Pandas'cubs: Discovery of Two New Dwarf Galaxies in the Surroundings
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Draft version October 30, 2018 A Preprint typeset using LTEX style emulateapj v. 08/22/09 PANDAS’ CUBS: DISCOVERY OF TWO NEW DWARF GALAXIES IN THE SURROUNDINGS OF THE ANDROMEDA AND TRIANGULUM GALAXIES.12 Nicolas F. Martin1, Alan W. McConnachie2, Mike Irwin3, Lawrence M. Widrow4, Annette M. N. Ferguson5, Rodrigo A. Ibata6, John Dubinski7, Arif Babul8, Scott Chapman 3, Mark Fardal9, Geraint F. Lewis10, Julio Navarro8 and R. Michael Rich11 Draft version October 30, 2018 ABSTRACT We present the discovery of two new dwarf galaxies, Andromeda XXI and Andromeda XXII, located in the surroundings of the Andromeda and Triangulum galaxies (M31 and M33). These discoveries stem from the first year data of the Pan-Andromeda Archaeological Survey (PAndAS), a photometric survey of the M31/M33 group conducted with the Megaprime/MegaCam wide-field camera mounted on the Canada-France-Hawaii Telescope. Both satellites appear as spatial overdensities of stars which, when plotted in a color-magnitude diagram, follow metal-poor, [Fe/H] = −1.8, red giant branches at the distance of M31/M33. Andromeda XXI is a moderately bright dwarf galaxy (MV = −9.9 ± 0.6), albeit with low surface brightness, emphasizing again that many relatively luminous M31 satellites still remain to be discovered. It is also a large satellite, with a half-light radius close to 1 kpc, making it the fourth largest Local Group dwarf spheroidal galaxy after the recently discovered Andromeda XIX, Andromeda II and Sagittarius around the Milky Way, and supports the trend that M31 satellites are larger than their Milky Way counterparts. Andromeda XXII is much fainter (MV = −6.5 ± 0.8) and lies a lot closer in projection to M33 than it does to M31 (42 vs. 224 kpc), suggesting that it could be the first Triangulum satellite to be discovered. Although this is a very exciting possibility in the context of a past interaction of M33 with M31 and the fate of its satellite system, a confirmation will have to await a good distance estimate to confirm its physical proximity to M33. Along with the dwarf galaxies found in previous surveys of the M31 surroundings, these two new satellites bring the number of dwarf spheroidal galaxies in this region to 20. Subject headings: Local Group — galaxies: dwarf 1. INTRODUCTION cluding only 6 dwarf spheroidal galaxies, while 12 new Systematic deep photometric observations of the sur- dwarf galaxies have been discovered to inhabit this re- roundings of the Andromeda galaxy have revolutionized gion of the Local Group sky over the last five years. A dedicated scan of the Sloan Digital Sky Survey (SDSS), our knowledge of its satellite system. Before 2004, 12 ◦ dwarf galaxies were known to be M31 companions, in- targeting a region within 2 of M31’s major axis, unveiled the presence of Andromeda IX and X (Zucker et al. 2004, Electronic address: [email protected] 2007). In parallel, a contiguous mapping of the region 1 Max-Planck-Institut f¨ur Astronomie, K¨onigstuhl 17, D-69117 within ∼ 50 kpc of Andromeda, performed with the Wide Heidelberg, Germany Field Camera on the Isaac Newton Telescope, revealed 2 NRC Herzberg Institute for Astrophysics, 5071 West Saanich Road, Victoria, British Columbia, Canada, V9E 2E7 the picture of a strikingly substructured inner stellar halo 3 Institute of Astronomy, Madingley Road, Cambridge, CB3 (Ferguson et al. 2002), and led to the discovery of An- 0HA, U.K. dromeda XVII (Irwin et al. 2008). 4 Department of Physics, Engineering Physics, and Astronomy, Mapping the outer regions of M31’s halo was the goal Queen’s University, Kingston, ON K7L 3N6, Canada 5 Institute for Astronomy, University of Edinburgh, Royal of an extension to this survey. Performed with the arXiv:0909.0399v3 [astro-ph.CO] 17 Sep 2009 Observatory, Blackford Hill, Edinburgh, EH9 3HJ, U.K. MegaPrime/MegaCam 1 deg2 camera on the Canada- 6 Observatoire de Strasbourg, 11, rue de l’Universit´e, F-67000, France-Hawaii Telescope (CFHT), it mapped a quarter Strasbourg, France 7 Department of Astronomy and Astrophysics, University of of the M31 stellar halo, out to ∼ 150 kpc (Ibata et al. Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada 2007). Confirming the clumpy nature of Andromeda’s 8 Department of Physics and Astronomy, University of Victoria, stellar halo, the survey has also revealed the presence 3800 Finnerty Rd., Victoria, BC V8W 3P6, Canada of numerous other dwarf galaxies: Andromeda XI, XII, 9 Astronomy Department, University of Massachusetts, Amherst, MA 01003, USA XIII, XV, XVI, XVIII, XIX and XX (Martin et al. 2006; 10 Institute of Astronomy, School of Physics, University of Ibata et al. 2007; McConnachie et al. 2008). At about Sydney, NSW 2006, Australia. the same time, Andromeda XIV was discovered serendip- 11 Physics and Astronomy Building, 430 Portola Plaza, Box 951547, Department of Physics and Astronomy, University of itously by Majewski et al. (2007), just outside of the edge California, Los Angeles, CA 90095-1547, USA. of the MegaCam survey. 12 Based on observations obtained with MegaPrime/MegaCam, Our understanding of these systems remains sparse, a joint project of CFHT and CEA/DAPNIA, at the Canada- mainly due to the distance at which they reside, trans- France-Hawaii Telescope (CFHT) which is operated by the Na- lating into difficult photometric observations that can- tional Research Council (NRC) of Canada, the Institut National des Science de l’Univers of the Centre National de la Recherche not easily reach much deeper than the horizontal branch Scientifique (CNRS) of France, and the University of Hawaii. at the distance of M31. Spectroscopic observations are 2 N. F. Martin et al. also limited to the handful of bright red giant branch separation only degrades at magnitudes fainter than (RGB) stars that can be targeted in each system. Al- g ∼ 25.0 and i ∼ 24.0. Data were preprocessed (de- though progress is expected along these lines in the com- biased, flat-fielded and fringe corrected) by the Elixir ing years, much can already be said of the generic prop- system at CFHT that also determines the photomet- erties of these new objects from the current survey data ric zero point of the observations, and then processed alone. Their absolute magnitude ranges from a very faint using a version of the Cambridge Astronomical Survey MV = −6.4 for And XII and And XX (Martin et al. Unit (CASU) photometry pipeline (Irwin & Lewis 2001), 2006; McConnachie et al. 2008) to a surprisingly bright adapted to CFHT/MegaCam observations. The pipeline < MV ∼ −9.7 for And XVIII (McConnachie et al. 2008), registers and stacks the images, and also generates cat- patently showing the incompleteness of the M31 satellite alogues with object morphological classification before luminosity function at the faint end, in regions that have creating band-merged g, i products. so far only been surveyed with photographic plates. In the following, dereddened magnitudes (g0 and i0) Given their relative faintness and significant sizes, have been determined from the Schlegel et al. (1998) these new systems are usually assumed to be dwarf E(B − V ) extinction maps, using the following correc- spheroidal galaxies, in other words dwarf galaxies that tion coefficients: g0 = g − 3.793E(B − V ) and i0 = are devoid of any significant amount of gas. This is cur- i − 2.086E(B − V ), listed in their Table 6. rently consistent with the analysis of Hi surveys but the upper limits on their Hi content remains relatively high 3. ANDROMEDA XXI AND ANDROMEDA XXII 5 (2−3×10 M⊙; Grcevich & Putman 2009). Therefore, it cannot be entirely ruled out that some of these galaxies 3.1. Discovery still contain non-negligeable amounts of gas. Two new dwarf galaxies were directly identified on Building upon the previous MegaCam survey, we matched-filter surface density maps of RGB candidate have initiated the Pan-Andromeda Archaeological Sur- stars selected to have colors consistent with the average vey (PAndAS), a Large Program using the CFHT Mega- (generally) metal-poor locus of M31 dwarf spheroidals. Cam imager to map the entire stellar halo of M31 and Satellites in the very sparsely populated outer halo re- M33 out to distances of ∼ 150 and ∼ 50kpc respec- gions correspond to stellar overdensities that are striking tively. Here, we report on the discovery of two new dwarf enough for them to be easily spotted, even down to mag- galaxies in the vicinity of the Andromeda and Triangu- nitudes of MV ∼ −6.5. This was the case for And XII lum galaxies based on the first year of PAndAS data. (Martin et al. 2006) and And XX (McConnachie et al. These systems, dubbed Andromeda XXI and XXII are 2008) and also here with Andromeda XXII. An auto- sparse but unmistakable overdensities of stars that are mated search is certainly warranted to better quantify also aligned along a RGB at the distance of M31. Sec- the detection limits of the survey in uncovering new tion 2 of this paper briefly summarizes the PAndAS data dwarf galaxies. However, it is beyond the scope of this while section 3 presents the new systems and details their initial discovery paper and will be presented elsewhere. properties. Section 4 briefly discusses the new discoveries The locations of the two new satellites are shown on and section 5 concludes the paper. the map of the PAndAS survey (Figure 1). As with many Throughout this paper, the distance moduli of M31 previous detections, these two systems are noticeably and M33 are assumed to be (m−M)0 = 24.47±0.07 and close to the edge of the survey, suggesting that the survey (m − M)0 = 24.54 ± 0.06, or 783 ± 25 and 809 ± 24kpc limit of 150 kpc in projected distance from M31 does not respectively (McConnachie et al.