Version: short worksheet
Rotation of Spiral Galaxies - Worksheets
Step 1: Measure the Galaxy Rotations
Table 1: Galaxy Rotation Data
Graph 1: The Tully-Fisher Relation
Step 2: Determine the Distance and Measure the Hubble
Data for M31: m = 1.04, vrot = 275 km/s, log(vrot) = 2.44, dM31 = 770
The apparent magnitude, md, if M31 were at a
distance of the 6 galaxies is __________
How many magnitudes dimmer is this than M31's real apparent
This is Δm in
the formula for d given
in the instructions.
The distance is then d = ___________ (See the
instructions for the appropriate formula.)
The average vexp for the 6 galaxies = ___________
H0 = vexp/d = __________ (The
usual units for this are km/s/Mpc, so convert d to Mpc first.)
For many years, a veritable blood feud went on between various astronomers who think that this number is either 50 or 100. More recently there seems to be a consensus developing that it is somewhere close to 75. Where does your value fit in?
- Why do you think we are using spiral galaxies that look
nearly edge-on? What kind of Doppler shift do you think
you would measure if the spiral galaxy were nearly
- It was mentioned that the six galaxies in this lab are
all nearly the same distance from us. Why is this
important? How would your graph have looked if the
galaxies were all at different distances.
- Why does the Tully-Fisher relation make sense? Think
about how the brightness of a galaxy relates to its mass,
and how its mass relates to it rotational velocity.
- All of the magnitudes that we're using in this lab are infrared
magnitudes (measured using infrared light). Why is it
advantageous to use the infrared light when studying
these edge-on spiral galaxies?
- You should have found M/L ratios significantly greater
than 1. Since stars typically have M/L values of around
1, what does this imply about these galaxies. What kind
of objects would increase M/L and why?
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