Astronomers always enjoy looking at incredibly violent places. Violence in the astronomical sense creates rare conditions that can explain a lot about our universe. One of the violent places astronomers love to study is the center of our Milky Way Galaxy. Now, astronomers at the Center for Astrophysics (CfA) at Harvard have created a new catalog of some of the most intense areas near the galactic core. They hope this will improve our understanding of these potential star-forming regions – and help explain why so few stars are actually being formed in them.
The area of the galaxy they studied, known as the “Central Molecular Zone” (CMZ), extends over the 1,600 light years closest to the supermassive black hole in the center of the Milky Way. That’s about 16 times closer to the galactic center than our own sun. Almost everything about this part of the galaxy is “more intense” than the further it goes. The molecular gas clouds that make it up have higher magnetic fields, radiation, and even pressures and temperatures. They also make up an estimated 60 million solar masses.
UT video on star formation
These extreme conditions are ideal locations for star formation, although there are several evolutionary steps that molecular gas clouds must go through before they end up as a ball of nuclear fusion. First, the molecular gas has to form “clumps” between 1 and 10 light years in diameter. When enough material accumulates in a given lump, it can move on to the next stage where it forms what is called a “core”. Kernels are about 10% the size of a lump, but they all contain the same material and therefore have a much higher temperature and pressure than their predecessors. Nuclei then occasionally give birth to individual stars.
This process is extremely slow and can take millions of years to move from one state to another. To really understand how these transitions might happen, it is best to examine as many different clumps as possible to observe discrete points in the evolution of the systems. However, foreground and background emissions (i.e., noise) have previously made it extremely difficult to determine the prevalence of clumps or nuclei anywhere near the center of the galaxy.
The arcs (L) and the quintuplet cluster (R) are two of the highest concentrations of young stars in the Milky Way. Both clusters are about 100 light years from the galactic center, right in the middle of the CMZ.
Credit: (L) ESO / P. Espinoza – (R) ESA / Hubble
What it took to subtract the signal from the noise was a considerable amount of time on a very powerful telescope. This is exactly what the research team received when they received 550 hours of observation time on the submillimeter array (SMA). With their data in hand, the team searched them to find 285 new stand-alone nuclei that could eventually form a star. In addition, 531 other potential core candidates are included in the data, but their presence was not conclusive enough to include them in the total without further confirmation.
Confirmation of another interesting observation was also hidden in the data – that nuclei form stars at a much slower rate than would be expected by modeling. Indeed, the SMA’s data suggest that star formation is as common elsewhere in the galaxy as it is in the cores at the violent center of the galaxy. This confuses astronomers as the higher pressures and temperatures in the center of the galaxy should guide the formation of stellar objects.
An image of the Terzan 5 cluster on one of the original building blocks of the galaxy.
Photo credit: NASA / ESA / Hubble / F. Ferraro
There are some theories as to why there is a discrepancy between the expected star formation rate of these regions and the rate observed by the SMA survey. However, more data must be collected before a hypothesis can be confirmed or refuted. However, the catalog collected by the CfA astronomers is a good place to start for collecting this data.
CfA – cold dust nuclei in the central zone of the Milky Way
CfA – high pressure star formation in the galactic center
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The Astrophysical Journal – CMZoom. II. Catalog of compact dust continuum sources in the sub-millimeter range in the central molecular zone of the Milky Way
False color image of the CMZ, delimited by the density and optical wavelength emitted by the clusters.
Photo credit: Battersby et al