Mon. Not. R. Astron. Soc. 000, 1–5 (2005) Printed 15 September 2018 (MN LaTEX style file v2.2) A radial velocity survey of low Galactic latitude structures: II. The Monoceros Ring behind the Canis Major dwarf galaxy Blair C. Conn1, Nicolas F. Martin2,4, Geraint F. Lewis1, Rodrigo A. Ibata2, Michele Bellazzini3 & Mike J. Irwin4 1Institute of Astronomy, School of Physics, A29, University of Sydney, NSW 2006, Australia: Email [email protected], [email protected] 2Observatoire de Strabourg, 11, rue de l’Universit´e, F-67000, Strasbourg, France: Email [email protected], [email protected] 3INAF - Osservatorio Astronomico di Bologna, Via Ranzani 1, 40127, Bologna, Italy: Email [email protected] 4Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, U.K.: Email [email protected] 15 September 2018 ABSTRACT An AAT/2dF Spectrograph Survey of low Galactic latitudes targeting the putative Canis Major dwarf galaxy, and the (possibly) associated tidal debris of stars known as the Monoceros Ring, covering Galactic coordinates 231.5◦ < l < 247.5◦ and -11.8◦ <b<-3.8◦, has revealed the presence of the Monoceros Ring in the background of the Canis Major dwarf galaxy. This detection resides at a galactocentric distance of ∼18.9±0.3kpc (13.5±0.3kpc heliocentric), exhibiting a velocity of ∼132.8±1.3 kms−1 with a dispersion of ∼22.7±1.7 kms−1; both of these comparable with previous measurements of the Monoceros Ring in nearby fields. This detection highlights the increasing complexity of structure being revealed in recent surveys of the Milky Way thick disk and Halo. Key words: Galaxy: structure – Galaxy: formation – galaxies: interactions 1 INTRODUCTION velocity of ∼220 km s−1 best fit the data. The velocity dispersion arXiv:astro-ph/0508366v1 17 Aug 2005 of their sample was 20±4 km s−1 which excludes it from being Galaxy formation based on a ΛCDM cosmology predicts larger of Galactic origins. As more detections of this structure were made structures being formed from the accumulation of smaller sys- (Yanny et al. 2004; Ibata et al. 2003; Rocha-Pinto et al. 2003) it be- tems (e.g. Searle & Zinn 1978; White & Rees 1978; White 1978; came clear that, as proposed by Newberg et al. (2002), this was in- Abadi, Navarro, Steinmetz, & Eke 2003a,b). One of the core out- deed another on-going accretion event. The search for the progen- comes of this model is that the halos of galaxies should be strewn itor of the stream in the 2MASS catalog led Martin et al. (2004a) with the debris from all these minor mergers. The first discov- to uncover the Canis Major dwarf galaxy (l,b) = ∼(240◦,-8◦). ery of such a merger within our own Milky Way, the Sagittar- Because of the low density of stream material on the sky ius dwarf galaxy (Ibata, Gilmore, & Irwin 1994), demonstrated that it has been necessary to employ the use of Wide-Field Cam- such mergers do occur and are in fact ongoing. eras to look deeply into Milky Way’s thick disk and Halo, not Insight into the complex nature of the Galactic Halo has only confirming and enhancing previous detections but probing been in part accomplished by recent all sky surveys, Sloan Dig- new regions leading to more detections of the Monoceros Ring ital Sky Survey (SDSS) and the Two Micron All Sky Survey and the Canis Major dwarf being made (Martin et al. 2004a; (2MASS). Investigating an overdensity of F-turnoff stars in the Bellazzini et al. 2004; Conn et al. 2005; Mart´inez-Delgado et al. SDSS, Newberg et al. (2002) discovered the presence of a stream 2005). Detections of an additional structure in Triangulum- of stars suggestive of an equatorial accretion event around the Andromedae (Rocha-Pinto et al. 2004; Majewski et al. 2004) are Milky Way. Unlike the Sagittarius dwarf galaxy which is on a polar revealing the increasing complexity of accretion events in the outer orbit, this accretion event may have implications on the formation regions of the Galaxy. Current debate on these detections centres on of key Galactic structures, such as the thick disk. The equatorial whether these represent separate accretion events or are the product orbit of the stream was investigated by Crane et al. (2003) who tar- of a single ongoing merger. The location of the progenitor is still geted 2MASS selected M-giant stars in the range l = 150◦ - 240◦, contentious but this paper will refer to the Canis Major overdensity although primarily in the Northern hemisphere, finding that a sim- as the Canis Major dwarf galaxy (CMa), following the conclusions ple circular orbit model with a galactocentric radius of 18 kpc and of Mart´inez-Delgado et al. (2005) and Martin et al. (2005a). c 2005 RAS 2 Blair Conn et al. While the positions of the streams on the sky are highly ac- of the spectra a custom-made reduction pipeline was constructed curate, the distance estimates to the stars are less so, requiring to minimise the errors this produced when comparing with radial the need for radial velocity kinematic measurements to increase velocity standard stars; a detailed presentation of the pipeline can and characterize the known properties of the streams. Distances be found in Martin et al. (2005b). This pipeline limits the system- are then estimated through photometric parallaxes as described in atic offsets in the radial velocity due to the LSF to ±5 km s−1, Martin et al. (2004b). To date, several kinematic surveys of the this value is not included in the errors stated. Determining the ra- Monoceros Stream have been undertaken using the data of 2MASS dial velocities, vr, using several artificial standard star templates and SDSS (Yanny et al. 2004; Crane et al. 2003; Rocha-Pinto et al. and taking a weighted average of the solutions avoids any system- 2004). These have confirmed a velocity gradient with Galactic lon- atic offset that may result from a peculiarity in one of the tem- −1 −1 gitude and also a low velocity dispersion of σ(vr) ∼20 km s for plates (Martin et al. 2005a,b). Stars then with σv > 5 km s (See the Monoceros Ring. Equation 2 of Paper I) are removed from the sample to avoid con- While most models of equatorial accretion onto disk-like taminating the results with poor determination of radial velocity. galaxies (e.g. Pe˜narrubia et al. 2005; Martin et al. 2005a) predict Dust extinction in this region typically is E(B-V)<0.4 (taken from the presence of multiply wrapped streams, no detections provide Schlegel, Finkbeiner, & Davis 1998) and only the field closest to conclusive evidence supporting this hypothesis. This is most likely the plane has higher extinction. There has been no significant im- due to the pencil beam nature of most fields with regard the en- pact on the 2MASS colours due to extinction. tire structure on the sky and given that confirmed detections of the stream are limited to galactic longitudes (120◦ < l < 240◦) and significant areas of this structure are yet to be sampled. This 2.2 Determining the Fit and the Associated Error new detection presented here is consistent with multiply wrapped streams. For each velocity and distance distribution, a Gaussian has been fitted to the profile using an amoeba routine (Press et al. 1992), obtaining a characteristic velocity, velocity dispersion, distance and distance dispersion which best represents the data. The errors on the 2 OBSERVATIONS AND REDUCTION fitted parameters were determined by bootstrap re-sampling and In April 2004, a survey was undertaken with the 2dF (Two-degree then refitting the data. The re-sampling was undertaken using the Field) multi-fibre spectrograph on the Anglo-Australian Telescope errors of each quantity namely the conservative ±1 kpc for the (AAT) with two aims: firstly, to determine the kinematics of the Ca- distance to each star and the root-mean-square value of the disper- nis Major dwarf, and secondly to try and locate the stars belonging sion. The errors cited then, are the standard deviations of the fit to the Monoceros Ring through kinematic constraints. Out of ∼15 determined using this method. There is no appreciable difference fields, eight were dedicated to this purpose, four focusing solely on between the galactocentric and heliocentric errors in this field. The the Canis Major region. This paper will focus primarily on results velocity errors also have an additional ±5kms−1 systematic error pertaining to the detection of the Monoceros Ring in the Canis Ma- on top of the stated errors. jor dwarf region, the same area studied by Martin et al. (2005a); hereafter Paper I. The 2dF instrument is capable of measuring velocities of ∼400 stars simultaneously within a two degree field of view. The 3 RESULTS field is divided between two spectrographs, the first using a 1200V grating (4600-5600A˚ at 1A/pixel);˚ the second using a 1200R grat- 3.1 The Monoceros Ring behind the Canis Major dwarf ˚ ˚ ing (8000-9000A at 1A/pixel). The configuration on the second The data in Figure 1 was taken from six fields of the 2dF spectro- spectrograph was chosen to target Red Clump stars in the distance graphic survey and shows a population of ∼38 RGB stars behind range 5-8 kpc, these stars fall outside the distance range expected the Canis Major overdensity. These stars are represented in the solid for the Monoceros Ring and thus are not discussed in this paper. histogram while the dashed histogram is the remaining stars in that < (Paper I discusses the results of the survey in the region 10 kpc.) region.