Thanks to the latest from the NASA Hubble Space Telescope astronomers may now be poised to understand the origins of our galaxy more clearly. A team of scientists led by astrophysicist Nicolas Lehner of the University of Notre Dame used the Hubble to find a massive gas halo surrounding the Andromeda Galaxy, our closest neighbor (at only 2.5 million light-years away).
The team estimates that the mass of this enormous halo is equal to that of half of Andromeda's stars. Like our own Milky Way, Andromeda is a giant spiral galaxy. On a clear night it can easily be seen from Earth, although it is the invisible halo that researchers are finding so compelling.
Scientists hope to study the halo which surrounds it, which is six times larger than they once believed, stretching approximately a million light years outward to be more than 2 million light years in diameter total. The researchers believe that understanding the halo will help us understand how galaxies like ours are structured, and how they evolve.
"Halos are the gaseous atmospheres of galaxies," Lehner says. "The properties of these gaseous halos control the rate at which stars form in galaxies."
Among galaxies in our Local Group-which contains not only the Milky Way but also around 45 other known galaxies-the most massive is Andromeda, known to astronomers as Messier 31 (M31). M31 is home to almost twice as many stars as the Milky Way; about one trillion stars. M31 is therefore, not surprisingly, more luminous than the Milky Way by about 25 percent.
The gas of the halo is the raw material that makes up new stars in Andromeda. Thanks to the presence of so much useful raw material, the galaxy has sustained a vigorous rate of new star creation. Nevertheless, Andromeda appears to be slowing its star birth rate, despite its presence among a gaseous ocean of raw materials.
Scientists hope to answer the questions of why and what is causing the galaxy to be unable to return this raw material to it in the form of new stars.
Predicting The Birth Of One Giant New Galaxy
This research shows that although the gas that makes up Andromeda's halo is "invisible," the halo itself is around 100 times larger than the moon's diameter, and is one of the galaxy's significant features. The team was able to locate the halo for study by searching for bright objects in the background to see how their luminosity is affected by the gas of the halo. Quasars, distant star-like objects, are very bright because of the way supermassive black holes pull gas into their cores. This high luminosity makes quasars perfect for this sort of research.
Co-investigator J. Christopher Howk, an associate professor of physics at Notre Dame, says that "As the light from the quasars travels toward Hubble, the halo's gas will absorb some of that light and make the quasar appear a little darker in just a very small wavelength range. By measuring the dip in brightness, we can tell how much halo gas from M31 there is between us and that quasar."
Using the Hubble Space Telescope, scientists have observed halos around other galaxies, but never before has such a massive halo been observed so near to Earth beyond the Milky Way. That proximity allows the team to use 18 quasars projected at different distances from Andromeda to determine its presence and size. "This is a new milestone because typically only one quasar is used to probe the halos of galaxies beyond the Local Group," Lehner says. "Here we have assembled a large sample of quasars that directly demonstrate the true extent of the halo of a single massive galaxy."
The Notre Dame team used five years' worth of data from the archive of the Hubble to complete this project. The Hubble Space Telescope is capable of high spectroscopic resolution, which makes it uniquely suited for the study of ultraviolet light. The team was able to model spectral features precisely, which in turn allowed them to glean important information about the qualities of gas halos like that of Andromeda. Ultimately the researchers hope to compile more data from more quasars using the same methods, thereby revealing more about the ways that galaxies and halos interact.
The scientists state that almost half of all heavy elements created by Andromeda's stars were expelled well beyond the galaxy's stellar disk over the course of the galaxy's lifetime. Andromeda's disk, which is about 200,000 light-year in diameter, may well resemble that of the Milky Way. If this is true, the scientists believe that if there is a similar halo surrounding the Milky Way, the massive halos of both galaxies may merge long before the collision of the two galaxies. This means that eventually, starting about 4 billion years from now, a giant elliptical galaxy will be born.
Spectral Signatures
The archives the team used in this work were from the COS-Halo project. COS-Halo analyzed similar gas halos from 44 galaxies, all much more distant than Andromeda, albeit in the same direction. The COS-Halo team, led by astronomer Jason Tumlinson from Baltimore's Space Telescope Science Institute, was able to describe some characteristics of the studied halos, but could not map their sizes and ranges. This was primarily due to the extreme distance of the studied halos.
Andromeda, however, is right next door in space terms, so this team was able to map the halo accurately using the ultraviolet spectral signatures of silicon. They also confirmed that the gas within the halo is bound to Andromeda by gravity, not rushing away from it. Predictive modeling then allowed researchers to place the birth of the halo at the same time as its galaxy, 9 billion years ago.
The very presence of carbon, oxygen, and the measured silicon in the halo depict billions of years of massive stars exploding these raw materials outward into the halo. How the halo affects or regulates the formation of stars is yet unclear-and it is these relationships that the team hopes to better understand with more study. Younger galaxies with more active star formation processes create gravitational pulls which draw raw materials inward from the halo. As galaxies age and evolve into large elliptical galaxies, this no longer holds true.
"When we observe huge elliptical galaxies, we find a lot of cold gas around these galaxies that should fall onto the galaxy, but doesn't," Lehner says.
Andromeda and its halo present a unique opportunity for astronomers. Many active, star-forming galaxies and older elliptical galaxies have been studied at great distances. In contrast, Andromeda is now shifting between stages, and is close enough to permit detailed study.
"From the Andromeda galaxy we may learn more with more data points."
Ultimately, however, even this detailed data will require supplementing information from other galaxies at other stages; this is the lone way we might understand the process in meaningful ways. Scientists are working to devise studies that are on point, because time is running out-the Hubble will end its time in orbit within the next ten years.
This study was published in the May 10, 2015, edition of The Astrophysical Journal. Its authors are Lehner, Howk, and Bart Wakkerfrom the University of Wisconsin Madison.
