S⁵: New insights from deep spectroscopic observations of the tidal tails of the globular clusters NGC 1261 and NGC 1904

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
² Bernoulli Institute for Mathematics, Computer Science and Artificial Intelligence, University of Groningen, 9700 AK Groningen, The Netherlands
³ Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada
⁴ Department of Physics, University of Surrey, Guildford GU2 7XH, UK
⁵ Universidad Technica Frederico de Santa Maria, Avenida Vicuña Mackenna 3939, San Joaquín, Santiago, Chile
⁶ University of Birmingham, School of Computer Science, B15 1TT, Birmingham, UK
⁷ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁸ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁹ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁰ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹¹ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
¹² Centre of Excellence for All-Sky Astrophysics in Three Dimensions (ASTRO 3D), Australia
¹³ Department of Astronomy & Astrophysics, University of Chicago, 5640 S Ellis Avenue, Chicago, IL 60637, USA
¹⁴ Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA
¹⁵ Lowell Observatory, 1400 W Mars Hill Rd, Flagstaff, AZ 86001, USA
¹⁶ Sydney Institute for Astronomy, School of Physics, A28, The University of Sydney, NSW 2006, Australia
¹⁷ McWilliams Center for Cosmology, Carnegie Mellon University, 5000 Forbes Ave, Pittsburgh, PA 15213, USA
¹⁸ School of Mathematical and Physical Sciences, Macquarie University, Sydney, NSW 2109, Australia
¹⁹ Macquarie University Research Centre for Astrophysics and Space Technologies, Sydney, NSW 2109, Australia
²⁰ Universidade de São Paulo, Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Departamento de Astronomia, SP 05508-090, São Paulo, Brazil
²¹ School of Physics, University of New South Wales, Sydney, NSW 2052, Australia

^★ Corresponding author; p.awad@rug.nl

Received: 19 August 2024
Accepted: 8 November 2024

Abstract

As globular clusters (GCs) orbit the Milky Way, their stars are tidally stripped and form tidal tails that follow the orbit of the cluster around the Galaxy. The morphology of these tails is complex and shows correlations with the phase of orbit and the orbital angular velocity, especially for GCs on eccentric orbits. Here we focus on two GCs, NGC 1261 and NGC 1904, that were potentially accreted alongside Gaia-Enceladus and that have shown signatures of having, in addition to tidal tails, structures formed by distributions of extra-tidal stars that are misaligned with the general direction of the clusters’ respective orbits. To provide an explanation for the formation of these structures, we made use of spectroscopic measurements from the Southern Stellar Stream Spectroscopic Survey (S⁵) as well as proper motion measurements from Gaia’s third data release (DR3), and applied a Bayesian mixture modelling approach to isolate high-probability member stars. We recovered extra-tidal features surrounding each cluster matching findings from previous work. We then conducted N-body simulations and compared the expected spatial distribution and variation in the dynamical parameters along the orbit with those of our potential member sample. Furthermore, we used Dark Energy Camera (DECam) photometry to inspect the distribution of the member stars in the colour-magnitude diagram (CMD). We find that potential members agree reasonably with the N-body simulations, and that the majority follow a simple stellar population distribution in the CMD, which is characteristic of GCs. We link the extra-tidal features with their orbital properties and find that the presence of the tails agrees well with the theory of stellar stream formation through tidal disruption. In the case of NGC 1904, we clearly detect the tidal debris escaping the inner and outer Lagrange points, which are expected to be prominent when at or close to the apocentre of its orbit. Our analysis allows for further exploration of other GCs in the Milky Way that exhibit similar extra-tidal features.