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Fig. 1:
Optical (left) and X-ray (right) images of elliptical
galaxy NGC1399 (central galaxy in the Fornax cluster)
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Fig. 2:
Potential determined from X-ray (blue curve) and from
optical (red line) data. Good agreement between two curves suggests
that the contribution of low energy cosmic rays to gas pressure does
not exceed 10%.
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Elliptical galaxies, consisting of up to a trillion (1012) stars,
have all the characteristics of old objects in the Universe, which
have not changed much during the last few billion years. In such
steady systems, the motions of stars obey well-established rules,
which are a direct consequence of classical mechanics. In particular,
if we know the velocity dispersion of stars and the size of the galaxy
we can immediately estimate the mass of the galaxy. However, this is
not the only way to measure the galaxy mass. Massive elliptical
galaxies often posses hot gaseous atmospheres, which are powerful
sources of X-ray emission. If the gas is in hydrostatic equilibrium -
the galaxy's gravity is balanced by the pressure of the gas, then we
can use the gas to make an independent estimate of the galaxy
mass. For this we need to measure accurately the gas temperature and
its spatial distribution.
With the launch of the Chandra X-ray
observatory, having angular resolution of order of 0.5 arc second, the
quality of X-ray images is comparable to ground based optical data
(Figure 1), and therefore the mass of the galaxy can be equally well
determined by each method. Any disagreement between the two methods is
a valuable tool to determine the characteristics of the gas. For
instance, it has been suggested that relativistic protons - an elusive
constituent of cosmic rays - are often mixed with the thermal plasma
that we see in X-rays. Due to their large mass, the protons, unlike
electrons, do not produce much radiation and we cannot easily detect
their presence in the gas. However, if relativistic protons make a
substantial contribution to the gas pressure, then the gas spatial
distribution will be broadened. The net result would be an error in
the mass determination - X-ray analysis is expected to give a lower
mass, compared to the mass determined from the optical data.
This comparison was made for a few well-studied elliptical
galaxies. To make the comparison less sensitive to observational
noise, the gravitational potential, rather than the mass itself, has
been calculated and compared as shown in Figure 2. The potentials
derived from X-ray and optical data came up remarkably close to each
other with the discrepancy amounting to less than 10%. This
immediately translates into a similarly small upper limit on the
contribution of cosmic rays to the gas pressure. Moreover, this 10%
limit is perhaps applicable to other hard to detect effects including
departures from hydrostatic equilibrium, gas motions, substantial
magnetic fields or incorrect modeling of stellar kinematics. Of
course it is possible that different effects have larger amplitudes,
but opposite signs so that they cancel each other. However, it would
be a remarkable coincidence that the residual discrepancy is so small.
The conclusion reached so far is that in the most round and
well-behaved systems the gas hydrostatic equilibrium is a good
approximation and the contribution of cosmic rays to the gas pressure
is small. The next step will be to look at systems that are more
complicated and use the X-ray observations to study the
characteristics of their stellar populations, which are difficult to
measure by other means.
Eugene Churazov, William Forman, Alexey Vikhlinin, Scott Tremaine,
Ortwin Gerhard, Christine Jones.
Publications
Eugene Churazov, William Forman, Alexey Vikhlinin, Scott Tremaine,
Ortwin Gerhard, Christine Jones,
"Non-thermal pressure in M87 and NGC 1399 gas: X-ray vs. optical potential profiles",
2008, submitted to MNRAS,
arXiv:0711.4686
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