oMEGACat. VII. Tracing Interstellar and Intracluster Medium of ω Centauri Using Sodium Absorptions

Wang, Z ORCID: 0000-0003-2512-6892, Seth, AC ORCID: 0000-0003-0248-5470, Latour, M ORCID: 0000-0002-7547-6180, Strader, J ORCID: 0000-0002-1468-9668, Häberle, M ORCID: 0000-0002-5844-4443, Neumayer, N ORCID: 0000-0002-6922-2598, Clontz, C ORCID: 0009-0005-8057-0031, Kamann, S ORCID: 0000-0001-6604-0505, Nitschai, MS ORCID: 0000-0002-2941-4480, Alfaro-Cuello, M ORCID: 0000-0002-1212-2844, Bellini, A ORCID: 0000-0003-3858-637X, Feldmeier-Krause, A ORCID: 0000-0002-0160-7221, Libralato, M ORCID: 0000-0001-9673-7397, Milone, AP ORCID: 0000-0001-7506-930X, Smith, PJ ORCID: 0000-0002-7489-5244, Souza, SO ORCID: 0000-0001-8052-969X and Ven, GVD ORCID: 0000-0003-4546-7731 (2025) oMEGACat. VII. Tracing Interstellar and Intracluster Medium of ω Centauri Using Sodium Absorptions. The Astrophysical Journal, 994 (2). ISSN 0004-637X

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Abstract

We investigate the foreground interstellar medium along the line of sight and intracluster medium of ω Centauri (ω Cen) by measuring the equivalent width of Na I D absorptions from MUSE observations. The large line-of-sight velocity difference between ω Cen and the foreground enables us to separate Na I D absorption contributed from atomic gas in the interstellar and intracluster medium. We find that small-scale substructures in the foreground Na I D distribution correlate with differential reddening derived from photometric methods. Using an empirical Na I D equivalent width-reddening relation, we determine an average reddening of E(B − V) = 0.153 ± 0.003 mag within the half-light radius of ω Cen. However, the Na I D-inferred differential reddening is significantly larger than photometric estimates. This is likely due to scatter in the Na I D-reddening relation. We find no evidence for intracluster atomic gas from spectra of horizontal branch stars, as there is no significant Na I D absorption at ω Cen’s systemic velocity. Given this nondetection, we place the strongest upper limit to date on the intracluster atomic gas column density in ω Cen of ≲2.17 × 1018 cm−2. We also estimate the ionized gas density from pulsar dispersion measure variations, which exceed the atomic gas limit by ∼50 times. Nevertheless, the strong correlation between dispersion measure and foreground Na I D suggests that much or all of this ionized gas resides in the foreground. Given ongoing mass loss from bright giant stars, our findings imply that the intracluster gas accumulation timescale is short and gas removal in the cluster is likely not tied to stripping as ω Cen passes through the galactic disk.

Item Type:	Article
Uncontrolled Keywords:	astro-ph.GA; astro-ph.GA; astro-ph.SR; 5101 Astronomical Sciences; 51 Physical Sciences; 5101 Astronomical Sciences; 51 Physical Sciences; 0201 Astronomical and Space Sciences; 0202 Atomic, Molecular, Nuclear, Particle and Plasma Physics; 0306 Physical Chemistry (incl. Structural); Astronomy & Astrophysics; 5101 Astronomical sciences; 5107 Particle and high energy physics; 5109 Space sciences
Subjects:	Q Science > QB Astronomy
Divisions:	Astrophysics Research Institute
Publisher:	American Astronomical Society
Date of acceptance:	27 September 2025
Date of first compliant Open Access:	8 June 2026
Date Deposited:	08 Jun 2026 13:09
Last Modified:	08 Jun 2026 13:09
DOI or ID number:	10.3847/1538-4357/ae102c
URI:	https://researchonline.ljmu.ac.uk/id/eprint/28775

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