ESO: Why distant galaxy surveys miss 90% of targets

[bystander]

Explained: Why many surveys of distant galaxies miss 90% of their targets
eso1013 - 24 March 2010

Astronomers have long known that in many surveys of the very distant Universe, a large fraction of the total intrinsic light was not being observed. Now, thanks to an extremely deep survey using two of the four giant 8.2-metre telescopes that make up ESO’s Very Large Telescope (VLT) and a unique custom-built filter, astronomers have determined that a large fraction of galaxies whose light took 10 billion years to reach us have gone undiscovered. The survey also helped uncover some of the faintest galaxies ever found at this early stage of the Universe.

Astronomers frequently use the strong, characteristic “fingerprint” of light emitted by hydrogen known as the Lyman-alpha line, to probe the amount of stars formed in the very distant Universe [1]. Yet there have long been suspicions that many distant galaxies go unnoticed in these surveys. A new VLT survey demonstrates for the first time that this is exactly what is happening. Most of the Lyman-alpha light is trapped within the galaxy that emits it, and 90% of galaxies do not show up in Lyman-alpha surveys.

• 
  This composite image of the GOODS-South field — the result of an extremely deep survey using two of the four giant 8.2-metre telescopes composing ESO’s Very Large Telescope (VLT) and a unique custom-built filter — shows some of the faintest galaxies ever seen. It also allows astronomers to determine that 90% of galaxies whose light took 10 billion years to reach us have gone undiscovered.
  
  The image is based on data acquired with the FORS and HAWK-I instruments on the VLT. It shows in particular two varieties of light emitted by excited hydrogen atoms, known as Lyman-alpha and H-alpha.
  
  (ESO/M. Hayes)

Escape of about five per cent of Lyman-α photons from high-redshift star-forming galaxies

[Pretzelogic]

The journal Nature, yesterday, published online an article about an effort to empirically test what fraction of Lyman-alpha photons actually "escape" absorption to the point of being detectable from Earth:

http://www.nature.com/nature/journal/v4 ... 08881.html

Nature's website only let me view the brief summary of the article (and as a presently unemployed wretch, I can't even afford to purchase the view of the entire article, much less a full subscription - and, I'm not at all sure how much of the full detail my non-expert mind would properly grasp, anyway). It sounds as though, from the summary ("We demonstrate that almost 90 per cent of star-forming galaxies emit insufficient Lyα to be detected by standard selection criteria") the popular press account I saw first may have correctly interpreted the implications: "Astronomers may have underestimated the tally of galaxies in some parts of the Universe by as much as 90 percent". (Ooops? )

http://preview.tinyurl.com/yzgpg87

I guess my question would be, if it's really possible that until now our estimates of the number of galaxies in the universe was off by so much (even assuming that the underestimate in some directions may have been less than in others due to differing amounts of intervening dust and such), what effect, if any, would that have on the hypotheses regarding dark matter and energy? Wouldn't the existence of that much more mass than we had previously suspected at least reduce the amount of dark matter that would be required to "balance things out"?

Thanks in advance for adding some clarity to a dusty universe...

[Chris Peterson]

I guess my question would be, if it's really possible that until now our estimates of the number of galaxies in the universe was off by so much (even assuming that the underestimate in some directions may have been less than in others due to differing amounts of intervening dust and such), what effect, if any, would that have on the hypotheses regarding dark matter and energy? Wouldn't the existence of that much more mass than we had previously suspected at least reduce the amount of dark matter that would be required to "balance things out"?

I don't think so. Dark matter "balances out" how things behave on a local scale, not a cosmological scale. The general understanding of the amount of normal matter is based on theoretical considerations, not observational. That is, we haven't depended on how many stars or galaxies we could see to determine the amount of normal matter. It is well known that a good deal of normal matter may remain unseen.