HI/OH/Recombination line survey of the inner Milky Way (THOR): data release 2 and H I overview

Aims. The first half of THOR data (l = 14.0°−37.9°, and l = 47.1°−51.2°, |b|≤ 1.25°) has been published in our data release 1 paper. With this data release 2 paper, we publish all the remaining spectral line data and Stokes I continuum data with high angular resolution (10′′–40′′), including a new H I dataset for the whole THOR survey region (l = 14.0−67.4° and |b|≤ 1.25°). As we published the results of OH lines and continuum emission elsewhere, we concentrate on the H I analysis in this paper. Methods. With the Karl G. Jansky Very Large Array (VLA) in C-configuration, we observed a large portion of the first Galactic quadrant, achieving an angular resolution of ≤40′′. At L Band, the WIDAR correlator at the VLA was set to cover the 21 cm H I line, four OH transitions, a series of Hnα radio recombination lines (RRLs; n = 151 to 186), and eight 128 MHz-wide continuum spectral windows, simultaneously. Results. We publish all OH and RRL data from the C-configuration observations, and a new H I dataset combining VLA C+D+GBT (VLA D-configuration and GBT data are from the VLA Galactic Plane Survey) for the whole survey. The H I emission shows clear filamentary substructures at negative velocities with low velocity crowding. The emission at positive velocities is more smeared-out, likely due to higher spatial and velocity crowding of structures at the positive velocities. Compared to the spiral arm model of the Milky Way, the atomic gas follows the Sagittarius and Perseus Arm well, but with significant material in the inter-arm regions. With the C-configuration-only H I+continuum data, we produce an H I optical depth map of the THOR areal coverage from 228 absorption spectra with the nearest-neighbor method. With this τ map, we corrected the H I emission for optical depth, and the derived column density is 38% higher than the column density with optically thin assumption. The total H I mass with optical depth correction in the survey region is 4.7 × 10⁸ M⊙, 31% more than the mass derived assuming the emission is optically thin. If we applied this 31% correction to the whole Milky Way, the total atomic gas mass would be 9.4–10.5 × 10⁹ M⊙. Comparing the H I with existing CO data, we find a significant increase in the atomic-to-molecular gas ratio from the spiral arms to the inter-arm regions. Conclusions. The high-sensitivity and resolution THOR H I dataset provides an important new window on the physical and kinematic properties of gas in the inner Galaxy. Although the optical depth we derive is a lower limit, our study shows that the optical depth correction issignificant for H I column density and mass estimation. Together with the OH, RRL and continuum emission from the THOR survey, these new H I data provide the basis for high-angular-resolution studies of the interstellar medium in different phases

OH absorption in the first quadrant of the Milky Way as seen by THOR

The hydroxyl radical (OH) is present in the diffuse molecular and partially atomic phases of the interstellar medium (ISM), but its abundance relative to hydrogen is not clear. We aim to evaluate the abundance of OH with respect to molecular hydrogen using OH absorption against cm-continuum sources over the first Galactic quadrant. This OH study is part of the HI/OH/Recombination line survey (THOR). THOR is a Karl G. Jansky Very Large Array large program of atomic, molecular and ionized gas in the range 15{\deg}$\leq$l$\leq$67{\deg} and |b|$\leq$1{\deg}. It is the highest-resolution unbiased OH absorption survey to date towards this region. We combine the derived optical depths with literature 13CO(1-0) and HI observations to determine the OH abundance. We detect absorption in the 1665 and 1667 MHz transitions for continuum sources stronger than $F_{\rm cont}\geq$0.1 Jy/beam. OH absorption is found against $\sim$15% of these continuum sources with increasing fractions for stronger sources. Most of the absorption is associated with Galactic HII regions. We find OH and 13CO gas to have similar kinematic properties. The OH abundance decreases with increasing hydrogen column density. The OH abundance with respect to the total hydrogen nuclei column density (atomic and molecular phase) is in agreement with a constant abundance for $A_V$ < 10-20. Towards the lowest column densities, we find sources that exhibit OH absorption but no 13CO emission, indicating that OH is a well suited tracer of the low column density molecular gas. We present spatially resolved OH absorption towards W43. The unbiased nature of the THOR survey opens a new window onto the gas properties of the ISM. The characterization of the OH abundance over a large range of hydrogen gas column densities contributes to the understanding of OH as a molecular gas tracer and provides a starting point for future investigations

THOR - The HI, OH, Recombination Line Survey of the Milky Way - The pilot study: HI observations of the giant molecular cloud W43

To study the atomic, molecular and ionized emission of Giant Molecular Clouds (GMCs), we have initiated a Large Program with the VLA: 'THOR - The HI, OH, Recombination Line survey of the Milky Way'. We map the 21cm HI line, 4 OH lines, 19 H_alpha recombination lines and the continuum from 1 to 2 GHz of a significant fraction of the Milky Way (l=15-67deg, |b|<1deg) at ~20" resolution. In this paper, we focus on the HI emission from the W43 star-formation complex. Classically, the HI 21cm line is treated as optically thin with column densities calculated under this assumption. This might give reasonable results for regions of low-mass star-formation, however, it is not sufficient to describe GMCs. We analyzed strong continuum sources to measure the optical depth, and thus correct the HI 21cm emission for optical depth effects and weak diffuse continuum emission. Hence, we are able to measure the HI mass of W43 more accurately and our analysis reveals a lower limit of M=6.6x10^6 M_sun, which is a factor of 2.4 larger than the mass estimated with the assumption of optically thin emission. The HI column densities are as high as N(HI)~150 M_sun/pc^2 ~ 1.9x10^22 cm^-2, which is an order of magnitude higher than for low mass star formation regions. This result challenges theoretical models that predict a threshold for the HI column density of ~10 M_sun/pc^2, at which the formation of molecular hydrogen should set in. By assuming an elliptical layered structure for W43, we estimate the particle density profiles. While at the cloud edge atomic and molecular hydrogen are well mixed, the center of the cloud is strongly dominated by molecular hydrogen. We do not identify a sharp transition between hydrogen in atomic and molecular form. Our results are an important characterization of the atomic to molecular hydrogen transition in an extreme environment and challenges current theoretical models

SEDIGISM: Structure, excitation, and dynamics of the inner Galactic interstellar medium

The origin and life-cycle of molecular clouds are still poorly constrained, despite their importance for understanding the evolution of the interstellar medium. Many large-scale surveys of the Galactic plane have been conducted recently, allowing for rapid progress in this field. Nevertheless, a sub-arcminute resolution global view of the large-scale distribution of molecular gas, from the diffuse medium to dense clouds and clumps, and of their relationshipto the spiral structure, is still missing. Aims. We have carried out a systematic, homogeneous, spectroscopic survey of the inner Galactic plane, in order to complement the many continuum Galactic surveys available with crucial distance and gas-kinematic information. Our aim is to combine this data set with recent infrared to sub-millimetre surveys at similar angular resolutions. © 2017 ESO

Fluctuations in active membranes

Active contributions to fluctuations are a direct consequence of metabolic energy consumption in living cells. Such metabolic processes continuously create active forces, which deform the membrane to control motility, proliferation as well as homeostasis. Membrane fluctuations contain therefore valuable information on the nature of active forces, but classical analysis of membrane fluctuations has been primarily centered on purely thermal driving. This chapter provides an overview of relevant experimental and theoretical approaches to measure, analyze and model active membrane fluctuations. In the focus of the discussion remains the intrinsic problem that the sole fluctuation analysis may not be sufficient to separate active from thermal contributions, since the presence of activity may modify membrane mechanical properties themselves. By combining independent measurements of spontaneous fluctuations and mechanical response, it is possible to directly quantify time and energy-scales of the active contributions, allowing for a refinement of current theoretical descriptions of active membranes.Comment: 38 pages, 9 figures, book chapte