About
eighty-five percent of the matter in the universe is in the form of
dark matter, whose nature remains a mystery. The rest of the matter in
the universe is of the kind found in atoms. Astronomers studying the
evolution of galaxies in the universe find that dark matter exhibits
gravity and, because it is so abundant, it dominates the formation of
large-scale structures in the universe like clusters of galaxies. Dark
matter is hard to observe directly, needless to say, and it shows no
evidence of interacting with itself or other matter other than via
gravity, but fortunately it can be traced by modeling sensitive
observations of the distributions of galaxies across a range of scales.
Galaxies generally reside at the centers of vast clumps of dark
matter called haloes because they surround the clusters of galaxies.
Gravitational lensing of more distant galaxies by dark matter haloes
offers a particularly unique and powerful probe of the detailed
distribution of dark matter. So-called strong gravitational lensing
creates highly distorted, magnified and occasionally multiple images of a
single source; so-called weak lensing results in modestly yet
systematically deformed shapes of background galaxies that can also
provide robust constraints on the distribution of dark matter within the
clusters.
CfA astronomers Annalisa Pillepich and Lars Hernquist and their
colleagues compared gravitationally distorted Hubble images of the
galaxy cluster Abell 2744 and two other clusters with the results of
computer simulations of dark matter haloes. They found, in agreement
with key predictions in the conventional dark matter picture, that the
detailed galaxy substructures depend on the dark matter halo
distribution, and that the total mass and the light trace each other.
They also found a few discrepancies: the radial distribution of the dark
matter is different from that predicted by the simulations, and the
effects of tidal stripping and friction in galaxies are smaller than
expected, but they suggest these issues might be resolved with more
precise simulations. Overall, however, the standard model of dark matter
does an excellent and reassuring job of describing galaxy clustering.
Reference(s):
"Mapping
Substructure in the HST Frontier Fields Cluster Lenses and in
Cosmological Simulations," Priyamvada Natarajan, Urmila Chadayammuri,
Mathilde Jauzac, Johan Richard, Jean-Paul Kneib, Harald Ebeling,
Fangzhou Jiang, Frank van den Bosch, Marceau Limousin, Eric Jullo, Hakim
Atek, Annalisa Pillepich,Cristina Popa, Federico Marinacci, Lars
Hernquist, Massimo Meneghetti, and Mark Vogelsberger, MNRAS 468, 1962, 2017.
