New Distances Support Idea of Dark Matter–less Galaxies

New distance measurements of two unusual galaxies within the constellation Cetus, the Whale, assist the notion that they’re virtually devoid of darkish matter — the mysterious stuff that constitutes 85% of all gravitating mass within the universe. Astronomers are at a loss to clarify the obvious absence of darkish matter within the two programs.

DF2 and DF4
A survey picture taken with the Dragonfly Telephoto Array exhibits objects inside the discipline of the elliptical galaxy NGC 1052 (heart). Among these objects are DF2 (backside left) and DF4 (high proper); each are darkish matter-deficient galaxies which might be comparable in dimension, luminosity, morphology, globular cluster inhabitants, and velocity dispersion.
P. van Dokkum (Yale University) / STScI / ACS

NGC 1052-DF2 and -DF4 (named after the Dragonfly Telephoto Array in New Mexico that imaged the objects) are prime examples of ultra-diffuse galaxies. They comprise comparatively few stars unfold out over a big quantity, making them onerous to detect. On the sky, the 2 faint smudges are near NGC 1052, the biggest member of a small galaxy group between 63 and 70 million light-years away.

If DF2 and DF4 are at an identical distance, measurements of the speed unfold of their member stars and orbiting globular clusters point out whole galaxy plenty which might be in good settlement with the noticed quantity of stars. This means that their dark matter fraction is a few percent at most, an oddity in comparison with most galaxies whose whole mass is dominated by darkish matter. Another puzzling factor is that the globular clusters of the 2 ultra-diffuse galaxies are exceptionally luminous.

The preliminary findings by the Dragonfly workforce, printed in 2018 and 2019, have been challenged by Ignacio Trujillo (Astrophysical Institute of the Canary Islands) and his colleagues. They argued that the 2 galaxies are a lot nearer, about 42 million light-years away, primarily based on archival observations of the constituent stars. In that case, there can be “plenty of room for dark matter”, the researchers wrote, and the globular clusters would have a traditional luminosity.

DF2, an apparently dark matter-less galaxy
The new Hubble Space Telescope picture of NGC 1052-DF2
Z. Shen et al. / arXiv

However, new, devoted Hubble Space Telescope observations of DF2 and DF4 now appear to assist the unique claims made by the Dragonfly workforce. Hubble imaged many hundreds of particular person red big stars within the outer components of each galaxies. The brightest red big stars in any galaxy have a well-defined luminosity, so measuring the obvious brightness of this “tip of the red giant branch” (TRGB) gives an estimate of the stars’ distance.

Shany Danieli (Yale University) and her colleagues printed preliminary outcomes for DF4 final year in Astrophysical Journal Letters. A workforce led by Zili Chen (additionally at Yale) posted a lot deeper observations of DF2  April ninth on the arXiv preprint server. The TRGB measurements point out a distance of 65.2 million mild years for DF4 and 72.0 million light-years for DF2. The researchers describe the gap as “definitive.”

Zooming in on red giants in DF2
This zoom in on DF2 exhibits resolved red big stars in a background of unresolved blue stars.
Z. Shen et al. / arXiv

However, Trujillo shouldn’t be but satisfied. In specific, he notes that Danieli, Chen, and their colleagues have omitted the innermost components of the 2 galaxies of their evaluation. “These regions contain 50% of the stars,” he says. “They should not have been thrown away so easily.” Trujillo additionally fears that the observations aren’t deep sufficient to confidently determine the tip of the red big department. Moreover, his workforce has claimed the invention of tidal streams of stars streaming away from DF4, suggesting gravitational interactions with one other galaxy, NGC 1035, which is a few 45 million light-years away.

If the bigger distances for the 2 ultra-diffuse galaxies stand the take a look at of time, astronomers must clarify how a galaxy can lose virtually all of its darkish matter. The velocity measurements of DF2 and DF4 would even doubtlessly pose an issue for MOND (Modified Newtonian Dynamics), which tries to clarify away darkish matter by introducing another description of gravity. If MOND is appropriate, each galaxy ought to behave as if it contained hundreds of darkish matter.

MOND theorists notice that the “MONDian” gravitational discipline of NGC 1052 would have an effect on the dynamics of close by DF2 in kind of the best way as to clarify the speed measurements. However, that leaves DF4 unexplained: The new Hubble observations point out that DF2 and DF4 are fairly far other than one another, to allow them to’t each lie within the bigger galaxy’s sphere of affect.

The two latest papers “are a useful addition to the distance discussion using deeper data,” says Trujillo’s colleague Michael Beasley. “However, I am sure that they are not the last word on the matter.” It might take future observations by the James Webb Space Telescope to settle the difficulty as soon as and for all.


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