5.3.3. Heating by ejection from galaxies
The presence of a nearly solar abundance of iron in the intracluster gas
(Sections 4.3.2 and
5.2.3) suggests that a
reasonable fraction of the gas may have come
from stars in galaxies within the clusters. The gas ejected from
galaxies is heated
in two ways. First, the gas may have some energy when it is ejected. Let
ej be
the total energy per unit mass of gas ejected from a galaxy in the rest
frame of that galaxy, but not including the cluster gravitational
potential, and define 3kTej / 2
µ
mp
ej.
Second, the gas will initially be moving relative to the cluster
center of mass at the galaxy's velocity. The ejected gas will collide with
intracluster gas and thermalize its kinetic energy. On average this will
give a temperature
![]() | (5.27) |
If the ejection energy can be ignored, the temperature is given by equation (5.14), in reasonable agreement with the observations (equation 4.10).
In a steady-state wind outflow from a galaxy, one expects
kTej
µ
mp
* 2, where
*
200 km/s is the
velocity dispersion of stars within the galaxy. If the
ejection temperature is near the lower limit given by this expression,
then this form of heating will not be very important because
*2 <<
r2.
However, the
ejection temperature could be considerably higher. For example, supernovae
within galaxies could both produce the heavy elements seen in cluster X-ray
spectra and heat the gas in galaxies until it was ejected. Supernovae
eject highly enriched gas at velocities of vSN
104
km/s. The highly
enriched, rapidly moving supernova ejecta would collide with the
interstellar medium in a galaxy and heat the gas. If
mSN is the mass of ejecta from
a supernova and mej is the resulting total gas mass
ejected from the galaxy, then Tej
2 ×
109 K(vSN / 104
km/s)2(MSN / Mej), which
will be significant if the supernova ejecta are diluted by less than a
factor of about 100.