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Hubble telescope in another tight spot: Between astrophysicists sparring over a 'dark matter deficient' galaxy
One team says DF2 is 72m LY away, another disagrees – we speak to both sides
Never mind memory errors from radiation. Another deteriorating part of the decades-old Hubble Space Telescope has found itself in a jam. This time its camera unit is once again in the middle of a clash between scientists over whether or not the galaxy NGC 1052-DF2 contains any dark matter.
When DF2 was written up in a Nature paper in 2018, the study immediately raised eyebrows. The paper concluded the galaxy was devoid of dark matter. How could that be possible?
The mysterious substance is thought to be present in all galaxies, holding together glittering stars, swirls of dust and gas, and more, giving the galaxies structure and shape. Dark matter is a crucial component in theories of galactic formation, and finding a galaxy without this hidden stuff, or a very low quantity of it, breaks all the rules. DF2 was heralded as a significant oddball.
About a year after the Nature paper the came out, researchers led by Spain's Instituto de Astrofísica de Canarias (IAC) dismissed the findings, arguing in the monthly notices of the Royal Astronomical Society that the earlier study got the distance to DF2 wrong. The galaxy is actually 42 million light years away, not 65 million as stated in the Nature paper, the IAC boffins said. And when you plug that closer distance into the formulas for calculating its mass, it turns out a large portion of DF2 is made up of dark matter, and there certainly isn't a shortage of it. In fact, DF2 would appear to be like any normal dark-matter-harboring galaxy.
Unswayed by that conclusion, the Nature paper's authors – led by astrophysicists at Yale University – now reckon DF2 is even further away than previously thought. In a new paper, published this month in The Astrophysical Journal, the team put their dark-matter-deficient DF2 at 72 million light years away.
“There’s a saying that extraordinary claims require extraordinary evidence, and the new distance measurement strongly supports our previous finding that DF2 is missing dark matter,” Zili Shen, a graduate student at Yale and first author of the latest paper, said.
That may be wishful thinking. Ignacio Trujillo, a researcher at the Tenerife-based IAC, for one, isn't backing down. “I think their analysis is far from being convincing,” he told The Register. "I think it is really poor."
By the way, just to give you an idea of how hotly fought this distance debate is, days after the IAC stated in 2019 that DF2 was 42 million light years away, members of the Yale team put out another paper concluding a neighboring galaxy, NGC 1052-DF4, was also short of dark matter and about the same distance away as DF2.
Where does Hubble fit into all of this? Well, the Yale team used the space telescope's Advanced Camera for Surveys (ACS) to calculate the distance between our corner of the Milky Way and DF2. This is done using the tip of red-giant branch technique, which involves using the brightness of stars to figure out how far away they are.
The ACS was installed in 2002, and repaired in 2006 following an electronics failure. At the time, the ACS was fitted with an instrument called the Wide Field Camera 3 (WFC3). That's the instrument that was briefly out of action in January 2019 after voltage levels were erroneously measured, requiring a telemetry circuit reset.
The WFC3's UVIS detector has sustained radiation damage from being in orbit, reducing its charge transfer efficiency, which scientists need to take into account and correct for especially when studying faint, faraway galaxies. Trujillo suspects this image sensor degradation coupled with the faintness of DF2 played a role in the Yale team miscalculating the distance by tens of millions of light years.
“They have neglected many important issues connected with the analysis of faint sources," he claimed. "This is key in a camera that is already so damaged as the Advanced Camera for Surveys (ACS) onboard the Hubble Space Telescope. In a reanalysis of the galaxy we are currently conducting, we find the galaxy is close contrary to what they claim.”
For what it's worth, in its 2019 paper, the IAC team said it calculated the distance of 42 million light years (13 million parsecs) after weighing up six sets of data from various telescopes and surveys of the sky, though it primarily relied on images from Hubble. It asserted that "up to five different redshift-independent distance measurements converge to a distance of 13 Mpc for the galaxy," and believes its approach is more accurate than the Yale team's estimation.
This row over the dark matter content of DF2, based on Hubble's images, comes at a make-or-break time for the space telescope, which has been operating for just over 30 years: it continues to suffer from memory errors, preventing it from carrying out any scientific work, despite NASA's ongoing efforts to fix the instrument. Earlier this year, a dodgy software update knocked it into safe mode, temporarily.
All in the details
Shen not only defended the telescope, she said the Yale team used longer exposure times for its latest paper to measure the luminosity of red-giant stars and thus DF2's distance. “Hubble is being used for incredible precise measurements, including the expansion rate of the universe, and its instruments are exquisitely calibrated," she told us. "Furthermore, with these long exposures, the red-giant stars are easily detected. The red giant stars are what matters for our distance measurement. This is a measurement that Hubble is very good at."
Hubble is being used for incredible precise measurements, including the expansion rate of the universe, and its instruments are exquisitely calibrated
“What’s going on is that the other team used our previous data from the same ACS camera to draw conclusions that are now shown to be incorrect with our new 20x longer exposures,” she added.
On the subject of the corrections needed for the deteriorated charge transfer efficiency, "it should not be a concern," said Shen. “The radiation causes a slow and well-understood degradation of the detectors over the years. It’s well-characterized, and it is corrected for in the analysis. We have a step called the Charge Transfer Efficiency (CTE) correction, and when performing photometry, there is a time-dependent zero point. These are standard data analysis steps that we followed.”
Yale astronomy professor Pieter van Dokkum, who coauthored the university's papers on DF2, also told us the CTE degradation is accounted for, adding that the long-exposure images cement the conclusion of a faraway, low-dark-matter galaxy. "This is a case where the Tenerife team had an interesting hypothesis, that the galaxy was much closer to us than we thought, but the new data rules it out with really no room for serious doubt," he told The Register.
Mireia Montes, an astronomer at the Space Telescope Science Institute in Maryland, who is working with Trujillo to disprove the suggestion of a dark-matter-free galaxy, acknowledged that longer exposures should, in theory, provide better results. However, she said the latest Yale paper is confusing, and seems to disregard the brightness of some of the red giants, an omission that may have skewed the final numbers.
“The analysis they made is not very clear to me," Montes told us. "They remove a lot of stars, which is critical for the method they use, with no clear explanation of why they do that. Another factor is the aging of the telescope. Hubble has been 31 years in orbit, and this camera has been in space since 2002.
"As the stars of this galaxy are so faint in the images, we expect some effect on the measurements."
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Shen, meanwhile, doesn’t believe there has been a significant change in the state of Hubble's camera. “We have not seen any official announcements regarding camera damage from the Hubble team. I'd be happy to see any documentation of said damage,” she told El Reg.
Montes and her team, which includes the IAC's Trujillo, said their followup paper on DF2 has been accepted for publication; a pre-print version is here. She believes the galaxy is more normal than people think, as not only is it held together with dark matter, it also has a disk and some structure, as one might expect.
Let the science continue. ®