The Isaac Newton Telescope Wide Field Camera survey of the Monoceros Ring: accretion origin or Galactic anomaly?

We present the results of a wide-field camera survey of the stars in the Monoceros Ring, thought to be an additional structure in the Milky Way of unknown origin. Lying roughly in the plane of the Milky Way, this may represent a unique equatorial accretion event which is contributing to the thick disc of the Galaxy. Alternatively, the Monoceros Ring may be a natural part of the disc formation process. With 10 pointings in symmetric pairs above and below the plane of the Galaxy, this survey spans 90° about the equator of the Milky Way. Signatures of the stream of stars were detected in three fields, (l, b) = (118°, +16°) and (150°, +15°) plus a more tentative detection at (150°, −15°). Galactocentric distance estimates to these structures gave ∼17, ∼17, and ∼13 kpc, respectively. The Monoceros Ring seems to be present on both sides of the Galactic plane, in a form different from that of the Galactic warp, suggestive of a tidal origin with streams multiply wrapping the Galaxy. A new model of the stream has shown a strong coincidence with our results and has also provided the opportunity to make several more detections in fields in which the stream is less significant. The confirmed detection at (l, b) = (123°, −19°) at ∼14, kpc from the Galactic Centre allows a re-examination revealing a tentative new detection with a Galactocentric distance of ∼21 kpc. These detections also lie very close to the newly discovered structure in Triangulum–Andromedae hinting at a link between the two. The remaining six fields are apparently non-detections although in light of these new models, closer inspection reveals tentative structure. With the overdensity of M giant stars in Canis Major being claimed both as a progenitor to the Monoceros Ring and alternatively a manifestation of the Milky Way warp, much is still unknown concerning this structure and its connection to the Monoceros Ring. Further constraints are needed for the numerical simulations to adequately resolve the increasingly complex view of this structure.

[1]  H. Rix,et al.  A Comprehensive Model for the Monoceros Tidal Stream , 2004, astro-ph/0410448.

[2]  M. Irwin,et al.  Why the Canis Major overdensity is not due to the Warp: analysis of its radial profile and velocities , 2004, astro-ph/0407391.

[3]  H. Susa,et al.  The Effects of Early Cosmic Reionization on the Substructure Problem in Galactic Halos , 2004, astro-ph/0406305.

[4]  U. Padova,et al.  Probing the Canis Major stellar over-density as due to the Galactic warp , 2004, astro-ph/0405526.

[5]  S. Majewski,et al.  Exploring Halo Substructure with Giant Stars: A Diffuse Star Cloud or Tidal Debris around the Milky Way in Triangulum-Andromeda , 2004, astro-ph/0405437.

[6]  D. Lamb,et al.  Erratum: “A Low-Latitude Halo Stream around the Milky Way” (ApJ, 588, 824 [2003]) , 2004 .

[7]  P. Frinchaboy,et al.  A Two Micron All Sky Survey View of the Sagittarius Dwarf Galaxy. II. Swope Telescope Spectroscopy of M Giant Stars in the Dynamically Cold Sagittarius Tidal Stream , 2004, astro-ph/0403701.

[8]  J. Strader,et al.  The Globular Cluster System of the Canis Major Dwarf Galaxy , 2004, astro-ph/0403136.

[9]  M. Irwin,et al.  Detection of the Canis Major galaxy at (l;b) = (244°; −8°) and in the background of Galactic open clusters , 2003, astro-ph/0311119.

[10]  P. Frinchaboy,et al.  Star Clusters in the Galactic Anticenter Stellar Structure and the Origin of Outer Old Open Clusters , 2003, astro-ph/0311101.

[11]  M. Bellazzini,et al.  A dwarf galaxy remnant in Canis Major: the fossil of an in-plane accretion on to the Milky Way , 2003, astro-ph/0311010.

[12]  A. Robin,et al.  A synthetic view on structure and evolution of the Milky Way , 2003, astro-ph/0401052.

[13]  S. Majewski,et al.  Tracing the Galactic Anticenter Stellar Stream with 2MASS M Giants , 2003 .

[14]  P. Frinchaboy,et al.  Exploring Halo Substructure with Giant Stars: Spectroscopy of Stars in the Galactic Anticenter Stellar Structure , 2003, astro-ph/0307505.

[15]  M. F. Skrutskie,et al.  A Two Micron All Sky Survey View of the Sagittarius Dwarf Galaxy. I. Morphology of the Sagittarius Core and Tidal Arms , 2003, astro-ph/0304198.

[16]  A. Helmi,et al.  On the Nature of the Ringlike Structure in the Outer Galactic Disk , 2003, astro-ph/0303305.

[17]  B. Gibson,et al.  Galactic Halo Stars in Phase Space: A Hint of Satellite Accretion? , 2003, astro-ph/0301596.

[18]  M. Irwin,et al.  One ring to encompass them all: a giant stellar structure that surrounds the Galaxy , 2003, astro-ph/0301067.

[19]  D. Lamb,et al.  A Low-Latitude Halo Stream around the Milky Way , 2003, astro-ph/0301029.

[20]  M. Steinmetz,et al.  Simulations of Galaxy Formation in a Λ Cold Dark Matter Universe. II. The Fine Structure of Simulated Galactic Disks , 2002, astro-ph/0212282.

[21]  M. Steinmetz,et al.  Simulations of Galaxy Formation in a Λ Cold Dark Matter Universe. I. Dynamical and Photometric Properties of a Simulated Disk Galaxy , 2002, astro-ph/0211331.

[22]  K. Freeman,et al.  The New Galaxy: Signatures of Its Formation , 2002, astro-ph/0208106.

[23]  Annette Ferguson,et al.  Evidence for Stellar Substructure in the Halo and Outer Disk of M31 , 2002, astro-ph/0205530.

[24]  Heather A. Rave,et al.  The Ghost of Sagittarius and Lumps in the Halo of the Milky Way , 2001, astro-ph/0111095.

[25]  A. Robin,et al.  Early galaxy evolution from deep wide field star counts - II. First estimate of the thick disc mass function , 2001, astro-ph/0105199.

[26]  R. Ibata,et al.  Galactic Halo Substructure in the Sloan Digital Sky Survey: The Ancient Tidal Stream from the Sagittarius Dwarf Galaxy , 2000, astro-ph/0004255.

[27]  R. Ibata,et al.  Great Circle Tidal Streams: Evidence for a Nearly Spherical Massive Dark Halo around the Milky Way , 2000, astro-ph/0004011.

[28]  P. Harding,et al.  Mapping the Galactic Halo. I. The “Spaghetti” Survey , 2000, astro-ph/0001492.

[29]  R. McMahon,et al.  The INT wide field imaging survey (WFS) , 2000, astro-ph/0001285.

[30]  P. T. de Zeeuw,et al.  Debris streams in the solar neighbourhood as relicts from the formation of the Milky Way , 1999, Nature.

[31]  D. Spergel,et al.  Tidal Streams as Probes of the Galactic Potential , 1998, astro-ph/9807243.

[32]  D. Schlegel,et al.  Maps of Dust Infrared Emission for Use in Estimation of Reddening and Cosmic Microwave Background Radiation Foregrounds , 1998 .

[33]  R. Ibata,et al.  Galactic Indigestion: Numerical Simulations of the Milky Way's Closest Neighbor , 1998, astro-ph/9802212.

[34]  D. Schlegel,et al.  Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds , 1997, astro-ph/9710327.

[35]  M. Irwin,et al.  A dwarf satellite galaxy in Sagittarius , 1994, Nature.

[36]  David Burstein,et al.  The discovery of a young radio galaxy at z = 2.390 - Probing initial star formation at z less than approximately 3.0 , 1991 .

[37]  R. Zinn,et al.  Compositions of halo clusters and the formation of the galactic halo , 1978 .

[38]  S. White,et al.  Simulations of merging galaxies. , 1978 .

[39]  M. Rees,et al.  Core condensation in heavy halos: a two-stage theory for galaxy formation and clustering , 1978 .

[40]  A. Sandage,et al.  Evidence from the motions of old stars that the Galaxy collapsed. , 1962 .

[41]  John E. Davis,et al.  Sloan Digital Sky Survey: Early Data Release , 2002 .

[42]  M. Irwin,et al.  INT WFS pipeline processing , 2001 .

[43]  submitted to the Astrophysical Journal Preprint typeset using L ATEX style emulateapj v. 04/03/99 WHERE ARE THE MISSING GALACTIC SATELLITES? , 1999 .