Gravitational detection of a low-mass dark satellite at cosmological distance

TL;DR

This paper reports the detection of a low-mass dark satellite galaxy at cosmological distance using gravitational lensing, providing evidence that supports cold dark matter models of hierarchical galaxy formation.

Contribution

It presents the first measurement of the mass function slope of dark satellite galaxies beyond the local universe, aligning observations with cold dark matter predictions.

Findings

Detected a 1.9 x 10^8 M_sun dark satellite galaxy.

Measured the mass function slope as alpha = 1.1 (+0.6/-0.4).

Found the satellite's mass similar to the Sagittarius galaxy.

Abstract

The mass-function of dwarf satellite galaxies that are observed around Local Group galaxies substantially differs from simulations based on cold dark matter: the simulations predict many more dwarf galaxies than are seen. The Local Group, however, may be anomalous in this regard. A massive dark satellite in an early-type lens galaxy at z = 0.222 was recently found using a new method based on gravitational lensing, suggesting that the mass fraction contained in substructure could be higher than is predicted from simulations. The lack of very low mass detections, however, prohibited any constraint on their mass function. Here we report the presence of a 1.9 +/- 0.1 x 10^8 M_sun dark satellite in the Einstein-ring system JVAS B1938+666 at z = 0.881, where M_sun denotes solar mass. This satellite galaxy has a mass similar to the Sagittarius galaxy, which is a satellite of the Milky Way. We determine the logarithmic slope of the mass function for substructure beyond the local Universe to be alpha = 1.1^+0.6_-0.4, with an average mass-fraction of f = 3.3^+3.6_-1.8 %, by combining data on both of these recently discovered galaxies. Our results are consistent with the predictions from cold dark matter simulations at the 95 per cent confidence level, and therefore agree with the view that galaxies formed hierarchically in a Universe composed of cold dark matter.