arXiv subjects

Göran Östlin

Publications and source records attributed to Göran Östlin.

At least 73 records · Page 4Linked to original sources

arXiv2023

The Low-Redshift Lyman Continuum Survey: Optically Thin and Thick Mg II Lines as Probes of Lyman Continuum Escape

The Mg II 2796, 2803 doublet has been suggested to be a useful indirect indicator for the escape of Ly-alpha and Lyman continuum (LyC) photons in local star-forming galaxies. However, studies to date have focused on small samples of galaxies with strong Mg II or strong LyC emission. Here we present the first study of Mg II probing a large dynamic range of galaxy properties, using newly obtained high signal-to-noise, moderate-resolution spectra of Mg II for a sample of 34 galaxies selected from the Low-redshift Lyman Continuum Survey. We show that the galaxies in our sample have Mg II profiles ranging from strong emission to P-Cygni profiles, and to pure absorption. We find there is a significant trend (with a possibility of spurious correlations of ~ 2%) that galaxies detected as strong LyC Emitters (LCEs) also show larger equivalent widths of Mg II emission, and non-LCEs tend to show evidence of more scattering and absorption features in Mg II We then find Mg II strongly correlates with Ly-alpha in both equivalent width and escape fraction, regardless of whether the emission or absorption dominates the Mg II profiles. Furthermore, we present that, for galaxies categorized as Mg II emitters (MgE), one can adopt the information of Mg II, metallicity, and dust to estimate the escape fraction of LyC within a factor of 3. These findings confirm that Mg II lines can be used as a tool to select galaxies as LCEs and to serve as an indirect indicator for the escape of Ly-alpha and LyC.

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arXiv2022

Reionization with star-forming galaxies: insights from the Low-z Lyman Continuum Survey

The fraction of ionizing photons escaping from galaxies, $f_{esc}$, is at the same time a crucial parameter in modelling reionization and a very poorly known quantity, especially at high redshift. Recent observations are starting to constrain the values of $f_{esc}$ in low-z star-forming galaxies, but the validity of this comparison remains to be verified. Applying at high-z the empirical relation between $f_{esc}$ and the UV slope trends derived from the Low-z Lyman Continuum Survey, we use the DELPHI semi-analytical galaxy formation model to estimate the global ionizing emissivity of high-z galaxies, which we use to compute the resulting reionization history. We find that both the global ionizing emissivity and reionization history match the observational constraints. Assuming that the low-z correlations hold during the epoch of reionization, we find that galaxies with $-16 \lesssim M_{UV} \lesssim -13.5$ are the main drivers of reionization. We derive a population-averaged $\langle f_{esc} \rangle \simeq 8\%, 10\%, 20\%$ at z=4.5, 6, 8.

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arXiv2022

Spatially resolved gas and stellar kinematics in compact starburst galaxies

The kinematics of galaxies provide valuable insights in their physics and assembly history. Kinematics are governed not only by the gravitational potential, but also by merger events and stellar feedback processes such as stellar winds and supernova explosions. Aims. We aim at identifying what governs the kinematics in a sample of SDSS selected nearby starburst galaxies, by obtaining spatially resolved measurements of the gas and stellar kinematics. We obtain near-infrared integral field K-band spectroscopy with VLT/SINFONI of 15 compact starburst galaxies. We derive the integrated as well as spatially resolved stellar and gas kinematics. The stellar kinematics are derived from the CO absorption bands, and Pa$α$ and Br$γ$ emission lines are used for the gas kinematics. Based on the integrated spectra we find that the majority of galaxies have gas and stellar velocity dispersion that are comparable. A spatially resolved comparison shows that the six galaxies that deviate show evidence for a bulge or stellar feedback. Two galaxies are identified as mergers based on their double peaked emission lines. In our sample, we find a negative correlation between the ratio of the rotational velocity over the velocity dispersion (vrot/$σ$) and the star formation rate surface density. We propose a scenario where the global kinematics of the galaxies are determined by gravitational instabilities that affect both the stars and gas. This process could be driven by mergers or accretion events. Effects of stellar feedback on the ionised gas are more localised and detected only in the spatially resolved analysis. The mass derived from the velocity dispersion provides a reliable mass even if the galaxy cannot be spatially resolved. The technique used in this paper is applicable to galaxies at low and high redshift with the next generation of infrared focussed telescopes (JWST, ELT).

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arXiv2022

Tracing Lyman-alpha and Lyman Continuum Escape in Galaxies with Mg II Emission

Star-forming galaxies are considered the likeliest source of the H I ionizing Lyman Continuum (LyC) photons that reionized the intergalactic medium at high redshifts. However, above z >~ 6, the neutral intergalactic medium prevents direct observations of LyC. Therefore, recent years have seen the development of indirect indicators for LyC that can be calibrated at lower redshifts and applied in the Epoch of Reionization. Emission from Mg II \ly\ly 2796, 2803 doublet has been proposed as a promising LyC proxy. In this paper, we present new Hubble Space Telescope/Cosmic Origins Spectrograph observations for 8 LyC emitter candidates, selected to have strong Mg II emission lines. We securely detect LyC emission in 50% (4/8) galaxies with 2$σ$ significance. This high detection rate suggests that strong Mg II emitters might be more likely to leak LyC than similar galaxies without strong Mg II. Using photoionization models, we constrain the escape fraction of Mg II as ~ 15 -- 60%. We confirm that the escape fraction of Mg II correlates tightly with that of Lyman-alpha (LyA), which we interpret as an indication that the escape fraction of both species is controlled by resonant scattering in the same low column density gas. Furthermore, we show that the combination of the Mg II emission and dust attenuation can be used to estimate the escape fraction of LyC statistically. These findings confirm that Mg II emission can be adopted to estimate the escape fraction of LyA and LyC in local star-forming galaxies and may serve as a useful indirect indicator at the Epoch of Reionization.

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arXiv2022

From giant clumps to clouds -- III. The connection between star formation and turbulence in the ISM

Supersonic gas turbulence is a ubiquitous property of the interstellar medium. The level of turbulence, quantified by the gas velocity dispersion ($σ_{\rm g}$), is observed to increase with the star formation rate (SFR) of a galaxy, but it is yet not established whether this trend is driven by stellar feedback or gravitational instabilities. In this work we carry out hydrodynamical simulations of entire disc galaxies, with different gas fractions, to understand the origins of the SFR-$σ_{\rm g}$ relation. We show that disc galaxies reach the same levels of turbulence regardless of the presence of stellar feedback processes, and argue that this is an outcome of the way disc galaxies regulate their gravitational stability. The simulations match the SFR-$σ_{\rm g}$ relation up to SFRs of the order of tens of M$_\odot$ yr$^{-1}$ and $σ_{\rm g}\sim 50$ km s$^{-1}$ in neutral hydrogen and molecular gas, but fail to reach the very large values ($>100$ km s$^{-1}$) reported in the literature for rapidly star-forming galaxies. We demonstrate that such high values of $σ_{\rm g}$ can be explained by 1) insufficient beam smearing corrections in observations, and 2) stellar feedback being coupled to the ionised gas phase traced by recombination lines. Given that the observed SFR-$σ_{\rm g}$ relation is composed of highly heterogeneous data, with $σ_{\rm g}$ at high SFRs almost exclusively being derived from H$α$ observations of high redshift galaxies with complex morphologies, we caution against analytical models that attempt to explain the SFR-$σ_{\rm g}$ relation without accounting for these effects.

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arXiv2022

The Low-Redshift Lyman Continuum Survey. Unveiling the ISM properties of low-$z$ Lyman continuum emitters

Combining 66 ultraviolet (UV) spectra and ancillary data from the Low-Redshift Lyman Continuum Survey (LzLCS) and 23 LyC observations by earlier studies, we form a statistical sample of star-forming galaxies at $z \sim 0.3$ to study the role of the cold interstellar medium (ISM) gas in the leakage of ionizing radiation. We first constrain the massive star content (ages and metallicities) and UV attenuation, by fitting the stellar continuum with a combination of simple stellar population models. The models, together with accurate LyC flux measurements, allow to determine the absolute LyC photon escape fraction for each galaxy ($f_{\rm esc}^{\rm abs}$). We measure the equivalent widths and residual fluxes of multiple HI and low-ionization state (LIS) lines, and the geometrical covering fraction adopting the picket-fence model. The $f_{\rm esc}^{\rm abs}$ spans a wide range, with a median (0.16, 0.84 quantiles) of 0.04 (0.02, 0.20), and 50 out of the 89 galaxies detected in the LyC. The HI and LIS line equivalent widths scale with the UV luminosity and attenuation, and inversely with the residual flux of the lines. The HI and LIS residual fluxes are correlated, indicating that the neutral gas is spatially traced by the LIS transitions. We find the observed trends of the absorption lines and the UV attenuation are primarily driven by the covering fraction. The non-uniform gas coverage demonstrates that LyC photons escape through low-column density channels in the ISM. The equivalent widths and residual fluxes of the UV lines strongly correlate with $f_{\rm esc}^{\rm abs}$: strong LyC leakers show weak absorption lines, low UV attenuation, and large Ly$α$ equivalent widths. We finally show that simultaneous UV absorption line and dust attenuation measurements can predict, on average, the escape fraction of galaxies and the method can be applied to galaxies across a wide redshift range.

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arXiv2022

Stellar feedback in M83 as observed with MUSE -- I. Overview, an unprecedented view of the stellar and gas kinematics and evidence of outflowing gas

We present a large VLT/MUSE mosaic (3.8 x 3.8 kpc) of the nearby spiral galaxy M83, with a spatial resolution ~20 pc. We obtained the kinematics of the stars and ionised gas, and compared them with molecular gas kinematics from ALMA CO(2-1). We separated the ionised gas into HII regions and diffuse ionised gas (DIG) and determined the fraction of Ha luminosity originating from the DIG (f_DIG). We observe that both stars and gas trace the galactic disk rotation, as well as a fast-rotating nuclear component, likely connected to secular processes driven by the galactic bar. In the gas kinematics, we observe a stream east of the nucleus, redshifted with respect to the disk. The stream is surrounded by an extended ionised gas region with enhanced velocity dispersion and a high ionisation state, which is largely consistent with being ionised by slow shocks. We interpret this feature as either the superposition of the disk and an extraplanar layer of DIG, or as a bar-driven inflow of shocked gas. A double Gaussian component fit to the Ha line also reveals the presence of a nuclear biconic structure whose axis of symmetry is perpendicular to the bar. The two cones appear blue- and redshifted along the line of sight and stand out for having an Ha emission separated by up to 200 km s-1 from that of the disk, and a high velocity dispersion ~80-200 km s-1. At the far end of the cones, we observe that the gas is consistent with being ionised by shocks. These features had never been observed before in M83; we postulate that they are tracing a starburst-driven outflow shocking into the surrounding ISM. Finally, we obtain f_DIG ~ 13% in our field of view. We inspect the emission of the HII regions and DIG in `BPT' diagrams, finding that in HII regions photoionisation accounts for 99.8% of the Ha flux, whereas the DIG has a mixed contribution from photoionisation (94.9%) and shocks (5.1%). [abridged]

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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$α$) halos, Ly$α$ morphology, and the star-forming characteristics of galaxies. We study how the morphology of Ly$α$ emission is related to other Ly$α$ observables. Furthermore, we study the interdependencies of Ly$α$ morphological quantities. We studied the spatial extent of Ly$α$ using surface brightness profiles in i) using circular apertures and ii) within faint Ly$α$ 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$α$, ultraviolet continuum, and I band maps. We found that galaxies with more extended star-forming regions possess larger Ly$α$ halos. Furthermore, galaxies with more elongated Ly$α$ morphology are also more extended in Ly$α$. Our data suggest that Ly$α$ bright galaxies appear rounder in their Ly$α$ morphology, and there is less of a contribution from their Ly$α$ halo to their overall luminosity. We compared our results with studies at high redshift and found that whilst the Ly$α$ extent in the inner regions of the galaxies in our sample are similar to the high$-z$ Ly$α$ emitters (LAEs), Ly$α$ halos are more extended in high$-z$ LAEs. Our analysis suggests that the Ly$α$ morphology affects the measurement of other observable quantities concerning Ly$α$ emission, and some of the conclusions drawn from high redshift LAEs might be biased towards galaxies with specific Ly$α$ shapes. In particular, faint Ly$α$ emitters have larger Ly$α$ scale lengths and halo fractions. This implies that faint Ly$α$ emitters are harder to detect at high redshift than previously believed.

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arXiv2021

The Low-redshift Lyman-continuum Survey: [S II]-deficiency and the leakage of ionizing radiation

The relationship between galaxy characteristics and the reionization of the universe remains elusive, mainly due to the observational difficulty in accessing the Lyman continuum (LyC) at these redshifts. It is thus important to identify low-redshift LyC-leaking galaxies that can be used as laboratories to investigate the physical processes that allow LyC photons to escape. The weakness of the [S II] nebular emission lines relative to typical star-forming galaxies has been proposed as a LyC predictor. In this paper, we show that the [S II]-deficiency is an effective method to select LyC-leaking candidates using data from the Low-redshift LyC Survey, which has detected flux below the Lyman edge in 35 out of 66 star-forming galaxies with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope. We show that LyC leakers tend to be more [S II]-deficient and that the fraction of their detections increases as [S II]-deficiency becomes more prominent. Correlational studies suggest that [S II]-deficiency complements other LyC diagnostics (such as strong Lyman-$α$ emission and high [O III]/[O II]). Our results verify an additional technique by which reionization-era galaxies could be studied.

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arXiv2021

Studying the ISM at $\sim$10 pc scale in NGC 7793 with MUSE -- II. Constraints on the oxygen abundance and ionising radiation escape

We study the ionised ISM in NGC 7793 with MUSE/AO, at a spatial resolution of $\sim$ 10 pc. The data are complemented with young star clusters (YSCs), O stars and GMCs observed with HST and ALMA. Using a strong-line method, we find a median $12 + \log(O/H) \sim 8.37$ with a scatter of 0.25 dex, in agreement with previous estimates. The abundance map is rich in substructures, surrounding clusters and massive stars, although clear degeneracies with photoionisation are present. We determine the observed total amount of ionising photons, $Q(H^0)$, from the extinction corrected H$α$ luminosity, and compare it to the expected $Q(H^0)$ obtained by summing the contributions of YSCs and massive stars, to obtain an escape fraction ($f_{esc}$). Overall, we find $f_{esc, HII} = 0.67_{-0.12}^{+0.08}$ for the population of HII regions. We also conclude that the sources of ionisation observed within the FoV are more than sufficient to explain the amount of diffuse ionised gas observed in this region of the galaxy. In general, we find that YSCs located in HII regions have a higher probability to be younger, less massive, and to emit a higher number of ionising photons than clusters in the rest of the field. Finally, we study the optical depth of the regions traced by [SII]/[OIII], finding no systematic trend between the resulting ionisation structure and $f_{esc}$. [abridged]

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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-$α$ emission and the properties of intervening gas. We find that all knots have bright Ly-$α$ 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-$α$ 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

Spatially resolved CIII]$λ$1909 emission in Haro 11

The CIII]1909 (hereafter, C III]) line is the strongest ultraviolet emission line after Ly$α$ and is therefore of interest to high redshift studies of star-forming (SF) galaxies near the epoch of reionization. It is thought that C III] emission is strongest in galaxies with subsolar metallicity and low mass, however, spectral observations of numerous such galaxies at high and low redshift produce inconclusive or even contradictory results. We present the first ever C III ] imaging, obtained with HST/STIS for the low-redshift SF galaxy Haro 11. Cluster parameters like stellar mass, dust fraction and dust attenuation, and ionization parameter, obtained through spectral energy distribution fitting, show no correlation with the CIII] equivalent width (EW), which may be due to a combination of the limitation of the models and the age-homogeneity of the cluster population. Comparing the ratio of C III] emission line flux from individual clusters to that of H$α$, [O III], and [O II] we find that the clusters with the highest EW(C III]) can be reconciled only with Cloudy models with an extremely high C/O ratio of $\ge$ 1.4(C/O)$_\odot$ for an ionizing population of single stars, binary stars, or a mixture of binary stars and active galactic nuclei. Given the point-like nature of strong C III], the integrated total strength of EW(C III]) becomes dependent on the morphology of the galaxy, which would explain the large scatter in EW(C III]) strengths, observed in galaxies with otherwise similar SF properties, and of similarly low metallicity and stellar mass.

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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 $α$ (Ly$α$) radiation from star-forming galaxies is a long standing problem in astrophysics. The ability to predict the Ly$α$ luminosity of galaxies would open up new ways of exploring the Epoch of Reionization (EoR), and to estimate Ly$α$ 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$α$. The derived relation predicts the Ly$α$ 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$α$ 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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arXiv2020

Studying the ISM at ~10 pc scale in NGC 7793 with MUSE -- I. Data description and properties of the ionised gas

Using MUSE AO data, we probe the ISM in the local spiral galaxy NGC 7793 at a spatial resolution of $\sim$ 10 pc. We identify HII regions and compile a catalogue of supernova remnants (SNRs), planetary nebulae (PNe) and Wolf Rayet stars (WR). We compute electron densities and temperatures from the [SII]6716/6731 and [SIII]6312/9069 line ratios. We study the properties of the ionised gas through BPT diagrams combined with gas velocity dispersion. We spectroscopically confirm 2 previously detected WR stars and a SNR and report the discovery of 7 WR, 1 SNR, and 2 PNe. The diffuse ionized gas (DIG) fraction is between $\sim$ 27 and 42% depending on the method used to define the HII region boundaries. In agreement with previous studies, we find that the DIG exhibits enhanced [SII]/H$α$ and [NII]/H$α$ ratios and a median temperature that is $\sim$ 3000 K higher than in HII regions. We also observe an apparent inverse correlation between temperature and H$α$ surface brightness. Overall, the observed [SII]6716/6731 ratio is consistent within 1$σ$ with $n_e$ < 30 cm$^{-3}$, with an almost identical distribution for the DIG and HII regions. The velocity dispersion of the ionised gas indicates that the DIG has a higher degree of turbulence than the HII regions. Comparison with photoionisation and shock models reveals that the diffuse component can only partially be explained via shocks and that it is most likely consistent with photons leaking from density bounded HII regions or with radiation from evolved field stars. [abridged]

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arXiv2019

Star-forming clumps in the Lyman Alpha Reference Sample of galaxies -- I. Photometric analysis and clumpiness

We study young star-forming clumps on physical scales of 10-500 pc in the Lyman-Alpha Reference Sample (LARS), a collection of low-redshift (z = 0.03-0.2) UV-selected star-forming galaxies. In each of the 14 galaxies of the sample, we detect clumps for which we derive sizes and magnitudes in 5 UV-optical filters. The final sample includes $\sim$1400 clumps, of which $\sim$600 have magnitude uncertainties below 0.3 in all filters. The UV luminosity function for the total sample of clumps is described by a power-law with slope $α= -2.03^{+0.11}_{-0.13}$. Clumps in the LARS galaxies have on average $Σ_{SFR}$ values higher than what observed in HII regions of local galaxies and comparable to typical SFR densities of clumps in z = 1-3 galaxies. We derive the clumpiness as the relative contribution from clumps to the UV emission of each galaxy, and study it as a function of galactic-scale properties, i.e. $Σ_{SFR}$ and the ratio between rotational and dispersion velocities of the gas ($v_s/σ_0$). We find that in galaxies with higher $Σ_{SFR}$ or lower $v_s/σ_0$, clumps dominate the UV emission of their host systems. All LARS galaxies with Ly$α$ escape fractions larger than 10% have more than 50% of the UV luminosity from clumps. We tested the robustness of these results against the effect of different physical resolutions. At low resolution, the measured clumpiness appears more elevated than if we could resolve clumps down to single clusters. This effect is small in the redshift range covered by LARS, thus our results are not driven by the physical resolution.

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arXiv2019

FIGS: Spectral fitting constraints on the star formation history of massive galaxies since Cosmic Noon

We constrain the stellar population properties of a sample of 52 massive galaxies, with stellar mass log Ms>10.5, over the redshift range 0.5<z<2 by use of observer-frame optical and near-infrared slitless spectra from HST's ACS and WFC3 grisms. The deep exposures (~100 ks) allow us to target individual spectra of massive galaxies to F160W=22.5AB. Our spectral fitting approach uses a set of six base models adapted to the redshift and spectral resolution of each observation, and fits the weights of the base models, including potential dust attenuation, via an MCMC method. Our sample comprises a mixed distribution of quiescent (19) and star-forming galaxies (33). We quantify the width of the age distribution (Dt) that is found to dominate the variance of the retrieved parameters according to Principal Component Analysis. The population parameters follow the expected trend towards older ages with increasing mass, and Dt appears to weakly anti-correlate with stellar mass, suggesting a more efficient star formation at the massive end. As expected, the redshift dependence of the relative stellar age (measured in units of the age of the Universe at the source) in the quiescent sample rejects the hypothesis of a single burst (aka monolithic collapse). Radial colour gradients within each galaxy are also explored, finding a wider scatter in the star-forming subsample, but no conclusive trend with respect to the population parameters.

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arXiv2019

An IFU investigation of possible Lyman continuum escape from Mrk 71/NGC 2366

Mrk 71/NGC 2366 is the closest Green Pea (GP) analog and candidate Lyman Continuum (LyC) emitter. Recently, 11 LyC-leaking GPs have been detected through direct observations of the ionizing continuum, making this the most abundant class of confirmed LyC-emitters at any redshift. High resolution, multi-wavelength studies of GPs can lead to an understanding of the method(s), through which LyC escapes from these galaxies. The proximity of Mrk 71/NCG 2366 offers unprecedented detail on the inner workings of a GP analog, and enables us to identify the mechanisms of LyC escape. We use 5825-7650Å integral field unit PMAS observations to study the kinematics and physical conditions in Mrk 71. An electron density map is obtained from the [S II] ratio. A fortuitous second order contamination by the [O II]3727 doublet enables the construction of an electron temperature map. Resolved maps of sound speed, thermal broadening, "true" velocity dispersion, and Mach number are obtained and compared to the high resolution magneto-hydrodynamic SILCC simulations. Two regions of increased velocity dispersion indicative of outflows are detected to the north and south of the super star cluster, knot B, with redshifted and blueshifted velocities, respectively. We confirm the presence of a faint broad kinematical component, which is seemingly decoupled from the outflow regions, and is fainter and narrower than previously reported in the literature. Within uncertainties, the low- and high-ionization gas move together. Outside of the core of Mrk 71, an increase in Mach numbers is detected, implying a decrease in gas density. Simulations suggest this drop in density can be as high as ~4 dex, down to almost optically thin levels, which would imply a non-zero LyC escape fraction along the outflows... [abridged]

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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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