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Between astrophysicists clashing on a galaxy “deficient in dark matter” • The Register

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Never mind the memory errors due to radiation. Another deteriorating part of the decades-old Hubble Space Telescope has found itself in a tight spot. This time, his camera unit is again in the middle of a clash between scientists over whether the galaxy NGC 1052-DF2 contains dark matter or not.

When DF2 was written in a Nature article in 2018, the study immediately raised eyebrows. The article concluded that the galaxy was devoid of dark matter. How could this be possible?

The mysterious substance is believed to be present in all galaxies, holding together twinkling stars, swirls of dust and gas, and more, giving galaxies structure and shape. Dark matter is a crucial element in our theories of galactic formation, and finding a galaxy without this hidden substance, or a very small amount of it, breaks all the rules. DF2 was presented as a major eccentric.

About a year after the Nature article appeared, researchers led by the Spanish Instituto de Astrofísica de Canarias (IAC) rejected the findings, arguing in the Royal Astronomical Society’s monthly notices that the previous study was was mistaken on the distance with DF2. The galaxy is actually 42 million light years away, not 65 million as stated in the Nature article, the IAC boffins said. And when you plug that closer distance into the formulas for calculating its mass, it turns out that a lot of DF2 is made up of dark matter, and there certainly isn’t a shortage of it. In fact, DF2 would look like any normal galaxy harboring dark matter.

Double

Unresponsive to this conclusion, the authors of the Nature article – led by astrophysicists at Yale University – now believe that DF2 is even further away than previously thought. In a new article, published this month in The Astrophysical Journal, the team placed their dark matter-deficient DF2 72 million light-years away.

“There is a saying that extraordinary claims require extraordinary evidence, and the new distance measure strongly supports our previous conclusion that DF2 lacks dark matter,” said Zili Shen, a graduate student at Yale and first author of the latter. article.

This can be wishful thinking. Ignacio Trujillo, IAC researcher based in Tenerife, is not backing down. “I think their analysis is far from convincing,” he said. The register. “I think it’s really poor.”

By the way, just to give you an idea of ​​how intense this debate is from a distance, days after the IAC said in 2019 that DF2 was 42 million light years away, team members from Yale published another article finding a nearby galaxy, NGC 1052-DF4, was also short of dark matter and at about the same distance as DF2.

Telescope damage

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 from our corner of the Milky Way to DF2. This is done using the red giant branch tip technique, which involves using the brightness of the stars to determine how far away they are.

The ACS was installed in 2002, and repaired in 2006 following an electronic failure. At the time, the ACS was equipped with an instrument called Wide Field Camera 3 (WFC3). It was the instrument that was briefly out of service in January 2019 after voltage levels were erroneously measured, requiring a reset of the telemetry circuit.

The WFC3’s UVIS detector has suffered radiation damage due to its orbit, reducing its charge transfer efficiency, which scientists must take into account and correct, especially when studying distant and weak galaxies. Trujillo suspects that this degradation of the image sensor coupled with the weakness of DF2 played a role in the Yale team’s erroneous calculation of the distance of tens of millions of light years.

“They overlooked many important issues related to weak source analysis,” he said. “This is the key to a camera that is already as damaged as the Advanced Camera for Investigations (ACS) on board the Hubble Space Telescope. In a reanalysis of the galaxy that we are currently conducting, we find that the galaxy is close. contrary to what they claim. ”

For what it’s worth, in their 2019 article, the IAC team said they calculated the distance of 42 million light years (13 million parsecs) after weighing six sets of data from various telescopes. and sky surveys, although it relied mainly on images. from Hubble. He claimed that “up to five independent redshift distance measurements converge at a distance of 13 Mpc for the galaxy,” and believes his approach is more accurate than the Yale team’s estimate.

This dispute over the dark matter content of DF2, based on the Hubble imagery, comes at a turning point for the Space Telescope, which has been in operation for just over 30 years: it continues to suffer from memory errors, the preventing scientific work from being carried out, despite NASA’s continued efforts to repair the instrument. Earlier this year, a questionable software update temporarily put it in safe mode.

Everything in the details

Shen not only defended the telescope, she said the Yale team used longer exposure times for their last article to measure the brightness of red giant stars and therefore the distance from DF2. “Hubble is used for incredibly precise measurements, including the rate of expansion of the universe, and its instruments are perfectly calibrated,” she told us. “Also, with these long exposures, the red giant stars are easily detected. The red giant stars are what matters to our distance measurement. This is a measurement at which Hubble is very good.”

Hubble is used for incredibly precise measurements, including the rate of expansion of the universe, and its instruments are perfectly calibrated

“What is happening is that the other team used our previous data from the same ACS camera to draw conclusions which have now been found to be incorrect with our new 20 times longer exposures,” she added. .

Regarding the corrections needed for the deteriorated charge transfer efficiency, “this shouldn’t be a problem,” Shen said. “Radiation causes slow and well understood degradation of detectors over the years. It is well characterized, and it is corrected in the analysis. We have a step called charge transfer efficiency (CTE) correction, and when performing photometry there is a time dependent zero point. These are standard data analysis steps that we have followed.

Yale astronomy professor Pieter van Dokkum, who co-authored the university papers on DF2, also told us that the degradation of CTE is explained, adding that the long-exposure images cement the conclusion of a distant galaxy with low dark matter content. “This is a case where the team from Tenerife had an interesting hypothesis, that the galaxy was much closer to us than we thought, but the new data definitely rules out it,” said he declared. The register.

Mireia Montes, an astronomer at the Space Telescope Science Institute in Maryland, who works with Trujillo to refute the suggestion of a galaxy without dark matter, agreed that longer exposures should, in theory, work better. However, she said the latest Yale article is confusing and appears to ignore the brightness of some of the red giants, an omission that may have skewed the final numbers.

“The analysis they did is not very clear to me,” Montes told us. “They remove a lot of stars, which is essential for the method they use, without clearly explaining why they are doing it. Another factor is the aging of the telescope. Hubble has been in orbit for 31 years and this camera has been in there. space since 2002.

“Since the stars in this galaxy are so faint in the images, we would expect some effect on the measurements.”

Shen, meanwhile, doesn’t think there has been a significant change in the state of Hubble’s camera. “We haven’t seen any official announcements regarding the damage to the camera from the Hubble team. I would be happy to see any documentation on this damage,” she said. El Reg.

Montes and his team, which includes IAC’s Trujillo, said their follow-up article on DF2 had been accepted for publication; a pre-printed version is here. She thinks the galaxy is more normal than people think, because not only is it tied to dark matter, but it also has a disk and structure, as one would expect.

Let science continue. ®


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