Ghosts of Milky Way's past: the globular cluster ESO 37-1 (E 3)

Abstract

Context. In the Milky Way, most globular clusters are highly conspicuous objects that were found centuries ago. However, a few dozen of them are faint, sparsely populated systems that were identified largely during the second half of the past century. One of the faintest is ESO 37-1 (E 3) and as such it remains poorly studied, with no spectroscopic observations published so far, although it was discovered in 1976. Aims. We investigate the globular cluster E 3 in an attempt to better constrain its fundamental parameters. Spectroscopy of stars in the field of E 3 is shown here for the first time. Methods. Deep, precise VI CCD photometry of E 3 down to V=26 mag is presented and analysed. Low-resolution, medium signal-to-noise ratio spectra of nine candidate members are studied to derive radial velocity and metallicity. Proper motions from the UCAC4 catalogue are used to explore the kinematics of the bright members of E 3. Results. Isochrone fitting indicates that E 3 is probably very old, with an age of about 13 Gyr; its distance from the Sun is nearly 10 kpc. It is also somewhat metal rich with [Fe/H]=-0.7. Regarding its kinematics, our tentative estimate for the proper motions is (-7.0+/-0.8, 3.5+/-0.3) mas/yr (or a tangential velocity of 382+/-79 km/s) and for the radial velocity is 45+/-5 km/s, in the solar rest frame. Conclusions. E 3 is one of the most intriguing globular clusters in the Galaxy. Having an old age and being metal rich is clearly a peculiar combination, only seen in a handful of objects like the far more conspicuous NGC 104 (47 Tucanae). In addition, its low luminosity and sparse population make it a unique template for the study of the final evolutionary phases in the life of a star cluster. Unfortunately, E 3 is among the most elusive and challenging known globular clusters because field contamination severely hampers spectroscopic studies.

7 pages, 6+1 figures, 2 tables. Accepted for publication in Astronomy and Astrophysics. Minor changes

References in corpus (4)

• The Fourth US Naval Observatory CCD Astrograph Catalog (UCAC4)
• The ACS Survey of Galactic Globular Clusters. I. Overview and Clusters Without Previous HST Photometry
• Whiting 1: the youngest globular cluster associated with the Sgr dSph
• The extended star formation history of the star cluster NGC 2154 in the Large Magellanic Cloud

Cited by in corpus (11)

• Accurate distances to Galactic globular clusters through a combination of Gaia EDR3, HST and literature data
• Homogeneous photometry VII. Globular clusters in the Gaia era
• The cumulative mass profile of the Milky Way as determined by globular cluster kinematics from Gaia DR2
• VVVX-Gaia Discovery of a Low Luminosity Globular Cluster in the Milky Way Disk
• Young stars in the periphery of the Large Magellanic Cloud
• Globular clusters: absolute proper motions and Galactic orbits
• Chemical abundance analysis of red giant branch stars in the globular cluster E3
• Towards a fully consistent Milky Way disk model -- IV. The impact of Gaia DR2 and APOGEE
• The disruption of the low-mass globular cluster E3
• Searching for the lost Unicorn: a prominent feature in the radial velocity distribution of stars in Vela from Gaia DR2 data
• Discovery of a hot post-AGB star in Galactic globular cluster E3