### Abstract

We use the sample of satellite galaxies presented by Zaritsky et al. (1993) to demonstrate the existence of extended massive dark halos around spiral galaxies. Isolated spirals with rotation velocities near 250 km s^{-1} have a typical halo mass within 200 kpc of 1.5-2.6 × 10^{12} M_{⊙} (90% confidence range for H_{0} = 75 km s^{-1} Mpc^{-1}). This result is most easily derived using standard mass estimator techniques, but such techniques do not account for the strong observational selection effects in the sample, nor for the extended mass distributions that the data imply. These complications can be addressed using scale-free models similar to those previously employed to study binary galaxies. When satellite velocities are assumed isotropic, both methods imply massive and extended halos. However, the derived masses depend sensitively on the assumed shape of satellite orbits. Furthermore, both methods ignore the fact that many of the satellites in the sample have orbital periods comparable to the Hubble time. The orbital phases of such satellites cannot be random, and their distribution in radius cannot be freely adjusted; rather these properties reflect ongoing infall onto the outer halos of their primaries. We use detailed dynamical models for halo formation to evaluate these problems, and we devise a maximum likelihood technique for estimating the parameters of such models from the data. The most strongly constrained parameter is the mass within 200-300 kpc, giving the confidence limits quoted above. The eccentricity, e, of satellite orbits is also strongly constrained, 0.50 < e < 0.88 at 90% confidence, implying a near-isotropic distribution of satellite velocities. The cosmic density parameter in the vicinity of our isolated halos exceeds 0.13 at 90% confidence, with preferred values exceeding 0.3.

Original language | English (US) |
---|---|

Pages (from-to) | 599-610 |

Number of pages | 12 |

Journal | Astrophysical Journal |

Volume | 435 |

Issue number | 2 |

State | Published - Nov 10 1994 |

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### Keywords

- Galaxies: kinematics and dynamics
- Galaxies: spiral
- Galaxies: structure

### ASJC Scopus subject areas

- Space and Planetary Science

### Cite this

*Astrophysical Journal*,

*435*(2), 599-610.

**The massive halos of spiral galaxies.** / Zaritsky, Dennis F; White, Simon D M.

Research output: Contribution to journal › Article

*Astrophysical Journal*, vol. 435, no. 2, pp. 599-610.

}

TY - JOUR

T1 - The massive halos of spiral galaxies

AU - Zaritsky, Dennis F

AU - White, Simon D M

PY - 1994/11/10

Y1 - 1994/11/10

N2 - We use the sample of satellite galaxies presented by Zaritsky et al. (1993) to demonstrate the existence of extended massive dark halos around spiral galaxies. Isolated spirals with rotation velocities near 250 km s-1 have a typical halo mass within 200 kpc of 1.5-2.6 × 1012 M⊙ (90% confidence range for H0 = 75 km s-1 Mpc-1). This result is most easily derived using standard mass estimator techniques, but such techniques do not account for the strong observational selection effects in the sample, nor for the extended mass distributions that the data imply. These complications can be addressed using scale-free models similar to those previously employed to study binary galaxies. When satellite velocities are assumed isotropic, both methods imply massive and extended halos. However, the derived masses depend sensitively on the assumed shape of satellite orbits. Furthermore, both methods ignore the fact that many of the satellites in the sample have orbital periods comparable to the Hubble time. The orbital phases of such satellites cannot be random, and their distribution in radius cannot be freely adjusted; rather these properties reflect ongoing infall onto the outer halos of their primaries. We use detailed dynamical models for halo formation to evaluate these problems, and we devise a maximum likelihood technique for estimating the parameters of such models from the data. The most strongly constrained parameter is the mass within 200-300 kpc, giving the confidence limits quoted above. The eccentricity, e, of satellite orbits is also strongly constrained, 0.50 < e < 0.88 at 90% confidence, implying a near-isotropic distribution of satellite velocities. The cosmic density parameter in the vicinity of our isolated halos exceeds 0.13 at 90% confidence, with preferred values exceeding 0.3.

AB - We use the sample of satellite galaxies presented by Zaritsky et al. (1993) to demonstrate the existence of extended massive dark halos around spiral galaxies. Isolated spirals with rotation velocities near 250 km s-1 have a typical halo mass within 200 kpc of 1.5-2.6 × 1012 M⊙ (90% confidence range for H0 = 75 km s-1 Mpc-1). This result is most easily derived using standard mass estimator techniques, but such techniques do not account for the strong observational selection effects in the sample, nor for the extended mass distributions that the data imply. These complications can be addressed using scale-free models similar to those previously employed to study binary galaxies. When satellite velocities are assumed isotropic, both methods imply massive and extended halos. However, the derived masses depend sensitively on the assumed shape of satellite orbits. Furthermore, both methods ignore the fact that many of the satellites in the sample have orbital periods comparable to the Hubble time. The orbital phases of such satellites cannot be random, and their distribution in radius cannot be freely adjusted; rather these properties reflect ongoing infall onto the outer halos of their primaries. We use detailed dynamical models for halo formation to evaluate these problems, and we devise a maximum likelihood technique for estimating the parameters of such models from the data. The most strongly constrained parameter is the mass within 200-300 kpc, giving the confidence limits quoted above. The eccentricity, e, of satellite orbits is also strongly constrained, 0.50 < e < 0.88 at 90% confidence, implying a near-isotropic distribution of satellite velocities. The cosmic density parameter in the vicinity of our isolated halos exceeds 0.13 at 90% confidence, with preferred values exceeding 0.3.

KW - Galaxies: kinematics and dynamics

KW - Galaxies: spiral

KW - Galaxies: structure

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M3 - Article

VL - 435

SP - 599

EP - 610

JO - Astrophysical Journal

JF - Astrophysical Journal

SN - 0004-637X

IS - 2

ER -