Explained: Why many surveys of distant galaxies miss 90 percent of their targets

Mar 24, 2010
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. Credit: ESO/M. Hayes

(PhysOrg.com) -- 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 known as the Lyman-alpha line, to probe the amount of stars formed in the very distant . Yet there have long been suspicions that many distant 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.

"Astronomers always knew they were missing some fraction of the galaxies in Lyman-alpha surveys," explains Matthew Hayes, the lead author of the paper, published this week in Nature, "but for the first time we now have a measurement. The number of missed galaxies is substantial."

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This video pans over the GOODS-South field, where astronomers have been able to detect, thanks to ESO’s Very Large Telescope, some of the faintest galaxies ever seen. Credit: ESO/M. Hayes

To figure out how much of the total luminosity was missed, Hayes and his team used the FORS camera at the VLT and a custom-built narrowband filter to measure this Lyman-alpha light, following the methodology of standard Lyman-alpha surveys. Then, using the new HAWK-I camera, attached to another VLT Unit , they surveyed the same area of space for light emitted at a different , also by glowing hydrogen, and known as the H-alpha line. They specifically looked at galaxies whose light has been travelling for 10 billion years (redshift 2.2), in a well-studied area of the sky, known as the GOODS-South field.

"This is the first time we have observed a patch of the sky so deeply in light coming from hydrogen at these two very specific wavelengths, and this proved crucial," says team member Göran Östlin. The survey was extremely deep, and uncovered some of the faintest galaxies known at this early epoch in the life of the Universe. The astronomers could thereby conclude that traditional surveys done using Lyman-alpha only see a tiny part of the total light that is produced, since most of the Lyman-alpha photons are destroyed by interaction with the interstellar clouds of gas and dust. This effect is dramatically more significant for Lyman-alpha than for H-alpha light. As a result, many galaxies, a proportion as high as 90%, go unseen by these surveys. "If there are ten galaxies seen, there could be a hundred there," Hayes says.

Different observational methods, targeting the light emitted at different wavelengths, will always lead to a view of the Universe that is only partially complete. The results of this survey issue a stark warning for cosmologists, as the strong Lyman-alpha signature becomes increasingly relied upon in examining the very first galaxies to form in the history of the Universe. "Now that we know how much light we've been missing, we can start to create far more accurate representations of the cosmos, understanding better how quickly stars have formed at different times in the life of the Universe," says co-author Miguel Mas-Hesse.

The breakthrough was made possible thanks to the unique camera used. HAWK-I, which saw first in 2007, is a state-of-the-art instrument. "There are only a few other cameras with a wider field of view than HAWK-I, and they are on telescopes less than half the size of the VLT. So only VLT/HAWK-I, really, is capable of efficiently finding galaxies this faint at these distances," says team member Daniel Schaerer.

Explore further: Astronomers release most detailed catalogue ever made of the visible Milky Way

More information: This research was presented in a paper to appear in Nature ("Escape of about five per cent of Lyman-a photons from high-redshift star-forming galaxies", by M. Hayes et al.).

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El_Nose
2.4 / 5 (7) Mar 24, 2010
Is it just me -- I fugred people would be all over this -- this states we were blinded to 90% of all visible matter -- WOW WOW WOW WOW WOW

so our understanding of visible matter just went from 4.9% of the observable universe to 49% of the observable universe -- WOW WOW

this has got to be the biggest monkey wrench in the world to dark matter theorists -- i have no opinion on that issue but this has GOT to be huge ..

right????
Caliban
3.2 / 5 (6) Mar 24, 2010
I was thinking the same thing. At the very least, this will require a revision in the estimate of visible/dark matter ratio, and possibly require scrapping the DM concept altogether.
Let's wait to hear from some of our more knowledgeable colleagues.
I have always felt a sense of horror at the thought of our little universe expiring from the increasing entropy gradient and eventual heat death scenario. I much prefer something along lines of Steady State- continuing rebirth though Big Bang/Big Crunch mechanism.
Glyndwr
5 / 5 (1) Mar 24, 2010
is it 49% accountability as there is a higher density of galaxies in the same 'patch of universe'
or just being able to see further into the universe and hence the actual proportions just remain 4.9%?
SincerelyTwo
not rated yet Mar 24, 2010
Good point Glyndwr, I guess it might mean our universe is older than expected ...
mattytheory
5 / 5 (2) Mar 25, 2010
^

"They specifically looked at galaxies whose light has been travelling for 10 billion years (redshift 2.2), in a well-studied area of the sky, known as the GOODS-South field."
lengould100
3 / 5 (4) Mar 25, 2010
This does sound like it should be significant to present estimates of normal to dark matter ratio, though 4.9 to 49 will be too large a change for THIS discovery.

A few years ago on this site there would have been at least a couple of expert astronomers commenting on this by now. Have the denialists pushed all of them away with stupid anti-AGW debates? Too bad.
danman5000
4 / 5 (1) Mar 25, 2010
From what I understand, one of the arguments for the existence of dark matter is the fact that galaxies are spinning faster than their visible matter would allow. This implies that there is a halo of 'dark matter' around them that adds mass which prevents them from flying apart. The number of galaxies in the visible universe probably wouldn't have an impact then, but I could be wrong. It might change our view of the ultimate fate of the universe though, as was mentioned above. With that much more matter, I would think we'd be much closer to the Big Crunch scenario.
El_Nose
3.7 / 5 (3) Mar 25, 2010
@ lengould100

i gave u a 5 -- it is a shame - and i remember when true astronomers would have piped in yesterday and cleared up this issue by now.

@danman5000

there was an article on here last month - no time to look it up that stated that we were missing appox 90% of just large stars because they were hidden - this applied to the ratio assumed of large stars : normal stars : dwarfs -- because the ratio we used was off the amount of large stars in other galaxies was terribly scewed on the order of being off by 90%, and was totally unrelated to this article

so we just missed 90% of all galaxies -- and under estimated the total number of big stars in galaxies -- though to you point this has nothing to due with our mass calculations because that is probably a bit more accurate as it is not based on light?

Science is ever evolving I grew up Pluto was a planet - now its not - we should not put too much stock in it when scientists are 100% sure -- but we should head warnings
frajo
5 / 5 (1) Mar 25, 2010
one of the arguments for the existence of dark matter is the fact that galaxies are spinning faster than their visible matter would allow.
This is a fact of observation.
This implies that there is a halo of 'dark matter' around them that adds mass which prevents them from flying apart.
This is one possible conclusion. It is the dominant conclusion, but it is not the only possible conclusion.
Au-Pu
3 / 5 (2) Mar 27, 2010
Dark matter has always had the uncomfortable feel of a band aid fix.
Hopefully this may get rid of the band aids.
The other concern is our propensity for anthropomorphism. We do it to everything our cars, boats(ships), planes, houses, animals. Everything appears to need to be cast in our own image.
I worry that the Big Bang theory might simply be another act of anthropomorphism, only this time we have applied it to the Cosmos.
Are we really as ignorantly arrogant as that?
RealScience
5 / 5 (2) Mar 28, 2010
No, this does not mean that our estimates of the amount of 'normal' (baryonic) matter in the universe are off by 10X.
Those estimates are based on the density of mater in space that we can observe well, and corroborated by the ratio of hydrogen, heavy hydrogen and helium.
(The matter estimates are, however, based on the assumption that space far away is a lot like the space that we can see).

What the article does imply is that there were a lot more galaxies around 10 billion years ago than we had been seeing.
This may have significant implications for models of galaxy formation (many of which models depend on dark matter).
Skeptic_Heretic
5 / 5 (1) Mar 28, 2010
Those estimates are based on the density of mater in space that we can observe well, and corroborated by the ratio of hydrogen, heavy hydrogen and helium


You beat me to it. Our baryonic estimates are most likely unaffected by this revelation, however, we're going to get a great many better "deep field" observations. This also bodes well for the search for galactic anomalies (ie: degenerate galaxies and stars).