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Some results from the Hubble Ultra-deep field

  1. Sep 23, 2004 #1

    Nereid

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    Hubble Approaches the Final Frontier: The Dawn of Galaxies, a PR earlier today, includes some interesting results, as well as a good insight into how five different teams analysed the data in different ways to arrive at more or less the same, or mutually re-inforcing, conclusions.

    From this PR: "Astronomers are now debating whether the hottest stars in these early galaxies may have provided enough radiation to "lift a curtain" of cold, primordial hydrogen that cooled after the big bang. This is a problem that has perplexed astronomers over the past decade, and NASA's Hubble Space Telescope has at last glimpsed what could be the "end of the opening act" of galaxy formation. These faint sources illustrate how astronomers can begin to explore when the first galaxies formed and what their properties might be.

    But even though Hubble has looked 95 percent of the way back to the beginning of time, astronomers agree that's not far enough. "For the first time, we at last have real data to address this final frontier — but we need more observations. We must push even deeper into the universe, unveiling what happened during the initial 5 percent of the remaining distance back to the big bang," said Richard Ellis of the California Institute of Technology in Pasadena, Calif.
    "
     
  2. jcsd
  3. Sep 23, 2004 #2
    Wow, stars with reshift z=20. i'm dying to see them
     
  4. Sep 24, 2004 #3

    turbo

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    I just scanned that PR and arrived at an understanding that is a "bit" less monolithinc than your description.

    One team said the early galaxies seen are insufficient to cause reionization.

    One teams said that the galaxies see are sufficient to cause reionisation.

    One team looked at the data, then came up with an "estimate" of how many galaxies are in the field, but can't be seen, and invoking the existence of the unseen galaxies, they then stated that the early galaxies were sufficient to cause reionization.

    The other two teams said that the presence of the early galaxies likely had an effect on ionization, and one of those teams comes to the conclusion that reionization happened at different rates and different times, based on the concentration of mass in each region. What effect would this have on the homogeneity of the universe? Interesting question.
     
  5. Sep 24, 2004 #4

    Nereid

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    We'll let PF readers decide for themselves, shall we?
    "The Bunker team identified a list of 50 probable distant galaxies in the Ultra Deep Field [...] Bunker's team claims that the combined ultraviolet light from the galaxies located in the Ultra Deep Field is insufficient to reionize the universe. Perhaps the physics of star formation was different at these early times, or a further, yet more distant population is responsible."
    "The Stiavelli team shows that the same objects would be sufficient to reionize the universe, if they possessed much fewer heavier elements — anything heavier than helium — than those of present-day galaxies, and if the early galaxies contained more massive stars. Both these assumptions are reasonable at early epochs, since astronomers know that stars make the metals that exist in the universe. Early on, before most of the stars we see today had been formed, the amount of elements must have been much lower." (my emphasis). That more distant objects (and older ones, locally) have lower metallicities is well established; the specifics for 'ordinary' galaxies at z >= 5 are poorly constrained.
    "The Yan and Windhorst team started from the objects that are seen, and then carefully estimated the fraction of fainter galaxies that are not seen, even in the Hubble Ultra Deep Field. They found that the number of dwarf galaxies rapidly increases at fainter levels in the HUDF. [...] Yan and Windhorst conclude that this steep increase of the faint dwarf galaxy population collectively generates enough ultraviolet light to finish reionizing the universe by redshift 6, even if the amount of heavier elements was similar to that of present-day galaxies." (my emphasis)
    "The HUDF NICMOS Treasury team (Thompson/Illingworth) has taken the UDF data and other ACS survey data to get the best possible estimate of the relative numbers of bright and faint galaxies around redshift 6, only 900 million years after the big bang. The papers, led by Rychard Bouwens, show that faint galaxies dominate at this epoch, compared to more recent times, and are likely to have played a significant role in the late stages of reionization. The team has also used the HUDF NICMOS data to detect a small sample of galaxies at higher redshifts (at z=7-8), 200 million years closer in time to the big bang. The amount of reionizing light at redshifts 7-8 appears to be lower than what is seen only 200 million years later at redshift 6."

    "The Malhotra and Rhoads team have found a "sheet" of galaxies in the HUDF. They find that the galaxy density near redshift z=5.9 (look-back time of 12.5 billion years) is four times the galaxy density in the rest of the surveyed HUDF "core sample." This supports theories of galaxy formation which predict that dense regions should be the first sites of galaxy formation. This evidence for an over density was bolstered by a complementary study, undertaken by Malhotra, Rhoads, and JunXian Wang, which uses the Cerro Tololo Inter-American Observatory to obtain a map of galaxies over a much wider area than the HUDF. Even with its lower sensitivity and more limited coverage in distance, this map shows that "extra" galaxies are spread like a sheet, with the HUDF located near one edge of the structure. "The presence of such structures doubtlessly affected the reionization of the universe, because the ultraviolet light that separated intergalactic hydrogen atoms into protons and electrons would have been more intense where galaxies are more common. It is then likely that reionization proceeded at different speeds in different regions of the early universe," says Rhoads."
    In the concordance model, just such inhomogeneity is predicted. You will find lots of papers showing that the inhomogeneity expected from analysis of WMAP data is consistent with that found in both 2dF surveys (galaxies and quasars) and with the first two SDSS data releases (both galaxies and quasars?).

    Oh, and the Hubble PR also has links to eight papers which discuss the various teams' findings in much more detail; my scanning of those eight papers leads me to an understanding that is consistent with my statement that 'five different teams analysed the data in different ways to arrive at more or less the same, or mutually re-inforcing, conclusions.'
     
  6. Sep 24, 2004 #5

    Garth

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    Thank you Neried for such a clear resume of those papers. One question - Had there been enough time for this structure to form?
    Garth
     
  7. Sep 25, 2004 #6

    Nereid

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    Sky and Telescope also has an article on these findings. There's a quote in this that you may find interesting: " Despite the lack of consensus on whether these dwarf galaxies alone could reionize the universe, independent commentator Richard Ellis (Caltech) pointed to the remarkable agreement on what has been seen. Theorists, Ellis noted, have speculated about this era of cosmic dawn for decades. "We're moving out of era of theory into an era of observation," he said. "This is a milestone. The sequence of deep fields from Hubble and other telescopes have changed the way we do astronomy. We have many unsolved issues, but we have a clear path forward." "
    This will certainly be a question to which much effort will be devoted! As several PF posters have noted, the details of how stars and galaxies formed, between z ~1000 (the surface of last scattering) and z = 5 or 6, are important parts of any cosmological model and observations in this regime should provide strong constraints on the models.

    What sort of progress on this question do PF members think will be made before the JWST?
     
  8. Sep 25, 2004 #7

    Garth

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    I have not crunched the numbers, however a gut feeling is that one problem will be the need for high density i.e. Dark Matter to assist quick gravitational collapse in a relatively hot and therefore relatively high pressure environment on the one hand and the counter effect of the 'anti-gravity' of Dark Energy on the other. It may be that Dark Energy has to be switched on and off as required to make the model work, which will be difficult to explain
    - Garth
     
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