Science News - USA (2022-06-18)

32 SCIENCE NEWS | June 18, 2022

ALL: P. VAN DOKKUM

ET AL

/NATURE

2022

SCIENCE VISUALIZED

In a patch of sky in the constellation Cetus, an appar-
ent row of dwarf galaxies seems to span more than
6 million light-years. This lineup (shown above) may
have formed in the aftermath of a long-ago galactic
crash. And that crash may explain why these and certain
other galaxies are surprisingly devoid of dark matter.
In 2018 and 2019, astronomer Pieter van Dokkum of
Yale University and colleagues reported on two dwarf
galaxies lacking dark matter (SN Online: 3/28/18).
Dark matter is thought to be the foundation of all
galaxies, gravitationally attracting gas that eventually
forms stars. So some process must have separated
these galaxies from their dark matter long ago.
It turns out those two galaxies, NGC1052-DF2 and
NGC1052-DF4 (numbers 2 and 9 in close-ups at right),
are racing away from each other as if they had come
from the same spot, van Dokkum and colleagues
report in the May 19 Nature. What’s more, the duo is
part of a chain of 11 galaxies, a structure that could
have formed in the aftermath of a collision of two
ancient dwarf galaxies, the team says.
That smashup could have split dark matter from
normal matter. In such a crash, the dark matter would
have continued on its path, because it doesn’t interact
with other matter. But the gas would have slammed
together, forming clumps that each became their own
galaxy free of dark matter. The newfound string of gal-
axies might have formed from such clumps.
Some scientists are skeptical. “I just don’t think the
bar has been met,” says astronomer Michelle Collins of
the University of Surrey in Guildford, England. More
measurements would help. She’s awaiting estimates on
how far each galaxy is from Earth, which would reveal
whether the lineup is real or just a chance overlap seen
from our viewpoint. — Emily Conover

Possible cosmic debris from a galactic collision

RCP21

RCP 32

RCP17

NGC1052-DF2

NGC1052-DF4

RCP28

LEDA4014647

RCP26

NGC1052-DF7

Ta21-12000 NGC1052-DF5

11

11

8

5

2

10

10

7

4

1

6

5

4

3

2

1

7

9

9

6

3

8