Molecular isotopologue measurements toward super star clusters and the relation to their ages in NGC 253 with ALCHEMI

¹ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
e-mail: butterworth@strw.leidenuniv.nl
² National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
³ European Southern Observatory, Alonso de Córdova, 3107, Vitacura, Santiago 763-0355, Chile
⁴ Joint ALMA Observatory, Alonso de Córdova, 3107, Vitacura, Santiago 763-0355, Chile
⁵ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁶ Institute of Astronomy and Astrophysics, Academia Sinica, 11F of AS/NTU Astronomy-Mathematics Building, No.1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan
⁷ Department of Astronomy, School of Science, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo 181-1855, Japan
⁸ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁹ Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
¹⁰ Department of Physics, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Yokohama, Kanagawa 223–8522, Japan
¹¹ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Magrans, 08193 Barcelona, Spain
¹² Centro de Astrobiología (CAB, CSIC-INTA), Ctra. de Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹³ Department of Astronomy, University of Virginia, PO Box 400325 530 McCormick Road, Charlottesville, VA 22904-4325, USA
¹⁴ Max-Planck-Institut für Radioastronomie, Auf-dem-Hügel 69, 53121 Bonn, Germany
¹⁵ Astron. Dept., Faculty of Science, King Abdulaziz University, PO Box 80203 Jeddah 21589, Saudi Arabia
¹⁶ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 830011 Urumqi, PR China
¹⁷ New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801, USA
¹⁸ National Radio Astronomy Observatory, PO Box O 1003 Lopezville Road, Socorro, NM 87801, USA
¹⁹ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
²⁰ DTU Space, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
²¹ Department of Physics and Astronomy, University of British Columbia, 6225 Agricultural Rd., Vancouver V6T 1Z1, Canada

Received: 29 November 2023
Accepted: 10 February 2024

Abstract

Context. Determining the evolution of the CNO isotopes in the interstellar medium (ISM) of starburst galaxies can yield important constraints on the ages of super star clusters (SSCs), or on other aspects and factors contributing to their evolution, such as the initial mass function (IMF). Due to the time-dependent nature of the abundances of isotopes within the ISM – as they are supplied from processes such as nucleosynthesis or chemical fractionation –, this provides the opportunity to test whether or not isotope ratios trace the ages of highly star-forming regions, such as SSCs.

Aims. The goal of this study is to investigate whether the isotopic variations in SSC regions within NGC 253 are correlated with their different ages as derived from stellar population modelling.

Methods. We measured abundance ratios of CO, HCN, and HCO⁺ isotopologues in six regions containing SSCs within NGC 253 using high-spatial-resolution (1.6″, ∼28 pc) data from the ALCHEMI (ALma Comprehensive High-resolution Extragalactic Molecular Inventory) ALMA Large program. We then analysed these ratios using RADEX radiative transfer modelling, with the parameter space sampled using the nested sampling Monte Carlo algorithm MLFriends. These abundance ratios were then compared to ages predicted in each region via the fitting of observed star-formation tracers (such as Brγ) to Starburst99 starburst stellar population evolution models.

Results. We determined the isotopic column density ratios across multiple regions of SSC activity in NGC 253 using non-LTE radiative transfer modelling. We do not find any significant trend with age for the CO and HCN isotopologue ratios on timescales of the ages of the SSC* regions observed. However, HCO⁺ may show a correlation with age over these timescales in ¹²C/¹³C.

Conclusions. The driving factors of these ratios within SSCs could be the IMF or fractionation effects. To further probe these effects in SSCs over time, a larger sample of SSCs must be observed spanning a larger age range.