The
center of our Milky Way galaxy lies about 27,000 light-years away in
the direction of the constellation of Sagittarius. At its core is a
black hole about four million solar masses in size. Around the black
hole is a donut-shaped structure about eight light-years across that
rings the inner volume of neutral gas and thousands of individual stars.
Around that, stretching out to about 700 light-years, is a dense zone
of activity called the Central Molecular Zone (CMZ). It contains almost
eighty percent of all the dense gas in the galaxy - a reservoir of tens
of millions of solar masses of material - and hosts giant molecular
clouds and massive star forming clusters of luminous stars, among other
regions many of which are poorly understood. For example, the CMZ
contains many dense molecular clouds that would normally be expected to
produce new stars, but which are instead eerily desolate. It also
contains gas moving at highly supersonic velocities - hundreds of
kilometers per second (hundreds of thousands of miles per hours).
Where did the CMZ come from? No place else in the Milky Way is
remotely like it (although there may be analogues in other galaxies).
How does it retain its structure as its molecular gas moves, and how do
those rapid motions determine its evolution? One difficulty facing
astronomers is that there is so much obscuring dust between us and the
CMZ that visible light is extinguished by factors of over a trillion.
Infrared, radio, and some X-ray radiation can penetrate the veil,
however, and they have allowed astronomers to develop the picture just
outlined.
CfA astronomers Cara Battersby, Dan Walker, and Qizhou Zhang, with
their team of colleagues, used the Australian Mopra radio telescope to
study the three molecules HNCO, N2H+, and HNC in the CMZ.
These
particular molecules were selected because they do a good job of tracing
the wide range of conditions thought to be present in the CMZ, from
shocked gas to quiescent material, and also because they suffer only
minimally from cluttering and extinction effects that complicate more
abundant species like carbon monoxide.
The scientists developed a new
computer code to analyze efficiently the large amounts of data they had.
The astronomers find, consistent with previous results, that the CMZ
is not centered on the black hole, but (for reasons that are not
understood) is offset; they also confirm that the gas motions throughout
are supersonic. They identify two large-scale flows across the region,
and suggest they represent one coherent (or at most two independent)
streams of material, perhaps even spiral-like arms. They also analyze
the gas in several previously identified zones of the CMZ, finding that
one shell-like region thought to be the result of supernova explosions
may instead be several regions that are physically unrelated, and that a
giant cloud thought to be independent is actually an extension of the
large-scale flows. The scientists note that this work is a first step in
unraveling an intrinsically complex galactic environment, and that
pending research will trace the gas motions to larger distances and try
to model the CMZ gas motions with computer simulations.
Reference(s):
"Molecular gas kinematics within the central 250 pc of the Milky Way,"
J. D. Henshaw, S. N. Longmore, J. M. D. Kruijssen, B. Davies, J. Bally,.
Barnes, C. Battersby, M. Burton, M. R. Cunningham, J. E. Dale, A.
Ginsburg, K. Immer, P. A. Jones, S. Kendrew, E. A. C. Mills, S.
Molinari, T. J. T. Moore, J. Ott, T. Pillai, J. Rathborne, P. Schilke,
A. Schmiedeke, L. Testi, D. Walker, A. Walsh and Q. Zhang, MNRAS 457, 2675, 2016.
