Aims. We continue the analysis of the data set of our spectroscopic observation campaign of M 31, whose ultimate goal is to provide an understanding of the three-dimensional structure of the bulge, its formation history, and composition in terms of a classical bulge, boxy-peanut bulge, and bar contributions. Methods. We derive simple stellar population (SSP) properties, such as age metallicity and α-element overabundance, from the measurement of Lick/IDS absorption line indices. We describe their two-dimensional maps taking into account the dust distribution in M 31. Results. We found 80% of the values of our age measurements are larger than 10 Gyr. The central 100 arcsec of M 31 are dominated by the stars of the classical bulge of M 31. These stars are old (11−13 Gyr), metal-rich (as high as [Z/H] ≈ 0.35 dex) at the center with a negative gradient outward and enhanced in α-elements ([α/Fe]≈ 0.28±0.01 dex). The bar stands out in the metallicity map, where an almost solar value of [Z/H] (≈0.02 ± 0.01 dex) with no gradient is observed along the bar position angle (55.7 deg) out to 600 arcsec from the center. In contrast, no signature of the bar is seen in the age and [α/Fe] maps, which are approximately axisymmetric, delivering a mean age and overabundance for the bar and boxy-peanut bulge of 10-13 Gyr and 0.25-0.27 dex, respectively. The boxy-peanut bulge has almost solar metallicity (−0.04 ± 0.01 dex). The mass-to-light ratio of the three components is approximately constant at M/L V ≈ 4.4−4.7 M /L . The disk component at larger distances is made of a mixture of stars, as young as 3-4 Gyr, with solar metallicity and smaller M/L V (≈3 ± 0.1 M /L ).Conclusions. We propose a two-phase formation scenario for the inner region of M 31, where most of the stars of the classical bulge come into place together with a proto-disk, where a bar develops and quickly transforms it into a boxy-peanut bulge. Star formation continues in the bulge region, producing stars younger than 10 Gyr, in particular along the bar, thereby enhancing its metallicity. The disk component appears to build up on longer timescales.

We examine the effects of gas-expulsion on initially substructured distributions of stars. We perform N-body simulations of the evolution of these distributions in a static background potential to mimic the gas. We remove the static potential instantaneously to model gas-expulsion. We find that the exact dynamical state of the cluster plays a very strong role in affecting a cluster's survival, especially at early times: they may be entirely destroyed or only weakly affected. We show that knowing both detailed dynamics and relative star-gas distributions can provide a good estimate of the postgas expulsion state of the cluster, but even knowing these is not an absolute way of determining the survival or otherwise of the cluster.

Aims. As the nearest large spiral galaxy, M31 provides a unique opportunity to learn about the structure and evolutionary history of this galaxy type in great detail. Among the many observing programs aimed at M31 are microlensing studies, which require good three-dimensional models of the stellar mass distribution. Possible non-axisymmetric structures like a bar need to be taken into account. Due to M31's high inclination, the bar is difficult to detect in photometry alone. Therefore, detailed kinematic measurements are needed to constrain the possible existence and position of a bar in M31. Methods. We obtained ≈ 220 separate fields with the optical IFU spectrograph VIRUS-W, covering the whole bulge region of M31 and parts of the disk. We derive stellar line-of-sight velocity distributions from the stellar absorption lines, as well as velocity distributions and line fluxes of the emission lines Hβ, [OIII] and [NI]. Our data supersede any previous study in terms of spacial coverage and spectral resolution. Results. We find several features that are indicative of a bar in the kinematics of the stars, we see intermediate plateaus in the velocity and the velocity dispersion, and correlation between the higher moment h3 and the velocity. The gas kinematics is highly irregular, but is consistent with non-triaxial streaming motions caused by a bar. The morphology of the gas shows a spiral pattern, with seemingly lower inclination than the stellar disk. We also look at the ionization mechanisms of the gas, which happens mostly through shocks and not through starbursts.

Context. The age–velocity dispersion relation is an important tool to understand the evolution of the disc of the Andromeda galaxy (M 31) in comparison with the Milky Way. Aims. We use planetary nebulae (PNe) to obtain the age–velocity dispersion relation in different radial bins of the M 31 disc. Methods. We separate the observed PNe sample based on their extinction values into two distinct age populations in the M 31 disc. The observed velocities of our high- and low-extinction PNe, which correspond to higher- and lower-mass progenitors, respectively, are fitted in de-projected elliptical bins to obtain their rotational velocities, Vϕ, and corresponding dispersions, σϕ. We assign ages to the two PN populations by comparing central-star properties of an archival sub-sample of PNe, that have models fitted to their observed spectral features, to stellar evolution tracks. Results. For the high- and low-extinction PNe, we find ages of ∼2.5 and ∼4.5 Gyr, respectively, with distinct kinematics beyond a deprojected radius RGC = 14 kpc. At RGC = 17–20 kpc, which is the equivalent distance in disc scale lengths of the Sun in the Milky Way disc, we obtain σϕ,  2.5 Gyr = 61 ± 14 km s−1 and σϕ,  4.5 Gyr = 101 ± 13 km s−1. The age–velocity dispersion relation for the M 31 disc is obtained in two radial bins, RGC = 14–17 and 17–20 kpc. Conclusions. The high- and low-extinction PNe are associated with the young thin and old thicker disc of M 31, respectively, whose velocity dispersion values increase with age. These values are almost twice and three times that of the Milky Way disc stellar population of corresponding ages, respectively. From comparison with simulations of merging galaxies, we find that the age–velocity dispersion relation in the M 31 disc measured using PNe is indicative of a single major merger that occurred 2.5–4.5 Gyr ago with an estimated merger mass ratio ≈1:5.

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.