Sky & Telescope - October 2021 - 27

Our Milky Way has torn a nearby galaxy to shreds. Find out how you can observe its remnants.
W
e've known for a long time that big galaxies grow
by cannibalizing little galaxies, and their eating
habits are quite messy. As they feast, they fling
their scraps across the sky, forming immense stellar streams.
In 1994, the team of Rodrigo Ibata, Mike Irwin, and Gerry
Gilmore (Institute of Astronomy at Cambridge University in
the UK) discovered a vivid example of life at the galactic dinner
table. During a spectroscopic study of the Milky Way's
central bulge, they noticed a group of stars moving together
at a uniform velocity in a field of random motions. They had
stumbled across our nearest neighbor, the Sagittarius Dwarf
Spheroidal Galaxy.
The Milky Way's strong gravitational grip is a relentless
force of demolition. The shredded debris of the dwarf spheroidal
is strung across the sky in two tidal tails that form a
prominent substructure in the galactic halo called the Sagittarius
Stream. Sloan Digital Sky Survey (SDSS) photometry
first revealed this feature in 2002, and a study of M-giant
halo stars in the Two Micron All-Sky Survey (2MASS) catalog
confirmed it. The leading and trailing tails of the stream
wrap over 360° in looping rosettes around the Milky Way in
a nearly vertical polar orbit, as shown in the diagram below.
Astronomers refer to the primary wraps as L (leading) and
T (trailing), while M represents the remnant core and its
associated objects.
The Milky Way has grown via mergers with several dwarf
galaxies, accreting stars, dark matter, and globular clusters
SNATCHED AWAY The Milky Way ripped globular cluster Palomar 12
from its former host galaxy some 1.7 billion years ago. The cluster nevertheless
still resides at a distance of around 60,000 light-years from the
solar system in the direction of the constellation Capricornus.
in the process. Each dwarf stellar population has a unique
chemical signature, helping astronomers trace their streams
across the sky. After the Sagittarius Dwarf Spheroidal Galaxy
stream, the largest are the Gaia-Enceladus, Sequoia, Koala,
and Helmi Streams - and a newly identified High-Energy
Group. A 2019 study (lead author Davide Massari, University
of Groningen) concluded that 35% of all Milky Way globulars
may be associated with these merger events.
The discovery of these streams, along with data from the
astrometric mission Gaia, has opened up a new research field
in galactic archaeology that provides valuable information
for astronomers piecing together the assembly history of our
Milky Way Galaxy (see, e.g., S&T: Mar. 2020, p. 40). This difficult
job requires precise stellar measurements of location,
age, kinematics, and chemistry. The Sagittarius Dwarf provides
a unique laboratory to study a galactic merger in detail.
At the beginning of its life, the youthful Sagittarius Dwarf
was the most massive of numerous spheroidal dwarf satellites
of the Milky Way. It has orbited our galaxy with periods ranging
from 550 to 950 million years perhaps as many as 10 times
during its billions of years as our hostage. During these orbits
the Sagittarius Dwarf elbowed its way through the Milky Way,
guided largely by its own massive body of dark matter. For
the past 1.8 billion years it looped over the galactic north and
south poles, and last punched through the disk 500 to 800
million years ago when it lost the last of its remaining gas.
A 2011 computer simulation (lead author Chris Purcell of
the University of Pittsburgh) suggests the last collision dramatically
impacted the Milky Way, triggering the emergence
of spiral arms and outer arcs or rings. More recent simulations
show that that crossing also rocked our galaxy, initiatSHREDS
OF A FORMER
GALAXY The Sagittarius
Stream wraps itself
around our own Milky
Way and consists of tidally
stripped stars from
our nearest neighbor. Multiple
close encounters in
the past few billion years
formed this structure.
The diagram shows the
locations of globular and
open clusters in the starry
trails, as well as the position
of the remnant core.
L
NGC 5634
T
Berkeley 29
Saurer 1
M
Sgr. core
Sun
Milky Way
disk
Direction of motion
Whiting 1
Palomar 12
sk yand tele scope .o r g * OCTOBER 2021 27
DAVID LAW

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Sky & Telescope - October 2021

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