PREPRINT
6EB786EC-8490-4ACD-8891-71AA1FD332A9

# Resolved SPLASH Chemodynamics in Andromeda's PHAT Stellar Halo and Disk: On the Nature of the Inner Halo Along the Major Axis

Ivanna Escala, Amanda C. N. Quirk, Puragra Guhathakurta, Karoline M. Gilbert, Lara Cullinane, Benjamin F. Williams, Julianne Dalcanton

Submitted on 16 September 2022

## Abstract

Stellar kinematics and metallicity are key to exploring formation scenarios for galactic disks and halos. In this work, we characterized the relationship between kinematics and photometric metallicity along the line-of-sight to M31's disk. We combined optical HST/ACS photometry from the Panchromatic Hubble Andromeda Treasury (PHAT) survey with Keck/DEIMOS spectra from the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH) survey. The resulting sample of 3536 individual red giant branch stars spans 4-19 projected kpc, making it a useful probe of both the disk and inner halo. We separated these stars into disk and halo populations by modeling the line-of-sight velocity distributions as a function of position across the disk region, where 70.9% stars have a high likelihood of belonging to the disk and 17.1% to the halo. Although stellar halos are typically thought to be metal-poor, the kinematically identified halo contains a significant population of stars (29.4%) with disk-like metallicity ([Fe/H]${}_{\mathrm{p}\mathrm{h}\mathrm{o}\mathrm{t}}\sim -0.10$). This metal-rich halo population lags the gaseous disk to a similar extent as the rest of the halo, indicating that it does not correspond to a canonical thick disk. Its properties are inconsistent with those of tidal debris originating from the Giant Stellar Stream merger event. Moreover, the halo is chemically distinct from the phase-mixed component previously identified along the minor axis (i.e., away from the disk), implying contributions from different formation channels. These metal-rich halo stars provide direct chemodynamical evidence in favor of the previously suggested "kicked-up'' disk population in M31's inner stellar halo.

## Preprint

Comment: Submitted to AJ. Conclusions on page 20. 18 figures, 2 tables, 4 appendices

Subject: Astrophysics - Astrophysics of Galaxies