The Velocity Field of the Outer Galaxy

Details The Velocity Field of the Outer Galaxy The Velocity Field of the Outer Galaxy

Aug 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...275...67b&link_type=abstract

Astronomy and Astrophysics, Vol.275, NO. 1/AUG(I), P. 67, 1993

Astronomy and Astrophysics

Astrophysics

549

Galaxy: Kinematics And Dynamics, Galaxy: Structure

Scientific paper

We present the velocity field of the outer Galaxy over a range in galactic longitude from 1=90° to 1=270°, out to a galactocentric radius R of about 17 kpc (2R0). The field also covers a region of the inner Galaxy, within 2-3 kpc from the Sun. The data set consists of a sample of H i: regions/reflection nebulae, for which we have (spectro-) photometric distances, and their associated molecular clouds (for which we have radial velocities). The velocity field includes non-circular motions, and consequently can be used to derive kinematic distances of objects for which only the position and radial velocity (Vlsr) are available.
Adding H I tangent point data to the sample of H II regions/reflection nebulae, the data set covers a range in R from R≍02R0 to R≍2R0. For R0=8.5 kpc and Θ0=220 km s-1, the best fit of a rotation curve of the form Θ/Θ0 = α1(R/R0)α2 + α3 is obtained with α1 = 1.00767, α2 = 0.0394, and α3 = 0.00712. The curve is approximately flat, but the outermost points indicate a slight rise in rotational velocity. We find that Oort's constant A = 12.6 km s-1 kpc-1. The mass of the Galaxy within R = 2R0 is found to be 4.1 1011 Msun.
The velocity residuals (observed Vlsr minus Vlsr expected from circular rotation) show a pattern which suggests deviations from circular rotation that are consistent with spiral density wave streaming. No clear evidence of spiral structure is seen in the spatial distribution of the early type stars in our sample. Streaming motions (i.e. the systematic component of the velocity residuals) are found, with a mean value of ≍ 12 km s-1, implying 2-D values of ≍ 17 km s-1. We find that the molecular gas is streaming past the LSR, from l=180° to l=0°, at about 3.8 km s-1.
The rotation curves fitted to Northern and Southern Hemisphere data separately, are found to differ only slightly (the difference between the rotational velocities at 20 kpc being about 5%); this implies that, to first order, the galactic velocity field is axisymmetric.
From a sample of local molecular clouds (between 0.7 and 2 kpc from the Sun), we derive the height of the Sun above the plane to be 13±7 pc. The scale height of these clouds is about 65 Pc.

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