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

Abstract

A set of diffuse interstellar clouds in the inner Galaxy within a few hundred pc of the Galactic plane has been observed at an angular resolution of ≈ 1. 0 combining data from the NRAO Green Bank Telescope and the Very Large Array.At the distance of the clouds the linear resolution ranges from ∼ 1.9 pc to ∼ 2.8 pc. These clouds have been selected to be somewhat out of the Galactic plane and are thus not confused with unrelated emission, but in other respects they are a Galactic population. They are located near the tangent points in the inner Galaxy, and thus at a quantifiable distance: 2.3 ≤ R ≤ 6.0 kpc from the Galactic Center, and −1000 ≤ z ≤ +610 pc from the Galactic plane. These are the first images of the diffuse neutral H I clouds that may constitute a considerable fraction of the ISM. Peak H I column densities range from N H I = 0.8 − 2.9 × 10 20 cm −2 . Cloud diameters vary between about 10 and 100 pc, and their H I mass spans the range from less than a hundred to a few thousands M . The clouds show no morphological consistency of any kind except that their shapes are highly irregular. One cloud may lie within the hot wind from the nucleus of the Galaxy, and some clouds show evidence of two distinct thermal phases as would be expected from equilibrium models of the interstellar medium.

DHIGLS: DRAO H i INTERMEDIATE GALACTIC LATITUDE SURVEY

“…We adopted a common cross calibration factor of 1.12± 0.01±0.03 for all seven DHIGLS regions. A correction of the same order was found by Pidopryhora et al (2015) for combining GBT and VLA data. This is mentioned not for direct comparison to our f cc , but to point out that a non-unity cross calibration factor is not unprecedented.…”

“…H I has been studied at even higher resolution (20 ) in the THOR survey (Beuther et al 2016), which supplements the VGPS with data from the VLA C-array configuration. Surveys at higher latitude are less common but some have been carried out at the Dominion Radio Astrophysical Observatory (DRAO 10 ) with the Synthesis Telescope (DRAO ST) (e.g., Martin et al 1994;Miville-Deschênes et al 2003a) and with the VLA closer to the plane (e.g., Pidopryhora et al 2015). As described in this paper we built on the legacy of the CGPS by using the DRAO ST to map H I in seven regions at intermediate Galactic latitude during the period 2004 to 2012.…”

Neutral Gas Accretion onto Nearby Galaxies

“…Often referred to as "disk-halo" clouds, they appear to be related both spatially and kinematically to the disk, though the population extends well into the lower halo (Lockman, 2002;Ford et al, 2010). Some of the larger Milky Way disk-halo clouds with M HI ≈ 500 M have been imaged at 2-3 pc resolution and show a variety of shapes with some sharp boundaries and evidence of a two-component thermodynamic structure (Pidopryhora et al, 2015). Note that from the above discussion on sensitivity, these disk-halo clouds could not be detected beyond the Milky Way as individual objects.…”

Comment on “Calorimetric Dark Matter Detection with Galactic Center Gas Clouds”

“…1, most of the Hi emission for this cloud is characterized by a line with a FWHM of 26.6 km/s (red line), with the narrow 1 km/s spike being a single-channel fluctuation (see [13]). For comparison, the spectrum of a robust cloud G33.4-8.0 [14], used in [15], is also shown.…”

“…An arrow marks the extremely narrow line quoted in the McG13 table, while the smooth curve shows a more appropriate Gaussian fit to the emission feature. Bottom: The corresponding spectrum for G33.4-8.0[14].…”