MOND implications for spectral line profiles of shell galaxies: shell formation history and mass-velocity scaling relations

Abstract

Context. Many ellipticals are surrounded by round stellar shells probably stemming from minor mergers. A new method for constraining gravitational potential in elliptical galaxies has recently been suggested. It uses the spectral line profiles of these shells to measure the circular velocity at the edge of the shell and the expansion velocity of the shell itself. MOND is an alternative to the dark matter framework aiming to solve the missing mass problem. Aims. We study how the circular and expansion velocities behave in MOND for large shells. Methods. The asymptotic behavior for infinitely large shells is derived analytically. The applicability of the asymptotic results for finitely sized shells is studied numerically on a grid of galaxies modeled with Sérsic spheres. Results. Circular velocity settles asymptotically at a value determined by the baryonic mass of the galaxy forming the baryonic TullyFisher relation known for disk galaxies. Shell expansion velocity also becomes asymptotically constant. The expansion velocities of large shells form a multibranched analogy to the baryonic Tully-Fisher relation, together with the galactic baryonic masses. For many -but not all -shell galaxies, the asymptotic values of these two types of velocities are reached under the effective radius. If MOND is assumed to work in ellipticals, then the shell spectra allow many details of the history to be revealed about the formation of the shell system, including its age. The results pertaining to circular velocities apply to all elliptical galaxies, not only those with shells.

From galactic bars to the Hubble tension: weighing up the astrophysical evidence for Milgromian gravity

“…A more uniform distribution of shell radii would be expected in the absence of CDM, which is broadly more in line with observations of NGC 3923 (Bílek et al 2016). More information can be gained by taking spectra and determining the LOS velocity distribution of the shells, which is expected to reveal a quadruply peaked profile (Bílek et al 2015b).…”

Type I Shell Galaxies as a Test of Gravity Models

“…Ebrova (2013) used the shell kinematics to reconstruct the parameters of the potential of the shell galaxies from the simulated data and extended its usage as an independent tool to determine the distribution of dark matter in these galaxies up to radii ≈ 100 kpc. Bílek et al (2013Bílek et al ( , 2014aBílek et al ( , 2015 pioneered the field by using these systems as a powerful tool to constrain MOdified Newtonian Dynamics (MOND) on galactic scales. In addition, observational evidence for the action of dynamical friction that is due to the expansive dark matter halos surrounding galaxies has been suggested as a powerful test for the existence of dark matter (Kroupa 2015).…”

“…Studying dynamics of giant elliptical galaxies is difficult because we lack kinematic tracers at large radii, where there is almost no way for a direct measurement of the gravitational potential (Richtler et al 2008;Samurović 2014;Bílek et al 2015). Shell galaxies are elliptical or S0 galaxies surrounded by faint arc-like structures made of stars.…”

The Newtonian and MOND dynamical models of NGC 5128: Investigation of the dark matter contribution

“…the work of Angus et al (2014) where the cosmological particle-mesh N-body code was used to investigate the feasibility of structure formation in a framework involving MOND and light sterile neutrinos, which means that an additional, hypothetical dark particle is needed. It is worth noting that an another interesting line of investigation of MOND is that based on the study of shell galaxies and promising results have been obtained (Bílek et al 2015) which may lead to further tests of the DM/MOND dichotomy in the future. Therefore, as will be shown below, since the number of early-type galaxies studied using MOND is still small, it is important to study as many interesting objects as possible, and NGC 5128 being the nearest large elliptical galaxy is certainly worth investigating.…”