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Molecular Gas in Low-Metallicity Starburst Galaxies

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Molecular Gas in Low-Metallicity Starburst Galaxies Astronomy & Astrophysics manuscript no. amorin-AA-2015-26397˙rev c ESO 2018 September 20, 2018 Molecular gas in low-metallicity starburst galaxies: ⋆ Scaling relations and the CO-to-H2 conversion factor R. Amor´ın1, C. Mu˜noz-Tu˜n´on2,3, J.A.L. Aguerri2,3, and P. Planesas4 1 INAF – Osservatorio Astronomico di Roma, Via Frascati 33, I-00040 Monte Porzio Catone, Roma, Italy 2 Instituto de Astrof´ısica de Canarias (IAC), V´ıa L´actea S/N, E-38200 La Laguna, Tenerife, Spain 3 Departamento de Astrof´ısica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain 4 Observatorio Astron´omico Nacional (IGN), Alfonso XII 3, E-28014 Madrid, Spain Received ..., ...; accepted ..., ... ABSTRACT 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 at studying 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-average properties. Results. We detect CO emission in 7 out of 10 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 ΣSFR, and therefore showing extremely low (< 0.1 Gyr) H2 and H2+Hi de- pletion timescales. The departure from the SK law is smaller when considering H2+Hi rather than H2 only,∼ and is larger for BCDs with lower metallicity and higher specific SFR. Thus, the molecular fraction (ΣH2 /ΣHI) and CO depletion timescale (ΣH2 /ΣSFR) 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 H2 depletion timescale of more metal-rich galaxies extends to lower masses, we derive a metallicity-dependent y CO-to-H2 conversion factor αCO,Z (Z/Z )− , 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 H2 traced by CO decreases by a factor of about 40 from Z Z to Z 0.1Z , leading to a strong underestimation of the H2 mass in ∼ ⊙ ∼ ⊙ metal-poor systems when a Galactic αCO,MW is considered. Adopting our metallicity-dependent conversion factor αCO,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 com- pared 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. Key words. galaxies: evolution – galaxies: general – galaxies: ISM – radio lines: ISM – starbursts 1. Introduction they are pristine galaxies experiencing at present the forma- tion of their first stellar population (Sargent & Searle, 1970). Blue Compact Galaxies (BCDs) are low luminosity gas-rich Subsequent work has shown, however, that most (>95%) systems with an optical extent of a few kpc (Thuan & Martin, BCDs are old systems (> 5 Gyr) that underwent previous star- 1981). They undergo intense bursts of star formation, as evi- burst episodes (Gerola et al., 1980; Davies & Phillipps, 1988; denced by their blue colours and strong nebular emission, with 1 S´anchez Almeida et al., 2008). This conclusion relies mostly arXiv:1512.06153v1 [astro-ph.GA] 18 Dec 2015 ongoing star formation rates (SFR) typically 0.1 10M yr− ∼ − ⊙ upon the detection of a more extended, evolved low-surface (e.g., GildePazetal., 2003). BCDs span in a wide range brightness host galaxy (Papaderos et al., 1996a,b; Cair´os et al., < < of sub-solar metallicities 0.02 Z/Z 0.5), including the 2001a; Cair´osetal., 2003; Noeskeetal., 2003, 2005; ∼ ⊙ ∼ most metal-poor star-forming galaxies known in the local Caon et al., 2005; Gil de Paz & Madore, 2005; Vaduvescu et al., ¨ Universe (e.g., Terlevich et al., 1991; Kunth & Ostlin, 2000; 2006; Hunter & Elmegreen, 2006; Amor´ın et al., 2007, 2009; Kniazevet al., 2004; Izotovetal., 2006; Papaderoset al., Michevaet al., 2013). Nonetheless, even though they are 2006, 2008; Morales-Luiset al., 2011; Filho etal., 2013). not pristine galaxies, nearby BCDs constitute ideal labora- These extreme properties lead originally to conjecture that tories to study in great detail vigorous star formation and galaxy evolution under physical conditions that are compa- Send offprint requests to: R. Amor´ın e-mail: rable to those present in low-mass galaxies at higher redshift [email protected] ⋆ (e.g., Amor´ınet al., 2015, 2014a,b; Masedaet al., 2014; Based on observations carried out with the IRAM 30m Telescope. de Barros et al., 2015). IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). 1 R. Amor´ın et al.: CO in low metallicity starbursts So far, our understanding of the main processes triggering Recent studies have identified the cold molecular gas den- and regulating star formation activity in BCDs remains very lim- sity to be the primary responsible for the observed star forma- ited. This is largely due to our poor knowledge of the physical tion rates in star-forming dwarf galaxies (e.g., Bigiel et al., 2008; mechanisms behind starburst activity, as well as of the feed- Schruba et al., 2011). This is in agreementwith the fact that mas- back induced by the massive star formation in the interstel- sive star clusters occur in giant molecular clouds. The accumu- lar medium (ISM). The strong UV radiation and the mechan- lation of large amounts of cold molecular gas would therefore ical energy from stellar winds and SNe are likely agents of be a prerequisite for the ignition of a starburst, leading to as- feedback in starbursts like those present in BCDs, leading to sume that galaxies with vigorous star formation must contain the ejection of metal-enriched gas into the galactic halo, limit- large amounts of H2. The knowledgeof the H2 mass, spatial dis- ing the star formation and shaping the large-scale structure and tribution and physical conditions is therefore essential for un- kinematics of the surrounding ISM (e.g., Mac Low & Ferrara, derstanding the star formation processes itself, its ISM chem- 1999; Silich & Tenorio-Tagle, 2001; Tenorio-Tagle et al., 2006; istry and the galaxy evolution as a whole. In spite of their rele- Recchi & Hensler, 2013). vance and the extensive observational effort conducted over the Likewise, the triggering of the starburst in BCDs remains last decade(see e.g.,Leroy et al., 2005, 2011; Bigiel et al., 2008; a puzzle: these low-mass, gas-rich galaxies generally lack den- Bolatto et al., 2011; Schruba et al., 2012; Boselli et al., 2014; sity waves and only a small fraction of them are seen in Cormier et al., 2014), these remain key open questions for star- strong tidal interactions or merging with more massive com- forming dwarf galaxies. panions (e.g., Campos-Aguilar et al., 1993; Telles & Terlevich, Due to the lack of a permanent dipole, H2 emission arises 1995; Pustilnik et al., 2001; Koulouridis et al., 2013). The ori- only from hot or warm gas. For this reason, indirect methods are gin of their ongoing star formation activity has been pre- used to estimate the mass of the cold H2 phase of the ISM. The dominantly associated with fainter interactions with low-mass most widely applied tracer is the CO rotational line emission. 12 companions (e.g., Noeske et al., 2001; Brosch et al., 2004; However, the cold phase of H2, as traced by the CO molecule Bekki, 2008), cold-gas accretion (e.g., S´anchez Almeida et al., at millimetre wavelengths, seems to be mostly elusive in galaxies 2014, 2015) and other, barely understood internal pro- with metallicity below 20% solar (e.g., Elmegreen et al., 2013; cesses (e.g., Papaderoset al., 1996b; van Zeeet al., 2001; Rubio et al., 2015). In∼ particular, BCDs are not only metal- Hunter & Elmegreen, 2004). poor but also strongly star-forming, a combination of properties Results from detailed surface photometry have shown that seems to disfavour high CO detection rates (<25%, Israel, that starbursting dwarfs are more compact, i.e., they have a 2005; Leroy et al., 2005). This problem becomes∼ particularly higher stellar concentration, than more quiescent dwarf irreg- severe in the lowest-metallicity BCDs (Z < 0.1 Z ) for which ulars (dIs; e.g., Papaderos et al., 1996b; Gil de Paz & Madore, only upper limits in CO luminosity exists∼ and for⊙ which the 2005; Amor´ın et al., 2009). Moreover, spatially resolved Hi CO-to-H2 conversion factor XCO seems to be extremely uncer- studies (van Zee et al., 1998, 2001; Ekta & Chengalur, 2010; tain, e.g., in the most metal-poor BCDs I Zw18 and SBS0335- L´opez-S´anchez et al., 2012; Lelli et al., 2014a,b) have shown 052 (Z 0.02 Z ), where even very deep searches for CO have both that the central gas surface density in BCDs can be a fac- proved∼ fruitless⊙ (Leroy et al., 2007; Hunt et al., 2014).
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