Annual Report 2011 Max Planck Institute for Astronomy

SFR Surface Density [M yr –1 kpc –2 ]
1
0.1
0.01
0.001
0.0001
0.1
SFR vs H,
0.1 Gyr
0.1 Gyr
10 Gyr SFR vs HH2
1 10 10 2 10 3 0.1 1 10
Gas Surface Density [M pc –2 ]
Fig. III.4.4: The star formation relation for different types of
gas: (left) atomic gas seen in HI, (middle) total neutral gas
(HIH 2 ), and (right) only molecular gas (H 2 ). We use sensitive
molecular CO gas data from heracles and a novel technique
to stack spectra that allows us to measure faint CO emission
with high significance in regions dominated by atomic gas
(HI). While the SFR is not correlated to HI, it does correlate
with H 2 and total gas column. However, the scaling is uniform
for all regimes only for H 2 .
Star-Formation Relations: Connecting stars and gas
While stars form out of molecular gas – a process that is
confirmed by observations of Giant Molecular Clouds in
the Galaxy and supported by the strong correlation of gas
and SFR in nearby galaxies (see e.g. Bigiel et al., 2008) –
there has been a long standing debate of the role atomic
gas has on star formation on large scales. The lack of a
clear correlation of HI and SFR in the inner parts of galaxy
disks offers circumstantial evidence that star formation
remains coupled to the molecular, rather than the total
gas even where the ISM is mostly atomic. However,
the exact relationship remained largely unexplored in the
HI dominated regime, until the thiNGs and heracles surveys
revealed the most sensitive view of the entire starforming
disks of nearby galaxies to date. Using the HI
and CO data from these surveys we applied a novel technique
to stack CO spectra and thus measure extremely
faint emission (Schruba et al. 2011). Using HI velocities
to shift the CO to a common velocity and then stacking
and radially average, significantly increases the signal-to
noise and allows to detect CO down to Σ H2 1M 0 pc –2 ,
or one order of magnitude deeper than previous studies.
Combining the far-UV and 24 µm emission from
the siNGs survey to measure the SFR, we compared the
role of different phases in the ISM to star formation
(Fig. III.4.4). We found that while HI and SFR are only
weakly correlated, H 2 and total gas column show strong
correlations with SFR. However, only the H 2 –SFR relation
can be parametrized by a unique (linear) function
0.1 Gyr
III.4 The Interstellar Medium of Nearby Galaxies 67
0.1 Gyr
10 Gyr SFR vs H2
0.1 Gyr
10 2 10 3 0.1 1 10 10 2 10 3
that is valid in both the H 2 and HI dominated regime.
Thus, star formation in molecular clouds appears to be
independent of environment (e.g., the local gas density),
however, the formation of molecular gas out of the atomic
gas shows systematic variations across galaxies and
a strong dependence on galactic environment.
Clouds and Clumps: Tracing the molecular ISM
structure in the Whirlpool galaxy.
0.1 Gyr
10 Gyr
The assembly of giant molecular clouds (GMCs) out of
the diffuse interstellar medium (ISM) and subsequent
onset of star formation is an active area of astrophysical
research. In normal galaxies, these GMCs are often
described as the basic unit of structure in the molecular
ISM, with typical GMC masses of 10 4 to 10 6 M 0
and sizes of 10 to 50 pc observed. Most Galactic star
formation activity appears to occur within GMCs, but
our knowledge of the processes that regulate the physical
and chemical properties of GMCs is far from being
complete.
To date, wide-field CO observations that can resolve
individual GMCs have been restricted to the Local
Group, mostly surveying low mass galaxies where atomic
gas dominates the interstellar medium (for a review,
see Fukui & Kawamura 2010). This is a major shortcoming
since massive disk galaxies dominate the mass and
light budget of star-forming galaxies and host most of
the star formation in the present-day universe. With paws
we resolve this issue, exploring a massive, molecular gas
dominated galaxy at high physical resolution. In Figure
III.4.5, we present the map of CO integrated intensity
within M 51 obtained by paws. For comparison, the
other panels of this figure show CO integrated intensity
maps of M 33 (Rosolowksy 2007) and the LMC (Wong
et al 2011), after smoothing the three datasets to the same
resolution and extrapolating them onto the same pixel
grid. Unlike the CO emission in the low-mass galaxies,
it is obvious that much of the emission in M 51 arises in
Credit: E. Schinnerer