Great Circle Tidal Streams: Evidence for a Nearly Spherical Massive Dark Halo around the Milky Way

By R. Ibata, Geraint F. Lewis, M. Irwin, E. Totten, Thomas R. Quinn. 2000

369 citations recorded by the source corpus.

An all-high-latitude sky survey for cool carbon giant stars in the Galactic halo has revealed 75 such stars, of which the majority are new detections. Of these, more than half are clustered on a great circle on the sky that intersects the center of Sagittarius dwarf galaxy and is parallel to its proper-motion vector, while many of the remainder are outlying Magellanic Cloud carbon stars. Previous numerical experiments regarding the disruption of the Sagittarius dwarf galaxy (the closest of the Galactic satellite galaxies) predicted that the effect of the strong tides, during its repeated close encounters with the Milky Way, would be to slowly disrupt that galaxy. Because of the small velocity dispersion of the disrupted particles, these disperse slowly along (approximately) the orbital path of the progenitor, eventually giving rise to a very long stream of tidal debris surrounding our Galaxy. The more recently disrupted fragments of this stream should contain a mix of stellar populations similar to that found in the progenitor, which includes giant carbon stars. Given the measured position and velocity of the Sagittarius dwarf, we first integrate its orbit assuming a standard spherical model for the Galactic potential and find both that the path of the orbit intersects the position of the stream and that the radial velocity of the orbit, as viewed from the solar position, agrees very well with the observed radial velocities of the carbon stars. We also present a pole-count analysis of the carbon star distribution, which clearly indicates that the great circle stream we have isolated is statistically significant, being a 5-6 σ overdensity. These two arguments strongly support our conclusion that a large fraction of the halo carbon stars originated in the Sagittarius dwarf galaxy. The stream orbits the Galaxy between the present location of the Sagittarius dwarf, 16 kpc from the Galactic center, and the most distant stream carbon star, at ~60 kpc. It follows neither a polar nor a Galactic plane orbit, so that a large range in both Galactic R- and z-distances is probed. That the stream is observed as a great circle indicates that the Galaxy does not exert a significant torque on the stream, so the Galactic potential must be nearly spherical in the regions probed by the stream. Furthermore, the radial mass distribution of the halo must allow a particle at the position and with the velocity of the Sagittarius dwarf galaxy to reach the distance of the furthest stream carbon stars. Thus, the Sagittarius dwarf galaxy tidal stream gives a very powerful means to constrain the mass distribution it resides in, that is, the dark halo. We present N-body experiments simulating this disruption process as a function of the distribution of mass in the Galactic halo. A likelihood analysis shows that, in the Galactocentric distance range 16 kpc < R < 60 kpc, the dark halo is most likely almost spherical. We rule out, at high confidence levels, the possibility that the halo is significantly oblate, with isodensity contours of aspect qm < 0.7. This result is quite unexpected and contests currently popular galaxy formation models.

Subfields: Atomic and Molecular Physics, and Optics, Instrumentation, Statistical and Nonlinear Physics

View Paper: https://www.semanticscholar.org/paper/49d3e92dbd7c66390cd4e8ec2955da6adf8c5d6e

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Citations in the corpus

On the life and death of satellite haloes (2003, background): “…of their stars should be present, but tracking such properties might be a daunting task C© 2003 RAS, MNRAS 341, 434–448 Downloaded from https://academic.oup.com/mnras/article-abstract/341/2/434/1103988 by guest on 29 July 2018 observationally if enough phase mixing occurs (Ibata et al. 2001a,b).”
A Two Micron All Sky Survey View of the Sagittarius Dwarf Galaxy. I. Morphology of the Sagittarius Core and Tidal Arms (2003, methodology): “The degree of coherence of tidal debris streams provide a powerful constraint on the lumpiness of the Galactic halo potential (Font et al. 2001; Johnston et al. 2002b; Ibata et al. 2001b;Mayer et al. 2001).”
The shape of dark matter haloes : dependence on mass, redshift, radius and formation (2005, background): “Investigations of Sagittarius’ tidal streams have led to the conclusion that the MW halo is oblate and nearly spherical with q 0.8 (Ibata et al. 2001; Majewski et al. 2003; Martı́nez-Delgado et al. 2004).”
Evidence for Stellar Substructure in the Halo and Outer Disk of M31 (2002, background): “The geometrical alignment of the two satellites with the southern stream and the broadly similar metal abundances initially suggested that either or both of them could be responsible for the tidal feature (Ibata et al. 2001a).”
The Dark Matter Crisis: Falsification of the Current Standard Model of Cosmology (2012, background): “Concerning the MW, the empirical evidence is that its DM halo may be somewhat flattened (oblate) with qd\ 0.8 within Rt 60 kpc (Olling & Merrifield 2000, 2001; Ibata et al. 2001; Majewski et al. 2003; Martı́nezDelgado et al. 2004).”
On the Accretion Origin of a Vast Extended Stellar Disk around the Andromeda Galaxy (2005, methodology): “This result shows that thick disks can have completely different kinematics from the thin disk, although this is not the case for the Milky Way.”
The Galaxy in Context: Structural, Kinematic, and Integrated Properties (2016, background): “The geometry of the stream on the sky has been shown to require an oblate near-spherical halo (Ibata et al. 2001, Johnston et al. 2005), whereas LOS velocities favor a prolate shape (Helmi 2004).”
A Two Micron All Sky Survey View of the Sagittarius Dwarf Galaxy. III. Constraints on the Flattening of the Galactic Halo (2004, background): “…given at least one estimate for the shape of the Milky Way’s dark matter halo that suggests that it could indeed be rather spherical ((c/a)ρ > 0.95, Ibata et al. 2001), placing it at the extreme of the extragalactic (c/a)ρ range (but cf. Mart́ınez-Delgado et al. 2004, who find (c/a)ρ ≈ 0.5).”
Lectures on Dark Matter Physics (2016, methodology): “In [14], it was shown that the a reasonable approximation for the Yukawa potential is a flat potential well whose width is determined by the characteristic length scale of the interaction, r0 = 1/m', Vbox(r) = U0⇥(r0 r).”
Stellar Abundance and Galactic Chemical Evolution through LAMOST Spectroscopic Survey (2006, background): “These include, among others, the “Spaghetti” survey (Morrison et al. 2000), the APM (Automatic Plate Measuring) carbon star survey (Totten & Irwin 1998; Totten et al. 2000; Ibata et al. 2001a) and the SDSS (Yanny et al. 2000; Ivezic et al. 2000; Ibata et al. 2001b).”
The Mass of the Milky Way: Limits from a newly assembled set of halo objects (2002, neutral): “Moreover, the implicit assumption behind the current method, that the sample stars have random distributions in location and in their space motions, may not be well satisfied if the halo is largely dominated by coherent structures such as tidal streams (e.g., Ibata et al. 2001).”
The Great disk of Milky Way satellites and cosmological sub-structures (2004, background): “The empirical evidence is that the MW dark halo is somewhat flattened (oblate) with qd >∼ 0.8 within R<∼ 60 kpc (Olling & Merrifield 2000, 2001; Ibata et al. 2001; Majewski et al. 2003; Martínez-Delgado et al. 2004).”
Colloquium: Annual modulation of dark matter (2013, neutral): “The Sloan Digital Sky Survey and the Two Micron All Sky Survey (Majewski et al., 2003; Newberg et al., 2003; Yanny et al., 2003) have traced the stellar component of the tidal stream (Dohm-Palmer et al., 2001; Ibata et al., 2001).”
Taking measure of the Andromeda halo: A Kinematic analysis of the giant stream surrounding M31 (2004, methodology): “This would suggest that And VIII is most likely part of the Andromeda Stream, situated in the region of highest over-density reported by Ibata et al. (2001b).”
The Haunted Halos of Andromeda and Triangulum: A Panorama of Galaxy Formation in Action (2007, methodology): “The limiting magnitude over the MegaCam region was chosen to be i0 = 23.5, and we kept a limit of i0 = 22.8 (S/N ∼ 10) for the INT survey, which gives rise to the obvious discontinuity around η ∼ −3◦.”
A Comprehensive Maximum Likelihood Analysis of the Structural Properties of Faint Milky Way Satellites (2008, background): “The best example of tidally induced elongation in the Local Group is given by the Sagittarius dwarf that is clearly being disrupted by the Milky Way as it leaves behind stellar streams wrapped around the Galaxy (Ibata et al. 2001; Majewski et al. 2003).”
THE NUCLEUS OF THE SAGITTARIUS DSPH GALAXY AND M54: A WINDOW ON THE PROCESS OF GALAXY NUCLEATION (2008, background): “The tidally stripped stars form a huge and coherent tidal tail system that can be observed over the whole celestial sphere (Sgr Stream, see Ibata et al. 2001; Majewski et al. 2003, and references therein).”
The local dark matter density (2014, background): “Early work on the stream suggested a near-spherical halo for the Milky Way (Ibata et al., 2001), but more recent data combined with more sophisticated modelling appear to favour a triaxial halo (Law & Majewski, 2010).”
THE SAGITTARIUS DWARF GALAXY: A MODEL FOR EVOLUTION IN A TRIAXIAL MILKY WAY HALO (2010, background): “Depending on which criterion a given study chose to focus, claims have been made in favor of an oblate halo (e.g., Johnston et al. 2005; Mart́ınez-Delgado et al. 2007), an approximately spherical halo (e.g., Ibata et al. 2001b; Fellhauer et al. 2006), and a prolate halo (e.g., Helmi 2004).”
A Two Micron All-Sky Survey View of the Sagittarius Dwarf Galaxy. IV. Modeling the Sagittarius Tidal Tails (2004, methodology): “Unfortunately, the angular extent of the 12 Carbon stars have been selected from Totten & Irwin (1998) subject to the requirement that both distance and velocity data have been measured and also subject to the photometric criteria employed by Ibata et al. (2001) that 11 < R < 17 and BJ R > 2:5.”