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Disk Scale Length Help

  1. Apr 5, 2005 #1
    Just a quickie

    I'm taking a slice of a galaxy picture and plotting luminosity vs kpc. I'm fitting a luminosity profile (de vacouleurs) and keeping disk scale length as a free parameter. However, textual values for this (referred to as h from now on) range between 3-4 kpc for most galaxies. h is defined as the radius when the luminosity decays to 1/e of its original value. My plots for most galaxies end up with h between 7-14 kpc.

    Is there something I'm doing wrong? Am I missing something fundamental? Can I just chalk this down to smeared pictures and dust effects?

    If this doesn't work - I'm gonna photometrically integrate and hope for the best :rolleyes:

    BTW: This is for a science fair (for anyone wondering)
  2. jcsd
  3. Apr 5, 2005 #2


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    By luminosity, I take it you mean "surface brightness". If not, make sure that's what you're measuring. What did you use to take these images? What's the width of your PSF? If it's comparable or greater than the scale length of your galaxy, you'll measure scale lengths that are too large.
  4. Apr 6, 2005 #3
    Yes, I do mean surface brightness (the reason I mentioned luminosity is that my values are in solar luminosity assuming a thin infinite disk). I didn't take the pictures myself, got it from an online database - the name evades me (I'm at school and my home comp. has the link). I scaled the pictures and preserved ratio, so I don't think it's a stretching error. They aren't the best quality in the world, but they're passable. I selected galaxies which were more or less face on - and sort of corrected for angle deviations. I don't however understand what you mean by PSF - care to elucidate?

    Thanks a million.
  5. Apr 6, 2005 #4


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    Not to split hairs or anything, but are you sure you're measuring the surface brightness? There's no need to convert things into physical units (like luminosity) if you're just measuring a surface brightness profile. I would expect your numbers to be in units of either intensity or magnitudes per square arcsecond. If you're measuring light integrated over the disk (as denoted by a luminosity), then you would expect a different shape of profile.

    When you get the info, let me know which database it is. It's not SDSS by any chance, is it?

    The PSF is the point spread function and it sort of describes the "blurring" of the light by the instrument. If there's a source of light that's emanating exactly from a point, then on your image it would be blurred out into something that doesn't look like a point (more like a circular halo), and this is described by the point spread function. A galaxy is not a point source, but its light will still be blurred somewhat by the PSF. The width of the PSF should be given approximately by the angular resolution of the instrument, so if you can find that (along with the distances to the galaxies you're looking at), I should be able to tell you right away whether or not it's a problem.
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