The Cosmic Web

A month or so ago I wrote about the galaxy NGC 4522 undergoing the process of ‘ram pressure stripping’ as it falls through a cluster of galaxies. Although the space between the galaxies seems pretty empty it has a thin gas floating in it - and there’s enough there that a galaxy ploughing through at hundreds of kilometers a second feels an enormous wind in its face, with the result that is gas is blasted off it into a trailing cloud. This stripping is a fairly interesting process with profound consequences for the galaxy in question, so astronomers are obviously interested in studying it. But - as is often the case when you study such a vast and slow-moving beast as a galaxy - some questions can be difficult to answer just by looking through a telescope.

A simulated galaxy undergoing ram stripping. The image is about a million light years top to bottom.

For example: what’s the most important property of the gas in determining the amount of stripping that goes on? Is it the density of the gas, or the speed at which the galaxy hits it? Then again, does the temperature of the gas have a bigger effect? To answer this question observationally we’d find lots of galaxies undergoing stripping, measure all the quantities we’re interested in, and look for a relationship. But it might not turn out to be as simple as all that. For instance, the gas tends to be hotter and denser in bigger clusters. So if we find a relationship between stripping and density, it could just be telling us about the cluster mass. Since cluster masses are often quite hard to measure accurately - and in fact we often measure them by looking at the gas temperature - sorting out which variables are affecting what can be tricky.

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