Continuum observations of M 51 and M 83 at 1.1 mm with AzTEC

Abstract

We observed the spiral galaxies M 51 and M 83 at 20" spatial resolution with the bolometer array AzTEC on the JCMT in the 1.1 mm continuum, recovering the extended emission out to galactocentric radii of more than 12 kpc in both galaxies. The 1.1 mm-continuum fluxes are 5.6±0.7 and 9.9±1.4 Jy, with associated gas masses estimated at 9.4 × 10 9 M and 7.2 × 10 9 M for M 51 and M 83, respectively. In the interarm regions of both galaxies the N(H 2 )/I(CO) (or X-factor) ratios exceed those in the arms by factors of ∼ 1.5-2. In the inner disks of both galaxies, the X-factor is about 1 × 10 20 cm −2 · (K · km · s −1 ) −1 . In the outer parts, the CO-dark molecular gas becomes more important.While the spiral density wave in M 51 appears to influence the interstellar medium and stars in a similar way, the bar potential in M 83 influences the interstellar medium and the stars differently. We confirm the result of Foyle et al. (2010) that the arms merely heighten the star formation rate and the gas surface density in the same proportion. Our maps reveal a threshold gas surface density for an SFR increase by two or more orders of magnitude. In both galaxy centers, the molecular gas depletion time is about 1 Gyr climbing to 10-20 Gyr at radii of 6-8 kpc. This is consistent with an inside-out depletion of the molecular gas in the disks of spiral galaxies.

Cloud-scale ISM Structure and Star Formation in M51

“…The figure shows that α CO ≈ α MW = 4.35 M ⊙ pc −2 (K km s −1 ) −1 yields an approximately flat dust-to-gas ratio as a function of radius. A lower conversion factor, as suggested by Nakai & Kuno (1995); Wall et al (2016) yields a strong gradient in dust-to-gas ratio as a function of radius. The right panel shows the formal results of minimizing scatter in DGR while varying α CO treating either each ring (black) or each θ = 30 ′′ line of sight (gray) as independent measurements.…”

“…Nakai & Kuno 1995;Wall et al 2016, though see our appendix). We discuss the impact of a lower conversion factor in the text.…”

Early Science with the Large Millimeter Telescope: discovery of the12CO(1–0) emission line in the ring galaxy VIIZw466

“…While it is currently beyond the scope of this paper to provide further constraints on the amount of dark CO gas, future observations of the millimetre dust continuum (e.g. Wall et al 2016), higher order CO transition lines, other ionised carbon lines such as [CII] (e.g. Herrera-Camus et al 2015;de Blok et al 2016) and high density tracers such as HCN or HNC can be used to help constrain the gas densities and possibly the existence of CO-dark gas in VII Zw466.…”

Small scale clumps of dark matter

“…The problem of the mechanisms and efficiency of destruction of substructures in galaxies was previously discussed in [198], and for large (galactic scale) dark matter subhalos, it was addressed later in many analytic and numerical studies. Similar studies of small-scale clumps started only recently.…”