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B Cosmic horseshoe is not the lucky beacon

  1. Oct 26, 2016 #1


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    October 26, 2016
    University of California - Riverside
    Although the universe started out with a bang it quickly evolved to a relatively cool, dark place. After a few hundred thousand years the lights came back on and scientists are still trying to figure out why.

    In a just-published paper, a team of researchers, led by a University of California, Riverside graduate student, used a direct detection method and found the previously used constraints have been overestimated by five times.

    So are galaxies alone responsible for the reionization of the universe?
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  3. Oct 26, 2016 #2


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    A factual quibble:
    The article states, "Astronomers know that reionization made the universe transparent by allowing light from distant galaxies to travel almost freely through the cosmos to reach us." This is false. Reionization made the universe less transparent. Light doesn't interact much at all with neutral atoms, but interacts readily with free electrons and ionized atoms. The reason why it's called the "dark ages" is that because before reionization, while the universe was extremely transparent, nothing was really emitting any light either. Effectively the only light around was that of the cosmic microwave background.

    That said, this is a single measurement of a single galaxy. As it's a new measurement technique, it's entirely plausible that they're missing something big that will bring this direct detection more in line with indirect detection methods. It's also plausible that this particular galaxy is just peculiar.
  4. Oct 27, 2016 #3
    Might this be related to the recent report that there were more small galaxies than previously thought:


    The study looked at galaxies down to 106 MSun (100,000 times less massive than the Milky Way) so perhaps a larger number of small sources would explain the required total flux. Does that make sense?
  5. Oct 27, 2016 #4


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    I don't think that study can really change the estimated total flux of stars in the early universe (if that study did, then we would have been able to detect those galaxies directly).

    Also note that it's plausible that AGN's and supernovae might have had a significant role to play, and it is exceedingly difficult to model the cosmological impact of either.
  6. Oct 28, 2016 #5
    In the abstract, they say "These results also reveal that the cosmic background light in the optical and near-infrared likely arise from these unobserved faint galaxies." so in a sense perhaps we have. I suspect the JWST should resolve these questions.

    This is the pre-print. There is a lot of discussion in the full paper, especially section 4.4, but this is typical "While this does not imply that these galaxies are reionizing the universe, it does show that this limit is required if UV emission from galaxies is the ionization culprit. This is also consistent with recent determinations of the cosmic background light which show that there are a factor of 10 more galaxies than we can presently observe, per unit area, needed to account for this light (e.g., Mitchell-Wynne et al. 2015).".

    Certainly, but I was really just wondering if the two papers essentially offset each other, individuals galaxies provide fewer photons than thought but there are more of them.
  7. Oct 28, 2016 #6


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    Maybe, but I doubt it. Bright galaxies are vastly brighter than dim ones.
  8. Oct 29, 2016 #7
    I'm not arguing here, just trying to broaden my knowledge.

    I would have thought that the luminosity of a galaxy would be roughly proportional to the number of stars as a first approximation, assuming the stars themselves are similar, and that a larger galaxy might have more of the core hidden by dust clouds outside the core. Hence I might expect that overall a large galaxy might actually dimmer than the same number of stars arranged in a large number of small galaxies.

    I'm thinking specifically of optical and UV (relevant to the topic of re-ionisation) rather than IR where the dust re-emission would be significant. Is that wrong?
  9. Oct 31, 2016 #8


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    The problem is that some stars are vastly, vastly brighter than others. And the brighter stars don't last for nearly as long. So there are also strong correlations between brightness and the overall density (which impacts star formation rates) and the age of the galaxy.

    We know that in the current universe, lower-mass galaxies also tend to not have nearly as much normal matter in them. One possible explanation is that when they first formed, supernovae and other energetic events were able to blow most of the gas out of these low-mass galaxies.

    Or in other words, galaxies are incredibly complicated beasts and it's hard to simplify them in such a way.
  10. Nov 1, 2016 #9
    Yes, I'm aware that Pop III for example are expected to be much larger, brighter and shorter lived than later generations for example.
    Ah, that I think is what I was missing, thanks. Is there a name for an empirical relationship between density and formation rate I could read up on?
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