arXiv:astro-ph/0508366v1 17 Aug 2005 o.Nt .Ato.Soc. Astron. R. Not. Mon. 5Spebr2018 September 15 li .Conn C. Blair galaxy dwarf Major Canis the behind structure Ring latitude Monoceros Galactic The low II. of survey velocity radial A aayfrainbsdo a on based formation Galaxy INTRODUCTION 1 ihl Bellazzini Michele ee MS eetdMgatsasi h range the in wh (2003) stars equat al. M-giant The et selected Crane disk. 2MASS thick by geted investigated the was stream as the of such for orbit structures, the on Galactic implications key have around of may p event event a on accretion is accretion this which orbit, galaxy equatorial dwarf Sagittarius an stre the Unlike a Way. of Milky of Survey suggestive presence Sky the i stars discovered All (2002) stars of al. Micron F-turnoff et Newberg Two of Di the overdensity SDSS, Sloan an and surveys, Investigating (SDSS) sky (2MASS). all Survey recent Sky by ital accomplished part in been Sagittar- ongoing. the fact in Way, are Milky and occur discov own demonstrat do 1994), mergers first our Irwin such & The within Gilmore, mergers. (Ibata, merger galaxy dwarf minor a ius str these such be all should of galaxies from ery of debris halos ou the the core that with is the 1978; model of White this One 1978; of 2003a,b). sy Rees comes & Eke smaller White & 1978; Steinmetz, of Navarro, Zinn Abadi, & accumulation Searle the (e.g. from tems formed being structures l iclrobtmdlwt aatcnrcrdu f1 k 18 of radius galactocentric a with model that orbit finding circular hemisphere, ple Northern the in primarily although
2 Email Email 1 4 Email 3 Email c bevtied taor,1,red l’Universit´e, F-670 de rue 11, Strabourg, de Observatoire nttt fAtooy colo hsc,A9 University A29, Physics, of School Astronomy, of Institute nttt fAtooy aige od abig,C30HA CB3 Cambridge, Road, Madingley Astronomy, of Institute NF-Osraoi srnmc iBlga i azn ,4 1, Ranzani Via Bologna, di Astronomico Osservatorio - INAF 05RAS 2005 nih notecmlxntr fteGlci aohas Halo Galactic the of nature complex the into Insight [email protected],[email protected] [email protected], [email protected] [email protected] [email protected] [email protected] [email protected], 1 ioa .Martin F. Nicolas , 000 – 20)Pitd1 etme 08(NL (MN 2018 September 15 Printed (2005) 1–5 , 3 Λ ieJ Irwin J. Mike & D omlg rdcslarger predicts cosmology CDM xiiigavlct of velocity a exhibiting oeigGlci oriae 231.5 coordinates of Galactic debris tidal covering associated (possibly) the and ta galaxy, latitudes dwarf Galactic low of Survey Spectrograph AAT/2dF An ABSTRACT rsneo h ooeo igi h akrudo h Canis the of of background distance galactocentric the a in at Ring resides detection Monoceros the of presence e words: Key Halo. and disk thick Way structu Milky of the complexity Monocer increasing the the of highlights detection measurements previous with comparable these l aay tutr aay omto aais interact galaxies: – formation Galaxy: – structure Galaxy: 150 = 0 tabug France: Strasbourg, 00, fSde,NW20,Australia: 2006, NSW Sydney, of 2 , 4 ean .Lewis F. Geraint , ◦ U.K.: , 17 oon,Italy: Bologna, 0127, 4 cand pc 240 - mation dthat ed sim- a the n tar- o orial ewn olar ∼ the am g- ◦ s- 132.8 t- - , ± tro h temi h MS aao e atne l (2004 al. et ( Martin galaxy dwarf led Major catalog Canis 2MASS the uncover the pr to in the stream for the search wa of The this itor event. (2002), al. accretion et on-going Newberg another deed 2003 by al. proposed et as Rocha-Pinto that, 2003; clear al. came et we Ibata structure 2004; this al. of et (Yanny detections more As origins. Galactic of ftersml a 20 was sample their of of velocity n h ai ao wr en ae(atne l 2004a; al. et Mart (Martin 2005; al. made et Conn being Ring 2004; Monoceros dwarf al. the Major et of Bellazzini Canis not detections probi Halo, the more but Cam- and and to detections Wide-Field disk leading previous thick of regions Way’s enhancing new use Milky and the into confirming employ deeply only look to to necessary eras been has it steCnsMjrdafglx Ca,floigteconclus the Mart following (CMa), of galaxy over dwarf Major Major is Canis Canis progenitor the the to the as refer of will paper location this The but merger. contentious ongoing single th a are or of events accretion separate ce represent detections these these whether on Triangulum debate Current Galaxy. a in the in 2004) of events regions al. accretion structure of et complexity Majewski increasing 2004; additional the revealing al. et an (Rocha-Pinto of Andromedae Detections 2005). 1.3 a ◦ ms km T eas ftelwdniyo temmtra ntesky the on material stream of density low the of Because E < tl l v2.2) file style X ´ nzDlaoe l 20)adMri ta.(2005a). al. et Martin and (2005) al. et inez-Delgado l − < ∼ 1 1 ihadseso of dispersion a with 220 247.5 org .Ibata A. Rodrigo , ∼ ms km 18.9 ◦ n -11.8 and − ± ± 1 .kc(13.5 0.3kpc ebigrvae nrcn uvy of surveys recent in revealed being re 4 etfi h aa h eoiydispersion velocity The data. the fit best tr nw steMncrsRing, Monoceros the as known stars ms km gtn h uaieCnsMajor Canis putative the rgeting − ◦ sRn nnab ed.This fields. nearby in Ring os 1 < ∼ hc xldsi rmbeing from it excludes which b ao wr aay This galaxy. dwarf Major 22.7 < ± -3.8 ions l .kcheliocentric), 0.3kpc ± , 2 b s: , = ) 1.7 ◦ a eeldthe revealed has , ´ nzDlaoe al. et inez-Delgado ∼ ms km (240 − ◦ 1 ,-8 ohof both ; product e h outer the emade re te on ntres ◦ density ). tbe- it ) ogen- in- s ions still ng a) re - 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. The stars have been selected using the following criteria, The Red Giant Branch (RGB) stars in this survey were chosen ◦ ◦ ◦ ◦ 231.5
c 2005 RAS, MNRAS 000, 1–5 A radial velocity survey of low Galactic latitude structures: II. 3
◦ ◦ ◦ ◦ Figure 1. Histogram of the 2dF RGB data in the range 231.5
c 2005 RAS, MNRAS 000, 1–5 4 Blair Conn et al.
Figure 2. Histogram of Besanc¸on data covering the same region of sky as the 2dF dataset; no distance cuts have been applied less than the 100 kpc cutoff of the model. The distances are galactocentric to allow easy comparison with the 2dF data as shown in Figure 1. The top panels show the thin disk population (stars in the age range 5-7 Gyrs) in this field fitted with a Gaussian profile at a distance of ∼11.1 kpc and a dispersion of ∼1.7 kpc. The velocity profile of the − − thin disk stars is vr= ∼71.4 km s 1 with a dispersion of ∼32.3 km s 1. In the bottom panels are the thick disk stars (noting that these stars have a different density profile than the thin disk stars) they too can be fitted with a Gaussian profile and reside at a galactocentric distance of 13.1 kpc with a dispersion − − of 5.0 kpc, the velocity and dispersion are found to be vr= 115.9 km s 1 and 47.7 km s 1 respectively. The smaller Gaussians represent those 15 stars in − − the same distance range as the MRi stars in Figure 1 with vr= 143.8 km s 1 and a dispersion of 28.2 km s 1; galactocentric distance of 19.7 kpc with a dispersion of 1.9 kpc. ering the number of stars that can be expected in the distance range 22.7±1.7 km s−1) is statistically similar to the Crane et al. (2003) of the MRi. The true number of Galactic thick disk stars would be value (20±4 km s−1). It can be seen on Figure 3, where our tar- much less than the ∼15 stars presented in Figure 2, especially since gets are plotted as filled circles, that the velocity of our sample most of the stars as presented in Paper I seem to belong to the CMa is only slightly higher than that expected by the circular model. structure and that confusion over whether they represent the warp Such a small discrepancy is not unexpected since the MRi pop- seems to have been sufficently discounted. ulation certainly does not follow a perfectly circular orbit (as is hinted by detections at different galactocentric distances within the 14
c 2005 RAS, MNRAS 000, 1–5 A radial velocity survey of low Galactic latitude structures: II. 5
Press, W. H., Teukolsky, S. A., Vetterling, W. T., & Flannery, B. P. 1992, Cambridge: University Press, —c1992, 2nd ed., Robin, A. C., Reyl´e, C., Derri`ere, S. & Picaud, S. 2003, A&A, 409, 523 Rocha-Pinto, H. J., Majewski, S. R., Skrutskie, M. F., Crane, J.D., & Patterson, R. J. 2004, ApJ, 615, 732 Rocha-Pinto H. J., Majewski S. R., Skrutskie M. F. & Crane J. D. 2003, ApJ, 594, L115 Schlegel D. J., Finkbeiner D. P. & Davis M. 1998, ApJ, 500, 525 Searle, L. & Zinn, R. 1978, ApJ, 225, 357 Taylor, K., Bailey, J., Wilkins, T., Shortridge, K., & Glazebrook, K. 1996, ASP Conf. Ser. 101: Astronomical Data Analysis Soft- ware and Systems V, 101, 195 White, S. D. M. 1978, MNRAS, 184, 185 White, S. D. M. & Rees, M. J. 1978, MNRAS, 183, 341 Yanny B., et al., 2004, ApJ, 605, 575 Figure 3. Distribution of the target M giant stars compared to previous detections of the MRi. Stars from our sample are shown as filled circles whereas SDSS detections are plotted as stars. The dashed line corresponds to the best circular orbit model from Crane et al. (2003) for a popula- tion orbiting the Milky Way at DGC =18 kpc with a rotational velocity − of vrot=220 km s 1
Marie Curie Early Stage Research Training Fellowship under con- tract MEST-CT-2004-504604.
REFERENCES Abadi, M. G., Navarro, J. F., Steinmetz, M., & Eke, V. R. 2003a, ApJ, 597, 21 Abadi, M. G., Navarro, J. F., Steinmetz, M., & Eke, V. R. 2003b, ApJ, 591, 499 Bellazzini, M., Ibata, R., Monaco, L., Martin, N., Irwin, M. J., & Lewis, G. F. 2004, MNRAS, 354, 1263 Bonifacio, P., Monai, S., & Beers, T. C. 2000, AJ, 120, 2065 Conn, B. C.,Lewis G. F.,Irwin M. J., Ibata, R. A., Irwin J. M., Ferguson, A. M. N. & Tanvir, N. R. 2005, MNRAS, accepted, astro-ph/0506522 Crane J. D., Majewski S. R., Rocha-Pinto H. J., Frinchaboy P. M., Skrutskie M. F. & Law D. R. 2003, ApJ, 594, L119 Ibata, R. A., Gilmore, G., & Irwin, M. J. 1994, Nature, 370, 194 Ibata R. A., Irwin M. J., Lewis G. F., Ferguson A. M. N.& Tanvir N. 2003, MNRAS, 340, L21 Majewski, S. R., Skrutskie, M. F., Weinberg, M. D., & Ostheimer, J. C. 2003, ApJ, 599, 1082 Majewski, S. R., Ostheimer, J. C., Rocha-Pinto, H. J., Patterson, R. J., Guhathakurta, P., & Reitzel, D. 2004, ApJ, 615, 738 Martin, N. F., Ibata, R. A., Bellazzini, M., Irwin, M. J., Lewis, G. F. & Dehnen, W. 2004a, MNRAS, 348, 12 Martin, N. F., Ibata, R. A., Conn, B. C., Lewis, G. F., Bellazzini, M., Irwin, M. J., & McConnachie, A. W. 2004b, MNRAS, 355, L33 Martin, N. F., Ibata, R. A., Conn, B. C., Lewis, G. F., Bellazzini M.& Irwin, M. J. 2005a, MNRAS, in press, astro-ph/0503705, (Paper I) Martin, N. F., Ibata, R. A., Conn, B. C.,Irwin, M. J & Lewis, G. F. 2005b, PASA, submitted Mart´inez-Delgado, D., Pe˜narrubia J., Dinescu, D. I., Butler, D. J., Rix H.W. 2005, in press, astro-ph/0506012 Newberg H. J. et al. 2002, ApJ, 569, 245 Pe˜narrubia, J., et al. 2005, ApJ, 626, 128
c 2005 RAS, MNRAS 000, 1–5