Barred spiral galaxies in modified gravity theories

Abstract

When bars form within galaxy formation simulations in the standard cosmological context, dynamical friction with dark matter (DM) causes them to rotate rather slowly. However, almost all observed galactic bars are fast in terms of the ratio between corotation radius and bar length. Here, we explicitly display an 8σ tension between the observed distribution of this ratio and that in the EAGLE simulation at redshift 0. We also compare the evolution of Newtonian galactic discs embedded in DM haloes to their evolution in three extended gravity theories: Milgromian Dynamics (MOND), a model of non-local gravity, and a scalar-tensor-vector gravity theory (MOG). Although our models start with the same initial baryonic distribution and rotation curve, the long-term evolution is different. The bar instability happens more violently in MOND compared to the other models. There are some common features between the extended gravity models, in particular the negligible role played by dynamical friction − which plays a key role in the DM model. Partly for this reason, all extended gravity models predict weaker bars and faster bar pattern speeds compared to the DM case. Although the absence of strong bars in our idealized, isolated extended gravity simulations is in tension with observations, they reproduce the strong observational preference for ‘fast’ bar pattern speeds, which we could not do with DM. We confirm previous findings that apparently ‘ultrafast’ bars can be due to bar-spiral arm alignment leading to an overestimated bar length, especially in extended gravity scenarios where the bar is already fast.

Fast galaxy bars continue to challenge standard cosmology

“…Our comparison with observations is done in terms of (i) the fraction of galaxies that have bars, and (ii) the pattern speeds of the bars. A similar analysis for EAGLE previously revealed an 8σ tension with ΛCDM (Roshan et al 2021). The main advantage of the present work is a homogeneous analysis of several ΛCDM cosmological simulations using techniques similar to those applied to real galaxies, along with further improvements on the observational side (Cuomo et al 2020).…”

“…After discarding measurements of Ωp with large errors and bars in the unstable regime (R < 1), Cuomo et al (2020) reported that all bars analysed so far with the TW method are compatible with being fast at the 95% confidence level. The small proportion of apparently ultrafast bars (responsible for a mean R < 1 in Table 1) is likely due to bar-spiral arm alignment leading to an overestimated R bar , as discussed in Hilmi et al (2020) and Roshan et al (2021) in the context of Newtonian and extended gravity theories, respectively. While a slight underestimation of R is possible in some cases, the bar must be quite fast intrinsically if it appears ultrafast in a careful analysis.…”

Probing the radial acceleration relation and the strong equivalence principle with the Coma cluster ultra-diffuse galaxies

“…This was a core prediction of the original MOND papers, and one of its most intriguing successes. MOND can also provide possible answers to various other puzzles in galaxy dynamics, such as the prevalence of bulgeless disks (Combes 2014) and of fast bars (Tiret & Combes 2007Roshan et al 2021), or the detailed kinematics of polar ring galaxies (Lüghausen et al 2013). Generally speaking, there now appears to be a clear and direct connection between the baryonic mass distribution and the rotation curve in most disk galaxies, known as the radial acceleration relation (RAR;McGaugh et al 2016;Lelli et al 2017), and this empirical relation is actually indistinguishable from the original MOND prescription (Li et al 2018).…”

The Kennicutt-Schmidt law and the main sequence of galaxies in Newtonian and Milgromian dynamics

“…This leads to differences in their stability and secular evolution, which are also partly driven by the dark matter halo being present in only the Newtonian context (e.g. Banik et al 2020;Roshan et al 2021).…”

“…The external field effect (EFE) predicted by MOND (Bekenstein & Milgrom 1984) and required for consistency with data on Solar neighbourhood wide binaries (Banik & Zhao 2018c; Pittordis & Sutherland 2019) has recently been confirmed at high significance by comparing galaxies in isolated and more crowded environments (Haghi et al 2016;Chae et al 2020). Detailed numerical simulations of disc galaxy secular evolution in MOND have recently been conducted for the case of M33 (Banik et al 2020) and for a Milky Way or M31-like surface density (Roshan et al 2021), while star formation has also been explored with high-resolution simulations (Renaud et al 2016). The possible cosmological context has been explored in e.g.…”