Scl-dE1 GC1: An Extended Globular Cluster in a Low-Luminosity Dwarf Elliptical Galaxy

arXiv:0903.0215v1 [astro-ph.GA] 2 Mar 2009 Accepted for Publication in the Astronomical Journal Scl-dE1 GC1: An Extended Globular Cluster in a Low-Luminosity Dwarf Elliptical Galaxy1 G. S. Da Costa1, E. K. Grebel2, H. Jerjen1, M. Rejkuba3, and M. E. Sharina4 ABSTRACT We report the discovery from Hubble Space Telescope ACS images of an extended globular cluster, denoted by Scl-dE1 GC1, in the Sculptor Group dwarf Elliptical galaxy Scl-dE1 (Sc22). The distance of the dE is determined as 4.3 ± 0.25 Mpc from the I magnitude of the tip of the red giant branch in the color-magnitude diagram. At this distance the half-light radius of Scl-dE1 GC1 is ∼22pc, placing it among the largest clusters known, particularly for globular clusters associated with dwarf galaxies. The absolute magnitude of Scl-dE1 GC1 is MV = –6.7 and, to within the photometric uncertainties of the data, the cluster stellar population appears indistinguishable from that of the dE. We suggest that there may be two modes of globular cluster formation in dwarf galaxies, a “normal” mode with half-light radii of typically 3 pc, and an “extended” mode with half-light radii of ∼10 pc or more. Subject headings: galaxies: dwarf — galaxies: star clusters — globular clusters: general 1. Introduction Globular clusters have generally been regarded as occupying distinctly different regions from dwarf galaxies in the multi-dimensional space formed from parameters such as central surface brightness, scale length, and total luminosity, mass or velocity dispersion. Recent discoveries, 1Research School of Astronomy & Astrophysics, The Australian National University, Mt Stromlo Observatory, via Cotter Rd, Weston, ACT 2611, Australia 2Astronomisches Rechen-Institut, Zentrum f¨ur Astronomie der Universit¨at Heidelberg, M¨onchhofstr. 12-14, D- 69120 Heidelberg, Germany 3European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei Mnchen, Germany 4Special Astrophysical Observatory, Nizhnii Arkhys 369167, Russia 1Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program GO-10503. – 2 – however, have blurred the previous clear-cut separation. For example, among the newly discovered dwarf galaxy satellites of the Milky Way, there are systems such as Willman 1 (Willman et al. 2005), Segue 1 (Belokurov et al. 2007) and Bo¨otes II (Walsh, Jerjen & Willman 2007) which have very low luminosities and small scale lengths that overlap with those of globular clusters (e.g. Gilmore et al. 2007; Martin, de Jong & Rix 2008, and the references therein). Equally, there is increasing recognition of the existence of extended globular clusters with comparatively large scale lengths; the M31 clusters discussed in Huxor et al. (2005) and Mackey et al. (2006), and the M33 extended star cluster M33-EC1 (Stonkut˙e et al. 2008) being prime examples. Note that we argue that extended globular clusters such as these are distinct from the diffuse stellar clusters discussed in Peng et al. (2006), which have notably red (metal-rich) colors and which are spatially associated with galactic disks (Peng et al. 2006). Similarly, we also regard extended globular clusters as distinct from the “faint fuzzies”, low luminosity (MV ≳–7), large (half-light radii between 7 and 15 pc) star clusters that are spatially and kinematically associated with the disks of lenticular galaxies (cf. Brodie & Larsen 2002). The origin of the extended globular clusters is not well understood. They are apparently comparatively rare objects, at least in luminous galaxies. For example, G´omez & Woodley (2007) indicate that in NGC 5128, extended clusters (defined as clusters with half-light radii, rh, ex- ceeding 8 pc) make up only ∼2.4% of their sample. Similarly, in the catalog of Milky Way globular cluster parameters (Harris 1996, 2003 Feb version), ∼9% of clusters have rh exceeding 10 pc. As has been known for some time (e.g. van den Bergh & Mackey 2004, Fig. 3), the ma- jority of these extended clusters are found at large Galactocentric radii. The results for four other spirals (M81, M83, NGC 6946, and M101) show similar small fractions of extended clusters (Chandar, Whitmore & Lee 2004), but intriguingly, in M51 ∼24% (8 of 34) of the cluster candidates have rh > 10 pc (Chandar, Whitmore & Lee 2004). Extended globular clusters are also known to occur in dwarf galaxies, for example, the Reticulum cluster in the LMC, Arp 2 in the Sagittarius dwarf, and cluster #1 in Fornax all have rh exceeding 10 pc (e.g. van den Bergh & Mackey 2004). Clearly to understand the formation processes for these extended clusters, and particularly to investigate any dependence on environment or host galaxy type, structural parameters for larger samples of such clusters are needed. In this context, as already im

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