Andromeda chained to the box - dynamical models for M31: bulge and bar

Andromeda is our nearest neighbouring disc galaxy and a prime target for detailed modelling of the evolutionary processes that shape galaxies. We analyse the nature of M31's triaxial bulge with an extensive set of N-body models, which include box/peanut (B/P) bulges as well as initial classical bulges (ICBs). Comparing with Infrared Array Camera (IRAC) 3.6 μm data, only one model matches simultaneously all the morphological properties of M31's bulge, and requires an ICB and a B/P bulge with 1/3 and 2/3 of the total bulge mass, respectively. We find that our pure B/P bulge models do not show concentrations high enough to match the Sérsic index (n) and the effective radius of M31's bulge. Instead, the best model requires an ICB component with mass M^ICB = 1.1 × 10¹⁰ M_⊙ and 3D half-mass radius r_half^ICB = 0.53 kpc (140 arcsec). The B/P bulge component has a mass of M^B/P = 2.2 × 10¹⁰ M_⊙ and a half-mass radius of r_half^B/P = 1.3 kpc (340 arcsec). The model's B/P bulge extends to r^B/P = 3.2 kpc (840 arcsec) in the plane of the disc as does M31's bulge. In this composite bulge model, the ICB component explains the velocity dispersion drop observed in the centre within R < 190 pc (50 arcsec), while the B/P bulge component reproduces the observed rapid rotation and the kinematic twist of the observed zero velocity line. This model's pattern speed is Ω_p = 38 km s^-1 kpc^-1, placing corotation at r_cor = 5.8 kpc (1500 arcsec). The outer Lindblad resonance (OLR) is then at r_OLR = 10.4 kpc, near the 10 kpc ring of M31, suggesting that this structure may be related to the bar's OLR. By comparison with an earlier snapshot, we estimate that M31's thin bar extends to r_bar^thin ∼ 4.0 kpc (1000 arcsec) in the disc plane, and in projection extends to R_bar^thin ∼ 2.3 kpc (600 arcsec).