The 21-SPONGE HI Absorption Survey I: Techniques and Initial Results

Abstract

We present methods and results from "21-cm Spectral Line Observations of Neutral Gas with the EVLA" (21-SPONGE), a large survey for Galactic neutral hydrogen (HI) absorption with the Karl G. Jansky Very Large Array (VLA). With the upgraded capabilities of the VLA, we reach median root-mean-square (RMS) noise in optical depth of ${\sigma }_{\tau }=9\times 10^{-4}$ per $0.42\mathrm{km}{\mathrm{s}}^{-1}$ channel for the 31 sources presented here. Upon completion, 21-SPONGE will be the largest HI absorption survey with this high sensitivity. We discuss the observations and data reduction strategies, as well as line fitting techniques. We prove that the VLA bandpass is stable enough to detect broad, shallow lines associated with warm HI, and show that bandpass observations can be combined in time to reduce spectral noise. In combination with matching HI emission profiles from the Arecibo Observatory ($\sim 3.5^{\prime }$ angular resolution), we estimate excitation (or spin) temperatures (${\mathrm{T}}_{\mathrm{s}}$) and column densities for Gaussian components fitted to sightlines along which we detect HI absorption (30/31). We measure temperatures up to ${\mathrm{T}}_{\mathrm{s}}\sim 1500\mathrm{K}$ for individual lines, showing that we can probe the thermally unstable interstellar medium (ISM) directly. However, we detect fewer of these thermally unstable components than expected from previous observational studies. We probe a wide range in column density between $\sim 10^{16}$ and $>10^{21}\mathrm{c}{\mathrm{m}}^{-2}$ for individual HI clouds. In addition, we reproduce the trend between cold gas fraction and average ${\mathrm{T}}_{\mathrm{s}}$ found by synthetic observations of a hydrodynamic ISM simulation by Kim et al. (2014). Finally, we investigate methods for estimating HI ${\mathrm{T}}_{\mathrm{s}}$ and discuss their biases.

Accepted for publication in ApJ; 24 pages, 14 figures

References in corpus (40)

• The Leiden/Argentine/Bonn (LAB) Survey of Galactic HI: Final data release of the combined LDS and IAR surveys with improved stray-radiation corrections
• The Neutral Atomic Phases of the ISM in the Galaxy
• At the heart of the matter: the origin of bulgeless dwarf galaxies and Dark Matter cores
• The Millennium Arecibo 21-CM Absorption Line Survey. II. Properties of the Warm and Cold Neutral Media
• The VLA Galactic Plane Survey
• Regulation of Star Formation Rates in Multiphase Galactic Disks: a Thermal/Dynamical Equilibrium Model
• An Origin of Supersonic Motions in Interstellar Clouds
• The Southern Galactic Plane Survey: HI Observations and Analysis
• Thermal condensation in a turbulent atomic hydrogen flow
• The GALFA-HI Survey: Data Release 1
• The Millennium Arecibo 21-cm Absorption Line Survey. I. Techniques and Gaussian Fits
• Implementing Molecular Hydrogen in Hydrodynamic Simulations of Galaxy Formation
• Vertical structure of a supernova-driven turbulent magnetized ISM
• The Spin Temperature of Warm Interstellar H I
• How long does it take to form a molecular cloud?
• The Distribution of Pressures in a Supernova-Driven Interstellar Medium. I. Magnetized Medium
• GASKAP -- The Galactic ASKAP Survey
• Fitting Together the HI Absorption and Emission in the SGPS
• The Outer Disk of the Milky Way Seen in 21-cm Absorption
• Formation of Cold Filamentary Structure from Wind Blown Superbubbles
• Gemini GMOS/IFU spectroscopy of NGC 1569 - I: Mapping the properties of a young star cluster and its environment
• Cold and warm atomic gas around the Perseus molecular cloud I: Basic Properties
• First results from the Arecibo Galactic HI Survey: the disk/halo interface region in the outer Galaxy
• Three Dimensional Hydrodynamic Simulations of Multiphase Galactic Disks with Star Formation Feedback: II. Synthetic HI 21 cm Line Observations
• The temperature of the diffuse HI in the Milky Way I: High resolution HI 21 cm absorption studies
• An HI column density threshold for cold gas formation in the Galaxy
• Supershells as Molecular Cloud Factories: Parsec Resolution Observations of HI and 12CO(J=1-0) in GSH 287+04-17 and GSH 277+00+36
• A multiwavelength investigation of the temperature of the cold neutral medium
• The thinnest cold HI clouds in the diffuse interstellar medium?
• Spatial and Temporal Variations in Small-Scale Galactic HI Structure Toward 3C~138
• The temperature of the WNM in the Milky Way
• Excitation temperature of the warm neutral medium as a new probe of the Lyman-α radiation field
• Accurate measurement of the HI column density from HI 21cm absorption-emission spectroscopy
• Detection of HI 21cm Absorption by the Warm Neutral Medium
• The thermally-unstable warm neutral medium: key for modeling the interstellar medium
• Tiny scale opacity fluctuations from VLBA, MERLIN and VLA observations of HI absorption toward 3C 138
• Detection of HI 21 cm-line absorption in the Warm Neutral Medium and in the Outer Arm of the Galaxy
• Spatial Variations in Galactic H I Structure on AU-Scales Toward 3C 147 Observed with the Very Long Baseline Array
• A High Galactic Latitude HI 21cm-line Absorption Survey using the GMRT: I. Observations and Spectra
• Galactic and Magellanic Evolution with the SKA

Cited by in corpus (57)

• Development of the Model of Galactic Interstellar Emission for Standard Point-Source Analysis of Fermi Large Area Telescope Data
• The 21-SPONGE HI Absorption Line Survey II: The temperature of Galactic HI
• GaussPy+: A fully automated Gaussian decomposition package for emission line spectra
• Dust-Gas Scaling Relations and OH Abundance in the Galactic ISM
• Cosmic rays, gas and dust in nearby anticentre clouds : I -- CO-to-H2 conversion factors and dust opacities
• Supernova Feedback and the Hot Gas Filling Fraction of the Interstellar Medium
• ROHSA: Regularized Optimization for Hyper-Spectral Analysis - Application to phase separation of 21 cm data
• The HI/OH/Recombination line survey of the inner Milky Way (THOR): data release 2 and HI overview
• Properties of cold and warm HI gas phases derived from a Gaussian decomposition of HI4PI data
• The Lognormal Probability Distribution Function of the Perseus Molecular Cloud: A Comparison of HI and Dust
• Kinematics of the Atomic ISM in M33 on 80 pc scales
• The HI/OH/Recombination line survey of the inner Milky Way (THOR)
• Atomic and Ionized Microstructures in the Diffuse Interstellar Medium
• 3D chemical structure of diffuse turbulent ISM. I. Statistics of the HI-to-H${}_2$ transition
• Optically thick HI does not dominate dark gas in the local ISM
• DHIGLS: DRAO H I Intermediate Galactic Latitude Survey
• The HI Probability Distribution Function and the Atomic-to-Molecular Transition in Molecular Clouds
• Extracting the cold neutral medium from HI emission with deep learning: Implications for Galactic foregrounds at high latitude
• Exploring the properties of warm and cold atomic hydrogen in the Taurus and Gemini regions
• Ly-alpha Radiative Transfer: Monte-Carlo Simulation of the Wouthuysen-Field Effect
• Recovering interstellar gas properties with HI spectral lines: A comparison between synthetic spectra and 21-SPONGE
• Calibrating the HISA temperature: Measuring the temperature of the Riegel-Crutcher cloud
• Synthetic Observations of 21cm HI Line Profiles from Inhomogeneous Turbulent Interstellar HI Gas with Magnetic Field
• Using an Artificial Neural Network to Classify Multi-component Emission Line Fits
• On the accuracy of HI observations in molecular clouds -- More cold HI than thought?
• Compressed magnetized shells of atomic gas and the formation of the Corona Australis molecular cloud
• Spectral index of synchrotron emission: insights from the diffuse and magnetised interstellar medium
• GASKAP-HI Pilot Survey Science III: An unbiased view of cold gas in the Small Magellanic Cloud
• GASKAP Pilot Survey Science II: ASKAP Zoom Observations of Galactic 21-cm Absorption
• The role of neutral hydrogen in setting the abundances of molecular species in the Milky Way's diffuse interstellar medium. I. Observational constraints from ALMA and NOEMA
• An ATCA Survey of HI Absorption in the Magellanic Clouds I: HI Gas Temperature Measurements in the Small Magellanic Cloud
• Using 21-cm absorption surveys to measure the average HI spin temperature in distant galaxies
• The Carina Nebula and Gum 31 molecular complex: II. The distribution of the atomic gas revealed in unprecedented detail
• Towards a multi-tracer timeline of star formation in the LMC -- I.\ Deriving the lifetimes of H\,{\sc i} clouds
• The "Maggie" filament: Physical properties of a giant atomic cloud
• Extragalactic HI 21-cm absorption line observations with the Five-hundred-meter Aperture Spherical radio Telescope
• Probing the cold neutral medium through HI emission morphology with the scattering transform
• Estimating kinetic temperature from H I 21 cm absorption studies: correction for the turbulence broadening
• Spinning nano-carbon grains: Viable origin for anomalous microwave emission
• OH as an Alternate Tracer for Molecular Gas: Excitation Temperatures of the OH 18-cm Main Lines in W5
• The Karl G. Jansky Very Large Array Local Group L-band Survey (LGLBS)
• Mapping the Thermal Condensation of Diffuse H I in the North Celestial Pole Loop
• Detection of the Galactic Warm Neutral Medium in HI 21cm absorption
• The validity of 21 cm spin temperature as a kinetic temperature indicator in atomic and molecular gas
• Optical Thickness, Spin Temperature, and Correction Factor for Density of the Galactic HI Gas
• The cosmic-ray content of the Orion-Eridanus superbubble
• A Comparison of Multi-Phase Magnetic Field Tracers in a High-Galactic Latitude Region of the Filamentary Interstellar Medium
• The ALMaQUEST Survey XV: The Dependence of the Molecular-to-Atomic Gas Ratios on Resolved Optical Diagnostics
• The influence of magnetic field on the CNM mass fraction and its alignment with density structures
• Probing galaxy evolution through HI 21-cm emission and absorption: current status and prospects with the Square Kilometre Array
• Spin Temperature and Density of Cold and Warm HI in the Galactic Disk - Hidden HI -
• Small-scale structure traced by neutral hydrogen absorption in the direction of multiple-component radio continuum sources
• A neutral hydrogen absorption study of cold gas in the outskirts of the Magellanic Clouds using the GASKAP-HI survey
• Wind of Change: Faraday Rotation in a Simulated Large Magellanic Cloud
• The MACH HI absorption survey I: Physical conditions of cold atomic gas outside of the Galactic plane
• The role of neutral hydrogen in setting the abundances of molecular species in the Milky Way's diffuse interstellar medium. II. Comparison between observations and theoretical models
• The Neutral Atomic Hydrogen in the solar neighborhood (NeAtHood) project I. Ghost in the shell: Neutral atomic hydrogen in the extended Orion nebula