Title: A New Milky Way Halo Star Cluster in the Southern Galactic Sky

Abstract

Here, we report on the discovery of a new Milky Way (MW) companion stellar system located at $$(\alpha _{J2000},\delta _{J2000}) = (22^{{\rm h}}10^{{\rm m}}43\buildrel{\mathrm{s}}\over{.}15, 14^\circ 56{^\prime }58{\buildrel{\prime\prime}\over{.}} 8)$$. The discovery was made using the eighth data release of SDSS after applying an automated method to search for overdensities in the Baryon Oscillation Spectroscopic Survey footprint. Follow-up observations were performed using Canada-France-Hawaii-Telescope/MegaCam, which reveal that this system is comprised of an old stellar population, located at a distance of $$31.9^{+1.0}_{-1.6}$$ kpc, with a half-light radius of $$r_h= 7.24^{+1.94}_{-1.29}$$ pc and a concentration parameter of c = log10(rt /rc ) = 1.55. A systematic isochrone fit to its color-magnitude diagram resulted in $$\log\; ({\rm age\,yr^{-1}}) = 10.07^{+0.05}_{-0.03}$$ and $$[{\rm Fe/H}] = -1.58^{+0.08}_{-0.13}$$. These quantities are typical of globular clusters in the MW halo. The newly found object is of low stellar mass, whose observed excess relative to the background is caused by 95 ± 6 stars. The direct integration of its background decontaminated luminosity function leads to an absolute magnitude of MV = –1.21 ± 0.66. The resulting surface brightness is μ V = 25.90 mag arcsec–2. Its position in the MV versus rh diagram lies close to AM4 and Koposov 1, which are identified as star clusters. The object is most likely a very faint star cluster—one of the faintest and lowest mass systems yet identified.

Authors:

Balbinot, E. ^[1]; Santiago, B. X. ^[1]; da Costa, L. ^[2]; Maia, M. A. G. ^[2]; Majewski, S. R. ^[3]; Nidever, D. ^[4]; Rocha-Pinto, H. J. ^[5]; Thomas, D. ^[6]; Wechsler, R. H. ^[7]; Yanny, B. ^[8]

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+ Show Author Affiliations

1. Instituto de Fisica, Porto Alegre, RS (Brazil); Lab. Interinstitucional de e-Astronomia-LIneA, Rio de Janeiro, RJ (Brazil)
2. Lab. Interinstitucional de e-Astronomia-LIneA, Rio de Janeiro, RJ (Brazil); Observatorio Nacional, Rio de Janeiro, RJ (Brazil)
3. Univ. of Virginia, Charlottesville, VA (United States)
4. Univ. of Michigan, Ann Arbor, MI (United States)
5. Lab. Interinstitucional de e-Astronomia-LIneA, Rio de Janeiro, RJ (Brazil); Univ. Federal do Rio de Janeiro, Rio de Janeiro, RJ (Brazil)
6. Univ. of Portsmouth, Portsmouth, NH (United States)
7. SLAC National Accelerator Lab., Menlo Park, CA (United States); Stanford Univ., Stanford, CA (United States)
8. Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)

Publication Date:

Mon Apr 01 00:00:00 EDT 2013

Research Org.:

SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States)

Sponsoring Org.:

USDOE

OSTI Identifier:

1477442

Grant/Contract Number:  

AC02-76SF00515

Resource Type:

Accepted Manuscript

Journal Name:

The Astrophysical Journal

Additional Journal Information:

Journal Volume: 767; Journal Issue: 2; Journal ID: ISSN 0004-637X

Publisher:

Institute of Physics (IOP)

Country of Publication:

United States

Language:

English

Subject:

79 ASTRONOMY AND ASTROPHYSICS; galaxies: dwarf; globular clusters: general; Local Group