Green Pea Galaxies Reveal Secrets of Lyα\alpha Escape

We analyze archival Ly$\alpha$ spectra of 12 "Green Pea" galaxies observed with the Hubble Space Telescope, model their Ly$\alpha$ profiles with radiative transfer models, and explore the dependence of Ly$\alpha$ escape fraction on various properties. Green Pea galaxies are nearby compact starburst galaxies with [OIII]$\lambda$5007 equivalent widths of hundreds of \AA. All 12 Green Pea galaxies in our sample show Ly$\alpha$ lines in emission, with a Ly$\alpha$ equivalent width distribution similar to high redshift Ly$\alpha$ emitters. Combining the optical and UV spectra of Green Pea galaxies, we estimate their Ly$\alpha$ escape fractions and find correlations between Ly$\alpha$ escape fraction and kinematic features of Ly$\alpha$ profiles. The escape fraction of Ly$\alpha$ in these galaxies ranges from 1.4% to 67%. We also find that the Ly$\alpha$ escape fraction depends strongly on metallicity and moderately on dust extinction. We compare their high-quality Ly$\alpha$ profiles with single HI shell radiative transfer models and find that the Ly$\alpha$ escape fraction anti-correlates with the derived HI column densities. Single shell models fit most Ly$\alpha$ profiles well, but not the ones with highest escape fractions of Ly$\alpha$. Our results suggest that low HI column density and low metallicity are essential for Ly$\alpha$ escape, and make a galaxy a Ly$\alpha$ emitter.Comment: 9 figures, ApJ accepte

Lyα\alpha profile, dust, and prediction of Lyα\alpha escape fraction in Green Pea Galaxies

We studied Lyman-$\alpha$ (Ly$\alpha$) escape in a statistical sample of 43 Green Peas with HST/COS Ly$\alpha$ spectra. Green Peas are nearby star-forming galaxies with strong [OIII]$\lambda$5007 emission lines. Our sample is four times larger than the previous sample and covers a much more complete range of Green Pea properties. We found that about 2/3 of Green Peas are strong Ly$\alpha$ line emitters with rest-frame Ly$\alpha$ equivalent width $>20$ \AA. The Ly$\alpha$ profiles of Green Peas are diverse. The Ly$\alpha$ escape fraction, defined as the ratio of observed Ly$\alpha$ flux to intrinsic Ly$\alpha$ flux, shows anti-correlations with a few Ly$\alpha$ kinematic features -- both the blue peak and red peak velocities, the peak separations, and FWHM of the red portion of the Ly$\alpha$ profile. Using properties measured from SDSS optical spectra, we found many correlations -- Ly$\alpha$ escape fraction generally increases at lower dust reddening, lower metallicity, lower stellar mass, and higher [OIII]/[OII] ratio. We fit their Ly$\alpha$ profiles with the HI shell radiative transfer model and found Ly$\alpha$ escape fraction anti-correlates with the best-fit $N_{HI}$. Finally, we fit an empirical linear relation to predict Ly$\alpha$ escape fraction from the dust extinction and Ly$\alpha$ red peak velocity. The standard deviation of this relation is about 0.3 dex. This relation can be used to isolate the effect of IGM scatterings from Ly$\alpha$ escape and to probe the IGM optical depth along the line of sight of each $z>7$ Ly$\alpha$ emission line galaxy in the JWST era.Comment: 15 pages, 11 figures, 3 tables, machine-readable tables included. ApJ in-pres

Diffuse Lyman Alpha Haloes around Lyman Alpha Emitters at z=3: Do Dark Matter Distributions Determine the Lyman Alpha Spatial Extents?

Using stacks of Ly-a images of 2128 Ly-a emitters (LAEs) and 24 protocluster UV-selected galaxies (LBGs) at z=3.1, we examine the surface brightness profiles of Ly-a haloes around high-z galaxies as a function of environment and UV luminosity. We find that the slopes of the Ly-a radial profiles become flatter as the Mpc-scale LAE surface densities increase, but they are almost independent of the central UV luminosities. The characteristic exponential scale lengths of the Ly-a haloes appear to be proportional to the square of the LAE surface densities (r(Lya) \propto Sigma(LAE)^2). Including the diffuse, extended Ly-a haloes, the rest-frame Ly-a equivalent width of the LAEs in the densest regions approaches EW_0(Lya) ~ 200 A, the maximum value expected for young (< 10^7 yr) galaxies. This suggests that Ly-a photons formed via shock compression by gas outflows or cooling radiation by gravitational gas inflows may partly contribute to illuminate the Ly-a haloes; however, most of their Ly-a luminosity can be explained by photo-ionisation by ionising photons or scattering of Ly-a photons produced in HII regions in and around the central galaxies. Regardless of the source of Ly-a photons, if the Ly-a haloes trace the overall gaseous structure following the dark matter distributions, it is not surprising that the Ly-a spatial extents depend more strongly on the surrounding Mpc-scale environment than on the activities of the central galaxies.Comment: 7 pages, 4 figures, accepted for publication in MNRA

What Makes Lyα\alpha Nebulae Glow? Mapping the Polarization of LABd05

"Ly$\alpha$ nebulae" are giant ($\sim$100 kpc), glowing gas clouds in the distant universe. The origin of their extended Ly$\alpha$ emission remains a mystery. Some models posit that Ly$\alpha$ emission is produced when the cloud is photoionized by UV emission from embedded or nearby sources, while others suggest that the Ly$\alpha$ photons originate from an embedded galaxy or AGN and are then resonantly scattered by the cloud. At least in the latter scenario, the observed Ly$\alpha$ emission will be polarized. To test these possibilities, we are conducting imaging polarimetric observations of seven Ly$\alpha$ nebulae. Here we present our results for LABd05, a cloud at $z$ = 2.656 with an obscured, embedded AGN to the northeast of the peak of Ly$\alpha$ emission. We detect significant polarization. The highest polarization fractions $P$ are $\sim$10-20% at $\sim$20-40 kpc southeast of the Ly$\alpha$ peak, away from the AGN. The lowest $P$, including upper-limits, are $\sim$5% and lie between the Ly$\alpha$ peak and AGN. In other words, the polarization map is lopsided, with $P$ increasing from the Ly$\alpha$ peak to the southeast. The measured polarization angles $\theta$ are oriented northeast, roughly perpendicular to the $P$ gradient. This unique polarization pattern suggests that 1) the spatially-offset AGN is photoionizing nearby gas and 2) escaping Ly$\alpha$ photons are scattered by the nebula at larger radii and into our sightline, producing tangentially-oriented, radially-increasing polarization away from the photoionized region. Finally we conclude that the interplay between the gas density and ionization profiles produces the observed central peak in the Ly$\alpha$ emission. This also implies that the structure of LABd05 is more complex than assumed by current theoretical spherical or cylindrical models.Comment: 11 pages, 8 figure

Lyα\alpha emission from Green Peas: the role of circumgalactic gas density, covering, and kinematics

We report Hubble Space Telescope/Cosmic Origins Spectrograph observations of the Ly$\alpha$ emission and interstellar absorption lines in a sample of ten star-forming galaxies at $z\sim0.2$. Selected on the basis of high equivalent width optical emission lines, the sample, dubbed "Green Peas," make some of the best analogs for young galaxies in an early Universe. We detect Ly$\alpha$ emission in all ten galaxies, and 9/10 show double-peaked line profiles suggestive of low HI column density. We measure Ly$\alpha$/H$\alpha$ flux ratios of 0.5-5.6, implying that 5% to 60% of Ly$\alpha$ photons escape the galaxies. These data confirm previous findings that low-ionization metal absorption (LIS) lines are weaker when Ly$\alpha$ escape fraction and equivalent width are higher. However, contrary to previously favored interpretations of this trend, increased Ly$\alpha$ output cannot be the result of varying HI covering: the Lyman absorption lines (Ly$\beta$ and higher) show a covering fraction near unity for gas with $N_{HI} \gtrsim 10^{16}$ cm$^{-2}$. Moreover, we detect no correlation between Ly$\alpha$ escape and the outflow velocity of the LIS lines, suggesting that kinematic effects do not explain the range of Ly$\alpha$/H$\alpha$ flux ratios in these galaxies. In contrast, we detect a strong anti-correlation between the Ly$\alpha$ escape fraction and the velocity separation of the Ly$\alpha$ emission peaks, driven primarily by the velocity of the blue peak. As this velocity separation is sensitive to HI column density, we conclude that Ly$\alpha$ escape in these Green Peas is likely regulated by the HI column density rather than outflow velocity or HI covering fraction.Comment: 27 pages, 26 figures; Accepted for publication in the Astrophysical Journa

Hubble Advanced Camera for Surveys Observations of the z=6.42 Quasar SDSS 1148+5251: A Leak in the Gunn-Peterson Trough

The Hubble Advanced Camera for Surveys has been used to obtain a narrow-band image of the weak emission peak seen at lambda=7205 A in the Gunn-Peterson Ly beta absorption trough of the highest redshift quasar, SDSS J1148+5251. The emission looks perfectly point-like; there is no evidence for the intervening galaxy that we previously suggested might be contaminating the quasar spectrum. We derive a more accurate astrometric position for the quasar in the two filters and see no indication of gravitational lensing. We conclude that the light in the Ly beta trough is leaking through two unusually transparent, overlapping windows in the IGM absorption, one in the Ly beta forest at z ~ 6 and one in the Ly alpha forest at z ~ 5. If there are significant optical depth variations on velocity scales small compared with our spectral resolution (~150 km/s), the Ly alpha trough becomes more transparent for a given Ly beta optical depth. Such variations can only strengthen our conclusion that the fraction of neutral hydrogen in the IGM increases dramatically at z>6. We argue that the transmission in the Ly beta trough is not only a more sensitive measure of the neutral fraction than is Ly alpha, it also provides a less biased estimator of the neutral hydrogen fraction than does the Ly alpha transmission.Comment: Submitted to the Astronomical Journa

The VANDELS survey: A strong correlation between Lyα\alpha equivalent width and stellar metallicity at 3≤z≤5\mathbf{3\leq z \leq 5}

We present the results of a new study investigating the relationship between observed Ly$\alpha$ equivalent width ($W_{\lambda}$(Ly$\alpha$)) and the metallicity of the ionizing stellar population ($Z_{\star}$) for a sample of $768$ star-forming galaxies at $3 \leq z \leq 5$ drawn from the VANDELS survey. Dividing our sample into quartiles of rest-frame $W_{\lambda}$(Ly$\alpha$) across the range -58 \unicode{xC5} \lesssim $W_{\lambda}$(Ly$\alpha$) \lesssim 110 \unicode{xC5} we determine $Z_{\star}$ from full spectral fitting of composite far-ultraviolet (FUV) spectra and find a clear anti-correlation between $W_{\lambda}$(Ly$\alpha$) and $Z_{\star}$. Our results indicate that $Z_{\star}$ decreases by a factor $\gtrsim 3$ between the lowest $W_{\lambda}$(Ly$\alpha$) quartile ($\langle$$W_{\lambda}$(Ly$\alpha$)\rangle=-18\unicode{xC5}) and the highest $W_{\lambda}$(Ly$\alpha$) quartile ($\langle$$W_{\lambda}$(Ly$\alpha$)\rangle=24\unicode{xC5}). Similarly, galaxies typically defined as Lyman Alpha Emitters (LAEs; $W_{\lambda}$(Ly$\alpha$) >20\unicode{xC5}) are, on average, metal poor with respect to the non-LAE galaxy population ($W_{\lambda}$(Ly$\alpha$) \leq20\unicode{xC5}) with $Z_{\star}$$_{\rm{non-LAE}}\gtrsim 2 \times$ $Z_{\star}$$_{\rm{LAE}}$. Finally, based on the best-fitting stellar models, we estimate that the increasing strength of the stellar ionizing spectrum towards lower $Z_{\star}$ is responsible for $\simeq 15-25\%$ of the observed variation in $W_{\lambda}$(Ly$\alpha$) across our sample, with the remaining contribution ($\simeq 75-85\%$) being due to a decrease in the HI/dust covering fractions in low $Z_{\star}$ galaxies.Comment: 10 pages, 6 figures, MNRAS accepte

The Lyman Alpha Reference Sample. VIII. Characterizing Lyman-Alpha Scattering in Nearby Galaxies

We examine the dust geometry and Ly{\alpha} scattering in the galaxies of the Lyman Alpha Reference Sample (LARS), a set of 14 nearby (0.02 < $z$ < 0.2) Ly{\alpha} emitting and starbursting systems with Hubble Space Telescope Ly{\alpha}, H{\alpha}, and H{\beta} imaging. We find that the global dust properties determined by line ratios are consistent with other studies, with some of the LARS galaxies exhibiting clumpy dust media while others of them show significantly lower Ly{\alpha} emission compared to their Balmer decrement. With the LARS imaging, we present Ly{\alpha}/H{\alpha} and H{\alpha}/H{\beta} maps with spatial resolutions as low as $\sim$ 40 pc, and use these data to show that in most galaxies, the dust geometry is best modeled by three distinct regions: a central core where dust acts as a screen, an annulus where dust is distributed in clumps, and an outer envelope where Ly{\alpha} photons only scatter. We show that the dust that affects the escape of Ly{\alpha} is more restricted to the galaxies' central regions, while the larger Ly{\alpha} halos are generated by scattering at large radii. We present an empirical modeling technique to quantify how much Ly{\alpha} scatters in the halo, and find that this "characteristic" scattering distance correlates with the measured size of the Ly{\alpha} halo. We note that there exists a slight anti-correlation between the scattering distance of Ly{\alpha} and global dust properties.Comment: 32 pages, 51 figures, accepted to Ap

Time-Dependent Behavior of Lymanα\alpha Photon Transfer in High Redshift Optically Thick Medium

With Monte Carlo simulation method, we investigate the time dependent behavior of Ly$\alpha$ photon transfer in optically thick medium of the concordance $\Lambda$CDM universe. At high redshift, the Ly$\alpha$ photon escaping from optically thick medium has a time scale as long as the age of the luminous object, or even comparable to the age of the universe. In this case, time-independent, or stationary solutions of the Ly$\alpha$ photon transfer with resonant scattering will overlook important features of the escaped Ly$\alpha$ photons in physical and frequency spaces. More seriously, the expansion of the universe leads to that the time-independent solutions of the Ly$\alpha$ photon transfer may not exist. We show that time-dependent solutions sometimes are essential for understanding the Ly$\alpha$ emission and absorption at high redshifts. For Ly\alpha photons from sources at redshift 1+z=10 and being surrounded by neutral hydrogen IGM of the $\Lambda$CDM universe, the escape coefficient is found to be always less, or much less than one, regardless of the age or life time of the sources. Under such environment, we also find that even when the Ly$\alpha$ photon luminosity of the sources is stable, the mean surface brightness is gradually increasing in the first 10^6 years, and then decreasing with a power law of time, but never approaches a stable, time-independent state. That is, all 1+z=10 sources in a neutral Hubble expanding IGM with Ly$\alpha$ luminosity L have their maximum of mean surface brightness ~ 10^{-21}(L/(10^{43}erg/s)) erg s^{-1} cm^{-2} arcsec^{-2} at the age of about 10^6 years. The time-dependent effects on the red damping wing profile are also addressed.Comment: 11 pages, 10 figures, accepted for publication in MNRA