A Two Micron All Sky Survey View of the Sagittarius Dwarf Galaxy. III. Constraints on the Flattening of the Galactic Halo

By K. Johnston, D. Law, S. Majewski. 2004

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M giants selected from the Two Micron All Sky Survey (2MASS) have been used to trace streams of tidal debris apparently associated with the Sagittarius dwarf spheroidal galaxy (Sgr) that entirely encircle the Galaxy. While the Sgr M giants are generally aligned with a single great circle on the sky, we measure a difference of 10.°4 ± 2.°6 between the mean orbital poles of the great circles that best fit debris leading and trailing Sgr, which can be attributed to the precession of Sgr's orbit over the range of phases explored by the data set. Simulations of the destruction of Sgr in potentials containing bulge, disk, and halo components best reproduce this level of precession along the same range of orbital phases if the potential contours of the halo are only slightly flattened, with the ratio of the axis length perpendicular to and in the disk in the range q = 0.90-0.95 (corresponding to isodensity contours with qρ ~ 0.83-0.92). Oblate halos are strongly preferred over prolate (qρ > 1) halos, and flattenings in the potential of q ≤ 0.85 (qρ ≤ 0.75) and q ≥ 1.05 (qρ ≥ 1.1) are ruled out at the 3 σ level. More extreme values of q ≤ 0.80 (qρ ≤ 0.6) and q ≥ 1.25 (qρ ≥ 1.6) are ruled out at the 7 and 5 σ levels, respectively. These constraints will improve as debris with larger separation in orbital phases is found.

Subfields: Astronomy and Astrophysics

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Citations in the corpus

The Causes of Halo Shape Changes Induced by Cooling Baryons: Disks versus Substructures (2007, background): “Observational constraints on halo shapes can be obtained from the Milky Way (Ibata et al. 2001; Johnston et al. 2005; Helmi 2004; Fellhauer et al. 2006), from polar ring galaxies (Schweizer et al. 1983; Sackett & Sparke 1990; Iodice et al. 2003), from X-ray isophotal shapes (Buote & Canizares 1994;…”
A Two Micron All-Sky Survey View of the Sagittarius Dwarf Galaxy. IV. Modeling the Sagittarius Tidal Tails (2004, methodology): “Helmi (2004) has presented evidence in favor of a prolate (q ¼ 1:25) halo using Sgr leading debris velocity trends, while in a companion paper (Johnston et al. 2005, hereafter Paper III) we have demonstrated that such prolate halos fail to reproduce the observed orbital pole precession of leading…”
Internal and External Alignment of the Shapes and Angular Momenta of ΛCDM Halos (2004, background): “Some authors have claimed that the thinness of the tidal stream associated with the Sagittarius dwarf spheroidal indicates that the halo of of the Milky Way is nearly spherical, with c/a & 0.8 (Ibata et al. 2001; Johnston et al. 2005; Mart́ınez-Delgado et al. 2004).”
Are the Magellanic Clouds on Their First Passage about the Milky Way? (2007, methodology): “On the other hand, Johnston et al. (2005) used the 2MASS M giants to trace the tidal streams and determined that the level of precession was best matched by oblate halos.”
THE LEAST-LUMINOUS GALAXY: SPECTROSCOPY OF THE MILKY WAY SATELLITE SEGUE 1 (2008, background): “Only models with strongly prolate (q = 1.25) halos produce streams that match the leading arm M-giant velocities (Law et al. 2005), while models with oblate halos (q = 0.9) best match the observed precession of the M-giant orbital plane (Johnston et al. 2005).”
THE SAGITTARIUS DWARF GALAXY: A MODEL FOR EVOLUTION IN A TRIAXIAL MILKY WAY HALO (2010, methodology): “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).”
CLUMPY STREAMS FROM CLUMPY HALOS: DETECTING MISSING SATELLITES WITH COLD STELLAR STRUCTURES (2010, methodology): “On global scales, they can be used to constrain the radial profile, shape and orientation of the Milky Way’s triaxial dark-matter halo (e.g. Johnston et al. 1999; Ibata et al. 2002; Johnston et al. 2005; Binney 2008; Eyre 2010; Koposov et al. 2010; Law & Majewski 2010).”
CONSTRAINING THE MILKY WAY POTENTIAL WITH A SIX-DIMENSIONAL PHASE-SPACE MAP OF THE GD-1 STELLAR STREAM (2009, background): “After initial searches for tidal tails of globular clusters (e.g. Grillmair et al. 1995) it was the extended Sagittarius tidal tail that first made deriving such constraints practical (see e.g. Ibata et al. 2001; Helmi 2004; Johnston et al. 2005; Law et al. 2005).”
The mass distribution and gravitational potential of the Milky Way (2016, result): “The study of the Sagittarius stream by Johnston et al.(2005) had best fitting models with masses in the range (3.8− 5.6)× 1011 within 50 kpc, which is in keeping with our prior.”
The Origin of the Bifurcation in the Sagittarius Stream (2006, methodology): “Hence, this model has been very widely used, particularly in many previous investigations of the Sgr stream (e.g., Helmi 2004a, 2004b; Johnston et al. 2005; Law et al. 2005).”
Is the SMC Bound to the LMC? The Hubble Space Telescope Proper Motion of the SMC (2006, background): “…Magellanic Stream with tidal streams from other Local Group dwarfs, such as the Sagittarius Stream (Ibata et al. 1994; Majewski et al. 2003), might further constrain the potential of the halo (e.g., Fellhauer et al. 2006; Belokurov et al. 2006; Johnston et al. 2005; Law et al. 2005; Helmi 2004).”
The shape of dark matter haloes in the Aquarius simulations: evolution and memory (2011, methodology): “…example, the positional information was used to argue that the Milky Way halo is nearly spherical (Ibata et al. 2001; Mart́ınez-Delgado et al. 2004; Johnston et al. 2005) whereas the kinematics of stars in the leading stream could only be fit in a prolate halo elongated perpendicular to the disc…”
VERTICAL STRUCTURE OF A SUPERNOVA-DRIVEN TURBULENT, MAGNETIZED INTERSTELLAR MEDIUM (2012, background): “The shape of the dark halo of the Galaxy is unknown; claims of an oblate, spheroidal, and prolate halo have all been made (e.g., Helmi 2004; Johnston et al. 2005; Fellhauer et al. 2006; Law & Majewski 2010).”