Wheels of Fire. Iv. Star Formation and the Neutral Interstellar Medium in the Ring Galaxy Am0644-741

Abstract

We combine data from the Australia Telescope National Facility and the Swedish ESO Submillimeter Telescope to investigate the neutral interstellar medium (ISM) in AM0644-741, a large and robustly star-forming ring galaxy. The galaxy's ISM is concentrated in the 42 kpc diameter starburst ring, but appears dominated by atomic gas, with a global molecular fraction (f mol ) of only 0.079 ± 0.005. Apart from the starburst peak, the gas ring appears stable against the growth of gravitational instabilities (Q gas = 2 − 7). Including the stellar component lowers Q overall, but not enough to make Q < 1 everywhere. The ring's global star formation efficiency (SFE) appears somewhat elevated relative to early spirals, but varies around the ring by more than an order of magnitude, peaking where star formation is most intense. AM0644-741's star formation law is peculiar: HI follows a Schmidt law while H 2 is uncorrelated with star formation rate density. Photodissociation models yield low volume densities in the ring, particularly in the starburst quadrant (n ≈ 2 cm −3 ), implying a warm neutral medium dominated ISM. At the same time, the ring's pressure and ambient far-ultraviolet radiation field lead to the expectation of a predominantly molecular ISM. We argue that the ring's peculiar star formation law, n, SFE, and f mol result from the ISM's 100 Myr confinement time in the starburst ring, which amplifies the destructive effects of embedded massive stars and supernovae. As a result, the ring's molecular ISM becomes dominated by small clouds where star formation is most intense, causing M H 2 to be underestimated by 12 CO line fluxes: in effect X CO ≫ X Gal despite the ring's solar metallicity. The observed large HI component is primarily a low density photodissociation product, i.e., a tracer rather than a precursor of massive star formation. Such an "over-cooked" ISM may be a general characteristic of evolved starburst ring galaxies.3 Visiting astronomer, European Southern Observatory. 4 Visiting astronomer, Cerro Tololo Inter-American Observatory.properties of the star-forming ISM in an evolved ring galaxy compare with other systems?This paper is organized as follows: The observations and data reduction are outlined in Section 2. The results derived using these observations are presented in Section 3, including the ring's metallicity, the distribution and kinematics of HI, the relative proportions of molecular and atomic gas, star formation efficiency, and their variations around the ring. We also consider the ring's gravitational stability, and whether or not star formation follows a Schmidt law. In Section 4 we consider the role played by pressure and ambient FUV radiation field in determining the ring's molecular fraction. We also discuss our results in the context of an alternate view of ring galaxies, in which the ring represents a self-sustaining propagating starburst. Our conclusions are given in Section 5. Throughout this paper we adopt a flat ΛCDM cosmology based on the 7 year WMAP results (Komatsu et al. 2011),...

Morphology and enhanced star formation in a Cartwheel-like ring galaxy

“…7) make it unlikely to detect star formation in these intermediate regions, in again a similar way as the bulk of classical tidal tails (except near their extremities where tidal dwarf galaxy can be found). If our interpretation of the features observed in AM 0644-741 corresponding to a vertical cylinder structure is correct (recall Section 3.2), the star formation activity detected there (Higdon et al 2011) would then support the idea presented above.…”

“…The azimuth angle is measured counter-clockwise in the xy plane, as shown in the cartoon representing the configuration of ring and the position of the nuclear region at t = 150 Myr. thest quadrant of its ring to the nuclei (Higdon et al 2011). We note however that the ring galaxy NGC 922 shows a stronger star formation activity on the side of the nucleus than on the opposite one (e.g.…”

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

“…This results in smaller molecular cloud fragments and photodissociated HI (e.g. Higdon et al 2011). If we assume a fiducial minimum and maximum ΣGMC of 40 M⊙ pc −2 and 188 M⊙ pc −2 , we find that Bolatto et al (2013)'s prescription results in αCO of 5.7 and 3.3 M⊙ (K km s −1 pc 2 ) −1 , respectively.…”

“…Atomic Hydrogen (HI) can provide the initial gas reservoir from which molecular clouds form, as well as the by-product of H2 photodissociation (e.g. Allen et al 2004;Higdon et al 2011). The HI-normalised star formation efficiency (SF EHI = SF R/MHI) is surprisingly uniform for nearby star-forming galaxies spanning across five orders of magnitude in stellar masses (e.g.…”

“…Horellou et al 1995). More recently, Higdon et al (2011) has found evidence for enhancement in the rate of photodissociation of the molecular H2 in these galaxies.…”