Milky way kinematics. II. A uniform inner galaxy H i terminal velocity curve

Using atomic hydrogen (H i) data from the VLA Galactic Plane Survey, we measure the H i terminal velocity as a function of longitude for the first quadrant of the Milky Way. We use these data, together with our previous work on the fourth Galactic quadrant, to produce a densely sampled, uniformly measured, rotation curve of the northern and southern Milky Way between $3\,\mathrm{kpc}\lt R\lt 8\,\mathrm{kpc}$. We determine a new joint rotation curve fit for the first and fourth quadrants, which is consistent with the fit we published in McClure-Griffiths & Dickey and can be used for estimating kinematic distances interior to the solar circle. Structure in the rotation curves is now exquisitely well defined, showing significant velocity structure on lengths of ~200 pc, which is much greater than the spatial resolution of the rotation curve. Furthermore, the shape of the rotation curves for the first and fourth quadrants, even after subtraction of a circular rotation fit shows a surprising degree of correlation with a roughly sinusoidal pattern between $4.2\lt R\lt 7$ kpc

Tracing the Milky Way nuclear wind with 21 cm atomic hydrogen emission

There is evidence in 21 cm H i emission for voids several kiloparsecs in size centered approximately on the Galactic center, both above and below the Galactic plane. These appear to map the boundaries of the Galactic nuclear wind. An analysis of H i at the tangent points, where the distance to the gas can be estimated with reasonable accuracy, shows a sharp transition at Galactic radii R lesssim 2.4 kpc from the extended neutral gas layer characteristic of much of the Galactic disk, to a thin Gaussian layer with FWHM ~ 125 pc. An anti-correlation between H i and γ-ray emission at latitudes $10^\circ \leqslant | b| \leqslant 20^\circ$ suggests that the boundary of the extended H i layer marks the walls of the Fermi Bubbles. With H i, we are able to trace the edges of the voids from $| z| \gt 2\,{\rm{kpc}}$ down to z ≈ 0, where they have a radius ~2 kpc. The extended Hi layer likely results from star formation in the disk, which is limited largely to R gsim 3 kpc, so the wind may be expanding into an area of relatively little H i. Because the H i kinematics can discriminate between gas in the Galactic center and foreground material, 21 cm H i emission may be the best probe of the extent of the nuclear wind near the Galactic plane

Distances to southern 6.7-GHz methanol masers through H i self-absorption

We present the kinematic distance ambiguity resolutions of over 400 6.7-GHz methanol masers. Using the data sets of the International Galactic Plane Survey we have extracted Hi spectra towards the sites of methanol maser emission between longitudes 270° and 67° and examined them for evidence of Hi self-absorption. Using well-established criteria we identify 442 resolutions (classified by reliability) with a 1.07:1.11 ratio between near and far kinematic distance assignments. For a sub-sample of 118, we compare our resolutions with those of the literature and find 85 ± 10 per cent agreement. Using the distances we examine the Galactocentric distribution and properties of the masers. We also compare 6.7-GHz maser velocities with those of CS, finding the maser mid-velocity is closely correlated with the CS velocity, typically to within 2.8kms-

Magnetic field draping around clumpy high-velocity clouds in galactic halo

Throughout the passage within the Galactic halo, high-velocity clouds (HVCs) sweep up ambient magnetic fields and form stretched and draped configurations of magnetic fields around them. Many earlier numerical studies adopt spherically symmetric uniform-density clouds as initial conditions for simplicity. However, observations demonstrate that HVCs are clumpy and turbulent. In this paper, we perform 3D magnetohydrodynamic simulations to study the evolution of clouds with initial density distributions described by power-law spatial power spectra. We systematically study the role of (i) the initial density structure, (ii) halo magnetic fields, and (iii) radiative cooling efficiency upon infalling HVCs. We find that (i) the clouds' density structure regulates mixing and mass growth. Uniform clouds grow from the onset of the simulations while clumpy clouds initially lose gas and then grow at later times. Along the same lines, the growth curve of clumpy clouds depends on the slope of the initial density power spectra. (ii) Magnetic fields suppress hydrodynamic instabilities and the growth of small-scale structures. As a result, magnetized clouds develop long filaments extended along the streaming direction whereas non-magnetized clouds are fragmented into many small clumps. (iii) Efficient cooling keeps the main cloud body more compact and produces decelerated dense clumps condensed from the halo gas. This work potentially helps us understand and predict the observed properties of HVCs such as the detectability of magnetized clouds, the presence of decelerated HI structures associated with HVC complexes and small-scale features, and a possible link between the origin and the fate of HVCs.Comment: 21 pages, 13 figures, Accepted to MNRA

Variability monitoring of the hydroxyl maser emission in G12.889+0.489

Through a series of observations with the Australia Telescope Compact Array we have monitored the variability of ground-state hydroxyl maser emission from G12.889+0.489 in all four Stokes polarisation products. These observations were motivated by the known periodicity in the associated 6.7-GHz methanol maser emission. A total of 27 epochs of observations were made over 16 months. No emission was seen from either the 1612 or 1720 MHz satellite line transitions (to a typical five sigma upper limit of 0.2 Jy). The peak flux densities of the 1665 and 1667 MHz emission were observed to vary at a level of ∼20% (with the exception of one epoch which dropped by 640%). There was no distinct flaring activity at any epoch, but there was a weak indication of periodic variability, with a period and phase of minimum emission similar to that of methanol. There is no significant variation in the polarised properties of the hydroxyl, with Stokes Q and U flux densities varying in accord with the Stokes I intensity (linear polarisation, P, varying by 620%) and the right and left circularly polarised components varying by 633% at 1665-MHz and 638% at 1667-MHz. These observations are the first monitoring observations of the hydroxyl maser emission from G12.889+0.489