Free Astronomy Magazine November-December 2020

22 NOVEMBER-DECEMBER 2020 SPACE CHRONICLES 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 deter- mined. “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 An- dromeda’s halo,” ex- plained 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 An- dromeda.” 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 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 back- ground quasars to probe their halo is usu- ally only one per galaxy. Spatial information is therefore essentially lost. With its close prox- imity to Earth, the gaseous halo of An- dromeda 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 informa- tion 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 gal- axy in the universe for which this experi- ment can be done now, and only with Hubble. Only with an ultraviolet-sensitive future space tele- scope will scientists be able to routinely undertake this type of experiment be- yond the approxi- mately 30 galaxies comprising the Local Group. “So Project AMIGA has also given us a glimpse of the future,” concluded Lehner. T his illustration shows the location of the 43 quasars scientists used to probe Andromeda’s gaseous halo. These quasars 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 ab- sorption 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)] T his diagram shows the light from a background quasar passing through the vast, gaseous halo around the neighboring Andromeda galaxy (M31), as spectroscopically measured by the Hubble Space Tele- scope. The colored regions show ab- sorption from two components that make up the halo. For ionized sili- con, a significant absorption is shown in both plots. The more highly ionized carbon is absorbed by only one component. Astronomers can tell the two components apart because their line-of-sight motions, known as radial velocity, cause a Doppler shift that changes the wavelength of light being absorbed. [NASA, ESA, and E. Wheatley (STScI)] ! two galaxies are quite similar. The two galaxies are on a collision course, and will merge to form a giant elliptical galaxy beginning