The Flattened Dark Matter Halo of M31 as Deduced from the Observed H<scp>i</scp>Scale Heights

Banerjee, Arunima; Jog, Chanda J.

doi:10.1086/591223

Cited by 56 publications

(58 citation statements)

References 28 publications

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“…D08 found that the halo shape changes by Δ( b / a ) ≳ 0.2 out to at least half the virial radius. This shape change reconciles the strongly prolate‐triaxial shapes found in collisionless N ‐body simulations of the hierarchal growth of haloes (Bardeen et al 1986; Barnes & Efstathiou 1987; Frenk et al 1988; Dubinski & Carlberg 1991; Jing & Suto 2002; Bailin & Steinmetz 2005; Allgood et al 2006) with observations, which generally find much rounder haloes (Schweizer, Whitmore & Rubin 1983; Sackett & Sparke 1990; Franx & de Zeeuw 1992; Huizinga & van Albada 1992; Buote & Canizares 1994; Franx, van Gorkom & de Zeeuw 1994; Kuijken & Tremaine 1994; Bartelmann, Steinmetz & Weiss 1995; Kochanek 1995; Olling 1995, 1996; Schoenmakers, Franx & de Zeeuw 1997; Koopmans, de Bruyn & Jackson 1998; Olling & Merrifield 2000; Andersen et al 2001; Buote et al 2002; Barnes & Sellwood 2003; Debattista 2003; Iodice et al 2003; Oguri, Lee & Suto 2003; Diehl & Statler 2007; Banerjee & Jog 2008).…”

Section: Introductionsupporting

confidence: 77%

The orbital evolution induced by baryonic condensation in triaxial haloes

Valluri

Debattista²,

Quinn

et al. 2010

Monthly Notices of the Royal Astronomical Society

117

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Using spectral methods, we analyse the orbital structure of prolate/triaxial dark matter (DM) haloes in N‐body simulations in an effort to understand the physical processes that drive the evolution of shapes of DM haloes and elliptical galaxies in which central masses are grown. A longstanding issue is whether the change in the shapes of DM haloes is the result of chaotic scattering of the major family of box orbits that serves as the backbone of a triaxial system, or whether they change shape adiabatically in response to the evolving galactic potential. We use the characteristic orbital frequencies to classify orbits into major orbital families, to quantify orbital shapes and to identify resonant orbits and chaotic orbits. The use of a frequency‐based method for distinguishing between regular and chaotic N‐body orbits overcomes the limitations of Lyapunov exponents which are sensitive to numerical discreteness effects. We show that regardless of the distribution of the baryonic component, the shape of a DM halo changes primarily due to changes in the shapes of individual orbits within a given family. Orbits with small pericentric radii are more likely to change both their orbital type and shape than orbits with large pericentric radii. Whether the evolution is regular (and reversible) or chaotic (and irreversible), it depends primarily on the radial distribution of the baryonic component. The growth of an extended baryonic component of any shape results in a regular and reversible change in orbital populations and shapes, features that are not expected for chaotic evolution. In contrast, the growth of a massive and compact central component results in chaotic scattering of a significant fraction of both box and long‐axis tube orbits, even those with pericentre distances much larger than the size of the central component. Frequency maps show that the growth of a disc causes a significant fraction of halo particles to become trapped by major global orbital resonances. We find that despite the fact that shape of a DM halo is always quite oblate following the growth of a central baryonic component, a significant fraction of its orbit population has the characteristics of its triaxial or prolate progenitor.

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Section: Introductionsupporting

confidence: 77%

The orbital evolution induced by baryonic condensation in triaxial haloes

Valluri

Debattista²,

Quinn

et al. 2010

Monthly Notices of the Royal Astronomical Society

117

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“…The joint Poisson equation in cylindrical coordinates for an axisymmetric system of stars and gas is given by where φ total is the net potential of the disc due to the stars, the H i gas and the dark matter halo, ρ i with i = 1 to 2 denotes the mass volume density for each of the disc components (stars, gas) while ρ h denotes the same for the halo, and is given by (de Zeeuw & Pfenniger 1988): where ρ 0 is the central core density of the halo, and R c is the core radius, as applicable for a pseudo‐isothermal density distribution. One may note here that since dwarf galaxies have rotation curves that are generally rising out to the last measured point (Salucci et al 2007), the radial term in cannot be ignored as in the case for large spirals with almost flat rotation curves (Narayan et al 2005; Banerjee & Jog 2008).…”

Section: Modelmentioning

confidence: 98%

Theoretical determination of H i vertical scale heights in the dwarf galaxies DDO 154, Ho II, IC 2574 and NGC 2366

Banerjee

Jog

Brinks

et al. 2011

Monthly Notices of the Royal Astronomical Society

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In this paper, we model dwarf galaxies as a two‐component system of gravitationally coupled stars and atomic hydrogen gas in the external force field of a pseudo‐isothermal dark matter halo, and numerically obtain the radial distribution of H i vertical scale heights. This is done for a group of four dwarf galaxies (DDO 154, Ho II, IC 2574 and NGC 2366) for which most necessary input parameters are available from observations. The formulation of the equations takes into account the rising rotation curves generally observed in dwarf galaxies. The inclusion of self‐gravity of the gas into the model at par with that of the stars results in scale heights that are smaller than what was obtained by previous authors. This is important as the gas scale height is often used for deriving other physical quantities. The inclusion of gas self‐gravity is particularly relevant in the case of dwarf galaxies where the gas cannot be considered a minor perturbation to the mass distribution of the stars. We find that three out of four galaxies studied show a flaring of their H i discs with increasing radius, by a factor of a few within several disc scale lengths. The fourth galaxy has a thick H i disc throughout. This flaring arises as a result of the gas velocity dispersion remaining constant or decreasing only slightly while the disc mass distribution declines exponentially as a function of radius.

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“…All applications of the flaring method have indicated highly flattened halo distributions with q ≤ 0.5 (Becquaert 1997;Becquaert & Combes 1997;Sicking 1997). Recently, Banerjee & Jog (2008) measured a flattening of q = 0.4 from flaring of the HI layer in M 31. This assumed a constant HI velocity dispersion with radius; if it is allowed to have a modest decline in the outer disk the flattening can be made less with q more like 0.5 to 0.6.…”

Section: Comparison To Other Workmentioning

confidence: 99%

The dark matter halo shape of edge-on disk galaxies

O’Brien¹,

Freeman²,

Kruit³

2010

A&A

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This is the fourth paper in a series in which we attempt to put constraints on the flattening of dark halos in disk galaxies. We observed for this purpose the HI in edge-on galaxies, where it is in principle possible to measure the force field in the halo vertically and radially from gas layer flaring and rotation curve decomposition respectively. As reported in earlier papers in this series we have for this purpose analysed the HI channel maps to accurately measure all four functions that describe as a function of galactocentric radius the planar HI kinematics and 3D HI distribution of a galaxy: the radial HI surface density, the HI vertical thickness, the rotation curve and the HI velocity dispersion. In this paper we analyse these data for the edge-on galaxy UGC 7321. We measured the stellar mass distribution (M = 3 × 10 8 M with M/L R < ∼ 0.2), finding that the vertical force of the gas disk dominates the stellar disk at all radii. Measurements of both the rotation curve and the vertical force field showed that the vertical force puts a much stronger constraint on the stellar mass-to-light ratio than rotation curve decomposition. Fitting of the vertical force field derived from the flaring of the HI layer and HI velocity dispersion revealed that UGC 7321 has a spherical halo density distribution with a flattening of q = c/a = 1.0 ± 0.1. However, the shape of the vertical force field showed that a non-singular isothermal halo was required, assuming a vertically isothermal HI velocity dispersion. A pseudo-isothermal halo and a gaseous disk with a declining HI velocity dispersion at high latitudes may also fit the vertical force field of UGC 7321, but to date there is no observational evidence that the HI velocity dispersion declines away from the galactic plane. We compare the halo flattening of UGC 7321 with other studies in the literature and discuss its implications. Our result is consistent with new n-body simulations which show that inclusion of hydrodynamical modelling produces more spherical halos.

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The Flattened Dark Matter Halo of M31 as Deduced from the Observed HiScale Heights

Cited by 56 publications

References 28 publications

The orbital evolution induced by baryonic condensation in triaxial haloes

The orbital evolution induced by baryonic condensation in triaxial haloes

Theoretical determination of H i vertical scale heights in the dwarf galaxies DDO 154, Ho II, IC 2574 and NGC 2366

The dark matter halo shape of edge-on disk galaxies

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