Abstract
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 radiusRGC = 14 kpc. AtRGC = 17–20 kpc, which is the equivalent distance in disc scale lengths of the Sun in the Milky Way disc, we obtainσϕ, 2.5 Gyr = 61 ± 14 km s−1andσϕ, 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.
Highlights
Discs in late-type galaxies contain two distinct dynamical populations: the “cold” thin disc and the “hot” thick disc, as found in the Milky Way (MW; e.g. Gilmore & Reid 1983) and nearby edge-on galaxies (Yoachim & Dalcanton 2006; Comerón et al 2019)
In the RGC = 14−17 kpc bin, Vφ for the high-extinction planetary nebulae (PNe) is in good agreement with that obtained by Quirk et al (2019) for older asymptotic giant branch (AGB) stars, but for the low-extinction PNe it is lower than that of red giant branch (RGB) stars by ∼30 km s−1
We classify the observed sample of PNe based on their measured extinction values into high- and low-extinction PNe which are associated with 2.5 Gyr and 4.5 Gyr parent populations, respectively
Summary
Discs in late-type galaxies contain two distinct dynamical populations: the “cold” thin disc and the “hot” thick disc, as found in the Milky Way (MW; e.g. Gilmore & Reid 1983) and nearby edge-on galaxies (Yoachim & Dalcanton 2006; Comerón et al 2019). It is well known that the velocity dispersion of a disc stellar population increases with age (Strömberg 1925; Wielen 1977). Kinematics of young and old stellar populations are well-traced by high- and low-mass giant stars, respectively, in the MW through their rotational velocity and velocity dispersion In the M 31 disc, different kinematics of younger and older stellar populations are expected to correlate with high- and low-extinction PNe, respectively. We obtain the rotational velocity curve and rotational velocity dispersion for the M 31 disc high- and low-extinction PNe. We assign ages to the two PNe populations by comparing modelled central star properties in Kwitter et al (2012; hereafter Kw+12) to the Miller Bertolami (2016) stellar evolution tracks.
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