Survey of Milky Way’s Central Molecular Zone Could Help Study Star Formation

The central molecular zone (CMZ) of the Milky Way galaxy covers the innermost 1600 light-years of the galaxy, including a huge complex of molecular clouds that contain nearly 60 million solar-masses of molecular gas.

A three-color image of the Milky Way’s central molecular zone in dense molecular gas (red), far infrared (green) and near infrared (blue); major regions are labeled. Astronomers have produced a new catalog of the densest, star-forming cores across the region.
A three-color image of the Milky Way’s central molecular zone in dense molecular gas (red), far infrared (green) and near infrared (blue); major regions are labeled. Astronomers have produced a new catalog of the densest, star-forming cores across the region. Image Credit: Battersby et al. 2020.

For a comparison of the size of the CMZ, the Sun is located 26,600 light-years away from the galactic center.

The gas in the molecular clouds occurs under more intense physical conditions compared to anywhere else in the galaxy, on average. The higher the temperatures and densities, the more and more extreme are the pressures, turbulence and magnetic fields, and the greater are the cosmic-ray abundances and ultraviolet and X-ray radiation.

Thus, the CMZ is an exclusive laboratory to study how stars form: apart from being rarely observed in the remaining part of the Milky Way, these conditions seem to be similar to those found in extremely luminous star-forming galaxies in the early universe and provide indirect insights for understanding the cosmic history of star formation not otherwise viable at present.

But there is a riddle: in the CMZ, the rate at which stars form is considerably less than what is expected, hardly one-tenth of a solar-mass per year.

In giant molecular clouds (GMCs), the birthplaces of stars are believed to be the densest regions known as 'clumps,' with characteristic sizes of 1 to 10 light-years.

These clumps divide further into gravitationally linked 'cores' with characteristic sizes of about 10 times smaller; and then from these cores, individual stellar systems can form.

Large surveys of star-forming and non-star-forming structures over the hierarchical continuum of relevant physical conditions and scales are required to find the nature of the transitions between these evolutionary stages.

Cara Battersby, Eric Keto, Daniel Callanan, Nimesh Patel, Qizhou Zhang and Volker Tolls—astronomers from CfA—and their collaborators have released the CMZoom survey, an unbiased and comprehensive map of the high density gas in the region.

High density regions are measured by the amount of molecular hydrogen gas along their lines-of-sight and present so much dust that the visible light is blocked entirely.

The survey was the outcome of a huge, 550-hour Submillimeter Array program and led to new catalogs of the compact cores in this region. In total, 285 separate cores were discovered unambiguously, and 531 more have highly tentative identifications.

Similar to anywhere else, the cores in the CMZ are promising locations for future star clusters. However, bright background and foreground emission toward the galactic center renders it hard to ascertain the masses of these cores accurately. This makes the critical predictor of star formation very much uncertain (the emission is also one of the reasons behind the catalog being so hard to prepare).

However, the astronomers could predict the maximum star formation capability of the cores in their catalog by carrying out general yet realistic assumptions of the temperatures, masses and other properties of the cores.

They have estimated a maximum prospective star formation rate ranging from 0.08 to 2.2 solar-masses per year, maybe even equivalent to the current average star formation rate in the whole galaxy. The findings stress the mysterious weakness of existing star formation in the CMZ.

Since the survey catalogs all the cores for study, it is a step ahead in the path toward gaining insights into star formation in the drastic environments that occur in the CMZ and the early universe.

Journal Reference:

Hatchfield, H. P., et al. (2020) CMZoom. II. Catalog of Compact Submillimeter Dust Continuum Sources in the Milky Way’s Central Molecular Zone. The Astrophysical Journal Supplement Series.


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