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Gas kinematics affect the radiative transfer and escape of hydrogen Lyman-$\alpha$ (Ly$\alpha$) emission from galaxies. We investigate this interplay empirically by relating the ionised gas kinematics of 42 galaxies in the extended Ly$\alpha$ Reference Sample with their Ly$\alpha$ escape fractions, $f_\rm{esc}$, Ly$\alpha$ equivalent widths, $\rm{EW}_\rm{Ly\alpha}$, and Ly$\alpha$ luminosities, $L_\rm{Ly\alpha}$. To this aim we use PMAS integral-field spectroscopic observations of the Balmer-$\alpha$ line. We calculate shearing velocities, $v_\rm{shear}$, and intrinsic velocity dispersions, $\sigma_0^\rm{obs}$ (empirically corrected for beam-smearing effects), as global kinematical measures for each galaxy. The sample is characterised by highly turbulent motions and more than half of the sample shows dispersion dominated kinematics. We uncover clear trends between Ly$\alpha$ observables and global kinematical statistics. We discuss statistically the importance of $v_\rm{shear}$, $\sigma_0^\rm{obs}$, and $v_\rm{shear}/\sigma_0^\rm{obs}$ for regulating the Ly$\alpha$ observables in comparison to other galaxy parameters. It emerges that $\sigma_0^\rm{obs}$ is the dominating parameter for $\rm{EW}_\rm{Ly\alpha}$ and that is as important as nebular extinction, gas covering fraction, and ionising photon production efficiency in regulating $f_\rm{esc}$. A simple scenario where the starburst age is simultaneously regulating turbulence, $\rm{EW}_\rm{Ly\alpha}$, and $f_\rm{esc}$ does not find support by our observations. However, we show that the small scale distribution of dust appears to be influenced by turbulence in some galaxies. In support of our observational result we discuss how turbulence is theoretically expected to play a significant role in modulating $f_\rm{esc}$. (abridged), Comment: Accepted version for publication in A&A

We report first results from a deep near infrared campaign with the Hubble Space Telescope to obtain late-epoch images of the Hubble Ultra-Deep Field (HUDF), 10-15 years after the first epoch data were obtained. The main objectives are to search for faint active galactic nuclei (AGN) at high redshifts by virtue of their photometric variability, and measure (or constrain) the comoving number density of supermassive black holes (SMBHs), n_{SMBH}, at early times. In this Letter we present an overview of the program and preliminary results concerning eight objects. Three variables are supernovae, two of which are apparently hostless with indeterminable redshifts, although one has previously been recorded at a z\approx 6 object precisely because of its transient nature. Two further objects are clear AGN at z= 2.0 and 3.2, based on morphology and/or infrared spectroscopy from JWST. Three variable targets are identified at z = 6-7, which are also likely AGN candidates. These sources provide a first measure of n_{SMBH} in the reionization epoch by photometric variability, which places a firm lower limit of 3 \times 10^{-4} cMpc^{-3}. After accounting for variability and luminosity incompleteness, we estimate n_{SMBH} \gtrsim 8 \times 10{-3} cMpc{-3}, which is the largest value so far reported at these redshifts. This SMBH abundance is also strikingly similar to estimates of n_{SMBH} in the local Universe. We discuss how these results test various theories for SMBH formation., Comment: In press at ApJL

Lyman $\alpha$ (Ly$\alpha$) is now routinely used as a tool for studying high-redshift galaxies and its resonant nature means it can trace neutral hydrogen around star-forming galaxies. Integral field spectrograph measurements of high-redshift Ly$\alpha$ emitters indicate that significant extended Ly$\alpha$ halo emission is ubiquitous around such objects. We present a sample of redshift 0.23 to 0.31 galaxies observed with the Hubble Space Telescope selected to match the star formation properties of high-$z$ samples while optimizing the observations for detection of low surface brightness Ly$\alpha$ emission. The Ly$\alpha$ escape fractions range between 0.7\% and 37\%, and we detect extended Ly$\alpha$ emission around six out of seven targets. We find Ly$\alpha$ halo to UV scale length ratios around 6:1 which is marginally lower than high-redshift observations, and halo flux fractions between 60\% and 85\% -- consistent with high-redshift observations -- when using comparable methods. However, our targets show additional extended stellar UV emission: we parametrize this with a new double exponential model. We find that this parametrization does not strongly affect the observed Ly$\alpha$ halo fractions. We find that deeper H$\alpha$ data would be required to firmly determine the origin of Ly$\alpha$ halo emission, however, there are indications that H$\alpha$ is more extended than the central FUV profile, potentially indicating conditions favorable for the escape of ionizing radiation. We discuss our results in the context of high-redshift galaxies, cosmological simulations, evolutionary studies of the circumgalactic medium in emission, and the emission of ionizing radiation., Comment: 20 page, 14 figures, 6 tables. Accepted for publication in MNRAS

We present Ly $\alpha$ imaging of 45 low redshift star-forming galaxies observed with the Hubble Space Telescope. The galaxies have been selected to have moderate to high star formation rates using far-ultraviolet (FUV) luminosity and \ha equivalent width criteria, but no constraints on Ly $\alpha$ luminosity. We employ a pixel stellar continuum fitting code to obtain accurate continuum subtracted Ly $\alpha$, H $\alpha$ and H $\beta$ maps. We find that Ly $\alpha$ is less concentrated than FUV and optical line emission in almost all galaxies with significant Ly $\alpha$ emission. We present global measurements of Ly $\alpha$ and other quantities measured in apertures designed to capture all of the Ly $\alpha$ emission. We then show how the escape fraction of Ly $\alpha$ relates to a number of other measured quantities (mass, metallicity, star formation, ionisation parameter, and extinction). We find that the escape fraction is strongly anti-correlated with nebular and stellar extinction, weakly anti-correlated with stellar mass, but no conclusive evidence for correlations with other quantities. We show that Ly $\alpha$ escape fractions are inconsistent with common dust extinction laws, and discuss how a combination of radiative transfer effects and clumpy dust models can help resolve the discrepancies. We present a star formation rate calibration based on Ly $\alpha$ luminosity, where the equivalent width of Ly $\alpha$ is used to correct for non-unity escape fraction, and show that this relation provides a reasonably accurate SFR estimate. We also show stacked growth curves of Ly $\alpha$ for the galaxies that can be used to find aperture loss fractions at a given physical radius., Comment: 48 pages, 21 figures, accepted for publication in The Astrophysical Journal Supplement Series. Multipanel figures for all 45 galaxies will be available in the online version of the published paper (and here: https://stockholmuniversity.box.com/s/ghvnvjyhxts64hpbqh90uwcj5dlczlhm ). Data products will be available as a MAST HLSP soon (placeholder DOI: 10.17909/pbe1-m743)

We demonstrate how the stellar and nebular conditions in star-forming galaxies modulate the emission and spectral profile of HI Lya emission line. We examine the net Lya output, kinematics, and in particular emission of blue-shifted Lya radiation, using spectroscopy from with the Cosmic Origins Spectrograph on HST, giving a sample of 87 galaxies at redshift z=0.05-0.44. We contrast the Lya spectral measurements with properties of the ionized gas (from optical spectra) and stars (from stellar modeling). We demonstrate correlations of unprecedented strength between the Lya escape fraction (and equivalent width) and the ionization parameter (p~10^-15). The relative contribution of blue-shifted emission to the total Lya also increases from ~0 to ~40% over the range of O_32 ratios (p~10^-6). We also find particularly strong correlations with estimators of stellar age and nebular abundance, and weaker correlations regarding thermodynamic variables. Low ionization stage absorption lines suggest the Lya emission and line profile are predominantly governed by the column of absorbing gas near zero velocity. Simultaneous multi-parametric analysis over many variables shows we can predict 80% of the variance on Lya luminosity, and ~50% on the EW. We determine the most crucial predictive variables, finding that for tracers of the ionization state and Hb luminosity dominate the luminosity prediction whereas the Lya EW is best predicted by Hb EW and the Ha/Hb ratio. We discuss our results with reference to high redshift observations, focussing upon the use of Lya to probe the nebular conditions in high-z galaxies and cosmic reionization., Comment: Accepted by MNRAS. 27 pages and 19 figures. ~90 online-only Figures are available here: https://ttt.astro.su.se/~mhaye/t/onlineonly_resub.pdf

Star formation is a clustered process that regulates the structure and evolution of galaxies. We investigate this process in the dwarf galaxy Haro 11, forming stars in three knots (A, B, C). The exquisite resolution of HST imaging allows us to resolve the starburst into tens of bright star clusters. We derive masses between $10^5$ and $10^7\,\rm M_{\odot}$ and ages younger than 20 Myr, using photometric modeling. We observe that the clustered star formation has propagated from knot C (the oldest) through knot A (in between) towards knot B (the youngest). We use aperture-matched ultraviolet and optical spectroscopy (HST + MUSE) to independently study the stellar populations of Haro 11 and determine the physical properties of the stellar populations and their feedback in 1 kpc diameter regions. We discuss these results in light of the properties of the ionised gas within the knots. We interpret the broad blue-shifted components of the optical emission lines as outflowing gas ($v_{max} \sim 400$ km/s). The strongest outflow is detected in knot A with a mass-rate of $\dot{M}_{out}\sim 10\,\rm M_{\odot}/yr$, ten times higher than the star-formation in the same region. Knot B hosts a young and not fully developed outflow, whereas knot C has likely been already evacuated. Because Haro 11 has properties similar to high-redshift unresolved galaxies, our work can additionally aid the understanding of star formation at high redshift, a window that will be opened by upcoming facilities., Comment: Accepted for publication in MNRAS on 22nd Dec 2022: 18 pages, 9 figures

We use Hubble space telescope data of 45 nearby star-forming galaxies to investigate properties of Lyman-alpha (Ly$\alpha$) halos, Ly$\alpha$ morphology, and the star-forming characteristics of galaxies. We study how the morphology of Ly$\alpha$ emission is related to other Ly$\alpha$ observables. Furthermore, we study the interdependencies of Ly$\alpha$ morphological quantities. We studied the spatial extent of Ly$\alpha$ using surface brightness profiles in i) using circular apertures and ii) within faint Ly$\alpha$ isophotes. We measured the average intensity and the size of the regions with a high star formation rate density. The morphology of the galaxies was quantified by computing centroid position, axis ratio, and position angle in the Ly$\alpha$, ultraviolet continuum, and I band maps. We found that galaxies with more extended star-forming regions possess larger Ly$\alpha$ halos. Furthermore, galaxies with more elongated Ly$\alpha$ morphology are also more extended in Ly$\alpha$. Our data suggest that Ly$\alpha$ bright galaxies appear rounder in their Ly$\alpha$ morphology, and there is less of a contribution from their Ly$\alpha$ halo to their overall luminosity. We compared our results with studies at high redshift and found that whilst the Ly$\alpha$ extent in the inner regions of the galaxies in our sample are similar to the high$-z$ Ly$\alpha$ emitters (LAEs), Ly$\alpha$ halos are more extended in high$-z$ LAEs. Our analysis suggests that the Ly$\alpha$ morphology affects the measurement of other observable quantities concerning Ly$\alpha$ emission, and some of the conclusions drawn from high redshift LAEs might be biased towards galaxies with specific Ly$\alpha$ shapes. In particular, faint Ly$\alpha$ emitters have larger Ly$\alpha$ scale lengths and halo fractions. This implies that faint Ly$\alpha$ emitters are harder to detect at high redshift than previously believed.

Understanding the escape of ionizing (Lyman continuum) photons from galaxies is vital for determining how galaxies contributed to reionization in the early universe. While directly detecting Lyman continuum from high redshift galaxies is impossible due to the intergalactic medium, low redshift galaxies in principle offer this possibility, but requirie observations from space. The first local galaxy for which Lyman continuum escape was found is Haro11 , a luminous blue compact galaxy at z=0.02, where observations with the FUSE satellite revealed an escape fraction of 3.3 %. However the FUSE aperture covers the entire galaxy, and it is not clear from where the Lyman continuum is leaking out. Here we utilize HST/COS spectroscopy in the wavelength range 1100-1700 A of the three knots (A, B, and C) of Haro11 to study the presence of Ly-$\alpha$ emission and the properties of intervening gas. We find that all knots have bright Ly-$\alpha$ emission. UV absorption lines, originating in the neutral interstellar medium, as well as lines probing the ionized medium, are seen extending to blue shifted velocities of 500 km/s in all three knots, demonstrating the presence of an outflowing multiphase medium. We find that knots A and B have large covering fractions of neutral gas, making LyC escape along these sightlines improbable, while knot C has a much lower covering fraction ($\lesssim50$%). Knot C also has the the highest Ly-$\alpha$ escape fraction and we conclude that it is the most likely source of the escaping Lyman continuum detected in Haro11., Comment: 19 pages, 7 figures, accepted for publication in ApJ

Lyman $\alpha$ (Ly$\alpha$) emission from star-forming galaxies is an important tool to study a large range of astrophysical questions: it has the potential to carry information about the source galaxy, its nearby circumgalactic medium, and also the surrounding intergalactic medium. Much observational and theoretical work has therefore focused on understanding the details of this emission line. These efforts have been hampered, however, by an absence of spectroscopic reference samples that can be used both as comparisons for observational studies and as critical tests for theoretical work. For this reason we have compiled a large sample of Ly$\alpha$ spectra, at both low and high redshift, and created a publicly available online database, at http://lasd.lyman-alpha.com. The Lyman alpha Spectral Database (LASD) hosts these spectra, as well as large set of spectral and kinematic quantities that have been homogeneously measured for the entire sample. These can easily be viewed online and downloaded in tabular form. The LASD has the capacity for users to easily upload their own Ly$\alpha$ spectra, and all the same spectral measurements will be made, reported, and ingested into the database. We actively invite the community to do so, and the LASD is intended to be a long-term community resource. In this paper we present the design of the database as well as descriptions of the underlying algorithms and the initial Ly$\alpha$ emitter samples that are in the database., Comment: Published in PASP. The LASD is available at http://lasd.lyman-alpha.com

We present the Hubble imaging Probe of Extreme Environments and Clusters (HiPEEC) survey. We fit HST NUV to NIR broadband and H$\alpha$ fluxes, to derive star cluster ages, masses, extinctions and determine the star formation rate (SFR) of 6 merging galaxies. These systems are excellent laboratories to trace cluster formation under extreme gas physical conditions, rare in the local universe, but typical for star-forming galaxies at cosmic noon. We detect clusters with ages of 1-500 Myr and masses that exceed $10^7$ M$_\odot$. The recent cluster formation history and their distribution within the host galaxies suggest that systems like NGC34, NGC1614, NGC4194 are close to their final coalescing phase, while NGC3256, NGC3690, NGC6052 are at an earlier/intermediate stage. A Bayesian analysis of the cluster mass function in the age interval 1-100 Myr provides strong evidence in 4 of the 6 galaxies that an exponentially truncated power law better describes the observed mass distributions. For two galaxies, the fits are inconclusive due to low number statistics. We determine power-law slopes $\beta \sim-1.5$ to $-2.0$, and truncation masses, M$_c$, between $10^6$ and a few times $10^7$ M$_\odot$, among the highest values reported in the literature. Advanced mergers have higher M$_c$ than early/intermediate merger stage galaxies, suggesting rapid changes in the dense gas conditions during the merger. We compare the total stellar mass in clusters to the SFR of the galaxy, finding that these systems are among the most efficient environments to form star clusters in the local universe., Comment: Accepted for publication in MNRAS; data and catalogues will be released at this URL http://dx.doi.org/10.17909/t9-cn0b-ht83

We demonstrate the redshift-evolution of the spectral profile of H i Lyman-alpha (Ly{\alpha}) emission from star-forming galaxies. In this first study we pay special attention to the contribution of blueshifted emission. At redshift z = 2.9-6.6, we compile spectra of a sample of 229 Ly{\alpha}-selected galaxies identified with the Multi-Unit Spectroscopic Explorer at the Very Large Telescope, while at low-z (< 0.44) we use a sample of 74 ultraviolet-selected galaxies observed with the Cosmic Origin Spectrograph onboard the Hubble Space Telescope. At low-z, where absorption from the intergalactic medium (IGM) is negligible, we show that the ratio of Ly{\alpha} luminosity bluewards and redwards of line center (L_B/R) increases rapidly with increasing equivalent width (W Ly{\alpha}). This correlation does not, however, emerge at z = 3-4, and we use bootstrap simulations to demonstrate that trends in L B/R should be suppressed by variations in IGM absorption. Our main result is that the observed blueshifted contribution evolves rapidly downwards with increasing redshift: L_B/R~30 % at z~0, but drops to 15 % at z~3, and to below 3 % by z~6. Applying further simulations of the IGM absorption to the unabsorbed COS spectrum, we demonstrate that this decrease in the blue-wing contribution can be entirely attributed to the thickening of intervening Ly{\alpha} absorbing systems, with no need for additional H i opacity from local structure, companion galaxies, or cosmic infall. We discuss our results in light of the numerical radiative transfer simulations, the evolving total Ly{\alpha} and ionizing output of galaxies, and the utility of resolved Ly{\alpha} spectra in the reionization epoch., Comment: 22 pages, 9 figures, in press at the ApJ

Understanding the production and escape of Lyman $\alpha$ (Ly$\alpha$) radiation from star-forming galaxies is a long standing problem in astrophysics. The ability to predict the Ly$\alpha$ luminosity of galaxies would open up new ways of exploring the Epoch of Reionization (EoR), and to estimate Ly$\alpha$ emission from galaxies in cosmological simulations where radiative transfer calculations cannot be done. We apply multivariate regression methods to the Lyman Alpha Reference Sample dataset to obtain a relation between the galaxy properties and the emitted Ly$\alpha$. The derived relation predicts the Ly$\alpha$ luminosity of our galaxy sample to good accuracy, regardless of whether we consider only direct observables (root-mean-square (RMS) dispersion around the relation of $\sim 0.19$ dex) or derived physical quantities (RMS $\sim 0.27$ dex). We confirm the predictive ability on a separate sample of compact star-forming galaxies and find that the prediction works well, but that aperture effects on measured Ly$\alpha$ luminosity may be important, depending on the redshift of the galaxy. We apply statistical feature selection techniques to determine an order of importance of the variables in our dataset, enabling future observations to be optimized for predictive ability. When using physical variables, we are able to determine that the most important predictive parameters are, in order, star formation rate, dust extinction, compactness and the gas covering fraction. We discuss the application of our results in terms of studying the EoR and intensity mapping experiments., Comment: Accepted for publication in ApJ. 23 pages, 6 figures

We report the large effort which is producing comprehensive high-level young star cluster (YSC) catalogues for a significant fraction of galaxies observed with the Legacy ExtraGalactic UV Survey (LEGUS) Hubble treasury program. We present the methodology developed to extract cluster positions, verify their genuine nature, produce multiband photometry (from NUV to NIR), and derive their physical properties via spectral energy distribution fitting analyses. We use the nearby spiral galaxy NGC628 as a test case for demonstrating the impact that LEGUS will have on our understanding of the formation and evolution of YSCs and compact stellar associations within their host galaxy. Our analysis of the cluster luminosity function from the UV to the NIR finds a steepening at the bright end and at all wavelengths suggesting a dearth of luminous clusters. The cluster mass function of NGC628 is consistent with a power-law distribution of slopes $\sim -2$ and a truncation of a few times $10^5$ M$_\odot$. After their formation YSCs and compact associations follow different evolutionary paths. YSCs survive for a longer timeframe, confirming their being potentially bound systems. Associations disappear on time scales comparable to hierarchically organized star-forming regions, suggesting that they are expanding systems. We find mass-independent cluster disruption in the inner region of NGC628, while in the outer part of the galaxy there is little or no disruption. We observe faster disruption rates for low mass ($\leq$ $10^4$ M$_\odot$) clusters suggesting that a mass-dependent component is necessary to fully describe the YSC disruption process in NGC628., Comment: accepted for publication in ApJ; 27 pages; complete stellar cluster catalogues will be released in June 2017 at https://legus.stsci.edu