Context.
Molecular gas is a necessary fuel for star formation. The CO (1−0) transition is often used to deduce the total molecular hydrogen but is challenging to detect in low-metallicity galaxies in ...spite of the star formation taking place. In contrast, the C
II
λ
158
μ
m is relatively bright, highlighting a potentially important reservoir of H
2
that is not traced by CO (1−0) but is residing in the C
+
-emitting regions.
Aims.
Here we aim to explore a method to quantify the total H
2
mass (
M
H
2
) in galaxies and to decipher what parameters control the CO-dark reservoir.
Methods.
We present Cloudy grids of density, radiation field, and metallicity in terms of observed quantities, such as O
I
, C
I
, CO (1−0), C
II
,
L
TIR
, and the total
M
H
2
. We provide recipes based on these models to derive total
M
H
2
mass estimates from observations. We apply the models to the
Herschel
Dwarf Galaxy Survey, extracting the total
M
H
2
for each galaxy, and compare this to the H
2
determined from the observed CO (1−0) line. This allows us to quantify the reservoir of H
2
that is CO-dark and traced by the C
II
λ
158
μ
m.
Results.
We demonstrate that while the H
2
traced by CO (1−0) can be negligible, the C
II
λ
158
μ
m can trace the total H
2
. We find 70 to 100% of the total H
2
mass is not traced by CO (1−0) in the dwarf galaxies, but is well-traced by C
II
λ
158
μ
m. The CO-dark gas mass fraction correlates with the observed
L
C
II
/
L
CO(1−0)
ratio. A conversion factor for C
II
λ
158
μ
m to total H
2
and a new CO-to-total-
M
H
2
conversion factor as a function of metallicity are presented.
Conclusions.
While low-metallicity galaxies may have a feeble molecular reservoir as surmised from CO observations, the presence of an important reservoir of molecular gas that is not detected by CO can exist. We suggest a general recipe to quantify the total mass of H
2
in galaxies, taking into account the CO and C
II
observations. Accounting for this CO-dark H
2
gas, we find that the star-forming dwarf galaxies now fall on the Schmidt–Kennicutt relation. Their star-forming efficiency is rather normal because the reservoir from which they form stars is now more massive when introducing the C
II
measures of the total H
2
compared to the small amount of H
2
in the CO-emitting region.
Context.
Finding and elucidating the properties of Lyman-continuum(LyC)-emitting galaxies is an important step in improving our understanding of cosmic reionization.
Aims.
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. Furthermore, it is unclear to what extent these emission lines can be used to distinguish LyC emitters.
Methods.
To address these questions we construct one- and two-zone photoionization models accounting for the observed LyC escape, which we compare to the observed emission line measurements. The main diagnostics used include lines of O
III
, O
II
, and O
I
plus sulfur lines (S
II
, S
III
) and a nitrogen line (N
II
), which probe regions of different ionization in the ISM.
Results.
We find that single (one-zone) density-bounded photoionization models cannot reproduce the emission lines of the LyC leakers, as pointed out by earlier studies, because they systematically underpredict the lines of species of low ionization potential, such as O
I
and S
II
. 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 of the LyC emitters are well reproduced. Furthermore, our model yields LyC escape fractions, which are in fair agreement with the observations and independent measurements. The O
I
λ
6300 excess, which is observed in some LyC leakers, can be naturally explained in this model, for example by emission from low-ionization and low-filling-factor gas. LyC emitters with a high escape fraction (
f
esc
≳ 38%) are deficient both in O
I
λ
6300 and in S
II
λ
λ
6716,6731. We also confirm that a S
II
λ
λ
6716,6731 deficiency can be used to select LyC emitter candidates, as suggested earlier. Finally, we find indications for a possible dichotomy in terms of escape mechanisms for LyC photons between galaxies with relatively low (
f
esc
≲ 10%) and higher escape fractions.
Conclusions.
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. This is in agreement with UV absorption line studies, which also show the co-existence of regions with high hydrogen column density (i.e., no escape of ionizing photons) and density-bounded or very low column density regions responsible for the observed escape of LyC radiation. These simple but consistent models provide a first step towards the use of optical emission lines and their ratios as quantitative diagnostics of LyC escape from galaxies.
ABSTRACT
Using new Very Large Telescope (VLT)/XShooter spectral observations we analyse the physical properties of five z ∼ 0.3–0.4 confirmed Lyman continuum (LyC) leakers. Strong resonant Mg ...ii λλ2796, 2803 Å emission lines (I(λλ2796, 2803)/I(Hβ) ≃ 10–38 per cent) and non-resonant Fe ii* λλ2612, 2626 Å emission lines are observed in spectra of five and three galaxies, respectively. We find high electron densities Ne ∼ 400 cm−3, significantly higher than in typical low-z, but comparable to those measured in z ∼ 2–3 star-forming galaxies (SFGs). The galaxies have a mean value of log N/O = –1.16, close to the maximum values found for SFGs in the metallicity range of 12 + log O/H ≃ 7.7–8.1. All 11 low-z LyC emitting galaxies found by Izotov et al., including the ones considered in this study, are characterized by high equivalent width (EW) (Hβ) ∼ 200–400 Å, high ionization parameter (log(U) = –2.5 to –1.7), high average ionizing photon production efficiency ξ = 1025.54 Hz erg−1, and hard ionizing radiation. On the Baldwin–Phillips–Terlevich (BPT) diagram we find the same offset of our leakers from low-z main-sequence SFGs as that for local analogues of Lyman-break galaxies (LBGs) and extreme SFGs at z ∼ 2–3. We confirm the effectiveness of the He i emission lines diagnostics proposed by Izotov et al. 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(H i) = 1017–5 × 1017 cm−2 that permit leakage of LyC radiation, likely due to their density-bounded H ii regions.
Local metal-poor galaxies stand as ideal laboratories for probing the properties of the interstellar medium (ISM) in chemically unevolved conditions. Detailed studies of this primitive ISM can help ...gain insights into the physics of the first primordial galaxies that may be responsible for the reionization. Quantifying the ISM porosity to ionizing photons in nearby galaxies may improve our understanding of the mechanisms leading to Lyman continuum photon leakage from galaxies. The wealth of infrared (IR) tracers available in local galaxies and arising from different ISM phases allows us to constrain complex models in order to estimate physical quantities.
Context. 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 that is not traced by CO. Constraining the relative importance of CO-bright versus CO-dark H2 star-forming reservoirs is crucial to understanding how star formation proceeds at low metallicity. Aims. We test classically used single component radiative transfer models and compare their results to those obtained on the assumption of an increasingly complex structure of the interstellar gas, mimicking an inhomogeneous distribution of clouds with various physical properties. Methods. Using the Bayesian code MULTIGRIS, we computed representative models of the interstellar medium as combinations of several gas components, each with a specific set of physical parameters. We introduced physically motivated models assuming power-law distributions for the density, ionization parameter, and the depth of molecular clouds. Results. This new modeling framework allows for the simultaneous reproduction of the spectral constraints from the ionized gas, neutral atomic gas, and molecular gas in 18 galaxies from the Dwarf Galaxy Survey. 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 II158 μm and, to a lesser extent, by C I 609 μm, rather than by CO(1–0). We examine the CO-to-H2 conversion factor (αCO) versus metallicity relation and find that its dispersion increases significantly when different geometries of the gas are considered. We define a “clumpiness” parameter that is anti-correlated with C II/CO and explains the dispersion of the αCO versus metallicity relation. We find that low-metallicity galaxies with high clumpiness parameters may have αCO values as low as the Galactic value, even at low metallicity. Conclusions. We identify the clumpiness of molecular gas as a key parameter for understanding variations of geometry-sensitive quantities, such as αCO. This new modeling framework enables the derivation of constraints on the internal cloud distribution of unresolved galaxies, based solely on their integrated spectra.
Context.
Spectroscopic observations of high-redshift galaxies slowly reveal the same complexity of the interstellar medium (ISM) as expected from resolved observations in nearby galaxies. While ...providing, in principle, a wealth of diagnostics concerning galaxy evolution, star formation, or the nature and influence of compact objects, such high-
z
spectra are often spatially and spectrally unresolved, and inferring reliable diagnostics represents a major obstacle. Bright, nearby, unresolved galaxies observed in the optical and infrared domains provide many constraints to design methods to infer ISM properties, but they have so far been limited to deterministic methods and/or with simple topological assumptions (e.g., single 1D model).
Aims.
It is urgent to build upon previous ISM multiphase and multicomponent methods by using a probabilistic approach that makes it possible to derive probability density functions for relevant parameters while also enabling a large number of free parameters with potential priors. The goal is to provide a flexible statistical framework that is agnostic to the model grid and that considers either a few discrete components defined by their parameter values and/or statistical distributions of parameters. In this paper, we present a first application with the objective to infer probability distributions of several physical parameters (e.g., the mass of H
0
, H
2
, escape fraction of ionizing photons, and metallicity) for the star-forming regions of the metal-poor dwarf galaxy I Zw 18 in order to confirm the low molecular gas content and high escape fraction of ionizing photons from H
ii
regions.
Methods.
We present a Bayesian approach to model a suite of spectral lines using a sequential Monte Carlo method provided by the Python package PyMC which combines several concepts such as tempered likelihoods, importance sampling, and independent Metropolis-Hastings chains. The algorithm, provided by the associated code MULTIGRIS, accepts a few components which can be represented as sectors around one or several stellar clusters, or continuous (e.g., power-law, normal) distributions for any given parameter. We applied this approach to a grid of models calculated with the photoionization and photodissociation code Cloudy in order to produce topological models of I Zw 18.
Results.
The statistical framework we present makes it possible to consider a large number of spectroscopic tracers, with the extinction and systematic uncertainties as potential additional random variables. We applied this technique to the galaxy I Zw 18 in order to reproduce and go beyond previous topological models specifically tailored to this object. While our grid is designed for global properties of low-metallicity star-forming galaxies, we were able to calculate accurate values for the metallicity, number of ionizing photons, masses of ionized and neutral hydrogen, as well as the dust mass and the dust-to-gas mass ratio in I Zw 18. We find a relatively modest amount of H
2
(~10
5
M
⊙
) which is predominantly CO-dark and traced by C
+
rather than C
0
. Nevertheless, more than 90% of the C
ii
emission is associated with the neutral atomic gas. Our models confirm the necessity to include an X-ray source with an inferred luminosity in good agreement with direct X-ray observations. Finally, we investigate the escape fraction of ionizing photons for different energy ranges. While the escape fraction for the main H
ii
region lies around 50–65%, we show that most of the soft X-ray photons are able to escape and may play a role in the ionization and heating of the circumgalactic or intergalactic medium.
Conclusions.
Multicomponent ISM models associate a complex enough distribution of matter and phases with a simple enough topological description to be constrained with probabilistic frameworks. Despite ignoring effects such as reflected light, the diffuse radiation field, or ionization by several non-cospatial sources, they remain well adapted to individual H
ii
regions and to star-forming galaxies dominated by one or a few H
ii
regions, and the improvement due to the combination of several components largely compensates for other secondary effects.
Context
. The O
iii
λ
88 µm line is observed in many galaxies including our neighboring Magellanic Clouds and is a well-known tracer of H
ii
regions, while the 24 µm continuum emission has often ...been used to trace warm dust in the ionized phases of galaxies. The association of both the O
iii
λ
88 µm line and 24 µm in galaxies to star formation motivates this study to determine their observational relation.
Aims
. This study explores the link between the O
iii
λ
88 µm and 24 µm continuum in star-forming regions in the Magellanic Clouds. We also explore the local conditions driving the relation between those tracers.
Methods
. We compared observations with 1D Cloudy models consisting of an H
ii
region plus a photodissociation region (PDR) component, varying the stellar age, the initial density (at the illuminated edge of the cloud), and the ionization parameter. We introduced a new parameter,
c
PDR
, to quantify the proportion of emission arising from PDRs and that with an origin in H
ii
regions along each line of sight. We used the ratio (C
ii
+O
i
)/O
iii
as a proxy for the ratio of PDR versus H
ii
region emission, and compared it to the O
iii
/24 µm ratio. The use of O
iii
/24 µm and O
iii
/70 µm together allowed us to constrain the models most efficiently.
Results
. We find a correlation over at least 3 orders of magnitude in O
iii
λ
88 µm and 24 µm continuum. This correlation is seen for spatially resolved maps of the Magellanic Cloud regions as well as unresolved galaxy-wide low metallicity galaxies of the Dwarf Galaxy Survey. We also find that most of the regions have low proportions of PDRs along the lines of sight (<12%), while a limited area of some of the mapped regions can reach 30–50%. For most lines of sight within the star-forming regions we have studied in the Magellanic Clouds, H
ii
regions are the dominant phase.
Conclusions
. We propose the use of the correlation between the O
iii
λ
88 µm and 24 µm continuum as a new predictive tool to estimate, for example, the O
iii
λ
88 µm when the 24 µm continuum is available or inversely. This can be especially useful to prepare for Atacama Large Milimeter Array (ALMA) observations of O
iii
λ
88 µm in high-z galaxies. The simple and novel method we developed may also provides a way to disentangle different phases along the line of sight, when other 3D information is not available.
Context.
Massive star formation leads to enrichment of the interstellar medium with heavy elements. On the other hand, the abundance of heavy elements is a key parameter with which to study the ...star-formation history of galaxies. Furthermore, the total molecular hydrogen mass, usually determined by converting CO or C
II
158 μm luminosities, depends on the metallicity as well. However, the excitation of metallicity-sensitive emission lines depends on the gas density of the H
II
regions where they arise.
Aims.
We used spectroscopic observations of the nuclear region of the starburst galaxy NGC 253 from SOFIA,
Herschel
, and
Spitzer
, as well as photometric observations from GALEX, 2MASS,
Spitzer
, and
Herschel
in order to derive physical properties such as the optical depth to correct for extinction, as well as the gas density and metallicity of the central region.
Methods.
Ratios of the integrated line fluxes of several species were utilised to derive the gas density and metallicity. The O
III
along with the S
III
and N
II
line flux ratios, for example, are sensitive to the gas density but nearly independent of the local temperature. As these line ratios trace different gas densities and ionisation states, we examined whether or not these lines could originate from different regions within the observing beam. The (Ne
II
13 μm + Ne
III
16 μm)/H
α
line flux ratio on the other hand is independent of the depletion onto dust grains but sensitive to the Ne/H abundance ratio and is used as a tracer for metallicity of the gas.
Results.
We derived values for gas phase abundances of the most important species, as well as estimates for the optical depth and the gas density of the ionised gas in the nuclear region of NGC 253. We obtained densities of at least two different ionised components (< 84 cm
−3
and ∼170−212 cm
−3
) and a metallicity of solar value.
We present the OIII52{\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 OIII52{\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 OIII88{\mu}m/OIII52{\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 SIII33{\mu}m/SIII18{\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.
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 II158um 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.
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