The Orthogonally Aligned Dark Halo of an Edge-on Lensing Galaxy in the Hubble Frontier Fields: A Challenge for Modified Gravity

TL;DR

This study uses a unique gravitational lensing observation of an edge-on galaxy to constrain dark matter halo properties, challenging modified gravity theories that link gravity to baryonic matter orientation.

Contribution

It provides detailed constraints on the shape and orientation of the dark matter halo of an edge-on galaxy using lensing data, testing modified gravity models.

Findings

Dark matter halo is nearly orthogonal to the galaxy plane.

Total mass of the lensing galaxy is about 3×10^11 solar masses.

Dark halo ellipticity is approximately 0.54.

Abstract

We examine a well resolved 8 arcsec lensed image that is symmetrically bent in the middle by an edge-on lenticular galaxy, in the Hubble Frontier Field (HFF) data of MACSJ0416.1-20403. The lengthy image is generated primarily by the large tangential shear of the cluster with a local secondary deflection by the member galaxy out to a limiting radius of about 18 kpc. The lensing lenticular galaxy is also well resolved and evidently lies nearly edge-on in projection. This fortuitous combination of a long arc intersecting an edge on galaxy provides us with an opportunity to place relatively strong constraints on the lensing effect of this galaxy. We can model the stellar lensing contribution using the observed pixels belonging to the galaxy, in 2D, and we add to this a standard parameterised dark halo component. Irrespective of the detailed choice of parameters we obtain a combined total mass of about 3E11 Msun. Depending on the dark halo parameters, the stellar contribution to this is limited to the range 5-15E10 Msun or 20-50 percent of the total mass, in good agreement with the independent stellar mass computed from the photometry of 5E10 Msun for a Chabrier IMF, or 8E10Msun for a Salpeter IMF. The major axis of the DM halo is constrained to be nearly orthogonal to the plane of the galaxy, within a range of about 15 degrees, and with an ellipticity e=015 corresponding to an axis ratio a/c=0.54. We show that these conclusions are very weakly dependent on the model of the cluster, or the additional influence of neighbouring galaxies or the properties of the lensed source. Alternative theories of gravity where the radial dependence is modified to avoid the need for DM are challenged by this finding since generically these must be tied to the baryonic component which here is a stellar disk oriented nearly orthogonally to the lensed image deflection.