Molecular Gas in Low-Metallicity Starburst Galaxies: Scaling Relations and the CO-To-H2 Conversion Factor R
Total Page：16
File Type：pdf, Size：1020Kb
A&A 588, A23 (2016) Astronomy DOI: 10.1051/0004-6361/201526397 & c ESO 2016 Astrophysics Molecular gas in low-metallicity starburst galaxies: Scaling relations and the CO-to-H2 conversion factor R. Amorín1, C. Muñoz-Tuñón2,3,J.A.L.Aguerri2,3, and P. Planesas4 1 INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Roma, Italy e-mail: [email protected] 2 Instituto de Astrofísica de Canarias (IAC), vía Láctea S/N, 38200 La Laguna, Tenerife, Spain 3 Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain 4 Observatorio Astronómico Nacional (IGN), Alfonso XII 3, 28014 Madrid, Spain Received 24 April 2015 / Accepted 16 December 2015 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 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 ΣSFR, and therefore showing extremely low (∼<0.1 Gyr) H2 and H2+Hi depletion timescales. The departure from the SK law is smaller when considering H2+Hi rather than H2 only, and is larger for Σ Σ Σ Σ BCDs with lower metallicity and higher specific SFR. Thus, the molecular fraction ( H2 / HI) and CO depletion timescale ( H2 / 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 H2 depletion timescale of more metal-rich galaxies extends to lower masses, we derive a metallicity- −y dependent 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 ∼ 0.1 Z, 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: ISM – radio lines: ISM – galaxies: starburst – galaxies: evolution – galaxies: general 1. Introduction however, that most (>95%) BCDs are old systems (>5Gyr) that have undergone previous starburst episodes (Gerola et al. Blue compact galaxies (BCDs) are low luminosity gas-rich sys- 1980; Davies & Phillipps 1988; Sánchez Almeida et al. 2008). tems with an optical extent of a few kpc (Thuan & Martin This conclusion relies mostly upon the detection of a more ex- 1981). They undergo intense bursts of star formation, as evi- tended, evolved low-surface brightness host galaxy (Papaderos denced by their blue colours and strong nebular emission, with ∼ − −1 et al. 1996a,b; Cairós et al. 2001a, 2003; Noeske et al. 2003, ongoing star-formation rates (SFR) typically 0.1 10 M yr 2005; Caon et al. 2005; Gil de Paz & Madore 2005; Vaduvescu (e.g., Gil de Paz et al. 2003). BCDs span a wide range of sub- < < et al. 2006; Hunter & Elmegreen 2006; Amorín et al. 2007, 2009; solar metallicities 0.02 ∼ Z/Z ∼ 0.5), including the most metal- Micheva et al. 2013). Nonetheless, even though they are not poor star-forming galaxies known in the local Universe (e.g., pristine galaxies, nearby BCDs constitute ideal laboratories for Terlevich et al. 1991; Kunth & Östlin 2000; Kniazev et al. 2004; studying vigorous star formation and galaxy evolution in great Izotov et al. 2006; Papaderos et al. 2006, 2008; Morales-Luis detail, under physical conditions that are comparable to those et al. 2011; Filho et al. 2013). These extreme properties orig- present in low-mass galaxies at higher redshift (e.g., Amorín inally lead to conjecture that they are pristine galaxies that, at et al. 2015, 2014a,b; Maseda et al. 2014; de Barros et al. 2016). present, are experiencing the formation of their first stellar pop- ulation (Sargent & Searle 1970). Subsequent work has shown, So far, our understanding of the main processes triggering and regulating star formation activity in BCDs remains very lim- Based on observations carried out with the IRAM 30m Telescope. ited. This is largely due to our poor knowledge of the physical IRAM is supported by INSU/CNRS (France), MPG (Germany) and mechanisms behind starburst activity, as well as of the feedback IGN (Spain). induced by the massive star formation in the interstellar medium Article published by EDP Sciences A23, page 1 of 17 A&A 588, A23 (2016) (ISM). The strong UV radiation and the mechanical energy from large amounts of cold molecular gas would therefore be a pre- stellar winds and SNe are likely agents of feedback in starbursts requisite for the ignition of a starburst, leading to the assumption like those present in BCDs, leading to the ejection of metal- that galaxies with vigorous star formation must contain large enriched gas into the galactic halo, which limits the star for- amounts of H2. The knowledge of the H2 mass, spatial distri- mation and shapes the large-scale structure and kinematics of bution and physical conditions is, therefore, essential for under- the surrounding ISM (e.g., Mac Low & Ferrara 1999; Silich standing the star-formation process itself, its ISM chemistry and & Tenorio-Tagle 2001; Tenorio-Tagle et al. 2006; Recchi & the galaxy evolution as a whole. In spite of their relevance and Hensler 2013). the extensive observational effort conducted over the last decade Likewise, the triggering of the starburst in BCDs remains a (see e.g., Leroy et al. 2005, 2011; Bigiel et al. 2008; Bolatto puzzle: these low-mass, gas-rich galaxies generally lack density et al. 2011; Schruba et al. 2012; Boselli et al. 2014; Cormier waves and only a small fraction of them are seen in strong tidal et al. 2014), these remain key open questions for star-forming interactions or merging with more massive companions (e.g., dwarf galaxies. Campos-Aguilar et al. 1993; Telles & Terlevich 1995; Pustilnik Owing to the lack of a permanent dipole, H2 emission only et al. 2001; Koulouridis et al. 2013). The origin of their ongoing arises from hot or warm gas. For this reason, indirect meth- star-formation activity has been predominantly associated with ods are used to estimate the mass of the cold H2 phase of fainter interactions with low-mass companions (e.g., Noeske the ISM. The most widely applied tracer is the CO rotational et al. 2001; Brosch et al. 2004; Bekki 2008), cold-gas accretion line emission. However, the cold phase of H2, as traced by the (e.g., Sánchez Almeida et al. 2014, 2015), and other, barely un- 12CO molecule at millimetre wavelengths, seems to be mostly derstood internal processes (e.g., Papaderos et al. 1996b; van Zee elusive in galaxies with metallicity below ∼20% solar (e.g., et al. 2001; Hunter & Elmegreen 2004). Elmegreen et al. 2013; Rubio et al. 2015). In particular, BCDs Results from detailed surface photometry have shown that are not only metal-poor but also strongly star-forming, a com- starbursting dwarfs are more compact, i.e., they have a higher bination of properties that seems to disfavour high CO detec- stellar concentration than more quiescent dwarf irregulars (dIs), tion rates (∼<25%, Israel 2005; Leroy et al. 2005). This prob- (e.g., Papaderos et al. 1996b; Gil de Paz & Madore 2005; i lem becomes particularly severe in the lowest-metallicity BCDs Amorín et al. 2009). Moreover, spatially resolved H studies (van (Z ∼< 0.1 Z) for which only upper limits in CO luminosity ex- Zee et al. 1998, 2001; Ekta & Chengalur 2010; López-Sánchez ists and for which the CO-to-H2 conversion factor XCO seems et al. 2012; Lelli et al. 2014a,b) have shown both that the cen- to be extremely uncertain, e.g., in the most metal-poor BCDs I tral gas surface density in BCDs can be a factor ∼>2 higher and ∼ i Zw 18 and SBS 0335-052 (Z 0.02 Z), where even very deep that their H gas kinematics is more disturbed than in dIs. Some searches for CO have proved fruitless (Leroy et al. 2007; Hunt of these studies suggest that inflows of gas onto a BCD might et al. 2014). lead to a critical gas density and, in turn, to the ignition of star formation on a small (∼1 kpc) spatial scale. In addition, it has Low metallicity environments imply lower C and O abun- been suggested that gravity-driven motions and torques induced dances and low dust-to-gas ratios (Draine et al.