THE CO-TO-H₂CONVERSION FACTOR AND DUST-TO-GAS RATIO ON KILOPARSEC SCALES IN NEARBY GALAXIES

Abstract

We present ∼kiloparsec spatial resolution maps of the CO-to-H 2 conversion factor (α CO ) and dust-to-gas ratio (DGR) in 26 nearby, star-forming galaxies. We have simultaneously solved for α CO and the DGR by assuming that the DGR is approximately constant on kiloparsec scales. With this assumption, we can combine maps of dust mass surface density, CO-integrated intensity, and H i column density to solve for both α CO and the DGR with no assumptions about their value or dependence on metallicity or other parameters. Such a study has just become possible with the availability of high-resolution far-IR maps from the Herschel key program KINGFISH, 12 CO J = (2-1) maps from the IRAM 30 m large program HERACLES, and H i 21 cm line maps from THINGS. We use a fixed ratio between the (2-1) and (1-0) lines to present our α CO results on the more typically used 12 CO J = (1-0) scale and show using literature measurements that variations in the line ratio do not affect our results. In total, we derive 782 individual solutions for α CO and the DGR. On average, α CO = 3.1 M pc −2 (K km s −1 ) −1 for our sample with a standard deviation of 0.3 dex. Within galaxies, we observe a generally flat profile of α CO as a function of galactocentric radius. However, most galaxies exhibit a lower α CO value in the central kiloparsec-a factor of ∼2 below the galaxy mean, on average. In some cases, the central α CO value can be factors of 5-10 below the standard Milky Way (MW) value of α CO,MW = 4.4 M pc −2 (K km s −1 ) −1 . While for α CO we find only weak correlations with metallicity, the DGR is well-correlated with metallicity, with an approximately linear slope. Finally, we present several recommendations for choosing an appropriate α CO for studies of nearby galaxies.

The Alma Spectroscopic Survey in the Hubble Ultra Deep Field: Molecular Gas Reservoirs in High-Redshift Galaxies

“…Using the dust as a proxy of the molecular gas does not require assumptions on CO excitation and on a CO . However, this approach relies on the assumption of a dust-to-gas mass ratio (DGR), which typically depends on the gas metallicity (Bolatto et al 2013;Sandstrom et al 2013;Genzel et al 2015). Recent ALMA results report substantially lower values of M gas than what are typically obtained in CObased studies (Scoville et al 2014(Scoville et al , 2016.…”

“…The Rayleigh-Jeans part of the dust emission is only weakly dependent on the dust temperature, thus it can be used to trace the mass of dust. Using the dust-to-gas scaling (see, e.g., Sandstrom et al 2013), it is possible to infer the gas mass via the dust mass. Groves et al (2015) compare CO-based gas masses with the monochromatic luminosity of the dust continuum in the Rayleigh-Jeans tail.…”

Inefficient star formation in extremely metal poor galaxies

“…Umin). Similar to studies of dust emission in spirals 15 , Umax was further fixed to the maximum of 10 6 . We then performed SED fitting with three free parameters for Sextans A and ESO 146-G14.…”

“…Extended Data Table 5 lists the result for five fits that are m1-MW, m2-MW, m3-MW, m1-SMC and m1-LMC. The associated uncertainties of Σgas are the quadratic sum of errors of dust mass measurements, errors of GDRs of diffuse regions contributed by uncertainties on HI and dust mass estimates of diffuse regions, 0.2 dex for the GDR variation across the disk based on studies of spiral galaxies 15 . The result of the m1-MW fit is used as a fiducial, as listed in Table 1 and shown in Fig.…”

“…As is usually done for nearby galaxies 7,15 the GDR in the star-forming clumps is taken as the ratio of atomic gas to dust in the diffuse region of the disk. This works because (1) the atomic gas dominates the total gas mass in the diffuse regions, and (2) the GDR is roughly constant in star-forming disks after removing the metallicity gradients 15,21 . Dwarf galaxies in general show little or no metallicity gradients across their disks 22 , including Sextans A, which has a variation of less than 0.1 dex 12 .…”