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01101001
2008-Feb-12, 10:40 PM
HubbleSite.org News: Astronomers Find One of the Youngest and Brightest Galaxies (http://hubblesite.org/newscenter/archive/releases/2008/08/full/)


NASA's Hubble and Spitzer space telescopes, with a boost from a natural "zoom lens," have uncovered what may be one of the youngest and brightest galaxies ever seen in the middle of the cosmic "dark ages," just 700 million years after the beginning of our universe.
[...]
"We certainly were surprised to find such a bright young galaxy 12.8 billion years in the past," said astronomer Garth Illingworth of the University of California, Santa Cruz, and a member of the research team. "This is the most detailed look to date at an object so far back in time."

Caltech Spitzer news release: Astronomers Eye Ultra-Young, Bright Galaxy in Early Universe (http://www.spitzer.caltech.edu/Media/releases/ssc2008-04/release.shtml)

ESA: Spacetelescope.org: Hubble finds strong contender for galaxy distance record (http://www.spacetelescope.org/news/html/heic0805.html)

Noclevername
2008-Feb-13, 01:25 AM
TooSeeked. (http://www.bautforum.com/universe-today-story-comments/70269-hubble-finds-one-earliest-brightest-galaxies-universe.html) Also, awesome.

Ken G
2008-Feb-15, 05:44 PM
That's clearly an important discovery (I wonder if they have metallicity measurements). I note in passing this rather curious paragraph from the first link:

Current theory holds that the dark ages began about 400,000 years after the Big Bang, as matter in the expanding universe cooled and formed clouds of cold hydrogen. These cold clouds pervaded the universe like a thick fog.
I can't help but wonder why they think that the dark ages "began" when hydrogen became neutral, and their choice of the word "fog" certainly makes it sound like it got dark because it was all shrouded in some thick dark fog like a foggy night. Of course that overlooks the fact that the universe was pervaded by a brightness approaching that of the Sun but from all directions during that epoch, and the formation of neutral hydrogen did not affect that "darkness" in any way, it only unfogged our view of that era. Sheesh. But probably the astronomers communicating to the non-astronomers were partly to blame for the erroneous terminology, a warning against choosing flowery but imprecise language (the media tends to run with it).

antoniseb
2008-Feb-15, 05:49 PM
I wonder if they have metallicity measurements ...

How many photons would we need to have collected to have enough of a spectrum to have statistically significant metallicity measurements? I bet we don't have 1% of the required photons from this object yet.

On the other hand, I would expect that the brightest parts of the galaxy are also the places with the most metal.

Ken G
2008-Feb-15, 06:01 PM
It certainly makes for an interesting target for an experiment that could collect enough light to make spectra of metals. But you're right-- there is an unfortunate tendency for metals to show up pretty quickly, it's hard to really catch a "pristine" stellar population. Depending on when the first stars really started forming, that galaxy might actually "still" contain some of the first stars to ever form in our universe-- I think we'll be hearing more about it.

matt.o
2008-Feb-15, 11:47 PM
How many photons would we need to have collected to have enough of a spectrum to have statistically significant metallicity measurements? I bet we don't have 1% of the required photons from this object yet.

On the other hand, I would expect that the brightest parts of the galaxy are also the places with the most metal.

I believe they use the lyman break dropout technique to measure the distance to these galaxies. This means they only have multi-band photometry, no spectra. It says down the bottom of the link that they plan to get infrared spectroscopy to confirm the distance (redshift). Basically (from memory) they image the galaxy in multiple wavebands and the waveband in which you don't detect the galaxy gives you a rought estimate of the redshift. The light of the galaxy is not detected because of absorption due to intervening gas blueward of the lyman alpha hydrogen line, hence you get a 'dropout' at some waveband roughly corresponding to the redshifted lyman break.

A quick google brought up this for more info:
http://www.nature.com/nature/journal/v427/n6969/full/nature02125.html

Ken G
2008-Feb-16, 02:41 AM
Is that the Lyman continuum break from the host galaxy, or the start of the Lyman alpha forest from that galaxy and all the intervening ones?

matt.o
2008-Feb-16, 04:42 AM
It is a combination of both. If you are interested, there is an Annual Review article on lyman break galaxies here (http://arjournals.annualreviews.org/doi/full/10.1146/annurev.astro.40.121301.111837?cookieSet=1).

You prpbably need to buy that article, so there is a shorter article here (http://www.astro.ku.dk/~jfynbo/LBG.html).

Ken G
2008-Feb-16, 10:41 PM
Actually, that article is strictly about the Lyman continuum. It makes sense that would dominate over Lyman lines, though I can see that in low resolution you could get some contribution from the lines which is I think what you are saying.

matt.o
2008-Feb-17, 12:47 AM
Actually, that article is strictly about the Lyman continuum. It makes sense that would dominate over Lyman lines, though I can see that in low resolution you could get some contribution from the lines which is I think what you are saying.

Actually, Ken, both articles specifically mention the contribution due to added effect of absorption in intervening clouds. I probably wasn't very specific in my initial post regarding the Ly alpha forest being a secondary effect to the lyman break. In any case, there is no spectra yet to measure metallicity.

Ken G
2008-Feb-17, 03:19 AM
You're right on the metallicity issue, it's clear we're a long way from that. But the Lyman alpha forest can't matter here (you are also right that the second article does explicitly reference Lyman lines in aspect #3). The Lyman alpha forest only exists in the range between the vacuum energy of the Lyman continuum limit and where the light source redshifts that limit. If the redshift is more than a z of 1 + 1215/912 = 2.3, there is no Lyman alpha forest from a light source with a strong Lyman break.

RussT
2008-Feb-17, 09:51 AM
You're right on the metallicity issue, it's clear we're a long way from that. But the Lyman alpha forest can't matter here (you are also right that the second article does explicitly reference Lyman lines in aspect #3). The Lyman alpha forest only exists in the range between the vacuum energy of the Lyman continuum limit and where the light source redshifts that limit. If the redshift is more than a z of 1 + 1215/912 = 2.3, there is no Lyman alpha forest from a light source with a strong Lyman break.

Then someone really needs to fix this in Wiki...

http://en.wikipedia.org/wiki/Lyman-alpha_forest



For quasars at higher redshift the number of lines in the forest is higher, until at a redshift of about 6, there is so much neutral hydrogen in the intergalactic medium that the forest turns into a Gunn-Peterson trough. This shows the end of the reionization of the universe.

Ken G
2008-Feb-17, 03:11 PM
No one needs to fix that relative to my remarks-- quasars aren't "sources with a strong Lyman break". I mentioned that in the final sentence, though it needs to be made clear that's what I meant. Quasar light is not self-absorbed by neutral hydrogen the way a Lyman break galaxy's is, presumably because the geometry of the emission due to the powerful central engine is much different from emission by young stars.

matt.o
2008-Feb-17, 11:09 PM
You're right on the metallicity issue, it's clear we're a long way from that. But the Lyman alpha forest can't matter here (you are also right that the second article does explicitly reference Lyman lines in aspect #3). The Lyman alpha forest only exists in the range between the vacuum energy of the Lyman continuum limit and where the light source redshifts that limit. If the redshift is more than a z of 1 + 1215/912 = 2.3, there is no Lyman alpha forest from a light source with a strong Lyman break.

Ah, I see what you are getting at now. In my original post, I was wrong in saying:



The light of the galaxy is not detected because of absorption due to intervening gas blueward of the lyman alpha hydrogen line, hence you get a 'dropout' at some waveband roughly corresponding to the redshifted lyman break.

It is not detected because of the lyman continuum break which forms in the atmospheres of stars. So there is actually no light from galaxies at this high redshift to be absorbed by intervening gas.

Ken G
2008-Feb-18, 02:46 AM
That's my sense of what "Lyman break" means, but I'm sure there can be intermediate situations and you were not wrong to mention Lyman lines. I was just trying to get my head around these distant objects and what kind of spectroscopy is actually possible with them. Apparently about all they have to go on is how "full" is the spectral bin that contains the Lyman break, so that's not a heck of a lot but you gotta make do.

matt.o
2008-Feb-19, 01:10 AM
Here is the paper on arxiv:

paper (http://arxiv.org/abs/0802.2506)

Discovery of a Very Bright Strongly-Lensed Galaxy Candidate at z ~ 7.6
Authors: L. D. Bradley, R. J. Bouwens, H. C. Ford, G. D. Illingworth, M. J. Jee, N. Benitez, T. J. Broadhurst, M. Franx, B. L. Frye, L. Infante, V. Motta, P. Rosati, R. L. White, W. Zheng
(Submitted on 18 Feb 2008)

Abstract: Using HST and Spitzer IRAC imaging, we report the discovery of a very bright strongly-lensed Lyman break galaxy (LBG) candidate at z~7.6 in the field of the massive galaxy cluster Abell 1689. The galaxy candidate, which we refer to as A1689-zD1, shows a strong z-J break of at least 2.2 magnitudes and is completely undetected (<1 sigma) in HST/ACS g, r, i, and z data. These properties, combined with the very blue J-H and H-[4.5] colors, are exactly the properties of an z~7.6 LBG and can only be reasonably fit by a star-forming galaxy at z=7.6 +/- 0.4. Attempts to reproduce these properties with a model galaxy at z<4 yield particularly poor fits. A1689-zD1 has an observed (lensed) magnitude of 25.3 AB (8 sigma) in the NICMOS J band and is ~1.2 magnitudes brighter than the brightest-known z-dropout galaxy. When corrected for the cluster magnification of 9.3 at z~7.6, the candidate has an intrinsic magnitude of J=27.7 AB, or about an 0.3 L* galaxy at z~7.6. The source plane deprojection shows that the star formation is occurring in compact knots of size ~<300 pc. The best-fit stellar population synthesis models yield a median redshift of 7.6, stellar masses (1.6-3.9) x 10^9 M_sun, stellar ages 45-320 Myr, star-formation rates ~<7.6 M_sun/yr, and low reddening with A_V<0.3. These properties are generally similar to those of LBGs found at z~5-6. The inferred stellar ages suggest a formation redshift of z~8-10 (t~<0.63 Gyr). A1689-zD1 is the brightest observed, highly reliable z>7.0 galaxy candidate found to date.

Ken G
2008-Feb-19, 05:01 PM
"When corrected for the cluster magnification of 9.3 at z~7.6, the candidate has an intrinsic magnitude of J=27.7 AB, or about an 0.3 L* galaxy at z~7.6."I wonder how they can know the lensing factor, there must be a chain of reasoning behind that too.

The source plane deprojection shows that the star formation is occurring in compact knots of size ~<300 pc.That means they can resolve 300 pc in something billions of parsecs away. That's like sseing if someone's garage door is open from the Moon. Remarkable, gravitational lensing is great stuff.


The best-fit stellar population synthesis models yield a median redshift of 7.6, stellar masses (1.6-3.9) x 10^9 M_sun, stellar ages 45-320 Myr, star-formation rates ~<7.6 M_sun/yr, and low reddening with A_V<0.3.Seems like they are pretty small, but must be bright because they have a high amount of massive stars. Perhaps the galaxy forms from higher density stuff than ours did, giving it a smaller Jeans mass, but it reaches higher temperatures in the ISM, leading to more massive stars.

matt.o
2008-Feb-19, 09:33 PM
I wonder how they can know the lensing factor, there must be a chain of reasoning behind that too.


Abell 1689 is an extensively studied cluster in terms of lensing mass distribution, so they have used a model from the literature to derive this.


That means they can resolve 300 pc in something billions of parsecs away. That's like sseing if someone's garage door is open from the Moon. Remarkable, gravitational lensing is great stuff.


It sure is!


Seems like they are pretty small, but must be bright because they have a high amount of massive stars. Perhaps the galaxy forms from higher density stuff than ours did, giving it a smaller Jeans mass, but it reaches higher temperatures in the ISM, leading to more massive stars.
Yes, apparently these lyman break galaxies are all starbursts, hence bright at shorter optical/uv wavelengths due to the massive stars.

Ken G
2008-Feb-20, 03:20 AM
Abell 1689 is an extensively studied cluster in terms of lensing mass distribution, so they have used a model from the literature to derive this.OK, that makes, sense, they have a model of the mass distribution and they are calculating the lensing. Thanks.