The case for a leaner Milky Way

Details The case for a leaner Milky Way The case for a leaner Milky Way

Dec 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997aas...191.9702o&link_type=abstract

American Astronomical Society, 191st AAS Meeting, #97.02; Bulletin of the American Astronomical Society, Vol. 29, p.1365

Astronomy and Astrophysics

Astronomy

Scientific paper

We present two new and entirely independent methods to determine our distance to the Galactic center (R_0), and the rotation speed at the Solar circle (Theta_0 ). Both methods indicate that the Galaxy rotates more slowly and is smaller than currently accepted: R_0 ~ 7.1 kpc and Theta_0 ~ 184 km s(-1) . As a bonus we find that Milky Way's dark matter (DM) distribution is rather round, with an axial ratio of q= 0.75 +/- 0.25. Declining rotation curves (RC) are consistent with kinematical constraints for small R_0's and Theta_0 's. Likewise are rising RCs, which have large dark matter densities (rho_ {DM}), R_0's, and Theta_0 's. Further, flattened halos have larger rho_ {DM}'s. We use two methods which are sensitive to rho_ {DM}, each a different dependence on the mass of the stellar disk (Sigma_ *). The thickness of the HI \ layer is smaller for flat halos, but is independent of the stellar mass at large radii (Olling 1995). In the Solar neighborhood, the total amount of matter within 1.1 kpc from the plane is well determined (Kuijken & Gilmore 1989,1991; KG), so that the amount of DM --and hence q-- depends strongly upon the value of Sigma_ *. Self-consistency requires that there is a value of Sigma_ * where both methods yield identical flattening. We can thus simultaneously determine q and Sigma_ * for a large number of Galaxy models. Assuming that the halo is oblate, we find an upper limit for Theta_0 (<= 191 - 5.8x(R_0-7)). The observed upper bound for Sigma_ * (KG) implies that R_0 <= 7.6 kpc. For these models, we also calculate the radial variation of the Oort constants A and B. Since the Oort functions vary substantially with radius, model and data have to be compared in the same regions. Hanson (1987) found ``small'' values from stars within ~ 1 kpc, while on a ~ 4 kpc scale Feast & Whitelock (1997) find substantially larger A and B values. These results can be reconciled when including the gravitational effects of a realistic HI \ distribution: it's peaks and troughs cause localized gradients in the RC, and hence A and B. However, only small Galactic constants do the job: R_0 = 7.1 +/- 0.4 kpc and Theta_0 = 184 +/- 4 km s(-1) .

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