Annual Report 2011 Max Planck Institute for Astronomy

Credit: E. Schinnerer
on GMC properties: clouds in M 51 are brighter, and
they have higher mass surface densities and larger velocity
dispersions relative to GMCs of comparable size
and mass in low-mass galaxies. These trends are also
observed within M 51 (Fig. III.4.6): GMCs in the spiral
arms and central region of M 51 tend to have higher CO
masses than GMCs located between the arms. One possible
explanation for the difference in CO brightness is that
the neutral ISM in the low-mass galaxies and inter-arm
region has a lower dust abundance and/or more clumpy
structure, enhancing the selective photodissociation of
CO molecules and reducing the filling factor of CO emission
on 50 pc scales. We are currently investigating
the physical processes, e.g. galactic rotation, spiral arm
streaming and feedback from star formation activity, that
potentially contribute to our measurement of a GMC’s
velocity dispersion within different M 51 environments.
ISM kinematics influence the formation of stars
Gas kinematics on the scales of Giant Molecular Clouds
(GMCs) are essential for probing the framework that
links the large-scale organization of interstellar gas to
cloud formation and subsequent star formation. The
M 51 paws observations permit the first such study in
a galaxy outside the Local Group, in an environment
which is dynamically rich and characterized by strong
non-circular gas flows. To interpret these motions we
combine the paws data with a profile of present-day spiral
arm torques newly derived from the stellar mass distribution
mapped with spitzer / irac 3.6 µm and 4.5 µm
images (Meidt et al. 2012). The observed gas motions
suggest a strong response to torquing by the stellar spiral
pattern, and dynamically distinct zones exhibit different
GMC properties as well as distinct patterns of
star formation.
log (n (M M) / kpc 2 )
2
1
0
–1
–2
Inter-arm (25 kpc 2 )
Spiral Arms (16 kpc 2 )
Central region (4 kpc 2 )
Full sample (47 kpc 2 )
5
5.5 6
log (M lum / M )
6.5 7
III.4 The Interstellar Medium of Nearby Galaxies 69
By comparing gas inflow and star formation rates
throughout the disk, we assemble a view of the spatialdependence
of gas depletion times for the current gas
reservoir (Fig. III.4.7). We find that the lowest azimuthally
averaged star formation efficiencies (highest depletion
times) coincide with zones of elevated radial gas
inflow. We interpret this as the dependence of GMC stabilization
on dynamical environment via the Bernoulli
principle, which raises the stable cloud mass in the presence
of strong spiral streaming. We find that this picture
can reproduce the observed pattern of star formation efficiency
where conventional sources of GMC stabilization,
such as shear and turbulence, fail. High streaming
motions along the spiral arm can reduce the cloud
surface pressure by an order of magnitude compared to
virialized clouds, with the outcome that there are fewer
clouds unstable to collapse per free-fall time along particular
segments of the spiral arm. Such dynamical effects
contribute to the observed scatter in the standard ‘cloud
equilibrium’ relations and star formation laws.
Magnetic fields in nearby galaxies
Understanding the role of magnetic fields in the appearance
and evolution of galaxies is an important concern
in modern astrophysics. Magnetic fields can significantly
shape the ISM, and the pressure provided by
magnetic fields and turbulent motions can be greater
than the thermal pressure provided by the different gaseous
phases (e.g., Tabatabaei et al. 2008). Radio synchrotron
emission, and its polarization and Faraday rotation,
are powerful tools in the study of the strength
and structure of magnetic fields in galaxies. Polarized
emission traces ordered magnetic fields, which can follow
a large-scale spiral pattern in grand-design, barred
and flocculent galaxies. Unpolarized emission traces
random magnetic fields which are strongest in spiral
arms and in central starburst regions. Our recent studies,
based on the nearby late type spiral galaxy and
KiNGfish target NGC 6946, show a power-law correlation
between star formation rate surface density and
the random magnetic field strength (Tabatabaei et al.,
subm). This is observational evidence of generation
Fig. III.4.6: Cumulative number surface density for M 51’s
Giant Molecular Clouds (GMCs) with masses greater than M’,
as identified within the paws field of view. The full sample
of 1507 GMCs (dark-blue squares) has been divided in three
main regions: central (red), spiral arm (light blue) and interarm
regions (green). The three regions encompass dynamically
distinct environments (see Meidt et al. 2012). The mass functions
show a clear change in both the slope and density between
different galactic environments. The vertical dashed line indicates
the completeness limit of the catalog set (3.6 10 5 M 0 ).
The surface area of each region over which the mass spectra is
determined as labeled.