“The sign of our extinction would be no more than a match flaring for a second in the heavens,” said film director Stanley Kubrick, about an image of the destruction of our planet to an alien observer in the Andromeda Galaxy, a massive spiral galaxy so close to our increasingly fragile Earth that it appears as a cigar-shaped smudge of light high in the autumn night sky. The Hubble Space Telescope has announced this week that it has captured the nearly invisible halo of our neighbor, M31–a veil of diffuse plasma extending 1.3 million light-years from the galaxy—about halfway to our Milky Way—and as far as 2 million light-years in some directions.
In the landmark study, scientists using NASA’s Hubble Space Telescope have mapped the immense halo of gas, surrounding the Andromeda galaxy, our nearest large galactic neighbor discovering a complex and dynamic inner shell triggered by from the impact of supernova activity, that extends to about a half million light-years. “The outer shell,” says study leader Nicolas Lehner, astrophysicist at the University of Notre Dame,”is smoother and hotter.”
Reveals a Layered Structure
The Hubble team also found that the halo has a layered structure, with two main nested and distinct shells of gas. “Understanding the huge halos of gas surrounding galaxies is immensely important,” explained co-investigator Samantha Berek of Yale University in New Haven, Connecticut. “This reservoir of gas contains fuel for future star formation within the galaxy, as well as outflows from events such as supernovae. It’s full of clues regarding the past and future evolution of the galaxy, and we’re finally able to study it in great detail in our closest galactic neighbor.”
A signature of this activity is the team’s discovery of a large amount of heavy elements in the gaseous halo of Andromeda. Heavier elements are cooked up in the interiors of stars and then ejected into space—sometimes violently as a star dies. The halo is then contaminated with this material from stellar explosions.
Ancient Light of 43 Quasars
Through a program called Project AMIGA (Absorption Map of Ionized Gas in Andromeda), the study examined the light from 43 quasars—the very distant, brilliant cores of active galaxies powered by black holes—located far beyond Andromeda. The quasars are scattered behind the halo, allowing scientists to probe multiple regions. Looking through the halo at the quasars’ light, the team observed how this light is absorbed by the Andromeda halo and how that absorption changes in different regions.
The immense Andromeda halo is made of very rarified and ionized gas that doesn’t emit radiation that is easily detectable. Therefore, tracing the absorption of light coming from a background source is a better way to probe this material.
The purple-hued illustration of Andromeda galaxy’s halo, with background quasars (shown with yellowish dots) scattered throughout. This illustration shows the location of the 43 quasars scientists used to probe Andromeda’s gaseous halo. These quasars—the very distant, brilliant cores of active galaxies powered by black holes—are scattered far behind the halo, allowing scientists to probe multiple regions. Looking through the immense halo at the quasars’ light, the team observed how this light is absorbed by the halo and how that absorption changes in different regions. By tracing the absorption of light coming from the background quasars, scientists are able to probe the halo’s material. (NASA, ESA, and E. Wheatley, STScI)
The researchers used the unique capability of Hubble’s Cosmic Origins Spectrograph (COS) to study the ultraviolet light from the quasars. Ultraviolet light is absorbed by Earth’s atmosphere, which makes it impossible to observe with ground-based telescopes. The team used COS to detect ionized gas from carbon, silicon, and oxygen. An atom becomes ionized when radiation strips one or more electrons from it.
The 2015 Probe
Andromeda’s halo has been probed before by Lehner’s team. In 2015, they discovered that the Andromeda halo is large and massive. But there was little hint of its complexity; now, it’s mapped out in more detail, leading to its size and mass being far more accurately determined.
“Previously, there was very little information—only six quasars—within 1 million light-years of the galaxy. This new program provides much more information on this inner region of Andromeda’s halo,” explained co-investigator J. Christopher Howk, also of Notre Dame. “Probing gas within this radius is important, as it represents something of a gravitational sphere of influence for Andromeda.”
Similar to Milky Way’s Halo
Because we live inside the Milky Way, scientists cannot easily interpret the signature of our own galaxy’s halo. However, they believe the halos of Andromeda and the Milky Way must be very similar since these two galaxies are quite similar. The two galaxies are on a collision course, and will merge to form a giant elliptical galaxy beginning about 4 billion years from now.
Scientists have studied gaseous halos of more distant galaxies, but those galaxies are much smaller on the sky, meaning the number of bright enough background quasars to probe their halo is usually only one per galaxy. Spatial information is therefore essentially lost. With its close proximity to Earth, the gaseous halo of Andromeda looms large on the sky, allowing for a far more extensive sampling.
“This is truly a unique experiment because only with Andromeda do we have information on its halo along not only one or two sightlines, but over 40,” explained Lehner. “This is groundbreaking for capturing the complexity of a galaxy halo beyond our own Milky Way.”
In fact, Andromeda is the only galaxy in the universe for which this experiment can be done now, and only with Hubble. Only with an ultraviolet-sensitive future space telescope will scientists be able to routinely undertake this type of experiment beyond the approximately 30 galaxies comprising the Local Group. “So Project AMIGA has also given us a glimpse of the future,” said Lehner.
The Daily Galaxy, Sam Cabot, via NASA’s Goddard Space Flight Center