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Estimates of galaxy numbers - re Herschel resuts

  1. Jan 24, 2013 #1
    In the January 2013 issue of Physics World there is an article by Steve Eales, illustrated with some remarkable images of galaxies obtained with the Herschel observatory. One, on page 31, is very appropriately captioned "Not stars; galaxies", because it looks very like star clouds, say in Sagittarius. It is a quite wonderful image that shows some 7000 galaxies; I'm told it's about four degrees on a side. If the entire celestial sphere were somehow to be imaged by Herschel in this way I figure one might say that the so observed universe would contain about 23 million galaxies. This prompts me to ask what, by comparison, is the actual estimated total number of galaxies in the observable galaxy-containing universe?
    Last edited: Jan 24, 2013
  2. jcsd
  3. Jan 24, 2013 #2


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    Most estimates I have seen conclude that there are about 10^11 galaxies in the observable universe. This link (http://en.wikipedia.org/wiki/Observable_universe) goes through a couple of calculations. Of course this is a rough number.

    Herschel is only seeing relatively nearby galaxies, which is why your number is so much smaller.
  4. Jan 24, 2013 #3


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    The Hubble Ultra Deep Field is the gold standard for galaxy counts. The are about 10,000 galaxies in that image covering about a 10 millionth of the entire sky. It took Hubble the better part of a year to collect this one small snapshot of the distant universe - http://www.universetoday.com/36610/how-many-galaxies-have-we-discovered/.
  5. Jan 24, 2013 #4
    Thanks, Phyzguy and Chronos. For me, that Herschel image is as impressive in its way as the Hubble images, as it incudes galaxies whose light has taken about 3/4 of our universe's age to reach us. Regarding these as "relatively nearby galaxies" truly emphasizes the immensity of our astronomical environment!
  6. Jan 25, 2013 #5
    link to the Herschel image please?

  7. Jan 25, 2013 #6
    Eric: I'm not sure if a link to this specific image is available. You could try herschel.cf.ac.uk or h-atlas.org which have lots of Herschel images.

    physicsworld.com is the link to the magazine where the image was published in the January 2013 issue, p.31. I saw the print edition, but access to the link may need membership of the Inst. of Physics.
  8. Jan 26, 2013 #7
    Ref the Cosmology thread, Estimating the number of galaxies, submitted by Jim Johnson
  9. Jan 27, 2013 #8
    Thanks, jimjohnson,for pointing me to this
    interesting thread . It fully answers my query about the total density of galaxies. The observed-by-Herschel density is only about 1/7000 part of this total but still gives the impression, rather like the star clouds in our Milky way, of an almost continuous, wall-to-wall distribution of galaxies. It first made me wonder why our night sky is not as bright as our Milky way --- galaxies are after all gravity-bound structures that don't expand along with the universe, so their per unit perceived- area surface brightness, like that of stars,
    should be independent of distance from us. I now realise that my above impression must be an artefact of the way the raw Herschel data was processed, which masks the dimming produced by the galaxy separations created by the universe’s expansion. I suspect that this expansion may be ruled by the balance of energies mandated by the Virial Theorem, which rules many other dynamic activities, such as the height attained by stones we toss upwards, the speeds of orbiting planets in our solar system, and of both stars or galaxies that are gravitationally bound together. This theorem is a true vis viva of systems.
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