Kinematical Modeling of the Resolved Stellar Outskirts of M32: Constraints on Tidal Stripping Scenarios

Abstract As the only compact elliptical close enough to resolve into individual stars, the satellite dwarf galaxy M32 provides a unique opportunity for exploring the origins of such rare galaxies. In this work, we combined archival and novel Keck/DEIMOS spectroscopy from a southern extension of the Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo (SPLASH) survey with optical HST imaging from the Panchromatic Hubble Andromeda Southern Treasury (PHAST) survey. The resulting sample of 2505 giant stars is unprecedented both in size and spatial coverage (0 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">′</mml:mi> </mml:mrow> </mml:mover> </mml:math> 9–15 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">′</mml:mi> </mml:mrow> </mml:mover> </mml:math> 5, or out to ∼23 r eff and ∼30 r eff along M32’s major and minor axes) for probing the resolved stellar outskirts of M32. Given the structurally complex region near M32 on the sky, we modeled M32’s line-of-sight kinematics simultaneously alongside M31’s rotating stellar disk and potential outliers corresponding to M31’s kinematically hot stellar halo and/or tidal substructure. Inside the radius corresponding to the observed twisting of isophotal contours in M32’s surface brightness profile ( R iso ∼ 5 r eff ∼ 150” or 0.56 kpc), M32 exhibits a line-of-sight velocity distribution characteristic of ordered rotation, transitioning to a distribution with heavier outliers beyond this radius. Within R iso , the rotational direction is aligned with M32’s major-axis rotation, but shifts to become roughly aligned with M32’s minor axis beyond R iso . We interpret these kinematical signatures in the stellar outskirts of M32 as evidence of tidal distortion from interactions with M31 and discuss their implications for M32 formation pathways.