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23 April 2024 |
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Article overview
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Molecular Gas and Star Formation in the Cartwheel | | James L. Higdon
; Sarah J. U. Higdon
; Sergio Martin Ruiz
; Richard J. Rand
; | | Date: |
23 Nov 2015 | | 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
$(2.69pm0.05) imes10^{9}$ M$_{odot}$ of H$_2$ associated with the inner ring
(72%) and nucleus (28%) for a Galactic I(CO)-to-N(H2) conversion factor
($alpha_{
m CO}$). The spokes and disk are not detected. Analysis of the
inner ring’s CO kinematics show it to be expanding ($V_{
m exp}=68.9pm4.9$ km
s$^{-1}$) implying an $approx70$ Myr age. Stack averaging reveals CO emission
in the starburst outer ring for the first time, but only where HI surface
density ($Sigma_{
m HI}$) is high, representing $M_{
m
H_2}=(7.5pm0.8) imes10^{8}$ M$_{odot}$ for a metallicity appropriate
$alpha_{
m CO}$, giving small $Sigma_{
m H_2}$ ($3.7$ M$_{odot}$
pc$^{-2}$), molecular fraction ($f_{
m mol}=0.10$), and H$_2$ depletion
timescales ($ au_{
m mol} approx50-600$ Myr). Elsewhere in the outer ring
$Sigma_{
m H_2}lesssim 2$ M$_{odot}$ pc$^{-2}$, $f_{
m mol}lesssim 0.1$
and $ au_{
m mol}lesssim 140-540$ Myr (all $3sigma$). The inner ring and
nucleus are H$_2$-dominated and are consistent with local spiral SF laws.
$Sigma_{
m SFR}$ in the outer ring appears independent of $Sigma_{
m H_2}$,
$Sigma_{
m HI}$ or $Sigma_{
m HI+H_2}$. 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 $Sigma_{
m SFR}$ 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. | | Source: | arXiv, 1511.7227 | | Services: | Forum | Review | PDF | Favorites | |
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