A study of globular clusters in a lenticular galaxy in Hydra I from deep HST/ACS photometry

Abstract

Aims. We take advantage of exquisitely deep optical imaging data from the Hubble Space Telescope’s Advanced Camera for Surveys (HST/ACS) in the F475W (gF475W) and F606W (VF606W) bands to study the properties of the globular cluster (GC) population in the intermediate-mass lenticular galaxy PGC 087327in the Hydra I galaxy cluster. Methods. We inspected the photometric (magnitudes and colour) and morphometric (compactness, elongation, etc.) properties of sources lying in an area of ∼19 × 19 kpc centred on PGC 087327 and compared them with four neighbouring fields over the same HST/ACS mosaic. This allowed us to identify a list of GC candidates and to inspect their properties using a background decontamination method. Results. Relative to four comparison fields, PGC 087327 shows a robust overdensity of GCs, NGC = 82 ± 9. At the estimated magnitude of the galaxy, this number implies a specific frequency of SN = 1.8 ± 0.7. In spite of the short wavelength interval available with the gF475W and VF606W passbands, the colour distribution shows a clear bimodality with a blue peak at ⟨gF475W − VF606W⟩ = 0.47 ± 0.05 mag and a red peak at ⟨gF475W − VF606W⟩ = 0.62 ± 0.03 mag. We also observe the typical steeper slope of the radial distribution of red GCs relative to blue ones. Thanks to the unique depth of the available data, we characterise the GC luminosity function (GCLF) well beyond the expected GCLF turnover. We find gTOMF475W = 26.54 ± 0.10 mag and VTOMF606W = 26.08 ± 0.09 mag, which after calibration yields a distance of DGCLF = 56.7 ± 4.3(statistical) ± 5.2(systematic) Mpc.