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.
The Herschel Dwarf Galaxy Survey Cormier, D; Madden, S C; Lebouteiller, V ...
Astronomy and astrophysics (Berlin),
06/2015, Letnik:
578
Journal Article
Recenzirano
Odprti dostop
The far-infrared (FIR) lines are important tracers of the cooling and physical conditions of the interstellar medium (ISM) and are rapidly becoming workhorse diagnostics for galaxies throughout the ...universe. Our goal is to explain the main differences and trends observed in the FIR line emission of dwarf galaxies compared to more metal-rich alaxies, and how this translates in ISM properties. We present Herschel/PACS spectroscopic observations of the CII 157 mum, OI 63 and 145 mum, OIII 88 mum, NII 122 and 205 mum, and NIII 57 mum fine-structure cooling lines in a sample of 48 low-metallicity star-forming galaxies of the guaranteed time key program Dwarf Galaxy Survey. We correlate PACS line ratios and line-to-LTIR ratios with LTIR, LTIR/LB, metallicity, and FIR color, and interpret the observed trends in terms of ISM conditions and phase filling factors with Cloudy radiative transfer models.
We report far-infrared and submillimeter observations of supernova 1987A, the star whose explosion was observed on 23 February 1987 in the Large Magellanic Cloud, a galaxy located 160,000 light years ...away. The observations reveal the presence of a population of cold dust grains radiating with a temperature of about 17 to 23 kelvin at a rate of about 220 times the luminosity of the Sun. The intensity and spectral energy distribution of the emission suggest a dust mass of about 0.4 to 0.7 times the mass of the Sun. The radiation must originate from the supernova ejecta and requires the efficient precipitation of all refractory material into dust. Our observations imply that supernovae can produce the large dust masses detected in young galaxies at very high redshifts.
Context. Star formation activity is an important driver of galaxy evolution and is influenced by the physical properties of the interstellar medium. Dwarf galaxies allow us to understand how the ...propagation of radiation and the physical conditions of the different ISM phases are affected by the low-metallicity environment. Aims. Our objective is to investigate the physical properties of the ionized gas of the low-metallicity dwarf galaxy, IC 10, at various spatial scales: from individual H II regions to the entire galaxy scale and examine whether diagnostics for integrated measurements introduce bias in the results. Methods. We modeled the ionized gas combining the mid- and far-infrared fine-structure cooling lines observed with Spitzer/IRS and Herschel/PACS, with the photoionization code CLOUDY. The free parameters of the models are the age of the stellar cluster, the density, and the ionization parameter of the ionized gas as well as the depth of the cloud. The latter is used to investigate the leakage of the ionizing photons from the analyzed regions of IC 10. We investigated H II 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 considered larger sizes on the scale of ~200 pc. Results. Most clumps have almost-identical properties, density ~102–102.6 cm−3, ionization 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 ionizing 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 H II regions they encompass, but with the larger regions tending to be more radiation bounded. Finally, we investigated the fraction of CII 157.7 μm, SiII 34.8 μm and FeII 25.9 μm emission arising from the ionized gas phase and we find that most of the emission originates from the neutral gas, not from the ionized gas.
Context. The low-metallicity interstellar medium (ISM) is profoundly different from that of normal systems, being clumpy with low dust abundance and little CO-traced molecular gas. Yet many dwarf ...galaxies in the nearby universe are actively forming stars. As the complex ISM phases are spatially mixed with each other, detailed modeling is needed to understand the gas emission and subsequent composition and structure of the ISM. Aims. Our goal is to describe the multi-phase ISM of the infrared bright low-metallicity galaxy Haro 11, dissecting the photoionised and photodissociated gas components. Methods. We present observations of the mid-infrared and far-infrared fine-structure cooling lines obtained with the Spitzer/IRS and Herschel/PACS spectrometers. We use the spectral synthesis code Cloudy to methodically model the ionised and neutral gas from which these lines originate. Results. We find that the mid- and far-infrared lines account for ~1% of the total infrared luminosity LTIR, acting as major coolants of the gas. Haro 11 is undergoing a phase of intense star formation, as traced by the brightest line, O iii 88 μm, with L O III /LTIR ~ 0.3%, and high ratios of Ne iii/Ne ii and S iv/S iii. Due to their different origins, the observed lines require a multi-phase modeling comprising: a compact H ii region, dense fragmented photodissociation regions (PDRs), a diffuse extended low-ionisation/neutral gas which has a volume filling factor of at least 90%, and porous warm dust in proximity to the stellar source. For a more realistic picture of the ISM of Haro 11 we would need to model the clumpy source and gas structures. We combine these 4 model components to explain the emission of 17 spectral lines, investigate the global energy balance of the galaxy through its spectral energy distribution, and establish a phase mass inventory. While the ionic emission lines of Haro 11 essentially originate from the dense H ii region component, a diffuse low-ionisation gas is needed to explain the Ne ii, N ii, and C ii line intensities. The O iii 88 μm line intensity is not fully reproduced by our model, hinting towards the possible presence of yet another low-density high-ionisation medium. The O i emission is consistent with a dense PDR of low covering factor, and we find no evidence for an X-ray dominated component. The PDR component accounts for only 10% of the C ii emission. Magnetic fields, known to be strong in star-forming regions, may dominate the pressure in the PDR. For example, for field strengths of the order of 100 μG, up to 50% of the C ii emission may come from the PDR.
Context. Observations of nearby starburst and spiral galaxies have revealed that molecular gas is the driver of star formation. However, some nearby low-metallicity dwarf galaxies are actively ...forming stars, but CO, the most common tracer of this reservoir, is faint, leaving us with a puzzle about how star formation proceeds in these environments. Aims. We aim to quantify the molecular gas reservoir in a subset of 6 galaxies from the Herschel Dwarf Galaxy Survey with newly acquired CO data and to link this reservoir to the observed star formation activity. Methods. We present CO(1-0), CO(2-1), and CO(3-2) observations obtained at the ATNF Mopra 22-m, APEX, and IRAM 30-m telescopes, as well as CII 157 mu m and OI 63 mu m observations obtained with the Herschel/PACS spectrometer in the 6 low-metallicity dwarf galaxies: Haro 11, Mrk 1089, Mrk 930, NGC 4861, NGC 625, and UM 311. We derived their molecular gas masses from several methods, including using the CO-to-H sub(2) conversion factor X sub(CO) (both Galactic and metallicity-scaled values) and dust measurements. The molecular and atomic gas reservoirs were compared to the star formation activity. We also constrained the physical conditions of the molecular clouds using the non-LTE code RADEX and the spectral synthesis code Cloudy. Results. We detect CO in 5 of the 6 galaxies, including first detections in Haro 11 (Z ~ 0.4 Zmiddot in circle), Mrk 930 (0.2 Zmiddot in circle), and UM 311 (0.5 Zmiddot in circle), but CO remains undetected in NGC 4861 (0.2Zmiddot in circle). The CO luminosities are low, while CII is bright in these galaxies, resulting in CII/CO(1-0) > or = 10 000. Our dwarf galaxies are in relatively good agreement with the Schmidt-Kennicutt relation for total gas. They show short molecular depletion timescales, even when considering metallicity-scaled X sub(CO) factors. Those galaxies are dominated by their HIgas, except Haro 11, which has high star formation efficiency and is dominated by ionized and molecular gas. We determine the mass of each ISM phase in Haro 11 using Cloudy and estimate an equivalent X sub(CO) factor that is 10 times higher than the Galactic value. Overall, our results confirm the emerging picture that CO suffers from significant selective photodissociation in low-metallicity dwarf galaxies.
We present new photometric data from our Herschel guaranteed time key programme, the Dwarf Galaxy Survey (DGS), dedicated to the observation of the gas and dust in low-metallicity environments. A ...total of 48 dwarf galaxies were observed with the PACS and SPIRE instruments onboard the Herschel Space Observatory at 70, 100, 160, 250, 350, and 500 mu m. The goal of this paper is to provide reliable far-infrared (FIR) photometry for the DGS sample and to analyse the FIR/submillimetre (submm) behaviour of the DGS galaxies. We focus on a systematic comparison of the derived FIR properties with more metal-rich galaxies and investigate the detection of a potential submm excess. The data reduction method is adapted for each galaxy in order to derive the most reliable photometry from the final maps. To study the variation in the dust properties with metallicity, we also include galaxies from the Herschel KINGFISH sample, which contains more metalrich environments, totalling 109 galaxies.
An Overview of the Dwarf Galaxy Survey Madden, S. C.; Rémy-Ruyer, A.; Galametz, M. ...
Publications of the Astronomical Society of the Pacific,
06/2013, Letnik:
125, Številka:
928
Journal Article
Recenzirano
Odprti dostop
ABSTRACT The Dwarf Galaxy Survey (DGS) program is studying low-metallicity galaxies using 230 hr of far-infrared (FIR) and submillimetre (submm) photometric and spectroscopic observations of the ...Herschel Space Observatory and draws from this a rich database of a wide range of wavelengths tracing the dust, gas and stars. This sample of 50 galaxies includes the largest metallicity range achievable in the local Universe including the lowest metallicity ( Z) galaxies, 1/50 Z⊙, and spans four orders of magnitude in star formation rates. The survey is designed to get a handle on the physics of the interstellar medium (ISM) of low metallicity dwarf galaxies, especially their dust and gas properties and the ISM heating and cooling processes. The DGS produces PACS and SPIRE maps of low-metallicity galaxies observed at 70, 100, 160, 250, 350, and 500 μm with the highest sensitivity achievable to date in the FIR and submm. The FIR fine-structure lines, CII 158 μm, OI 63 μm, OI 145 μm, OIII 88 μm, NIII 57 μm, and NII 122 and 205 μm have also been observed with the aim of studying the gas cooling in the neutral and ionized phases. The SPIRE FTS observations include many CO lines ( J = 4-3 to J = 13-12), NII 205 μm, and CI lines at 370 and 609 μm. This paper describes the sample selection and global properties of the galaxies and the observing strategy as well as the vast ancillary database available to complement the Herschel observations. The scientific potential of the full DGS survey is described with some example results included.
We present the molecular cloud properties of N55 in the Large Magellanic Cloud using 12CO(1-0) and 13CO(1-0) observations obtained with Atacama Large Millimeter Array. We have done a detailed study ...of molecular gas properties, to understand how the cloud properties of N55 differ from Galactic clouds. Most CO emission appears clumpy in N55, and molecular cores that have young stellar objects (YSOs) show larger linewidths and masses. The massive clumps are associated with high and intermediate mass YSOs. The clump masses are determined by local thermodynamic equilibrium and virial analysis of the 12CO and 13CO emissions. These mass estimates lead to the conclusion that (a) the clumps are in self-gravitational virial equilibrium, and (b) the 12CO(1-0)-to-H2 conversion factor, X CO , is 6.5 × 1020 cm−2 (K km s−1)−1. This CO-to-H2 conversion factor for N55 clumps is measured at a spatial scale of ∼0.67 pc, which is about two times higher than the X CO value of the Orion cloud at a similar spatial scale. The core mass function of N55 clearly show a turnover below 200 M , separating the low-mass end from the high-mass end. The low-mass end of the 12CO mass spectrum is fitted with a power law of index 0.5 0.1, while for 13CO it is fitted with a power law index 0.6 0.2. In the high-mass end, the core mass spectrum is fitted with a power index of 2.0 0.3 for 12CO, and with 2.5 0.4 for 13CO. This power law behavior of the core mass function in N55 is consistent with many Galactic clouds.
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