Unravelling stellar populations in the Andromeda Galaxy

Abstract

To understand the history and formation mechanisms of galaxies, it is crucial to determine their current multidimensional structure. In this work, we focus on the properties that characterise stellar populations, such as metallicity and [α/Fe] enhancement. We devised a new technique to recover the distribution of these parameters using spatially resolved, line-of-sight averaged data. Our chemodynamical method is based on the made-to-measure framework and results in an N-body model for the abundance distribution. Following a test on a mock data set we found that the radial and azimuthal profiles were well-recovered, however, only the overall shape of the vertical profile matches the true profile. We applied our procedure to spatially resolved maps of mean [Z/H] and [α/Fe] for the Andromeda Galaxy, using an earlier barred dynamical model of M 31. We find that the metallicity is enhanced along the bar, with a possible maxima at the ansae. In the edge-on view, the [Z/H] distribution has an X shape due to the boxy/peanut bulge; the average vertical metallicity gradient is equal to −0.133 ± 0.006 dex kpc−1. We identify a metallicity-enhanced ring around the bar, which also has relatively lower [α/Fe]. The highest [α/Fe] is found in the centre, due to the classical bulge. Away from the centre, the α-overabundance in the bar region increases with height, which could be an indication of a thick disc. We argue that the galaxy assembly resulted in a sharp peak of metallicity in the central few hundred parsecs and a more gentle negative gradient in the remaining disc, but no [α/Fe] gradient. The formation of the bar leads to the re-arrangement of the [Z/H] distribution, causing a flat gradient along the bar. Subsequent star formation close to the bar ends may have produced the metallicity enhancements at the ansae and the [Z/H] enhanced lower-α ring.

The Panchromatic Hubble Andromeda Treasury. XX. The Disk of M31 is Thick

“…Finally, there are tentative hints of a thickened disk in the 3D chemical structure of M31, as inferred from "made-to-measure" chemo-dynamical modeling of integral field spectroscopy of M31 (Gajda et al 2021). The authors find flaring in the overall metallicity, and a tendency toward larger α-enhancement at larger scale heights, which they argue is consistent with thickening due to a major merger as advocated for by Hammer et al (2018), D'Souza & Bell (2018b), and Bhattacharya et al (2019.…”

The Milky Way's middle-aged inner ring

“…This difference leads to a pronounced horizontal metallicity gradient along the bar's major axis (Wylie et al 2021). The Milky Way (MW) is not unique in this structure, bars with ends that are more metal rich and/or younger than their central regions have been observed in a few other galaxies as well (Seidel et al 2016), including M31 (Gajda et al 2021).…”

Large-scale Hydrodynamical Shocks as the Smoking-gun Evidence for a Bar in M31

“…Moreover, Saglia et al (2018) found that the stellar metallicity is enhanced along the proposed bar structure. Based on the m2m models in Blaña Díaz et al (2018), Gajda et al (2021) constrained the 3D distribution of metallicity and α-enrichment using observed [Z/H] and [α/Fe] maps (Saglia et al 2018), and an X-shaped metallicity distribution is found in the bulge region. These results imply that M31 is a barred galaxy.…”

“…Moreover, Saglia et al (2018) found that the stellar metallicity is enhanced along the proposed bar structure. Based on the m2m models in Blaña Díaz et al (2018), Gajda et al (2021) constrained the 3D distribution of metallicity and α-enrichment using observed [Z/H] and [α/Fe] maps (Saglia et al 2018), and an X-shaped Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.…”