The additional possibility is that such a large [OIII]/[OII] ratio is in part due to density rather than ionization bound nebular regions, which would imply a high escape fraction of ionizing photons. We discuss how further observations could confirm this possibility. Clearly LAEs with intense [OIII] emission represent a promising analog of those z>7 sources with similarly strong lines which are thought to be an important contributor to cosmic reionization.
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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 present Keck/MOSFIRE observations of UV metal lines in four bright gravitationally-lensed z~6-8 galaxies behind the cluster Abell 1703. The spectrum of A1703-zd6, a highly-magnified star forming galaxy with a Lyman-alpha redshift of z=7.045, reveals a confident detection of the nebular CIV emission line (unresolved with FWHM < 125 km/s). UV metal lines are not detected in the three other galaxies. At z~2-3, nebular CIV emission is observed in just 1% of UV-selected galaxies. The presence of strong CIV emission in one of the small sample of galaxies targeted in this paper may indicate hard ionizing spectra are more common at z~7. The total estimated equivalent width of the CIV doublet (38 A) and CIV/Lyman-alpha flux ratio (0.3) are comparable to measurements of narrow-lined AGNs. Photoionization models show that the nebular CIV line can also be reproduced by a young stellar population, with very hot metal poor stars dominating the photon flux responsible for triply ionizing carbon. Regardless of the origin of the CIV, we show that the ionizing spectrum of A1703-zd6 is different from that of typical galaxies at z~2, producing more H ionizing photons per unit 1500A luminosity and a larger flux density at 30-50 eV. If such extreme radiation fields are typical in UV-selected systems at z>7, it would indicate that reionization-era galaxies are more efficient ionizing agents than previously thought. Alternatively, we suggest that the small sample of Lyman-alpha emitters at z>7 may trace a rare population with intense radiation fields capable of ionizing their surrounding hydrogen distribution. Additional constraints on high ionization emission lines in galaxies with and without Lyman-alpha detections will help clarify whether hard ionizing spectra are common in the reionization era.
We present a novel method to investigate cosmic reionization, using joint spectral information on high redshift Lyman Alpha Emitters (LAE) and quasars (QSOs). Although LAEs have been proposed as reionization probes, their use is hampered by the fact their Ly{alpha} line is damped not only by intergalactic HI but also internally by dust. Our method allows to overcome such degeneracy. First, we carefully calibrate a reionization simulation with QSO absorption line experiments. Then we identify LAEs in two simulation boxes at z=5.7 and z=6.6 and we build synthetic images/spectra of a prototypical LAE. At redshift 5.7, we find that the Ly{alpha} transmissivity (T_LAE) ~ 0.25, almost independent of the halo mass. This constancy arises from the conspiracy of two effects: (i) the intrinsic Ly{alpha} line width and (ii) the infall peculiar velocity. At higher redshift, z=6.6, where the transmissivity is instead largely set by the local HI abundance and LAE transmissivity consequently increases with halo mass from 0.15 to 0.3. Although outflows are present, they are efficiently pressure-confined by infall in a small region around the LAE; hence they only marginally affect transmissivity. Finally, we cast LOS originating from background QSOs passing through foreground LAEs at different impact parameters, and compute the quasar transmissivity (T_QSO). At smaller impact parameters, d < 1 cMpc, a positive correlation between T_QSO and halo mass is found at z = 5.7, which tends to become less pronounced (i.e. flatter) at larger distances. By cross-correlating T_LAE and T_QSO, we can obtain a HI density estimate unaffected by dust. At z= 5.7, the cross-correlation is relatively weak,whereas at z = 6.6 we find a clear positive correlation. We conclude by briefly discussing the perspectives for the application of the method to existing and forthcoming data.
In the last few years, prominent high-ionization nebular emission lines (i.e., OIII], CIII], CIV, HeII) have been observed in the deep UV spectra of z~5-7 galaxies, indicating that extreme radiation fields characterize reionization-era systems. These lines have been linked to the leakage of Lyman continuum photons both theoretically and observationally. Consequently, high-ionization UV emission lines present our best probe to detect and characterize the most distant galaxies that we will observe in the coming years, and are key to understanding the sources of reionization, yet the physics governing their production is poorly understood. Here we present recent high-resolution Hubble Space Telescope spectra of two nearby extreme UV emission-line galaxies, J104457 and J141851. We report the first observations of intense nebular HeII and double-peaked, resonantly-scattered CIV emission, a combination that suggests these galaxies both produce and transmit a significant number of very high-energy ionizing photons (E>47.89 eV) through relatively low column densities of high-ionization gas. This suggests that, in addition to photons at the H-ionizing edge, the very hard ionizing photons that escape from these galaxies may provide a secondary source of ionization that is currently unconstrained observationally. Simultaneous radiative transfer models of LyA and CIV are needed to understand how ionizing radiation is transmitted through both low- and high-ionization gas. Future rest-frame FUV observations of galaxies within the epoch of reionization using the JWST or ELTs will allow us to constrain the escape of helium-ionizing photons and provide an estimate for their contribution to the reionization budget.
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.
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