8. SUMMARY AND CONCLUSIONS
We have compared the properties of GMCs in 5 galaxies, four of which
have been surveyed in their entirety: the LMC, the SMC, M33, IC 10.
M31 was observed over a very limited area. The
interstellar medium of all five galaxies is dominated by the atomic phase.
- The GMCs do not, in general, show any
relationship to the stellar content of the galaxies except for the O stars
born in the GMCs.
- There is a very good correlation between the
locations of the
GMCs and filaments of H I. Many filaments contain little of no
molecular gas even though they have similar surface densities compared
to those that are rich in GMCs. This suggests that clouds form from
the H I rather than vice-versa.
- There appears to be a clear evolutionary trend
molecule formation GMC
formation. It is not clear however, whether the condensations that
form GMCs are first formed in the atomic filaments, or only after the
molecules have formed.
- We derive XCO for all of the
galaxies assuming that the GMCs are virialized. Although there is some
variation, a value of XCO =
4 × 1020 cm-2 (K km
s-1)-1 is a representative value
to within about 50% except for the SMC, which has a value more than 3
times higher. There is no clear trend of
XCO with metallicity.
- The discrepancy between the virial value and
the value determined from
observations in the Milky Way suggests that the GMCs are not virialized,
applicable to other galaxies in the Local Group. In that case, a value
of XCO = 2 × 1020 cm-2 (K
km s-1)-1 may be more appropriate.
- The GMCs in our sample appear to satisfy the
relation for the Milky Way, but with an offset in the constant of
proportionality. This offset may be due, at least in part, to the
different data analysis techniques for the MW and extragalactic data
sets. For a given line width, the extragalactic clouds appear to be
about 50 % larger. Despite the systematic offset, there are small
but significant differences in the line width-size relationship among
GMCs in different galaxies.
- The GMCs within a particular galaxy have a
roughly constant surface density. If the value of XCO
we derive for each galaxy is
applied, the surface densities of the sample as a whole, have a
scatter of less than a factor of two.
- The mass spectra for the GMCs in all of the
galaxies can be
characterized as a power law with a slope of ~ -1.7, with the
exception of M33, which has a slope of -2.5.
- The ratio of H2 to H I on a
pixel-by-pixel basis in galaxies
appears to be determined by the hydrostatic pressure in the disk.
- About 1/4 - 1/3 of the GMCs in the LMC and M33
appear to be devoid of high-mass star formation.
- The association of stars and H2 regions in the
a lifetime for the GMCs of about 27 Myr, with a quiescent phase that
is about 25% of the age of the GMCs. In M33, a lifetime of ~ 20
Myr is measured. For GMCs in these galaxies we estimate that typical
lifetimes are roughly 20-30 Myr. Both lifetimes are uncertain by
Acknowledgments. This work is partially supported by US National
Science Foundation under grants AST-0228963 and AST-0502605, a
Grant-in-Aid for Scientific Research from the Ministry of Education,
Culture, Sports, Science and Technology of Japan (No. 15071203), and
from JSPS (No. 14102003). The NANTEN project is based on a mutual
agreement between Nagoya University and the Carnegie Institution of
Washington (CIW). We greatly appreciate the hospitality of all the
staff members of the Las Campanas Observatory of CIW. We are thankful
to many Japanese public donors and companies who contributed to the
realization of the project. We would like to acknowledge
Drs. L. Stavely-Smith and M. Filipovic for the kind use of their
radio continuum data prior to publication.