Full-disc 13CO(1–0) mapping across nearby galaxies of the EMPIRE survey and the CO-to-H2 conversion factor

Abstract

Carbon monoxide (CO) provides crucial information about the molecular gas properties of galaxies. While 12 CO has been targeted extensively, isotopologues such as 13 CO have the advantage of being less optically thick and observations have recently become accessible across full galaxy discs. We present a comprehensive new dataset of 13 CO(1-0) observations with the IRAM 30-m telescope of the full discs of 9 nearby spiral galaxies from the EMPIRE survey at a spatial resolution of ∼1.5 kpc. 13 CO(1-0) is mapped out to 0.7 − 1 r 25 and detected at high signal-to-noise throughout our maps. We analyse the 12 CO(1-0)-to-13 CO(1-0) ratio (ℜ) as a function of galactocentric radius and other parameters such as the 12 CO(2-1)-to-12 CO(1-0) intensity ratio, the 70-to-160 µm flux density ratio, the star-formation rate surface density, the star-formation efficiency, and the CO-to-H 2 conversion factor. We find that ℜ varies by a factor of 2 at most within and amongst galaxies, with a median value of 11 and larger variations in the galaxy centres than in the discs. We argue that optical depth effects, most likely due to changes in the mixture of diffuse/dense gas, are favored explanations for the observed ℜ variations, while abundance changes may also be at play. We calculate a spatially-resolved 13 CO(1-0)-to-H 2 conversion factor and find an average value of 1.0 × 10 21 cm −2 (K km s −1 ) −1 over our sample with a standard deviation of a factor of 2. We find that 13 CO(1-0) does not appear to be a good predictor of the bulk molecular gas mass in normal galaxy discs due to the presence of a large diffuse phase, but it may be a better tracer of the mass than 12 CO(1-0) in the galaxy centres where the fraction of dense gas is larger.

CO Multi-line Imaging of Nearby Galaxies (COMING). III. Dynamical effect on molecular gas density and star formation in the barred spiral galaxy NGC 4303

“…While several studies have reported that R 12/13 is high in starburst galaxies, luminous and ultra luminous infrared galaxies (LIRG, ULIRG; e.g., Aalto et al 1991 andAalto et al 1995), it ranges 5-18 for normal spiral galaxies (Paglione et al 2001, Cao et al 2017, Cormier et al 2018. Measured R 12/13 from stacked spectra in each region of NGC 4303 are consistent with such normal galaxies.…”

The ALMaQUEST Survey – II. What drives central starbursts at z ∼ 0?

“…Downes & Solomon 1998;Bryant & Scoville 1999;Papadopoulos et a. 2012;Sargent et al 2014), and resolved studies of gas and dust have indicated lower values of α CO are also common at small galactic radii (Sandstrom et al 2013;Cormier et al 2018). It is therefore likely that our assumption of a fixed α CO is incorrect, with smaller values likely to be more appropriate for the inner starbursting spaxels of our sample (e.g.…”

“…Similar to the process described above for ∆Σ SFR , ∆SFE is then computed as the offset of a given spaxel's Σ SFR from those spaxels in the control sample that are matched within 0.1 dex in Σ H 2 , 0.1 dex in total stellar mass (of the host galaxy), 0.1 in R/R e and 0.1 dex in O/H. These matching criteria will mitigate dependences of α CO on metallicity and galactocentric radius (Narayanan et al 2011;Bolatto, Wolfire & Leroy 2013;Sandstrom et al 2013;Cormier et al 2018). ∆SFE is then the offset of Σ SFR of a given spaxel relative the median Σ H 2 of its control spaxels, with positive values of ∆SFE indicating greater SFEs (or shorter depletion times).…”

Atomic and molecular gas in IllustrisTNG galaxies at low redshift

“…An analysis of similar galaxies in IllustrisTNG could perhaps explain their low star formation efficiency. Finally, we have refrained from a detailed comparison of the spatial distribution of molecular gas, which is increasingly being constrained observationally (e.g., Cormier et al 2016Cormier et al , 2018Bolatto et al 2017).…”