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Fraser
2006-Jan-13, 05:46 AM
SUMMARY: Astronomers think they might have found a "dark galaxy", that has no stars and emits no light. Although the galaxy itself, located 50 million light years from Earth, is practically invisible, it contains a small amount of neutral hydrogen which emits radio waves. If astronomers are correct, this galaxy contains ten billion times the mass of Sun, but only 1% of this is hydrogen - the rest is dark matter.

View full article (http://www.universetoday.com/am/publish/pparc_darkmatter_virgohi.html)
What do you think about this story? post your comments below.

Fr. Wayne
2006-Jan-13, 09:56 AM
With this dark galaxy at 50 million lyrs toward Virgo and your "Giant Galactic Companion http://www.bautforum.com/showthread.php?t=36938also in Virgo at 30 million lyrs. , we either got a battle of beaus going on here or something happened of an immoral nature I can't repeat on this family forum that took place about 5 billion years ago.

P.S. Virgo was in ancient times often regarded as Proserpina, a daughter of Ceres(Demeter) who was abducted by Pluto in his chariot (Libra?) as well as Isis, the thousand-named goddess, who dropped wheat ears to form the Milky Way. Even Sir William Herschel was struck by her 500+ "nebula"... - seems our modern astronomers are getting infatuated with her too.

Relmuis
2006-Jan-13, 11:53 AM
I have sometimes read that star formation must be triggered by a nearby supernova. And of course no supernovae can occur in the absence of stars.
This leads me to think that a special, and perhaps improbable event must occur for the first stars in a galaxy to form. (Perhaps the galaxy must be "seeded" by a star which escaped from another galaxy, and goes supernova while passing through. This would lead me to supect that the existence of metagalactic clusters and voids might be more apparent than real: a chain reaction of galaxies which have, so to speak, lit each other up.)

So, it might not be unusual for a galaxy to fail to produce stars. For all I know, a vast majority of all galaxies might be starless, and therefore nearly invisible.

This might (partially) solve the riddle of the Dark Matter, the invisible stuff that is supposed to dominate our universe. But why should it be something else than hydrogen?

If most galaxies fail to produce stars, we should be able to find galaxies where star formation has recently begun. What would these galaxies look like? That depends on the lifetimes of the first stars and the rotation time of the galaxy. If the first stars are heavy, and have short lifetimes, the newly enstarred galaxy should look like part of a "normal" galaxy, say a segment of a single spiral arm. If the first stars are less heavy, the newly enstarred galaxy should look like a "tenuous" galaxy, with a small number of stars spread over its entire extent.

Greg
2006-Jan-13, 06:18 PM
I thought that 1-2 percent of the mass of the universe is light matter so I do not see anything particuarly interesting about the mass breakdown figure (1 percent of the mass of the galaxy being hydrogen.) The oddity is that the hydrogen associated with this galaxy has not formed stars. This would not be the first such discovery of delayed star development in a galaxy. There are several articles of dwarf-sized galaxies that have not begun forming stars until very recently. The really interesting question is why some galaxies take so long to form stars. The possible answers had to do with the density and concentration of the initial hydrogen cloud. If it is too diffuse and spread out when it first forms and nothing disturbs it sufficiently later, the hydrogen doesn't concentrate enough to begin forming stars. The question of why and how many clouds formed and developed in this fashion as opposed to how "normal" galaxies formed would require looking back further in time than our most powerful telescopes currently can to see just what was happening in the early universe.

Grey
2006-Jan-13, 08:00 PM
I thought that 1-2 percent of the mass of the universe is light matter so I do not see anything particuarly interesting about the mass breakdown figure (1 percent of the mass of the galaxy being hydrogen.) The oddity is that the hydrogen associated with this galaxy has not formed stars.Not quite. You'll sometimes see a breakdown of the components of the universe in the concordance model as about 70% dark energy, 25% dark matter, and 4% or 5% baryonic matter. But the dark energy component is really only relevant on cosmological scales (it's spread evenly through the universe, including intergalactic space, so it won't be concentrated in a galaxy), so a typical galaxy will be about 80% to 85% dark matter and 15% to 20% baryonic. So having only 1% hydrogen is indeed much lower than the ratio for a typical galaxy.

Blob
2006-Jan-13, 08:33 PM
Hum,
It should be noted that the hydrogen gas in VIRGOHI 21 appears to be rotating, implying a dark galaxy...
it may actually be two separate gas clouds passing each other...no galaxy...