46 ASTRONOMY • MARCH 2020
Then, based on the Keck data, van
Dokkum’s team identified globular clus-
ters (large, spherical groups of old stars)
within DF2. Surprisingly, they found the
globular clusters were moving at a snail’s
pace compared to what we would see if
the galaxy were chock-full of dark mat-
ter. If DF2 had more dark matter, van
Dokkum says, the increased gravitational
pull would cause these clusters to move
about three times faster than they do.
The realization that DF2 seems to
have very little, if any, dark matter caught
the researchers off guard because it’s the
first galaxy found lacking the pervasive
material. “There is no theory that pre-
dicted these types of galaxies. The galaxy
is a complete mystery, as everything
about it is strange,” said van Dokkum.
“How you actually go about forming one
of these things is completely unknown.”
However, not everyone agrees that
DF2 is missing its dark matter.create a novel telescope that’s particularly adept at imaging
expansive and extremely faint structures. When the researchers
observed DF2 with Dragonfly, they noticed the galaxy looked
different than it did in images taken with the Sloan Digital Sky
Survey (SDSS). In the Dragonf ly images, DF2 looked like a blob
of dim and diffuse light, but in the SDSS images, it appeared as
a group of pointlike sources.
To investigate this discrepancy, van Dokkum’s team went on
to observe DF2 using the Advanced Camera for Surveys on the
Hubble Space Telescope, and performed follow-up spectro-
scopic observations with the Keck Telescope.
“I spent an hour just staring at the Hubble image,” said van
Dokkum. “It’s so rare, particularly these days after so many years
of Hubble, that you get an image of something and you say, ‘I’ve
never seen that before.’ This thing is astonishing — a gigantic
blob that you can look through. It’s so sparse that you see all
of the galaxies behind it. It is literally a see-through galaxy.”
Using the Hubble data, van Dokkum and his team mea-
sured the galaxy’s surface brightness f luctuations, which is a
rudimentary distance indicator based on the fact that more
distant galaxies appear more uniform in brightness. From this,
the researchers calculated DF2 is located about 65 million
light-years away.
“
I spentan hour
just
staring
at the
Hubble
image.
Pieter van Dokkum”