Fast galaxy bars continue to challenge standard cosmology

Abstract

Many observed disc galaxies harbour a central bar. In the standard cosmological paradigm, galactic bars should be slowed down by dynamical friction from the dark matter halo. This friction depends on the galaxy’s physical properties in a complex way, making it impossible to formulate analytically. Fortunately, cosmological hydrodynamical simulations provide an excellent statistical population of galaxies, letting us quantify how simulated galactic bars evolve within dark matter haloes. We measure bar lengths and pattern speeds in barred galaxies in state-of-the-art cosmological hydrodynamical simulations of the IllustrisTNG and EAGLE projects, using techniques similar to those used observationally. We then compare our results with the largest available observational sample at z = 0. We show that the tension between these simulations and observations in the ratio of corotation radius to bar length is 12.62σ (TNG50), 13.56σ (TNG100), 2.94σ (EAGLE50), and 9.69σ (EAGLE100), revealing for the first time that the significant tension reported previously persists in the recently released TNG50. The lower statistical tension in EAGLE50 is actually caused by it only having 5 galaxies suitable for our analysis, but all four simulations give similar statistics for the bar pattern speed distribution. In addition, the fraction of disc galaxies with bars is similar between TNG50 and TNG100, though somewhat above EAGLE100. The simulated bar fraction and its trend with stellar mass both differ greatly from observations. These dramatic disagreements cast serious doubt on whether galaxies actually have massive cold dark matter haloes, with their associated dynamical friction acting on galactic bars.

The High Fraction of Thin Disk Galaxies Continues to Challenge ΛCDM Cosmology

“…Consequently, the angular momentum problem persists in the hierarchical cosmological ΛCDM framework and is unlikely to be solved by improving the resolution. This conclusion can also be reached independently through observed galaxy bars being fast with no sign of slowdown through Chandrasekhar dynamical friction on the hypothetical dark matter halo (Roshan et al 2021b). ΛCDM also faces many problems on other scales (Kroupa 2012(Kroupa , 2015Pawlowski 2021;Banik & Zhao 2022).…”

“…In addition, secular processes like disk-halo angular momentum exchange could also drive the formation of significant bars and bulges in ΛCDM (Athanassoula 2002;Sellwood et al 2019), but perhaps not in the real universe (Banik et al 2020;Roshan et al 2021a) where the fraction of bars differs substantially from ΛCDM expectations (Reddish et al 2021). There is also a highly significant discrepancy between the pattern speeds of bars in observations and in ΛCDM simulations, where the results seem to have converged with respect to the ratio of bar length to corotation radius (Roshan et al 2021b). The tension is caused by the fact that bars are expected to be slowed down by dynamical friction with the dark matter halo, but observed bars are fast.…”

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

“…This argument is statistical in nature, leaving plenty of room for some galaxies to have fast bars in ΛCDM. Indeed, many such cases are evident in figures 5 and 6 of Roshan et al (2021b). It is therefore not completely surprising that some studies found fast bars in the ΛCDM paradigm (e.g.…”

“…The final result of Roshan et al (2021b) is their figure 8, reproduced here as our Figure 18. The different ΛCDM simulations all give consistent results, but the region they converge on is strongly excluded observationally.…”

“…To check if the EAGLE100-based falsification of ΛCDM persists with more recent and higher resolution simulations, Roshan et al (2021b) analysed all z = 0 disc galaxies in EAGLE50, EAGLE100, TNG50, and TNG100 using the TW method, thereby allowing a direct comparison with measurements of R in real galaxies. Their figure 2 illustrates the steps involved in determining R b and Ωp for a single simulated galaxy with identifier number 229935 in TNG50.…”