MIT: Why isn’t the universe as bright as it should be?

[bystander]

Why isn’t the universe as bright as it should be?
Massachusetts Institute of Technology | 2015 Mar 04

This Hubble Space Telescope image of galaxy NGC 1275 reveals the fine, thread-like filamentary structures in the gas surrounding the galaxy. The red filaments are composed of cool gas being suspended by a magnetic field, and are surrounded by the 100-million-degree Fahrenheit gas in the center of the Perseus galaxy cluster. The filaments are dramatic markers of the feedback process through which energy is transferred from the central massive black hole to the surrounding gas. (Courtesy of NASA (edited by Jose-Luis Olivares/MIT))

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A handful of new stars are born each year in the Milky Way, while many more blink on across the universe. But astronomers have observed that galaxies should be churning out millions more stars, based on the amount of interstellar gas available.

Now researchers from MIT, Columbia University, and Michigan State University have pieced together a theory describing how clusters of galaxies may regulate star formation. They describe their framework this week in the journal Nature.

When intracluster gas cools rapidly, it condenses, then collapses to form new stars. Scientists have long thought that something must be keeping the gas from cooling enough to generate more stars — but exactly what has remained a mystery.

For some galaxy clusters, the researchers say, the intracluster gas may simply be too hot — on the order of hundreds of millions of degrees Celsius. Even if one region experiences some cooling, the intensity of the surrounding heat would keep that region from cooling further — an effect known as conduction. ...

For so-called “cool core” galaxy clusters, the gas near the center may be cool enough to form some stars. However, a portion of this cooled gas may rain down into a central black hole, which then spews out hot material that serves to reheat the surroundings, preventing many stars from forming — an effect the team terms “precipitation-driven feedback.” ...

Regulation of star formation in giant galaxies by precipitation, feedback and conduction - G. M. Voit et al

• Nature (online 04 Mar 2015) DOI: 10.1038/nature14167
  arXiv.org > astro-ph > arXiv:1409.1598 > 04 Sep 2014 (v1), 03 Mar 2015 (v3)

[bystander]

Cosmic Showers Halt Galaxy Growth
NASA | MSFC | SAO | Chandra X-ray Observatory | 2015 Mar 04

X-ray: NASA/CXC/Michigan State Univ/G.Voit et al; Optical: NASA/STScI & DSS; H-alpha: Carnegie Obs./Magellan/W.Baade Telescope

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This galaxy cluster comes from a sample of over 200 that were studied to determine how giant black holes at their centers affect the growth and evolution of their host galaxy, as reported in our latest press release. This study revealed that an unusual form of cosmic precipitation enables a feedback loop of cooling and heating, stifling star formation in the middle of these galaxy clusters.

Abell 2597, shown here, is a galaxy cluster located about one billion light years from Earth. This image contains X-rays from NASA's Chandra X-ray Observatory (blue), optical data from the Hubble Space Telescope and the Digitized Sky Survey (yellow) and emission from hydrogen atoms (red) from the Walter Baade Telescope in Chile.

According to this new study, the regulation of the largest black hole and their host galaxies works as follows: in some galaxies, such as NGC 2597, hot gas is able to quickly cool through radiation and energy loss, in a process called precipitation. The clouds of cool gas that result then fall into the central supermassive black hole, producing jets that heat the gas and prevent further cooling. ...

A pre-print of the Nature study by Mark Voit (Michigan State University), Megan Donahue (Michigan State), Greg Bryan (Columbia University), and Michael McDonald (Massachusetts Institute of Technology) is available online; the study builds on work by Voit and Donahue that was published in the January 20th, 2015 issue of The Astrophysical Journal Letters and is available online.

Cooling Time, Freefall Time, and Precipitation in the Cores of ACCEPT Galaxy Clusters - G. M. Voit, M. Donahue

• Astrophysical Journal Letters 799(1) L1 (2015 Jan 20) DOI: 10.1088/2041-8205/799/1/L1
  arXiv.org > astro-ph > arXiv:1409.1601 > 04 Sep 2014