arXiv subjects

Daniel Kunth

Publications and source records attributed to Daniel Kunth.

At least 19 recordsLinked to original sources

arXiv2024

Haro 11: The Spatially Resolved Lyman Continuum Sources

As the nearest confirmed Lyman continuum (LyC) emitter, Haro 11 is an exceptional laboratory for studying LyC escape processes crucial to cosmic reionization. Our new HST/COS G130M/1055 observations of its three star-forming knots now reveal that the observed LyC originates in Knots B and C, with $903 - 912~\r{A}$ luminosities of $1.9\pm1.5 \times 10^{40}~\rm erg~s^{-1}$ and $0.9\pm0.7 \times 10^{40}~\rm erg~s^{-1}$, respectively. We derive local escape fractions $f_{\rm{esc, 912}} = 3.4\pm2.9\%$ and $5.1\pm4.3\%$ for Knots B and C, respectively. Our Starburst99 modeling shows dominant populations on the order of $\sim1-4$ Myr and $1-2\times10^7 \rm~M_\odot$ in each knot, with the youngest population in Knot B. Thus, the knot with the strongest LyC detection has the highest LyC production. However, LyC escape is likely less efficient in Knot B than in Knot C due to higher neutral gas covering. Our results therefore stress the importance of the intrinsic ionizing luminosity, and not just the escape fraction, for LyC detection. Similarly, the Ly$\alpha$ escape fraction does not consistently correlate with LyC flux, nor do narrow Ly$\alpha$ red peaks. High observed Ly$\alpha$ luminosity and low Ly$\alpha$ peak velocity separation, however, do correlate with higher LyC escape. Another insight comes from the undetected Knot A, which drives the Green Pea properties of Haro 11. Its density-bounded conditions suggest highly anisotropic LyC escape. Finally, both of the LyC-leaking Knots, B and C, host ultra-luminous X-ray sources (ULXs). While stars strongly dominate over the ULXs in LyC emission, this intriguing coincidence underscores the importance of unveiling the role of accretors in LyC escape and reionization.

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arXiv2023

The Lyman $\alpha$ Reference Sample XIV: Lyman $\alpha$ imaging of 45 low redshift star-forming galaxies and inferences on global emission

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.

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arXiv2021

The Lyman Alpha Reference Sample XII: Morphology of extended Lyman alpha emission in star-forming galaxies

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.

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arXiv2021

The source of leaking ionizing photons from Haro11 -- Clues from HST/COS spectroscopy of knots A, B and C

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.

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arXiv2020

Lyman Alpha Reference Sample: X. Predicting Lyman alpha output from starforming galaxies using multivariate regression

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.

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arXiv2018

The Lyman Alpha Reference Sample IX: Revelations from deep surface photometry

The Lyman Alpha Reference Sample (LARS) of 14 star-forming galaxies offers a wealth of insight into the workings of these local analogs to high-redshift star-forming galaxies. The sample has been well-studied in terms of LyA and other emission line properties, such as HI mass, gas kinematics, and morphology. We analyze deep surface photometry of the LARS sample in UBIK broadband imaging obtained at the Nordic Optical Telescope and the Canada-France-Hawaii Telescope, and juxtaposition their derived properties with a sample of local high-redshift galaxy analogs, namely, with blue compact galaxies (BCGs). We construct radial surface brightness and color profiles with both elliptical and isophotal integration, as well as RGB images, deep contours, color maps, a burst fraction estimate, and a radial mass-to-light ratio profile for each LARS galaxy. Standard morphological parameters like asymmetry, clumpiness, the Gini and M20 coefficients are [...] analyzed, as well as isophotal asymmetry profiles for each galaxy. [...] We compare the LARS to the properties of the BCG sample and highlight the differences. Several diagnostics indicate that the LARS galaxies have highly disturbed morphologies even at the level of the faintest isophotes [...]. The ground-based photometry [...] reveals previously unexplored isophotes [...]. The burst fraction estimate suggests a spatially more extended burst region in LARS than in the BCGs. [...] The galaxies in the LARS sample appear to be in earlier stages of a merger event compared to the BCGs. Standard morphological diagnostics like asymmetry, clumpiness, Gini and M20 coefficients cannot separate the two samples, although an isophotal asymmetry profile successfully captures the average difference in morphology. These morphological diagnostics do not show any correlation with the equivalent width or the escape fraction of Lyman Alpha. [abridged]

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arXiv2017

The Lyman Alpha Reference Sample. VIII. Characterizing Lyman-Alpha Scattering in Nearby Galaxies

We examine the dust geometry and Lyα scattering in the galaxies of the Lyman Alpha Reference Sample (LARS), a set of 14 nearby (0.02 < $z$ < 0.2) Lyα emitting and starbursting systems with Hubble Space Telescope Lyα, Hα, and Hβ imaging. We find that the global dust properties determined by line ratios are consistent with other studies, with some of the LARS galaxies exhibiting clumpy dust media while others of them show significantly lower Lyα emission compared to their Balmer decrement. With the LARS imaging, we present Lyα/Hα and Hα/Hβ maps with spatial resolutions as low as $\sim$ 40 pc, and use these data to show that in most galaxies, the dust geometry is best modeled by three distinct regions: a central core where dust acts as a screen, an annulus where dust is distributed in clumps, and an outer envelope where Lyα photons only scatter. We show that the dust that affects the escape of Lyα is more restricted to the galaxies' central regions, while the larger Lyα halos are generated by scattering at large radii. We present an empirical modeling technique to quantify how much Lyα scatters in the halo, and find that this "characteristic" scattering distance correlates with the measured size of the Lyα halo. We note that there exists a slight anti-correlation between the scattering distance of Lyα and global dust properties.

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arXiv2015

The Lyman alpha Reference Sample VI: Lyman alpha escape from the edge-on disk galaxy Mrk1486

While numerical simulations suggest that the strength of the Lyman alpha (Lya) line of star-forming disk galaxies strongly depends on the inclination at which they are observed (i.e. from edge-on to face-on, we expect to see a change from an attenuated Lya line to a strong Lya emission line), recent observations with the Hubble space telescope (HST) have highlighted few low-redshift highly inclined (edge-on) disk galaxies that breaks this trend. We aim to understand how a strong Lya emission line is able to escape from one of those inclined disk galaxies, named Mrk1486 (z=0.0338). For that purpose we used a large set of HST imaging and spectroscopic data to investigate both the ISM structure and the dominant source of Lya radiation inside Mrk1486. Moreover, we used a 3D Monte Carlo Lya radiation transfer code to study the radiative transfer of Lya and UV continuum photons inside a 3D geometry of neutral hydrogen (HI) and dust that models the ISM structure at the galaxy center. The analysis of IFU Halpha spectroscopic data of Mrk1486 indicates the presence of two bipolar galactic winds of HI gas above and bellow the disk plane of Mrk1486. Furthermore, comparing different diagnostic diagrams (such as [OIII]5007/Hbeta versus [OI]6300/Halpha) to photo- and shock-ionization models, we find that the Lya production of Mrk1486 is dominated by photoionization inside the galaxy disk. From this perspective, our numerical simulations succeed in reproducing the strength and spectral shape of the observed Lya line of Mrk1486 by assuming a scenario in which the Lya photons are produced inside the disk, travel along the galactic winds and scatter on cool HI materials toward the observer. As bipolar galactic winds are ubiquitous in star-forming disk galaxies, this mechanism may explain the origin of strong Lya emission lines recently observed from highly inclined galaxies at high-redshift.

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arXiv2015

The Lyman alpha reference sample. VII. Spatially resolved H$α$ kinematics

We present integral field spectroscopic observations with the Potsdam Multi Aperture Spectrophotometer of all 14 galaxies in the $z\sim 0.1$ Lyman Alpha Reference Sample (LARS). We produce 2D line of sight velocity maps and velocity dispersion maps from the Balmer $α$ (H$α$) emission in our data cubes. These maps trace the spectral and spatial properties of the LARS galaxies' intrinsic Ly$α$ radiation field. We show our kinematic maps spatially registered onto the Hubble Space Telescope H$α$ and Lyman $α$ (Ly$α$) images. Only for individual galaxies a causal connection between spatially resolved H$α$ kinematics and Ly$α$ photometry can be conjectured. However, no general trend can be established for the whole sample. Furthermore, we compute non-parametric global kinematical statistics -- intrinsic velocity dispersion $σ_0$, shearing velocity $v_\mathrm{shear}$, and the $v_\mathrm{shear}/σ_0$ 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). $v_\mathrm{shear}/σ_0$ values range from 0.5 to 3.2 with an average of 1.5. Noteworthy, five galaxies of the sample are dispersion dominated systems with $v_\mathrm{shear}/σ_0 <1$ and are thus kinematically similar to turbulent star forming galaxies seen at high redshift. When linking our kinematical statistics to the global LARS Ly$α$ properties, we find that dispersion dominated systems show higher Ly$α$ equivalent widths and higher Ly$α$ escape fractions than systems with $v_\mathrm{shear}/σ_0 > 1$. Our result indicates that turbulence in actively star-forming systems is causally connected to interstellar medium conditions that favour an escape of Ly$α$ radiation.

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arXiv2015

PopIII-star siblings in IZw18 and WRs in metal-poor galaxies unveiled from integral field spectroscopy

Here we highlight our recent results from the IFS study of Mrk178, the closest metal-poor WR galaxy, and of IZw18, the most metal-poor star-forming galaxy known in the local Universe. The IFS data of Mrk178 show the importance of aperture effects on the search for WR features, and the extent to which physical variations in the ISM properties can be detected. Our IFS data of IZw18 reveal its entire nebular HeII4686-emitting region, and indicate for the first time that peculiar, very hot (nearly) metal-free ionizing stars (called here PopIII-star siblings) might hold the key to the HeII-ionization in IZw18.

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arXiv2015

The Lyman Alpha Reference Sample: V. The impact of neutral ISM kinematics and geometry on Lyman Alpha escape

We present high-resolution far-UV spectroscopy of the 14 galaxies of the Lyman Alpha Reference Sample; a sample of strongly star-forming galaxies at low redshifts ($0.028 < z < 0.18$). We compare the derived properties to global properties derived from multi band imaging and 21 cm HI interferometry and single dish observations, as well as archival optical SDSS spectra. Besides the Lyman $α$ line, the spectra contain a number of metal absorption features allowing us to probe the kinematics of the neutral ISM and evaluate the optical depth and and covering fraction of the neutral medium as a function of line-of-sight velocity. Furthermore, we show how this, in combination with precise determination of systemic velocity and good Ly$α$ spectra, can be used to distinguish a model in which separate clumps together fully cover the background source, from the "picket fence" model named by Heckman et al. (2011). We find that no one single effect dominates in governing Ly$α$ radiative transfer and escape. Ly$α$ escape in our sample coincides with a maximum velocity-binned covering fraction of $\lesssim 0.9$ and bulk outflow velocities of $\gtrsim 50$ km s$^{-1}$, although a number of galaxies show these characteristics and yet little or no Ly$α$ escape. We find that Ly$α$ peak velocities, where available, are not consistent with a strong backscattered component, but rather with a simpler model of an intrinsic emission line overlaid by a blueshifted absorption profile from the outflowing wind. Finally, we find a strong anticorrelation between H$α$ equivalent width and maximum velocity-binned covering factor, and propose a heuristic explanatory model.

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arXiv2014

The Lyman-alpha Reference Sample: I. Survey outline and first results for Markarian 259

The Lyman-alpha reference sample (LARS) is a program with the Hubble Space Telescope (HST) that provides a sample of local universe laboratory galaxies in which to study the astrophysics of the visibility and strength of the Lyman-alpha (Lya) line of hydrogen. This article presents an overview of the survey, its selection function and HST imaging observations. The sample was selected from the GALEX+SDSS catalogue at z=0.028-0.19, in order to allow Lya to be captured with combinations of long pass filters in the Solar Blind Channel (SBC) of HST/ACS. In addition, LARS utilises Halpha and Hbeta narrow, and U, B, i broad-band imaging with ACS and WFC3. In order to study galaxies in which large numbers of Lya photons are produced we demanded an Halpha equivalent width > 100Å. The sample of 14 galaxies covers far UV (FUV) luminosities that overlaps with those of high-z Lya emitters and Lyman Break Galaxies (LBGs). We present the reduction steps used to obtain the Lya images, including our LARS eXtraction software (LaXs) which utilises pixel-by-pixel spectral synthesis fitting to subtract the continuum at Lya. We demonstrate that the use of SBC long pass filters dramatically increase the signal to noise compared to the nominal Lya filter in SBC. To exemplify, we present results for Mrk 259. This irregular galaxy shows extended but strongly asymmetric Lya emission. Spectroscopy from HST/COS show a moderate outflow in the neutral ISM and an asymmetric Lya profile. Radiative transfer modeling is able to reproduce the Lya line profile, confirming the presence of an outflow. The integrated photometry provides a Lya luminosity of 1.3E42 erg/s an equivalent width W(Lya) =45Å and a FUV absolute magnitude -19.2. Mrk 259 would hence be detectable in high-z Lya and LBG surveys. The total Lya escape fraction is 12%, similar to that observed at z>4. (shortened)

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arXiv2014

The Lyman Alpha Reference Sample: III. Properties of the Neutral ISM from GBT and VLA Observations

We present new H I imaging and spectroscopy of the 14 UV-selected star-forming galaxies in the Lyman Alpha Reference Sample (LARS), aimed for a detailed study of the processes governing the production, propagation, and escape of Ly$α$ photons. New H I spectroscopy, obtained with the 100m Green Bank Telescope (GBT), robustly detects the H I spectral line in 11 of the 14 observed LARS galaxies (although the profiles of two of the galaxies are likely confused by other sources within the GBT beam); the three highest redshift galaxies are not detected at our current sensitivity limits. The GBT profiles are used to derive fundamental H I line properties of the LARS galaxies. We also present new pilot H I spectral line imaging of 5 of the LARS galaxies obtained with the Karl G. Jansky Very Large Array (VLA). This imaging localizes the H I gas and provides a measurement of the total H I mass in each galaxy. In one system, LARS 03 (UGC 8335 or Arp 238), VLA observations reveal an enormous tidal structure that extends over 160 kpc from the main interacting systems and that contains $>$10$^9$ M$_{\odot}$ of H I. We compare various H I properties with global Ly$α$ quantities derived from HST measurements. The measurements of the Ly$α$ escape fraction are coupled with the new direct measurements of H I mass and significantly disturbed H I velocities. Our robustly detected sample reveals that both total H I mass and linewidth are tentatively correlated with key Ly$α$ tracers. Further, on global scales, these data support a complex coupling between Ly$α$ propagation and the H I properties of the surrounding medium.

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arXiv2013

Are Wolf-Rayet stars able to pollute the interstellar medium of galaxies? Results from the integral field spectroscopy

We investigate the spatial distribution of chemical abundances in a sample of low metallicity Wolf-Rayet (WR) galaxies selected from the SDSS. We used the integral field spectroscopy technique in the optical spectral range (3700-6850 AA) with PMAS attached to the CAHA 3.5 m telescope. Our statistical analysis of the spatial distributions of O/H and N/O, as derived using the direct method or strong-line parameters consistent with it, indicates that metallicity is homogeneous in five out of the six analysed objects in scales of the order of several kpc. Only in the object WR404, a gradient of metallicity is found in the direction of the low surface brightness tail. In contrast,we found an overabundance of N/O in spatial scales of the order of hundreds of pc associated with or close to the positions of the WR stars in 4 out of the 6 galaxies. We exclude possible hydrodynamical causes, such as the metal-poor gas inflow, for this local pollution by means of the analysis of the mass-metallicity relation (MZR) and mass-nitrogen-to-oxygen relation (MNOR) for the WR galaxies catalogued in the SDSS.

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arXiv2013

On the influence of physical galaxy properties on Lyman-alpha escape in star-forming galaxies

[abridged] Among the different observational techniques used to select high-redshift galaxies, the hydrogen recombination line Lyman-alpha (Lya) is of particular interest as it gives access to the measurement of cosmological quantities such as the star formation rate of distant galaxy populations. However, the interpretation of this line and the calibration of such observables is still subject to serious uncertainties. Therefore, it important to understand under what conditions the Lya line can be used as a reliable star formation diagnostic tool. We use a sample of 24 Lya emitters at z ~ 0.3 with an optical spectroscopic follow-up to calculate the Lya escape fraction and its dependency upon different physical properties. We also examine the reliability of Lya as a star formation rate indicator. We combine these observations with a compilation of Lya emitters selected at z = 0 - 0.3 to assemble a larger sample. The Lya escape fraction depends clearly on the dust extinction following the relation fesc(Lya) = C(Lya) x 10^(-0.4 E(B-V) k(Lya)), but with a shallower slope than previously reported, with k(Lya) ~ 6.67 and C(Lya) = 0.22. However, the correlation does not follow the expected curve for a simple dust attenuation. We explore the various mechanisms than lead to fesc(Lya) values above the continuum extinction curve, i.e. to an enhancement of the Lya output. We also observe that the strength of Lya and the escape fraction appear unrelated to the galaxy metallicity. Regarding the reliability of Lya as a star formation rate (SFR) indicator, we show that the deviation of SFR(Lya) from the true SFR (as traced by the UV continuum) is a function of the observed SFR(UV), which can be seen as the decrease of fesc(Lya) with increasing UV luminosity. Moreover, we observe a redshift-dependence of this relationship revealing the underlying evolution of fesc(Lya) with redshift.

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arXiv2013

The Lyman alpha Reference Sample: II. HST imaging results, integrated properties and trends

We report upon new results regarding the Lya 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 Lya, Halpha, and UV, and maps of Halpha/Hbeta, Lya equivalent width (EW), and Lya/Halpha. We present Lya and UV light profiles and show they are well-fitted by Sérsic profiles, but Lya profiles show indices systematically lower than those of the UV (n approx 1-2 instead of >~4). This reveals a general lack of the central concentration in Lya that is ubiquitous in the UV. Photometric growth curves increase more slowly for Lya 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 Lya 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 Lya emitters, based upon their luminosity and EW. We discuss the results in the context of high-z Lya and UV samples. A few galaxies have EWs above 50 AA, and one shows f_escLya of 80%; such objects have not previously been reported at low-z.

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arXiv2013

Chemical enrichment and physical conditions in I Zw 18

Abridged. Low-metallicity star-forming dwarf galaxies are prime targets to understand the chemical enrichment of the interstellar medium. The HI region provides important constraints on the dispersal and mixing of heavy elements released by successive star-formation episodes. Our primary objective is to study the enrichment of the HI region and the interplay between star-formation history and metallicity evolution. We observed the most metal-poor star-forming galaxy in the Local Universe, I Zw 18, with Hubble/COS. The abundances in the neutral gas are derived from far-UV absorption-lines (HI, CII, CII*, NI, OI, ...) and are compared to the abundances in the HII region. Models are constructed to calculate the ionization structure and the thermal processes. We investigate the gas cooling in the HI region through physical diagnostics drawn from the fine-structure level of C+. We find that HI region abundances are lower by a factor of ~2 as compared to the HII region. There is no differential depletion on dust between the HI and HII region. Using sulfur as a metallicity tracer, we calculate a metallicity of 1/46 solar (vs. 1/31 in the HII region). From the study of abundance ratios, we propose that C, N, O, and Fe are mainly produced in massive stars. We argue that the HI envelope may contain pockets of pristine gas with a metallicity essentially null. Finally, we derive the physical conditions in the HI region by investigating the CII* absorption line. The cooling rate derived from CII* is consistent with collisions with H atoms in the diffuse neutral gas. We calculate the star-formation rate from the CII* cooling rate assuming that photoelectric effect on dust is the dominant gas heating mechanism. Our determination is in good agreement with the values in the literature if we assume a low dust-to-gas ratio (~2000 times lower than the Milky Way value).

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arXiv2013

The Lyman alpha Reference Sample: Extended Lyman alpha Halos Produced at Low Dust Content

We report on new imaging observations of the Lyman alpha emission line (Lya), performed with the Hubble Space Telescope, that comprise the backbone of the Lyman alpha Reference Sample (LARS). We present images of 14 starburst galaxies at redshifts 0.028 < z < 0.18 in continuum-subtracted Lya, Halpha, and the far ultraviolet continuum. We show that Lya is emitted on scales that systematically exceed those of the massive stellar population and recombination nebulae: as measured by the Petrosian 20 percent radius, RP20, Lya radii are larger than those of Halpha by factors ranging from 1 to 3.6, with an average of 2.4. The average ratio of Lya-to-FUV radii is 2.9. This suggests that much of the Lya light is pushed to large radii by resonance scattering. Defining the "Relative Petrosian Extension" of Lya compared to Halpha, ξ_ext = RP20_Lya / RP20_Ha, we find ξ_ext to be uncorrelated with total Lya luminosity. However ξ_ext is strongly correlated with quantities that scale with dust content, in the sense that a low dust abundance is a necessary requirement (although not the only one) in order to spread Lya photons throughout the interstellar medium and drive a large extended Lya halo.

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