From Sky & Telescope : “Astronomers Look into the Past of Local Dwarf Galaxies” 

From Sky & Telescope

June 14, 2021
Camille M. Carlisle

A combination of simulations and observations indicates that galaxies like the Large Magellanic Cloud control when punier dwarfs plunge into large galaxies [is this really news?].

When midsize galaxies merge with large ones, they drag a lot of little galaxies with them, Eric Bell (University of Michigan (US), Ann Arbor) reported June 9th at the virtual summer meeting of the American Astronomical Society (US).

This visible-light mosaic shows the Large and Small Magellanic Clouds below the Milky Way plane.
Axel Mellinger, Central Michigan University (US).

To investigate the dwarf galaxy population of the Local Group in which the Milky Way and Andromeda galaxies live, Bell and Richard D’Souza (Vatican Observatory (VCS), Italy) simulated the clouds of dark matter in which Milky Way–mass galaxies reside. By following the interactions of 48 of these simulated haloes with smaller surrounding ones (stand-ins for dwarf galaxies), the astronomers discovered something interesting: Satellites tended to merge in groups with the big galaxy, with smaller galaxies tagging along [by gravity] when a midsize one merged with the central leviathan. The effect didn’t increase the number of infalling galaxies; it only changed when they fell in.

To test these predictions, the team looked at when star formation shut off in real-life, smaller galaxies surrounding the Milky Way and Andromeda. Starbirth, the astronomers reasoned, is a good proxy for merger time, because the little galaxies would likely be stripped of their star-forming gas when they fell into the big, hot halo of the central galaxy. And indeed, observations show that star formation shut off in the Milky Way satellites around two periods of time when a sizable galaxy was falling in — about 2 billion years ago for the Large Magellanic Cloud and 10 billion years ago for Gaia-Enceladus.

A visualization of the moment of impact between the MW’s progenitor and the Gaia-Enceladus dwarf galaxy. Credit: Instituto de Astrofísica de Canarias (ES).

Andromeda Galaxy Messier 31 with Messier 32 -a satellite galaxy. Credit:Terry Hancock-DownUnderObservatory.

Conversely, there’s only one turn-off time for Andromeda satellites: about 6 billion years ago. That suggests that Andromeda’s largest companion, Messier 33, might have started its nosedive into Andromeda’s halo around then. The timeline would problematize others’ suggestion that Messier 33 is on its first flyby past Andromeda and, like the LMC, had only arrived in the last couple billion years. A closer look at Andromeda’s satellite family could elucidate just when Messier 33 showed up for its visit.

Astronomers are now able to explore the satellites of large galaxies out to several tens of millions of light-years from us, and if borne out, these results may enable them to discern such galaxies’ histories more clearly. The team’s work appears in the July MNRAS.

See the full article here .


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Sky & Telescope, 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.”