From Sky & Telescope: “Astronomers Map Andromeda’s Halo”

From Sky & Telescope

September 10, 2020
Monica Young

This illustration depicts what Andromeda’s gaseous halo might look like if it were visible to humans on Earth. At three times the size of the Big Dipper, the halo would be “easily the biggest feature on the nighttime sky,” per the NASA statement. (NASA, ESA, J. DePasquale and E. Wheatley (STScI), and Z. Levay (background image)).

This illustration shows the location of the 43 distant background quasars scientists used to probe Andromeda’s outermost regions. The purple shows an artist’s concept of what the hot gaseous halo might look like.
NASA / ESA / E. Wheatley (STScI).

Astronomers have observed 43 quasars in back of our sister galaxy, Andromeda, using the distant beacons to map its halo of hot gas.

If it’s clear tonight, you might go outside and find the Andromeda Galaxy between the Cassiopeia and Andromeda constellations. Our galactic sibling is currently 2.2 million light-years away, so it appears as a spindle-shape bit of fuzz that requires dark skies to see. A few billion years from now, our descendants will have an even better view, as we’re headed almost straight for it.

Even today, though, new observations show the two galaxies are already “touching.” New research on the hot gas surrounding Andromeda shows that it’s likely already overlapping the Milky Way’s halo.

Milkdromeda -Andromeda on the left-Earth’s night sky in 3.75 billion years-NASA.

“We know galaxies rotate,” says Roeland van der Marel (Space Telescope Science Institute), who led a team in publishing the results in The Astrophysical Journal, 2019. But, he adds, this rotation has never been measured in three dimensions and on the level of individual stars. “But with less than two years of Gaia data, we were able to measure the 250 million-year revolution of Andromeda.” To put this in perspective, van der Marel says, the stars that Gaia was staring at move at the same speed that a human hair would grow as seen at the distance of the Moon.

Hot Gas Hiding in Plain Sight

Every large galaxy is surrounded by gas so hot (between 10,000 and 100,000 degrees) that it hasn’t settled yet. But this gas is hard to see — it doesn’t emit much light itself, so astronomers see it mostly as part of foreground “clouds” that absorb light from background sources.

In The Astrophysical Journal, September 1st, Nicolas Lehner (University of Notre Dame) and colleagues used an unprecedented number of such background sources to map the hot halo around Andromeda.

Quasars are the bright centers of galaxies, where black holes devour gas — they’re luminous enough to shine from the earliest ages of the universe. Nevertheless, they’re points of light, and most foreground galaxies are quite small on the sky. So, despite the trillions of galaxies in our cosmos, there aren’t typically a lot of quasars in back of any one galaxy — maybe one or two.

But Andromeda is close enough — and therefore appears large enough — that Lehner and colleagues identified and observed 43 distant quasars near it on the sky. (This study extends previous research by the same team using 18 quasars [The Astrophysical Journal]). Using the Hubble Space Telescope, the researchers measured spectra of these background quasars, essentially taking 43 different “core samples” through the halo to map out its structure.

Andromeda is a big one and has had its share of galactic scrapes. So it’s not surprising that its halo is a jumbled mess. But with the new map, Lehner’s team sees the halo essentially has two distinct regions: a disturbed inner shell and a smoother outer section. The outer section represents gas leftover from the galaxy’s formation; the inner shell has been stirred up in the more recent past, likely by supernovae in the galaxy’s disk.

A Common Halo

The team also found that the halo is huge — it extends almost 2 million light-years from the galaxy’s center. If we could see it, it would be three times the width of the Big Dipper!

If the Milky Way’s halo has a similar size, and there’s no reason to think it shouldn’t, then the two halos actually overlap in space. That’s not to say that their collision has begun: The halo gas is so spread out, and the relative velocities so low, that the gas doesn’t collide per se. Instead, the Milky Way and Andromeda might share a common halo, as shown in simulations.

See the full article here .


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Sky & Telescope magazine, founded in 1941 by Charles A. Federer Jr. and Helen Spence Federer, has the largest, most experienced staff of any astronomy magazine in the world. Its editors are virtually all amateur or professional astronomers, and every one has built a telescope, written a book, done original research, developed a new product, or otherwise distinguished him or herself.

Sky & Telescope magazine, now in its eighth decade, came about because of some happy accidents. Its earliest known ancestor was a four-page bulletin called The Amateur Astronomer, which was begun in 1929 by the Amateur Astronomers Association in New York City. Then, in 1935, the American Museum of Natural History opened its Hayden Planetarium and began to issue a monthly bulletin that became a full-size magazine called The Sky within a year. Under the editorship of Hans Christian Adamson, The Sky featured large illustrations and articles from astronomers all over the globe. It immediately absorbed The Amateur Astronomer.

Despite initial success, by 1939 the planetarium found itself unable to continue financial support of The Sky. Charles A. Federer, who would become the dominant force behind Sky & Telescope, was then working as a lecturer at the planetarium. He was asked to take over publishing The Sky. Federer agreed and started an independent publishing corporation in New York.

“Our first issue came out in January 1940,” he noted. “We dropped from 32 to 24 pages, used cheaper quality paper…but editorially we further defined the departments and tried to squeeze as much information as possible between the covers.” Federer was The Sky’s editor, and his wife, Helen, served as managing editor. In that January 1940 issue, they stated their goal: “We shall try to make the magazine meet the needs of amateur astronomy, so that amateur astronomers will come to regard it as essential to their pursuit, and professionals to consider it a worthwhile medium in which to bring their work before the public.”