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We present the [OIII]52{\mu}m map of the dwarf galaxy IC10, obtained with the Field-Imaging Far-Infrared Line Spectrometer (FIFI-LS) on board the Stratospheric Observatory for Infrared Astronomy (SOFIA). We combine the [OIII]52{\mu}m map with Herschel and Spitzer observations, to estimate the electron density distribution of the brightest HII regions of IC10. We find that the line ratio [OIII]88{\mu}m/[OIII]52{\mu}m gives electron density (n_e) values (n_e_OIII) that cover a broad range, while the n_e values obtained using the line ratio [SIII]33{\mu}m/[SIII]18{\mu}m (n_e_SIII) are all similar within the uncertainties. n_e_OIII is similar to n_e_SIII for the M1, M2 and A1 regions, and it is higher than n_e_SIII for the two regions, A2 and M1b, which are the brightest in the 24{\mu}m continuum emission. These results suggest that for these regions the two ions, O++ and S++, trace two different ionised gas components, and that the properties of the ionised gas component traced by the O++ ion are more sensitive to the local physical conditions. In fact, while the gas layer traced by [SIII] does not keep track of the characteristics of the radiation field, the n_e_OIII, correlates with the star formation rate (SFR), the dust temperature and the 24{\mu}m. Therefore, n_e_OIII is an indicator of the evolutionary stage of the HII region and the radiation field, with higher n_e_OIII, found in younger SF regions and in more energetic environments., Comment: 19 pages, 13 figures, 5 tables. Paper accepted for publication in ApJ

Low-metallicity dwarf galaxies often show no or little CO emission, despite the intense star formation observed in local samples. Both simulations and resolved observations indicate that molecular gas in low-metallicity galaxies may reside in small dense clumps, surrounded by a substantial amount of more diffuse gas, not traced by CO. Constraining the relative importance of CO-bright versus CO-dark H2 star-forming reservoirs is crucial to understand how star formation proceeds at low metallicity. We put to the test classically used single component radiative transfer models and compare their results to those obtained assuming an increasingly complex structure of the interstellar gas, mimicking an inhomogeneous distribution of clouds with various physical properties. We compute representative models of the interstellar medium as combinations of several gas components, each with a specific set of physical parameters. We introduce physically-motivated models assuming power-law distributions for the density, ionization parameter, and the depth of molecular clouds. We confirm the presence of a predominantly CO-dark molecular reservoir in low-metallicity galaxies. The predicted total H2 mass is best traced by [C II]158um and, to a lesser extent, by [CI] 609um, rather than by CO(1-0). We examine the CO-to-H2 conversion factor vs. metallicity relation and find that its dispersion increases significantly when different geometries of the gas are considered. We define a clumpiness parameter that anti-correlates with [CII]/CO and explains the dispersion of the CO-to-H2 conversion factor vs. metallicity relation. We find that low-metallicity galaxies with high clumpiness may have CO-to-H2 conversion factor as low as the Galactic value. We identify the clumpiness of molecular gas as a key parameter to understand variations of geometry-sensitive quantities, such as CO-to-H2 conversion factor., Comment: Accepted for publication in A&A, 24 pages, 14 figures

(abridged) Quantifying the ISM porosity to ionizing photons in nearby galaxies may improve our understanding of the mechanisms leading to Lyman Continuum photons leakage from galaxies. Primitive galaxies with low metal and dust content have been shown to host a more patchy and porous ISM than their high-metallicity counterparts. To what extent this peculiar structure contributes to the leakage of ionizing photons remains to be quantitatively studied. To address these questions we build a refined grid of models including density-bounded regions and a possible contribution of an X-ray source. Using MULTIGRIS, a new Bayesian code based on Monte Carlo sampling, we combine the models as sectors under various assumptions to extract the probability density distributions of the parameters and infer the corresponding escape fractions from H II regions (fesc,HII). We apply this new code to a sample of 39 well-know local starbursting dwarf galaxies from the Dwarf Galaxy Survey. We confirm previous results hinting at an increased porosity to ionizing photons of the ISM in low-metallicity galaxies and provide, for the first time, quantitative predictions for fesc,HII. The predicted fesc,HII for low-metallicity objects span a large range of values, up to 60%, while the values derived for more metal-rich galaxies are globally lower. We also examine the influence of other parameters on the escape fractions, and find that the specific star-formation rate correlates best with fesc,HII . Finally, we provide observational line ratios which could be used as tracers of the photons escaping from density-bounded regions. Although this multi-sector modelling remains too simple to fully capture the ISM complexity, it can be used to preselect galaxy samples with potential leakage of ionizing photons based on current and up-coming spectral data in unresolved surveys of local and high-redshift galaxies., Comment: 39 pages, 24 figures, accepted for publication in A&A

Finding and elucidating the properties of Lyman-continuum(LyC)-emitting galaxies is an important step for our understanding of cosmic reionization. Although the z - 0.3-0.4 LyC emitters found recently show strong optical emission lines, no consistent quantitative photoionization model taking into account the escape of ionizing photons and inhomogenous interstellar medium (ISM) geometry of these galaxies has yet been constructed. We hence construct one- and two-zone photoionization models accounting for the observed LyC escape, which we compare to the observed emission line measurements. We find that one-zone density-bounded photoionization models cannot reproduce the emission lines of the LyC leakers because they systematically underpredict the lines of species of low ionization potential, as [OI] and [SII]. Introducing a two-zone model, with differing ionization parameter and a variable covering fraction and where one of the zones is density-bounded, we show that the observed emission line ratios and escape fractions of the LyC emitters are well reproduced. The [OI] excess, which is observed in some LyC leakers, can be naturally explained in this model, e.g., by emission from low-ionization and low-filling-factor gas. LyC emitters with a high escape fraction (fesc > 38%) are deficient both in [OI]6300A and in [SII]6716,6731A. We also confirm that a [SII] deficiency can be used to select LyC emitter candidates. Finally, we find indications for a possible dichotomy in terms of escape mechanisms for LyC photons between galaxies with relatively low (fesc < 10%) and higher escape fractions. We conclude that two-zone photoionization models are sufficient and required to explain the observed emission line properties of z - 0.3-0.4 LyC emitters. These models provide a first step towards the use of optical emission lines and their ratios as quantitative diagnostics of LyC escape from galaxies., Comment: 16 pages, 11 figures, accepted for publication in Astronomy & Astrophysics, revised by language editor

While the CO(1-0) transition is often used to deduce the total molecular hydrogen in galaxies, it is challenging to detect in low metallicity galaxies, in spite of the star formation taking place. In contrast, the [CII] 158 micron line is relatively bright, highlighting a potentially important reservoir of H2 that is not traced by CO(1-0), but residing in the C+ - emitting regions. We explore a method to quantify the total H2 mass (MH2) in galaxies and learn what parameters control the CO-dark gas reservoir. We present Cloudy grids of density, radiation field and metallicity in terms of observed quantities, such as [OI], [CI], CO(1-0), [CII], total infrared luminosity and the total MH2 and provide recipes based on these models to derive total MH2 mass estimates from observations. The models are applied to the Herschel Dwarf Galaxy Survey, extracting the total MH2 for each galaxy which is compared to the H2 determined from the observed CO(1-0) line. While the H2 traced by CO(1-0) can be negligible, the [CII] 158 micron line can trace the total H2. 70% to 100% of the total H2 mass is not traced by CO(1-0) in the dwarf galaxies, but is well-traced by [CII] 158 micron line. The CO-dark gas mass fraction correlates with the observed L[CII]/LCO(1-0) ratio. A conversion factor for [CII] luminosity to total H2 and a new CO-to-total-MH2 conversion factor, as a function of metallicity, is presented. A recipe is provided to quantify the total mass of H2 in galaxies, taking into account the CO and [CII] observations. Accounting for this CO-dark H2 gas, we find that the star forming dwarf galaxies now fall on the Schmidt-Kennicutt relation. Their star-forming efficiency is rather normal, since the reservoir from which they form stars is now more massive when introducing the [CII] measures of the total H2, compared to the little amount of H2 in the CO-emitting region., Comment: Accepted for publication in A&A

Using new VLT/XShooter spectral observations we analyse the physical properties of five z~0.3-0.4 confirmed LyC leakers. Strong resonant MgII 2796,2803 emission lines (I(2796,2803)/I(Hbeta)=10-38 per cent) and non-resonant FeII* 2612,2626 emission lines are observed in spectra of five and three galaxies, respectively. We find high electron densities Ne~400cm-3, significantly higher than in typical low-z, but comparable to those measured in z~2-3 star-forming galaxies. The galaxies have a mean value of logN/O=-1.16, close to the maximum values found for star-forming (SF) galaxies in the metallicity range of 12+logO/H=7.7-8.1. All 11 low-z LyC emitting galaxies found by Izotov et al. (2016, 2018), including the ones considered in the present study, are characterised by high EW(Hbeta)~200-400A, high ionisation parameter (log(U)=-2.5 to -1.7), high average ionising photon production efficiency \xi= 10^{25.54} Hz erg-1 and hard ionising radiation. On the BPT diagram we find the same offset of our leakers from low-$z$ main-sequence SFGs as that for local analogues of LBGs and extreme SF galaxies at z~2-3. We confirm the effectiveness of the HeI emission lines diagnostics proposed by Izotov et al. (2017) in searching for LyC leaker candidates and find that their intensity ratios correspond to those in a median with low neutral hydrogen column density N(HI)=10^{17}-5x10^{17} cm-2 that permit leakage of LyC radiation, likely due to their density-bounded HII regions., Comment: 20 pages, 10 figures, accepted for publication in the Monthly Notices of the Royal Astronomical Society