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The Question
(Submitted March 26, 1997)
I've read that the Sunyaev-Zeldovich Effect (SZE) can be used as
an independent means to calculate the Hubble Constant. My question
is:
What is the SZE and what observing programs are either ongoing or
planned to measure it and the Hubble Constant? Also, I'd appreciate
any references you can point me to for further reading.
The Answer
The Sunyaev-Zeldovich effect refers to the scattering of cosmic
microwave background (CMB) photons off of hot gas. It can be used as
a cosmological measure because clusters of galaxies have hot gas
associated with them, at temperatures of 10^7 K or more. When
the CMB photons pass through the hot gas, the photons scatter off
of the hot gas and gain energy (this is called inverse-Compton scattering).
This distorts the spectrum of the CMB in the directions of clusters.
The amount of distortion depends on the size, temperature, and density
of the hot cluster gas. We measure the temperature and density of the
gas with X-ray observations. Using the X-ray temp., density and the distortion
of the CMB we can estimate the size of the gas-emitting region. This
size estimate along with the brightness gives an estimate of the distance
to the cluster. This is the key ingredient to measure the Hubble constant.
This discussion is a little simplified but it gives the basic idea.
Using radio and X-ray observations, the SZE has been used to estimate
Hubble's constant. Measurements reported from 1990 to 1995 give values
ranging from 32 to 82 km/s/Mpc. Relevant X-ray measurements are
continuing to be done using the ASCA and ROSAT satellites.
For a recent review, written at a level for professional astronomers,
see "Comptonization of the Cosmic Microwave Background: The Sunyaev-
Zeldovich Effect" by Y. Rephaeli in Annual Review of Astronomy and
Astrophysics, Vol 33 (1995), p. 541.
We're not aware of any discussion in a popular level magazine, like
Scientific American or Sky and Telescope, but you might check their
indices.
Jim Lochner & Andy Ptak
for Imagine the Universe!
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