Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A probable galaxy at z=7

  1. Mar 5, 2005 #1

    wolram

    User Avatar
    Gold Member

    http://ernie.ecs.soton.ac.uk/opcit/cgi-bin/pdf?id=oai:arXiv.org:astro-ph/0402319
    Reionization seems to be a fundamental process to the formation of our universe, but when did it occur? I have seen papers state z=6, but this seems
    to be a lowest limit, as this paper suggests a galaxy at z=7, a search of
    citebase found a possible z=10, I can only imagine that better observations
    will find galaxies upto or beyond z=16. So to fit in with our model of the
    U what is the upper, lower bound for reionization?
     
  2. jcsd
  3. Mar 5, 2005 #2

    SpaceTiger

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Actually, the paper is suggesting that the galaxy has a "Gunn-Peterson trough" -- in fact, that seems to be how they're estimating the redshift. This is a feature that arises in the spectra of objects seen before reionization because it comes from absorption by neutral hydrogen. Thus, it would suggest z<~7 for reionization.

    Be cautious of reports like these, however, as they often turn out to be bogus (usually due to misindentification).

    NOTE: The Gunn-Peterson trough is also present in the hypothetical z~10 galaxy.
     
    Last edited: Mar 5, 2005
  4. Mar 5, 2005 #3

    wolram

    User Avatar
    Gold Member

  5. Mar 5, 2005 #4

    wolram

    User Avatar
    Gold Member

    This clip is from Wikipidia
    http://en.wikipedia.org/wiki/Reionization

    As of 2004, this has created a puzzle due to observations from WMAP, which measured the total number of electrons present in the universe since recombination. This measurement of the Thomson scattering optical depth to electrons implies that if reionization occurred abruptly, it had to begin and end at a much earlier time, z = 17 (about 200 million years after the big bang). This indicates that: (1) either the Gunn-Peterson or WMAP results are being mis-interpreted OR (2) that reionization does not happen abruptly but is long and complex. It would seem the best bet is that the theoretical models were too simple and that reionization is long and complex. Astronomers are in a frenzy to understand this.
     
  6. Mar 5, 2005 #5

    wolram

    User Avatar
    Gold Member

    I am not sure if scientists are in a frenzy or not, it just seems that
    the more i look the more ambiguous the answer.
     
  7. Mar 5, 2005 #6

    wolram

    User Avatar
    Gold Member

    Actually, the paper is suggesting that the galaxy has a "Gunn-Peterson trough" -- in fact, that seems to be how they're estimating the redshift. This is a feature that arises in the spectra of objects seen before reionization because it comes from absorption by neutral hydrogen. Thus, it would suggest z<~7 for reionization.
    ------------------------------------------------------------------------------------------------
    This is the crux if GPT is dependant on reionization era, and reionization
    era is ambiguous, how can we determine high redshift objects "distance"
    beyond GPT.
     
  8. Mar 5, 2005 #7

    SpaceTiger

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I wasn't suggesting that we were sure about the epoch of reionization, just that these observations didn't indicate that it happened at higher redshifts. In fact, galaxies at these redshifts are often selected based on their Gunn-Peterson breaks (it's hard to measure the redshift otherwise). This implies that we're selectively observing galaxies passing through a lot of neutral hydrogen, so it's not particularly telling about reionization as a whole. This is why the authors of those papers were hesitant to draw any strong conclusions about reionization itself.

    I suspect that you're right about it being long and complex, but there's a professor in this department (Renyue Cen) who has a theory which suggests that there might have been two reionizations, one by the initial Pop III stars and another by the active newly-formed galaxies, much like those in the papers you referenced. In fact, that was the motivation of their study, according to the abstract.
     
  9. Mar 5, 2005 #8

    SpaceTiger

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    We would need another spectral feature that was strong enough to be observed with a reasonable amount of spectroscopic observing time. Unfortunately, the high redshift objects are really dim, so the spectra we get are really crude. Gunn-Peterson breaks are by far the easiest to pick out of all the major features in a high-z galaxy.
     
  10. Mar 5, 2005 #9

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    Multiple inflationary epochs might explain reionization.
     
  11. Mar 5, 2005 #10

    turbo

    User Avatar
    Gold Member

    But how would we explain the "multiple inflationary epochs?" Some folks have trouble accepting one of them.
     
  12. Mar 5, 2005 #11

    wolram

    User Avatar
    Gold Member

    I can see that many of you are seeking some "truth", in all the
    data, where it will come from is unknown to date, but one thing
    is for sure, and that is we know zip.
     
  13. Mar 5, 2005 #12

    turbo

    User Avatar
    Gold Member

  14. Mar 5, 2005 #13

    wolram

    User Avatar
    Gold Member

    So present reionization z=6 is predicted by the results of a very
    small data set, Thanks for the link Turbo.
     
  15. Mar 5, 2005 #14

    turbo

    User Avatar
    Gold Member

    There are quite a number of papers on these lines, indicating that reionization in the BB model was essentially completed by z~6-6.5. Could it have been completed earlier (in the BB model), or is reionization even necessary (not necessary in other models)?

    When Webb (extended infrared capabilities) and the Large Binocular Telescope (monster aperture with adaptive optics!) come on line, there will be some serious horsepower trained on "early" objects. It is here that the rubber will meet the road. The standard model (with its heirarchical model of galactic evolution) is already being seriously constrained by observations, since we are currently observing z>6 quasars with BH masses of several billion suns residing in galaxies massing hundreds of billions or even trillions of suns. It is hard to imagine how these tremendously massive structures might have formed in about a half-billion years after the big bang. Why are these monsters found at such high redshifts, while none are in our locality? Perhaps quasars are local, and their redshift is intrinsic and not cosmological.... If the redshift of these objects is not due to cosmological expansion, they do not have to be the most luminous objects in the universe, they do not have to reside in the most massive galaxies in the universe, and they do not have to assume the wierd abundance ratios dictated by the standard model, nor will it be suprising when we measure their spectra, and find that they generally exhibit super-solar metallicities. This last one is a BIG problem for the standard model.

    The insistance that redshift=cosmological expansion/distance will prove to be the undoing of the standard model. As more and more highly redshifted quasars are observed, their apparent masses and luminosities will rise. This is because we do not properly understand the causes of redshift. By gauging every redshift as if it were the product of a Doppler-type or GR expansion effect, we have put ourselves into a box that will soon become untenable.
     
    Last edited: Mar 5, 2005
  16. Mar 6, 2005 #15

    wolram

    User Avatar
    Gold Member

    http://citebase.eprints.org/cgi-bin/citations?id=oai:arXiv.org:astro-ph/0406260
    The observational evidence for a population of quasars powered by supermassive black holes of mass \geq 10^9 M_sun at redshifts z\geq 6 poses a great challenge for any model describing the formation of galaxies. Assuming uninterrupted accretion at the Eddington limit, seed black holes of at least 1000 M_sun are needed at z \approx 15. Here I study whether these seeds could be primordial black holes (PBHs) which have been produced in the very early universe by the collapse of primordial density fluctuations. In particular, I study the expected number densities of PBHs in the relevant mass range for several classes of spectra of primordial density fluctuations and confront the results with observational data. While it seems to be possible to produce the required PBHs with spectra showing large enhancements of fluctuations on a certain scale, our hypothesis can be clearly disproved for a scale free spectrum of primordial fluctuations described by a power-law slope consistent with recent observations.
     
  17. Mar 6, 2005 #16

    wolram

    User Avatar
    Gold Member

    I see what you mean Turbo, super massive BH at z=6, sort of makes
    billy bunter a rank outsider for gluttony, i have not found an estimate
    on how much of the mass of the U is beyond z=6, something that may
    be of interesting.
     
  18. Mar 6, 2005 #17

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    As a matter of fact, as suggested by a number of studies, reionization may have been complete by z=10. The only problem that poses is in the realm of stellar evolution. The SMBH observations are not problematic. In the early universe, there was a great deal of unattached mass to draw from. Huge stars and huge black holes are not the least bit unexpected. And if you do not care for cosmological redshift, propose an alternative explanation that holds water. Early metallicity is not that big a problem. Primordial elemental abundance is, however, a huge problem in any model that does not include a hot big bang.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?