Galaxy Zoo Talk

No Galaxy?

  • endless18mass by endless18mass

    Is there a galaxy here?

    Posted

  • ElisabethB by ElisabethB moderator

    Hi endless18mass and welcome to the Zoo

    No, there is no galaxy here. The programme messed up ! 😄 Just classify as star/artifact and move on to the next one!

    Happy hunting ! 😄

    Posted

  • JeanTate by JeanTate

    Myself, I'm not so sure this is a glitch.

    In current consensus cosmological models, there should be at least some 'dark halos', local concentrations of dark matter with little baryonic matter (i.e. gas, dust, stars) in them. There are, IIRC, one or two objects that might be such dark halos (our very own stellar190 wrote about one candidate, in this GZ forum OOTD: December the 11th 2010 - A Dark Secret in Virgo).

    As Illustris faithfully follows the prescription for the makeup of the universe (per the best cosmological models), it may well be that there are dark halos in Illustris. And as the images that we get to classify are created - blindly - to be centered on mass concentrations in Illustris, this may be one such dark halo! 😄

    I hope a Science Team member will drop by sometime to comment on this ...

    Posted

  • KWillett by KWillett scientist, admin, translator

    Both of those are definitely plausible explanations. You could try looking at the expected properties for this halo in both the Illustris Explorer and in viewing data on the subhalo (you can get there by clicking through the links in GZ Examine to the left). What is the listed dark matter mass compared to the baryon mass for this particular source? Is it actively forming stars? And can we see the same source from different viewpoints?

    Posted

  • JeanTate by JeanTate in response to KWillett's comment.

    Thanks Kyle.

    For this particular field, "glitch" seems more appropriate. For example:

    • Absolute is -19.88 (I assume this is, say, rest-frame r-band magnitude), which would make it very visible at z=0.05 (I think)
    • stellar mass is 10.691 (log sols), which is a lowish mass galaxy, but still should be visible at 0.05
    • SFR is 0.028 (sols/yr), which means it should be pretty 'dead and red'

    The "View data on Illustris Project" gives a lot of interesting data (see below); however, the various "mass" parameters do not seem to have easily-guessable units 😦 I guess they're somewhere in the project website.

    Interesting data:

    • "mass_log_msun": 10.688807736699475" (not the same as 10.691)
    • "mass": 3.43859", "mass_gas": 0.12821, "mass_dm": 2.33896, "mass_stars": 0.971427, giving a dm to baryonic mass ratio of only 2.12 (very low, right?)

    Time to check all the other "blank Illustris" fields in my Collection! 😃

    Posted

  • KWillett by KWillett scientist, admin, translator

    The mass_log_msun is the total mass of the system, including stars, gas, and dark matter altogether.

    The units of "mass", "mass_gas", "mass_dm", and "mass_stars" are all in units of 10^10 M_solar / h. We keep it in units like these so that different cosmological parameters (such as the Hubble parameter h = H0 / 100 km/s/Mpc) can be applied after the fact. So the stellar mass of this system, for example, should be:

    M = 10^10 * (0.971427) / 0.704

    assuming a Hubble constant of H0 = 70.4 km/s/Mpc.

    Posted