In the disks of our nearest spiral neighbors, M31 and M33, we can resolve populations and kinematics of Galactic giant molecular cloud (GMC) analogs. Their proximity also requires on the order of 1000 pointings for a typical millimeter telescope or interferometer to cover the angular size subtended by the star-forming disk. Therefore, complete studies of these individual galaxies can rightly be called surveys, and they provide the means to study cloud populations in external galaxies in an unbiased way.
M31 is the nearest spiral to our own, offering an alternative view of a large, bright spiral galaxy. The large inclination of M31 motivates the study of molecular gas at high angular resolution, in order to separate arm and interarm regions and assess the properties and development of spiral structure. Recent efforts to attain a complete assessment of the molecular gas in M31 have seen a dramatic improvement in spatial resolution, from the first complete CO survey at 1.7 kpc resolution in 1993 (with the CfA 1.2m, [13]) to a higher resolution (200 pc) survey of the southwestern half of the galaxy (with the FCRAO 14m, [14], [15]) and a full survey at better than 90pc resolution (with the IRAM 30m, [16]) in the last few years. We can now assess the properties of the spiral structure and molecular cloud population in M31, and compare it with those of the Milky Way (see presentation by M. Guelin, this volume). Furthermore, the wealth of kinematic information over a wide range of radii can be used to constrain galaxy properties such as the shape and rotation speed of the bulge [17], [18].
M33 offers a contrasting view of the properties of a
nearby, later-type spiral. After a seminal study of the properties of
molecular gas in the nuclear region of M33
[19],
[20],
it has taken a decade to expand on this important work. The recently
completed 759-field survey covering the star-forming disk of M33 at 50 pc resolution (with BIMA,
[21])
is the first flux-limited
sample of distinguishable GMCs in a spiral galaxy, and creates a database
of 148 GMCs with which to study the properties of molecular clouds in
M33. In turn, it has enabled a higher resolution (20
pc) study of a high-mass sample of giant molecular clouds in M33
[22].
These studies confirm that M33 seems to be devoid of the most massive
GMCs seen in the Galaxy, and enable comparisons with other tracers
such as HI and H
which
can be used to constrain GMC lifetimes,
a gas depletion timescale, and cloud formation mechanisms.