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Take a look at any galaxy in the universe with the help of a telescope or images captured by observatories and you may have a good idea of its shape.
Think about it again: about half of the matter of a galaxy is invisible. But scientists have since discovered this missing mass in cold gas halos surrounding galaxies. Understanding their dynamics, however, remains an open question.
Today, a team of astronomers led by Crystal Martin, a professor at the University of Santa Barbara, and graduate student Stephanie Ho have published an article on the interactions between these halos and the rest of their galaxies. The study appears in The Astrophysical Journal.
"When you look at galaxies, you see light and that's what catches your eye, and you think that's where the content is," Martin said. "What's interesting to me is that most of the normal materials associated with a galaxy are not bright at all."
Astronomers can measure the total mass of a galaxy by observing the movement and rotation of the galaxy. But that does not separate normal matter from dark matter. Fortunately, scientists have good measurements of the ratio of these two values on a cosmic scale. About 84% of the mass of the universe consists of dark matter and 16% is conventional matter made up of protons, electrons, neutrons and their cousins. With this knowledge, researchers realized that about half of the normal matter of galaxies could not be found.
Much previous work has focused on finding this missing material, Martin said. "Our study was trying to measure the evolution of gas in the halo," she said.
To find the missing material, astronomers needed something incredibly brilliant across the diffuse clouds. Some galaxies have extremely active black holes in their center, called quasars, that emit radiation beacons into the universe and are bright enough to help scientists detect diluted gas halos.
As quasar light crosses these galactic halos, dust and gases absorb specific wavelengths of light depending on its composition. The research team has collected the distribution and composition of this missing material by comparing the light spectra of these enveloped quasars to those they can see directly.
"This method gives you a lot of information, but it's all in a line of sight," Martin said. And few galaxies have more than one quasar behind them.
To get around this challenge, Martin and his colleagues combined data from 50 similar galaxies, each with a single quasar. This gave a model of an average galaxy with fifty quasars behind, sufficient coverage to get an accurate picture of the system.
Now that researchers knew the size of the halo and what it was made of, they wanted to study the behavior of this gas. Fortunately, the movement of the halo gas modifies the spectra in a predictable way: the material moving towards us produces more blue spectra and the receding materials are redder. This is the same effect as the change in pitch you hear in an ambulance siren when she crosses you on the road.
Martin and Ho discovered that these gaseous halos were spinning with the rest of the galaxy, but not fast enough to prevent materials from falling slowly.
This dynamic creates a circulation, where the gas falls into the galaxy and feeds new stars, which melt light elements into heavier elements. Eventually, some of this material, now enriched with heavier elements, is thrown off the galaxy, where it can restart the cycle. In fact, astronomers believe that this circulation dictates the composition of the material that forms new stars. In addition, stars enriched in heavier elements seem more likely to form planetary systems than those composed only of light elements, according to Martin.
"This circulation is the engine of the galactic ecosystem," she said.
In addition to highlighting planetary systems like ours, this study also sheds light on how our galaxy works. Most of the 50 galaxies studied were similar to those of the Milky Way about 250 million years ago, a fairly short period on a cosmic scale. It's also about as long as our galaxy takes to make a revolution, explained Martin, which is the minimum time scale for any significant galactic shift.
Martin and Ho plan to study how the rate of gas falling in a galaxy is comparable to the rate of star formation. This will better understand the evolution of galaxies that form stars over billions of years. Martin is investigating whether galaxies with more star formation have more disturbed halo gas above and below the plane of the disc, as could be expected with more supernovae.
Reference:
"Circumgalactic gas kinematics: galaxies and feedback", Crystal L. Martin et al., 2019, June 18, Astrophysical Journal [https://iopscience.iop.org/article/10.3847/1538-4357/ab18ac, preprint: https://arxiv.org/abs/1901.09123].
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