Gemini/Keck: Massive Galaxy Made of 99.99% Dark Matter

[bystander]

Massive Galaxy Made of 99.99% Dark Matter
Gemini Observatory | W.M. Keck Observatory | 2016 Aug 25

The dark galaxy Dragonfly 44. The image on the left is a wide view of the
galaxy taken with the Gemini North telescope using the Gemini Multi-Object
Spectrograph (GMOS). The close-up on the right is from the same very deep
image, revealing the large, elongated galaxy, and halo of spherical clusters
of stars around the galaxy’s core, similar to the halo that surrounds our Milky
Way Galaxy. Dragonfly 44 is very faint for its mass, and consists almost
entirely of Dark Matter. Image Credit: Pieter van Dokkum, Roberto Abraham,
Gemini Observatory, Sloan Digital Sky Survey, AURA.

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Using the world's most powerful telescopes, an international team of astronomers has discovered a massive galaxy that consists almost entirely of Dark Matter. Using the W. M. Keck Observatory and the Gemini North telescope – both on Maunakea, Hawaii – the team found a galaxy whose mass is almost entirely Dark Matter.

Even though it is relatively nearby, the galaxy, named Dragonfly 44, had been missed by astronomers for decades because it is very dim. It was discovered just last year when the Dragonfly Telephoto Array observed a region of the sky in the constellation Coma. Upon further scrutiny, the team realized the galaxy had to have more than meets the eye: it has so few stars that it quickly would be ripped apart unless something was holding it together.

To determine the amount of Dark Matter in Dragonfly 44, astronomers used the DEIMOS instrument installed on Keck II to measure the velocities of stars for 33.5 hours over a period of six nights so they could determine the galaxy’s mass. The team then used the Gemini Multi-Object Spectrograph (GMOS) on the 8-meter Gemini North telescope on Maunakea in Hawaii to reveal a halo of spherical clusters of stars around the galaxy’s core, similar to the halo that surrounds our Milky Way Galaxy. ...

Scientists Discover a 'Dark' Milky Way
Yale University | 2016 Aug 25

A High Stellar Velocity Dispersion and ~100 Globular Clusters for the Ultra Diffuse Galaxy Dragonfly 44 - Pieter van Dokkum et al

• Astrophysical Journal Letters 828(1):L6 (2016 Sep 01) DOI: 10.3847/2041-8205/828/1/L6
  arXiv.org > astro-ph > arXiv:1606.06291 > 20 Jun 2016 (v1), 15 Jul 2016 (v2)

[Ann]

Jim Shelton of Yale University wrote:
“We have no idea how galaxies like Dragonfly 44 could have formed,” said Abraham.

Perhaps the question is not how that blob of dark matter came together, but why its baryonic matter formed so few stars.

Dragonfly 44 sounds quite a bit like a low surface brightness galaxy, somewhat similar to a galaxy like Malin 1.

“The Gemini data show that a relatively large fraction of the stars is in the form of very compact clusters, and that is probably an important clue. But at the moment we’re just guessing.”

Indeed, that could be an important clue.

Those compact clusters are most likely globular clusters, or some other kind of old clusters. At least to me, it looks as if this galaxy contains very few young stars, and therefore very few young clusters.

The spheroidal Fornax Dwarf galaxy contains six globular clusters. Dragonfly 44, which is much larger than the Fornax Dwarf, could nevertheless be somewhat similar to it.

Ann

[neufer]

[b][color=#0000FF]This image shows the nearby spiral galaxy Messier 101 and the seven newly discovered dwarf galaxies. Image credit: Allison Merritt et al.[/color][/b]

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Jim Shelton of Yale University wrote:

“We have no idea how galaxies like Dragonfly 44 could have formed,” said Abraham.

Perhaps the question is not how that blob of dark matter came together, but why its baryonic matter formed so few stars.

Perhaps it (as well as dark matter dwarf galaxies) evolved out of the debris of Bullet Nebula type collisions.

[bystander]

Puzzle of Strange Galaxy Made of 99.99% Dark Matter Solved
Canary Islands Institute of Astrophysics (IAC) | 2020 Oct 13

Image and amplification (in colour) of the ultra-diffuse galaxy Dragonfly 44 taken
with the Hubble space telescope. Credit: Teymoor Saifollahi and NASA/HST.

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At present, the formation of galaxies is difficult to understand without the presence of a ubiquitous, but mysterious component, termed dark matter. Astronomers have measure how much dark matter there is around galaxies, and have found that it varies between 10 and 300 times the quantity of visible matter. However, a few years ago, the discovery of a very diffuse object, named Dragonfly 44, changed this view. It was found that this galaxy has 10,000 times more dark matter than the stars. Taken back by this finding, astronomers have made efforts to see whether this object is really anomalous, or whether something went wrong in the analysis of the observations. Now we have the answer.

An international team ... has found that the total number of globular clusters around Dragonfly 44 and, therefore, the dark matter content, is much less than earlier findings had suggested, which shows that this galaxy is neither unique nor anomalous. ...

The galaxy Dragonfly 44 was discovered in a deep survey of the Coma cluster, a cluster with several thousand galaxies. From the start, the galaxy was considered remarkable by the researchers because the quantity of dark matter they inferred was almost as much as that in the Milky Way, the equivalent of a billion solar masses.

However, instead of containing around a hundred thousand million stars, as has the Milky Way, DF44 has only a hundred million stars, a thousand times fewer. This means that the amount of dark matter was ten thousand times greater than that of its stars. If this had been true, it would have been a unique object, with almost 100 times as much dark matter as that expected from the number of its stars.

Nevertheless, by an exhaustive analysis of the system of globular cluster around Dragonfly 44, the researchers have detected that the total number of globular clusters is only 20, and that the total quantity of dark matter is around 300 times that of the luminous matter, which means that it is not way outside the normal value for this type of galaxies. ...

The Number of Globular Clusters Around the Iconic UDG DF44
is as Expected for Dwarf Galaxies ~ Teymoor Saifollahi et al

• Monthly Notices of the RAS (online 08 Oct 2020) DOI: 10.1093/mnras/staa3016
• arXiv.org > astro-ph > arXiv:2006.14630 > 25 Jun 2020 (v1), 29 Sep 2020 (v4)