Galaxy clusters are the biggest gravitationally certain constructions within the universe. They’re additionally essentially the most large, containing not solely the celebs and gasoline we see in a whole bunch of galaxies, but in addition the frustratingly invisible darkish matter in and round them.
Within the September 11th Science (paper available here), Massimo Meneghetti (Nationwide Institute of Astrophysics, Italy, and Caltech) and colleagues current a brand new approach of visualizing the unseen darkish matter in clusters. And the outcomes seem to contradict what theorists would have anticipated.
Lumps within the Dough
Theorists have lengthy recognized that darkish matter isn’t clean. As an alternative, like a botched baking mission, the darkish matter halos that encompass galaxies — and galaxy clusters — have lumps, or as astronomers time period them, subhalos.
Like kneaded dough, neither halo nor subhalos keep nonetheless. “[Subhalos] transfer on orbits across the cluster heart, interacting with the bigger halo they’re embedded in, and sometimes colliding and merging with different subhalos,” Meneghetti explains.
Between the a whole bunch of galaxies in a cluster, there ought to be plenty of subhalos — simulations specify what number of we’d anticipate. A few of them may have stars and will even be faintly seen as dwarf galaxies. Others is likely to be completely darkish. Most of them, in any case, could be troublesome to look at.
Meneghetti’s group got here up with a approach round that. As a result of galaxy clusters are so large, they act as gravitational magnifying glasses, lensing the sunshine from background galaxies into arcs or a number of pictures. A galaxy cluster’s mass might create pictures separated by tens of arcseconds, however a subhalo in simply the fitting place within the cluster may create further pictures separated by only some arcseconds.
The group used detailed pictures and information from the Hubble Area Telescope and the European Southern Observatory’s Very Giant Telescope in Chile to look at a number of galaxy clusters, counting up the variety of these small-scale lenses that solely subhalos could make. Then they checked out what cosmological simulations predict.
To the researchers’ astonishment, the simulations predict far fewer halos than have been noticed. And take a look at as they could to elucidate the distinction, there have been no straightforward solutions.
“To me personally, detecting a gnawing hole — an element of 10 discrepancy on this case — between an statement and theoretical prediction could be very thrilling,” says group member Priyamvada Natarajan (Yale).
Frank van den Bosch (additionally at Yale), who was not concerned within the research, calls the outcomes “probably groundbreaking.”
“The authors tackle all of the ‘apparent’ candidates for what may clarify this,” he says. “I agree with them that none of them appear to have the ability to reconcile the massive discrepancy discovered.” He provides that the result’s prone to spawn plenty of follow-up research that study the abundance, construction, and different properties of darkish matter lumps.
The Reverse of Lacking Satellites
By coincidence, the darkish matter “downside” that Meneghetti and colleagues have uncovered is the precise reverse of a decade-old, now-solved predicament often called the “missing satellite problem.”
Earlier on, cosmological simulations had modeled solely the conduct of darkish matter, as that’s what dominates the universe gravitationally. However these simulations predicted that galaxies just like the Milky Manner should have far more satellites. At the same time as telescopes improved and astronomers found further, fainter dwarf galaxies, they weren’t sufficient to make up the distinction. It wasn’t till simulations started taking common ol’ matter under consideration — together with all of the bombastic results of supernovae, jet-throwing black holes, and all the remaining — that the predicted numbers began matching observations.
Now, although, astronomers have the other downside – there are too many subhalos as an alternative of too few. But it surely’s additionally on a wholly totally different scale — galaxy clusters as an alternative of galaxies, large subhalos as an alternative of minuscule ones.
“The lacking satellites downside offers with the small-mass subhalos in galaxies,” Meneghetti says, “whereas right here we’re trying on the highest mass tail of the subhalo distribution.”
Nonetheless, the analogy means that this new “downside” may assist us study extra about darkish matter. “There are a lot of points associated to galaxy formation and evolution in cluster environments within the context of different darkish matter fashions which might be but unexplored,” Meneghetti says, “particularly concerning the interaction between extraordinary and darkish matter.”
For now, there’s extra work to be finished in extending the evaluation to further clusters, to not point out exploring different darkish matter candidates. And within the years to come back, the European Area Company’s Euclid mission, the Vera C. Rubin Observatory, and NASA’s Nancy Grace Roman Area Telescope will supply the chance to probe the darkish matter construction of galaxy clusters as by no means earlier than. Says Meneghetti, “These observations might be a gold mine for us!”