HI Imaging of Galactic ISM


Neutral atomic Hydrogen (HI) radiates at a wavelength of 21cm (1420.4 MHz frequency) due to a hyper-fine transition of the electron’s ground state. When the electron’s and the proton’s spins are aligned in parallel the atom is in a higher energy state than when they are antiparallel. Due to quantum mechanical effects, the electron in a hydrogen atom will occasionally switch its spin, and emit a photon of light with a wavelength of 21cm. Hydrogen is the dominant component of our universe, and the inter-stellar medium in particular.  So even though an individual HI atom may only emit a photon every 10 million years on average, there are so many HI atoms in interstellar space that we can see this emission almost everywhere in our galaxy using radio telescopes. The goal of HIFAST is to create the best map of the HI gas in our galaxy as well as many of the nearby galaxies in the Local Group.

The Five-hundred meter Aperture Spherical Telescope (FAST), constructed in 2016 in the Guizhou region of China, can observe this emission with unprecedented sensitivity and detail. The Commensal Radio Astronomy FAST Survey (CRAFTS) is expected to begin in 2018. CRAFTS will map the entire sky visible from FAST at frequencies ranging from 1.05 to 1.45 GHz with a 19-beam receiver. As part of CRAFTS, HIFAST will observe simultaneously with other projects ( Extragalactic HI survey, Pulsar search, Fast Radio Burst search, etc.) to produce a detailed map the HI gas in our galaxy. With an angular resolution of 3 arcminutes, and a frequency resolution of approximately 550Hz, HIFAST will aim to produce the largest HI map to date with over 10 billion voxels. This map will serve astronomers for many decades to come. Some of the more prominent scientific goals for HIFAST are described below.
Assist in extragalactic and cosmological studies: HI can absorb light at higher frequencies (such as X-rays), and is present throughout our galaxy. This can complicate the efforts of astronomers who study distant sources at those frequencies as they need to know how much HI there is along a particular line of sight in order to account for that absorption.
The Andromeda Bridge: The nearby galaxies M31 and M33 have interacted with each other gravitationally in the the past leaving behind a trail of HI clouds between them. HIFAST will be able to map that trail in high spectral resolution giving clues not only to the past interaction between those two galaxies, but also the environmental properties of the inter-galactic medium.
The missing HI problem: Astronomers can see that our galaxy is scooping up dust and gas from the inter-galactic medium (IGM) by absorbing High-velocity Clouds (HVCs), and other material. These HVCs are most prominent when observed through their HI emission. However, the amount of HVCs that have been observed to date account for only about one third of the mass accretion rate necessary for our galaxy to maintain its current rate of star formation. Either our models of star formation are wrong, or current telescopes are only sensitive enough to detect the most massive HVCs. Using FAST’s large collective area HIFAST will achieve greater sensitivity than any previous survey, hoping to shed light on this mystery.
The Galactic Rotation Curve: The observed frequency of the 21cm HI line is shifted through the doppler effect by the velocity of the object being studied. In this way astronomers were able to see our own galaxy’s spiral arms and develop the galactic rotation curve which serves as a sort of galactic map. With increased sensitivity HIFAST aims not only to improve the current galactic rotation curve, but possibly find new structures besides the already known spiral arms.
Bubbles, Shells, and Ejections: High energy events within our galaxy (such as supernovae) will often produce shocks that will disrupt the ISM, and even eject material out of our galaxy. We can trace many of these events by studying the HI gas, yielding a greater understanding of our own galaxy’s evolution.

Filaments everywhere: As our telescopes improved in the last few decades, astronomers could see that the gas in our galaxy (particularly HI) was not formed into spherical clumps (as was previously assumed), but instead was formed into countless thin filaments. How and why these filaments formed is still poorly understood. With such a large, detailed map HIFAST will be able to study the filaments on a large scale to search for patterns in their properties and orientation that might yield clues to their origins.
Orion: One of HIFAST’s first priority targets will be the Orion Molecular Cloud Complex. This is an active, nearby stellar nursery where thousands of stars are being born within a large nebula of dust and gas. HIFAST will provide the highest resolution map of Orion to date. By studying the HI we will gain understanding on not only the kinematics of Orion, but the chemical processes therein.
Molecular Clouds: In the diffuse interstellar medium, most of the gas is composed of individual atoms. Under gravity, or shocks, this gas condenses into dense clouds. As it does so, the gas cools and the HI atoms bond to form molecular Hydrogen. Eventually these clouds may form stars, but by then they must convert all their HI into molecular Hydrogen. By mapping the remaining HI in thousands of nearby clouds, HIFAST may be able to determine their individual ages and thus construct a map of nearby active star forming regions.

HIFAST Survey Planning Committee Membership:

HIFAST represents a large, multi-year endeavor with a large international collaboration. Therefore we have formed an initial team to assist in leading the project to conclusion. The following is an incomplete list of Chinese and international scientists who are committed to, or have expressed interest in participating in the HIFAST project.

Marko Krčo (HIFAST Lead Organizer)
John Dickey
Paul F. Goldsmith
Carl Heiles
George Hobbs
Bo Hu
Zhang Kai
Min-Young Lee
Di Li
Mengting Liu
Felix J. Lockman
Naomi McClure-Griffiths
Joshua Peek
Mary Putman
Lei Qian
Keping Qiu
Tim Robishaw
Erin Scott
Snezana Stanimirovic
Lister Staveley-Smith
Ningyu Tang
Jing Wang
Benjamin Winkel
Tao-Chung Ching