Sky & Telescope - December 2023 - 16

Galactic Portrait
According to the new idea, Andromeda's gravity tore stars
off the large intruder - stars that wound up in Andromeda's
stellar halo. That explains why Andromeda's stellar halo is so
massive: The disrupted galaxy itself was big, carrying around
25 billion solar masses' worth of stars. And since larger
galaxies are fairly metal-rich, that explains why Andromeda's
stellar halo is, too.
The impact stirred up the stars in Andromeda's disk, making
it thick. " There is no other way to reproduce this thing
but by doing a recent major merger, " Hammer says. Earlier
simulations had successfully created the Giant Stellar Stream
by invoking a collision with a much smaller galaxy, but Hammer
says a small galaxy can't shake up Andromeda's disk sufficiently.
The big collision also drove gas clouds together, causing
them to collapse and spawn new stars across Andromeda's
disk, triggering the global starburst 2 to 4 billion years ago.
Like all large galaxies, the intruder had satellite galaxies
of its own, and those lesser galaxies now orbit Andromeda.
These satellite galaxies share a common trait: " They stopped
forming stars about 6 billion years ago, when all of them fell
in and when their gas was stripped from them, " Bell says.
That, he says, dates the epoch when the intruder trespassed
into Andromeda's sphere of influence.
Being far larger than its satellites, the intruder galaxy
retained enough gas to continue forming stars up until more
recently. Indeed, Andromeda's halo has stars that are just
2 billion years old, " very unlike our Milky Way, " Bell says.
That must be when star formation in the intruder finally got
snuffed out, as it lost the last of its gas.
M32: The Survivor?
D'Souza and Bell believe the remnant of the large galaxy that
hit Andromeda still exists. The original galaxy, they say, was
a spiral whose arms were peeled off by Andromeda's gravity,
leaving behind only a dense core: the small, compact elliptical
galaxy M32.
M32 is just 24 arcminutes from Andromeda's center, closer
than any of the dozens of other galaxies that orbit the large
galaxy. If the two galaxies are equidistant from us, then they
would be only 17,000 light-years from each other. That's just
two-thirds of the distance between the Sun and the Milky
Way's center.
But no one knows just how far M32 lies in front of or
behind the Andromeda Galaxy, so M32's true distance
from Andromeda's center is uncertain. No dust lanes from
Andromeda blot out M32, which suggests that the smaller
galaxy is in the foreground.
Whatever its exact location, M32 is hardly your typical
elliptical, stuffed with old stars. " M32 has young stars, "
Bell says. " It's always been one of these very weird outliers: a
young, super compact, solar-metallicity elliptical galaxy that's
less massive than the Large Magellanic Cloud. It made no
sense. " D'Souza notes M32's strange composition: " It has a
metallicity of a galaxy about 100 times larger. "
It all makes perfect sense, of course, if M32 was once a
much larger galaxy. Back in 1972, Sandra Faber (University of
California, Santa Cruz), who first discovered M32's metalrich
nature, speculated that Andromeda had indeed torn
away most of the smaller galaxy's stars.
What's more, a 2012 study found that M32 had a starburst
of its own between 2 and 5 billion years ago - about
the same time that Andromeda did. " It has a sharp cut-off at
2 billion years ago, " D'Souza says. Thus, M32 stopped forming
stars at the same time that Andromeda's disk puffed up.
The 2 billion-year-old figure also marks the age of the youngest
stars in Andromeda's stellar halo.
Compact elliptical galaxies such as
100%
Centaurus A
Andromeda
32%
NGC 891
NGC 4945
10%
Milky Way
M83
M94
3.2%
M101
M81
NGC 7814
NGC 3115
NGC 253
NGC 4565
Sombrero Galaxy
M32 " are extremely rare, " D'Souza says.
The next nearest example is a small
galaxy orbiting M87, the giant elliptical
galaxy at the heart of the Virgo Cluster,
54 million light-years from Earth. That
small galaxy, he says, likely owes its odd
features to a past interaction with its
huge host.
D'Souza and Bell also point at galaxies
closer to home that might someday
become compact ellipticals resembling
1%
0.032
0.1
0.32
16 DECEMBER 2023 * SKY & TELESCOPE
1
3.2
10
32
Stellar halo mass (in billions of solar masses)
100
DIVERSE STELLAR HALOS Giant galaxies
sport stellar halos of widely varying masses
and metallicities, depending on the sizes of the
galaxies that have fallen into them. Galaxies
that have accreted large amounts of material,
such as Andromeda (shown in red) and the
Sombrero Galaxy, have massive, metal-rich
stellar halos, whereas spiral galaxies that have
led quieter lives, such as the Milky Way (shown
in blue) and M101, have low-mass, metal-poor
stellar halos.
Stellar halo metallicity relative to the Sun's
KATYA GOZMAN ET AL. / ASTROPHYSICAL JOURNAL 2023
Sky & Telescope - December 2023

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