Annual Report 2011 Max Planck Institute for Astronomy

64 III. Selected Research Areas
III.4 The Interstellar Medium of Nearby Galaxies
The temperature and corresponding phase transitions of
the interstellar medium (ISM) play key roles in the formation
of stars, and thus galaxy evolution. Yet the heating
and cooling of the ISM and associated transitions
between phases are still not fundamentally understood.
This section describes some of our ongoing multiwavelength
work at MPIA to understand these processes and
the fundamental connections between the stars and ISM
of galaxies.
To understand galaxies we must first understand the
physical processes that regulate their evolution: the cooling
and corresponding phase transitions in the interstellar
medium (ISM), the formation of stars from the cold
ISM, and the return of radiant and mechanical energy
from those stars, heating the ISM. Together, the structure
and composition of the ISM are tracers of, and direct results
from, the formation of and the feedback from stars.
The ISM is generally considered to be in three phases;
ionized, atomic and, molecular, each with a range of densities
and temperatures. Throughout these phases exists
interstellar dust, thought to be composed mostly of carbonaceous
grains and silicates, and ranging from micronsized
grains to large molecules, such as polycyclic aromatic
hydrocarbons (PAHs). This ISM has a still poorly
understood structure, consisting of diffuse gas, clumpy
clouds, and large scale features such as spiral arms.
The heating in the ISM is dominated by young, massive
stars, with large UV fluxes that ionize interstellar
gas, eject photoelectrons from dust grains, and heat the
same dust grains to high temperatures. These stars also
have a large mechanical energy input into the ISM due to
their strong winds during their lifetimes, and the supernova
at the end of their lives. ISM cooling is dominated
by recombination and forbidden lines within ionized regions,
and far-infrared continuum and line emission (e.g.
from HI and CO) in neutral gas and in the cold molecular
clouds from which stars form. It is this emission that allows
us to trace the ISM structure and its phases.
The processes that shape the ISM occur on varying
scales; the galactic scale spiral density waves observable
at both optical and infrared wavelengths, the large scale
outflows driven by starbursts and supernovae, the small
scale molecular clouds that form stars, and the individual
HII regions and the photo-dissociation regions that surround
them. While observations of individual interstellar
clouds and star-forming regions within the Milky Way
provide the highest spatial resolution, studying the various
scales of energy and heating balance within our own
Galaxy proves difficult as line-of-sight reddening in the
optical and UV, uncertain distances, and background/
foreground confusion lead to enormous complications.
Observations of external galaxies avoid these problems
and in addition allow one to explore a significantly
wider range of physical properties, such as metallicity,
ISM densities, and star formation rates (SFR). In particular,
nearby galaxies provide a vital bridge between
in-depth, resolved studies in our Galaxy and the globally
integrated measurements of distant galaxies. In nearby
galaxies it is possible to explore the ways and the scales
over which different stellar populations affect the surrounding
ISM in different regions of galaxies (i.e. spiral
arms and inter arm regions, bulge, and disk, etc.) and at
different metallicities, as well as large ranges in gas and
dust column densities.
Nearby Galaxies at all Wavelengths
MPIA has conducted several multiwavelength surveys
of nearby galaxies, and is part of several more through
collaborations. These multiwavelength surveys enable
the delineation of both the stars and multi-phase ISM in
these galaxies. We detail a fraction of these below that
are ongoing.
• The Andromeda Galaxy (Messier 31) is the nearest
massive spiral galaxy to our own, and thus provides the
best spatial resolution while still giving an integrated
galaxy view. We have recently obtained herschel Space
Observatory images from 70 – 500 µm of Andromeda
(see Fig. III.4.1) and in association with existing multiwavelength
observations, this allows us to directly connect
the dust with stars and all phases of gas. By including
a MPIA-involved heritage hubble survey of M 31
that resolves individual stars (phat; Dalcanton et al.,
2012) we can directly determine ISM heating input from
stars to dust.
• The Whirlpool Galaxy (Messier 51) is an interacting
grand-design spiral galaxy that is face-on (i 23), extremely
gas rich, with a high current rate of star formation.
With the Plateau de Bure Interferometer Arcsecond
Whirlpool Survey (paws) we have mapped 12 CO(1−0)
in the central 11 8 kpc of M 51, detecting molecular
clouds down to 40 pc scales and masses down to 10 5
M 0 , typical values for Galactic GMCs. In association
with an existing multiwavelength dataset, this allows us to
directly connect the molecular gas clouds with dust, stars,
star-formation (i.e. HII regions) and atomic (HI) gas.
• The SingS/KingfiSh/ThingS/heracleS sample
MPIA is part of and has contributed to the largest multiwavelength,
resolved sample of galaxies with the combination
of the Infrared siNGs (spitzer; Kennicutt et al.,
2003) and KiNGfish (herschel; Kennicutt et al., 2011)
surveys and the HI thiNGs (Walter et al., 2008) and iraM