A bag of tricks: Using proper motions of Galactic stars to identify the Hercules ultra-faint dwarf galaxy members ^⋆,⋆⋆

¹ INAF – Osservatorio Astronomico di Teramo, via Mentore Maggini s.n.c., 64100 Teramo, Italy
e-mail: fabrizio@oa-teramo.inaf.it
² INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone (RM), Italy
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Dipartimento di Fisica e Astronomia, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁵ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁶ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁷ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁸ Dipartimento di Fisica, Università di Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
⁹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received: 11 March 2014
Accepted: 13 August 2014

Abstract

Context. Discovered in the last decade as overdensities of resolved stars, the ultra-faint dwarfs (UFDs) are among the least luminous, most dark-matter dominated, and most metal-poor galaxies known today. They appear as sparse, loose objects with high mass-to-light ratios. Hercules is the prototype of the UFD galaxies. To date, there are still no firm constraints on its total luminosity due to the difficulty of disentangling Hercules bona-fide stars from the severe Galactic field contamination.

Aims. To better constrain Hercules properties, we aim at removing foreground and background contaminants in the galaxy field using the proper motions of the Milky Way stars and the colour–colour diagram.

Methods. We have obtained images of Hercules in the r_Sloan , B_Bessel and U_spec bands with the Large Binocular Telescope (LBT) and LBC-BIN mode capabilities. The r_Sloan new dataset combined with data from the LBT archive span a time baseline of about 5 yr, allowing us to measure proper motions of stars in the Hercules direction for the first time. The U_spec data along with existing LBT photometry allowed us to use colour–colour diagram to further remove the field contamination.

Results. Thanks to a highly-accurate procedure to derive the r_Sloan -filter geometric distortion solution for the LBC-red, we were able to measure stellar relative proper motions to a precision of better than 5 mas yr^-1 down to r_Sloan≃ 22 mag and disentangle a significant fraction (>90%) of Milky Way contaminants. We ended up with a sample of 528 sources distributed over a large portion of the galaxy body (~0.12 deg²). Of these sources, 171 turned out to be background galaxies and additional foreground stars from the analysis of the U_spec − B_Bessel vs. B_Bessel − r_Sloan colour–colour diagram. This leaves us with a sample of 357 likely members of the Hercules UFD. We compared the cleaned colour-magnitude diagram (CMD) with evolutionary models and synthetic CMDs, confirming the presence in Hercules of an old population (t = 12 ± 2 Gyr) with wide spread metallicity (−3.3 < [Fe/H] < − 1.8).

Conclusions. Our procedure to estimate star proper motions proved to be a very effective way to identify likely members for stellar systems as far as 130 kpc. The present selection, based on both proper motions and colour–colour diagram, provides a robust identification of Hercules members and a new target list for more spectroscopic investigations.