Properties of extra-planar H I clouds in the outer part of the Milky Way

Abstract

Context. There is mounting evidence for an extra-planar gas layer around the Milky Way disk, similar to the anomalous H i gas detected in a few other galaxies. As much as 10% of the gas may be in this phase. Aims.We analyze H i clouds located in the disk-halo interface outside the solar circle to probe the properties of the extra-planar H i gas, which is following Galactic rotation. Methods. We use the Leiden/Argentine/Bonn (LAB) 21-cm line survey to search for H i clouds which take part in the rotation of the Galactic plane, but are located above the disk layer. Selected regions are mapped with the Effelsberg 100-m telescope. Two of the H i halo clouds are studied in detail for their small scale structure using the Westerbork Synthesis Radio Telescope (WSRT) and the NRAO Very Large Array (VLA) .Results. Data from the 100 m telescope allow for the parameterization of 25 distinct H i halo clouds at Galactocentric radii 10 kpc < R <15 kpc and heights 1 kpc < z < 5 kpc. The clouds have a median temperature of 620 K, column densities of N HI ∼ 10 19 cm −2 , and most of them are surrounded by an extended envelope of warmer H i gas. Interferometer observations for two selected regions resolve the H i clouds into several arc-minute sized cores. These cores show narrow line widths (FWHM ∼ 3 km s −1 ), they have volume densities of n > 1.3 cm −3 , masses up to 24 M , and are on average in pressure equilibrium with the surrounding envelopes. Pressures and densities fall within the expectations from theoretical phase diagrams (P vs. n h ). The H i cores tend to be unstable if one assumes a thermally bistable medium, but are in better agreement with models that predict thermal fragmentation driven by a turbulent flow.

High-Resolution Images of Diffuse Neutral Clouds in the Milky Way. I. Observations, Imaging, and Basic Cloud Properties

“…The situation is quite different, however, in the lower halo of the inner Galaxy, where there is a population of discreet H I clouds whose velocities are consistent with circular rotation, but whose location several hundred pc from the place separates them from unrelated emission (Lockman 2002(Lockman , 2004 1 . Similar clouds can be seen at low Galactic 1 First detections of a few prominent representatives of this population date back to Prata latitude when their random velocity is large enough to remove confusion (Stil et al 2006); others are detected in the outer Galaxy (Strasser et al 2007;Stanimirović et al 2006;Dedes & Kalberla 2010). The clouds in the inner Galaxy are likely the product of H I supershells as their abundance and scale height are linked to the large-scale pattern of star formation in the disk (Ford et al 2008(Ford et al , 2010.…”

A Catalog of Ultra-Compact High Velocity Clouds From the Alfalfa Survey: Local Group Galaxy Candidates?

“…Previous studies have uncovered a population of compact clouds associated with the Galactic halo (e.g. Lockman 2002;Lockman & Pidopryhora 2005;Stil et al 2006;Stanimirović et al 2006;Ford et al 2010;Dedes & Kalberla 2010). While well separated from the Galactic hydrogen, these clouds typically have low v LSR values, and they generally appear to be consistent with Galactic rotation.…”

Virialization of the Inner CGM in the FIRE Simulations and Implications for Galaxy Disks, Star Formation, and Feedback

“…This can be understood by comparing the CGM thermal pressure with the expected pressure in SNe-powered superbubbles. Assume a CGM characteristic of the Milky Way with pressure P h /k ∼ 1000 cm −3 K (estimated from the pressure of high-velocity clouds, Dedes & Kalberla 2010) at the disc scale of R d ≈ R circ ≈ 10 kpc. The work required to lift the virialized CGM beyond the scale of the disc is ∼ (4π/3)R 3 d P h ∼ 2 • 10 55 erg.…”