Molecular Depletion Times and the CO-To-H2 Conversion Factor in Metal-Poor Galaxies ? L

Astronomy & Astrophysics manuscript no. dwarfsmoldep˙v6˙nobold c ESO 2018 April 8, 2018 Molecular depletion times and the CO-to-H2 conversion factor in metal-poor galaxies ? L. K. Hunt1, S. Garc´ıa-Burillo2, V. Casasola1, P. Caselli3, F. Combes4, C. Henkel5;6, A. Lundgren7, R. Maiolino8, K. M. Menten5, L. Testi9, and A. Weiss5 1 INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125, Firenze, Italy e-mail: [email protected] 2 Observatorio Astronomico´ Nacional (OAN)-Observatorio de Madrid, Alfonso XII, 3, 28014-Madrid, Spain 3 Max-Planck-Institut fur¨ extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany 4 Observatoire de Paris, LERMA, College de France, CNRS, PSL, Sorbonne University UPMC, F-75014, Paris, France 5 Max-Planck-Institut fur¨ Radioastronomie, Auf dem Hugel¨ 69, 53121 Bonn, Germany 6 Astronomy Department, King Abdulaziz University, P.O. Box 80203, Jeddah, Saudia Arabia 7 ALMA JAO, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago, Chile 8 Cavendish Laboratory, University of Cambridge, 19 J.J. Thomson Avenue, Cambridge CB3 0HE, UK 9 ESO, Karl Schwarzschild str. 2, 85748 Garching bei Munchen,¨ Germany Received 2015/ Accepted 2015 ABSTRACT Tracing molecular hydrogen content with carbon monoxide in low-metallicity galaxies has been exceedingly difficult. Here we present a new effort, with IRAM 30-m observations of 12CO(1–0) of a sample of 8 dwarf galaxies having oxygen abundances ranging from 12+log(O/H)∼7.7 to 8.4. CO emission is detected in all galaxies, including the most metal-poor galaxy of our sample (0.1 Z ); to 12 our knowledge this is the largest number of CO(1–0) detections ever reported for galaxies with 12+log(O/H)∼<8 (0.2 Z ) outside the Local Group. We calculate stellar masses, Mstar, and star-formation rates (SFRs), and analyze our results in conjunction with galaxy samples from the literature. Extending previous results for a correlation of the molecular gas depletion time, τdep, with Mstar and 3 specific SFR (sSFR), we find a variation in τdep of a factor of 200 or more (from ∼<50 Myr to ∼10 Gyr) over a spread of ∼ 10 in sSFR and Mstar. We exploit the variation of τdep to constrain the CO-to-H2 mass conversion factor αCO at low metallicity, and assuming −2 a power-law variation find αCO / Z=Z , similar to results based on dust continuum measurements compared with gas mass. By including H I measurements, we show that the fraction of total gas mass relative to the baryonic mass is higher in galaxies that are metal poor, of low mass, and with high sSFR. Finally, comparisons of the data with star-formation models of the molecular gas phases show that the models are generally quite successful, but at metallicities Z=Z ∼<0.2, there are some discrepancies. Key words. Galaxies: starburst — Galaxies: dwarf — Galaxies: star formation — Galaxies: ISM — ISM: general 1. Introduction “CO-dark” molecular gas in the interstellar medium (ISM) (e.g., Wolfire et al. 2010; Leroy et al. 2011; Planck Collaboration et Dense clumps in cool molecular clouds are thought to be the al. 2011; Paradis et al. 2012; Lebouteiller et al. 2012; Pineda “cradles” of star formation, but finding molecules at low metal- et al. 2014). Such a component would result from a diffuse or licities has been exceedingly difficult. In dwarf galaxies with 1 metal-poor ISM in which CO suffers from enhanced photodis- nebular oxygen abundances ∼< 0:3 Z (12+log(O/H)∼<8.2) , de- sociation relative to H2 (e.g., Maloney & Black 1988; Bolatto spite vigorous on-going star formation, there seems to be very et al. 1999; Glover & Mac Low 2011; Narayanan et al. 2012). little CO (Sage et al. 1992; Taylor et al. 1998; Gondhalekar et H2 can effectively avoid the photodissociation effects of UV ra- al. 1998; Barone et al. 2000; Leroy et al. 2005; Buyle et al. diation through self-shielding, even at relatively low gas surface 2006; Leroy et al. 2007; Schruba et al. 2012; Cormier et al. densities, Σ . On the other hand, CO needs higher Σ to self- 2014). Intriguingly, this is the same metallicity “threshold” be- ISM ISM shield, thus implying that the CO-to-H2 ratio should vary even arXiv:1509.04870v1 [astro-ph.GA] 16 Sep 2015 low which little or no polycyclic aromatic hydrocarbon (PAH) within an individual molecular cloud (e.g., Rollig¨ et al. 2006; emission is detected in metal-poor galaxies (Engelbracht et al. Leroy et al. 2013a; Hughes et al. 2013). Under conditions of low 2005; Wu et al. 2006; Madden et al. 2006; Hunt et al. 2010). ΣISM and low dust extinction AV , most of the carbon would be in The reason for this deficit in CO at low metallicities is not other forms such as C I or Cii, rather than in CO. yet completely understood, although there is strong evidence for Nevertheless, even under extreme conditions in a metal-poor Send offprint requests to: L. K. Hunt ISM, it is likely that stars form in cool, dense molecular clouds ? Based on observations carried out with the IRAM 30m; IRAM is (e.g., Krumholz et al. 2011). However, H2, the dominant molec- supported by the INSU/CNRS (France), MPG (Germany), and IGN ular component of giant molecular clouds, is not directly observ- (Spain). able, but rather inferred from line emission of rotational transi- 1 Throughout this work, we will rely on the Asplund et al. (2009) tions of 12CO(1–0), unfortunately deficient at low metallicity. solar abundance calibration of 12+log(O/H) = 8.69. Constraining the properties of metal-poor molecular clouds ob- 1 Hunt et al.: Molecular depletion times at low metallicity servationally is difficult because the factor, αCO, relating CO lu- very clumpy morphology as seen in Fig. 1. The radiation field is 0 minosity LCO to H2 mass MH2 can vary dramatically, because sufficiently hard to excite [Fev], a line that requires ionizing ra- of the presence of CO-dark gas. αCO can be more than 50 times diation with photon energies exceeding 4 Ryd (54.4 eV) (Thuan higher than the Galactic value in metal-poor environments and & Izotov 2005). Although a population of Wolf-Rayet stars has in regions of low AV and low ΣISM (Israel 1997; Leroy et al. been detected in this galaxy (Zhang et al. 2007), these are insuffi- 2009b, 2011; Pineda et al. 2010; Bolatto et al. 2011; Schruba et cient to power the high-excitation emission lines, thought instead al. 2012; Sandstrom et al. 2013). Although the relation between to be due to fast radiative shocks (Thuan & Izotov 2005). Hunt et CO and H2 has been the subject of much debate (see Bolatto et al. (2010) found evidence for rotational H2 emission, and despite al. 2013, for a recent review), it is clear that at some low metal- the low metallicity of CGCG 007−025, also 7.7 µm and 11.3 µm licity, probably ∼< 0:05 − 0:1 Z , CO ceases to become a useful emission from PAHs. Judging from the relatively high density of −3 tracer of total H2 mass. its ionized gas (∼300 cm , Izotov et al. 2007), CGCG 007−025 In this paper, we begin to explore the limits to which CO can is an example of an “active” starburst, forming stars in compact trace H2 in metal-poor star-forming galaxies. We first describe dense regions (Hirashita & Hunt 2004). the targets and ancillary data in Sect. 2, and the 12CO(1–0) ob- ? II Zw 40 (UGCA 116) servations and data analysis in Sect. 3. In Sect. 4, we compare our results with previous work, and assess the dependence of CO Although we did not observe this source in 12CO(1–0), it is in- luminosity on star-formation rate (SFR). Sect. 5 describes con- cluded in our CO survey of higher-J lines (Hunt et al., in prep.); straints on αCO and τdep, the molecular gas depletion time, i.e., thus we consider it in our sample here. From its discovery, to- the time required to consume the H2 gas reservoir. We adopt a gether with I Zw 18, as an extragalactic Hii region (Sargent & variation of the method proposed by Saintonge et al. (2011b), Searle 1970), II Zw 40 has been the subject of much scrutiny. extending it to low metallicity in order to separate the effects of Its two-tailed optical morphology and kinematics suggest that it τdep variations from variations of αCO. In Sect.6, we analyze the is the result of a collision between two gas-rich dwarf galaxies H2 content of our sample and compare it with H I for a compila- which triggered the current starburst (Baldwin et al. 1982; van tion of galaxies taken from the literature. We discuss the impli- Zee et al. 1998). Near-infrared recombination lines show that cations of our results in Sect. 7, and, in Sect. 8, summarize our more than half the 2 µm flux comes from free-free emission, results and present our conclusions. and warm dust contributes at least ∼25% to the 3-4 µm fluxes (Joy & Lester 1988). Emission from the ro-vibrational transitions of H2 at 2 µm was discovered by Vanzi et al. (1996), and 2. The targets and the ancillary data later found to be in a cloud distinct from the primary Hii region (Vanzi et al. 2008). Visual extinction A is spatially variable in The observing sample was defined mainly on the basis of de- V II Zw 40 and can be as high as ∼>10 mag in the nucleus (Vanzi et tected H2 emission, either in the ro-vibrational transitions at al.

Load more