Sky & Telescope - September 2022 - 9

COLLISION SCENARIO: P. VAN DOKKUM (YALE UNIVERSITY);
WHITE DWARF AND COMPANION: ESO / M. KORNMESSER
GALAXIES
Collision site
Did a Cosmic Collision
Rob Two Dwarfs of
Dark Matter?
WHEN TWO DWARF galaxies were
found apparently devoid of dark matter,
they made headlines (S&T: July 2019, p.
11). Dark matter is key to galactic formation,
so how had these galaxies come
to be without it?
In the May 19th Nature, Pieter van
Dokkum (Yale) and team propose an
answer: The two dwarfs, dubbed DF2
and DF4, were born sans dark matter as
a result of a long-ago collision near the
bright elliptical galaxy NGC 1052.
Using the dwarfs' present-day positions
and velocities and DF2's estimated
age, the team traced their motions back
in time to a common origin. The team
hypothesizes that some 8 billion years
ago, a satellite of NGC 1052 collided
with an unbound galaxy. While the galaxies'
stars and dark matter slipped past
one another, interacting only weakly
through gravity, the high-speed crash
slowed down the galaxies' gas.
STARS
X-ray Flash from White Dwarf Fireball Observed
FOR THE FIRST TIME, astronomers
have spotted the X-ray flash that precedes
a nova, as reported in the May
12th Nature. The flare was the first sign
that a white dwarf's entire surface had
ignited in a colossal, expanding fireball.
Astronomer Robert McNaught
noticed the " new star " on July 15, 2020:
The nova, YZ Reticuli, ultimately brightened
thousands-fold to magnitude 3.7
before fading back into obscurity.
Further observations revealed the
system consists of a white dwarf whipping
around a companion star every
three hours, so close that the white
dwarf steals its companion's outer
layers. The gas piles up on the white
dwarf's surface until, once the pressure
has reached the tipping point, the entire
top layer of hydrogen goes aflame in a
runaway thermonuclear reaction.
The fusion created the visible light
that astronomers monitored as the
second-brightest nova of the decade. But
even before the visible light spiked, theory
said the conflagration ought to emit
a brief flash of X-rays. Such a flash had
never been recorded, but in this case the
space telescope EROSITA happened to
be imaging in the right direction at the
right time to catch 35.8 seconds of the
flare. Ole König (Friedrich-Alexander
University Erlangen-Nürnberg, Germany)
and team recognized the " nowyou-see-it,
now-you-don't " X-ray source
as the nova's incendiary flash.
The X-rays indicated that the white
dwarf's fireball had expanded quickly,
becoming much bigger than the white
dwarf itself. By the time EROSITA
observed it, the layer of fusing hydrogen
had already mushroomed to more
(~8 billion years ago)
Progenitor 1
(unbound)
Progenitor 2
(initially bound)
DF2
NGC 1052
Remnant of
progenitor 2
(DF7?)
DF4
Remnant of
progenitor 1
(RCP 32?)
pIn the collision scenario, an infalling, unbound galaxy (progenitor 1) crashed into a satellite
galaxy of NGC 1052 (progenitor 2), leaving two dark remnants (RCP 32 and DF7) and several darkmatter-free
galaxies, including DF2 and DF4. The latter two lie roughly 7 million light-years apart.
That gas was then free to form new
galaxies. " The gas got strung out into a
whole bunch of clumps that then, under
their own gravity, collapsed and formed
new galaxies without dark matter, " van
Dokkum explains. These dark matter-deficient
galaxies would now line up
like a string of pearls. Indeed, when the
team searched for galaxies around NGC
1052, they found 11, including DF2 and
DF4, lying in a row. At the far ends,
beyond DF2 and DF4, are two peculiar
galaxies, RCP 32 and DF7 - perhaps the
remains of the original collision.
" For me, it comes across as a nice
idea, but with a number of significant
issues, " says Michelle Collins (University
of Surrey, UK), who wasn't involved
in the study. The team will need more
data, she adds, to demonstrate that the
galactic string they've found isn't just a
chance alignment on the sky.
Van Dokkum agrees: His team is
already planning to point an army of
ground- and space-based telescopes at
the string-of-pearl galaxies to settle
their origin.
¢BENJAMIN SKUSE
pIn this artist's view, a white dwarf (left) robs
gas from its companion star, material that ultimately
ignites in a thermonuclear blast.
than five times Earth's diameter. " The
fact that we caught the event when it
was still so close to the white dwarf was
quite lucky, " König says.
The observation validates long-established
predictions for what occurs when
novae start burning. " This is a spectacular
result! " says Simone Scaringi
(Durham University, UK), who wasn't
involved in the study. " The detection of
the X-ray flash will clearly aid in testing
and refining the physical models yielding
thermonuclear explosions. "
¢MONICA YOUNG
sk yand tele scope .o r g * SEPTEMBER 2022
9

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