ERAU Prescott Observatory

Galactic Hydrogen Observations

Hydrogen is the most abundant element (90% of interstellar gas) in the universe, and in a stable or ground state does not emit detectable radiation at radio frequencies.  However, hydrogen becomes detectable via the spin-flip transition when a electron flips to the anti-parallel spin direction from a parallel spin direction. Radiation energy is given off at a precise frequency of 1420.405751… Mhz (a wavelength of approximately 21-cm).  This radiation is called by astronomers the “21-cm line radiation”.  The 21-cm line radiation provides an extremely useful tool for studying gas in galaxies and the interactions of galaxies.


HI Redshift due to the Doppler Effect

HI redshift (lower in frequency) happens when hydrogen atoms are moving away from the observer or blueshift (higher in frequency) when hydrogen atoms are moving toward the observer. When observing a galaxy, the received radiation is not a single frequency but a band of radiation energy centered around the 21-cm line.

Redsift of GalaviesWhen observing a galaxy (such as a spiral galaxy), some radiating hydrogen gas will be moving toward the observer and some away from the observer with each visible radiating hydrogen atom revealing a hydrogen line.  The only difference between each of these lines is their Doppler shift.  By calculating the relative speed of each arm of the galaxy the rotational speed of the galaxy can be determined.  By using the plot of the rotation curve the velocity and distance of each point within the galaxy can be determined.  The rotation curve of our galaxy has been extensively mapped using the 21-cm hydrogen line. 

HI Scan of the Galactic Edge made from the ERAU 4-meter HI Dish

Multiple scans at different parts of the Galactic Edge.
Galatic Edge HI Scan

Cosmological Redshift

Cosmological redshift is a similar effect to the more familiar Doppler redshift, however, there is a difference. In Doppler Shift, the wavelength of the emitted radiation depends on the motion of the object at the instant the photons are emitted.  In cosmological redshift, the wavelength at which the radiation is originally emitted is lengthened (lower frequency) as it travels through expanding space.  Cosmological redshift results from the expansion of the universe itself and not from the motion of an individual body.  Because the cosmological redshift occurs due to the expansion of the universe (and hence the recession velocity of the object), with distant HI sources the redshift corresponds to the rate of increase in their distance from the observer.  The further the distance to the system, the longer the emitted photons have travelled through expanding space and therefore a larger cosmological redshift.

Epoch of Reionization (EoR)

The Epoch of Reionization (EoR) is a term used to describe the period during which the gas in the Universe went from being almost completely neutral to a state in which it became almost completely ionized. This occurred when the Universe was a few hundred million years old (about 1/20 of its current age) and the first radiating objects formed. This time frame is not well understood and is linked to many fundamental questions in cosmology.

A large amount of theoretical effort, guided by very limited observational evidence, is currently dedicated to understanding the physics that trigger this epoch, ruled its evolution, and what effects it had on subsequent structural formations within the Universe.  The 21-cm line is at this point the only observable characteristics of the gas during the EoR.  Due to the extreme distance from us, the 21-cm line is shifted from a frequency of 1420 MHz down to a frequency in the range of approximately 150 MHz.