Digital Universe Guide - Hayden Planetarium

3.3. MILKY WAY DATA GROUPS 113
3.3.22 Atomic Hydrogen Survey
Group Name mwH
Reference Atlas of Galactic Neutral Hydrogen (Hartmann+, 1997)
Prepared by Brian Abbott (AMNH/Hayden)
Labels No
Files mw-21cm.speck
Dependencies 02-21cm-512.sgi
Wavelength 21 cm
Frequency 1.42 GHz
Warm neutral hydrogen radiates in the radio spectrum at a wavelength of 21 centimeters. In the
hydrogen atom, the electron and proton are magnetized, giving each a north and south pole just like a
bar magnet. Any particular neutral hydrogen atom can exist in two configurations: a lower energy state,
in which the north poles of the electron and proton are pointing in the same direction, and a higher
energy state, in which they point in opposite directions.
The warm interstellar gas provides the energy to boost the atom into this higher energy state. Once
the atom returns to its lower energy state, it gives off energy at a wavelength of 21 cm. The low-energy
light in this survey reflects the small difference between these atomic states.
This radiation is important because it penetrates the dust in the interstellar medium, allowing us to
see it across the galaxy. The 21-cm light is perhaps the most important tracer we have for determining
the structure of our Galaxy.
The false colors in the survey represent the density of atomic hydrogen along a line of sight. White
and red represent highly dense regions of hydrogen gas, while blue and purple are regions of lower
density. These measurements, combined with the Doppler shift of the light, give us an idea of the
structure of the Milky Way’s hydrogen gas clouds.