Radial Velocities in the Outermost Disk toward the Anticenter
- Instituto de Astrofisica de Canarias, E-38205 La Laguna, Tenerife (Spain)
- Istituto dei Sistemi Complessi Consiglio Nazionale delle Ricerche, Via dei Taurini 19, I-00185 Rome (Italy)
- Argelander-Institut für Astronomie, Auf dem Hügel 71, D-53121, Bonn (Germany)
We measure the mean Galactocentric radial component of the velocity of stars (v {sub R}) in the disk at 8 kpc < R < 28 kpc in the direction of the anticenter. For this, we use the Apache Point Galactic Evolution Experiment. Furthermore, we compare the result with H i maps along the same line of sight. We find an increase in positive (expansion) v {sub R} at R ≈ 9–13 kpc, reaching a maximum of ≈6 km s{sup −1}, and a decrease at large values of R, reaching a negative (contraction) value of ≈−10 km s{sup −1} for R > 17 kpc. Negative velocities are also observed in 21 cm H i maps, possibly dominated by local gas emission. Among the possible dynamical causes for these non-zero v {sub R}, factors such as the effect of the Galactic bar, streams, or mergers do not seem appropriate to explain our observations. An explanation might be the gravitational attraction of overdensities in a spiral arm. As a matter of fact, we see a change of regime from positive to negative velocities around R ≈ 15 kpc, in the position where we cross the Outer spiral arm in the anticenter. The mass in spiral arms necessary to produce these velocities would be about 3% of the mass of the disk, consistent with our knowledge of the spiral arms. Another scenario that we explore is a simple class of out-of-equilibrium systems in which radial motions are generally created by the monolithic collapse of isolated self-gravitating overdensities.
- OSTI ID:
- 22897245
- Journal Information:
- The Astronomical Journal (Online), Journal Name: The Astronomical Journal (Online) Journal Issue: 1 Vol. 157; ISSN 1538-3881
- Country of Publication:
- United States
- Language:
- English
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