fossil groups

As I have discussed earlier, there is a hypothesis out there that groups with a large magnitude gap between brightest and second-brightest galaxies are considered likely candidates to be fossil groups, in which multiple group members have (in the past) merged into a large, remnant galaxy. I say considered because this is far from demonstrated. In fact, the distribution of magnitude gaps is very close to what one would expect from a Poisson sampling of an envionment-dependent luminosity function. But we find that there are groups with anomalously large magnitude gaps. The question is, in the face of small redshift incompleteness (and usually spectrograph constraints cause the incompleteness to be higher in compact groups), what is the most conservative estimate of a group's magnitude gap? I think I figured that out today. I will implement it next week.


  1. hi David,

    this is yeong, erica's post-doc and michael's student.
    sorry for being a lurker!
    i'm so glad that at least some people is interested in this "gap" business! after my thesis posted, not even Brinkman want to be a co-author!

    did you read our paper on lrg gap paper?
    we find bigger gap in groups compare to clusters? as you and michael had quickly figured out, statistically, from a purely Poisson sampling, group will have a larger gap (as was discussed in our paper, also predicted by works by Scott, Peebles, Geller, Tremaine -- in the early days)
    Our results appeared to be in contradiction to Marc Postman's Thesis (with Geller) where they find smaller "gap" for groups.
    Our feeling was that you will find larger group gaps if you only look for gaps in the "red sequence" as we have done with the LRG groups, different from the CfA groups.

    So I'm looking forward to your results.

    Also, I had such a interesting time reading Berlind et al 2005 (groups) as well as Berlind et al 2005(color,luminosity, environment). The later figure 2 (top left) best illustrate the problem of galaxy formation today -- of all plots i've seen for the last few year!

    while the current hot idea is radio-galaxy-agn, i think it is the understanding of the "gap" that is going to give us the solution.

  2. Hi David!

    this issue of "fossil groups" is really an interesting stuff.

    The magnitude gap is not the only characteristic of this kind of structure, the large X-ray halo (larger than expected) is another indicator.

    Some observational results can be seen here:


    There are also recent simulations showing that dynamical evolution leads to this kind of structure naturally:


    And some Ponman and colaborators works.

    Tell me one thing. Where can I get information about this additive contamination to the images?? Thanks for the suggestion, btw!

  3. Yeong and Cris, Thanks for great comments!

    Cris, I answered your question here.