Confirmation of two new Galactic bulge globular clusters: FSR 19 and FSR 25

¹ Depto. de Ciencias Físicas, Facultad de Ciencias Exactas, Universidad Andres Bello, Av. Fernandez Concha 700, Las Condes, Santiago, Chile
e-mail: c.obasi@uandresbello.edu
² Centre for Basic Space Science, University of Nigeria Nsukka, Nsukka, Nigeria
³ Vatican Observatory, V00120 Vatican City State, Italy
⁴ Centro de Astronomía (CITEVA), Universidad de Antofagasta, Av. Angamos 601, Antofagasta, Chile
⁵ Millennium Institute of Astrophysics, Nuncio Monseñor Sotero Sanz 100, Of. 104, Providencia, Santiago, Chile

Received: 17 May 2021
Accepted: 14 June 2021

Abstract

Context. Globular clusters (GCs) in the Milky Way (MW) bulge are very difficult to study for the following reasons: (i) they suffer from the severe crowding and Galactic extinction, which are characteristic of these inner Galactic regions; (ii) they are more prone to the effects of dynamical processes. Therefore, they are relatively faint and difficult to map. However, deep, near-infrared photometry like that provided by the VISTA variables in the Via Láctea Extended Survey (VVVX) allows us to map GCs in this crucial yet relatively uncharted region.

Aims. Our main goals are to study the true nature of the GC candidates FSR 19 and FSR 25 and measure their physical parameters.

Methods. We used the near-infrared VVVX database, in combination with the Two Micron All Sky Survey and Gaia EDR3 proper motions (PMs) and photometry to study ages, metallicities, distances, reddening, mean PMs, sizes, and integrated luminosities for FSR 19 and FSR 25. A robust combination of selection criteria allowed us to effectively clean interlopers among our samples.

Results. Our results confirm with a high level of confidence that both FSR 19 and FSR 25 are genuine MW bulge GCs. Each of the performed tests and resulting parameters provide clear evidence of the GC nature of these targets. We derive distances of 7.2 ± 0.7 kpc and D = 7.0 ± 0.6 (corresponding to distance moduli of 14.29 ± 0.08 and 14.23 ± 0.07) for FSR 19 and FSR 25, respectively. Their ages and metallicities are 11 Gyr and [Fe/H] = −0.5 dex for both clusters, which were determined from Dartmouth and PARSEC isochrone fitting. The integrated luminosities are M_Ks(FSR 19) = −7.72 mag and M_Ks(FSR 25) = −7.31 mag, which places them in the faint tail of the GC luminosity function. By adopting a King profile for their number distribution, we determine their core and tidal radii (r_c, r_t). For FSR 19, r_c = 2.76 ± 0.36 pc and r_t = 5.31 ± 0.49 pc, while FSR 25 appears more extended with r_c = 1.92 ± 0.59 pc and r_t = 6.85 ± 1.78 pc. Finally, their mean GC PMs (from Gaia EDR3) are μ_α^* = −2.50 ± 0.76 mas yr⁻¹, μ_δ = −5.02 ± 0.47 mas yr⁻¹ for FSR 19 and μ_α^* = −2.61 ± 1.27 mas yr⁻¹, μ_δ = −5.23 ± 0.74 mas yr⁻¹ for FSR 25.

Conclusion. We demonstrate and confirm, based on the measured astrophysical parameters, that the two target clusters are indeed genuine and of low luminosity relatively metal-rich old GCs in the bulge of the MW.