Hubble telescope in another tight spot: Between astrophysicists sparring over a 'dark matter deficient' galaxy
- Reference: 1624944728
- News link: https://www.theregister.co.uk/2021/06/29/hubble_galaxy_distance_dark_matter/
- Source link:
When DF2 was [1]written up in a [2]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.
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About a year after the Nature paper the came out, researchers led by Spain's Instituto de Astrofísica de Canarias (IAC) dismissed the findings, [4]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.
Doubling down
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 [5]paper , [6]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, [7]said .
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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 [10]paper concluding a neighboring galaxy, NGC 1052-DF4, was also short of dark matter and about the same distance away 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 between our corner of the Milky Way and DF2. This is done using the [11]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 [12]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 [13]erroneously measured, requiring a telemetry circuit reset.
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The WFC3's UVIS detector has [15]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.
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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 [17]memory errors , preventing it from carrying out any scientific work, despite NASA's ongoing efforts to fix the instrument. Earlier this year, a [18]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."
[19]Hubble’s cosmic science is mind-blowing, but its soul celebrates something surprising about us
[20]James Webb Space Telescope runs one last dress rehearsal for its massive golden mirrors before heading to launchpad
[21]A borked bit of code sent the Hubble Space Telescope into safe mode, revealing a bunch of other glitches
[22]Behold, the Ultimately Large Telescope: A revived proposal for a 100-metre liquid-mirror star scanner on the Moon
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 [23]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. ®
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[1] https://arxiv.org/abs/1803.10237
[2] https://www.nature.com/articles/nature25767
[3] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_offbeat/science&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=2&c=2YNrvRvGaToaZQnS3MZfv3wAAAEY&t=ct%3Dns%26unitnum%3D2%26raptor%3Dcondor%26pos%3Dtop%26test%3D0
[4] https://academic.oup.com/mnras/article/486/1/1192/5380810
[5] https://arxiv.org/abs/2104.03319
[6] https://iopscience.iop.org/article/10.3847/2041-8213/ac0335
[7] https://news.yale.edu/2021/06/17/going-distance-confirm-galaxy-almost-no-dark-matter
[8] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_offbeat/science&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=4&c=44YNrvRvGaToaZQnS3MZfv3wAAAEY&t=ct%3Dns%26unitnum%3D4%26raptor%3Dfalcon%26pos%3Dmid%26test%3D0
[9] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_offbeat/science&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=3&c=33YNrvRvGaToaZQnS3MZfv3wAAAEY&t=ct%3Dns%26unitnum%3D3%26raptor%3Deagle%26pos%3Dmid%26test%3D0
[10] https://iopscience.iop.org/article/10.3847/2041-8213/ab0d92/meta
[11] https://en.wikipedia.org/wiki/Tip_of_the_red-giant_branch
[12] https://esahubble.org/about/history/acs_repair/
[13] https://www.nasa.gov/feature/goddard/2019/wide-field-camera-3-anomaly-on-hubble-space-telescope
[14] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_offbeat/science&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=4&c=44YNrvRvGaToaZQnS3MZfv3wAAAEY&t=ct%3Dns%26unitnum%3D4%26raptor%3Dfalcon%26pos%3Dmid%26test%3D0
[15] https://hst-docs.stsci.edu/wfc3ihb/chapter-6-uvis-imaging-with-wfc3/6-9-charge-transfer-efficiency
[16] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_offbeat/science&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=3&c=33YNrvRvGaToaZQnS3MZfv3wAAAEY&t=ct%3Dns%26unitnum%3D3%26raptor%3Deagle%26pos%3Dmid%26test%3D0
[17] https://www.theregister.com/2021/06/28/hubble_backup_borked/
[18] https://www.theregister.com/2021/03/12/hubble_bork/
[19] https://www.theregister.com/2021/06/28/hubbles_cosmic_science_is_mindblowing/
[20] https://www.theregister.com/2021/05/13/james_webb_last_checks/
[21] https://www.theregister.com/2021/03/12/hubble_bork/
[22] https://www.theregister.com/2020/11/18/moon_ultimate_telescope/
[23] https://arxiv.org/abs/2106.10283
[24] https://whitepapers.theregister.com/
Forgive my ignorance but...
I thought dark matter was still "theoretical". So, in a nutshell, what we have here is assorted bunches of people arguing the toss over something that might not even exist. Science is truly baffling!
Re: Forgive my ignorance but...
EVERYTHING is still theoretical. The current theory holds until something bigger and better disproves, or (usually) improves upon it, like Einstein's relativity improved on Newton's gravity.
Even the facts are at conflict in this story. Just how far away is the galaxy... a simple fact, no?
Dark matter is something convenient until it's better supported (AKA "proved") or disproved like phlogiston.
Re: Forgive my ignorance but...
You don't need an alternative to reject a flawed hypothesis. Dark matter models are already contradicted by observations, but rather than drop them and focus on developing alternatives, astronomers are adding the modern equivalent of epicycles to keep them alive.
The default position of science is "I do not know". Not "I assume I'm right", as is the case with this failed hypothesis.
Re: Forgive my ignorance but...
Dark matter models are already contradicted by observations,
Citation needed.
The theory of Dark Matter is actually pretty well agreed upon, as in there does not appear to be enough Mass in galaxies (based on current understandings) to maintain their current forms and shapes. That's pretty much universally agreed upon.
What isnt agreed upon is why? Or in what form that missing matter takes? Thats where you get into the hypothetical. Is it just that were bad at estimating the mass of galaxies? Is it WIMP's (weakly interacting massive particles)? Neutrinos? Fuzzy Dark matter? Axions?
There are a lot of hypotheses, and only some very vague evidence at the moment to support one or the other theories, but the fact remains most scientists agree something is missing from our understanding. Adding Mass to galaxies, explains with extreme precision their movement, shapes and make-up. As such, by observation we can say a galaxy should weigh this much, but our calculations only add up this much. Hence why dark matter is added. Maybe we are just bad at estimating the mass in a galaxy but there's a lot of very smart people doing very detailed calculations and they all seem to be finding mass missing.
Discussions like this are therefore highly intriguing because if these galaxies are really further away and light on dark matter, than that opens up possibilities for more study opportunities, more things to look at to say why are these galaxies different compared to others we have studied. Anomalies are great for advancing science and I look forward to hearing more on this topic in the future.
Re: Forgive my ignorance but...
Dark matter is 'theoretical' in the sense that we don't have direct observations of it – in the same sense, say, that gravitational waves were theoretical until LIGO heard the first one. As with gravitational waves pre GW150914 we have very good indirect evidence for something: for gravitational waves that was things like the spin-down of Hulse-Taylor, for dark matter / modified gravity it's pretty much every galaxy we look at.
In particular the discovery of galaxies without dark matter would be an enormous boost to the the idea that there is actually missing mass: in other words that what we're looking for is indeed dark matter and not a modified theory of gravity.
The reason for that is that it's a pretty safe assumption that gravity works the same everywhere. Indeed, if this isn't true – if the laws of physics vary from place to place – then so many assumptions about pretty much everything fail that it's hard to describe: momentum is not conserved, for instance. Under that assumption then if there is no dark matter but instead a modified theory of gravity then all galaxies should obey this modified theory of gravity. If we observe that some don't obey it we can say with very high confidence that a modified theory of gravity can't explain what we observe: we need missing mass, aka dark matter, as well. And Occam's razor would then say that, since we need dark matter then we should not assume modified gravity until we are sure that dark matter alone cannot explain what we see. And there's very little evidence that it can't.
Dark matter believers
I didn't realise that dark matter could be explained away so easily. It just needs to be shown that all inter-galactic distances are wrong, presumably due to an inaccurate assumption about something that is used in those calculations.
This science stuff is easy - although I'll leave it up to someone else to find what that something is.
Re: Dark matter believers
Galaxy distances are normally definitively measured by the yardstick of Type 1a supernova. This is meant to be the best way and yet it has pretty massive error bars, similar to the ones mentioned above. My Granddad was a pretty keen amateur astronomer (I believe the Carver scope in my barn inspired an Astronomer Royal in his hands (the scope!)) and he had a spectrometer that attached to this thing and he would use that to try and calculate stars redshift. That was hard enough for local stars. I've love to see in detail how Hubble try this on ones buried in distant galaxies. But not as much as getting hold of the fuck that stole the spectrometer.
Re: Dark matter believers
But not as much as getting hold of the fuck that stole the spectrometer.
The sad thing is that the thief probably didn't know what they had stolen and when they realized it wasn't a shiny smartphone they probably just threw it in the bin.
Re: Dark matter believers
The requirement for dark matter or a modified theory of gravity doesn't generally depend on getting distances correct. The reason you need dark matter is that the velocity curves of galaxies make no sense otherwise.
If you consider a spiral galaxy then it's really a large collection of stars orbiting around their common centre of mass. Well, you can estimate the mass within some radius from the common centre of mass simply by looking at how many stars there are and their types and looking for dust & gas &c. And you can use that mass estimate to compute what the orbital velocity of a star should be at some radius from the centre of mass. If you're confused about the distance all of this will be off by some constant factor perhaps, but it will only be off by a constant factor. The variation of orbital velocity with radius is the velocity curve of the galaxy.
Then you can measure the velocities of the stars from spectrographic data (Doppler shifts in spectral lines really). And that data will also be offset by recessional velocity and whatever the peculiar velocity of the galaxy is, but you can correct for those offsets because you know where the spectral lines really are because you can measure them locally.
And that data gives you the actual velocity curve of the galaxy. And it's different than the predicted curve, and critically it's not different by a constant factor: the curve is a different shape (it's flatter than it should be, with distant stars orbiting faster than predicted). That's something you can't fix by simply adjusting the distance.
I think the case with galaxies like this one, which are ultra-diffuse, is different. I'm not sure but I suspect you can't get a good measurement of the velocity curve so you rely on a rather small number of observations to give you a mass estimate. And in this case things do depend on knowing the distance.
But you can't fix the dark matter / modified law of gravity problem in general by any fix to distance scales.
Candid question
I guess there are other ways than using able to measure galaxies distance, as it was something done before Hubble launch, right?
So using these alternative means could provide the answer to the question are Hubble's ASC data flawed or not, couldn't it?
Remember kids, that scientsts rarely agree 100% on something. And this is a good thing. This is what helps science move forwards: People questioning things.