The flattened dark halo of polar ring galaxy NGC 4650A: A conspiracy of shapes?

Sackett, Penny D.; Rix, Hans─Walter; Jarvis, B. J.; Freeman, K. C.

doi:10.1086/174938

Cited by 122 publications

(140 citation statements)

References 29 publications

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“…The halo gradient dK z,h /dz is modelled using the equation for the vertical force K z of a flattened pseudo-isothermal halo given in Sackett et al (1994).…”

Section: Methods For Fitting the Halo Shapementioning

confidence: 99%

“…By evaluating the radial force K R (R, z) of a flattened pseudoisothermal halo with density distribution Sackett et al (1994) show that the corresponding rotation curve in the midplane is…”

Section: Rotation Curve Decompositionmentioning

confidence: 99%

“…4 we measure q by fitting the gradient of the vertical force dK z /dz, where the gradient of the flattened pseudoisothermal halo is also given by Sackett et al (1994). As K z (R, z) also depends on the same parameters q, halo core radius R c and halo central density ρ h,0 , we can use the asymptotic halo rotation v h,∞ and the core radius R c which are well measured from rotation curve decomposition, and just perform a single parameter fit to the flattening q, as the central density ρ h,0 is tied to v h,∞ , R c and q.…”

Section: Rotation Curve Decompositionmentioning

confidence: 99%

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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 halo gradient dK z,h /dz is modelled using the equation for the vertical force K z of a flattened pseudo-isothermal halo given in Sackett et al (1994).…”

Section: Methods For Fitting the Halo Shapementioning

confidence: 99%

“…By evaluating the radial force K R (R, z) of a flattened pseudoisothermal halo with density distribution Sackett et al (1994) show that the corresponding rotation curve in the midplane is…”

Section: Rotation Curve Decompositionmentioning

confidence: 99%

Section: Rotation Curve Decompositionmentioning

confidence: 99%

See 1 more Smart Citation

The dark matter halo shape of edge-on disk galaxies

O’Brien¹,

Freeman²,

Kruit³

2010

A&A

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show abstract

“…The axis ratio of the halo is not significantly constrained by the observations discussed below, and we arbritraily set it to q h = 0.8 (our standard value). However, we will also consider models with q h = 0.3 motivated by (i) possible evidence that the halos of external galaxies might be that flat (Olling 1996;Sackett et al 1994), and (ii) Sciama's (1990) decaying neutrino model for dark matter that requires a high local neutrino density (∼ 0.04 M⊙ pc −2 ) only achievable with a flat neutrino halo.…”

Section: Bulge and Halomentioning

confidence: 99%

Mass models of the Milky Way

Dehnen

Binney

1998

Monthly Notices RAS

516

734

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A parameterized model of the mass distribution within the Milky Way is fitted to the available observational constraints. The most important single parameter is the ratio of the scale length R d, * of the stellar disk to R 0 . The disk and bulge dominate v c (R) at R < ∼ R 0 only for R d, * /R 0 < ∼ 0.3. Since the only knowledge we have of the halo derives from studies like the present one, we allow it to contribute to the density at all radii. When allowed this freedom, however, the halo causes changes in assumptions relating to R ≪ R 0 to affect profoundly the structure of the best-fitting model at R ≫ R 0 . For example, changing the disk slightly from an exponential surface-density profile significantly changes the form of v c (R) at R ≫ R 0 , where the disk makes a negligible contribution to v c . Moreover, minor changes in the constraints can cause the halo to develop a deep hole at its centre that is not physically plausible. These problems call into question the proposition that flat rotation curves arise because galaxies have physically distinct halos rather than outwards-increasing mass-to-light ratios.The mass distribution of the Galaxy and the relative importance of its various components will remain very uncertain until more observational data can be used to constrain mass models. Data that constrain the Galactic force field at z > ∼ R and at R > R 0 are especially important.

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“…There are now very good constraints on two such galaxies. For NGC 4650A, Sackett et al (1994) find an axis ratio 10:4 to 10:3, corresponding to E6-E7. For A0136-0801, Rick Pogge has obtained a spectacular Fabry-Perot velocity cube with 2700 independent data points.…”

Section: Other Evidencementioning

confidence: 87%

Mass near the Sun

Gould¹

1996

Symp. - Int. Astron. Union

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Abstract.I review the dynamical measurements of mass in the solar neighborhood and show that, within 1 kpc, ~ 40% is unaccounted for by known stars and gas. I discuss several interpretations of the data including the 'standard model' where the extra mass is due to a spherical dark halo. I argue that the evidence for a radically different picture of an ultra-flat distribution is at least as compelling as the standard model.

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The flattened dark halo of polar ring galaxy NGC 4650A: A conspiracy of shapes?

Cited by 122 publications

References 29 publications

The dark matter halo shape of edge-on disk galaxies

The dark matter halo shape of edge-on disk galaxies

Mass models of the Milky Way

Mass near the Sun

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