The Milky Way's dark matter halo appears to be lopsided

Abstract

The atomic hydrogen gas (HI) disk in the outer region (beyond ~10 kpc from the centre) of Milky Way can provide valuable information about the structure of the dark matter halo. The recent 3-D thickness map of the outer HI disk from the all sky 21-cm line LAB survey, gives us a unique opportunity to investigate the structure of the dark matter halo of Milky Way in great detail. A striking feature of this new survey is the North-South asymmetry in the thickness map of the atomic hydrogen gas. Assuming vertical hydrostatic equilibrium under the total potential of the Galaxy, we derive the model thickness map of the HI gas. We show that simple axisymmetric halo models, such as softened isothermal halo (producing a flat rotation curve with V_c ~ 220 km/s) or any halo with density falling faster than the isothermal one, are not able to explain the observed radial variation of the gas thickness. We also show that such axisymmetric halos along with different HI velocity dispersion in the two halves, cannot explain the observed asymmetry in the thickness map. Amongst the non-axisymmetric models, it is shown that a purely lopsided (m=1, first harmonic) dark matter halo with reasonable HI velocity dispersion fails to explain the North-South asymmetry satisfactorily. However, we show that by superposing a second harmonic (m=2) out of phase onto a purely lopsided halo e.g. our best fit and more acceptable model A (with parameters ε_{h}^{1}=0.2, ε_{h}^{2}=0.18 and σ_{HI}=8.5 km/s) can provide an excellent fit to the observation and reproduce the North-South asymmetry naturally. The emerging picture of the asymmetric dark matter halo is supported by the ΛCDM halos formed in the cosmological N-body simulation.

37 pages, 8 figures, accepted for publication in ApJ

References in corpus (8)

• The Milky Way's Circular Velocity Curve to 60 kpc and an Estimate of the Dark Matter Halo Mass from Kinematics of ~2400 SDSS Blue Horizontal Branch Stars
• The Vertical Structure of the Outer Milky Way HI Disk
• The Spiral Structure of the Outer Milky Way in Hydrogen
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• Asymmetries in the inner regions of LCDM haloes
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• Global lopsided instability in a purely stellar galactic disc
• The thickness of HI in galactic discs under MOND: theory and application to the Galaxy

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• Fermi-LAT Observations of Gamma-Ray Emission Towards the Outer Halo of M31
• Dark Matter Interpretation of the Fermi-LAT Observations Toward the Outer Halo of M31
• Warp and flare of the Galactic disc revealed with supergiants by Gaia EDR3
• Angular momentum transport and evolution of lopsided galaxies
• A Synthetic 21-cm Galactic Plane Survey of an SPH Galaxy Simulation
• Effect of dark matter halo substructures on galaxy rotation curves