Elemental Abundances in M31: A Comparative Analysis of Iron and Alpha Element Abundances in the Outer Disk, Giant Stellar Stream, and Inner Halo of M31

Emily C. Cunningham; Evan N. Kirby; Ivanna Escala; Jennifer Wojno; Karoline M. Gilbert; Puragra Guhathakurta

REVIEW 3 major objections 5 minor 31 references

Elemental Abundances in M31: A Comparative Analysis of Iron and Alpha Element Abundances in the Outer Disk, Giant Stellar Stream, and Inner Halo of M31

T0 review · 3 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

Challenge this review Re-run · record.json Download PDF Read on arXiv ↗ Review JSON

Pith's one-line read This paper claims that all four sampled regions of M31—the inner halo, the Giant Stellar Stream, and the outer disk—are alpha-enhanced, and that the metal-rich smooth inner halo is chemically inconsistent with assembly from present-day…

desk verdict Solid new abundance measurements with a careful treatment of systematics, but the headline satellite-inconsistency claim rests on a kinematic decomposition that is too fragile to carry it without extra validation. read the letter →

arxiv 1909.00006 v2 pith:5DWORDFD submitted 2019-08-30 astro-ph.GA

Ivanna Escala , Karoline M. Gilbert , Evan N. Kirby , Jennifer Wojno , Emily C. Cunningham , Puragra Guhathakurta This is my paper · ORCID

classification astro-ph.GA

keywords M31stellarhalosalpha-elementabundancesGiantStreamspectralsynthesiskinematicdecompositiongalaxyformationdwarfsatellitegalaxies

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper tries to establish a chemical map of the inner parts of our nearest large galaxy, Andromeda (M31), using abundances of iron and $\alpha$ elements (magnesium, silicon, calcium) in 70 individual red giant stars. It argues that every region sampled—two halo fields at 12 and 23 kpc, the Giant Stellar Stream at 22 kpc, and the outer disk at 26 kpc—formed under rapid star formation, since all are $\alpha$-enhanced with mean $[\alpha/\mathrm{Fe}]$ between 0.41 and 0.58. It further argues that the metal-rich part of the smooth inner stellar halo could not have been assembled from the disruption of dwarf galaxies like the ones orbiting M31 today; the probability of that match is below one percent. If right, this distinguishes M31's halo-building history from a simple dwarf-accretion picture and points to massive progenitors or in situ star formation in the inner regions.

What carries the argument

The argument rests on spectral synthesis of low-resolution ($R \sim 2500$) Keck/DEIMOS spectra, comparing measured spectra against synthetic grids to extract $[\mathrm{Fe}/\mathrm{H}]$ and $[\alpha/\mathrm{Fe}]$ for individual red giants. To isolate stellar populations, the paper fits each field's heliocentric velocity distribution as a Gaussian mixture, fixing the halo component's mean and dispersion to previously measured M31 halo values and assigning each star a probability of belonging to halo versus substructure; abundances are then weighted by those probabilities. For the satellite-assembly test, the paper constructs a simulated stellar halo by drawing from the abundance distributions of M31 dwarf galaxies, weighted by the satellite luminosity function, and compares the resulting $[\alpha/\mathrm{Fe}]$ distributions to the observed one with two-sample Kolmogorov–Smirnov tests.

What would settle it

Measure radial velocities for a larger sample in these four fields and fit the halo component freely instead of holding it fixed; if the freely fitted halo mean and dispersion differ from the adopted values by more than the uncertainties, the claims built on the decomposition would need revision. A direct chemical test would be to obtain $[\alpha/\mathrm{Fe}]$ for a larger sample of metal-rich ($[\mathrm{Fe}/\mathrm{H}] > -1.5$) inner-halo stars: if the resulting distribution overlaps the luminosity-weighted satellite-based distribution, the claimed $p < 1\%$ inconsistency would disappear.

Watch

Extended reading notes

Core claim

On the paper's own terms, the central discovery is that M31's inner stellar populations are uniformly $\alpha$-enhanced: the 23 kpc halo, 12 kpc halo, 22 kpc Giant Stellar Stream, and 26 kpc outer disk have inverse-variance mean $[\alpha/\mathrm{Fe}]$ of 0.43, 0.50, 0.41, and 0.58, respectively, while the two halo fields are metal-poor ($[\mathrm{Fe}/\mathrm{H}] = -1.54$ and $-1.30$) and the stream and disk are metal-rich ($-0.84$ and $-0.92$). After removing substructure kinematically, the metal-rich ($[\mathrm{Fe}/\mathrm{H}] > -1.5$) smooth stellar halo at projected radii within 26 kpc has an $[\alpha/\mathrm{Fe}]$ distribution that is 0.43–0.50 dex more $\alpha$-enhanced than a simulated halo built from present-day M31 satellites, and the two distributions are inconsistent at $p < 1\%$. The paper also finds no $\alpha$ gradient along the Giant Stellar Stream between 17 and 22 kpc, tentative evidence for a negative $\alpha$ gradient in the stellar halo, and a cold, highly $\alpha$-enhanced outer disk that supports a merger-triggered star formation episode.

Load-bearing premise

The kinematic separation assumes that the stellar halo's mean velocity and velocity dispersion in each pointed field are the fixed values adopted from earlier wide-field M31 measurements; if the true halo velocities in these specific fields differ, the component-separated abundances and the satellite comparison are biased.

Editorial extensions

If this is right

All four M31 fields being alpha-enhanced means the sampled stars formed rapidly, with core-collapse supernova yields dominating over Type Ia; this holds for halo, stream, and disk alike.
The consistency of the Giant Stellar Stream abundances at 17 and 22 kpc rules out a strong chemical gradient along the stream's high-surface-brightness core, so the stream's chemistry can be treated as a single-progenitor signature.
The $p < 1\%$ mismatch between the metal-rich inner halo and present-day satellites implies the inner halo was built by more massive progenitors, by in situ star formation, or by a mix of both.
The outer disk's high alpha enhancement at 26 kpc supports a rapid, likely merger-induced star formation episode for M31's extended disk rather than slow secular build-up.
The tentative negative alpha gradient from inner to outer halo, if confirmed, indicates different progenitors or formation mechanisms at different radii.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

The paper's satellite-assembly test only uses present-day satellites; a natural extension is to model the expected abundance distribution of the earlier, more massive progenitors that theory predicts were disrupted long ago, which could either strengthen or weaken the claimed mismatch.
If the alpha enhancement of the 26 kpc disk is confirmed with more stars and additional azimuths, it would provide a chemically testable link between the proposed major merger and M31's 2–4 Gyr old star formation burst.
The apparent alpha difference between the Southeast shelf and the Giant Stellar Stream core, if real, could serve as a tracer of a radial metallicity gradient inside the stream's progenitor galaxy.
The same kinematic-decomposition plus abundance technique could be applied to other Local Group spirals to ask whether uniformly alpha-enhanced inner halos are a general feature or peculiar to M31.

Share X Bluesky LinkedIn Reddit HN

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Two things to know. First, this is the first [alpha/Fe] measurement in M31's outer disk and a genuine expansion of the inner-halo and GSS samples, with 70 new RGB stars across four fields. Second, the central observational result—all four fields are alpha-enhanced, [alpha/Fe] roughly 0.4–0.6—is probably solid. The satellite-inconsistency conclusion in Section 6.2 is the fragile part, and the paper's own footnote concedes the main vulnerability.

What the paper does well: the abundance pipeline is inherited from E19a but re-checked, including a 600ZD-versus-1200G comparison that shows broad consistency and identifies the blue-wavelength scatter source; the TiO star removal is quantified as a color bias and its effect on the MDFs is tested; the kinematic Gaussian mixture is standard and the halo parameters are anchored to Gilbert+18, which is sensible given the truncated velocity window. The bootstrap error treatment and the explicit comparison of field samples to the full M31 RGB population are careful. This is a solid observational contribution.

Where it gets soft. The p<1% inconsistency between the metal-rich smooth halo and present-day M31 satellites depends on assigning stars to the halo via p<0.5, with halo mean and dispersion fixed from global profiles rather than fitted per field. In the 22 kpc GSS field the halo is poorly constrained and the final abundance sample under-represents it by ~10%. Metal-rich, alpha-enhanced substructure leaking into the halo bin would exactly produce the observed offset, and the authors acknowledge this in the Section 6.2 footnote. That does not sink the paper, but it means the satellite claim is modest, not robust. The unknown systematic offset between 600ZD and 1200G abundances (the dSph sample shows -0.13 dex in [Fe/H]) is also not propagated into the satellite comparison, which mixes both gratings. And 70 stars is 70 stars; the field-to-field differences are suggestive, not definitive.

Who this is for: anyone working on M31's assembly history or on resolved abundances in the Local Group. It deserves a serious referee, not a desk reject. I would push for a revision that either fits the halo kinematics per field or demonstrates stability of the Section 6.2 result across plausible halo parameter choices, and that quantifies the 600ZD/1200G offset effect on the conclusions.

Referee Report

3 major / 5 minor

Summary. The paper presents spectral-synthesis measurements of [Fe/H] and [alpha/Fe] for 70 individual red giant branch stars in four M31 fields: a 12 kpc inner halo field, a 22 kpc Giant Stellar Stream (GSS) field, a 26 kpc outer disk field, and a re-analysis of the 23 kpc smooth halo field from Escala et al. (2019a). The data are Keck/DEIMOS 600ZD low-resolution spectra, analyzed with a synthetic spectral grid and calibrated against globular clusters and a 600ZD-versus-1200G comparison. The authors find that all four fields are alpha-enhanced, with inverse-variance means [alpha/Fe] = 0.50, 0.41, 0.58, and 0.43 for the 12 kpc halo, 22 kpc GSS, 26 kpc disk, and 23 kpc halo, respectively (Table 5). They separate substructure from the smooth stellar halo using a Gaussian mixture in velocity, report no [alpha/Fe] gradient along the GSS between 17 and 22 kpc, find tentative evidence for a negative radial [alpha/Fe] gradient in the halo, and conclude that the metal-rich ([Fe/H] > -1.5) smooth inner halo is inconsistent with being built from present-day M31 satellite galaxies at the p < 1% level. The 26 kpc disk is interpreted as part of an extended, rapidly star-forming disk, possibly induced by a major merger.

Significance. If the conclusions survive scrutiny, this is one of the first multi-field chemical-abundance maps of M31's inner halo, GSS, and outer disk from individual stars, and it provides important observational constraints on M31's assembly history. The paper is careful in several respects: abundances are measured against a synthetic grid with external globular cluster and dwarf spheroidal calibration, the 600ZD and 1200G measurement systems are compared star-by-star (Appendix A), selection biases are analyzed quantitatively (Section 3.5), and the satellite comparison in Section 6.2 uses published satellite abundance data rather than fitting the target conclusion. The claim that all four fields are alpha-enhanced is simple and, being based on whole-field averages, is largely independent of the kinematic decomposition. The more ambitious claim that the metal-rich inner halo is inconsistent with present-day M31 satellites, however, depends on the kinematic decomposition and on a small, kinematically uncertain halo sample; the paper's own footnotes concede that substructure contamination could affect this result.

major comments (3)

[Section 6.2, Section 4.2, Table 4] The p < 1% inconsistency between the metal-rich inner halo and present-day M31 satellites is computed from a 'smooth halo' sample defined by p < 0.5 in Eq. (3), where the halo mean velocity and dispersion are fixed to the Gilbert et al. (2018) values rather than fitted to each field. In the 22 kpc GSS field, the final abundance sample under-represents the fitted halo fraction by about 10% and over-represents the secondary cold component (Section 5.2), and Section 5.2.2 states that the stellar halo in this field cannot be robustly characterized. The footnote in Section 6.2 already concedes that the difference in means may result from substructure contamination in the stellar halo at [Fe/H] > -0.8. The authors should demonstrate that the p < 1% result survives (a) varying mu_halo and sigma_halo over the uncertainties of Gilbert et al. (2018), or fitting them, and (b) excluding the poorly constrained 22 kpc halo stars from the comparison. If the result is not robust to these checks, it should be presented as tentative rather than as a headline conclusion.
[Section 3.4, Section 3.5, Table 5] The TiO-star exclusion removes 41%, 44%, 34%, and 39% of the reliable measurements in fields H, S, D, and f130_2, respectively, and biases the final sample against red, presumably metal-rich stars. Section 3.5 shows that the final sample is biased toward lower photometric [Fe/H] by 0.2-0.4 dex, and footnote 2 states that if the TiO-star abundances are valid, the final sample could be biased toward lower [alpha/Fe] by 0.1-0.2 dex. Since the headline claim that all four fields are alpha-enhanced is based on means of 0.41-0.58 (Table 5) and the paper's own summary uses [alpha/Fe] > 0.35 as the alpha-enhanced threshold, the quoted bias is large enough to move the 22 kpc GSS field mean below that threshold. The authors should either validate abundances for TiO stars with an appropriate linelist and calibration sample, or soften the alpha-enhanced claim to the TiO-free subset and quantify the maximum plausible downward revision of each field mean.
[Section 6.2, Figure 15] The satellite comparison in Section 6.2 rests on only N = 29 halo stars across five fields, split into three metallicity bins, and the p < 1% KS result is obtained by resampling this small parent distribution. The paper does not report how many stars fall in the metal-rich bin or whether the result survives removing any single field. Because the 22 kpc halo component is the least secure, a jackknife or leave-one-field-out test is needed to show that the p < 1% conclusion is not driven by a handful of stars or by one field. The authors should report the per-bin sample sizes and the corresponding p-values with each field excluded.

minor comments (5)

[Title] The title contains a duplicated 'the': 'in the the Outer Disk' should be 'in the Outer Disk'.
[Section 4.2, Section 4.4] There are several typographical errors, including 'velocitiy distributions' in Section 4.2 and 'M31's systemtic velocity' in Section 4.4; these do not affect the science but should be corrected.
[Figure 16 caption] The caption cites 'Kirby et al., in prep' for the And I, And III, And V, and And VII abundances, while the reference list contains Kirby et al. (2019); the citation should be made consistent.
[Appendix A] The Appendix reports a -0.13 +/- 0.02 dex offset in [Fe/H] between 600ZD and 1200G measurements for the dSph calibration sample. Because Section 6.2 directly compares 600ZD-based halo abundances with 1200G-based satellite abundances, the paper should state explicitly whether this offset is applied or corrected, and what effect it would have on the simulated-versus-observed comparison.
[Table 5] The 'Halo' row for the 22 kpc GSS field reports a mean [Fe/H] = -0.66 and [alpha/Fe] = 0.49, but Section 5.2.2 emphasizes that the stellar halo in this field cannot be robustly characterized. Consider adding a table footnote alerting the reader to the low number of halo stars and the under-representation of the fitted halo fraction in this field.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the abundance measurements and the satellite comparison are built on external calibrations and published data, not on the paper's conclusions.

full rationale

The paper's central claims are not forced by construction. Abundances of [Fe/H] and [alpha/Fe] are measured by spectral synthesis against a synthetic grid, and the technique is validated against external globular-cluster and dwarf-spheroidal samples, including an Appendix A comparison between 1200G and 600ZD measurements. The kinematic decomposition fixes the stellar halo velocity parameters from Gilbert et al. (2018), a separate observational measurement, rather than fitting those parameters to the abundance data or to the target conclusions; this is an input assumption, not a prediction derived from itself. The field-averaged alpha-enhancement claim does not depend on the decomposition at all. The Section 6.2 satellite comparison is a genuine forward model: the simulated halo is constructed from published satellite abundance catalogs (Vargas et al. 2014a; Kirby et al. 2019) weighted by the observed M31 satellite luminosity function, while the observed halo sample is a separate set of field stars assigned by the velocity model. The KS test compares two independently assembled distributions, and nothing in the satellite data is tuned to reproduce the halo's high-metallicity alpha-enhancement. Self-citations to the companion papers E19a and G19 and to Gilbert et al. (2018) supply the method, a prior observed field, and prior kinematic measurements; none of these is a uniqueness theorem or an ansatz that forbids alternatives, and none contains the paper's final conclusion. The paper's own footnote acknowledging possible substructure contamination in the metal-rich halo bin is a robustness caveat about the kinematic decomposition, not evidence that the claimed inconsistency is definitionally equal to its inputs. Overall, no reduction of a prediction to a fitted parameter or to a self-citation chain was found.

Assumptions & free parameters 5 free parameters · 7 assumptions · 0 invented entities

The core claims rest on the spectral synthesis pipeline, adopted isochrone assumptions, kinematic decomposition, and the forward model from satellite abundances. I list the main chosen inputs and domain assumptions; no new physical entities are introduced.

free parameters (5)

Assumed stellar age for photometric Teff and logg = 9 Gyr (H, S, D); 12 Gyr (f130_2)
Determines placement on PARSEC isochrones and thus Teff,phot and logg, inputs to the abundance fit (Section 3.1).
Assumed distance modulus (m-M) = 24.63
Used to convert photometry to absolute quantities for isochrone fitting; uncertainty 0.20 mag (Section 3.1).
Photometric metallicity assumes [alpha/Fe] = 0 = 0
PARSEC isochrones at [alpha/Fe]=0 used for [Fe/H]phot and CMD selection; biases photometric metallicities but not the spectral fit (Section 3.1, Figure 2).
Spectral resolution scale factor f = mask-averaged
Free parameter in line-profile fitting, averaged per slitmask to correct for seeing; fixed in final abundance determination (Section 3.2).
Fixed stellar halo velocity parameters = mu_halo = -315 to -319 km/s; sigma_halo = 98 to 108 km/s
Adopted from Gilbert et al. 2018 rather than fitted, to avoid truncation bias; basis for component assignment (Table 4, Section 4.2).

assumptions (7)

domain assumption M31 membership criteria (Na I, CMD, vhelio cuts) leave 2-5% MW contamination in H, S, f130_2 and 5-10% in D.
Section 4.1; contamination fractions adopted from Gilbert et al. 2006/2007. If larger, abundance distributions include foreground stars.
domain assumption Excluding TiO-affected stars (34-44% of reliable measurements) does not systematically bias the [alpha/Fe] comparison between fields.
Section 3.5 discusses color and photometric metallicity bias; residual alpha bias is estimated at 0.1-0.2 dex but not corrected.
domain assumption 600ZD low-resolution abundances are on the same scale as 1200G abundances.
Appendix A finds the GC sample consistent within 1.3 sigma but a -0.13 dex [Fe/H] offset for dSphs; paper treats offsets as not altering conclusions.
domain assumption Fixed halo velocity parameters from Gilbert et al. 2018 are valid at each field.
Table 4 and Section 4.2; basis of kinematic decomposition and halo probability weights.
domain assumption PARSEC isochrones and assumed ages/distance give reliable Teff,phot and logg for RGB stars; logg is fixed to photometric value.
Sections 3.1-3.4; incorrect logg would shift abundance measurements.
domain assumption Present-day M31 satellite galaxies with N>20 abundance stars and LV > 10^5 Lsun are representative of ancient halo-building progenitors.
Section 6.2; used to construct simulated halo via Eq. 4. If progenitors were more massive or chemically different, the conclusion changes.
domain assumption Gaussian mixture likelihood with AIC/BIC determines the correct number of kinematic components; H is forced to two components despite AIC preferring one.
Section 4.2; the two-component choice is justified by expected halo dispersion and a KS test, but it is a modeling assumption.

how reviews work

0 comments

Cite this review

Pith. "Pith review of Elemental Abundances in M31: A Comparative Analysis of Iron and Alpha Element Abundances in the Outer Disk, Giant Stellar Stream, and Inner Halo of M31." pith.science (2026). https://pith.science/paper/5DWORDFD

@misc{pith2026190900006,
  author       = {Pith},
  title        = {Pith review of: Elemental Abundances in M31: A Comparative Analysis of Iron and Alpha Element Abundances in the Outer Disk, Giant Stellar Stream, and Inner Halo of M31},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/5DWORDFD}},
  note         = {Machine review of arXiv:1909.00006}
}

abstract

We measured [Fe/H] and [$\alpha$/Fe] using spectral synthesis of low-resolution stellar spectroscopy for 70 individual red giant branch stars across four fields spanning the outer disk, Giant Stellar Stream (GSS), and inner halo of M31. Fields at M31-centric projected distances of 23 kpc in the halo, 12 kpc in the halo, 22 kpc in the GSS, and 26 kpc in the outer disk are $\alpha$-enhanced, with $\langle$[$\alpha$/Fe]$\rangle$ = 0.43, 0.50, 0.41, and 0.58, respectively. The 23 kpc and 12 kpc halo fields are relatively metal-poor, with $\langle$[Fe/H]$\rangle$ = $-$1.54 and $-$1.30, whereas the 22 kpc GSS and 26 kpc outer disk fields are relatively metal-rich with $\langle$[Fe/H]$\rangle$ = $-$0.84 and $-$0.92, respectively. For fields with substructure, we separated the stellar populations into kinematically hot stellar halo components and kinematically cold components. We did not find any evidence of an [$\alpha$/Fe] gradient along the high surface brightness core of the GSS between $\sim$17$-$22 kpc. However, we found tentative suggestions of a negative [$\alpha$/Fe] gradient in the stellar halo, which may indicate that different progenitor(s) or formation mechanisms contributed to the build up of the inner versus outer halo. Additionally, the [$\alpha$/Fe] distribution of the metal-rich ([Fe/H] $>$ $-$1.5), smooth inner stellar halo (r$_{\rm{proj}}$ $\lesssim$ 26 kpc) is inconsistent with having formed from the disruption of progenitor(s) similar to present-day M31 satellite galaxies. The 26 kpc outer disk is most likely associated with the extended disk of M31, where its high $\alpha$-enhancement provides support for an episode of rapid star formation in M31's disk, possibly induced by a major merger.

Figures

Figures reproduced from arXiv: 1909.00006 by the authors.

**Figure 1.** The location of M31 DEIMOS fields observed with the 600ZD grating (§ 2 of this work, E19a; magenta rectangles), the 1200G grating (Kalirai et al. 2006a; Gilbert et al. 2007, 2009b; yellow rectangles), and HST/ACS fields (Brown et al. 2009; black stars) in M31-centric coordinates, overlaid on the PAndAS star count map (McConnachie et al. 2018). The dashed magenta line corresponds to 50 projected kpc. The ACS fields … view at source ↗

**Figure 2.** (i 0 0, g 0 0 − i 0 0) and (V0 − I0, I0) color-magnitude diagrams for all stars (M31 RGB stars and MW foreground dwarf stars) in the 12 kpc inner halo field (H), 22 kpc GSS field (S), the 26 kpc outer disk field (D), and the 23 kpc smooth halo field (f130 2). The points are color coded according to the photometric metallicity estimated for each star from the PARSEC (Marigo et al. 2017) isochrones (−2.2 < [Fe/H] < +0… view at source ↗

**Figure 3.** Color magnitude diagrams of M31 RGB stars (§ 4.1) reflecting selection effects in the 12 kpc inner halo field (H), 22 kpc GSS field (S), 26 kpc outer disk field (D), and 23 kpc smooth halo field (f130 2). Stars are color-coded according to probability of belonging to any substructure component in a given field (§ 4.3). Magenta (blue) points are likely (unlikely) to be associated with substructure. For each field, we… view at source ↗

Figures from the paper (15 more)

**Figure 4.** Figure 4: Metallicity distribution functions (MDFs), represented in terms of probability density, for subsets of our total sample of M31 RGB stars with abundance measurements (§ 3.5). The MDFs are weighted according to the inverse variance of the total measurement uncertainty in…

**Figure 5.** Figure 5: Heliocentric radial velocity versus photometric metallicity (§ 3.1) for stars with successful velocity measurements in the 12 kpc halo (H), 22 kpc GSS (S), 26 kpc disk (D), and 23 kpc halo (f130 2) M31 fields. The velocity errors represent only the random component of …

**Figure 6.** Figure 6: Heliocentric radial velocity distributions of stars with successful velocity measurements (§ 3.3, top panels, black histograms), including foreground MW dwarf stars (§ 4.1), and velocity distributions for M31 RGB stars (grey filled histograms) in the 12 kpc (H), 22 kpc…

**Figure 7.** Figure 7: [α/Fe] versus [Fe/H] for RGB stars in M31 (§ 5.1). We show abundance distributions for the inner stellar halo at 12 kpc (H), the GSS at 22 kpc (S), the outer disk at 26 kpc (D), and the smooth inner stellar halo at 23 kpc (f130 2). We present measurements for 70 stars …

**Figure 8.** Figure 8: [α/Fe] versus [Fe/H] for M31 RGB stars in fields with substructure (i.e., excluding the 23 kpc smooth halo field, f130 2), color-coded as in [PITH_FULL_IMAGE:figures/full_fig_p012_8.png]

**Figure 9.** Figure 9: Spectroscopic [Fe/H] (top panels) and [α/Fe] (bottom panels) versus heliocentric radial velocity for the same samples and color-coding as [PITH_FULL_IMAGE:figures/full_fig_p013_9.png]

**Figure 10.** Figure 10: [α/Fe] versus [Fe/H] for M31 RGB stars with δ([α/Fe]) < 0.5 in the 12 kpc inner halo field (H; § 5.2.1) and 26 kpc outer disk field (D; § 5.2.3). We separated each field into its kinematic components by assigning stars to the component to which it has the highest pro…

**Figure 12.** Figure 12: h[α/Fe]i versus h[Fe/H]i for all M31 fields (§ 5). The data are presented in [PITH_FULL_IMAGE:figures/full_fig_p015_12.png]

**Figure 13.** Figure 13: [α/Fe] versus [Fe/H] for M31 RGB stars in the 22 kpc GSS field (S; colored squares, § 5.2.2) compared to a 17 kpc GSS field (grey triangles; G19). We present abundances for all M31 RGB stars in a given field (top panel), the GSS core (middle panel), and the KCC of unk…

**Figure 14.** Figure 14: illustrates h[Fe/H]i and h[α/Fe]i as a function of projected radius from the center of M31 for the stellar halo component (§ 4.2) in each field. We referred to the stellar halo components in each field as belonging to the “inner halo” based on their projected radius …

**Figure 15.** Figure 15: The construction of the inner stellar halo of M31 from present-day M31 satellite galaxies (§ 6.2). (Left panel) V-band luminosity function of satellite galaxies within 300 kpc of M31, where absolute V-band magnitudes were taken from the compilation by McConnachie (201…

**Figure 16.** Figure 16: illustrates a subset of this comparison. We classified M31 RGB stars as belonging to substructure if they were more likely to be associated with substructure than the stellar halo (§ 4.3). In the case of the GSS field, we do not distinguish between the GSS core and t…

**Figure 17.** Figure 17: Heliocentric velocity versus projected distance of M31 RGB stars (§ 6.4). The 12 kpc field (H) corresponds to circles color-coded according to probability of belonging to substructure (§ 4.3), where M31 RGB stars in our final abundance sample (§ 3.4) are outlined in b…

**Figure 18.** Figure 18: A star-by-star comparison between [Fe/H] and [α/Fe] measurements for giant stars from a sample of MW globular clusters (Kirby et al. 2016; Escala et al. 2019a) using spectra obtained with the 1200G and 600ZD gratings on DEIMOS. The dashed line represents a one-to-one …

**Figure 19.** Figure 19: A star-by-star comparison between [Fe/H] and [α/Fe] for 30 giant stars from a sample of MW dSphs (Kirby et al. 2010; Escala et al. 2019a) using spectra obtained with the 1200G and 600ZD gratings on DEIMOS. The dashed line represents a one-to-one relation. As in the ca…

Discussion (0). Continue with ORCID to comment.

Reference graph

Works this paper leans on

31 extracted references · 29 canonical work pages

[1]

G., Navarro, J

Abadi, M. G., Navarro, J. F., Steinmetz, M., et al. 2003, ApJ, 597, 21 Adibekyan, V. Z., Figueira, P., Santos, N. C., et al. 2013, A&A, 554, A44 An, D., Beers, T. C., Johnson, J. A., et al. 2013, ApJ, 763, 65 Anders, F., Chiappini, C., Santiago, B. X., et al. 2014, A&A, 564, A115 Astropy Collaboration, Robitaille, T. P., Tollerud, E. J., et al. 2013, A&A,...

work page 2003
[15]

A., Babul, A., Guhathakurta, P., et al

Fardal, M. A., Babul, A., Guhathakurta, P., et al. 2008, ApJ, 682, L33 Fern´ andez-Alvar, E., Allende Prieto, C., Schlesinger, K. J., et al. 2015, A&A, 577, A81 Fern´ andez-Alvar, E., Carigi, L., Allende Prieto, C., et al. 2017, MNRAS, 465, 1586 Ferguson, A. M. N., Johnson, R. A., Faria, D. C., et al. 2005, ApJ, 622, L109 Foreman-Mackey, D., Hogg, D. W., ...

work page 2008
[18]

de Jong, J. T. A., Yanny, B., Rix, H.-W., et al. 2010, ApJ, 714, 663 De Rijcke, S., Prugniel, P., Simien, F., et al. 2006, MNRAS, 369, 1321 30 Escala et al. de Vaucouleurs, G. 1958, ApJ, 128, 465 Deason, A. J., Mao, Y.-Y., & Wechsler, R. H. 2016, ApJ, 821, 5 Deason, A. J., Belokurov, V., Koposov, S. E., et al. 2018, ApJ, 862, L1 Dotter, A., Chaboyer, B., ...

work page 2010
[19]

Collins, M. L. M., Chapman, S. C., Ibata, R. A., et al. 2011, MNRAS, 413,

work page 2011
[20]

M., Guhathakurta, P., Beaton, R

Gilbert, K. M., Guhathakurta, P., Beaton, R. L., et al. 2012, ApJ, 760,

work page 2012
[23]

2010, PASJ, 62, 143 Ishigaki, M

Ishigaki, M., Chiba, M., & Aoki, W. 2010, PASJ, 62, 143 Ishigaki, M. N., Chiba, M., & Aoki, W. 2012, ApJ, 753, 64 Ishigaki, M. N., Aoki, W., & Chiba, M. 2013, ApJ, 771, 67 Johnston, K. V., Bullock, J. S., Sharma, S., et al. 2008, ApJ, 689, 936 Kalirai, J. S., Guhathakurta, P., Gilbert, K. M., et al. 2006, ApJ, 641,

work page 2010
[24]

N., Gilbert, K

Escala, I., Kirby, E. N., Gilbert, K. M., et al. 2019, ApJ, 878, 42 Faber, S. M., Phillips, A. C., Kibrick, R. I., et al. 2003, Instrument Design and Performance for Optical/infrared Ground-based Telescopes,

work page 2019
[55]

Miki, Y., Mori, M., & Rich, R. M. 2016, ApJ, 827, 82 Mikolaitis, ˇS., Hill, V., Recio-Blanco, A., et al. 2014, A&A, 572, A33 Minchev, I., Chiappini, C., & Martig, M. 2014, A&A, 572, A92 Monachesi, A., G´ omez, F. A., Grand, R. J. J., et al. 2019, MNRAS, 485, 2589 Mori, M., & Rich, R. M. 2008, ApJ, 674, L77 Myeong, G. C., Vasiliev, E., Iorio, G., et al. 20...

arXiv 2016

Show all 31 references

[76]

M., Kalirai, J

Gilbert, K. M., Kalirai, J. S., Guhathakurta, P., et al. 2014, ApJ, 796, 76 Gilbert, K. M., Tollerud, E., Beaton, R. L., et al. 2018, ApJ, 852,

2014
[77]

2015, MNRAS, 451, 2230 Majewski, S

Martig, M., Rix, H.-W., Silva Aguirre, V., et al. 2015, MNRAS, 451, 2230 Majewski, S. R., Ostheimer, J. C., Rocha-Pinto, H. J., et al. 2004, ApJ, 615, 738 McCarthy, I. G., Font, A. S., Crain, R. A., et al. 2012, MNRAS, 420, 2245 McConnachie, A. W., Irwin, M. J., Ferguson, A. M...

2015
[89]

E., Guhathakurta, P., Fardal, M

Dorman, C. E., Guhathakurta, P., Fardal, M. A., et al. 2012, ApJ, 752,

2012
[103]

E., Guhathakurta, P., Seth, A

Dorman, C. E., Guhathakurta, P., Seth, A. C., et al. 2015, ApJ, 803,

2015
[128]

M., Kirby, E

Gilbert, K. M., Kirby, E. N., Escala, I., et al. 2019, ApJ, 883, 128 Gilmore, G., & Wyse, R. F. G. 1998, AJ, 116, 748 Gontcharov, G. A. 2006, Astronomical and Astrophysical Transactions, 25, 145 Gratton, R. G., Carretta, E., Desidera, S., et al. 2003, A&A, 406, 131 Gregersen, ...

2019
[147]

E., Widrow, L

Dorman, C. E., Widrow, L. M., Guhathakurta, P., et al. 2013, ApJ, 779,

2013
[152]

S., & Johnston, K

Bullock, J. S., & Johnston, K. V. 2005, ApJ, 635, 931 Butler, D. J., & Mart´ ınez-Delgado, D. 2005, AJ, 129, 2217 Carollo, D., Beers, T. C., Lee, Y. S., et al. 2007, Nature, 450, 1020 Carollo, D., Beers, T. C., Chiba, M., et al. 2010, ApJ, 712, 692 Carollo, D., Chiba, M., Ishi...

2005
[268]

S., Gilbert, K

Kalirai, J. S., Gilbert, K. M., Guhathakurta, P., et al. 2006, ApJ, 648,

2006
[287]

F., Irwin, M., et al

[α/Fe] in M31’s Halo, Stream, and Disk 31 Ibata, R., Martin, N. F., Irwin, M., et al. 2007, ApJ, 671, 1591 Ibata, R. A., Lewis, G. F., McConnachie, A. W., et al. 2014, ApJ, 780,

2007
[306]

S., Johnston, K

Font, A. S., Johnston, K. V., Bullock, J. S., et al. 2006, ApJ, 646, 886 Font, A. S., Johnston, K. V., Ferguson, A. M. N., et al. 2008, ApJ, 673, 215 Font, A. S., McCarthy, I. G., Crain, R. A., et al. 2011, MNRAS, 416, 2802 Fulbright, J. P. 2002, AJ, 123, 404 Gaia Collaboratio...

2006 arXiv
[313]

D., Monelli, M., Weisz, D

Skillman, E. D., Monelli, M., Weisz, D. R., et al. 2017, ApJ, 837, 102 Solway, M., Sellwood, J. A., & Sch¨ onrich, R. 2012, MNRAS, 422, 1363 Starkenburg, E., Hill, V., Tolstoy, E., et al. 2010, A&A, 513, A34 Tissera, P. B., White, S. D. M., & Scannapieco, C. 2012, MNRAS, 420, ...

2017
[323]

M., Smith, E., Ferguson, H

Brown, T. M., Smith, E., Ferguson, H. C., et al. 2009, The Astrophysical Journal Supplement Series, 184,

2009
[328]

N., Guhathakurta, P., Simon, J

Kirby, E. N., Guhathakurta, P., Simon, J. D., et al. 2010, ApJS, 191, 352 Kirby, E. N., Cohen, J. G., Guhathakurta, P., et al. 2013, ApJ, 779, 102 Kirby, E. N., Gilbert, K. M., Escala, I., et al. 2019, arXiv e-prints, arXiv:1912.02186 Kirby, E. N., Guhathakurta, P., Zhang, A. ...

2010 arXiv
[389]

S., Zucker, D

Kalirai, J. S., Zucker, D. B., Guhathakurta, P., et al. 2009, ApJ, 705, 1043 Kirby, E. N., Guhathakurta, P., & Sneden, C. 2008, ApJ, 682,

2009
[403]

2008, The Astrophysical Journal Supplement Series, 178,

Dotter, A., Chaboyer, B., Jevremovi´ c, D., et al. 2008, The Astrophysical Journal Supplement Series, 178,

2008
[523]

Roederer, I. U. 2009, AJ, 137, 272 Rocha-Pinto, H. J., Majewski, S. R., Skrutskie, M. F., et al. 2004, ApJ, 615, 732 Schiavon, R. P., Barbuy, B., Rossi, S. C. F., et al. 1997, ApJ, 479, 902 Schlegel, D. J., Finkbeiner, D. P., & Davis, M. 1998, ApJ, 500, 525 Schlaﬂy, E. F., & F...

2009
[597]

B., Lehman, E

Kwitter, K. B., Lehman, E. M. M., Balick, B., et al. 2012, ApJ, 753, 12 Lee, Y. S., Beers, T. C., An, D., et al. 2011, ApJ, 738, 187 Li, T. S., Sheﬃeld, A. A., Johnston, K. V., et al. 2017, ApJ, 844, 74 Lianou, S., Grebel, E. K., & Koch, A. 2011, A&A, 531, A152 Mackereth, J. T...

2012 arXiv
[851]

Gwyn, S. D. J. 2008, PASP, 120, 212 Hammer, F., Puech, M., Chemin, L., et al. 2007, ApJ, 662, 322 Hammer, F., Yang, Y. B., Wang, J. L., et al. 2018, MNRAS, 475, 2754 Hawkins, K., Jofr´ e, P., Masseron, T., et al. 2015, MNRAS, 453, 758 Hayden, M. R., Holtzman, J. A., Bovy, J., ...

2008
[1188]

M., Fardal, M., Kalirai, J

Gilbert, K. M., Fardal, M., Kalirai, J. S., et al. 2007, ApJ, 668,

2007
[1217]

N., Guhathakurta, P., Bolte, M., et al

Kirby, E. N., Guhathakurta, P., Bolte, M., et al. 2009, ApJ, 705,

2009
[1275]

M., Guhathakurta, P., Kollipara, P., et al

Gilbert, K. M., Guhathakurta, P., Kollipara, P., et al. 2009, ApJ, 705,

2009
[1548]

P., Zaritsky, D., et al

Conroy, C., Naidu, R. P., Zaritsky, D., et al. 2019, arXiv e-prints, arXiv:1909.02007 Cooper, A. P., Cole, S., Frenk, C. S., et al. 2010, MNRAS, 406, 744 Cooper, M. C., Griﬃth, R. L., Newman, J. A., et al. 2012, MNRAS, 419, 3018 Cunningham, E. C., Deason, A. J., Guhathakurta, ...

2019 arXiv
[1657]

A., Babul, A., Geehan, J

Fardal, M. A., Babul, A., Geehan, J. J., et al. 2006, MNRAS, 366, 1012 Fardal, M. A., Guhathakurta, P., Babul, A., et al. 2007, MNRAS, 380,

2006

Pith tools

Reviewed August 14, 2026 · model on record in the stance chip above.

[1] [1]

G., Navarro, J

Abadi, M. G., Navarro, J. F., Steinmetz, M., et al. 2003, ApJ, 597, 21 Adibekyan, V. Z., Figueira, P., Santos, N. C., et al. 2013, A&A, 554, A44 An, D., Beers, T. C., Johnson, J. A., et al. 2013, ApJ, 763, 65 Anders, F., Chiappini, C., Santiago, B. X., et al. 2014, A&A, 564, A115 Astropy Collaboration, Robitaille, T. P., Tollerud, E. J., et al. 2013, A&A,...

work page 2003

[2] [15]

A., Babul, A., Guhathakurta, P., et al

Fardal, M. A., Babul, A., Guhathakurta, P., et al. 2008, ApJ, 682, L33 Fern´ andez-Alvar, E., Allende Prieto, C., Schlesinger, K. J., et al. 2015, A&A, 577, A81 Fern´ andez-Alvar, E., Carigi, L., Allende Prieto, C., et al. 2017, MNRAS, 465, 1586 Ferguson, A. M. N., Johnson, R. A., Faria, D. C., et al. 2005, ApJ, 622, L109 Foreman-Mackey, D., Hogg, D. W., ...

work page 2008

[3] [18]

de Jong, J. T. A., Yanny, B., Rix, H.-W., et al. 2010, ApJ, 714, 663 De Rijcke, S., Prugniel, P., Simien, F., et al. 2006, MNRAS, 369, 1321 30 Escala et al. de Vaucouleurs, G. 1958, ApJ, 128, 465 Deason, A. J., Mao, Y.-Y., & Wechsler, R. H. 2016, ApJ, 821, 5 Deason, A. J., Belokurov, V., Koposov, S. E., et al. 2018, ApJ, 862, L1 Dotter, A., Chaboyer, B., ...

work page 2010

[4] [19]

Collins, M. L. M., Chapman, S. C., Ibata, R. A., et al. 2011, MNRAS, 413,

work page 2011

[5] [20]

M., Guhathakurta, P., Beaton, R

Gilbert, K. M., Guhathakurta, P., Beaton, R. L., et al. 2012, ApJ, 760,

work page 2012

[6] [23]

2010, PASJ, 62, 143 Ishigaki, M

Ishigaki, M., Chiba, M., & Aoki, W. 2010, PASJ, 62, 143 Ishigaki, M. N., Chiba, M., & Aoki, W. 2012, ApJ, 753, 64 Ishigaki, M. N., Aoki, W., & Chiba, M. 2013, ApJ, 771, 67 Johnston, K. V., Bullock, J. S., Sharma, S., et al. 2008, ApJ, 689, 936 Kalirai, J. S., Guhathakurta, P., Gilbert, K. M., et al. 2006, ApJ, 641,

work page 2010

[7] [24]

N., Gilbert, K

Escala, I., Kirby, E. N., Gilbert, K. M., et al. 2019, ApJ, 878, 42 Faber, S. M., Phillips, A. C., Kibrick, R. I., et al. 2003, Instrument Design and Performance for Optical/infrared Ground-based Telescopes,

work page 2019

[8] [55]

Miki, Y., Mori, M., & Rich, R. M. 2016, ApJ, 827, 82 Mikolaitis, ˇS., Hill, V., Recio-Blanco, A., et al. 2014, A&A, 572, A33 Minchev, I., Chiappini, C., & Martig, M. 2014, A&A, 572, A92 Monachesi, A., G´ omez, F. A., Grand, R. J. J., et al. 2019, MNRAS, 485, 2589 Mori, M., & Rich, R. M. 2008, ApJ, 674, L77 Myeong, G. C., Vasiliev, E., Iorio, G., et al. 20...

arXiv 2016