Sculpting Andromeda – made-to-measure models for M31’s bar and composite bulge: dynamics, stellar and dark matter mass

Abstract

The Andromeda galaxy (M31) contains a box/peanut bulge (BPB) entangled with a classical bulge (CB) requiring a triaxial modelling to determine the dynamics, stellar and dark matter mass. We construct made-to-measure models fitting new VIRUS-W IFU bulge stellar kinematic observations, the IRAC-3.6µm photometry, and the disc's H I rotation curve. We explore the parameter space for the 3.6µm mass-to-light ratio (Υ 3.6 ), the bar pattern speed (Ω p ), and the dark matter mass in the composite bulge (M B DM ) within 3.2 kpc. Considering Einasto dark matter profiles, we find the best models for Υ 3.6 = 0.72±0.02 M L −1 , M B DM = 1.2 +0.2 −0.4 × 10 10 M and Ω p = 40 ± 5 km s −1 kpc −1 . These models have a dynamical bulge mass of M B dyn =4.25 +0.10 −0.29 ×10 10 M including a stellar mass of M B =3.09 +0.10 −0.12 ×10 10 M (73%), of which the CB has M CB =1.18 +0.06 −0.07 × 10 10 M (28%) and the BPB M BPB =1.91 ± 0.06 × 10 10 M (45%). We also explore models with NFW haloes finding that, while the Einasto models better fit the stellar kinematics, the obtained parameters agree within the errors. The M B DM values agree with adiabatically contracted cosmological NFW haloes with M31's virial mass and radius. The best model has two bulge components with completely different kinematics that only together successfully reproduce the observations (µ 3.6 , υ los , σ los , h3, h4). The modelling includes dust absorption which reproduces the observed kinematic asymmetries. Our results provide new constraints for the early formation of M31 given the lower mass found for the classical bulge and the shallow dark matter profile, as well as the secular evolution of M31 implied by the bar and its resonant interactions with the classical bulge, stellar halo and disc.

Stellar populations of the central region of M 31

“…(6) and Table 4; the dotted red and green lines the assumed profiles for the CB and B/P components, respectively. The blue dashed lines show the age and M/L V profiles obtained using the age values denoted with "20%" in Table 4. outer region the age of the B/P in the model of Blaña Díaz et al (2018) is much smaller than previously assumed (7 or 5 Gyr, see Table 4) to reproduce the observed values of the age and M/L V .…”

“…In order to interpret the features visible in the maps of the Lick indices presented below, we took the distribution of the dust in M 31 into account. Blaña Díaz et al (2018) have produced a map projected onto the plane of the sky of the ratio R dust of the three-dimensional model luminosities measured with and without dust using the dust maps of Draine et al (2014), converted to A V following Draine & Li (2007). According to this map, the light integrated along the near side of M 31 is dominated by the outer part of the disk, since the rest is highly absorbed (R dust < 0.8).…”

“…It is worth comparing the properties of the M 31 B/P bulge we discussed above with those of the MW bulge. To start, the MW has a strong B/P shape (Wegg & Gerhard 2013), while this is not the case for M 31 (Blaña Díaz et al 2018). The vertical metallicity gradient in the MW is ≈−0.4 dex kpc −1 (Gonzalez et al 2011) and is believed to be due to the fact that stars with higher metallicities are concentrated more to the Galactic plane and contribute less at high latitudes (Zoccali et al 2008;Ness et al 2013;Portail et al 2017b).…”

“…However, inside this region much of what Kormendy & Bender (1999) referred to as the bulge component is, in reality, the bar plus B/P structure. As a consequence, the CB component of Blaña Díaz et al (2018) model has a much shorter scale length than that of Kormendy & Bender (1999). Figure 20 shows the ratios BP/CB between the projected V-band luminosity densities, taking into account dust attenuation, of these two components for the four regions shown in Fig.…”

“…Furthermore, only determining the ages and metallicities of the stellar populations of the bar, the classical and B/P bulge can clarify when these different structures formed and constrain their mass-to-light ratios. Presently, the modeling approach of Blaña Díaz et al (2018) assumes that they all have the same uniform value and this assumption needs verification. Finally, a plausible formation scenario should emerge from a joint dynamical and stellar population modeling approach.…”