Hubble Space Telescope Takes Another Look at ‘Weird’ Galaxy That Seems to Lack Dark Matter

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

The extremely-diffuse galaxy DF2 appears to be like strangely devoid of sad topic.
Image: SCIENCE: NASA, ESA, STScI, Zili Shen (Yale), Pieter van Dokkum (Yale), Shany Danieli (IAS) IMAGE PROCESSING: Alyssa Pagan (STScI)

In 2018, a team of astronomers reported that a galaxy regarded as if it could well well don’t have any sad topic, the invisible, mysterious stuff that is assumed very finest from its gravitational outcomes. The team’s work used to be met with skepticism, but now, they’ve doubled-down on the hypothesis after extra observations of the galaxy utilizing the Hubble Save Telescope.

Many galaxies are huge spiral formations, admire our Milky Scheme, or elliptical, admire M87 of dim hole popularity. Nonetheless this galaxy, referred to as DF2 for short, lacks any of that density. It’s a long way extremely-diffuse, that arrangement that it has the mass of a smaller galaxy but a long way more unfold out. DF2 is as huge because the Milky Scheme but incorporates appropriate half of a percent as many stars. A evaluate team in actual fact helpful in 2018 that DF2’s extremely-diffuseness is presumably explained by an absence of sad topic, which is step by step ceaselessly referred to as the “glue” retaining galaxies collectively. Nonetheless other astronomers weren’t convinced, arguing that the data wasn’t extra special ample to attain any such conclusion.

The team’s 2d spherical of outcomes had been printed this month in the Astrophysical Journal Letters, and they refine the dimension of the galaxy’s distance from Earth, which has implications for the arrangement sad topic-much less DF2 in actuality is presumably.

“We went out on a limb with our initial Hubble observations of this galaxy in 2018,” acknowledged Pieter van Dokkum, an astronomer at Yale College and a co-author of the fresh paper, in a NASA notify. “I reveal folks had been staunch to question it attributable to it’s such an uncommon consequence. It can well also be fine if there had been a straightforward clarification, admire a negative distance. Nonetheless I reveal it’s more fun and more provocative if it in actual fact is a odd galaxy.”

Darkish topic has yet to be exact now noticed (and there are many candidates to exhibit it, from primordial dim holes to theoretical particles admire axions), but astronomers look the gravitational signature of the stuff after they detect at how quickly galaxies rotate when when in contrast with how great viewed topic they’ve. That can’t be acknowledged for DF2, even if, where the observable stars seem to tale for plenty of of the galaxy’s gravity.

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Some criticisms of the unique paper centered on the galaxy’s brightness: If the researchers had gotten the gap negative of their measurements, the galaxy could perchance well be nearer than thought and therefore have fewer stars than implied by its brightness. Fewer stars would indicate that much less of DF2’s gravity could perchance well be attributed to viewed topic, and thus it will have to have more sad topic to defend the construction collectively. Nonetheless that’s no longer what the researchers came all the arrangement thru of their most up-to-date observations.

“For nearly every galaxy we detect at, we mumble that we can’t look lots of the mass attributable to it’s sad topic,” van Dokkum acknowledged. “What you look is awfully finest the tip of the iceberg with Hubble. Nonetheless on this case, what you look is what you procure. Hubble in actual fact presentations all the ingredient. That’s it. It’s no longer appropriate the tip of the iceberg, it’s the total iceberg.”

So for the fresh paper, the researchers took more measurements of DF2’s distance, utilizing 40 orbits of Hubble (the well-known voice telescope, sadly, is presently malfunctioning). The estimated distance of the galaxy used to be sophisticated to about 22 megaparsecs, or 72 million gentle-years, making it a long way more distant than its old estimated distance of 65 million gentle-years and defying the root that its seeming lack of sad topic could perchance well be explained by a miscalculation of its distance. For now, the mystery of DF2’s “lacking” sad topic continues.

Extra: Heated Debate Surrounds Galaxy Seeming to Lack Darkish Topic

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