Hidden Figures of Globular Clusters: Integrated Stellar Populations Impacted by Hot Subdwarfs

Pacheco, TA ORCID: 0000-0002-8139-7278, Coelho, PRT ORCID: 0000-0003-1846-4826, Martins, LP ORCID: 0000-0002-3666-2810, Schiavon, RP ORCID: 0000-0002-2244-0897, de Lima, EVR ORCID: 0000-0002-6268-8600, Diaz, MP ORCID: 0000-0002-6040-0458, Nardiello, D ORCID: 0000-0003-1149-3659, Levenhagen, RS ORCID: 0000-0003-2499-9325, Riffel, R ORCID: 0000-0002-1321-1320, Bonatto, CJ ORCID: 0000-0002-4102-1751 and Chies-Santos, AL ORCID: 0000-0003-3220-0165 (2025) Hidden Figures of Globular Clusters: Integrated Stellar Populations Impacted by Hot Subdwarfs. The Astrophysical Journal, 992 (1). p. 151. ISSN 0004-637X

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Abstract

Globular clusters (GCs) are fundamental for understanding the integrated light of old stellar populations and galaxy assembly processes. However, the role of hot, evolved stars, such as horizontal branch (HB), extreme HB, and blue stragglers, remains poorly constrained. These stars are often underrepresented or entirely excluded from stellar population models, despite their dominant contribution to the ultraviolet (UV) flux. Their presence can bias age estimates by mimicking the spectral signatures of younger populations. We examined the impact of evolved hot stars on the models using two well-studied Galactic GCs with high-quality Hubble Space Telescope photometry and integrated spectra from the International Ultraviolet Explorer and the Blanco Telescope. NGC 2808 and NGC 7089 (M 2) have extended HBs and are proxies for old stellar populations. Integrated spectra were constructed using a color magnitude diagram–based (CMD-based) method, matching observed stars to evolutionary phases and then to appropriate synthetic stellar libraries, enabling the HB morphology to be taken into account. Our findings show that the inclusion of evolved hot stars significantly improves the agreement between the model and observed spectra from the UV to the optical. The inclusion of these phases reduced the residuals in spectral comparisons. Our results reinforce that comprehensive stellar population models incorporating evolved hot components are essential to accurately date unresolved systems and to robustly trace formation histories of extragalactic galaxies.

Item Type:	Article
Uncontrolled Keywords:	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 Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	American Astronomical Society
Date of acceptance:	24 August 2025
Date of first compliant Open Access:	16 October 2025
Date Deposited:	16 Oct 2025 15:19
Last Modified:	16 Oct 2025 15:30
DOI or ID number:	10.3847/1538-4357/adff5f
URI:	https://researchonline.ljmu.ac.uk/id/eprint/27358

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