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Chapter 1 Introduction

5.4 Discussion

5.4 Discussion

5.4.1 Interpreting the abundance gradients in M 31

We find that the high- and low-extinction PNe are both kinematically (Paper II) and chemically distinct (Section 5.3.2), forming the dynamically colder thin disc and dynamically hotter thicker disc respectively. The piece-wise fits to the radial abundance measurements in M 31 allow for the interpretation of the abundance gradients at different radial ranges. With both argon and oxygen abundance measurement, we also check for the influence of surface oxygen modification in the AGB phase on the radial abundance gradients.

For the thin disc, the radial distribution of oxygen abundances from individual PNe (Fig-ure 5.10) and also for the median oxygen abundances (Fig(Fig-ure 5.11) are best fitted by three line-segments showing a trough in oxygen abundances values at RGC∼10 kpc and a crest at RGC∼ 21 kpc. The trough is also seen in the radial distribution of median argon abundances values at sim-ilar radial ranges (Figure 5.13) but is not prominent enough to appear in the argon abundances from individual PNe (Figure 5.12). This trough corresponds with the 10 kpc star-forming ring in M 31 (Dalcanton et al. 2015) and indicates that the thin disc stellar population in this ring is less metal-rich than the adjacent radial regions. Williams et al. (2017) from isochrone-fitting of resolved stars in theHubble Space Telescopeobservations in the Pan-Hubble Andromeda Trea-sury (PHAT- Dalcanton et al. 2012) also found in some of their isochrone fits (see their Figure 20) that the 10 kpc star-forming ring in M 31 has stars with lower metallicity than the adjacent radial regions.

The crest in the thin disc radial distribution of oxygen abundance measurements does not occur in the argon abundance distributions. There is no influence from any substructure PNe at RGC ∼ 21 kpc nor any star forming-ring. We thus check if this is an effect of surface oxygen modification in AGB stars effecting the PN oxygen abundances that influences the abundance gradients in the M 31 thin disc. The top panel of Figure 5.14 shows the galactocentric radial dis-tribution of log(Ar/O) values for the high-extinction thin disc PNe in M 31. The oxygen-enriched PNe appear only around RGC=14–23 kpc while the oxygen-depleted PNe appear intermittently at various radial distances within 18 kpc. Oxygen enrichment thus influences the fitted radial oxygen abundance gradients for the thin disc PNe. Unlike in the MW (Stanghellini & Haywood 2018), even the median oxygen abundance gradient is influenced by oxygen enrichment and thereby does not reflect the ISM conditions in the RGC=14–23 kpc radial range. We thus consider the median argon abundance gradients of−0.013±0.003 and−0.016±0.003 in RGC=2.31–13.06 kpc and RGC=13.89–27.69 kpc respectively as the abundance gradients for the thin disc of M 31.

For the thick disc, the radial distribution of oxygen abundances from individual PNe (Fig-ure 5.10) is best fit by a single line for the entire radial range with a positive slope (Table 5.3).

However, the radial distribution of median oxygen abundances (Figure 5.11) are best fitted by three line-segments showing a trough in oxygen abundances values at RGC ∼ 8 kpc but with a metallicity gradient consistent with being flat (Table 5.3) in the RGC=9.56–28.78 kpc radial range. For the radial distribution of argon abundances from individual PNe in the M 31 thick disc (Figure 5.12), the entire radial range is best fit by a single line but having a negative slope that is consistent with being flat (Table 5.4). However, the radial distribution of median

ar-Figure 5.14: The galactocentric radial distribution of log(Ar/O) values for the [top] high- and [bottom] low-extinction PNe in M 31. The green and yellow zones are the same as in Figure 5.7.

PNe with log(Ar/O) values above and below the green band in this plot are oxygen depleted and enriched respectively (as also marked in Figure 5.7).

gon abundances (Figure 5.13), similar to the median oxygen abundances, shows the trough at RGC∼ 8 kpc and is best fitted by two linear functions. The rise in abundances from RGC∼ 8–10 kpc, captured by the middle segment in the median oxygen abundance radial distribution, is not captured in the median argon abundances simply due to lower number of PNe with argon abun-dances in this radial range. A slight trough at RGC ∼ 8 kpc is also observed in the photometric metallicity ([M/H]) gradient calculated by Gregersen et al. (2015) from fitting resolved∼ 4 Gyr old RGB stars in the PHAT footprint (see their Figure 9) but may be negligible within errors.

Given its appearance in the median argon abundance distribution, this trough is a reflection of the ISM conditions at the epoch of formation of the thick disc PNe. In Section 5.4.3, we discuss the possible origin of this through with respect to the major merger scenario of M 31.

The argon abundance gradient beyond RGC ∼ 9 kpc is negative (Table 5.4) and thus differs slightly from the flat median oxygen abundance gradient. As seen in the lower panel of Fig-ure 5.14, oxygen-enriched PNe in the thick disc appear mainly beyond RGC=18 kpc while the oxygen-depleted ones appear intermittently at various radial distances. The slightly negative ar-gon abundance gradient appears as flat in the oxygen abundance gradient since the thick disc PNe

5.4 Discussion 117

beyond RGC=18 kpc have so low intrinsic abundances that their surface oxygen is enriched in the TDU process (Ventura et al. 2017). Oxygen enrichment thus influences the fitted radial oxygen abundance gradients also for the M 31 thick disc PNe and thus does not reflect the ISM con-ditions in the radial range beyond RGC=18 kpc. We thus consider the median argon abundance gradients of−0.02±0.012 and−0.009±0.007 in RGC=1.94–8.61 kpc and RGC=8.8–28.78 kpc respectively as the abundance gradients for the thick disc of M 31. Thus, compared to previous attempts at fitting the abundance gradient in M 31 (Sanders et al. 2012; Kwitter et al. 2012; Pe˜na

& Flores-Dur´an 2019), where the oxygen abundance was found to be near-flat for the complete PN sample, we find a steeper abundance gradient for the outer thin disc at RGC=13.89–27.69 kpc, while the near-flat abundance gradient is found for the outer thick disc at RGC=8.8–28.78 kpc.

5.4.2 The radial elemental abundance distribution and the merger sce-nario in M 31

In a minor merger scenario in M 31 as advocated by Fardal et al. (2013), a satellite galaxy (mass ratio ∼ 1:20) infalls along the giant stream on to the M 31 disc ∼1 Gyr ago. Such a satellite however would not disrupt the thicker disc by the velocity dispersion measured in Paper II and would additionally not form a thin disc (Martig et al. 2014). Following the major merger scenario described by Hammer et al. (2018), however, the pre-merger disc in M 31 would be perturbed by the a massive satellite (mass ratio> 1:4.5) in a highly retrograde orbit. The first pericenter passage of the satellite takes place∼ 7 Gyr ago while the second pericenter passage occurs∼ 3 Gyr ago and the satellite then has multiple passages in the inner regions of M 31 before finally coalescing with the bulge. The thin disc is formed from the gas brought in by the satellite along with a burst of star formation.

The observed thin disc PNe are∼2.5 Gyr old (or younger; Paper II) and likely capture the thin disc during its formation while the major merger is still ongoing in the inner regions and soon after. While Hammer et al. (2018) do not analyse the radial abundance gradients in their simula-tions, the secular evolution of a thin disc after formation has been shown in other cosmological models (e.g. Gibson et al. 2013; Zinchenko et al. 2015) to result in a negative radial abundance gradient, consistent with the observed negative abundance gradient for the thin disc PNe.

A near-flat abundance gradient, as observed here for the M 31 thick disc, has been seen in cosmological simulations of merging galaxies with merger mass ratio≥1:10 (Tissera et al. 2019).

Zinchenko et al. (2015) studied the effect of mergers on the radial elemental abundance profiles of MW mass galaxies using N-body simulations (no new star formation). They found that the amount of flattening of the radial abundance gradient at large radii depends on the mass and inclination of the in-falling satellite, with flatter gradients observed for more massive mergers.

Furthermore, chemodynamical simulations of merging galaxies (e.g. Perez et al. 2011) find that gas from the merging satellite galaxy falls into the center of the massive host galaxy during the first pericenter passage, lowering the mean-metallicity of the stars in the central regions of the host galaxy which are formed soon after. If the first pericenter passage occurred∼7 Gyr ago as predicted by Hammer et al. (2018), these lower elemental abundance stars in the central regions of M 31 would appear as the low-extinction PNe in M 31 which are older than∼4.5 Gyr, thereby

lowering the central abundance measured from the low-extinction PNe and resulting in flatter abundance gradients. It is thus likely that the thick disc of M 31 exhibits the near-flat abundance gradient as a result of the recent massive merger.

It is possible that the trough observed at RGC ∼ 8 kpc results from stars from the satellite galaxy, which would be more metal poor compared to the pre-merger M 31 disc PNe following the mass-metallicity relation (e.g. Zahid et al. 2017). The elemental abundance distribution from PNe thus acts as constraints for merger simulations in M 31. While the fairly major merger simulations by Hammer et al. (2018) do predict the formation of distinct thin and thick discs as observed, predictions of the abundance gradients from such simulations must conform to the observed values.

5.4.3 Inferences on chemical evolution of galaxies

Simulations of chemical evolution in galaxies make predictions on the variation of the radial abundance gradient over time depending on the choice of physical mechanisms, particularly feedback prescriptions, that govern the enrichment of elements into the ISM (e.g. Gibson et al.

2013; Moll´a et al. 2019). In secular evolution of galaxy discs, such simulations generally predict either an initial flat gradient that steepens over time or an initial steep one that flattens over time (Gibson et al. 2013). These predictions can be constrained from radial abundance gradients measurements of stars formed in different epochs in a galaxy, as carried out using the PNe in the MW by Stanghellini & Haywood (2018).

In the case of M 31, the thick disc PNe have a flatter abundance gradient than the thin disc PNe and provide two epochs of star formation where the chemical evolution of M 31 can be constrained. The present-day epoch for such comparisons is provided by the abundance gradient from direct oxygen abundance measurements in HII regions in M 31. This is found by Esteban et al. (2020) within RGC ∼8–18 kpc to be−0.014±0.01 dex, consistent with both the thin and thick disc PN abundances. Thus constraints to chemical evolution models of M31 are subject to a more accurate measurement of the direct oxygen abundance gradient for HII regions in M 31.

However, the abundance gradients from the PNe (both thin and thick disc) and HII regions are flatter than predicted by secular evolution models of galaxy evolution by Gibson et al. (2013), regardless of the enrichment prescriptions used. Since merger events typically flatten radial metallicity gradients, the flat metallicity gradient of M 31 compared to secular evolution models underscores the influence of the recent merger on the radial metallicity gradient of M 31. Further insight on the chemical evolution of M 31 may be obtained from comparison with simulations having different chemical evolution recipes for merging galaxies (e.g. Rupke et al. 2010).