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2015 Featured Story #1: Galaxy Cluster Star Formation Modelled as "Rain"

Composite image Abell 2597 galaxy cluster - visible, H-Alpha, Chandra X-ray

The majority of the hundreds of millions of galaxies in the universe are members of giant clusters of galaxies, ranging from small groupings of 50 or so (such as the Local Cluster that our own Milky Way galaxy is part of) to larger collections of over 1000 galaxies. Observations over the years of the rates of star formation in the central bright galaxies in these clusters have shown a wide variation. It has not been clear why these galaxies in some clusters have formed millions of stars in the past million years (and seem to have done so at similar rate for a good part of their lifetimes), while similar, central galaxies in other clusters appear to have formed no new stars for a substantial fraction of their lifetimes. There are some clusters whose brightest galaxy lies between these extremes as well, showing evidence of ample star formation during some periods of time and little star formation at other times.

MSU Physics & Astronomy Professors Mark Voit and Megan Donahue are part of a research team also including astronomers from MIT and Columbia University which has recently published an analysis of over 200 similar galaxy clusters in order to find features which can distinguish between low-production clusters and high-production clusters (and those which seem to oscillate between the two states), and then to come up with an explanation which makes sense according to the laws of physics as to why they differ. They chose galaxy clusters which have the common configuration of a large primary galaxy at the center of a group of other galaxies.

Star formation occurs under certain conditions of gas density and temperature which the research group makes an analogy of how precipitation occurs in Earth's atmosphere. On earth, if there is too little water vapor in an atmospheric layer, or it's too hot, it is not easy for the water molecules to clump up to form raindrops at that time, but if more is added, or what is there gets a chance to cool, then rain may fall. The group finds that interstellar gas behaves in a similar fashion, with the behavior of gas clouds with respect to the supermassive black hole (SMBH) at the center of the primary galaxy as the main indicator of whether the galaxies in the cluster have the right conditions of density and temperature to allow stars to form.

The researchers supplemented visible-light observations of the clusters of galaxies with detailed X-ray studies from NASA's Chandra X-ray Observatory satellite in order to identify the cooler and hotter gas, respectively, around the cluster and near the primary galaxy's SMBH (the picture above is a composite made from bands of X-ray and visible light wavelengths of cluster Abell 2597, about a billion light years away). They have calculated the rates at which gas heats up by passing near the SMBH and cools down over time after leaving its vicinity, leading it to clump up and either form stars or fall back in towards the SMBH to heat up again. Galaxy clusters which their calculations show have the heating and cooling processes nearly in balance are seen to have star formation regions, just as balanced water vapor densities and temperatures allow for rain on Earth. Clusters of galaxies where the processes are out of balance do not. In some cases, the imbalance appears to be temporary and the researchers believe that star formation will resume in the future after the hot gases have a chance to cool back down and restart the "precipitation" cycle. In other cases, however, it appears that some past event, perhaps a galaxy-galaxy collision, generated enough excess heat to "dry up" the supply of cooling interstellar gas which might be used for star production, possibly permanently.

For further information, see this MSU Today article or this Chandra X-ray Observatory article.

Two scholarly articles related to these studies and computations are:
"Cooling Time, Freefall Time, and Precipitation in the Cores of ACCEPT Galaxy Clusters", G. Mark Voit & Megan Donahue, 2015 Astrophysical Journal Letters 799 L1 and
"Regulation of star formation in giant galaxies by precipitation, feedback and conduction", G. M. Voit, M. Donahue, G. L. Bryan, M. McDonald, Nature 2015/03/04/online (advance online publication)