Modeling 237237 Lyman-α\alpha spectra of the MUSE-Wide survey

We compare $237$ Lyman-$\alpha$ (Ly$\alpha$) spectra of the "MUSE-Wide survey" (Herenz et al. 2017) to a suite of radiative transfer simulations consisting of a central luminous source within a concentric, moving shell of neutral gas, and dust. This six parameter shell-model has been used numerously in previous studies, however, on significantly smaller data-sets. We find that the shell-model can reproduce the observed spectral shape very well - better than the also common `Gaussian-minus-Gaussian' model which we also fitted to the dataset. Specifically, we find that $\sim 94\%$ of the fits possess a goodness-of-fit value of $p(\chi^2)>0.1$. The large number of spectra allows us to robustly characterize the shell-model parameter range, and consequently, the spectral shapes typical for realistic spectra. We find that the vast majority of the Ly$\alpha$ spectral shapes require an outflow and only $\sim 5\%$ are well-fitted through an inflowing shell. In addition, we find $\sim 46\%$ of the spectra to be consistent with a neutral hydrogen column density $<10^{17}\,\mathrm{cm}^{-2}$ - suggestive of a non-negligible fraction of continuum leakers in the MUSE-Wide sample. Furthermore, we correlate the spectral against the Ly$\alpha$ halo properties against each other but do not find any strong correlation.Comment: 10 pages, 7 figures; data can be downloaded at http://bit.ly/a-spectra-of-M

A systematic study of Lyman-Alpha transfer through outflowing shells: Model parameter estimation

Outflows promote the escape of Lyman-$\alpha$ (Ly$\alpha$) photons from dusty interstellar media. The process of radiative transfer through interstellar outflows is often modelled by a spherically symmetric, geometrically thin shell of gas that scatters photons emitted by a central Ly$\alpha$ source. Despite its simplified geometry, this `shell model' has been surprisingly successful at reproducing observed Ly$\alpha$ line shapes. In this paper we perform automated line fitting on a set of noisy simulated shell model spectra, in order to determine whether degeneracies exist between the different shell model parameters. While there are some significant degeneracies, we find that most parameters are accurately recovered, especially the HI column density ($N_{\rm HI}$) and outflow velocity ($v_{\rm exp}$). This work represents an important first step in determining how the shell model parameters relate to the actual physical properties of Ly$\alpha$ sources. To aid further exploration of the parameter space, we have made our simulated model spectra available through an interactive online tool.Comment: 10 pages, 6 figures. Matches version published in ApJ. Our grid of Lyman alpha spectra can be accessed at http://bit.ly/man-alpha through an interactive online too

Connecting Faint End Slopes of the Lyman-α\alpha emitter and Lyman-break Galaxy Luminosity Functions

We predict Lyman-$\alpha$ (Ly$\alpha$) luminosity functions (LFs) of Ly$\alpha$-selected galaxies (Ly$\alpha$ emitters, or LAEs) at $z=3-6$ using the phenomenological model of Dijkstra & Wyithe (2012). This model combines observed UV-LFs of Lyman-break galaxies (LBGs, or drop out galaxies), with constraints on their distribution of Ly$\alpha$ line strengths as a function of UV-luminosity and redshift. Our analysis shows that while Ly$\alpha$ LFs of LAEs are generally not Schechter functions, these provide a good description over the luminosity range of $\log_{10}( L_{\alpha}/{\rm erg}\,{\rm s}^{-1})=41-44$. Motivated by this result, we predict Schechter function parameters at $z=3-6$. Our analysis further shows that (i) the faint end slope of the Ly$\alpha$ LF is steeper than that of the UV-LF of Lyman-break galaxies, (with a median $\alpha_{Ly\alpha} < -2.0$ at $z\gtrsim 4$), and (ii) a turn-over in the Ly$\alpha$ LF of LAEs at Ly$\alpha$ luminosities $10^{40}$ erg s$^{-1}<L_{\alpha}\lesssim 10^{41}$ erg s$^{-1}$ may signal a flattening of UV-LF of Lyman-break galaxies at $-12>M_{\rm UV}>-14$. We discuss the implications of these results - which can be tested directly with upcoming surveys - for the Epoch of Reionization.Comment: 8 pages, 6 figures, submitted to MNRAS (after revision following referee's report

Resonant line transfer in a fog: Using Lyman-alpha to probe tiny structures in atomic gas

Motivated by observational and theoretical work which both suggest very small scale ($\lesssim 1\,$pc) structure in the circum-galactic medium of galaxies and in other environments, we study Lyman-$\alpha$ (Ly$\alpha$) radiative transfer in an extremely clumpy medium with many "clouds" of neutral gas along the line of sight. While previous studies have typically considered radiative transfer through sightlines intercepting $\lesssim 10$ clumps, we explore the limit of a very large number of clumps per sightline (up to $f_{\mathrm{c}} \sim 1000$). Our main finding is that, for covering factors greater than some critical threshold, a multiphase medium behaves similar to a homogeneous medium in terms of the emergent Ly$\alpha$ spectrum. The value of this threshold depends on both the clump column density and on the movement of the clumps. We estimate this threshold analytically and compare our findings to radiative transfer simulations with a range of covering factors, clump column densities, radii, and motions. Our results suggest that (i) the success in fitting observed Ly$\alpha$ spectra using homogeneous "shell models" (and the corresponding failure of multiphase models) hints towards the presence of very small-scale structure in neutral gas, in agreement within a number of other observations; and (ii) the recurrent problems of reproducing realistic line profiles from hydrodynamical simulations may be due to their inability to resolve small-scale structure, which causes simulations to underestimate the effective covering factor of neutral gas clouds.Comment: 18 pages, 21 figures; submitted to A&A; animations available at http://bit.ly/a-in-a-fo

Molecular Shattering

Recent observations suggest galaxies may ubiquitously host a molecular component to their multiphase circumgalactic medium (CGM). However, the structure and kinematics of the molecular CGM remains understudied theoretically and largely unconstrained observationally. Recent work suggests molecular gas clouds with efficient cooling survive acceleration in hot winds similar to atomic clouds. Yet the pressure-driven fragmentation of molecular clouds when subjected to external shocks or undergoing cooling remains unstudied. We perform radiative, inviscid hydrodynamics simulations of clouds perturbed out of pressure equilibrium to explore the process of hydrodynamic fragmentation to molecular temperatures. We find molecular clouds larger than a critical size can shatter into a mist of tiny droplets, with the critical size deviating significantly from the atomic case. We find that cold clouds shatter only if the sound crossing time exceeds the local maximum of the cooling time ~8000 K. Moreover, we find evidence for a universal mechanism to 'shatter' cold clouds into a 'mist' of tiny droplets as a result of rotational fragmentation -- a process we dub 'splintering.' Our results have implications for resolving the molecular phase of the CGM in observations and cosmological simulations.Comment: 5 pages, 4 figures, submitted to MNRAS