Both absorption and emission line studies show that cold gas around galaxies is commonly outflowing at speeds of several hundred km s −1 . This observational fact poses a severe challenge to our theoretical models of galaxy evolution since most feedback mechanisms (e.g., supernovae feedback) accelerate hot gas, and the timescale it takes to accelerate a blob of cold gas via a hot wind is much larger than the time it takes to destroy the blob. We revisit this long-standing problem using three-dimensional hydrodynamical simulations with radiative cooling. Our results confirm previous findings, that cooling is often not efficient enough to prevent the destruction of cold gas. However, we also identify regions of parameter space where the cooling efficiency of the mixed, 'warm' gas is sufficiently large to contribute new comoving cold gas which can significantly exceed the original cold gas mass. This happens whenever, t cool,mix /t cc < 1, where t cool,mix is the cooling time of the mixed warm gas and t cc is the cloud-crushing time. This criterion is always satisfied for a large enough cloud. Cooling 'focuses' stripped material onto the tail where mixing takes place and new cold gas forms. A sufficiently large simulation domain is crucial to capturing this behavior.

We studied Lyman-α (Lyα) escape in a statistical sample of 43 Green Peas with HST/COS Lyα spectra. Green Peas are nearby star-forming galaxies with strong [OIII]λ5007 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 Lyα line emitters with rest-frame Lyα equivalent width > 20Å. The Lyα profiles of Green Peas are diverse. The Lyα escape fraction, defined as the ratio of observed Lyα flux to intrinsic Lyα flux, shows anti-correlations with a few Lyα kinematic features -both the blue peak and red peak velocities, the peak separations, and FWHM of the red portion of the Lyα profile. Using properties measured from SDSS optical spectra, we found many correlations -Lyα escape fraction generally increases at lower dust reddening, lower metallicity, lower stellar mass, and higher [OIII]/[OII] ratio. We fit their Lyα profiles with the HI shell radiative transfer model and found Lyα escape fraction anti-correlates with the best-fit N HI . Finally, we fit an empirical linear relation to predict f Lyα esc from the dust extinction and Lyα 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 Lyα escape and to probe the IGM optical depth along the line of sight of each z > 7 Lyα emission line galaxy in the JWST era.

We present VLT/X-Shooter and MUSE spectroscopy of a faint F814W = 28.60 ± 0.33 ( = -M 17.0 UV F ), consistent with a dust-free and young 20 Myr galaxy. Line ratios suggest an oxygen abundance 12 + log(O/H) < 7.8. We are witnessing an early episode of star formation in which a relatively low N H I and negligible dust attenuation might favor a leakage of ionizing radiation. This galaxy currently represents a unique low-luminosity reference object for future studies of the reionization epoch with the James Webb Space Telescope.

The escape of ionizing Lyman continuum (LyC) photons requires the existence of low-N H I sightlines, which also promote escape of Lyα. We use a suite of 2500 Lyα Monte-Carlo radiative transfer simulations through models of dusty, clumpy interstellar ("multiphase") media from Gronke & Dijkstra, and compare the escape fractions of Lyα ( a f esc Ly ) and LyC radiation ( f esc ion ). We find that f esc ion and a f esc Ly are correlated: galaxies with a low a f esc Ly consistently have a low f esc ion , while galaxies with a high a f esc Ly exhibit a large dispersion in f esc ion . We argue that there is increasing observational evidence that Lyα escapes more easily from UV-faint galaxies. The correlation between f esc ion and a f esc Ly then implies that UV-faint galaxies contribute more to the ionizing background than implied by the faint-end slope of the UV luminosity function. In multiphase gases, the ionizing escape fraction is most strongly affected by the cloud covering factor, f cl , which implies that f esc ion is closely connected to the observed Lyα spectral line shape. Specifically, LyC-emitting galaxies typically having narrower, more symmetric line profiles. This prediction is qualitatively similar to that for "shell models."

Outflows promote the escape of Lyman-α (Lyα) photons from dusty interstellar media. The process of radiative transfer through interstellar outflows is often modelled by a spherically symmetric, geometrically thin shell of neutral gas that scatters photons emitted by a central Lyα source. Despite its simplified geometry, this 'shell model' has been surprisingly successful at reproducing observed Lyα 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 HI ) and outflow velocity (v exp ). This work represents an important first step in determining how the shell model parameters relate to the actual physical properties of Lyα sources. To aid further exploration of the parameter space, we have made our simulated model spectra available through an interactive online tool.

We analyze archival Lyα spectra of 12 "Green Pea" galaxies observed with the Hubble Space Telescope, model their Lyα profiles with radiative transfer models, and explore the dependence of Lyα escape fraction on various properties. Green Pea galaxies are nearby compact starburst galaxies with [OIII]λ5007 equivalent widths of hundreds ofÅ. All 12 Green Pea galaxies in our sample show Lyα lines in emission, with a Lyα equivalent width distribution similar to high redshift Lyα emitters. Combining the optical and UV spectra of Green Pea galaxies, we estimate their Lyα escape fractions and find correlations between Lyα escape fraction and kinematic features of Lyα profiles. The escape fraction of Lyα in these galaxies ranges from 1.4% to 67%. We also find that the Lyα escape fraction depends strongly on metallicity and moderately on dust extinction. We compare their high-quality Lyα profiles with single HI shell radiative transfer models and find that the Lyα escape fraction anticorrelates with the derived HI column densities. Single shell models fit most Lyα profiles well, but not the ones with highest escape fractions of Lyα. Our results suggest that low HI column density and low metallicity are essential for Lyα escape, and make a galaxy a Lyα emitter.

The existence of fast moving, cold gas ubiquitously observed in galactic winds is theoretically puzzling, since the destruction time of cold gas is much smaller than its acceleration time. In Gronke & Oh (2018), we showed that cold gas can accelerate to wind speeds and grow in mass if the radiative cooling time of mixed gas is shorter than the cloud destruction time. Here, we study this process in much more detail, and find remarkably robust cloud acceleration and growth in a wide variety of scenarios. Radiative cooling, rather than the Kelvin-Helmholtz instability, enables self-sustaining entrainment of hot gas onto the cloud via cooling-induced pressure gradients. Indeed, growth peaks when the cloud is almost co-moving. The entrainment velocity is of order the cold gas sound speed, and growth is accompanied by cloud pulsations. Growth is also robust to the background wind and initial cloud geometry. In an adiabatic Chevalier-Clegg type wind, for instance, the mass growth rate is constant. Although growth rates are similar with magnetic fields, cloud morphology changes dramatically, with low density, magnetically supported filaments which have a small mass fraction but dominate by volume. This could bias absorption line observations. Cloud growth from entraining and cooling hot gas can potentially account for the cold gas content of the CGM. It can also fuel star formation in the disk as cold gas recycled in a galactic fountain accretes and cools halo gas. We speculate that galaxy-scale simulations should converge in cold gas mass once cloud column densities of N ∼ 10 18 cm −2 are resolved.1 Note that by Eq. (3), clouds which should survive by our criterion are nonetheless disrupted in the simulations of Armillotta et al. (2017). We attribute this to the difference between 2D and 3D simulations: disruption is easier in 2D and continues at cloud sizes which survive in 3D simulations (see § 5.5).

In spite of their conjectured importance for the Epoch of Reionization, the properties of low-mass galaxies are currently still very much under debate. In this article, we study the stellar and gaseous properties of faint, low-mass galaxies at z > 3. We observed the Frontier Fields cluster Abell S1063 with MUSE over a 2 arcmin 2 field, and combined integral-field spectroscopy with gravitational lensing to perform a blind search for intrinsically faint Lyα emitters (LAEs). We determined in total the redshift of 172 galaxies of which 14 are lensed LAEs at z = 3-6.1. We increased the number of spectroscopically-confirmed multiple-image families from 6 to 17 and updated our gravitational-lensing model accordingly. The lensing-corrected Lyα luminosities are with L Lyα 10 41.5 erg/s among the lowest for spectroscopically confirmed LAEs at any redshift. We used expanding gaseous shell models to fit the Lyα line profile, and find low column densities and expansion velocities. This is, to our knowledge, the first time that gaseous properties of such faint galaxies at z 3 are reported. We performed SED modelling to broadband photometry from the U band through the infrared to determine the stellar properties of these LAEs. The stellar masses are very low (10 6−8 M), and are accompanied by very young ages of 1-100 Myr. The very high specific star-formation rates (∼100 Gyr −1) are characteristic of starburst galaxies, and we find that most galaxies will double their stellar mass in 20 Myr. The UV-continuum slopes β are low in our sample, with β < −2 for all galaxies with M < 10 8 M. We conclude that our low-mass galaxies at 3 < z < 6 are forming stars at higher rates when correcting for stellar mass effects than seen locally or in more massive galaxies. The young stellar populations with high star-formation rates and low H i column densities lead to continuum slopes and LyC-escape fractions expected for a scenario where low mass galaxies reionise the Universe.