Sky & Telescope - December 2023 - 15

The Milky Way also has a much older stellar disk - the
thick disk - whose stars venture about three times farther
above and below the galactic plane than most thin-disk stars
do. These thick-disk stars therefore have a higher velocity
dispersion. In the Milky Way, however, most disk stars, such
as the Sun, belong to the thin disk rather than the thick disk.
Not so in Andromeda. " The Andromeda disk is not a thin
disk at all, " Hammer says. " It is a thick disk. "
That revelation came in 2015, when Claire Dorman, then
a graduate student at the University of California, Santa
Cruz, and her colleagues reported Doppler shifts of stars
in Andromeda's disk, which indicate how fast the stars are
moving toward and away from us. While those measurements
show that the youngest stars have a fairly low velocity
dispersion, they also, incredibly, reveal that stars older
than just 2 billion years have a very high velocity dispersion.
That means they must rise far above and below Andromeda's
plane, constituting a thick disk.
In 2023, Julianne Dalcanton (Flatiron Institute) and her
colleagues measured Andromeda's stellar disk to be about
5,000 light-years thick - comparable to the Milky Way's
thick disk. But the youthful stars in Andromeda's thick disk
indicate it must have formed much more recently, a mere
2 billion years ago.
COSMIC COINCIDENCE
The dwarf elliptical M32
formed lots of stars 2 to
5 billion years ago, about
the same time Andromeda
did. This coincidence suggests
that the lesser galaxy
may have triggered the
larger galaxy's starburst.
Moreover, just as Andromeda was getting stirred up, a stellar
wildfire swept through the galaxy's entire disk. " Around
2015, people were starting to notice that just about wherever
Hubble looked, they would pretty much always find an overabundance
of stars with ages of 2 to 4 gigayears, " says Benjamin
Williams (University of Washington), who led one such
study. At that time, Andromeda must have been a spectacle,
with brilliant young stars and supernova explosions sparkling
across its disk.
The Great Galactic Intruder
In 2018 Hammer and his colleagues, and independently
D'Souza and Bell, proposed a theory to explain all of
Andromeda's many oddities: its massive, metal-rich stellar
halo; its agitated, puffed-up stellar disk; and its recent starburst.
The idea was simple. A large galaxy with perhaps 40%
the stellar mass of the Milky Way looped around Andromeda
and then smashed into its disk 2 billion years ago.
" The balance of evidence is really shifting toward indicating
that Andromeda may have actually undergone a major
merger, " says Karoline Gilbert (Space Telescope Science Institute),
who has studied Andromeda but was not part of either
team proposing the scenario. It's surprising, she adds, because
a major collision could have destroyed Andromeda's disk.
DWARF ON THE OUTSKIRTS
The weird elliptical dwarf galaxy
M32, which appears to sit
in front of Andromeda, might
be the remains of a much
larger galaxy that crashed
into the giant spiral some
2 billion years ago.
YOUNG HALO
Unlike the Milky Way,
Andromeda has a halo
with stars as young as just
2 billion years, around the
same time its disk puffed
up after a large galaxy apparently
smashed into it.
STREAM OF YOUTH
The most dramatic feature in
Andromeda's halo, the Giant
Stellar Stream, spans a wide
range of ages and includes
stars that formed just 2 billion
years ago. They likely
came from the large galaxy
that hit Andromeda around
that same time.
sk yand tele scope .o r g * DECEMBER 2023 15
FABRIZIO FRANCIONE / CC BY-SA 4.0
https://skyandtelescope.org/
Sky & Telescope - December 2023

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