The Milky Way’s Central Molecular Zone –“There’s No Place in the Galaxy Remotely Like It” (Monday’s Most Popular)

 

 

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"Complex organic molecules formed in interstellar space are undoubtedly the fundamental building blocks of astrobiology. The complete inventory of such molecules in this cloud will produce a tremendous advance in our understanding of the physical conditions in that cloud and of the first chemical steps toward life," said Phil Jewell, of the NRAO.


The enigmatic 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).

The CMZ 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).

 

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An infrared and multi-wavelength image above of the Central Molecular Zone in the Milky Way. Dense gas is shown in red, and warm and cold dust in green and blue, respectively. Several key objects in the region are labelled, along with a set of embedded young stellar clusters seen at 24 microns. (C. Battersby).

The ALMA observation of ‘The Brick’ is shown below, which in terms of its density is the most extreme giant molecular cloud in the Milky Way. Within a radius of about ten lightyears, it hosts over a hundred thousand solar masses of gas. The Brick is thought to be the progenitor of a massive stellar cluster, and provides the best opportunity to date to understand the cluster formation process in detail. (Jill Rathborne/CSIRO/JCMT/ALMA).

 

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Harvard-Smithsonian 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 analyse efficiently the large amounts of data they had.

As molecules rotate and vibrate, they emit radio waves at specific frequencies. Each molecule has a unique pattern of such frequencies, called spectral lines, that constitutes a "fingerprint" identifying that molecule. Laboratory tests can determine the pattern of spectral lines that identifies a specific molecule.

Most past discoveries came from identifying a molecule's pattern in the laboratory, then searching with a radio telescope for that set of spectral lines in a region of sky. So far, more than 140 different molecules have been found that way in interstellar space.

"Clouds like this one are the raw material for new stars and planets," said Anthony Remijan, of the National Radio Astronomy Observatory (NRAO). "We know that complex chemistry builds prebiotic molecules in such clouds long before the stars and planets are formed. There is a chance that some of these interstellar molecules may find their way to the surface of young planets such as the early Earth, and provide a head start for the chemistry of life. For the first time, we now have the capability to make a very thorough and methodical search to find all the chemicals in the cloud."

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.

The astronomers found, 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.

The CfA team also analyzed 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 Daily Galaxy via https://www.cfa.harvard.edu/ and NRAO

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