We report upon new results regarding the Lyα output of galaxies, derived from the Lyman alpha Reference Sample (LARS), focusing on Hubble Space Telescope imaging. For 14 galaxies we present intensity images in Lyα, Hα, and UV, and maps of Hα/Hβ, Lyα equivalent width (EW), and Lyα/Hα. We present Lyα and UV light profiles and show they are well-fitted by Sérsic profiles, but Lyα profiles show indices systematically lower than those of the UV (n ≈ 1 − 2 instead of 4). This reveals a general lack of the central concentration in Lyα that is ubiquitous in the UV. Photometric growth curves increase more slowly for Lyα than the FUV, showing that small apertures may underestimate the EW. For most galaxies, however, flux and EW curves flatten by radii ≈ 10 kpc, suggesting that if placed at high-z , only a few of our galaxies would suffer from large flux losses. We compute global properties of the sample in large apertures, and show total luminosities to be independent of all other quantities. Normalized Lyα throughput, however, shows significant correlations: escape is found to be higher in galaxies of lower star formation rate, dust content, mass, and several quantities that suggest harder ionizing continuum and lower metallicity. Eight galaxies could be selected as high-z Lyα emitters, based upon their luminosity and EW. We discuss the results in the context of high-z Lyα and UV samples. A few galaxies have EWs above 50Å, and one shows f Lyα esc of 80%; such objects have not previously been reported at low-z.

The Magellanic Stream and Leading Arm of HI that stretches from the Large and Small Magellanic Clouds (LMC and SMC) and over 200 • of the Southern sky is thought to be formed from multiple encounters between the LMC and SMC. In this scenario, most of the gas in the Stream and Leading Arm is stripped from the SMC, yet recent observations have shown a bifurcation of the Trailing Arm that reveals LMC origins for some of the gas. Absorption measurements in the Stream also reveal an order of magnitude more gas than in current tidal models. We present hydrodynamical simulations of the multiple encounters between the LMC and SMC at their first pass around the Milky Way, assuming that the Clouds were more extended and gas rich in the past. Our models create filamentary structures of gas in the Trailing Stream from both the LMC and SMC. While the SMC trailing filament matches the observed Stream location, the LMC filament is offset. In addition, the total observed mass of the Stream in these models is underestimated of a factor of four when the ionized component is accounted for. Our results suggest that there should also be gas stripped from both the LMC and SMC in the Leading Arm, mirroring the bifurcation in the Trailing Stream. This prediction is consistent with recent measurements of spatial variation in chemical abundances in the Leading Arm, which show that gas from multiple sources is present, although the nature is still uncertain.

We present integral field spectroscopic observations with the Potsdam Multi-Aperture Spectrophotometer of all 14 galaxies in the z ∼ 0.1 Lyman Alpha Reference Sample (LARS). We produce 2D line-of-sight velocity maps and velocity dispersion maps from the Balmer α (Hα) emission in our data cubes. These maps trace the spectral and spatial properties of the LARS galaxies' intrinsic Lyα radiation field. We show our kinematic maps that are spatially registered onto the Hubble Space Telescope Hα and Lyman α (Lyα) images. We can conjecture a causal connection between spatially resolved Hα kinematics and Lyα photometry for individual galaxies, however, no general trend can be established for the whole sample. Furthermore, we compute the intrinsic velocity dispersion σ 0 , the shearing velocity v shear , and the v shear /σ 0 ratio from our kinematic maps. In general LARS galaxies are characterised by high intrinsic velocity dispersions (54 km s −1 median) and low shearing velocities (65 km s −1 median). The v shear /σ 0 values range from 0.5 to 3.2 with an average of 1.5. It is noteworthy that five galaxies of the sample are dispersion-dominated systems with v shear /σ 0 < 1, and are thus kinematically similar to turbulent star-forming galaxies seen at high redshift. When linking our kinematical statistics to the global LARS Lyα properties, we find that dispersion-dominated systems show higher Lyα equivalent widths and higher Lyα escape fractions than systems with v shear /σ 0 > 1. Our result indicates that turbulence in actively star-forming systems is causally connected to interstellar medium conditions that favour an escape of Lyα radiation.

The Lyman-alpha reference sample (LARS) is a substantial program with the Hubble Space Telescope (HST) that provides a sample of local universe laboratory galaxies in which to study the detailed astrophysics of the visibility and strength of the Lyman-alpha (Lyα) line of neutral hydrogen. Lyα is the dominant spectral line in use for characterizing high redshift (z) galaxies. This article presents an overview of the survey, its selection function and HST imaging observations. The sample was selected from the combined GALEX+SDSS catalogue at z = 0.028−0.19, in order to allow Lyα to be captured with combinations of long pass filters in the Solar Blind Channel (SBC) of the Advanced Camera for Surveys (ACS) on board HST. In addition, LARS utilises Hα and Hβ narrow, and u, b, i broad-band imaging with ACS and the Wide Field Camera 3 (WFC3). In order to study galaxies in which large numbers of Lyα photons are produced (whether or not they escape) we demanded an Hα equivalent width W(Hα)≥ 100Å. The final sample of 14 galaxies covers far UV (FUV, λ ∼ 1500Å) luminosities that overlaps with those of high-z Lyα emitters and Lyman Break Galaxies (LBGs), making LARS a valid comparison sample. We present the reduction steps used to obtain the Lyα images, including our LARS eXtraction software (LaXs) which utilises pixel-by-pixel spectral synthesis fitting of the energy distribution to determine and subtract the continuum at Lyα. We demonstrate that the use of SBC long pass filter combinations increase the signal to noise with an order of magnitude compared to the nominal Lyα filter available in SBC. To exemplify the science potential of LARS, we also present some first results for a single galaxy, Mrk 259 (LARS #1). This irregular galaxy shows bright extended (indicative of resonance scattering), but strongly asymmetric Lyα emission. Spectroscopy from HST/COS centered on the brightest UV knot show a moderate outflow in the neutral interstellar medium (probed by low ionization stage absorption features), and Lyα emission with an asymmetric profile. Radiative transfer modeling is able to reproduce the essential features of the Lyα line profile, and confirms the presence of an outflow. From the integrated photometry we measure a Lyα luminosity of L Lyα = 1.3 × 10 42 erg/s an equivalent width W(Lyα)=45Å and a far UV absolute magnitude M FUV = −19.2 (AB). Mrk 259 would hence be detectable in high-z Lyα and LBG surveys. The total Lyα escape fraction is 12 %. This number is higher than the low-z average, but similar to that at z > 4 demonstrating that LARS provides a valid comparison sample for high-z galaxy studies.