JAC Style guide: template
 Physical Conditions in Molecular Clouds in M33
We have used the JCMT to observe the 12CO J=2-1, 13CO J=2-1, and 12CO J=3-2 lines in a sample of seven
giant molecular clouds in the Local Group spiral galaxy M33. The clouds were chosen to cover a wide
variety of star formation conditions, from clouds with no optical H II regions to a cloud located in the
brightest giant HII region in the galaxy. Determining the physical conditions inside molecular clouds is
important for understanding the link between the properties of the molecular gas and the types and amount
of stars that are formed. Cloud properties that could affect the star formation process include the
temperature and density of the molecular gas, as well as the mass fraction in high density gas. In return,
star formation, particularly massive star formation, can affect conditions inside molecular clouds by
compressing the gas at the boundaries of stellar wind or supernova shocks and heating the gas by increasing
the ultraviolet radiation field.
The 12CO/13CO J=2-1 line ratios are very uniform, with an average value of 7.3 and an rms
dispersion of 1.3 (18%). This average line ratio compares quite well with values measured in other spiral
galaxies, and is slightly smaller than the value measured in the starburst galaxies (13 ± 5, Aalto et
al. 1995). The 12CO J=3-2/J=2-1 line ratios show more scatter. In particular, the line ratio obtained
for NGC 604-2 (1.07) is significantly higher than the value for any other cloud. Excluding NGC 604-2, the
average 12CO J=3-2/J=2-1 line ratio is 0.69 with an rms dispersion of 0.15 (21%). Thus we
conclude that the 12CO J=3-2/J=2-1 line ratio is uniform across six of the seven clouds in our sample, but is
significantly enhanced in NGC 604-2. This average line ratio for the six clouds in M33 is in good
agreement with values measured in other starburst and normal galaxies, while higher line ratios similar to
the value for the cloud NGC 604-2 are seen in the nuclei of both NGC 253 (Wall et al. 1991) and
M51 (Garcia-Burillo et al. 1993).
If we compare the CO line ratios for the three clouds without H II regions (MC19, MC32, and NGC 604-4)
with those for the clouds with normal H II regions (MC 1, MC 13, and MC 20), the average line ratios for
the two sets of clouds agree very well. The one cloud with a large 12CO J=3-2/J=2-1 line ratio, NGC 604-2,
is located in the very intense star formation environment of the giant H II region NGC 604. The difference
between the two clouds (NGC 604-2 and NGC 604-4) in and around this giant H II region is striking. The
projected separation of the two clouds is only 120 pc (30"), and yet their 12CO J=3-2/J=2-1 line ratios are
very different. Only the cloud in the immediate vicinity of the giant H II region and its powering OB
association has an unusual line ratio.
The high 12CO J=3-2/J=2-1 line ratio in NGC 604-2 may be a clue to unusual physical conditions in the
molecular clouds from which the H II region NGC 604 formed, i.e. a pre-star formation difference
in the physical conditions that may have caused the formation of the giant H II region. Alternatively, the
high line ratio may be due to heating of the gas by the massive stars, i.e. a post-star formation
change in the physical conditions in the molecular cloud. This interpretation implies that the giant H II
region has a relatively small sphere of influence over which its intense radiation field can change the
properties of the dense molecular gas. It appears that both an unusually intense radiation field and a cloud
in close proximity to the source of the ionizing radiation are required to produce a significant change in the
CO line ratios.
We used a large velocity gradient code to estimate the physical conditions in the molecular clouds in M33.
To provide better constraints for the models, we include the 12CO/13CO J=1-0 line ratio observed for these
clouds in a 55" beam (Wilson & Walker 1994). We performed separate fits for the average line ratios
for the six clouds and for NGC 604-2. The results from the LVG analysis show that there is a significant
difference in the kinetic temperature of NGC 604-2 (TK =< 100 K) compared to the other clouds (TK =< 30
K). Thus the large 12CO J=3-2/J=2-1 line ratio in NGC 604-2 is likely the result of the higher kinetic
temperature of this cloud. In addition, the average solution suggests that NGC 604-2 has a higher column
density by about an order of magnitude compared to the six other clouds. The molecular gas in the six
clouds is quite dense (10(3) - 3x10(4) cm(-3)). The volume-averaged densities for the six normal clouds are 40
- 290 cm(-3), significantly lower than the densities derived from the LVG analysis. Thus the volume filling
factor of the dense gas within the clouds is <10%. For NGC 604-2 the volume-averaged density is
somewhat higher, 500 cm(-3), which combined with the somewhat lower density from the LVG model of 1 - 3x10(3) cm(-3) gives a filling factor for the dense gas of 17 - 50%.
Figure 1: 12CO J=2-1, 13CO J=2-1, and 12CO J=3-2 spectra for three of the seven giant molecular
clouds in M33. The spectra are binned to 1 km/s resolution and scaled to the main beam temperature scale.
The 13CO J=2-1 spectrum has been scaled up by a factor of 3. Note in particular the varying strength of the
12CO J=3-2 line relative to the J=2-1 line.
Thus NGC 604-2 is distinguished from the other clouds by a larger column density, volume filling factor of
dense gas, and kinetic temperature. The kinetic temperature of the gas is probably the physical parameter
most easily affected by the presence of the intense burst of star formation in the giant H II region. The gas
column density could be increased through shock-initiated merging of two or more molecular clouds or
through a partial collapse of the molecular cloud due to an increase in the external gas pressure. However,
the molecular column density could also be decreased through photo-dissociation of the molecular gas and
thus it is difficult to predict the net effect on the column density from the formation of the giant H II region.
It is tempting to speculate that the higher column density and filling factor of the dense gas in NGC 604-2
(or clouds like it that have since been destroyed) could have played a role in initiating the burst of star
formation that formed the giant H II region.
The uniformity of the line ratios of the six molecular clouds observed in the normal disk of M33 suggests
that similar observations of ensembles of molecular clouds in more distant galaxies are likely to produce
meaningful measurements of the average physical conditions of the molecular gas. The relatively small
sphere of influence (100 pc) of the giant H II region NGC 604 suggests that in normal galaxies only the
most intense star forming regions may produce significant changes in the molecular gas. The change in the
line ratios is likely to be measurable only in relatively nearby galaxies (< 10 Mpc), where the warm
molecular gas in the H II region is not diluted by emission from cooler gas included in the beam. It would
be interesting to test whether similarly uniform line ratios are observed in individual clouds in the intense
ultraviolet field of a starburst galaxy, but such observations must await the construction of an imaging
submillimeter interferometer, such as the Smithsonian Submillimeter Array.
References
Aalto, S., Booth, R. S., Black, J. H., & Johansson, L. E. B., 1995, A&A, 300, 369
Garcia-Burillo,S., Guelin, M., & Cernicharo, J., 1993, A&A, 274, 123
Wall, W. F., Jaffe, D. T., Bash, F. N., & Israel, F. P., 1991, ApJ, 380, 384
Wilson, C. D., & Walker, C. E., 1994, ApJ, 432, 148
C. D. Wilson, McMaster University
C. E. Walker, University of Arizona
M. D. Thornley, University of Maryland
Last Modification Date 1996/08/13 - Last Modification Author: gdw
Graeme Watt (gdw)
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