The first scientific light from the Extremely Large Telescope (ELT) is expected in March 2029. One of the instruments being designed for it - NEXUS@SHARP - could become the key to understanding the nature of ultra-diffuse galaxies (UDGs). These are faint and very sparse stellar systems that challenge the generally accepted cosmological model - lambda-CDM (a model combining cold dark matter and the accelerated expansion of the Universe).
Ultra-diffuse galaxies are comparable in size to the Milky Way but contain thousands of times fewer stars. Their existence is poorly consistent with expectations: the size of a galaxy is thought to be determined by its dark matter halo, yet observed UDGs look too puffed up for their halos. To make sense of this, astronomers study globular clusters - dense groups of old stars that are usually brighter and more compact than the UDGs themselves and can serve as beacons for measuring the mass and formation history of galaxies.
The problem is that current spectrographs are not sensitive enough to study globular clusters around distant UDGs in detail. In a paper published in the journal New Astronomy, a group led by Marco Mirabile of the Italian National Institute for Astrophysics (INAF-OAAb) and the Gran Sasso Science Institute (GSSI) proposes a solution. NEXUS@SHARP is a near-infrared multi-object spectrograph that will be able to observe many globular clusters simultaneously with high spatial resolution.
The authors show that with NEXUS@SHARP it will be possible to efficiently characterize the globular cluster population in UDGs at distances of up to 60-70 megaparsecs in just a few hours of observations on the ELT. This will make it possible to measure the velocities of the clusters and their chemical composition, and therefore to obtain independent estimates of the dark halo mass and reconstruct the star formation history.
The researchers consider two scenarios for the formation of UDGs. According to the first, they are failed massive galaxies that stopped forming stars early but retained a massive dark matter halo. Such objects should be red, old, metal-poor and rich in globular clusters. The second scenario: UDGs are puffed-up dwarf galaxies with low mass that for some reason expanded in space. They should be bluer, have a longer star formation history and a dark matter content typical of dwarfs.
By comparing the dynamical mass (from cluster velocities) and the mass estimated from the number of globular clusters, astronomers will be able to distinguish between these scenarios. If both estimates are high and agree, this will support the first scenario; if they are low, the second. If the dynamical mass turns out to be low while the number of clusters is large, this will point to unusually efficient cluster formation under the influence of the environment.
For now NEXUS@SHARP exists only as a concept, and the paper argues the case for building it. If the instrument is built, it will help either confirm or refute the predictions of lambda-CDM for ultra-diffuse galaxies. As the authors note, systematic spectroscopic surveys of globular clusters in such faint systems will become a powerful tool for constraining the formation channels of the most elusive galaxies in the local Universe.