Astronomy 201

Assignment #2: Due February 16, 2018

 

Please answer the following from the practice sections of the on-line course notes. Each question is worth 3 marks so write a brief paragraph (but not longer) explaining as concisely and completely as you can. If mathematical calculations are required be sure to show your work and explain what you are doing.

  1. When quasars were first discovered some astronomers suggested that they were small masses ejected from the nuclei of galaxies. What piece of observational evidence argues against this hypothesis? (Winter lecture 14.2)
    There are no blue-shifted quasars! If they were ejected from nearby galaxies some would have to be directed toward us.
  2. If the Hydrogen "H-beta"  line is observed to occur with a wavelength of 600 nm instead of its normal 486.1 nm what redhsift z-value and velocity should you record for this quasar? (Winter lecture 14.2)
    The Z-value is just (600 nm - 486.1 nm)/(486.1 nm) = 0.23. Since this is quite large you need to use the relativistic form

    plugging the numbers you get v/c = 0.20, so v = 60 000 km/s.
  3. Explain why it is possible for a nearby galaxy to have a blue shift while all very distant galaxies show red shifts. (Winter lecture 15.2)
    Nearby the expansion rate of the universe is smaller than the individual velocities of the galaxies - some will be moving toward us and others away. It is only when the expansion velocity becomes bigger than the average motion of galaxies in clusters that we begin to notice the distance-redshift effect.
  4. Explain why elliptical galaxies are so poor in gas and dust. (Winter lecture 16.3)
    We think that ellipticals formed through galactic collisions and mergers. These events would have depleted dust & gas through massive starburst activity a well and the direct dispersal of gas and dust into inter-galactic space.
  5. In the early 1930's Hubble's Constant was closer to 500 km/s/Mpc - why was this "problematic'? (Winter lecture 18.1)
    The age of the universe is inversely related to the Hubble constant so a large value for H leads to a small age of the universe. A value of 500 km/s/Mpc would imply that the universe was less than 2 billion years old - younger than the earth!

Problem (10 marks)

  1. The Seyfert Class galaxy NGC1275 is shown below along with a spectrum of the galaxy taken at The King's University Observatory on October 27, 2017. The spectrum shows strong emission lines for a number of elements and this is plotted for you and tabluated below. Your task is to:
    1. Measure the redshifts (ie - "z-values") for the lines identified in the table and provide an average of these "z" values.
    2. Determine the recessional velocity of NGC 1275 from your z-value.
    3. Use the Hubble expansion law and your average z-value determined velocity to estimate the distance to NGC 1275. Use H = 73 km/s/MPc.

 
NGC 1275 - Hubble Image
Emission spectrum for NGC 1275 - King's University Observatory Oct 27, 2017
Spectral inensity plot for NGC 1275 - King's University Observatory
Element
λrest (A)
λobserved (A)
Z
V
Hβ (Hydrogen)
4861
4945
 0.017 5184
OIII (Oxygen)
5007
5089
6678
6862
 0.016 4913
Hα (Hydrogen)
6563
 0.018 5257
SII (Sulphur)
6757
 0.015 4662
   
average
 0.0165 5004

Use v = Hd and re-write as d = v/H = (5004 km/s)/(73 km/s/Mpc) = 68.5 Mpc or 223 million light years!