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The ultra-diffuse galaxy DF2 seems strangely devoid of dark matter.

The ultra-diffuse galaxy DF2 appears surprisingly devoid of darkish matter.
Picture: SCIENCE: NASA, ESA, STScI, Zili Shen (Yale), Pieter van Dokkum (Yale), Shany Danieli (IAS) IMAGE PROCESSING: Alyssa Pagan (STScI)

In 2018, a group of astronomers reported {that a} galaxy appeared to don’t have any darkish matter, the invisible, mysterious stuff that’s identified solely from its gravitational results. The group’s work was met with skepticism, however now, they’ve doubled-down on the speculation after extra observations of the galaxy utilizing the Hubble Area Telescope.

Many galaxies are large spiral formations, like our Milky Approach, or elliptical, like M87 of black gap fame. However this galaxy, known as DF2 for brief, lacks any of that density. It’s ultra-diffuse, that means that it has the mass of a smaller galaxy however rather more unfold out. DF2 is as large because the Milky Approach however accommodates simply half a % as many stars. A analysis group recommended in 2018 that DF2’s ultra-diffuseness may be defined by an absence of darkish matter, which is also known as the “glue” holding galaxies collectively. However different astronomers weren’t satisfied, arguing that the info wasn’t sturdy sufficient to achieve such a conclusion.

The group’s second spherical of outcomes have been published this month within the Astrophysical Journal Letters, they usually refine the measurement of the galaxy’s distance from Earth, which has implications for the way darkish matter-less DF2 actually may be.

“We went out on a limb with our preliminary Hubble observations of this galaxy in 2018,” stated Pieter van Dokkum, an astronomer at Yale College and a co-author of the brand new paper, in a NASA statement. “I feel individuals have been proper to query it as a result of it’s such an uncommon outcome. It might be good if there have been a easy clarification, like a unsuitable distance. However I feel it’s extra enjoyable and extra fascinating if it truly is a bizarre galaxy.”

Darkish matter has but to be instantly noticed (and there are numerous candidates to elucidate it, from primordial black holes to theoretical particles like axions), however astronomers see the gravitational signature of the stuff once they have a look at how briskly galaxies rotate in comparison with how a lot seen matter they’ve. That can’t be stated for DF2, although, the place the observable stars appear to account for a lot of the galaxy’s gravity.

Some criticisms of the unique paper centered on the galaxy’s brightness: If the researchers had gotten the gap unsuitable of their measurements, the galaxy might be nearer than thought and subsequently have fewer stars than implied by its brightness. Fewer stars would imply that much less of DF2’s gravity might be attributed to seen matter, and thus it will need to have extra darkish matter to carry the construction collectively. However that’s not what the researchers discovered of their newest observations.

“For nearly each galaxy we have a look at, we are saying that we are able to’t see a lot of the mass as a result of it’s darkish matter,” van Dokkum stated. “What you see is barely the tip of the iceberg with Hubble. However on this case, what you see is what you get. Hubble actually exhibits your complete factor. That’s it. It’s not simply the tip of the iceberg, it’s the entire iceberg.”

So for the brand new paper, the researchers took extra measurements of DF2’s distance, utilizing 40 orbits of Hubble (the well-known house telescope, sadly, is at present malfunctioning). The estimated distance of the galaxy was refined to about 22 megaparsecs, or 72 million light-years, making it much more distant than its earlier estimated distance of 65 million light-years and defying the concept its seeming lack of darkish matter might be defined by a miscalculation of its distance. For now, the thriller of DF2’s “lacking” darkish matter continues.

Extra: Heated Debate Surrounds Galaxy Seeming to Lack Darkish Matter

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