Archive for September 14th, 2011

September 14, 2011

Black holes act as galactic thermostats

The supermassive black holes at the centers of most medium to large size galaxies may drive a periodic self-regulating boom-bust cycle. This may be like a pendulum which swings only so far in one direction before it reaches a limit, turns around, and eventually reaches a limit in the other direction. In this case it would be interstellar gas acting like the pendulum. Starting from a very hot state the gas cools by emission of X rays, falls in towards the black hole, and a portion of it forms into an energetic active galactic nucleus (AGN) – which reheats gas further out until the AGN burns itself out and the cycle begins all over again. That is, at least, the scenario explored in more detail in research being reported this week.

Black holes act as galactic thermostats –

The supermassive black hole at the centre of a massive galaxy or galaxy cluster acts as a furnace, pumping heat into its surroundings. But astronomers have struggled to understand how a steady temperature is maintained throughout the whole galaxy when the black hole only appears to interact with nearby gas. Now, researchers in Canada and Australia believe the answer could be a feedback loop in which gravity causes gas to accumulate around the black hole until its density reaches a tipping point. Then, the gas rushes into the black hole, temporarily turning up the heat.

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September 14, 2011

Herschel paints new story of galaxy evolution

ESA’s Herschel infrared satellite telescope has clarified the circumstances for rapid star formation in galaxies. In the present (nearby) universe, rapid star formation is observed only in galaxies that have experienced collisions with other galaxies. However, the new findings make it clear that such collisions aren’t necessary for rapid star formation. By careful study of the spectra of much more distant galaxies – which are seen as they were at an earlier time in the universe – it’s been shown that the rate of star formation is directly proportional to the amount of interstellar gas contained in galaxies not undergoing collisions. Since nearby galaxies are significantly older, they have already used up most of their interstellar gas, and only the disturbance caused by a collisions is sufficient to reinvigorate the rate of star formation.

Herschel paints new story of galaxy evolution – ESA

Gas is the raw building material for stars and this work reveals a simple link: the more gas a galaxy contains, the more stars are born.

“It’s only in those galaxies that do not already have a lot of gas that collisions are needed to provide the gas and trigger high rates of star formation”, says Dr Elbaz.