Dissecting the 3D chemo-dynamical structures of NGC 1381: A galaxy hosting an ancient slow bar with an accretion-related bulge and thick disc

Jin, Y, Zhu, L, Mao, S, Martig, M ORCID: 0000-0001-5454-1492, Pinna, F, van de Ven, G and Ding, Y (2026) Dissecting the 3D chemo-dynamical structures of NGC 1381: A galaxy hosting an ancient slow bar with an accretion-related bulge and thick disc. Astronomy & Astrophysics, 715. pp. 1-21. ISSN 0004-6361

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Abstract

We applied the barred population-orbit superposition method developed and validated in our previous 2025 papers to construct 3D chemo-dynamical models for the barred S0 galaxy NGC 1381 (FCC 170) in the Fornax cluster. Based on the properties of stellar orbits in the models, we decomposed NGC 1381 into six components with distinct kinematics and morphologies: (1) a dynamically warm nuclear disc; (2) a rigidly rotating, boxy, peanut, or X-shaped bar; (3) a dynamically hot, spheroidal bulge; (4) a dynamically cold thin disc; (5) a vertically extended thick disc with slightly slower rotation than the thin disc; and (6) a dynamically hot, spatially diffuse stellar halo. The luminosity fractions of the dynamical components indicate that the luminosity of NGC 1381 is dominated by the bar (f_ ̊m bar and thin disc (f_ ̊m thin with notable contributions from the thick disc (f_ ̊m thick and bulge (f_ ̊m bulge and only minor contributions from the nuclear disc (f_ ̊m nucl and halo (f_ ̊m halo The nuclear disc, bar, and thin disc are metal-rich ($ Z/̊m H α-poor (rm Mg/Fe łesssim0.2), and old (sim13̊m,Gyr), consistent with in situ formation in the early Universe. The bulge, thick disc, and stellar halo are metal-poor Z/̊m H łesssim0$), α-rich (rm Mg/Fe and younger than or comparable in age to the in situ components, suggesting that they are related to ex situ formation through the accretion of minor mergers. Further analysis of stellar population gradients for these ex situ-related components reveals that the thick disc and stellar halo, with their flat metallicity and Mg/Fe gradients, are likely dominated by a similar ex situ population. In contrast, the bulge exhibits a negative metallicity gradient (∇ Z/̊m H _ bulge <0), which might originate from either predominantly ex situ stars or a mixture of in situ and ex situ stars; the bulge's ex situ fraction cannot be quantified with the current data. Our modelling also reveals the presence of a slow bar (mathcal R =2.40_ -0.27 ^ +0.54 ), with a bar pattern speed of ̊mΩ_p=34_ -7 ^ +4 ,km,s^ -1 ,kpc^ -1 , a bar length of R_ ̊m bar =2.24_ -0.22 ^ +0.43 ̊m,kpc, and a corotation radius of R_ ̊m CR =5.38_ -0.28 ^ +1.59 ̊m,kpc, which is consistent with its ancient formation time.

Item Type:	Article
Uncontrolled Keywords:	5109 Space Sciences; 51 Physical Sciences; 5109 Space Sciences; 51 Physical Sciences; 0201 Astronomical and Space Sciences; Astronomy & Astrophysics; 5101 Astronomical sciences; 5107 Particle and high energy physics; 5109 Space sciences
Subjects:	Q Science > QB Astronomy
Divisions:	Astrophysics Research Institute
Publisher:	EDP Sciences
Date of acceptance:	29 September 2026
Date of first compliant Open Access:	4 November 2026
Date Deposited:	04 Nov 2026 13:15
Last Modified:	04 Nov 2026 13:15
DOI or ID number:	10.1051/0004-6361/202661758
URI:	https://researchonline.ljmu.ac.uk/id/eprint/29813

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