galaxies, cosmology, black holes
Galaxies are stunningly beautiful. Each one is like an ecosystem that turns gas into stars, which ultimately recycle much of their gas into interstellar space. This recycling process makes life possible, as stars produce the necessary elements and galaxies turn them into solar systems like our own. Studying how galaxies evolve through time therefore sheds light on how we got to be here, with the opportunity to admire the universe’s beauty. Curiosity about that evolution process motivates much of Mark’s research.
Galaxies form because gravity continually pulls the universe’s matter into clumps of ever increasing size. Galaxy clusters, like the one in the picture, are the largest clumps to have settled into distinct structures. Nearly all of the yellow blobs of light are galaxies belonging to the cluster. The purple glow represents X-ray emission from hot gas filling the spaces between the galaxies. Analyses of galaxy clusters tell us that about 2% of the universe’s matter consists of stars, another 13% is gaseous, and the remaining 85% is “dark matter” that would otherwise be undetectable. And the amount of dark matter determines how rapidly galaxies and galaxy clusters form.
Black Hole Feedback
Why hasn’t more of the hot gas in galaxy clusters turned into stars? The answer has something to do with the supermassive black holes at the centers of galaxy clusters, which are the largest in the universe. This composite photo shows energetic particles erupting from the vicinity of a central black hole (red bubbles) interacting with the hot gas filling the galaxy cluster (blue glow). Eruptions like this one tend to happen in clusters that contain cooler gas capable of star formation (red filaments) and appear to limit the number of stars that form. Understanding these interactions is a central goal of our research group at Michigan State.
Much of what we have learned from galaxy cluster studies may also apply to galaxies like our own Milky Way. Each one has an extended atmosphere, sometimes called its circumgalactic medium. The atmosphere supplies the gas that sustains star formation, and eruptions of energy from the galaxy regulate the gas supply. Black holes appear necessary to power eruptions from large galaxies. Supernova explosions seem to regulate star formation in smaller galaxies. The photo shows bubbles of hot gas (yellow) erupting above and below the disk (black) of our own galaxy.
For more details, see Mark’s most recent scientific papers.