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About the hot gas around NGC 4555

  1. Dec 1, 2011 #1


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    Upon reading about this galaxy I was curious about the gas surrounding it. This galaxy is a field galaxy so is not under the influence of any nearby galaxies. It is elliptical so I do not think it has any average rotation. It is approximately 40,000 parsecs in diameter and is surrounded by a 10,000,000 degree C cloud of gas with a diameter of 120,000 parsecs.

    According to various descriptions about this galaxy, there is not enough mass in the galaxy itself to hold the gas cloud in place wiithout adding in dark matter, and the amount of dark matter required is considerable, about 10 times the mass of the galaxy.

    My question is why would this gas would go anywhere (even without dark matter) considering the isolation of the galaxy. It's temperature would cause it to stay a considerable distance from the galaxy, but why would it leave the vicinity of the galaxy at all when there would be no nearby sideways solar winds to blow it off or nearby gravity to pull it away?
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  3. Dec 1, 2011 #2


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    Because the gas is hot, it has a pressure. The pressure is forcing the gas outward, and unless there is enough gravitational force pulling it inward, it will simply blow away into space. Basically, without the dark matter, the velocity of the gas particles is greater than the escape velocity of the galaxy, so they just leave. This is the same reason why there is no hydrogen in the Earth's atmosphere, because the thermal velocity of hydrogen molecules at the temperature of the Earth is greater than escape velocity.
  4. Dec 1, 2011 #3


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    Thanks for the reply. Of course, escape velocity of particles being ejected due to heat! I had my head stuck in obiting gas and forgot individual particles could be ejected at high speeds.
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