First Large Absorption Survey in HI

First Large Absorption Survey in HI
Name	First Large Absorption Survey in HI
Acronym	FLASH
Type	Astronomical survey
Wavelength	21 cm
Telescope	Parkes Radio Telescope
Start	2018
Status	Completed

Contents

First Large Absorption Survey in HI The First Large Absorption Survey in HI was a radio astronomical program to map 21-centimeter hydrogen absorption across extragalactic sightlines using the Parkes Observatory, coordinated by teams at the Commonwealth Scientific and Industrial Research Organisation, the Australian National University, and international partners including the National Radio Astronomy Observatory and the Max Planck Institute for Radio Astronomy. It aimed to detect neutral hydrogen absorption against background sources identified with the Very Large Array, the Atacama Large Millimeter/submillimeter Array, and the Karl G. Jansky Very Large Array. The survey linked observational strategy to theoretical frameworks developed at the Institute of Astronomy, Cambridge, the Harvard–Smithsonian Center for Astrophysics, and the University of Cambridge.

Introduction

The survey targeted intervening and associated 21 cm absorption toward bright continuum sources catalogued by the Faint Images of the Radio Sky at Twenty-Centimeters and the NRAO VLA Sky Survey, leveraging sensitivity improvements pioneered by the Square Kilometre Array pathfinder projects such as the Australian Square Kilometre Array Pathfinder and the MeerKAT. FLASH built on precursor work from the HI Parkes All Sky Survey, the Westerbork Synthesis Radio Telescope absorption studies, and single-dish programs at the Green Bank Telescope and Arecibo Observatory. Major collaborators included researchers affiliated with the European Southern Observatory, the Max Planck Society, and the CSIRO Astronomy and Space Science division.

The design used wideband receivers at the Parkes Observatory to cover redshifted 21 cm lines over a large frequency range, informed by source lists from the Sloan Digital Sky Survey, the Two Micron All Sky Survey, and catalogs from the Fermi Gamma-ray Space Telescope. The methodology combined spectral-line observing modes used by the Westerbork Synthesis Radio Telescope, calibration pipelines developed at the National Radio Astronomy Observatory, and sky models derived from the Planck (spacecraft) results. Teams from the University of Oxford, the University of Sydney, and the University of California, Berkeley implemented automated flagging algorithms influenced by techniques at the European Southern Observatory and statistical tools from the Max Planck Institute for Astrophysics.

Observations employed on-source integration strategies similar to campaigns by the Very Long Baseline Array and the Giant Metrewave Radio Telescope, with contemporaneous monitoring using the Australia Telescope Compact Array and the Arcminute Microkelvin Imager. Data reduction pipelines adapted software developed at the National Radio Astronomy Observatory and the Commonwealth Scientific and Industrial Research Organisation, integrating spectral-line tools from the CASA (software) environment promoted by the National Radio Astronomy Observatory and imaging algorithms validated by the Atacama Pathfinder Experiment. Quality control procedures referenced calibration catalogs maintained by the International Astronomical Union and standards from the Royal Astronomical Society.

The survey reported detections of intervening and associated HI absorption in sightlines toward radio sources identified with the 2dF Galaxy Redshift Survey, the Gaia (spacecraft) astrometric catalog, and active nuclei cataloged by the Sloan Digital Sky Survey. Analysis compared column densities and spin temperatures with measurements from the Herschel Space Observatory and molecular line observations from the Atacama Large Millimeter/submillimeter Array. Statistical assessments used techniques associated with the Institute of Cosmology and Gravitation and modeling frameworks from the Max Planck Institute for Astronomy. The FLASH dataset enabled cross-correlation with surveys such as the Dark Energy Survey and the Wide-field Infrared Survey Explorer, facilitating demographic studies tied to host properties cataloged by the Two Degree Field Galaxy Redshift Survey.

Findings informed theories of gas accretion, feedback, and circumgalactic medium properties championed by groups at the Kavli Institute for Cosmology, Cambridge and the Flatiron Institute. Results bear on models of galaxy evolution developed at the California Institute of Technology, the Princeton University astrophysics group, and the Max Planck Institute for Extraterrestrial Physics. Observed absorber incidence and kinematics were discussed in the context of simulations from the Illustris project and the EAGLE (simulation) collaboration. The survey provided observational constraints relevant to energetic processes studied at the Harvard–Smithsonian Center for Astrophysics and chemical enrichment scenarios considered by researchers at the University of Chicago and the Massachusetts Institute of Technology.

Limitations included single-dish resolution constraints familiar from work at the Arecibo Observatory and sensitivity floors addressed by the Square Kilometre Array program. Future work plans referenced follow-up high-resolution imaging with the Very Long Baseline Array, deeper integrations with MeerKAT, and synergy with optical spectroscopy from the European Southern Observatory facilities and the Keck Observatory. Continued collaboration with institutions such as the Australian National University, the National Radio Astronomy Observatory, and the Max Planck Institute for Radio Astronomy aims to integrate FLASH results into next-generation surveys including the SKA Observatory.

Category:Astronomical surveys