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Missing mass

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Afternoon all.

Not been on in a while so hello.

I was watching the BBC2 Horizon program last night which was covering an array of telescopes (notably SOFIA, JWST, ALMA and DORIS) and how they can observe different types of wavelengths.

Subsequently I remembered a problem relating to dark matter which I believe may or may not be related to the different wavelengths. Having studied dark matter in a little depth at university I should know the answer but I am after your thoughts.

Little history...... in the early part of the 20th century a number of astronomers observed galaxies and galaxy clusters in visible wavelengths (as this was the only real means of doing so at the time) and tried to account for their mass. However, one of them (namely Fritz Zwicky) couldn't relate the luminous output of these clusters to the gravitational binding of the galaxies, he inferred that there was some 'missing mass' which was driving the additional gravitational potential of the system, a few decades later we are all familiar (sort of) with the concept of dark matter.

However, viewing the program last night I was impressed to see that modern telescopes such as SOFIA (which shows infra-red - dust clouds and low energy gas), ALMA (which collects (radio) waves and shows higher energy output jets) and indeed the Chandra X-Ray observatory can show far more material interacting and held within galaxies, namely the galaxy of Centaurus A.

So this got me thinking, if Zwicky only observed in the visible surely he wouldn't of seen the non-visibly-luminous material such as those found by SOFIA, ALMA and Chandra (dusty nebulae, star forming regions and high energy outputs such as those now being seen in the x-ray, infra-red and radio wavelengths). So could some of this account for the 'missing mass' ?

Answers on a postcard.

My own personal experience with dark matter would probably rule out this question as particle physics is accounting for exotic micro particles that could be used to explain dark matter and even dark energy, but it doesn't stop me wondering.

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I vaguely remember a long time back someone talking about dusty stuff around a galaxy being necessary to balance the mass/spin/light equations.

No need for exotic new forms of matter, or forces.

Don't ask me for details though. Still, someone knowledgable will be alng shortly, I hope.

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In clusters of galaxies, the mass contribution from hot gas (seen in X-rays) may indeed outweigh that from the optically luminous material by a factor of 2 or more. However it still leaves a substantial fraction of the dynamical mass missing (unless you happen to believe in an exotically low value for the Hubble constant, as the X-ray mass is quite strongly dependent on said constant).


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Sorry about the length of this post. The following is a slightly modified version of something I wrote for elsewhere, and repeats some things that have already been said.

Dark matter is needed to explain the motions of stars within galaxies, and the motions of galaxies within clusters of galaxies. For example, stars revolve around the centres of galaxies, with our Sun taking about 250 million years to complete one orbit about the centre of our Milky Way galaxy. Because the speed at which a star orbits the centre of a galaxy depends on the gravitational force that holds the star in orbit, and this gravitational force depends on the amount of matter in the galaxy, measurements of orbital speeds of stars can be used to "weigh" galaxies. These measurements indicate that most of the masses of galaxies are dark, that is, not visible as hot, glowing matter. Studies of the motions of galaxies in clusters of galaxies also indicate this. If there weren't gravitational attraction from dark matter, stars (galaxies) are moving so fast that they would "boil off" from galaxies (clusters of galaxies).

It might seem that dark matter is to be expected, as galaxies conceivably could contain a lot of material that is too cold to glow. (An electric stove element that is off can't be seen in the dark, but it still has mass.) However, theoretical studies of the production of chemical elements after the Big Bang, together with observations of cosmic abundances of chemical elements today, show most of the dark matter is not made of the same type of matter that make up ordinary stuff. If dark matter were made of normal stuff, nuclear reactions shortly after the Big Bang would have occurred at rates that would not have produced the relative abundances of the least massive elements (hydrogen, deuterium, helium) that we see today.

By ordinary stuff, I mean things like people, planets, and stars, for which protons and neutrons make up the majority of their masses. This also rules black holes that formed from the collapse of stars, as the stars were originally made of protons and neutrons. Physicists think that dark matter requires particles that have yet to be observed directly. Protons and neutrons are examples of subatomic particles called baryons, so physicists think that dark matter is non-baryonic.

Edited by George Jones

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