Molecular depletion times and the CO-to-H<sub>2</sub>conversion factor in metal-poor galaxies

Hunt, L. K.; García‐Burillo, S.; Casasola, V.; Caselli, P.; Combes, F.; Henkel, C.; Lundgren, Andreas; Maiolino, R.; Menten, K. M.; Testi, L.; Weiß, A.

doi:10.1051/0004-6361/201526553

Cited by 115 publications

(179 citation statements)

References 160 publications

(344 reference statements)

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“…We note that in the process of refereeing the present paper, a work by Hunt et al (2015), which tackles similar goals, has just been published. Their results on the CO emission of eight lowmetallicity dwarf galaxies are certainly complementary to the work presented here, and their conclusions on the molecular depletion timescales of these galaxies appear in good agreement with ours.…”

Section: Discussionmentioning

confidence: 86%

Molecular gas in low-metallicity starburst galaxies:

Amorín

Muñoz–Tuñón

Aguerri

et al. 2016

A&A

121

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Context. Tracing the molecular gas-phase in low-mass star-forming galaxies becomes extremely challenging due to significant UV photo-dissociation of CO molecules in their low-dust, low-metallicity ISM environments. Aims. We aim to study the molecular content and the star-formation efficiency of a representative sample of 21 blue compact dwarf galaxies (BCDs), previously characterized on the basis of their spectrophotometric properties. Methods. We present CO (1-0) and (2-1) observations conducted at the IRAM-30m telescope. These data are further supplemented with additional CO measurements and multiwavelength ancillary data from the literature. We explore correlations between the derived CO luminosities and several galaxy-averaged properties. Results. We detect CO emission in seven out of ten BCDs observed. For two galaxies these are the first CO detections reported so far. We find the molecular content traced by CO to be correlated with the stellar and Hi masses, star formation rate (SFR) tracers, the projected size of the starburst, and its gas-phase metallicity. BCDs appear to be systematically offset from the Schmidt-Kennicutt (SK) law, showing lower average gas surface densities for a given Σ SFR , and therefore showing extremely low ( < ∼ 0.1 Gyr) H 2 and H 2 +Hi depletion timescales. The departure from the SK law is smaller when considering H 2 +Hi rather than H 2 only, and is larger for BCDs with lower metallicity and higher specific SFR. Thus, the molecular fraction (Σ H 2 /Σ HI ) and CO depletion timescale (Σ H 2 /Σ SFR ) of BCDs is found to be strongly correlated with metallicity. Using this, and assuming that the empirical correlation found between the specific SFR and galaxy-averaged H 2 depletion timescale of more metal-rich galaxies extends to lower masses, we derive a metallicitydependent CO-to-H 2 conversion factor α CO,Z ∝ (Z/Z ) −y , with y = 1.5(±0.3) in qualitative agreement with previous determinations, dust-based measurements, and recent model predictions. Consequently, our results suggest that in vigorously star-forming dwarfs the fraction of H 2 traced by CO decreases by a factor of about 40 from Z ∼ Z to Z ∼ 0.1 Z , leading to a strong underestimation of the H 2 mass in metal-poor systems when a Galactic α CO,MW is considered. Adopting our metallicity-dependent conversion factor α CO,Z we find that departures from the SK law are partially resolved. Conclusions. Our results suggest that starbursting dwarfs have shorter depletion gas timescales and lower molecular fractions compared to normal late-type disc galaxies, even accounting for the molecular gas not traced by CO emission in metal-poor environments, raising additional constraints to model predictions.

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Section: Discussionmentioning

confidence: 86%

Molecular gas in low-metallicity starburst galaxies:

Amorín

Muñoz–Tuñón

Aguerri

et al. 2016

A&A

121

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“…Leroy et al 2005;Lisenfeld et al 2011;Young et al 2011;Amorín et al 2016), SFR (as traced by the FIR luminosity e.g. Sanders & Mirabel 1985;Tacconi & Young 1987;Verter 1988;Solomon & Sage 1988;Young & Scoville 1991;Solomon et al 1997;Gao & Solomon 2004a;Leroy et al 2005;Chung et al 2009;Lisenfeld et al 2011;Schruba et al 2011;Hunt et al 2015;Amorín et al 2016), gas-phase metallicity (e.g. Schruba et al 2012;Amorín et al 2016;Kepley et al 2016), and M HI (e.g.…”

Section: Co Line Luminosity As a Function Of Galaxy Propertiesmentioning

confidence: 99%

The final data release of ALLSMOG: a survey of CO in typical local low-M_∗star-forming galaxies

Cicone

Bothwell

Wagg³

et al. 2017

A&A

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We present the final data release of the APEX low-redshift legacy survey for molecular gas (ALLSMOG), comprising CO(2-1) emission line observations of 88 nearby, low-mass (10 8.5 < M * [M ] < 10 10 ) star-forming galaxies carried out with the 230 GHz APEX-1 receiver on the APEX telescope. The main goal of ALLSMOG is to probe the molecular gas content of more typical and lower stellar mass galaxies than have been studied by previous CO surveys. We also present IRAM 30 m observations of the CO(1-0) and CO(2-1) emission lines in nine galaxies aimed at increasing the M * < 10 9 M sample size. In this paper we describe the observations, data reduction and analysis methods and we present the final CO spectra together with archival Hi 21 cm line observations for the entire sample of 97 galaxies. At the sensitivity limit of ALLSMOG, we register a total CO detection rate of 47%. Galaxies with higher M * , SFR, nebular extinction (A V ), gas-phase metallicity (O/H), and Hi gas mass have systematically higher CO detection rates. In particular, the parameter according to which CO detections and non-detections show the strongest statistical differences is the gas-phase metallicity, for any of the five metallicity calibrations examined in this work. We investigate scaling relations between the CO(1-0) line luminosity (L CO(1−0) ) and galaxy-averaged properties using ALLSMOG and a sub-sample of COLD GASS for a total of 185 sources that probe the local main sequence (MS) of star-forming galaxies and its ±0.3 dex intrinsic scatter from M * = 10 8.5 M to M * = 10 11 M . L CO(1−0) is most strongly correlated with the SFR, but the correlation with M * is closer to linear and almost comparably tight. The relation between L CO(1−0) and metallicity is the steepest one, although deeper CO observations of galaxies with A V < 0.5 mag may reveal an as much steep correlation with A V . Our results suggest that star-forming galaxies across more than two orders of magnitude in M * obey similar scaling relations between CO luminosity and the galaxy properties examined in this work. Besides SFR, the CO luminosity is likely most fundamentally linked to M * , although we note that stellar mass alone cannot explain all of the variation in CO emission observed as a function of O/H and M HI .

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“…As the average metallicity is subsolar by only a factor of two and the morphology remains that of a rotating disk, M33 represents a stepping stone towards lower metallicity and less regular objects. Measuring the link between CO and H 2 is particularly important given the evidence that the conversion of H 2 into stars becomes more efficient at lower metallicities (Gardan et al 2007;Gratier et al 2010a;Druard et al 2014;Hunt et al 2015).…”

Section: Notesmentioning

confidence: 99%

The molecular gas mass of M 33

Gratier

Braine

Schüster

et al. 2017

A&A

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Do some environments favor efficient conversion of molecular gas into stars? To answer this, we need to be able to estimate the H2 mass. Traditionally, this is done using CO observations and a few assumptions but the Herschel observations which cover the Far-IR dust spectrum make it possible to estimate the molecular gas mass independently of CO and thus to investigate whether and how the CO traces H2. Previous attempts to derive gas masses from dust emission suffered from biases. Generally, dust surface densities, H i column densities, and CO intensities are used to derive a gas-to-dust ratio (GDR) and the local CO intensity to H2 column density ratio (XCO), sometimes allowing for an additional CO-dark gas component (K dark ). We tested earlier methods, revealing degeneracies among the parameters, and then used a sophisticated Bayesian formalism to derive the most likely values for each of the parameters mentioned above as a function of position in the nearby prototypical low metallicity (12 + log(O/H) ∼ 8.4) spiral galaxy M33. The data are from the IRAM Large Program mapping in the CO(2-1) line along with high-resolution H i and Herschel dust continuum observations. Solving for GDR, XCO, and K dark in macropixels 500 pc in size, each containing many individual measurements of the CO, H i, and dust emission, we find that (i) allowing for CO dark gas (K dark ) significantly improves fits; (ii) K dark decreases with galactocentric distance; (iii) GDR is slightly higher than initially expected and increases with galactocentric distance; (iv) the total amount of dark gas closely follows the radially decreasing CO emission, as might be expected if the dark gas is H2 where CO is photodissociated. The total amount of H2, including dark gas, yields an average XCO of twice the galactic value of 2 × 10 20 cm −2 / K km s −1 , with about 55% of this traced directly through CO. The rather constant fraction of dark gas suggests that there is no large population of diffuse H2 clouds (unrelated to GMCs) without CO emission. Unlike in large spirals, we detect no systematic radial trend in XCO, possibly linked to the absence of a radial decrease in CO line ratios.

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Molecular depletion times and the CO-to-H₂conversion factor in metal-poor galaxies

Cited by 115 publications

References 160 publications

Molecular gas in low-metallicity starburst galaxies:

Molecular gas in low-metallicity starburst galaxies:

The final data release of ALLSMOG: a survey of CO in typical local low-M_∗star-forming galaxies

The molecular gas mass of M 33

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Molecular depletion times and the CO-to-H2conversion factor in metal-poor galaxies

Cited by 115 publications

References 160 publications

Molecular gas in low-metallicity starburst galaxies:

Molecular gas in low-metallicity starburst galaxies:

The final data release of ALLSMOG: a survey of CO in typical local low-M∗star-forming galaxies

The molecular gas mass of M 33

Contact Info

Product

Resources

About

Molecular depletion times and the CO-to-H₂conversion factor in metal-poor galaxies

The final data release of ALLSMOG: a survey of CO in typical local low-M_∗star-forming galaxies