Molecular Gas and Star Formation in the Cartwheel

Abstract

Atacama Large Millimeter/submillimeter Array (ALMA) 12CO(J=1-0) observations are used to study the cold molecular ISM of the Cartwheel ring galaxy and its relation to HI and massive star formation (SF). CO moment maps find $(2.69\pm 0.05)\times 10^9$ M${}_{\odot }$ of H${}_2$ associated with the inner ring (72%) and nucleus (28%) for a Galactic I(CO)-to-N(H2) conversion factor (${\alpha }_{\mathrm{CO}}$). The spokes and disk are not detected. Analysis of the inner ring's CO kinematics show it to be expanding ($V_{\exp }=68.9\pm 4.9$ km s${}^{-1}$) implying an $\approx 70$ Myr age. Stack averaging reveals CO emission in the starburst outer ring for the first time, but only where HI surface density (${\Sigma }_{\mathrm{HI}}$) is high, representing $M_{{\mathrm{H}}_2}=(7.5\pm 0.8)\times 10^8$ M${}_{\odot }$ for a metallicity appropriate ${\alpha }_{\mathrm{CO}}$, giving small ${\Sigma }_{{\mathrm{H}}_2}$ ($3.7$ M${}_{\odot }$ pc${}^{-2}$), molecular fraction ($f_{\mathrm{mol}}=0.10$), and H${}_2$ depletion timescales (${\tau }_{\mathrm{mol}}\approx 50-600$ Myr). Elsewhere in the outer ring ${\Sigma }_{{\mathrm{H}}_2}\lesssim 2$ M${}_{\odot }$ pc${}^{-2}$, $f_{\mathrm{mol}}\lesssim 0.1$ and ${\tau }_{\mathrm{mol}}\lesssim 140-540$ Myr (all $3\sigma$). The inner ring and nucleus are H${}_2$-dominated and are consistent with local spiral SF laws. ${\Sigma }_{\mathrm{SFR}}$ in the outer ring appears independent of ${\Sigma }_{{\mathrm{H}}_2}$, ${\Sigma }_{\mathrm{HI}}$ or ${\Sigma }_{\mathrm{HI}+{\mathrm{H}}_2}$. The ISM's long confinement in the robustly star forming rings of the Cartwheel and AM0644-741 may result in either a large diffuse H${}_2$ component or an abundance of CO-faint low column density molecular clouds. The H${}_2$ content of evolved starburst rings may therefore be substantially larger. Due to its lower ${\Sigma }_{\mathrm{SFR}}$ and age the Cartwheel's inner ring has yet to reach this state. Alternately, the outer ring may trigger efficient SF in an HI-dominated ISM.

10-pages text; 5-figures

References in corpus (5)

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Cited by in corpus (10)

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• Quasar Variability in the Mid-Infrared
• Morphology and enhanced star formation in a Cartwheel-like ring galaxy
• Ring Galaxies in the EAGLE Hydrodynamical Simulations
• The X-Ray Luminosity Function of Ultra Luminous X-Ray Sources in Collisional Ring Galaxies
• A giant galaxy in the young Universe with a massive ring
• Discovery of a near-infrared bar and a pseudobulge in the collisional ring galaxy Cartwheel
• Early Science with the Large Millimeter Telescope: discovery of the 12CO(1-0) emission line in the ring galaxy, VIIZw466
• Unravelling the Post-Collision Properties of the Cartwheel Galaxy: A MUSE Exploration of its Bar and Inner Region
• Nebular abundance gradient in the Cartwheel galaxy using MUSE data