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Observing galaxies: area of sky would I need to survey

  1. Mar 15, 2019 at 10:51 PM #1
    1. The problem statement, all variables and given/known data
    Given that there are 10-2 Ellipticals per Mpc3 and my garden telescope can reach to 14 mag. How large an area of sky would I need to survey to find 100 Elliptical galaxies ? (assume the typical absolute magnitude for an Elliptical galaxy is -21 mag).


    2. Relevant equations
    d=100.2(m-M-25)
    n=N/V

    3. The attempt at a solution
    by substituting m=14 and M=-21 into the first equation I found d=100Mpc,

    which indicates the volume of sky I could possibly observed is 4/3⋅π⋅d3 = 4188790 Mpc3,

    using the second equation n=N/V , where n=number density, I have 10-2 = 100/V,

    thus V=10000Mpc3, which is the volume I need to observed to find 100 galaxies.

    with these values, how can I find how large the area of sky I need to survey?
     
  2. jcsd
  3. Mar 16, 2019 at 9:47 PM #2
    You have almost finished.... What units are you going to use to report your answer (solid angle, fraction of the hemisphere sphere......)?
     
  4. Mar 17, 2019 at 12:29 AM #3
    How can I find solid angle relative to the 10000MPc3?
     
  5. Mar 17, 2019 at 12:38 AM #4
    The solid angle for a whole sphere is 4pi....for half sphere 2pi....how much of the sphere do you need to view?
     
  6. Mar 17, 2019 at 12:46 AM #5
    Thank you for the quick reply,

    the total volume of the sphere is 4188790 Mpc3
    ("which indicates the volume of sky I could possibly observed is 4/3⋅π⋅d3 = 4188790 Mpc3")

    and the volume I need to view is 10000Mpc3
    ("thus V=10000Mpc3, which is the volume I need to observed to find 100 galaxies")

    with these values how can I find solid angle?
     
  7. Mar 17, 2019 at 6:44 AM #6
    What fraction of the total sphere volume do you need to observe?
     
  8. Mar 17, 2019 at 7:06 AM #7
    I think that would be (volume I need to observe) / (total sphere volume) = (10000) / (4188790) = 2.39⋅10-3
     
  9. Mar 17, 2019 at 7:18 AM #8
  10. Mar 17, 2019 at 7:42 AM #9
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