Shells, Bubbles and Holes: the Porosity of the Interstellar Medium in Galaxies
Abstract
We present an analysis of the properties of HI holes detected in 20 galaxies that
are part of “The HI Nearby Galaxy Survey” (THINGS). We detected more than
1000 holes in total in the sampled galaxies. Where they can be measured, their
sizes range from about 100 pc (our resolution limit) to about 2 kpc, their expansion
velocities range from 4 to 36 km/s, and their ages are estimated to range between
3 and 150 Myr. The holes are found throughout the discs of the galaxies, out to
the edge of the HI disc; 23% of the holes fall outside R25. We find that shear
limits the age of holes in spirals; shear is less important in dwarf galaxies which
explains why HI holes in dwarfs are rounder, on average than in spirals. Shear,
which is particularly strong in the inner part of spiral galaxies, also explains why
we find that holes outside R25 are larger and older. We derive the scale height of
the HI disc as a function of galactocentric radius and find that the disc flares at
large radii in all galaxies. We proceed to derive the surface and volume porosity
(Q2D and Q3D) and find that this correlates with the type of the host galaxy: later
Hubble types tend to be more porous.
The size distribution of the holes in our sample follows a power law with a slope
of a=−2.9. Assuming that the holes are the result of massive star formation, we
derive values for the supernova rate (SNR) and star formation rate (SFR) which
scales with the SFR derived based on other tracers. If we extrapolate the observed
number of holes to include those that fall below our resolution limit, down to holes
created by a single supernova, we find that our results are compatible with the
hypothesis that HI holes result from star formation.
We use HI data from THINGS, 8μm, 24μm, 70μm and HI maps from SINGS,
CO(2–1) data from HERACLES and FUV data from NGS to present a visual
comparison of these maps with respect to the locations of HI holes. We find that
the vast majority of HI holes are also prominent in the 8μm map and to some
extent in the 24μm map. There is a lack of molecular gas from the interior of
nearly all the holes, which is consistent with the idea that the latter are filled
with hot gas. About 60% of young holes have FUV emission detected in their
interiors highlighting the presence of the parent OB association. In addition,
FUV is detected on the rims of some of the older HI holes, presumably due to
the dispersion of the OB association with respect to the gas. We describe the
development of a 2–D cross-correlation method to compare multi-wavelength maps
in a quantitative way (quantified by Ccoef ) and give some first results from the
application of this method to the nearby galaxy NGC2403. We find that the all
the dust tracers are well correlated (Ccoef > 0.7) with the 8μm–24μm correlation
being the highest (Ccoef > 0.88). Similarly all the star formation tracers are
well linked as expected (Ccoef > 0.6). With respect to the relations between star
formation and dust tracers we found that most are well matched (Ccoef > 0.7) as
dust grains are heated by radiation in star forming regions. At smaller scales (15")
FUV correlates poorly (Ccoef ~ 0.3) with the dust tracers, a direct consequence
of the absorption of FUV photons by dust. We find that the HI is reasonably
well correlated with the 8μm emission (Ccoef ~ 0.6) illustrating the fact that HI is
mixed with PAH’s. Interestingly, the HI map shows some correlation with the SF
map (Ccoef ~ 0.4) even though FUV and HI emissions were found to be completely
uncorrelated (Ccoef ~ 0).