MNRAS 501, 1690–1700 (2021) doi:10.1093/mnras/staa2655 Advance Access publication 2020 October 14 A revised view of the Canis Major stellar overdensity with DECam and Gaia: new evidence of a stellar warp of blue stars Downloaded from https://academic.oup.com/mnras/article/501/2/1690/5923573 by Consejo Superior de Investigaciones Cientificas (CSIC) user on 15 March 2021 Julio A. Carballo-Bello ,1‹ David Mart´ınez-Delgado,2 Jesus´ M. Corral-Santana ,3 Emilio J. Alfaro,2 Camila Navarrete,3,4 A. Katherina Vivas 5 and Marcio´ Catelan 4,6 1Instituto de Alta Investigacion,´ Universidad de Tarapaca,´ Casilla 7D, Arica, Chile 2Instituto de Astrof´ısica de Andaluc´ıa, CSIC, E-18080 Granada, Spain 3European Southern Observatory, Alonso de Cordova´ 3107, Casilla 19001, Santiago, Chile 4Millennium Institute of Astrophysics, Santiago, Chile 5Cerro Tololo Inter-American Observatory, NSF’s National Optical-Infrared Astronomy Research Laboratory, Casilla 603, La Serena, Chile 6Instituto de Astrof´ısica, Facultad de F´ısica, Pontificia Universidad Catolica´ de Chile, Av. Vicuna˜ Mackenna 4860, 782-0436 Macul, Santiago, Chile Accepted 2020 August 27. Received 2020 July 16; in original form 2020 February 24 ABSTRACT We present the Dark Energy Camera (DECam) imaging combined with Gaia Data Release 2 (DR2) data to study the Canis Major overdensity. The presence of the so-called Blue Plume stars in a low-pollution area of the colour–magnitude diagram allows us to derive the distance and proper motions of this stellar feature along the line of sight of its hypothetical core. The stellar overdensity extends on a large area of the sky at low Galactic latitudes, below the plane, and in the range 230◦ <<255◦. According to the orbit derived for Canis Major, it presents an on-plane rotation around the Milky Way. Moreover, additional overdensities of Blue Plume stars are found around the plane and across the Galaxy, proving that these objects are not only associated with that structure. The spatial distribution of these stars, derived using Gaia astrometric data, confirms that the detection of the Canis Major overdensity results more from the warped structure of the Milky Way disc than from the accretion of a dwarf galaxy. Key words: Galaxy: disc – Galaxy: formation – Galaxy: halo. Martin et al. (2004a) by analysing the distribution of M-giant stars 1 INTRODUCTION below and above the Galactic plane. Such a remarkable Milky Way The Galactic halo is partially the result of the continuous merging halo substructure is spreading over a wide area (∼100 deg2)ofthe and accretion of minor satellites (e.g. Font et al. 2011; Rodriguez- sky and seems to be located at only <d> ∼ 7 kpc (Bellazzini, Ibata Gomez et al. 2016). Large-scale sky surveys, such as Sloan Digital & Ferraro 2004; Martin et al. 2004b;Mart´ınez-Delgado et al. 2005; Sky Survey (SDSS), Two Micron All-Sky Survey (2MASS), and Bellazzini et al. 2006; Butler et al. 2007). Gaia, have shown us the heterogeneous origin of the outer Milky Way For many years, the origin of that overdensity generated an intense by unveiling past accretion events (e.g. Helmi et al. 2018; Myeong debate. One of the hypotheses about its nature is that CMa is the et al. 2019; Torrealba et al. 2019), a population of stellar tidal streams core of a dwarf galaxy accreted at low Galactic latitudes. The first and overdensities (e.g. Bernard et al. 2016; Shipp et al. 2018), and estimates, based on M-giant stars counts, suggested that CMa and faint galaxies (e.g. Drlica-Wagner et al. 2015; Koposov et al. 2015; the Sagittarius dwarf galaxy have a similar total mass (Martin et al. Koposov, Belokurov & Torrealba 2017; Torrealba et al. 2018). One 2004a). Moreover, its absolute magnitude (MV ∼−14.4) and central −2 of the main evidence of hierarchical formation in our Galaxy is surface brightness (μV,0∼ 24 mag arcsec ) values seemed to locate the tidal stream generated by the accretion of the Sagittarius dwarf this hypothetical galaxy in the same region of the luminosity–size galaxy (Ibata, Gilmore & Irwin 1994;Mart´ınez-Delgado et al. 2001; and MV–μV planes where other Milky Way satellites are located Majewski et al. 2003; Belokurov et al. 2006; Koposov et al. 2012), (Mart´ınez-Delgado et al. 2005; Bellazzini et al. 2006; Butler et al. which is orbiting the Milky Way in almost a polar orbit. Since its 2007). Moreover, a group of peculiar globular clusters (NGC 1851, discovery, a handful of new stellar overdensities, clumps, and streams NGC 1904, NGC 2298, and NGC 2808, among others) has been have been discovered in the halo and this family will keep growing as associated with CMa, what initially reinforced its definition as the coverage of the sky by new projects [e.g. Large Synoptic Survey possible accreted galaxy within the inner Galactic halo (Forbes, Telescope (LSST)] and their completenesses increase. Strader & Brodie 2004; Martin et al. 2004a; Forbes & Bridges The so-called Canis Major overdensity (CMa), an elliptical-shaped 2010). Those globulars present extended haloes and seem to be stellar overdensity centred at (, b) = (240◦, −8◦), was discovered by surrounded by an unexpected stellar population, what might confirm their accreted origin (see Carballo-Bello et al. 2018, and references therein), as also suggested by their position in the age–metallicity E-mail: [email protected] distribution of Galactic globular clusters (see Kruijssen et al. 2019). C 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society A revised view of the Canis Major overdensity 1691 In addition, this family of globulars has been associated with the Gaia think that gravitational interaction between Galactic subsystems is Sausage, a structure in the velocity space formed after the disruption the main mechanism that drives the warp formation (Poggio et al. of a massive galaxy (see Myeong et al. 2018; Forbes 2020,and 2018). references therein). The scarce analyses of chemical abundances of Moreover, the lack of an excess of RR Lyrae stars at the position of CMa members found that some of those stars displayed abundance CMa, as previously reported for other well-studied dwarf galaxies, Downloaded from https://academic.oup.com/mnras/article/501/2/1690/5923573 by Consejo Superior de Investigaciones Cientificas (CSIC) user on 15 March 2021 ratios clearly unusual for Galactic stars (e.g. Sbordone et al. 2005), was pointed out in the last work on this overdensity for the last decade thus confirming the extra-Galactic nature of that halo substructure. and discarded once again its extra-Galactic origin (Mateu et al. 2009). The excess of bright blue stars in the colour–magnitude diagrams The absence of RR Lyrae stars would imply that the alleged galaxy (CMDs) obtained for the central regions of CMa with respect to would have an old population that is either negligible or with very the ones derived for the symmetric fields above the Galactic plane unique properties. Either case implies the stellar population of CMa has been suggested as an additional evidence for the extra-Galactic is at odds with what is observed in the rest of dwarf spheroidal origin of this overdensity (Bellazzini et al. 2004;Mart´ınez-Delgado galaxies in the Local Group and around other nearby stellar systems. et al. 2005; de Jong et al. 2007). This possibly young (1–2 Gyr) To add more complexity to the interpretation of the CMa nature, stellar population, namely the Blue Plume (BP), is bright enough early numerical simulations of the Monoceros Ring, a vast stellar to be detected even in shallow wide-field photometry and is found halo substructure surrounding the Milky Way (Newberg et al. 2002), in a section of the CMD with very low levels of contamination by placed the overdensity in a projected position compatible with that fore/background stars. Dinescu et al. (2005) measured the proper of the stream (Penarrubia˜ et al. 2005). However, this identification motions for a sample of BP stars in direction of the CMa core of CMa as the accreted progenitor galaxy of Monoceros was not −1 −1 (μ cos (b) =−1.47 mas yr , μb =−1.07 mas yr ) and derived, conclusive and even the origin of the ring as a possible Galactic disc together with the radial velocity measured for M-giant stars in the perturbation has recently been object of discussion (e.g. Xu et al. −1 same region (Martin et al. 2004b, vr = 109 km s ), a tentative orbit 2015). for this hypothetical dwarf galaxy. Their results show that CMa might In this paper, we combine new photometric data from our wide- have an in-plane rotation, similar to that of the thick-disc stars in that field Dark Energy Camera (DECam) survey of CMa with parallaxes direction of the Galaxy but with a remarkable motion perpendicular and proper motions provided by the Gaia Data Release 2 (DR2) to to the plane (W =−49 km s−1). An alternative interpretation for map the extend of CMa and shed more light on its real nature. This the detection of BP stars along this line of sight suggests that this data set allows us to study the three-dimensional structure of this population, which is also detected at different longitudes across the stellar overdensity by tracing BP stars across the area covered by our third Galactic quadrant, might be associated with an out-of-plane DECam survey. We also extend this study to the whole sky based on spiral arm of the Milky Way instead of being part of the debris the photometry obtained by this space mission, which is crucial to from a disrupted dwarf galaxy (Carraro 2005; Moitinho et al.