Shell galaxies as laboratories for testing MOND

摘要

Tests of MOND in elliptical galaxies are relatively rare because they often lack kinematic tracers in the regions where MOND effects are significant. Stellar shells observed in many elliptical galaxies offer a promising way to constrain their gravitational fields. Shells appear as glowing arcs around the host galaxy, with radii observed up to similar 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, if we overcome several problems (e.g., multiple shell generations present in the system, shells missed by observations, blurry shell edges, dynamical friction during the merger), 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 Bilek et al. (Astron. Astrophys. 559, A110 (2013)), where we demonstrated the consistency of shell radii in an elliptical NGC 3923 with MOND, and Bilek et al. (arXiv:1404.1109. 2014), in which we predicted a giant (similar to 200 kpc), yet undiscovered shell of NGC 3923. We explain the shell identification method that was used in these two papers. We further describe the expected shape of line profiles in shell spectra in MOND, which is very special because of the direct relation of the gravitational field and baryonic matter distribution (Bilek et al. 2014, in preparation).