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

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

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

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

“We went out on a limb with our initial Hubble observations of this galaxy in 2018,” mentioned Pieter van Dokkum, an astronomer at Yale University and a co-author of the brand new paper, in a NASA statement. “I think people were right to question it because it’s such an unusual result. It would be nice if there were a simple explanation, like a wrong distance. But I think it’s more fun and more interesting if it actually is a weird galaxy.”

Dark matter has but to be immediately 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 after they have a look at how briskly galaxies rotate in comparison with how a lot seen matter they’ve. That can’t be mentioned for DF2, although, the place the observable stars appear to account for many of the galaxy’s gravity.

Some criticisms of the unique paper centered on the galaxy’s brightness: If the researchers had gotten the space fallacious of their measurements, the galaxy could possibly be nearer than thought and due to this fact have fewer stars than implied by its brightness. Fewer stars would imply that much less of DF2’s gravity could possibly be attributed to seen matter, and thus it should have extra darkish matter to carry the construction collectively. But that’s not what the researchers discovered of their newest observations.

“For almost every galaxy we look at, we say that we can’t see most of the mass because it’s dark matter,” van Dokkum mentioned. “What you see is only the tip of the iceberg with Hubble. But in this case, what you see is what you get. Hubble really shows the entire thing. That’s it. It’s not just the tip of the iceberg, it’s the whole 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 the moment 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 that its seeming lack of darkish matter could possibly be defined by a miscalculation of its distance. For now, the thriller of DF2’s “missing” darkish matter continues.

More: Heated Debate Surrounds Galaxy Seeming to Lack Dark Matter