Planned Giant Telescope Could Probe Dark Matter in Puffy, Dim Galaxies

A proposed instrument for the Extremely Large Telescope (ELT) could help astronomers weigh the dark matter around some of the strangest galaxies known, according to a new study in the journal New Astronomy. Universe Today reported on the paper, which argues that the instrument could study these galaxies efficiently, with only a few hours of telescope time per target.

The work is a projection of what the instrument could do, not a set of new observations. The ELT's first light is scheduled for March 2029, according to Universe Today, and the instrument in question has been proposed but is not yet confirmed.

The puzzle of ultra-diffuse galaxies

Ultra-diffuse galaxies, or UDGs, are about as large as the Milky Way but can contain thousands of times fewer stars, Universe Today reported. Their stars are spread thin, which makes them dim. They are as faint as dwarf galaxies but sprawl like large ones.

That poses a problem for the leading model of cosmology, known as Lambda-CDM. A galaxy's size is shaped by its dark matter halo, and the halos expected for these galaxies should not produce such puffy systems. The paper's authors say UDGs challenge galaxy formation models in this framework, especially regarding their dark matter content and how they formed. An earlier idea, that exploding stars blew the galaxies' stars outward, has not held up, according to Universe Today.

Why globular clusters are the key

The new paper, titled "SHARP(ening the) view of unresolved globular clusters in ultra diffuse galaxies," is led by Marco Mirabile, a PhD student at the INAF-Astronomical Observatory of Abruzzo and the Gran Sasso Science Institute in Italy.

The team focuses on globular clusters, the dense balls of stars found around all galaxies, including the Milky Way. Universe Today explained why they are useful probes of UDGs:

• They are brighter than the faint galaxies that host them, so they are easier to observe.
• They extend farther out than the galaxy's stars, which helps map the dark matter halo and tighten estimates of its mass.
• Their number scales with halo mass, so counting them offers another route to the halo's mass.
• Their spectra reveal ages and metal content, which show how quickly the cluster's stars formed and so what kind of UDG astronomers are looking at.

The obstacle, the authors write, is that current spectroscopic facilities cannot reach the depth needed to fully characterize globular cluster populations beyond the local neighborhood.

What the proposed instrument would add

The instrument is called NEXUS@SHARP. SHARP stands for Spectrograph for High Angular Resolution Physics, and NEXUS is a multi-object spectrograph designed to work with it. It is a near-infrared, multi-slit instrument proposed for the ELT, whose segmented mirror is 39.3 meters across.

Multi-object spectrographs, which can capture data from many targets at once, already exist on telescopes such as Keck, the VLT, Gemini and the James Webb Space Telescope, Universe Today noted. The authors argue that NEXUS would combine this ability with high spatial resolution and the ELT's size. They say it could characterize globular cluster populations in UDGs in different environments, out to 60 to 70 Mpc (megaparsecs), with a few hours of ELT time.

That would allow precise measurements of the clusters' motions and stellar populations, the authors write. In turn, that would support solid estimates of a galaxy's mass and help distinguish between competing explanations for how UDGs formed.

Two competing explanations

One scenario treats UDGs as failed massive galaxies that stopped forming stars early. They kept the modest stellar mass of a dwarf galaxy but sit inside the heavier dark matter halo typical of a regular galaxy. The authors say such galaxies would look red, old and metal-poor, be dominated by dark matter and host unusually rich globular cluster populations.

Why it matters

For general readers, the study is a look at how a future telescope might be used on a long-standing question: what dark matter is and whether the standard cosmological model can explain oddball galaxies. Universe Today notes that scientists still do not know what dark matter is, and that some doubt it exists at all.

Nothing in the paper changes what is known about UDGs today. Its value is in showing that a few hours of observing time per galaxy could make a large survey of globular clusters practical, which a handful of measurements around a few galaxies cannot do. Whether that happens depends on the instrument being built, as Universe Today put it, and on the telescope starting work as scheduled.

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Around Prague, based on reporting by Universe Today