50 ASTRONOMY • MARCH 2020
location, and distance. But, most impor-
tantly, DF4 likewise has an apparent
dearth of dark matter.
Using the Keck Telescope to measure
the motion and speed of DF4’s diffuse
gas, as well as seven of its globular clus-
ters, van Dokkum’s team calculated that
DF4’s distance is similar to DF2’s — hov-
ering around 65 million light-years away,
give or take about 9 million light-years.
“We conclude that NGC 1052-DF2 is
not an isolated case, but that a class of
such objects exists,” van Dokkum’s team
wrote in their DF4 discovery paper. “The
origin of these large, faint galaxies with
an excess of luminous globular clusters
and an apparent lack of dark matter is, at
present, not understood.”
But, yet again, Trujillo and his team
calculated their own distance to DF4.
Based on Hubble data available at the
time, they identified what they think is
DF4’s TRGB. This led them to conclude
that DF4 is about 46 million light-years
away, which would mean its globular
clusters aren’t actually that strange, and
instead are pretty similar to those found
in the Milky Way and elsewhere.
“All in all,” Trujillo’s team concluded
in their response paper, “the proposition
that both NGC 1052-DF2 and NGC 1052-
DF4 are ‘missing dark matter’ is still far
from being placed on sure footing.”
Hubble takes another look
In the summer of 2019, in order to deter-
mine whether Trujillo’s team had identi-
fied DF4’s true TRGB, van Dokkum’s
group used Hubble’s keen eye to collect
new, deep images of DF4. On October 16,
they posted another paper, which has been
submitted to The Astrophysical Journal
Letters, on the preprint site arXiv. Based
on the fresh Hubble data, which picked up
many more, much fainter stars, the paper
claims the short-distance camp again mis-
identified DF4’s brightest red giant stars,
leading to a closer derived distance.
“In the new data, there really is no
ambiguity,” says study author Shany
Danieli of Yale, who is van Dokkum’s
colleague. “We think the new data really
rule out the [shorter-distance] option.”
“I think this is definitive,” says van
Dokkum. “The TRGB cannot be argued
with: It is caused by well-understood stel-
lar physics, and [is] as direct as distance
indicators get.”
But after reviewing the new research, Trujillo is still
dubious of the study’s conclusion. “At the moment, I
remain enormously skeptical about their outcome of a
long distance for DF4,” Trujillo says. “The first thing
you should note is that this paper has still not [been]
accepted by the referee and/or the journal,” he
explains. “It has been only submitted; therefore, fur-
ther changes of its content can be expected after a
careful reading by the referee.”
Then there’s also the matter of how van Dokkum’s
long-distance camp selected which of DF4’s stars they
would include in their TRGB analysis, which is yet
another point of contention. “I think there are a num-
ber of choices [van Dokkum’s group] have used that
have not been justified,” Trujillo says. “All of these
choices seem to be selected to favor a larger distance
than what the data suggests.”
One such choice, Trujillo explains, is that the quality
cuts van Dokkum’s team used to pick which stars were
included in their analysis seem to ignore many stars.
Another, he adds, is that van Dokkum’s team made “a
preselection of their stars based on the colors” and they
do not explain why or how that impacts their outcome.
Both of these choices, Trujillo says, could greatly
impact the derived distance to DF4.What’s next?
So, at this stage, the answer to whether DF2 and DF4
have dark matter or not is still largely up in the air.“
At themoment,
I remain
enormously
skeptical
about their
outcome
of a long
distance
fo r DF4.
Ignacio Trujillo”
The Hubble Space Telescope has
remained one of the most prolific work-
horses in astronomy for three decades.
Although it’s approaching the end of its
life, it is still performing great science. NASA