Letter to the Editor

Discovery of a kinematically distinct component in the central region of the collisional ring galaxy AM0644-741

¹ Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), No. 1, Section 4, Roosevelt Road, Taipei 106319, Taiwan
² Inter-University Centre for Astronomy and Astrophysics, Ganeshkhind, Post Bag 4, Pune 411007, India
³ Indian Institute of Astrophysics, Koramangala II Block, Bangalore 560034, India

^⋆ Corresponding author: cmondal@asiaa.sinica.edu.tw; mondalchayan1991@gmail.com

Received: 25 August 2024
Accepted: 19 May 2025

Abstract

We present the discovery of a peculiar central stellar structure in the collisional ring galaxy AM0644-741 using HST imaging and MUSE integral field unit (IFU) data. We identified two Sérsic components with a Sérsic index of 1.72 (inner part) and 1.11 (outer part) in the HST F814W band optical image using GALFIT. We utilized the MUSE data cube to construct stellar line-of-sight velocity (V_LOS), velocity dispersion (σ_LOS), h₃ and h₄ velocity moments, and stellar population age maps using the GIST pipeline for further investigating both Sérsic components, which have a difference of ∼60° in their position angle. The inner component, with an effective radius of ∼1 kpc, shows a strong anticorrelation between V_LOS/σ_LOS and h₃, indicating the presence of a rotating stellar structure. In addition, the inner component shows a higher velocity dispersion (average values reaching up to ∼240 km s⁻¹) along with disky isophotes and a stronger Mg b line strength, which all together highlight a peculiar dynamical state of AM0644-741’s central region. Our analysis suggests that the recent encounter has had a smaller impact on the stellar orbits within the inner component. In contrast, it has specifically affected the stellar orbits of the progenitor’s outer disk when forming the star-forming ring. The Baldwin, Phillips and Terlevich (BPT) analysis of the unresolved nuclear source shows a low-ionization nuclear emission-line region (LINER) type ionization, hinting at active galactic nucleus (AGN) activity in the galaxy. Our study projects the dynamical evolution of collisional systems and provides scope for simulations to explore the central region in greater detail.