The Global Stability of M33 in MOND

Abstract

The dynamical stability of disk galaxies is sensitive to whether their anomalous rotation curves are caused by dark matter halos or Milgromian dynamics (MOND). We investigate this by setting up a MOND model of M33. We first simulate it in isolation for 6 Gyr, starting from an initial good match to the rotation curve (RC). Too large a bar and bulge form when the gas is too hot, but this is avoided by reducing the gas temperature. A strong bar still forms in 1 Gyr, but rapidly weakens and becomes consistent with the observed weak bar. Previous work showed this to be challenging in Newtonian models with a live dark matter halo, which developed strong bars. The bar pattern speed implies a realistic corotation radius of 3 kpc. However, the RC still rises too steeply, and the central line-of-sight velocity dispersion (LOSVD) is too high. We then add a constant external acceleration field of 8.4 × 10−12 m s−2 at 30° to the disk as a first-order estimate for the gravity exerted by M31. This suppresses buildup of material at the center, causing the RC to rise more slowly and reducing the central LOSVD. Overall, this simulation bears good resemblance to several global properties of M33, and highlights the importance of including even a weak external field on the stability and evolution of disk galaxies. Further simulations with a time-varying external field, modeling the full orbit of M33, will be needed to confirm its resemblance to observations.

Barred spiral galaxies in modified gravity theories

“…In our simulations, we see that the bar amplitude starts to decrease after a sharp maximum, and then stays almost constant. Observationally, this means that MOND predicts very strong bars for some spiral galaxies, but weakly barred galaxies are also expected (see also Banik et al 2020).…”

“…por has previously been used to investigate polar ring galaxies (Lüghausen et al 2013), shell galaxies (Bílek et al 2015), and the tidal streams of Sagittarius (Thomas et al 2017) and Palomar 5 (Thomas et al 2018). Recently, it was used in hydrodynamical simulations of collapsing gas clouds to naturally yield exponential disc galaxies (Wittenburg et al 2020), and to obtain a quite realistic morphology for M33 after secularly evolving it for 10 Gyr (Banik et al 2020). Galaxy interactions have also been simulated in por with hydrodynamics (Renaud et al 2016) and without it (Bílek et al 2018).…”

“…3 por is the most widely used publicly available N -body solver for MOND. Banik et al (2020) provides further details regarding por and its application to thin disc galaxies, including links to download the algorithms used to prepare the initial conditions for both hydrodynamical and stellaronly thin disc simulations (Section 3.3.1). A user guide for por simulations has recently been published, including details on how to initialize isolated or interacting disc galaxy simulations consisting of only stars or also including hydrodynamics (Nagesh et al 2021).…”

“…We set up a Milgromian disc using a code we make publicly available. 5 The method was previously used to simulate M33 (Banik et al 2020), but we briefly describe it here. We use an adapted version of the Newtonian code Disk Initial Conditions Environment (dice, Perret et al 2014).…”

The weak lensing radial acceleration relation: Constraining modified gravity and cold dark matter theories with KiDS-1000

“…(11). The main exception to this rule is the possible influence by neighbouring mass distributions through the external field effect (EFE), predicted by Milgrom (1983) and studied analytically, observationally and in simulations by Banik & Zhao (2018), Banik et al (2020), Chae et al (2020). Since we explicitly selected isolated galaxies in this work (see Appendix A), this effect is minimised as much as possible.…”