Annual Report 2011 Max Planck Institute for Astronomy

68 III. Selected Research Areas
M51
Fig. III.4.5: Maps of CO integrated intensity in M 51 (Schinnerer
et al, in preparation), M 33 (Rosolowsky et al. 2007), and the
LMC (Wong et al. 2011) after matching the spatial and spectral
resolution of the data-cubes and interpolating them onto a
pixel grid with the same physical dimensions. For all panels,
the telescope beam is shown as the small red circle in the bot-
bright kiloparsec-sized structures that bear little resemblance
to the discrete isolated clouds that are present in
the low-mass galaxies. Bright CO emission is also clearly
detected between the spiral arms, both in cloud-like
structures and as thin filaments that appear to span the
inter-arm region.
Assuming that CO integrated intensity is a reliable
tracer of molecular gas column density, our observations
indicate that regions of high gas column density develop
within M 51’s spiral arms and in a central region that we
identify as the “molecular ring”, a zone where molecular
gas accumulates due to the action of opposing torques
from the nuclear stellar bar and first spiral arm pattern.
Gas in the inter-arm, by contrast, achieves lower maximum
column densities and the probability density function
(PDF) of line intensity tends towards a characteristic
lognormal shape, suggesting that the gas density distribution
in the inter-arm region is determined by physical
processes occurring on relatively small scales. Overall,
1.3 kpc
0.6
LMC 0.84kpc M33 1.5 kpc
60
6
150
tom left corner, and the image scale by the bar and text in the
top right. The maps are presented using a square-root intensity
scale with the limits of the color stretch the same to highlight
the significant difference in CO brightness between M 51 and
the other galaxies.
our results confirm the standard argument that lognormal
PDFs in galactic disks emerge via the central limit theorem
from the combined action of independent physical
processes that modify the gas density distribution locally.
What was less expected from numerical models is that
galactic structure (i.e. M 51’s stellar spiral arms and bar)
clearly plays an important role in organizing the density
distribution of the molecular ISM on 50 pc scales.
Using the paws data and a novel finding method, we
have identified over 1500 GMCs within the inner disk
of M 51. This is the largest GMC catalogue that has ever
been constructed for any galaxy including the Milky Way.
We have studied the physical properties of GMCs located
in different dynamical environments within M 51, and
compared the properties of GMCs in M 51 to the GMC
populations of M 33 and the LMC. Contrary to previous
comparative studies that analyzed modest, observationally
heterogeneous samples of GMCs (e.g. Bolatto et al
2008), we find clear evidence for environmental effects
I CO [K km s –1 ]
100
50
0
Credit: E. Schinnerer