Max Planck Institute for Astronomy - Annual Report 2007

66 III. Selected Research Areas
With all data in hand, the th i n g S team focused on
science in 2007. The data is now fueling more than a
half dozen PhD theses and the first round of research includes
over a dozen science papers now at or near submission.
A sample of the questions being addressed by
the th i n g S team:
• What is the dark matter distribution in each galaxy?
From the motions of the gas measured by th i n g S, one
can infer the distribution of mass in each galaxy and,
after comparison with starlight and the gas distribution,
infer the distribution of dark matter.
• Where do galaxies end? th i n g S maps are both extremely
sensitive and very big, allowing team members
to measure where a galaxyʼs gas reservoir ends. This
yields clues to both the origins of galactic structure and
the conditions, particularly the radiation field, in the
space between galaxies.
• What is the distribution of holes and shells in the ISM?
A casual inspection of the th i n g S maps reveals a wealth
of such structures and systematically measuring their
properties may offer a clue to their origins, for example
from comparison to the distributions of young stellar
clusters.
• Do the deepest hydrogen absorption lines seen along the
line of sight towards high redshift quasars come from
galaxies like those around us? By simulating the effect
of random lines of sight drawn through the th i n g S galaxies,
one can hope to explain the distribution of these
so-called “damped Lyman alpha systems”.
In addition to the four MPIA-based projects described
in detail later in this chapter a half-dozen more projects
are nearing completion. Even with this rich first round
of science, the potential of th i n g S is only beginning to
be tapped. The second round of projects are already underway,
complementary molecular line data are being
collected, and the sample will be targeted as part of a
he r S c h e l Space Observatory legacy program.
A New Atlas of Molecular Gas Maps Using the ir a m
30 m telescope
The atomic hydrogen probed by th i n g S is a key component
of the ISM but does not represent a complete
inventory of the gas. Particularly in the centers of spiral
galaxies, molecular (rather than atomic) gas is often
the dominant component of the ISM. This is also the
phase observed to relate most directly to star formation.
Therefore any study hoping to understand why stars form
where they do must take into account the molecular gas.
Unfortunately, the H 2 that makes up most of the molecular
phase lacks a dipole moment and is therefore not
readily observed under the conditions where it is found
in the ISM (cold, dark clouds). Therefore emission from
other molecules is often used to infer the distribution and
abundance of molecular gas. The most common and commonly
used of these tracer molecules is CO. Although
relatively abundant and bright, mapping CO still remains
a challenging undertaking and the lack of extended, sensitive
maps of CO emission have remained a serious gap
(although the pioneering Bi m a Survey of Nearby Galaxies
took a major step forward in this regard).
Following a pilot project in the winter of 2006, 2007
saw an MPIA-lead team undertake a large project using
the ir a m 30 m telescope to map the molecular gas in a
subset of the th i n g S /Si n g S sample. The project will use
more than 300 hours of telescope time to map CO line
emission from 20 th i n g S galaxies. The enabling technology
is the he r a focal plane detector array, a 9-element
dual-polarization receiver array. he r a allows fast
on-the-fly mapping of the CO (2 → 1) transition, making
it possible to obtain sensitive maps of CO emission
over wide areas while still yielding an excellent angular
resolution of 11 arcseconds. Because it simultaneously
maps emission from nine positions on the sky, he r a offers
almost an order of magnitude increase in efficiency,
making it possible to accomplish in only 300 hours a
project that would have required almost 3000 hours just
five years ago.
The he r a survey will wrap up in the early spring of
2008 but is already yielding exciting results. In particular,
one already observes a striking, nearly one-to-one
correspondence between CO line emission and the rate
of star formation inferred from ultraviolet and infrared
maps. The result suggests that molecular gas may form
stars in roughly the same way in all of the spiral galaxies
that the team studied.
Science Highlights
First Science With the LBT: Stellar Feedback in IC 2574
The Large Binocular Telescope (LBT), with its large aperture,
wide field of view, and excellent sensitivity to
blue light, will be a powerful tool to study star formation
in nearby galaxies. Some of the first science results
to come out of the LBT already demonstrate this capability.
MPIA researchers led an effort using the LBT to
make sensitive maps of U-, B-, and V-band light from
the nearby dwarf irregular galaxy IC 2574, also a target
of th i n g S. With extended star formation and an HI distribution
characterized by dozens of known holes and
shells, IC 2574 is an ideal locale to study the interaction
between star formation as viewed by the LBT and the
ISM seen by th i n g S.
From the LBT data (Fig. III.1.1a), the researchers
were able to estimate the number of young stars at each
location in the galaxy and how recently these stars were
formed. They also calculated how much energy these
stars create in the form of winds, supernova explosions,
and ionizing photons. Comparing this information to the