Gas dynamics and large-scale morphology of the Milky Way galaxy

Abstract

We present a new model for the gas dynamics in the galactic disk inside the Sun's orbit. Quasi-equilibrium flow solutions are determined in the gravitational potential of the deprojected COBE NIR bar and disk, complemented by a central cusp and, in some models, an outer halo. These models generically lead to four-armed spiral structure between corotation of the bar and the solar circle; their large-scale morphology is not sensitive to the precise value of the bar's pattern speed, to the orientation of the bar with respect to the observer, and to whether or not the spiral arms carry mass.Our best model provides a coherent interpretation of many observed gas dynamical features. Its four-armed spiral structure outside corotation reproduces quantitatively the directions to the five main spiral arm tangents at |l| ≤ 60 • observed in a variety of tracers. The 3-kpc-arm is identified with one of the model arms emanating from the ends of the bar, extending into the corotation region. The model features an inner gas disk with a cusped orbit shock transition to an x 2 orbit disk of radius R ∼ 150 pc.The bar's corotation radius is fairly well-constrained at R c ≃ 3.5 ± 0.5 kpc. The best value for the orientation angle of the bar is probably 20 − 25 • , but the uncertainty is large since no detailed quantitative fit to all features in the observed (l, v) diagrams is yet possible. The Galactic terminal velocity curve from HI and CO observations out to l ≃ ±45 • (∼ 5 kpc) is approximately described by a maximal disk model with constant mass-to-light ratio for the NIR bulge and disk.

Ground-state ammonia and water in absorption towards Sgr B2

“…More detailed modelling of the large-scale Galactic gas dynamics, using a barred potential, reproduces existing H i and CO position-velocity data (Englmaier & Gerhard 1999;Bissantz et al 2003), but the model resolution is insufficiently high to make accurate predictions about the central 1 kpc of the Galaxy. However, the model does support the idea that the high-velocity gas (|V| > 80 km s −1 ) along the lineof-sight towards the Galactic centre is confined to this central region.…”

The orbital motion of the Arches cluster: clues on cluster formation near the Galactic Center

“…Clouds on stable orbits in the central molecular zone (r GC ≤ 200 pc) are suggested to follow spheroidal x2 orbits in the Milky Way bar potential (e.g. Binney et al 1991;Englmaier and Gerhard 1999). Molecular clouds with sufficiently high densities to allow for fragmentation and the formation of a massive cluster display line-of-sight velocities of v los < 120 km s −1 in radio CS surveys (Dame et al 2001), in agreement with velocities of x2 orbits in the bar model.…”

“…Therefore, the derived proper motion of 212 km s −1 is a lower limit to the absolute cluster velocity. Combination with the radial velocity of 95 ± 8 km s −1 (Figer et al 2002) results in a 3D space motion of 232 ± 30 km s −1 for the Arches cluster.…”

The orbital motion of the Arches cluster — clues on cluster formation near the galactic center

“…With ages as young as a few Myr, the central starburst clusters should still move with the inherited velocity from their natal clouds. Clouds on stable orbits in the central molecular zone (r GC ≤ 200 pc) are suggested to follow spheroidal x2 orbits in the Milky Way bar potential (e.g., Binney et al 1991, Englmaier andGerhard 1999). Molecular clouds with high densities capable of massive cluster formation display line-of-sight velocities of v los < 120 km/s in radio CS surveys (Dame et al 2001).…”