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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

It has long been discussed whether stellar feedback in the form of winds and/or radiation can shred the nascent molecular cloud, thereby controlling the star formation rate. However, directly probing and quantifying the impact of stellar feedback on the neutral gas of the nascent clouds is challenging. We present an investigation doing exactly that toward the RCW 79 HII region using the ionized carbon line at 158 $\mu$m ([CII]) from the FEEDBACK Legacy Survey. We combine this data with information on the dozen ionizing O stars responsible for the evolution of the region, and observe in [CII] for the first time both blue- and red-shifted mostly neutral high-velocity gas which reaches velocities up to 25 km s$^{-1}$ relative to the bulk emission of the molecular cloud. This high-velocity gas mostly contains neutral gas and partly forms a fragmented shell, similar to recently found shells in a few Galactic HII regions. However, this shell does not account for all of the observed neutral high-velocity gas. We also find high-velocity gas streaming out of the nascent cloud through holes and obtain a range of dynamical timescales below 1.0 Myr for the high-velocity gas which is well below the 2.3$\pm$0.5 Myr age of the OB cluster. This suggests a different scenario for the evolution of RCW 79, where the high-velocity gas is not solely stemming from a spherical expanding bubble, but also from gas recently ablated at the edge of the turbulent molecular cloud into the surrounding interstellar medium through low-pressure holes or chimneys. The resulting mass ejection rate estimate for the cloud is 0.9-3.5$\times$10$^{-2}$ M$_{\odot}$~yr$^{-1}$, which leads to short erosion timescales, i.e. $<$5 Myr, for the nascent molecular cloud. This finding provides direct observational evidence of rapid molecular cloud dispersal., Comment: 9 pages, 7 figures, accepted in A&A Letters

(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

The origin of the cold gas in central galaxies in groups is still a matter of debate. We present Multi-Unit Spectroscopic Explorer (MUSE) observations of 18 optically selected local Brightest Group Galaxies (BGGs) to study the kinematics and distribution of the optical emission-line gas. MUSE observations reveal a distribution of gas morphologies including ten complex networks of filaments extending up to 10 kpc to two compact (<3 kpc) and five extended (>5 kpc) disk-dominated structures. Some rotating disks show rings and elongated structures arising from the central disk. The kinematics of the stellar component is mainly rotation-dominated, which is very different from the disturbed kinematics and distribution found in the filamentary sources. The ionized gas is kinematically decoupled from the stellar component for most systems, suggesting an external origin for the gas. We also find that the Halpha luminosity correlates with the cold molecular mass. By exploring the thermodynamical properties of the hot atmospheres, we find that the filamentary sources and compact disks are found in systems with small central entropy values and tcool/teddy ratios. This suggests that, like for Brightest Cluster Galaxies in cool core clusters, the ionized gas are likely formed from hot halo gas condensations, consistently with the Chaotic Cold Accretion simulations (as shown via the C-ratio, Tat, and k-plot). We note that gaseous rotating disks are more frequent than in BCGs. An explanation for the origin of the gas in those objects is a contribution to gas fueling by mergers or group satellites, as qualitatively hinted by some sources of the present sample. Nonetheless, we discuss the possibility that some extended disks could also be a transition stage in an evolutionary sequence including filaments, extended disks and compact disks, as described by hot gas condensation models of cooling flows., Comment: 38 pages, and 16 figures. Resubmitted to A&A after referee report

The excitation of the filamentary gas structures surrounding giant elliptical galaxies at the center of cool-core clusters, a.k.a BCGs (brightest cluster galaxies), is key to our understanding of active galactic nucleus feedback, and of the impact of environmental and local effects on star formation. We investigate the contribution of the thermal radiation from the cooling flow surrounding BCGs to the excitation of the filaments. We explore the effects of small levels of extra-heating (turbulence), and of metallicity, on the optical and infrared lines. Using the Cloudy code, we model the photoionization and photodissociation of a slab of gas of optical depth AV{\leq}30mag at constant pressure, in order to calculate self-consistently all of the gas phases, from ionized gas to molecular gas. The ionizing source is the EUV and soft X-ray radiation emitted by the cooling gas. We test these models comparing their predictions to the rich multi-wavelength observations, from optical to submillimeter. These models reproduce most of the multi-wavelength spectra observed in the nebulae surrounding the BCGs, not only the LINER-like optical diagnostics: [O iii]{\lambda} 5007 {\AA}/H\b{eta}, [N ii]{\lambda} 6583 {\AA}/H{\alpha} and ([S ii]{\lambda} 6716 {\AA}+[S ii]{\lambda} 6731 {\AA})/H{\alpha} but also the infrared emission lines from the atomic gas. The modeled ro-vib H2 lines also match observations, which indicates that near and mid-IR H2 lines are mostly excited by collisions between H2 molecules and secondary electrons produced naturally inside the cloud by the interaction between the X-rays and the cold gas in the filament. However, there is still some tension between ionized and molecular line tracers (i.e. CO), which requires to optimize the cloud structure and the density of the molecular zone., Comment: 16 pages, 9 figures, 3 tables. Accepted for publication in A&A

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

Investigating star formation requires precise knowledge of the properties of the dense molecular gas. The low metallicity and wide range of star formation activity of the Large and Small Magellanic Clouds make them prime laboratories to study how local physical conditions impact the dense gas reservoirs. The aim of the Dense Gas Survey for the Magellanic Clouds (DeGaS-MC) project is to expand our knowledge of the relation between dense gas properties and star formation activity by targeting the LMC and SMC observed in the HCO+(2-1) and HCN(2-1) transitions. We carried out a pointing survey toward 30 LMC and SMC molecular clouds using the SEPIA180 instrument installed on the APEX telescope and a follow-up mapping campaign in 13 star-forming regions. This first paper provides line characteristic catalogs and integrated line-intensity maps of the sources. HCO+(2-1) is detected in 20 and HCN(2-1) in 8 of the 29 pointings observed. The dense gas velocity pattern follows the line-of-sight velocity field derived from the stellar population. The HCN emission is less extended than the HCO+ emission. The HCO+(2-1)/HCN(2-1) brightness temperature ratios range from 1 to 7, which is consistent with the large ratios commonly observed in low-metallicity environments. A larger number of young stellar objects are found at high HCO+ intensities and lower HCO+/HCN flux ratios, and thus toward denser lines of sight. The dense gas luminosities correlate with the star formation rate traced by the total infrared luminosity over the two orders of magnitude covered by our observations, although substantial region-to-region variations are observed., Comment: Accepted for publication in A&A; 25 pages, 13 figures, 5 tables

(abridged) The ambiguous origin of [CII] 158um in the interstellar medium complicates its use for diagnostics concerning the star-formation rate and physical conditions in photodissociation regions (PDRs). We observed the giant HII region N11 in the Large Magellanic Cloud with SOFIA/GREAT in order to investigate the origin of [CII] to obtain the total H2 gas content, the fraction of CO-dark H2 gas, and the influence of environmental effects such as stellar feedback. We present an innovative spectral decomposition method that allows statistical trends to be derived. The [CII] line is resolved in velocity and compared to HI and CO, using a Bayesian approach to decompose the profiles. A simple model accounting for collisions in the neutral atomic and molecular gas was used in order to derive the H2 column density traced by C+. The profile of [CII] most closely resembles that of CO, but the integrated [CII] line width lies between that of CO and that of HI. Using various methods, we find that [CII] mostly originates from the neutral gas. We show that [CII] mostly traces the CO-dark H2 gas but there is evidence of a weak contribution from neutral atomic gas preferentially in the faintest components. Most of the molecular gas is CO-dark. The fraction of CO-dark H2 gas decreases with increasing CO column density, with a slope that seems to depend on the impinging radiation field from nearby massive stars. Finally we extend previous measurements of the photoelectric-effect heating efficiency, which we find is constant across regions probed with Herschel, with [CII] and [OI] being the main coolants in faint and diffuse, and bright and compact regions, respectively, and with PAH emission tracing the CO-dark H2 gas heating where [CII] and [OI] emit. Our study highlights the importance of velocity-resolved PDR diagnostics and higher spatial resolution for HI observations., Comment: Accepted for publication in A&A

The sensitive infrared telescopes, Spitzer and Herschel, have been used to target low-metallicity star-forming galaxies, allowing us to investigate the properties of their interstellar medium (ISM) in unprecedented detail. Interpretation of the observations in physical terms relies on careful modeling of those properties. We have employed a multiphase approach to model the ISM phases (HII region and photodissociation region) with the spectral synthesis code Cloudy. Our goal is to characterize the physical conditions (gas densities, radiation fields, etc.) in the ISM of the galaxies from the Herschel Dwarf Galaxy Survey. We are particularly interested in correlations between those physical conditions and metallicity or star-formation rate. Other key issues we have addressed are the contribution of different ISM phases to the total line emission, especially of the [CII]157um line, and the characterization of the porosity of the ISM. We find that the lower-metallicity galaxies of our sample tend to have higher ionization parameters and galaxies with higher specific star-formation rates have higher gas densities. The [CII] emission arises mainly from PDRs and the contribution from the ionized gas phases is small, typically less than 30% of the observed emission. We also find correlation - though with scatter - between metallicity and both the PDR covering factor and the fraction of [CII] from the ionized gas. Overall, the low metal abundances appear to be driving most of the changes in the ISM structure and conditions of these galaxies, and not the high specific star-formation rates. These results demonstrate in a quantitative way the increase of ISM porosity at low metallicity. Such porosity may be typical of galaxies in the young Universe., Comment: Accepted in A&A. 20 pages, 12 figures, 5 tables plus appendices

Our objective is to investigate the physical properties of the ionised gas of the low-metallicity dwarf galaxy, IC 10, at various spatial scales: from individual HII regions to the entire galaxy scale and examine whether diagnostics for integrated measurements introduce bias in the results. We modeled the ionised gas combining the mid- and far-infrared fine-structure cooling lines observed with Spitzer/IRS and Herschel/PACS, with the photoionisation code Cloudy. The free parameters of the models are the age of the stellar cluster, the density and the ionisation parameter of the ionised gas as well as the depth of the cloud. The latter is used to investigate the leakage of the ionising photons from the analysed regions of IC 10. We investigate HII regions in the main star-forming body, on scales of ~25 pc, three in the main star-forming region in the center of the galaxy and two on the first arc. We then consider larger sizes on the scale of ~200 pc. We find that most clumps have nearly identical properties, density ~10$^{2.} $ - 10$^{2.6}$ cm$^{-3}$, ionisation parameter between 10$^{-2.2}$ and 10$^{-1.6}$ and age of the stellar cluster ~5.5 Myr. All of them are matter-bounded regions, allowing ionising photons to leak. The relatively uniform physical properties of the clumps suggest a common origin for their star formation activity, which could be related to the feedback from stellar winds or supernovae of a previous generation of stars. The properties derived for ~200 pc size "zones" have similar properties as the HII regions they encompass, but with the larger regions tending to be more radiation-bounded. Finally, we investigate the fraction of [CII] 157.7 {\mu}m, [SiII] 34.8 {\mu}m, and [FeII] 25.9 {\mu}m, emission arising from the ionised gas phase and we find that most of the emission originates from the neutral gas, not from the ionised gas., Comment: accepted for publication in A&A

We present the discovery of diffuse optical line emission in the Centaurus cluster seen with the MUSE IFU. The unparalleled sensitivity of MUSE allows us to detect the faint emission from these structures which extend well beyond the bounds of the previously known filaments. Diffuse structures (emission surrounding the filaments, a northern shell and an extended Halo) are detected in many lines typical of the nebulae in cluster cores ([NII]$_{\lambda 6548\&6583}$ ,[SII]$_{\lambda 6716\&6731}$, [OI]$_{\lambda 6300}$, [OIII]$_{\lambda 4959\&5007}$ etc.) but are more than an order of magnitude fainter than the filaments, with the faint halo only detected through the brightest line in the spectrum ([NII]$_{\lambda 6583}$). These structures are shown to be kinematically distinct from the stars in the central galaxy and have different physical and excitation states to the filaments. Possible origins are discussed for each structure in turn and we conclude that shocks and/or pressure imbalances are resulting in gas dispersed throughout the cluster core, formed from either disrupted filaments or direct cooling, which is not confined to the bright filaments., Comment: 17 pages, 19 figures, Published in MNRAS

(abridged) The dominant thermal mechanisms in the neutral interstellar medium, which acts as a star-forming gas reservoir, are uncertain in extremely metal-poor galaxies. Our objective is to identify the heating mechanisms in one such galaxy, IZw18, and assess the diagnostic value of fine-structure cooling lines. We also seek to constrain the mass of H$_2$, which, despite being an important catalyst and tracer of star formation, remains elusive in this object. Building on a previous photoionization model within a multi-sector topology, we provide additional constraints from the [CII] and [OI] lines and the dust mass recently measured with Herschel. The heating of the HI region appears to be mainly due to photoionization by radiation from a bright X-ray binary source, while photoelectric effect (PE) is negligible. The [CII] and [OI] lines imply an average X-ray luminosity of $4\times10^{40}$ erg s$^{-1}$, while the [NeV] upper limits bring strong constraints to the soft X-ray flux arising from the binary. A negligible amount of H$_2$ is predicted, but $\lesssim10^7$ M$_\odot$ of H$_2$ may be hidden in sufficiently dense clouds of order $\lesssim10$ pc in size. Regardless of the presence of significant amounts of H$_2$, [CII] and [OI] do not trace the so-called CO-dark gas, but the almost purely atomic medium. Although the [CII]+[OI]/TIR ratio is close to values found in more metal-rich sources, it cannot be safely used as a PE heating efficiency proxy. This ratio seems to be kept stable due to a correlation between the X-ray luminosity and the star-formation rate. We propose that X-ray heating is an important process in extremely metal-poor sources. The weak PE heating due to the low dust-to-gas ratio could be compensated for by the larger occurrence and power of X-ray binaries in low-metallicity galaxies. We speculate that X-ray heating may quench star formation., Comment: Accepted for publication in A&A. Language editing completed in version 2