arXiv:astro-ph/0408197v1 11 Aug 2004

eeldapoietoedniyo aosaswihhas which stars data halo these of F-stars, overdensity of prominent colours a the have revealed to selected and tor, n aosasdanfo h la iia k Survey. Sky Digital examin- Sloan of while band the narrow (2002) from a of al. in drawn Taken et population stars Newberg halo halo additional ing by found an was of stars indication Galaxy first the around The ring The 2 properties its of understanding current of our discovery and the the reviews object within this paper place This Way. taking Milky is the of that plane event part accretion being current as curious a interpreted a been of has uncovered that scale have stars is large however, of overdensity Recent systems, region, Way. 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Nicolas ean .Lewis F. Geraint NF-Osraoi srnmc iBlga i azn ,4 1, Ranzani Via Bologna, di Email: 0HA Astronomico CB3 Osservatorio Cambridge, - Road, INAF F-6700 Madingley l’Universite, Astronomy, de of Rue Institute University 11 A29, Strasbourg, Physics, de of Observatoire School Astronomy, of Institute ∼ PUBLISHING saprn ihteMlyWy ihrcn ob- recent with Way, Milky the with apparent is Gr Iaae l 02 aesie l 2003), al. et Majewski 2002; al. et (Ibata 7Gyrs [email protected] aay tutr aay vlto aais dwarf galaxies: – evolution Galaxy: – structure Galaxy: ∼ 1 2 , 2 5 . 5 ihl Bellazzini Michele , org .Ibata A. Rodrigo , o bu h eeta equa- celestial the about isn satellite missing ulctoso h srnmclSceyo Australia of Society Astronomical the of Publications 2 ihe .Irwin J. Michael , 4 . li Conn Blair & ci ao h oainadeeta eieo Canis of demise eventual and location the halo, actic h itnet h temrne from that ranges estimates stream study the this to this field, distance of each the sequence main in ac- the population stream Fitting stellar Monoceros Galaxy. the the that rings suggest tually (2003) al. et Ibata vrtesy within sky, the over h ooeo tempplto nanme faddi- extend of to number population a this revealing in fields, population tional identified stream also study Monoceros This popula- Monoceros. the of stellar vicinity distinct the in the tion of (2003) identification al. the archive. the et confirmed data Ibata in diagrams, Survey stars colour-magnitude Field of of Analysis Wide stream Telescope Monoceros Newton the Isaac of signature the for unknown yet a of but that galaxy, stream, suggested companion tidal disrupting (2002) a represented al. et too Newberg this out, ruled clusively width fMncrs taglcoeti itneof distance galactocentric directio a the at in Monoceros, anti-centre, of Galactic the within towards identified halo, was addi- the stars One of Galaxy. overdensity Dwarf prominent Sagittarius tional the of from stream the debris through tidal slicing survey the as interpreted been -trcniae,ti td bandsetai eea r a upon several in Focusing of spectra SDSS. obtained analysis study the this from candidates, kinematic F-star drawn a stars presented halo Galactic (2003) al. et Yanny in resonance a via disk. Galactic generated is the flare/warp spiral data unknown outer their an or that including arm hypotheses out other pointed with a also consistent was they population dwarf, stellar this accreting that possibility the consider iha paetsaehih of scale-height apparent an with sdafglx ersnsteol ao ongoing major only the represents galaxy dwarf us a crto vn,teCnsMjrdafgalaxy. dwarf Major Canis the event, accretion Way kdisk. ck rt fselrdbi dnie hr.Ti paper This there. identified debris stellar of ority fSde,NW20,Australia 2006, NSW Sydney, of tabug France Strasbourg, ci ln n per ob na equatorial an on be to appears and plane actic pre yti icvr,Iaae l 20)searched (2003) al. et Ibata discovery, this by Spurred ttesm iea h bt ta.(03 results, (2003) al. et Ibata the as time same the At < 17 oon,Italy Bologna, 0127, U.K. , kc hl aatcoii ol o econ- be not could origin Galactic a While 6kpc. 1 3 , ∼ 30 o fteGlci qao,and equator, Galactic the of ∼ 0 . 5p.Wiethey While 75kpc. 00 0,000–000 000, 0000, ∼ 5to 15 ∼ 8p and 18kpc ∼ ∼ 20kpc, 100 e- n n o 2 Lewis et al.

Figure 1. Binned M-giant counts about the Galactic plane. The three panels denote different magnitude (and hence distance) ranges, with the galactocentric distance modulus labeled upon each panel. Also in each panel, the counts represent the asymmetry between the north and south (ie - southern data subtracted from the north and vice-versa). The main body of Canis Major and prominent structures in the M-giant distribution are noted (Figure 5 from Martin et al. (2004)). gions, allowing the determination of their kinemtatic prop- Way. With a galactocentric distance of 18 ± 2kpc, they erties. Accounting for the Galactic contribution, the Mono- find the arc of material possesses an angular extent of at o ceros population was found to have a velocity dispersion of ∼ 170 , with the presence of M giants indicating the stel- ∼ 25 − 30km/s. While of similar order to the tidal debris lar population of the Monoceros stars possesses a higher F e torn from the Sagittarius dwarf, this velocity dispersion is metallicity than previously estimated ([ H ] ∼ 0.4 ± 0.3). quite distinct from the spheroid, thick disk or any known Crane et al. (2003) extended this work, obtaining veloci- o warp or flare. From these velocities, Yanny et al. (2003) ties of 2MASS selected stars over ∼ 100 of the Mono- deduce that the orbital velocity of the stream of stars is ceros stream. While confirming a velocity dispersion of prograde and (assuming circular orbits) is 215 ± 25km/s, ∼ 20km/s, these data also indicate the stars in the Mono- [An erratum to the original result of 110±25kms/s was pre- ceros stream orbit the Galaxy in a prograde fashion, with sented in Yanny et al. (2004)], and that the stars appear little eccentricity. While this is somewhat worrisome for the F e to be relatively metal-poor ([ H ] = −1.6). Yanny et al. tidally disrupting dwarf galaxy hypothesis, Crane et al. (2003) conclude by proposing a simple model for the Mono- (2003) point out that such features are apparent in numer- ceros stream as a disrupting dwarf, orbiting the Milky Way ical simulations of in-plane dwarf accretion(Helmi et al. at a distance of ∼ 18kpc; the dwarf galaxy’s stars are un- 2003). Furthermore, this study identifies four globular clus- dulating above and below the plane of the Milky Way by ters that are spatially and kinematically aligned with the ∼ 6kpc. putative stellar stream; such an alignment argues against a Several additional programs have focused upon the Galactic origin for the stream, such as a spiral arm. Finally, Monoceros stream 1. Using M giant star candidates drawn Frinchaboy et al. (2004) noted that five globular clusters from 2MASS, Rocha-Pinto et al. (2003) further confirmed aligned with the Monoceros stream, as well as ∼ 15 outer, the existence of the Monoceros Stream as a distinct stel- old stellar clusters that may also be part of the same pop- lar population beyond the edge of the disk of the Milky ulation; these clusters lie in a plane which is significantly tilted (∼ 17o) to that of the Milky Way. The population of globular clusters is reminiscent of the Sagittarius Dwarf 1 The stream of stars has acquired several names since its dis- galaxy which has appears to have deposited a similar num- covery. For the sake of consistency, in this paper it is referred ber of globular clusters into the halo of the Milky Way, bol- to solely as the Monoceros Stream.

c Astronomical Society of Australia 0000 The Canis Major Dwarf Galaxy 3 stering the argument that the Monoceros stream represets the mass distribution of the Milky Way (Dehnen & Binney a similarly disrupting dwarf galaxy. 1998), these simulations involved following the dynami- The extensive nature of the material strongly sug- cal dissociation of dwarf galaxies (modeled as King pro- gested that its origin lay in a disrupting system, in an files) as they orbited the Galaxy. The results of this pro- event similar to that of Sagittarius, but occurring in the cedure favoured a dwarf galaxy with an initial mass of 8 plane of the Galaxy. This, however, was not the only ex- ∼ 5 × 10 M⊙, with a orbital period of ∼ 0.4Gyrs. While planation for its origin as it may represent a previously such simulations can reasonably account for the observed unidentified aspect of Galactic structure. If the disrupt- distribution of M-stars not only in the body of Canis Ma- ing system hypothesis is correct, and its destruction is not jor, but also along the extensive arcs above and below the yet complete, we should expect to identify some remnant Galactic plane, the current data does not allow a defini- of the original dwarf galaxy. However, the expected loca- tive differentiation between prograde and retrograde or- tion, observed against the plane of the Galaxy, makes the bits. Martin et al. (2004), however, point out that the re- detection of such a remnant difficult. sultant orbits of the debris in such an encounter closely mimic those of stars in the thick disk. Given that the es- timated mass of this single dwarf is roughly ∼ 10% of the 3 The Canis Major Dwarf entire thick disk, then the thick disk could be formed via only a few such accretion events. As with Rocha-Pinto et al. (2003), Martin et al. (2004) Further observational evidence for the nature of the employed 2MASS to search for a signature of Mono- o Canis Major dwarf came from the study of Bellazzini et al. ceros stream of stars. Pushing the search to |b| ∼ 5 , (2004) who identified the main-sequence and red giant Martin et al. (2004) mapped the density of M-giant stars branch populations of Canis Major in the background to around the Galactic equator. This reveals a strong asym- the Galactic open clusters NGC2477, Tombaugh 1 and metry about the Galactic plane which is interpreted as being the Monoceros ring of stars snaking around the out- Berkeley 33. Analysis of this population suggested it is somewhat metal-rich (−0.7 < [ F e ] < 0.0), with an age of skirts of the Milky Way. Several prominent features are H ∼ 2 − 7Gyrs, although an apparent blue plume of stars noted, including a strong Northern arc and weaker South- ern arc, both of which extend for more than ∼ 100o on the is taken as evidence of a more recent episode of star for- mation. This study also finds a photometric parallax for sky (see Figure 1). the main body of Canis Major of ∼ 8.3 ± 1.2kpc, larger Intriguingly, Martin et al. (2004) also identified a than the M-giant study of (Martin et al. 2004), whose strong, elliptical overdensity of M-stars at (l,b) = o o smaller distance determination they put down to poorer (240 , −8 ), aligned somewhat with the Galactic disk. The systematics in the M-giant photometric parallax. Finally, (heliocentric) distance to this stellar overdensity is D⊙ = Bellazzini et al. (2004) also suggest that as well as the iden- 7.1 ± 0.1kpc, with a major axis is ∼ 4.2kpc. With ∼ 2300 o tified globular cluster population, two old open clusters, M-giant stars within 10 of its centre, this overdensity con- AM-2 and Tombaugh 2, are possibly associated with the tains a similar number of M-giants to the Sagittarius Dwarf Canis Major dwarf. galaxy, a system which we know is currently being canni- balised by the Milky Way. Given that this implies that the Utilizing the Second U.S. Naval Observatory CCD As- 8 9 mass of the Canis Major overdensity is ∼ 10 − 10 M⊙, trograph Catalog (UCAC2), Ibata et al. (2004) examined Martin et al. (2004) concluded that it too represents a the proper motions of stars in the vicinity of the Canis dwarf galaxy also undergoing tidal disruption, and pos- Major dwarf galaxy. Cross-identifying M-giant stars drawn sibly representing the progenitor of the Monoceros stream from the 2MASS catalog, the Galactic-longitudinal motion of stars. of Canis Major was found to be µl = −4.0 ± 0.4mas/yr, Unlike Sagittarius, which passes over the poles of the with no measurable motion in Galactic latitude. At a dis- Milky Way, the identification of the Canis Major dwarf tance of D = 8.3kpc, this corresponds to transverse veloc- galaxy (as this overdensity will now be referred to) repre- ity of ∼ 238 ± 28km/s, with Canis Major on a prograde sents the first detection of an accretion occurring within orbit about the Milky Way. Since this value is higher than the plane of the Galaxy. that predicted by Martin et al. (2004), drawn from their Martin et al. (2004) also noted that possibly five glob- numerical simulation, it is clear the full three-dimensional ular clusters were associated in phase-space with the Ca- velocity of Canis Major will be required for the detailed nis Major dwarf galaxy. This is a similar number to the numerical modeling of this accretion event. globular cluster population currently associated with the Recently, Forbes et al. (2004) further examined the Sagittarius dwarf galaxy, providing further evidence for the globular cluster population associated with the Canis Ma- origin of Canis Major. Furthermore, Martin et al. (2004) jor dwarf galaxy, finding that the age-metallicity relation- noted the phase-space grouping of a number of four open ship for these is distinct from the main globular cluster galactic clusters directly associated with the main body of 2 population of the Milky Way. Furthermore, these globu- Canis Major . lar clusters are somewhat smaller than expected if they In an attempt to understand the observed distribu- were drawn from the Galactic population. Both these lines tion of M-stars, Martin et al. (2004) also undertook a se- of evidence add further weight to the possibility that the ries of numerical simulations. Utilizing detailed models for globular clusters, and Canis Major itself, were formed else- where, were drawn in presumably as the initial orbit de- 2 Several of these clusters were also noted by Frinchaboy et al. cayed by dynamical friction, and represent an on-going ac- (2004) as being part of the Monoceros stream. cretion event onto the Milky Way galaxy.

c Astronomical Society of Australia 0000 4 Lewis et al.

4 Further Observations While the tidally disrupting dwarf galaxy is the favoured explanation for the observed overdensity of star in Canis Major, current observations are not yet completely conclu- sive, and the possibility that the Monoceros Ring and Ca- nis major dwarf galaxy actually represent some unknown aspect of Galactic structure has not been ruled out. Hence, several observational programs are underway to address this issue. The first is an extension of the earlier study by Ibata et al. (2003), systematically mapping above and below the Galactic plane with wide-field camera observa- tions. This has been completed in the north, using the Wide-Field Camera on the Isaac Newton Telescope; these data are being analyzed and will be published shortly (Conn et al. in preparation). The southern survey, with the Wide-Field Imager at the Anglo-Australian Telescope, is currently underway and will be completed by mid-2004. These will provide important probes of the extent of the stellar material, as well as constraining its distance (via main sequence fitting). To augment these studies, 2dF observations of the main body of Canis Major and the extensive stream of stars are being conducted to obtain stellar kinematics, via the calcium triplet, over ∼ 100o of the sky. Coupled with the spatial data obtained with the wide-field cam- era surveys, these data should test the hypotheses that Canis Major represents a truly disrupting dwarf galaxy, or (equally interesting) is a currently unknown aspect of Galactic structure. Furthermore, if the dwarf galaxy is con- firmed, then these data will provide important constraints to numerical simulations of its orbit and eventual demise. Figure 2. A numerical simulation of the demise of the Canis Major dwarf galaxy. The top panel presents the view from above the Milky Way [with the Sun located at (x,y,z)= −8, 0, 0 kpc]. The lower panel presents the side-on view of the debris. The ACKNOWLEGEMENTS colour coding in both denoted the heliocentric velocity of the stars (as given in the lower key), while the symbols repre- GFL thanks Joss Bland-Hawthorn for inviting him to the sent data from the INT survey and 2MASS [Figure 14 from Little Bay Meeting. The anonymous referee is thanked for Martin et al. (2004)]. their constructive comments.

Ibata R. A., Irwin M. J., Lewis G. F., Ferguson A. M. N., Tanvir N., 2003, MNRAS, 340, L21 REFERENCES Ibata R., Bellazzini, M., Irwin M., Lewis G. F., Martin, N. F., 2004, MNRAS (Submitted) Bellazzini M., Ibata R., Monaco L., Martin N., Ir- Ibata R. A., Lewis G. F., Irwin M. J., Cambr´esy L., 2002, win M. J., Lewis G. F., 2004, MNRAS in press, MNRAS, 332, 921 astro-ph/0311119 Ibata R., Lewis G. F., Irwin M., Totten E., Quinn T., Brook C. B., Kawata D., Gibson B. K., Flynn C., 2003, 2001, ApJ, 551, 294 ApJ, 585, L125 Klypin A., Kravtsov A. V., Valenzuela O., Prada F., 1999, Crane J. D., Majewski S. R., Rocha-Pinto H. J., Frinch- ApJ, 522, 82 aboy P. M., Skrutskie M. F., Law D. R., 2003, ApJ, 594, Majewski S. R., Skrutskie M. F., Weinberg M. D., Os- L119 theimer J. C., 2003, ApJ, 599, 1082 Dehnen W., Binney J., 1998, MNRAS, 294, 429 Martin N. F., Ibata R. A., Bellazzini M., Irwin M. J., Forbes, D. A., Strader, J., Brodie, J.P., 2004, Lewis G. F., Dehnen W., 2004, MNRAS, 348, 12 astro-ph/0403136 Newberg H. J. et al., 2002, ApJ, 569, 245 Frinchaboy P. M., Majewski S. R., Crane J. D., Reid I. N., Rocha-Pinto H. J., Majewski S. R., Skrutskie M. F., Rocha-Pinto H. J., Phelps R. L., Patterson R. J., Mu˜noz Crane J. D., 2003, ApJ, 594, L115 R. R., 2004, ApJ, 602, L21 Yanny B. et al., 2003, ApJ, 588, 824 Helmi A., Navarro J. F., Meza A., Steinmetz M., Eke Yanny, B., et al. 2004, ApJ, 605, 575 V. R., 2003, ApJ, 592, L25 Helmi A., White S. D. M., de Zeeuw P. T., Zhao H., 1999, Natur, 402, 53

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