News from ‘El Gordo’: Study suggests dark matter may have collisional properties after all

Opposite to what’s established by the usual mannequin, dark matter could certainly be self-interacting. This was the conclusion of a chunk of analysis published in Astronomy & Astrophysics and carried out by Riccardo Valdarnini of SISSA’s Astrophysics and Cosmology group. Utilizing numerical simulations, the examine analyzed what occurs inside “El Gordo” (actually “The Fats One” in Spanish), an enormous cluster merger seven billion gentle years away from us.

The calculations indicated that on this cluster the noticed bodily separation between the factors of most density of Darkish Matter and people of the opposite mass parts may be defined utilizing the so-called SIDM (Self-Interacting dark matter) mannequin, versus the usual one. This analysis makes an necessary contribution in favor of the SIDM mannequin, based on which dark matter particles alternate vitality by collisions, with fascinating astrophysical repercussions.

‘El Gordo’: A huge cosmic construction for the examine of dark matter

“Based on the currently-accepted normal cosmological mannequin, the current baryonic matter density of the universe can account for under 10% of its total matter content material. The remaining 90% is within the type of Darkish Matter,” explains Valdarnini, writer of the analysis.

“It’s typically thought that this matter is non baryonic and made from chilly collisionless particles, which reply solely to gravity. Therefore the identify ‘Chilly Darkish Matter’ (CDM). Nonetheless, there are nonetheless a variety of observations which haven’t but been defined utilizing the standard model,” says the researcher. “To reply these questions, a number of authors recommend an alternate mannequin, referred to as SIDM.”

Proving the collisional properties of dark matter and, extra typically, different theories to the usual cosmological mannequin one, could be very sophisticated. He explains, “There are, nonetheless, distinctive laboratories that may show very helpful for this goal, many gentle years away from us. These are the large galaxy clusters, gigantic cosmic constructions that, upon collision, decide probably the most energetic occasions for the reason that Huge Bang.

“With a mass of about 10¹⁵ solar masses, El Gordo is likely one of the largest galaxy clusters we all know. On account of its peculiarities, El Gordo has been the topic of quite a few research, each theoretical and observational.”

Darkish matter may very well be collisional

Based on the usual paradigm, throughout a cluster merger the conduct of the collisional fuel mass element will differ from that of the opposite two parts—galaxies and dark matter. On this state of affairs, the fuel will dissipate a part of its preliminary vitality.

“For this reason, after the collision, the height of fuel mass density will lag behind these of dark matter and galaxies,” explains Valdarnini. With the SIDM mannequin, nonetheless, a peculiar phenomenon needs to be noticed, specifically the bodily separation of dark matter centroids—its most density factors—from these of different mass parts with peculiarities that signify a real “Signature of SIDM fashions.” Based on observations, that is precisely what occurs inside “El Gordo.”

Observing El Gordo

“Allow us to begin with observations,” explains Valdarnini. El Gordo consists of two huge subclusters, respectively denominated northwestern (NW) and southeastern (SE). The X-ray picture of the “El Gordo” cluster exhibits a single X-ray emission peak within the SE subcluster and two faint tails elongated past the X-ray peak. A noteworthy function is the height location of the totally different mass parts.

At variance with what may be seen within the Bullet Cluster, one other necessary instance of a colliding cluster, the X-ray peak precedes the SE dark matter peak. Furthermore, the Brightest Cluster Galaxy (BCG) is just not solely trailing the X-ray peak, nevertheless it additionally seems to be spatially offset from the SE mass centroid. One other notable facet may be seen within the NW cluster, the place the galaxy quantity density peak is spatially offset from the corresponding mass peak.

Analysis findings recommend Collisional Darkish Matter as a proof for the phenomena noticed in ‘El Gordo’

With a view to clarify his findings and validate the SIDM fashions, Valdarnini used a big set of so-called N-body/hydrodynamical simulations. Thus, he carried out a scientific examine aimed toward reproducing the observational options of “El Gordo.”

“Probably the most important results of this simulation examine is that the relative separations noticed between the totally different mass centroids of the ‘El Gordo’ cluster are naturally defined if the dark matter is self-interacting,” states Valdarnini.

“Because of this, these findings present an unambiguous signature of a dark matter conduct that reveals collisional properties in a really energetic high-redshift cluster collision. There are, nonetheless, inconsistencies, because the SIDM cross part values obtained from these simulations are larger than current higher limits, that are of order unity at cluster scales.

“This means that current SIDM fashions needs to be thought-about as solely a low order approximation, and that the underlying bodily processes that describe the interplay of dark matter in main cluster mergers are extra complicated than may be adequately represented by the commonly-assumed strategy primarily based on the scattering of dark matter particles.

“The examine makes a compelling case for the potential of self-interacting dark matter between colliding clusters as a substitute for the usual collisionless dark matter paradigm.”