Shell galaxies as laboratories for testing MOND

TL;DR

Shell galaxies provide a unique observational method to test Modified Newtonian Dynamics (MOND) in elliptical galaxies by analyzing shell structures and spectra, extending tests to larger radii where dark matter effects are usually inferred.

Contribution

This paper reviews the use of shell galaxies as a novel way to test MOND, including previous work on shell radii consistency and predictions of undiscovered shells in NGC 3923.

Findings

Shell radii in NGC 3923 are consistent with MOND predictions.

Predicted a large, undiscovered shell at ~200 kpc in NGC 3923.

Shell spectral line profiles have distinctive shapes in MOND.

Abstract

Tests of MOND in ellipticals are relatively rare because these galaxies often lack kinematic tracers in the regions where the MOND effects are significant. Stellar shells observed in many elliptical galaxies offer a promising way to constrain their gravitational field. Shells appear as glowing arcs around their host galaxy. They are observed up to ~100 kpc. The stars in axially symmetric shell systems move in nearly radial orbits. The radial distributions of shell locations and the spectra of stars in shells can be used to constrain the gravitational potential of their host galaxy. The symmetrical shell systems, being especially suitable for these studies, occur in approximately 3% of all early-type galaxies. Hence the shells substantially increase the number of ellipticals in which MOND can be tested up to large radii. In this paper, we review our work on shell galaxies in MOND. We summarize the paper B\'{i}lek et al. (2013), where we demonstrated the consistency of shell radii in an elliptical NGC 3923 with MOND, and the work B\'{i}lek et al. (2014), in which we predicted a giant (~200 kpc), as yet undiscovered shell of NGC 3923. We explain the shell identification method, which was used in these two papers. We further describe the expected shape of line profiles in shell spectra in MOND which is very special due to the direct relation of the gravitational field and baryonic matter distribution (B\'{i}lek et al., 2014, in preparation).