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Register a new userNeutral gas properties of Lyman continuum emitting galaxies: column densities and covering fractions from UV absorption lines
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The processes allowing the escape of ionizing photons from galaxies into the intergalactic medium are poorly known. To understand how Lyman continuum (LyC) photons escape galaxies, we constrain the HI covering fractions and column densities using ultraviolet HI and metal absorption lines of 18 star-forming galaxies which have Lyman series observations. Nine of these galaxies are confirmed LyC emitters. We fit the stellar continuum, dust attenuation, metal, and HI properties to consistently determine the UV attenuation, as well as the column densities and covering factors of neutral hydrogen and metals. We use synthetic interstellar absorption lines to explore the systematics of our measurements. Then we apply our method to the observed UV spectra of low-redshift and z-2 galaxies. The observed HI lines are found to be saturated in all galaxies. An indirect approach using OI column densities and the observed O/H abundances yields HI column densities of 18.6 to 20 cm-2. These columns are too high to allow the escape of ionizing photons. We find that the known LyC leakers have HI covering fractions less than unity. Ionizing photons escape through optically thin holes/channels in a clumpy interstellar medium. Our simulations confirm that the HI covering fractions are accurately recovered. The SiII and HI covering fractions scale linearly, in agreement with observations from stacked Lyman break galaxy spectra at z-3. Thus, with an empirical correction, the SiII absorption lines can also be used to determine the HI coverage. Finally, we show that a consistent fitting of dust attenuation, continuum and absorption lines is required to properly infer the covering fraction of neutral gas and subsequently to infer the escape fraction of ionizing radiation. These measurements can estimate the LyC escape fraction, as we demonstrate in a companion paper.
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The neutral intergalactic medium above redshift 6 is opaque to ionizing radiation, therefore one needs indirect measurements of the escape fraction of ionizing photons from galaxies of this epoch. Low-ionization state absorption lines are a common feature in the spectrum of galaxies, showing a diversity of strengths and shapes. Since these lines indicate the presence of neutral gas in front of the stars, they have been proposed to carry information on the escape of ionizing radiation from galaxies. We study which processes are responsible for the shape of the absorption lines, to better understand their origin. We then explore whether the absorption lines can be used to predict the escape fractions. Using a radiation-hydrodynamical zoom-in simulation and the radiative transfer code RASCAS, we generate mock CII 1334 and LyB lines of a virtual galaxy at redshift 3 as seen from many directions of observation. We also compute the escape fraction of ionizing photons in those directions and look for correlations between the lines and the escape fractions. We find that the resulting mock absorption lines are comparable to observations and that the lines and the escape fractions vary strongly depending on the direction of observation. Gas velocity and dust always affect the absorption profile significantly. We find no strong correlations between observable LyB or CII 1334 and the escape fraction. After correcting the continuum for attenuation by dust to recover the intrinsic continuum, the residual flux of CII 1334 correlates well with the escape fraction for directions with a dust corrected residual flux larger than 30%. For other directions, the relations have a strong dispersion, and the residual flux overestimates the escape fraction for most cases. Concerning LyB, the residual flux after dust correction does not correlate with the escape fraction but can be used as a lower limit. (abridged)
We report the discovery of J0121+0025, an extremely luminous and young star-forming galaxy (M_UV = -24.11, log[L_Lya / erg s^-1] = 43.8) at z = 3.244 showing copious Lyman continuum (LyC) leakage (f_esc,abs ~ 40%). High signal-to-noise ratio rest-frame UV spectroscopy with the Gran Telescopio Canarias reveals a high significance (7.9 sigma) emission below the Lyman limit (< 912A), with a flux density level f_900A = 0.78 +/- 0.10 uJy, and strong P-Cygni in wind lines of OVI 1033A, NV 1240A and CIV 1550A that are indicative of a young age of the starburst (<10 Myr). The spectrum is rich in stellar photospheric features, for which a significant contribution of an AGN at these wavelengths is ruled out. Low-ionization ISM absorption lines are also detected, but are weak (EW0 ~ 1A) and show large residual intensities, suggesting a clumpy geometry of the gas with a non-unity covering fraction or a highly ionized ISM. The contribution of a foreground and AGN contamination to the LyC signal is unlikely. Deep optical to Spitzer/IRAC 4.5um imaging show that the spectral energy distribution of J0121+0025 is dominated by the emission of the young starburst, with log(M*/Msun) = 9.9 +/- 0.1 and SFR = 981 +/- 232 Msun yr^-1. J0121+0025 is the most powerful LyC emitter known among the star-forming galaxy population. The discovery of such luminous and young starburst leaking LyC radiation suggests that a significant fraction of LyC photons can escape in sources with a wide range of UV luminosities and are not restricted to the faintest ones as previously thought. These findings might shed further light on the role of luminous starbursts to the cosmic reionization.
We examine halo gas cross sections and covering fractions, f_c, of intermediate redshift MgII absorption selected galaxies. We computed statistical absorber halo radii, R_x, using current values of dN/dz and Schechter luminosity function parameters, and have compared these values to the distribution of impact parameters and luminosities from a sample of 37 galaxies. For equivalent widths W_r(2796) > 0.3 Ang, we find 43 < R_x < 88 kpc, depending on the lower luminosity cutoff and the slope, beta, of the Holmberg-like luminosity scaling, R propto L^beta. The observed distribution of impact parameters, D, are such that several absorbing galaxies lie at D > R_x and several non-absorbing galaxies lie at D < R_x. We deduced f_c must be less than unity and obtain a mean of <f_c> ~ 0.5 for our sample. Moreover, the data suggest halo radii of MgII absorbing galaxies do not follow a luminosity scaling with beta in the range of 0.2-0.28, if f_c= 1 as previously reported. However, provided f_c~0.5, we find that halo radii can remain consistent with a Holmberg-like luminosity relation with beta ~ 0.2 and R* = R_x/sqrt(f_c)= 110 kpc. No luminosity scaling (beta=0) is also consistent with the observed distribution of impact parameters if f_c < 0.37. The data support a scenario in which gaseous halos are patchy and likely have non-symmetric geometric distributions about the galaxies. We suggest halo gas distributions may not be govern primarily by galaxy mass/luminosity but also by stochastic processes local to the galaxy.
Population III stars can regulate star formation in the primordial Universe in several ways. They can ionize nearby halos, and even if their ionizing photons are trapped by their own halos, their Lyman-Werner (LW) photons can still escape and destroy H$_2$ in other halos, preventing them from cooling and forming stars. LW escape fractions are thus a key parameter in cosmological simulations of early reionization and star formation but have not yet been parametrized for realistic halos by halo or stellar mass. To do so, we perform radiation hydrodynamical simulations of LW UV escape from 9--120 M$_{odot}$ Pop III stars in $10^5$ to $10^7$ M$_{odot}$ halos with ZEUS-MP. We find that photons in the LW lines (i.e. those responsible for destroying H$_{2}$ in nearby systems) have escape fractions ranging from 0% to 85%. No LW photons escape the most massive halo in our sample, even from the most massive star. Escape fractions for photons elsewhere in the 11.18--13.6~eV energy range, which can be redshifted into the LW lines at cosmological distances, are generally much higher, being above 60% for all but the least massive stars in the most massive halos. We find that shielding of H$_2$ by neutral hydrogen, which has been neglected in most studies to date, produces escape fractions that are up to a factor of three smaller than those predicted by H$_2$ self-shielding alone.
Motivated by the discovery of the ultra-strong emission line starburst galaxies (EELGs) known as green pea galaxies, we consider here their contribution to the intergalactic flux of ionizing UV at high redshifts. Most galaxies that have been observed show a precipitous drop in their flux blueward of the Lyman limit. However, recent observations of EELGs have discovered that many more Lyman continuum photons escape from them into intergalactic space than was previously suspected. We calculate their contribution to the extragalactic background light (EBL). We also calculate the effect of these photons on the absorption of high energy $gamma$-rays. For the more distant $gamma$-ray sources, particularly at $z ge 3$, the intergalactic opacity above a few GeV is significantly higher than previous estimates which ignored the Lyman continuum photons. We calculate the results of this increased opacity on observed $gamma$-ray spectra, which produces a high-energy turnover starting at lower energies than previously thought, and a gradual spectral steepening that may also be observable.
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