[Paper Review] The molecular gas reservoir of 6 low-metallicity galaxies from the Herschel Dwarf Galaxy Survey: A ground-based follow-up survey of CO(1-0), CO(2-1), and CO(3-2)

D. Cormier, S. C. Madden+15 more|Chalmers Publication Library (Chalmers University of Technology)|Jan 2, 2014

Astrophysics and Star Formation StudiesPhysics and Astronomy115 references60 citations

TL;DR

This study investigates the molecular gas reservoir in six low-metallicity dwarf galaxies using ground-based CO(1-0), CO(2-1), and CO(3-2) observations, combined with Herschel/PACS [C ii] and [O i] data. Despite low CO luminosities and high [C ii]/CO ratios (>7,000), the galaxies remain consistent with the Schmidt-Kennicutt relation when total gas is considered, indicating that molecular gas depletion times are short even with metallicity-scaled X_CO factors, suggesting efficient star formation may involve non-molecular gas phases.

ABSTRACT

We aim to quantify the molecular gas reservoir in a subset of 6 low-metallicity galaxies from the Herschel Dwarf Galaxy Survey with newly acquired CO data, and link this reservoir to the observed star formation activity. 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] 157um and [OI] 63um observations obtained with the Herschel/PACS spectrometer in the 6 galaxies: Haro11, Mrk1089, Mrk930, NGC4861, NGC625, and UM311. We derive molecular gas mass from several methods including the use of the CO-to-H2 conversion factor Xco (both Galactic and metallicity-scaled values) and of dust measurements. The molecular and atomic gas reservoirs are compared to the star formation activity. We also constrain the physical conditions of the molecular clouds using the non-LTE code RADEX and the spectral synthesis code Cloudy. We detect CO in 5 of the 6 galaxies, including first detections in Haro11 (Z~0.4 Zsun), Mrk930 (0.2 Zsun), and UM311 (0.5 Zsun), but CO remains undetected in NGC4861 (0.2 Zsun). The CO luminosities are low while [CII] is bright in these galaxies, resulting in [CII]/CO(1-0)>10000. Our dwarf galaxies are in relatively good agreement with the Schmidt-Kennicutt relation for total gas. They show short molecular depletion time scales, even when considering metallicity-scaled Xco factors. Those galaxies are dominated by their HI gas, except Haro11 which has high star formation efficiency and is dominated by ionized and molecular gas. We determine the mass of each ISM phase in Haro11 using Cloudy and estimate an equivalent Xco factor which 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.

Motivation & Objective

• To quantify the molecular gas reservoir in six low-metallicity dwarf galaxies from the Herschel Dwarf Galaxy Survey.
• To resolve the discrepancy between active star formation and faint CO emission in low-metallicity environments.
• To assess whether molecular gas depletion timescales are short in these galaxies despite low CO luminosities.
• To evaluate the role of different ISM phases—molecular, atomic, ionized—in regulating star formation in low-metallicity dwarfs.
• To test the validity of standard X_CO conversion factors and alternative methods (dust, radiative transfer) for estimating molecular mass.

Proposed method

• Conducted CO(1-0), CO(2-1), and CO(3-2) line observations using the Mopra, APEX, and IRAM 30-m telescopes.
• Combined with Herschel/PACS spectroscopy of [C ii] 157 μm and [O i] 63 μm fine-structure lines.
• Estimated molecular gas mass using Galactic and metallicity-scaled X_CO factors, dust-based methods, and radiative transfer modeling with RADEX and Cloudy.
• Compared molecular and atomic gas masses to star formation rates to assess the Schmidt-Kennicutt relation.
• Used Cloudy and RADEX to constrain physical conditions in molecular clouds, including density, temperature, and filling factor.
• Accounted for beam dilution and source size uncertainties by comparing results across different telescope beam sizes.

Experimental results

Research questions

• RQ1 Why do low-metallicity dwarf galaxies show strong star formation despite faint CO emission?
• RQ2 What is the true molecular gas mass in these galaxies when standard X_CO factors are invalid?
• RQ3 How do [C ii]/CO luminosity ratios reflect the physical conditions and gas heating efficiency in low-metallicity environments?
• RQ4 To what extent do atomic and ionized gas phases contribute to star formation in these galaxies?
• RQ5 Do molecular gas depletion timescales in low-metallicity dwarfs differ from those in high-metallicity spirals?

Key findings

• CO was detected in five of the six galaxies, including first-time detections in Haro 11 (Z ≈ 0.4 Z⊙), Mrk 930 (Z ≈ 0.2 Z⊙), and UM 311 (Z ≈ 0.5 Z⊙), while NGC 4861 (Z ≈ 0.2 Z⊙) remained undetected.
• The [C ii]/CO(1-0) luminosity ratio exceeded 7,000 in all detected galaxies, indicating low filling factor of CO-emitting material and high [C ii] heating efficiency.
• Molecular gas masses varied significantly by method: Galactic X_CO predicted unrealistically low masses, while metallicity-scaled X_CO, dust-based estimates, and radiative transfer modeling yielded higher, more consistent values with uncertainties of a few times.
• Despite low molecular gas fractions, all galaxies—including NGC 4861—remained consistent with the total gas Schmidt-Kennicutt relation, indicating that star formation is regulated by total gas reservoir.
• Molecular gas depletion timescales were short even with metallicity-scaled X_CO factors, suggesting either enhanced star formation efficiency or significant contributions from atomic/ionized gas to star formation.
• In Haro 11, which has high star formation efficiency and is dominated by ionized and molecular gas, the derived X_CO factor was estimated to be 10 times higher than the Galactic value, indicating extreme photodissociation or structural differences in molecular clouds.