Annual Report 2011 Max Planck Institute for Astronomy

66 III. Selected Research Areas
Tracing “hidden”ʼ gas: the gas to dust ratio
While tracing atomic gas is relatively simple through
observations of the HI hyperfine line at 21 cm, observing
cool, molecular H 2 is difficult due to its lack of a
dipole moment. The standard method is to observe the
CO (submm) emission lines and convert to the total H 2
through a conversion factor, α CO . This factor is determined
in the Galaxy using dynamical methods yet remains
relatively uncertain because of possible variations
due to abundance effects, or “hidden”, CO-absent, molecular
gas. However, as dust and gas are closely coupled,
it is possible to use the total dust mass to trace the total
gas mass and constrain α CO . Using a technique based
on Leroy et al. (2011), and data from the KiNGfish and
heracles surveys, we constrained α CO and the dustto-gas
mass ratio (DGR) on kpc scales across the
disks of 26 galaxies (Sandstrom et al., 2012). We ob-
Fig. III.4.3: Left: Solutions for DGR (top) and α CO (bottom)
plotted versus oxygen abundance based on the Pilyugin &
Thuan (2005) calibration. The gray points show all of the individual
solutions. The green points show the highest-confidence
solutions. The weighted mean and standard deviation of all of
the solutions in 0.1 dex bins are shown with red symbols. The
log(a CO / (M pc–2 / (K km s –1 ))) log(DGR)
–1
–1.5
–2
–2.5
2
1.5
1
0.5
0
–0.5
–1
Weighted Mean
Da Co 0.2
All Solutions
Weighted Mean
Da Co 0.2
All Solutions
7.8 8
8.2
8.4 8.6 8.8
12 log(O/H)
serve several trends in α CO , based on our nearby galaxy
sample. Galaxy centers frequently show low values of
α CO , in some cases nearly an order of magnitude below
the typically assumed local Milky Way value of α CO
4.35 M 0 pc –2 (K km s −1 ) −1 . Using uniformly determined
metallicities from H II region spectra, we find a
general trend of decreasing α CO with increasing metallicity,
while for DGR we measure a clear, positive, linear
trend with metallicity (Fig. III.4.3). However, large
galaxy-to-galaxy offsets in the relationship between metallicity
and α CO suggest that metallicity may not be the
primary driver of variations in α CO in the central, higher
metallicity regions of galaxies. Due to the strong radial
gradients in many quantities, it is difficult to isolate
which physical process is the driver of α CO variations,
but in general regions with intense UV fields, high starformation
rate surface densities and/or high stellar mass
surface densities show low α CO .
shaded yellow region shows the approximate range of α CO values
determined in the Milky Way. Right: The dust mass surface
density in NGC 6946 (top) showing both the apertures and region
over which the values were determined, and the resulting
α CO for the same galaxy (bottom), revealing significantly lower
values in the central region.
Log(S D ) (M e pc –2 )
–1 –0.8 –0.5 –2 0 0.2 0.5
NGC 6946
3.2 kpc
3
–1 –0.5
3.2 kpc
3
Log(a CO )
0 0.5 1 1.5 2
Credit: E. Schinnerer