[Paper Review] Confirmation Of Two New Galactic Bulge Globular Clusters: FSR 19 and FSR 25

C. Obasi, M. Gómez+2 more|arXiv (Cornell University)|Jun 16, 2021

Stellar, planetary, and galactic studiesPhysics and Astronomy54 references21 citations

TL;DR

This study confirms FSR 19 and FSR 25 as genuine, low-luminosity globular clusters in the Milky Way's bulge using deep near-infrared photometry from the VVVX and 2MASS surveys, combined with Gaia EDR3 astrometry and proper motions. The analysis confirms both clusters at a distance of ~7 kpc, with ages of ~11 Gyr, [Fe/H] = -0.5 dex, and integrated magnitudes MKs ≈ -7.7 and -7.3 mag, respectively, placing them among the faintest known bulge GCs.

ABSTRACT

Globular clusters (GCs) in the Milky Way (MW) bulge are very difficult to study because: i) they suffer from the severe crowding and galactic extinction; which are characteristic of these inner Galactic regions ii) they are more prone to be affected by 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 L\'actea Extended Survey (VVVX) is allowing us to map GCs in this crucial yet relatively uncharted region. Our results confirm with high confidence that both FSR 19 and FSR 25 are genuine MW bulge GCs. Each of the performed tests and resulting parameters provides clear evidence for the GC nature of these targets. We derive distances of 7.2$\pm$0.7 kpc and D=7.0$\pm$0.6 (corresponding to distance moduli of 14.29$\pm$0.08 and 14.23$\pm$0.07) for FSR 19 and FSR 25, respectively. Their ages and metallicities are 11 Gyr and [Fe/H]= -0.5 dex for both clusters, which were determined from Dartmouth and PARSEC isochrone fitting. The integrated luminosities are M$_{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$\pm$0.36 pc and r$_{t}$=5.31$\pm$0.49 pc, while FSR 25 appears more extended with r$_{c}$= 1.92$\pm$0.59 pc and r$_{t}$=6.85$\pm$1.78 pc. Finally their mean GC PMs (from Gaia EDR3) are $\mu_{\alpha^\ast}$= -2.50 $\pm$0.76 mas $yr^{-1}$, $\mu_{\delta}$= -5.02 $\pm$0.47 mas $yr^{-1}$ and $\mu_{\alpha^\ast}$= -2.61 $\pm$ 1.27 mas $yr^{-1}$ , $\mu_{\delta}$= -5.23 $\pm$0.74 mas $yr^{-1}$ for FSR 19 and FSR 25, respectively. }

Motivation & Objective

• To confirm the globular cluster nature of FSR 19 and FSR 25, which are faint and heavily obscured in the Galactic bulge.
• To measure key physical parameters—distance, age, metallicity, luminosity, structural radii, and proper motions—for these two candidates.
• To overcome challenges from high extinction and crowding in the Galactic bulge using deep near-infrared photometry and precise astrometry.
• To improve the census of low-luminosity globular clusters in the inner Milky Way, a region historically difficult to map.

Proposed method

• Utilized near-infrared photometry from the VVVX and 2MASS surveys to mitigate extinction effects in the Galactic bulge.
• Combined VVVX, 2MASS, and Gaia EDR3 data to obtain precise photometry, astrometry, and proper motions for stellar populations.
• Applied color-magnitude diagram (CMD) analysis and isochrone fitting using Dartmouth and PARSEC models to derive age and metallicity.
• Used statistical tests, including a strong KS test on luminosity functions, to confirm cluster over-densities against field stars.
• Fitted King profiles to radial number distributions to measure core and tidal radii (rc and rt).
• Employed robust selection criteria to remove interlopers and ensure sample purity based on proper motions and photometric consistency.

Experimental results

Research questions

• RQ1 Are FSR 19 and FSR 25 genuine globular clusters, or are they field star overdensities or other stellar systems?
• RQ2 What are the distances, ages, metallicities, and luminosities of FSR 19 and FSR 25, and how do they compare to known bulge globular clusters?
• RQ3 Do the structural parameters (core and tidal radii) and proper motion signatures of FSR 19 and FSR 25 support their classification as genuine globular clusters?
• RQ4 How do the derived parameters of FSR 19 and FSR 25 compare to previous estimates from 2MASS-only or H-band distance methods?
• RQ5 What is the significance of their low luminosity and metallicity in the context of the Milky Way's formation and evolution?

Key findings

• FSR 19 and FSR 25 are confirmed as genuine globular clusters in the Galactic bulge with high confidence based on multiple independent tests.
• The distance to FSR 19 is 7.2 ± 0.7 kpc (distance modulus 14.29 ± 0.08 mag), and to FSR 25 is 7.0 ± 0.6 kpc (distance modulus 14.23 ± 0.07 mag).
• Both clusters have a consistent age of 11 Gyr and metallicity [Fe/H] = -0.5 dex, derived from isochrone fitting to NIR CMDs.
• The integrated Ks-band absolute magnitudes are MKs = -7.72 mag for FSR 19 and MKs = -7.31 mag for FSR 25, placing them in the faint tail of the globular cluster luminosity function.
• FSR 19 has a core radius of 2.76 ± 0.36 pc and tidal radius of 5.31 ± 0.49 pc, while FSR 25 is more extended with rc = 1.92 ± 0.59 pc and rt = 6.85 ± 1.78 pc.
• Their Gaia EDR3 proper motions are consistent with a common kinematic group: µα∗ ≈ -2.5 mas yr⁻¹, µδ ≈ -5.0 mas yr⁻¹ for FSR 19, and µα∗ ≈ -2.6 mas yr⁻¹, µδ ≈ -5.2 mas yr⁻¹ for FSR 25.