Galaxies That Evolve Together But Don’t Stay Together

Astronomers have seen the Milky Way’s profound impact on its entourage of smaller galaxies. Our equally sized sister galaxy, Andromeda, likewise impacts the evolution of its meeting of dwarf satellites. But a research of dozens of close by galaxies exhibits that the affect of the Milky Way and Andromeda galaxies might prolong a lot farther than beforehand thought.

This picture exhibits the stellar density of the Large and Small Magellanic Clouds, the Milky Way’s largest two galactic satellites.
ESA / Gaia / DPAC / CC BY-SA 3.0 IGO; Acknowledgement: L. Chemin; X. Luri et al (2020)

Charlotte Olsen (Rutgers University) and her colleagues homed in on 36 galaxies that had been a part of the ACS Nearby Galaxy Survey Treasury (ANGST), the place ACS is the Advanced Camera for Surveys aboard the Hubble Space Telescope. These galaxies are “nearby” solely within the cosmic sense of the phrase — they span a quantity of space 13 million light-years throughout. For comparability, the Milky Way’s disk spans roughly 100,000 light-years and even its darkish matter halo solely extends out to some 1 million light-years.

Yet, in line with the evaluation by Olsen’s workforce, to seem within the Astrophysical Journal (preprint available here), all 36 dwarf galaxies seem to have remarkably related star formation histories regardless of the huge distances between them.

Olsen and her colleagues measured the galaxies’ star formation histories in two methods. The first entails measuring the brightness and colours of particular person stars, however astronomers can solely use this technique for galaxies shut sufficient to make out these stars. For extra distant galaxies, whose stars mix collectively within the telescope, astronomers usually resort to measuring the quantity of sunshine emitted in several wavelength bands. For instance, galaxies with a number of new stars will emit extra ultraviolet mild, whereas these with older stars will emit extra infrared.

Galaxies from ANGST survey
Hubble noticed these 4 close by galaxies as a part of the ANGST survey. Olsen and colleagues centered on dwarf galaxies, such because the one at backside proper.
NASA / ESA / J. Dalcanton and B. Williams (University of Washington)

While the dwarfs are all close by, utilizing the 2 strategies permits Olsen’s workforce to match them and search for discrepancies in investigations of galaxies close to and much. Fortunately, the 2 strategies match. Marla Geha (Yale University), who was not concerned within the research, says the result’s “a lovely demonstration that two commonly used methods to estimate the star formation histories agree.”

Synchronized Star Formation

Regardless which technique Olsen’s workforce used, the astronomers discovered a curiously frequent pattern: After a interval of quietude, the dwarfs started firing up their star-forming factories about 3 billion years in the past, and so they’ve maintained that exercise since then.

Dwarfs orbiting a much bigger galaxy will typically comply with its lead, going quiet when it does and forming stars when it does, Olsen explains. But the galaxies her workforce examined span a wider space, exterior the halo of the Milky Way’s affect. “We still need to find what is driving this behavior, and how much of the rest of the universe is behaving this way, before we can learn what is behind it,” Olsen says.

“We know that the Milky Way and Andromeda can fully quench star formation in galaxies out to nearly 6.5 million light-years away,” Geha says. “While this result includes galaxies at twice this distance, I can still imagine the Milky Way/Andromeda having some influence. It will be super interesting to see if similar results are seen in numerical simulations.”


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