Sky & Telescope - December 2023 - 17

DEEP DIVE This panorama was taken as part of the Panchromatic Hubble Andromeda Treasury (PHAT), one of several projects that have given
astronomers a close look at Andromeda's thick disk and stellar halo.
M32. The first is the starburst galaxy M82, which is 12 million
light-years from Earth. M82 orbits the giant spiral galaxy
M81, whose gravity stirs up gas clouds in its neighbor and
probably causes the intense star formation there.
More than twice as far away, at a distance of 28 million
light-years, the Whirlpool Galaxy (M51) sports spectacular
spiral arms whipped up by a satellite galaxy at the edge of the
great galaxy's stellar disk. The Whirlpool's gravity is tearing
stars off, converting its companion into what D'Souza and
Bell say will also one day be a compact, M32-like elliptical.
However, not all astronomers are convinced that M32 was
the culprit behind the recent drama in Andromeda. " M32
is very special, " Hammer acknowledges. But he thinks the
Andromeda Galaxy obliterated the intruder. " It has been
completely destroyed. "
As Hammer explains, such a massive satellite galaxy suffers
a drag force that moons orbiting planets do not experience:
dynamical friction. A massive satellite revolving around
a giant galaxy speeds through the dark matter in the giant's
dark halo. Never mind that no one yet knows what dark matter
is; it both feels and exerts gravitational force. Thus, as the
satellite moves through the dark-matter particles, they lurch
toward the satellite. But by the time the particles reach where
the satellite was, it has moved on, so the dark-matter particles
bunch up behind the satellite. The gravitational pull of this
bunch tries to yank the satellite backward, slowing it down
and causing it to plummet into the larger galaxy.
The more massive the satellite galaxy, the faster it spirals
in and meets its maker - and its master. That's why the
Milky Way's two most massive satellite galaxies, the Magellanic
Clouds, will eventually merge with the Milky Way.
Andromeda's big satellite galaxy faced ferocious dynamical
friction, because the satellite had about 10 times as many
stars as the Large Magellanic Cloud. Thus, Hammer says, the
intruder quickly plunged into the Andromeda Galaxy and
ceased to exist. For that reason, M32 can't be the intruder
galaxy's remnant.
Still, M32 did have a starburst at the same time that
Andromeda did. And M32's metal-rich nature does suggest it
was once a much larger galaxy.
Gilbert calls this evidence for M32's interference in
Andromeda circumstantial. " But it's an intriguing circumstantial, "
she admits. Nevertheless, the M32 idea faces a
problem. The small galaxy may be in the wrong place to have
been the guilty party: It's south of Andromeda's disk, whereas
simulations suggest any surviving remnant of the intruder
should be east of Andromeda in order to produce its Giant
Stellar Stream.
" I think we will have the answer within a few years, "
Hammer says, " because the proper motion will tell us the
truth. " A measurement of M32's proper motion - its apparent
sideways movement on the sky - will come from the
Hubble Space Telescope and Gaia spacecraft, showing whether
the odd galaxy was in the right place at the right time to stage
the drama in Andromeda.
Whatever the verdict is for M32, Andromeda's life story
seems to be nothing like the Milky Way's, which has been
quiet for the past 8 billion years. " You learn as a student that
M31 is sort of like a Milky Way twin, " Williams says. " But
you study it in detail and realize it has a lot of pretty significant
differences with the Milky Way. "
¢ KEN CROSWELL is the author of a book about our galaxy,
The Alchemy of the Heavens: Searching for Meaning in the Milky
Way. His feature article on the Milky Way's life story appeared
in the August 2023 issue.
sk yand tele scope .o r g * DECEMBER 2023 17
NASA / ESA / J. DALCANTON (UNIVERSITY OF WASHINGTON) /
THE PHAT TEAM / R. GENDLER

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Sky & Telescope - December 2023

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