Prof John Dickey (University of Minnesota)

Why Aren't Galaxies Brighter at 21 cm ? - Prof John Dickey Colloquium

ATNF Marsfield Lecture Theatre

Abstract

Using data from the Southern Galactic Plane Survey,
I will try to answer the question of what sets the
limit on the brightness temperature of the 21-cm line
emission from a galaxy. This involves studying the
abundance of both the warm phase and the cool phase
of the neutral interstellar medium, traced by 21-cm
emission and absorption respectively. Along the way
we attempt to separate the phases in order to
interpret emission-absorption spectrum pairs to
derive the temperature of the cool phase gas. We
also look at the Galactic distribution of the cool gas,
cast in terms of the mean opacity of the 21-cm line
as a function of Galactic radius. Finally we do
some simple radiative transfer to find that the
Milky Way must have peak brightness in the 21-cm
line of about 125 to 140 K. Other galaxies show
somewhat different values; this can be explained by
changing the mixture of warm and cool phase gas.