Evidence of broadside collision with dwarf galaxy discovered in Milky
Way
'Shell structures' are first of their kind found in the galaxy

Date:
October 20, 2020
Source:
Rensselaer Polytechnic Institute
Summary:
Astrophysicists have discovered a series of telltale shell-like
formations of stars in the vicinity of the Virgo constellation,
evidence of a radial merger between a dwarf galaxy and the Milky
Way, and the first such 'shell structures' to be found in the
Milky Way.



FULL STORY ========================================================================== Nearly 3 billion years ago, a dwarf galaxy plunged into the center of
the Milky Way and was ripped apart by the gravitational forces of the collision.

Astrophysicists announced today that the merger produced a series of
telltale shell-like formations of stars in the vicinity of the Virgo constellation, the first such "shell structures" to be found in the
Milky Way. The finding offers further evidence of the ancient event,
and new possible explanations for other phenomena in the galaxy.


========================================================================== Astronomers identified an unusually high density of stars called the
Virgo Overdensity about two decades ago. Star surveys revealed that
some of these stars are moving toward us while others are moving away,
which is also unusual, as a cluster of stars would typically travel
in concert. Based on emerging data, astrophysicists at Rensselaer
Polytechnic Institute proposed in 2019 that the overdensity was the
result of a radial merger, the stellar version of a T- bone crash.

"When we put it together, it was an 'aha' moment," said Heidi Jo Newberg, Rensselaer professor of physics, applied physics, and astronomy,
and lead author of the The Astrophysical Journal paper detailing the
discovery. "This group of stars had a whole bunch of different velocities, which was very strange. But now that we see their motion as a whole, we understand why the velocities are different, and why they are moving the
way that they are." The newly announced shell structures are planes of
stars curved, like umbrellas, left behind as the dwarf galaxy was torn
apart, literally bouncing up and down through the center of the galaxy
as it was incorporated into the Milky Way, an event the researchers have
named the "Virgo Radial Merger." Each time the dwarf galaxy stars pass
quickly through the galaxy center, slow down as they are pulled back
by the Milky Way's gravity until they stop at their farthest point,
and then turn around to crash though the center again, another shell
structure is created. Simulations that match survey data can be used to calculate how many cycles the dwarf galaxy has endured, and therefore,
when the original collision occurred.

The new paper identifies two shell structures in the Virgo Overdensity
and two in the Hercules Aquila Cloud region, based on data from the Sloan Digital Sky Survey, the European Space Agency's Gaia space telescope,
and the LAMOST telescope in China. Computer modeling of the shells and
the motion of the stars indicates that the dwarf galaxy first passed
through the galactic center of the Milky Way 2.7 billion years ago.

Newberg is an expert on the halo of the Milky Way, a spherical cloud
of stars that surrounds the spiral arms of the central disk. Most if
not all of those stars appear to be "immigrants," stars that formed
in smaller galaxies that were later pulled into the Milky Way. As the
smaller galaxies coalesce with the Milky Way, their stars are pulled by so-called "tidal forces," the same kind of differential forces that make
tides on Earth, and they eventually form a long cord of stars moving in
unison within the halo. Such tidal mergers are fairly common and have
formed much of Newberg's research over the past two decades.

More violent "radial mergers" are considered far less common. Thomas
Donlon II, a Rensselaer graduate student and first author of the paper,
said that they were not initially seeking evidence of such an event.

"There are other galaxies, typically more spherical galaxies, that have
a very pronounced shell structure, so you know that these things happen,
but we've looked in the Milky Way and hadn't seen really obvious, gigantic shells," said Donlon, who was lead author on the 2019 paper that first
proposed the Virgo Radial Merger. As they modeled the movement of the
Virgo Overdensity, they began to consider a radial merger. "And then
we realized that it's the same type of merger that causes these big
shells. It just looks different because, for one thing, we're inside
the Milky Way, so we have a different perspective, and also this is a
disk galaxy and we don't have as many examples of shell structures in
disk galaxies." The finding poses potential implications for a number
of other stellar phenomena, including the Gaia Sausage, a formation
of stars believed to have resulted from the merger of a dwarf galaxy
between 8 and 11 billion years ago.

Previous work supported the idea that the Virgo Radial Merger and the Gaia Sausage resulted from the same event; the much lower age estimate for the
Virgo Radial Merger means that either the two are different events or the
Gaia Sausage is much younger and could not have caused the creation of the thick disk of the Milky Way, as previously claimed. A recently discovered spiral pattern in position and velocity data for stars close to the sun, sometimes called the Gaia Snail, and a proposed event called the Splash,
may also be associated with the Virgo Radial Merger.

"There are lots of potential tie-ins to this finding," Newberg
said. "The Virgo Radial Merger opens the door to greater understanding
of other phenomena that we see and don't fully understand, and that
could very well have been affected by something having fallen right
through the middle of the galaxy less than 3 billion years ago." View_a_video_simulation_of_the_formation_of_the_stellar_shell_structures.


========================================================================== Story Source: Materials provided by
Rensselaer_Polytechnic_Institute. Original written by Mary
L. Martialay. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Thomas Donlon, Heidi Jo Newberg, Robyn Sanderson, Lawrence
M. Widrow. The
Milky Way's Shell Structure Reveals the Time of a Radial
Collision. The Astrophysical Journal, 2020; 902 (2): 119 DOI:
10.3847/1538-4357/abb5f6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201020105542.htm

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