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

Tan and blue galaxies

  1. May 16, 2007 #1
    Today's Astronomy picture of the day is visually appealing as well as scientifically revealing.


    The text points out that the foreground galaxies are tan in color while the background distorted ones are blue. Does anyone know why this should be so? Is it because the background galaxies, being further away are being viewed at a time when they were younger and in an earlier stage of development?
  2. jcsd
  3. May 16, 2007 #2
    Correct. As we look to higher redshifts, we see the cosmic star formation rate increases, and a high portion of galaxies are forming stars at a rate many times that of our own milky way. The light emitted from these "starbursting" galaxies is dominated by young, hot O-type stars which are very blue.

    Within clusters, the cores are known to be dominated by elliptical type galaxies which in general have very small amounts of star formation and are dominated by old, red stars. There is a well known result that star formation is quenched in dense regions and determining which physical mechanism(s) cause this is an active area of research (ie. whether there is some pre-processing before the infall into the cluster, or whether some cluster specific mechanism is responsible for this transform).
  4. May 16, 2007 #3


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    that's fascinating. thanks to you both for bringing it up and
    the clear explanation
  5. May 16, 2007 #4
    Thanks matt.o. So in the apod, we are looking at the dense center of the foreground cluster and that's why there is so little blue star formation? Or is it just that the cluster has outgrown the prolific star formation era?
  6. May 16, 2007 #5
    Yes, we are looking at the cluster centre where the majority of galaxies are not forming stars. However, the issue is a little more complicated in that this cluster is in the process of merging with another cluster. In fact, this cluster is one of the original Butcher-Oemler clusters, which have a much higher fraction of blue galaxies compared to clusters in the local universe. This higher fraction may be caused by the cluster mergers impact on the galaxies in this environment, however again, this is an ongoing area of research.

    Generally, galaxies in clusters are very deficient in gas, hence star formation doesn't happen (no fuel). The fact that this is a merging cluster confuses the issue a little bit, since the impact on star formation due to the merger is not well understood and depends on a few things.

    As for the prolific star formation era - this occured at much earlier timaes than the epoch at which this cluster is observed.
  7. May 16, 2007 #6
    Thanks again matt.o
  8. May 16, 2007 #7
    I should add, it is important to remember that not all lensed objects will be blue. So if we look at another lensing cluster, the lensed galaxy may indeed be redder than the cluster galaxies.
  9. May 17, 2007 #8
    I should point out that most of what I know about lensing, I learned from apod (and now from you). Until now, it seems that the only distorted galaxies I have seen have been blue. That's what prompted me to ask about yesterday's image. I will ask professor Nemiroff to look in his files for an image of a non-blue distorted galaxy. Actually, the words of yesterday's caption could be interpretted to mean that there were some in that image, but I couldn't find any. Perhaps the one at 1 o'clock could be an example, but I can't be sure because it may not be distored, just edge on.

    You mentioned that galaxies in clusters are deficient in gas, and yet I had thought that almost all galaxies were found within clusters. What percentage of galaxies are not in clusters?
  10. May 17, 2007 #9
    In fact, most galaxies are found in the field or in groups of galaxies. Clusters are rare (in fact only about 1 rich cluster is found per 100000Mpc^3). I think it is something like 1 in 10 galaxies are found in clusters, but I would have to look that up.
  11. May 17, 2007 #10
    No need to look it up unless you want to do it, I'll take 10% as a rough figure. I had no idea it was like that. How about me. Am I in a cluster?
  12. May 17, 2007 #11


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    It depends, at least somewhat, on the definition of 'cluster'.

    I think the 10% figure refers to 'rich clusters'.

    There are also galaxy groups (such as the one we're in - the Local Group), and clusters which are not so rich. As usual, the definitions are, to some extent, arbitrary ... but there is quite some effort, at least by some astronomers, to make the classifications meaningful, in terms of the underlying physics as well as the observational precision.

    The nearest cluster is the Virgo cluster, centred on M87; what is the relationship between the Local Group and the Virgo cluster? Is the Virgo cluster relaxed? is it virialised? and so on ...
  13. May 17, 2007 #12
    In general, groups are loosely defined as 2-50 bound members, whilst clusters have 50 - 1000s of members. Although, this is a loose definition!

    Yes, that's right. Although, it is important to emphsise that most galaxies in the universe do not live in clusters (rich or otherwise).
    Last edited: May 17, 2007
  14. May 17, 2007 #13
    I see. When matt.o spoke of galaxies outside of clusters, I thought he meant singletons. That's why I was surprised at the 10% figure. Any idea on the percentage of galaxies that are not bound to any other galaxy?
  15. May 17, 2007 #14
    Sorry for the confusion.

    Well, this is a tough question, since most massive galaxies like our own Milky Way will have some small satellites (like the LMC and SMC) and we have found ~30 around the milky way. In saying this, using a group finding algorithm in the 2dFGRS (two-degree field glaxy redshift survey) Eke et al. found around 50% of galaxies were bound to groups containing 2 or more members. Remember though that the 2dFGRS is a flux limited catalogue, so some galaxies could be in groups but the other members are too faint to detect in the 2dFGRS. This 50% number would be a lower limit, depending on how you want to define a group!!

    So it is difficult to define a truly isolated sample of galaxies, although work has been done to try to do this.
  16. May 17, 2007 #15
    No sweat, you dispell much more than you create.
    So the other 50% would be singletons. I had no idea. Thanks so much for this discussion.
  17. May 17, 2007 #16
    Well, I am not so sure about that. I am not sure that study included clusters as groups or not. You also have to remember the caveat noted above about magnitude limits (and a few others not mentioned). For instance, if there was a galaxy of magnitude 19.5 (the limit of the 2dFGRS) which lies in a group of ten galaxies of which all have magnitudes fainter than 19.5 then the bright galaxy would be detected in the survey, however the fainter ones would not. That would mean this group would not be detected in the Eke et al. study.

    I think the definition of an isolated galaxy is one which has no nearby luminous galaxies, but may well have faint satellites. My guestimate would be that this would occur 10 per cent of the time, bu not as high as 50 per cent.
    Last edited: May 17, 2007
  18. May 31, 2007 #17
    matt.o, you mentioned that among several caveats, there is the problem of magnitude limits. It looks like we are getting better at finding the faint ones.
    I wonder what percentage of galaxies will prove to be singletons under the following floating definition of singleton:

    Any galaxy that is detectable on earth using the best equipment available at the moment, but has no detectable bound partners.

    I expect that with this definition, the percentage would change from time to time as the equipment and usage techniques change. The answer I would expect to get is something like: The percentage is x%, er, no wait a minute.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook