Topic of the Month: August 2014. Galaxy Morphology

  1. EigenState
    In his presentation on Galaxy Classification, Professor William Keel, University of Alabama at Tuscaloosa states:

    "It is important to keep in mind that existing schemes are based on the appearance of a galaxy at optical wavelengths (usually in blue light) ...",

    "The original definition of classification criteria in blue light also has implications for use on galaxies at substantial redshifts, where we typically view in the emitted ultraviolet."

    Please consult the link for further details and illustrative examples.

    Thus, what is the physical significance of morphological classifications based upon observations in visible light?
  2. ngc3314
    Those are fighting words with some people!

    (It appears that I've already had my say in the links, so will wait for anyone else to chime in first.)
  3. EigenState

    Different spectral frequency domains sample different stellar populations. Cosmological redshift alters the stellar population observed within any given spectral frequency domain.

    Conclusion: morphological classifications based only upon observations in visible light are of questionable physical significance because they implicitly assume scrutiny of a restricted subset of stellar populations and a dynamics determined by that restricted subset.

    Best regards,
  4. Jean Tate
    Jean Tate
    In some 'spectral frequency domains' stars are invisible (or almost so); what's detected is dust and the ISM (interstellar medium, or some of its 'phases'), PNe (planetary nebulae), SNR (supernova remnants), the AGN (active galactic nucleus), AGN jets, ...

    Conclusion: in some spectral frequency domains what optical astronomers call 'galaxies' do not exist.
  5. ngc3314
    It's not all that bad. We now study stellar populations in X-rays, since Chandra can count X-ray binaries, and of course spiral structure still shows up in H I at 21 cm. The thing that is sort of special about the optical and near-infrared ranges is that they are the only places that the old stellar populations are easy to see (everything else across the spectrum mostly traces star formation supernova remnants, or phenomena related to the central black holes). Our good fortune with such systems as Hubble's is that it was easy to include both young and old populations (crudely, bulges and spiral arms) with comparable weight. Had we started in the UV, it might have been a revelation eventually that bulges are important as a stellar component, because they almost disappear.

    Of course, when it comes down to it, which spectral view of galaxies is "true"? None of them - and all of them.
  6. ngc3314
    That post taught me that these have a 1000-character limit...
  7. Jean Tate
    Jean Tate
    Define a window as a region of the EM spectrum where the upper bound is SQRT(10) times that of the lower bound, wavelength-wise.

    In how many windows are external galaxies - as ensembles of stars - visible? Assume BOTE and OOM (please check my arithmetic!)

    Take the high-energy limit as 10 TeV; galaxies are invisible in the 20 windows to 0.1 keV. In the next two windows the (extra-galactic) sky is opaque. Being generous, galaxies are visible in the next four windows, roughly from the Lyman limit to the mid-IR. And they're invisible in the next 13+ windows (from here on the Earth, the sky is opaque at ~30m; from space a few more windows would be open).

    Certainly some extraordinarily rare stars are visible, in external galaxies, outside the four central windows, but these are vastly outnumbered at essentially all accessible fluxes (so far), in all windows, by AGNs, rich clusters, etc.
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