Cosmic rays can help sterilise small galaxies, preventing them from forming new stars, new simulations suggest. The effect could help explain why we see fewer dwarf galaxies than expected in orbit around the Milky Way.
Standard theories of dark matter and galactic evolution predict that small galaxies should merge over time to form large ones, and that hundreds or even thousands of these undersized, unmerged galaxies should be spiralling around the Milky Way today. But observations have turned up just 35, a gap known as the 'missing satellite problem'.
One possibility is that many of the missing satellite galaxies are much fainter than expected, making them mostly invisible to past surveys – some extremely faint objects spotted recently seem to point in this direction.
But that begs the question: why do these galaxies contain so few stars?
Some scientists have suggested the smallest galaxies never formed many stars because they were zapped by radiation that suffused the early universe in a process called reionisation. According to this idea, reionisation heated and expanded gas in small galaxies until their gravity could no longer hold onto it.
But recent research suggests reionisation does not inhibit star formation enough to account for all the 'missing' galaxies. "[It] doesn't suppress the formation of satellite galaxies as much as we thought," says Andrew Benson of Caltech.
Using 600,000 hours of computing time, they performed detailed simulations of the formation of Milky Way-like galaxies. "Previous simulations followed dark matter only and did not include the far more messy details of stars and gas," comments Marla Geha of Yale University in New Haven, Connecticut.
The team found that reionisation did make some of the puniest galaxies lose their gas early on. Others held onto their gas and formed some stars, but still ended up being very dim.
That is because stars that formed within them early on exploded in supernovae, shooting out charged particles called cosmic rays. The particles infused the surrounding gas, creating an outward pressure that prevented the gas from collapsing to form stars.
This un-collapsed gas was then vulnerable to being stripped away by wind resistance as the small galaxies moved through the medium of gas between them and larger galaxies.
The simulations also show that even when small galaxies managed to form stars, they tended to lose some of them when they approached bigger galaxies. The gravitational pull of the behemoths pulled stars away from the smaller objects, lowering their brightness.
"Our results show that even rather massive and gas-rich satellites at early times can end up as dark objects" that have no chance of being observed, Wadepuhl told New Scientist.
"It shows how complex the process of galaxy formation is and how we have to consider many different physical processes to arrive at the correct answer," says Benson.
Journal reference: Study abstract
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The Extra Power Of Cosmic Rays
Sat May 08 10:59:47 BST 2010 by Mark Bridger
I have a theory as to why solar and stellar winds are very powerful and the sun's corona is so hot. The same physics could explain the heating of gas near galaxies and the power of cosmic rays (that appear to reduce the star making near many galaxies)
Basically the gravity redshift of the light that leaves an object means a loss of the energy of the light. That has to be accounted for according to the conservation of energy principle and so maybe means the deposting of energy in the gravitational space. That energy is picked up by the particles that enter the space, heating them up. Hence the corona heats up, expands and accelerates outwardly. The same physics would apply to the gravity fields of radiant galaxies (though with dark gravity objects that receive more radiation than they emit there would be mostly a blueshifting of the light which, by the same principles would cool the space and stuff that went near the object (and here help to make stars).
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