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J/ApJ/761/37 CO observations in giant molecular clouds of M33 (Miura+, 2012)
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Beginning of ReadMe :J/ApJ/761/37 CO observations in giant molecular clouds of M33 (Miura+, 2012)
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Giant molecular cloud evolutions in the nearby spiral galaxy M33.
Miura R.E., Kohno K., Tosaki T., Espada D., Hwang N., Kuno N.,
Okumura S.K., Hirota A., Muraoka K., Onodera S., Minamidani T., Komugi S.,
Nakanishi K., Sawada T., Kaneko H., Kawabe R.
<Astrophys. J., 761, 37 (2012)>
=2012ApJ...761...37M
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ADC_Keywords: Galaxies, nearby ; Molecular clouds ; Cross identifications ;
Associations, stellar ; Millimetric/submm sources
Keywords: galaxies: individual: M33; ISM: clouds; ISM: molecules;
submillimeter: ISM
Abstract:
We present a giant molecular cloud (GMC) catalog of M33 (NGC598),
containing 71 GMCs in total, based on wide-field and high-sensitivity
CO(J=3-2) observations with a spatial resolution of 100pc using the
ASTE 10m telescope. Employing archival optical data, we identify 75
young stellar groups (YSGs) from the excess of the surface stellar
density, and estimate their ages by comparing with stellar evolution
models. A spatial comparison among the GMCs, YSGs, and H II regions
enable us to classify GMCs into four categories: Type A, showing no
sign of massive star formation (SF); Type B, being associated only
with H II regions; Type C, with both H II regions and <10Myr old YSGs;
and Type D, with both H II regions and 10-30Myr YSGs. Out of 65 GMCs
(discarding those at the edges of the observed fields), 1 (1%), 13
(20%), 29 (45%), and 22 (34%) are Types A, B, C, and D, respectively.
We interpret these categories as stages in a GMC evolutionary
sequence. Assuming that the timescale for each evolutionary stage is
proportional to the number of GMCs, the lifetime of a GMC with a mass
>10^5^M_sun_ is estimated to be 20-40Myr. In addition, we find that
the dense gas fraction as traced by the CO(J=3-2)/CO(J=1-0) ratio is
enhanced around SF regions. This confirms a scenario where dense gas
is preferentially formed around previously generated stars, and will
be the fuel for the next stellar generation. In this way, massive SF
gradually propagates in a GMC until gas is exhausted.
Description:
We chose eight regions covering the northern and southern spiral arms
as well as the galaxy center, where the CO(J=1-0) emission is
prominent. The CO(J=3-2) (rest frequency: 345.796GHz) mapping
observations were performed between 2011 June and November, using the
Atacama Submillimeter Telescope Experiment (ASTE) 10m dish. The
observed area covers in total about 80% of the CO(J=1-0) based
molecular gas mass in Tosaki et al. (2011PASJ...63.1171T).
The CO(J=1-0) emission was observed in a 31'x36' area (corresponding
to 7.6kpcx8.8kpc) toward the disk of M33 using the NRO 45m telescope.
Among the 71 GMCs in our catalog, only one GMC (GMC-50) is a new
identification.