The HI/OH/Recombination line survey of the inner Milky Way (THOR)

Abstract

Context. The past decade has witnessed a large number of Galactic plane surveys at angular resolutions below 20 . However, no comparable high-resolution survey exists at long radio wavelengths around 21 cm in line and continuum emission. Aims. We remedy this situation by studying the northern Galactic plane at ∼20 resolution in emission of atomic, molecular, and ionized gas. Methods. Employing the Karl G. Jansky Very Large Array (VLA) in the C-array configuration and a large program, we observe the HI 21 cm line, four OH lines, nineteen Hnα radio recombination lines as well as the continuum emission from 1 to 2 GHz in full polarization over a large part of the first Galactic quadrant. Results. Covering Galactic longitudes from 14.5 to 67.4 deg and latitudes between ±1.25 deg, we image all of these lines and the continuum at ∼20 resolution. These data allow us to study the various components of the interstellar medium (ISM): from the atomic phase, traced by the HI line, to the molecular phase, observed by the OH transitions, to the ionized medium, revealed by the cm continuum and the Hnα radio recombination lines. Furthermore, the polarized continuum emission enables magnetic field studies. In this overview paper, we discuss the survey outline and present the first data release as well as early results from the different datasets. We now release the first half of the survey; the second half will follow later after the ongoing data processing has been completed. The data in fits format (continuum images and line data cubes) can be accessed through the project web-page. Conclusions. The HI/OH/Recombination line survey of the Milky Way (THOR) opens a new window to the different parts of the ISM. It enables detailed studies of molecular cloud formation, conversion of atomic to molecular gas, and feedback from Hii regions as well as the magnetic field in the Milky Way. It is highly complementary to other surveys of our Galaxy, and comparing the different datasets will allow us to address many open questions.

[C II] 158 μm self-absorption and optical depth effects

“…In the case of M17 SW, we can extract an H i absorption spectrum from the H i/OH/Recombination line survey of the inner Milky Way (THOR, Beuther et al 2016), and compare the velocity profile with its respective counterparts in [C ii] and [N ii]. The position corresponds to [C ii] position 0 ( Fig.…”

Galactic supernova remnant candidates discovered by THOR

“…It was conducted in VLA C-configuration, with a resolution of ∼ 20 . More survey details are given in Beuther et al (2016). When the THOR continuum data are combined with 20 cm VGPS continuum data, taken with the VLA in D-configuration at a resolution of 60 and data taken with the 100 m Effelsberg telescope at a resolution of 9 , the resulting data product is the most sensitive radio continuum Galactic plane survey in existence covering both large and small spatial scales.…”

“…We call this combined data set "THOR+VGPS." The THOR+VGPS data have an angular resolution of 25 because of smoothing we apply to the THOR data (see Beuther et al, 2016). Due to the coverage of the VGPS, the THOR+VGPS data set is restricted to > 17.5 • , so our final longitude range is 67.4 • > > 17.5 • .…”

“…Here, we identify extended sources of emission in radio continuum data from The H I, OH, Recombination line survey of the Milky Way (THOR; Beuther et al, 2016) combined with the 1.4 GHz radio continuum data from the VLA Galactic Plane Survey (VGPS Stil et al, 2006). In the identification process, we first use the WISE Catalog of Galactic H II Regions (Anderson et al, 2014) to separate thermal and non-thermal extended emission.…”

H2O Southern Galactic Plane Survey (HOPS): Paper III – properties of dense molecular gas across the inner Milky Way

“…Further analysis of the kinematics of the dense gas and surrounding lower density envelopes from existing survey data (e.g. Jackson et al 2013;Bihr et al 2015;Beuther et al 2016;Jordan et al 2015;Rathborne et al 2016;Rigby et al 2016;Bihr et al 2016), and fragmentation at higher resolution (e.g. Longmore et al 2006;Beuther, Walsh & Longmore 2009;Longmore et al 2011;Henshaw et al , 2017 will form the basis of a subsequent paper.…”