Gas dynamics in the Milky Way: second pattern speed and large-scale morphology

Abstract

We present new gas flow models for the Milky Way inside the solar circle. We use smoothed particles hydrodynamics (SPH) simulations in gravitational potentials determined from the near‐infrared (NIR) luminosity distribution of the bulge and disc, assuming a constant NIR mass‐to‐light ratio, with an outer halo added in some cases. The luminosity models are based on the COBE/DIRBE maps and on clump giant star counts in several bulge fields and include a spiral arm model for the disc. Gas flows in models that include massive spiral arms clearly match the observed 12CO (l, v) diagram better than if the potential does not include spiral structure. Furthermore, models in which the luminous mass distribution and the gravitational potential of the Milky Way have four spiral arms are better fits to the observed (l, v) diagram than two‐armed models. Besides single‐pattern speed models we investigate models with separate pattern speeds for the bar and spiral arms. The most important difference is that in the latter case the gas spiral arms go through the bar corotation region, keeping the gas aligned with the arms there. In the (l, v) plot this results in characteristic regions that appear to be nearly devoid of gas. In single‐pattern speed models these regions are filled with gas because the spiral arms dissolve in the bar corotation region. Comparing with the 12CO data we find evidence for separate pattern speeds in the Milky Way. From a series of models the preferred range for the bar pattern speed is Ωp= 60 ± 5 Gyr−1, corresponding to corotation at 3.4 ± 0.3 kpc. The spiral pattern speed is less well constrained, but our preferred value is Ωsp≈ 20 Gyr−1. A further series of gas models is computed for different bar angles, using separately determined luminosity models and gravitational potentials in each case. We find acceptable gas models for 20°≲ϕbar≲ 25°. The model with (ϕbar= 20°, Ωp= 60 Gyr−1, Ωsp= 20 Gyr−1) gives an excellent fit to the spiral arm ridges in the observed (l, v) plot.

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.…”

High-mass X-ray binaries in the Milky Way

“…Lin et al, 1969) rotate with angular velocities varying between Ω ∼ 20−60 rad/Gyr, in the outer and inner galactic regions, respectively (Bissantz et al, 2003). The inner part of the spiral galactic structure is probably corotating with the stars up to a distance of ∼ 3.4 kpc, corresponding approximately to the inner extremity of the Norma arm.…”

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

“…The orbit is integrated for 2.5 Myr backwards to the suspected origin of the Arches cluster (solid lines), and 360 degrees into the future (dashed lines). The dotted line approximates the location of the outer boundary of x2 orbits in the bar (Bissantz et al 2003), assuming a bar rotation angle of 25 degrees (Rattenbury et al 2007). Note that for these orbits with galacto-centric distances inside ∼ 100 pc, an origin near the x2-x1 orbital boundary (dotted line) is possible, while a cluster origin near the x2 boundary becomes increasingly unlikely at larger galacto-centric distances.…”