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Register a new userOn the redshift-evolution of the Lyman-alpha escape fraction and the dust content of galaxies
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Matthew Hayes
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The Ly-alpha emission line has been proven a powerful tool by which to study evolving galaxies at the highest redshifts. However, in order to use Lya as a physical probe of galaxies, it becomes vital to know the Lya escape fraction (fescLya). Unfortunately, due to the resonant nature of Lya, fescLya may vary unpredictably and requires empirical measurement. Here we compile Lya luminosity functions between redshift z=0 and 8 and, combined with H-alpha and ultraviolet data, assess how fescLya evolves with redshift. We find a strong upwards evolution in fescLya over the range z=0.3-6, which is well-fit by the power-law fescLya propto (1+z)^xi, with xi =(2.57_-0.12^+0.19). This predicts that fescLya should reach unity at z=11.1. By comparing fescLya and E(B-V) in individual galaxies we derive an empirical relationship between fescLya and E(B-V), which includes resonance scattering and can explain the redshift evolution of fescLya between z=0 and 6 purely as a function of the evolution in the dust content of galaxies. Beyond z~6.5, fescLya drops more substantially; an effect attributed to either ionizing photon leakage, or an increase in the neutral gas fraction of the intergalactic medium. While distinguishing between those two scenarios may be extremely challenging, by framing the problem this way we remove the uncertainty of the halo mass from Lya-based tests of reionization. We finally derive a new method by which to estimate the dust content of galaxies based purely upon the observed Lya and UV LFs. These data are characterized by an exponential with an e-folding redshift of ~3.5.
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We study the far-infrared properties of 498 Lyman Alpha Emitters (LAEs) at z=2.8, 3.1 and 4.5 in the Extended Chandra Deep Field-South, using 250, 350 and 500 micron data from the Herschel Multi-tiered Extragalactic Survey (HerMES) and 870 micron data from the LABOCA ECDFS Submillimeter Survey (LESS). None of the 126, 280 or 92 LAEs at z=2.8, 3.1 and 4.5, respectively, are individually detected in the far-infrared data. We use stacking to probe the average emission to deeper flux limits, reaching $1sigma$ depths of ~0.1 to 0.4 mJy. The LAEs are also undetected at $ge3sigma$ in the stacks, although a $2.5sigma$ signal is observed at 870 micron for the z=2.8 sources. We consider a wide range of far-infrared spectral energy distributions (SEDs), including a M82 and an Sd galaxy template, to determine upper limits on the far-infrared luminosities and far-infrared-derived star-formation rates of the LAEs. These star-formation rates are then combined with those inferred from the Ly$alpha$ and UV emission to determine lower limits on the LAEs Ly$alpha$ escape fraction ($f_{rm esc}($Ly$alpha$)). For the Sd SED template, the inferred LAEs $f_{rm esc}($Ly$alpha$) are $gtrsim30%$ ($1sigma$) at z=2.8, 3.1 and 4.5, which are all significantly higher than the global $f_{rm esc}($Ly$alpha$) at these redshifts. Thus, if the LAEs $f_{rm esc}($Ly$alpha$) follows the global evolution then they have warmer far-infrared SEDs than the Sd galaxy template. The average and M82 SEDs produce lower limits on the LAE $f_{rm esc}($Ly$alpha$) of ~10 to 20% ($1sigma$), all of which are slightly higher than the global evolution of $f_{rm esc}($Ly$alpha$) but consistent with it at the 2 to 3$sigma$ level.
Lyman-alpha (Ly{alpha}) photons from ionizing sources and cooling radiation undergo a complex resonant scattering process that generates unique spectral signatures in high-redshift galaxies. We present a detailed Ly{alpha} radiative transfer study of a cosmological zoom-in simulation from the Feedback In Realistic Environments (FIRE) project. We focus on the time, spatial, and angular properties of the Ly{alpha} emission over a redshift range of z = 5-7, after escaping the galaxy and being transmitted through the intergalactic medium (IGM). Over this epoch, our target galaxy has an average stellar mass of $M_{rm star} approx 5 times 10^8 {rm M}_odot$. We find that many of the interesting features of the Ly{alpha} line can be understood in terms of the galaxys star formation history. The time variability, spatial morphology, and anisotropy of Ly{alpha} properties are consistent with current observations. For example, the rest frame equivalent width has a ${rm EW}_{{rm Ly}alpha,0} > 20 {rm AA}$ duty cycle of 62% with a non-negligible number of sightlines with $> 100 {rm AA}$, associated with outflowing regions of a starburst with greater coincident UV continuum absorption, as these conditions generate redder, narrower (or single peaked) line profiles. The lowest equivalent widths correspond to cosmological filaments, which have little impact on UV continuum photons but efficiently trap Ly{alpha} and produce bluer, broader lines with less transmission through the IGM. We also show that in dense self-shielding, low-metallicity filaments and satellites Ly{alpha} radiation pressure can be dynamically important. Finally, despite a significant reduction in surface brightness with increasing redshift, Ly{alpha} detections and spectroscopy of high-$z$ galaxies with the upcoming James Webb Space Telescope is feasible.
We have obtained the first constraints on extended Ly-alpha emission at z ~ 1 in a sample of five radio galaxies. We detect Ly-alpha emission from four of the five galaxies. The Ly-alpha luminosities range from 0.1 - 4 times 10^43 erg/s and are much smaller than those observed for halos around higher redshift radio galaxies. If the z ~ 1 radio galaxies are the descendents the z >~ 2 radio galaxies, then their Ly-alpha luminosities evolve strongly with redshift as ~(1+z)^5. There do not appear to be strong correlations between other parameters, such as radio power, suggesting that this observed evolution is real and not an observational artifact or secondary correlation. We speculate that this evolution of luminous halos may be due to gas depletion (as gas cools, settles, and forms stars) accompanied by an overall rise in the mean gas temperature and a decrease in specific star-formation rate in and around these massive galaxies.
The Lyman-alpha (Lya) recombination line is a fundamental tool for galaxy evolution studies and modern observational cosmology. However, subsequent interpretations are still prone to a number of uncertainties. Besides numerical efforts, empirical data are urgently needed for a better understanding of Lya escape process. We empirically estimate the Lyman-alpha escape fraction fesc(Lya) in a statistically significant sample of z ~ 0 - 0.3 galaxies in order to calibrate high-redshift Lyman-alpha observations. An optical spectroscopic follow-up of a sub-sample of 24 Lyman-alpha emitters (LAEs) detected by GALEX at z ~ 0.2-0.3, combined with a UV-optical sample of local starbursts, both with matched apertures, allow us to quantify the dust extinction through Balmer lines, and to estimate the Lyman-alpha escape fraction from the Halpha flux corrected for extinction in the framework of the recombination theory. The global escape fraction of Lyman-alpha radiation spans nearly the entire range of values, from 0.5 to 100 %, and fesc(Lya) clearly decreases with increasing nebular dust extinction E(B-V). Several objects show fesc(Lya) greater than fesc(continuum) which may be an observational evidence for clumpy ISM geometry or for an aspherical ISM. Selection biases and aperture size effects may still prevail between z ~ 0.2-0.3 LAEs and local starbursts, which may explain the difference observed for fesc(Lya).
We present the optical spectroscopic follow-up of 31 z=0.3 Lyman-alpha (Lya) emitters, previously identified by Deharveng et al. (2008). We find that 17% of the Lya emitters have line ratios that require the hard ionizing continuum produced by an AGN. The uniform dust screen geometry traditionally used in studies similar to ours is not able to simultaneously reproduce the observed high Lya/Halpha and Halpha/Hbeta line ratios. We consider different possibilities for the geometry of the dust around the emitting sources. We find that also a uniform mixture of sources and dust does not reproduce the observed line ratios. Instead, these are well reproduced by a clumpy dust screen. This more realistic treatment of the geometry results in extinction corrected (Lya/Halpha)_C values consistent with Case B recombination theory, whereas a uniform dust screen model would imply values (Lya/Halpha)_C higher than 8.7. Our analysis shows that there is no need to invoke ad-hoc multi phase media in which the Lya photons only scatter between the dusty clouds and eventually escape.
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