APOD: 2020 September 3 - A Halo for Andromeda

Good morning, Rallypoint. Today's Astronomy Picture of the Day (APOD) is titled "A Halo for Andromeda." The Andromeda Galaxy, also known as M31, NGC 224, or the Andromeda Nebula, is a barred spiral galaxy approximately 2.5 million light-years from Earth. It is the nearest major galaxy to our own (the Milky Way). But what's this about a halo? That's where our story picks up this morning.

Quasars are found at the center of extremely active galaxies. They are powered by immense black holes estimated to be millions of times more massive than our Sun. Quasars emit electromagetic radiation that can be detected throughout the observable spectrum. Investigators have identified 43 quasars located beyond the Andromeda Galaxy. Project AMIGA (Absorption Map of Ionized Gas in Andromeda) examined the light from these 43 sources, and found that much of it was absorbed as it passed through different regions near M31. Why?

The Cosmic Origins Spectrograph (COS) flying aboard the Hubble Space Telescope (HST) provided the answer. Ionized gas from carbon, oxygen, and silicon was detected by the COS in a radius of 1.3 million light years around the Andromeda Galaxy. The presence of this gas matched the observed absorption of electromagnetic radiation from the 43 quasars. Our own Milky Way Galaxy has a similar gaseous halo. Given the size of the halo about M31, the two are probably already interacting with each other.

Back to today's APOD. The halo about the Andromeda Galaxy has been digitally enhanced to show the two layers detected by Project AMIGA. The inner shell that extends to about a half million light-years is far more complex and dynamic. The outer shell is smoother and hotter, likely due to the impact of supernova activity in the galaxy’s disk more directly affecting the inner halo. This gas surrounding the Andromeda Galaxy provides clues about its past and future, and may offer insight into the evolution of our own Milky Way galaxy as well.