BBC Sky at Night - UK (2021-08)

BULLETIN

The latest astronomy and space news, written by Ezzy Pearson

by Chris Lintott

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A distant galaxy could throw our understanding of

galactic formation into question, after recent Hubble

Space Telescope measurements have bolstered

previous claims that it’s free of dark matter.

According to our current understanding, galaxies

are held together by a mysterious substance known

as dark matter. Spheroidal galaxy NGC 1052-DF

(DF2 for short), however, seems to be an exception

to this rule. Although it is as wide as the Milky Way,

the galaxy contains 200 times fewer stars, and initial

measurements (made in 2018) suggest that it has

400 times less dark matter than astronomers think it

should have – according to our current understanding

of how galaxies grow and evolve.

An alternate possibility, however, is that the galaxy

is closer than its initial measurement of 65 million

lightyears. This would make it intrinsically fainter,

meaning it has less mass and less dark matter. To

Since its discovery,

DF2 has caused

controversy.

Detecting the

LQƆXHQFHRIGDUN

matter on small

systems is a

GLIƅFXOWWDVNDQG

showing it isn’t

there is very hard.

This also adds

strength to claims

about the dark

matter-free status

of another satellite

of NGC 1052-DF4,

which has similar

properties to DF2.

These results

challenge our

‘standard’

conception of how

galaxies form in a

Universe dominated

by dark matter, and

alternative ideas

that remove any

need for mysterious

SDUWLFOHVE\ƅGGOLQJ

with the theory

of gravity.

With new clues

needed by both

sides of that great

debate, I’d expect

more observations

of this intriguing

system in the

near future.

Chris Lintott

co-presents

The Sky at Night

The find challenges our knowledge of how galaxies form and stay together

test if this was the case, astronomers used

Hubble’s Advanced Camera for Surveys to look at

the red giant stars within DF2. Red giant stars all

peak with the same brightness, so astronomers

can use them to work out how far away the

objects they lie within are.

The new observations reveal that not only is the

galaxy as distant as initially thought, but it’s

actually further away – at a distance of 72 million

lightyears from the Milky Way.

“What you see is normally only the tip of the

iceberg with Hubble,” says Pieter van Dokkum

from Yale University, who led the Hubble

observations. “But in this case, what you see is

what you get. Hubble really shows the entire thing.

That’s it: it’s not just the tip of the iceberg, it’s the

whole iceberg.”

https://hubblesite.org

Comment

Dark matter-free galaxy confirmed

Determining the amount of dark

matter in galaxy NGC 1052-DF

hinges on getting measurements

of its distance from the Milky Way

10 BBC Sky at Night Magazine August 2021