Distances to Galaxies

Description

In this lab, students use Hubble's Law to calculate the distance to eight different galaxy clusters using data from the Sloan Digital Sky Survey.

Procedure

In this lab you will use Hubble's Law to find the distances to eight different clusters of galaxies. The galaxy clusters you will use are all Abell Clusters, which are (relatively) nearby groups of galaxies named after the astronomer who studied them: George Abell.

As you learned, Hubble's Law relates the recessional velocity (v) in km/s to distance (d) in megaparsecs (Mpc) for galaxies in our universe:

The redshifts of most galaxies in each cluster are easily available from their spectra. The video below shows how to go about measuring these redshifts, which we then average in order to estimate the redshift of the entire cluster. 

You will then convert each cluster's redshift to velocity, and then to distance using Hubble's Law. Finally, you will find the lookback time to the most distant cluster., 

• 1. Navigate to Voyages: Distances to Galaxies.

  2. For each cluster in turn, click its link under "First, Observe and Appreciate" to open it in the SkyServer Navigate tool. Follow the directions under "Record the Redshift to Each Cluster" to get to redshifts of members of the cluster. I suggest that you calculate the redshift to each cluster by averaging five galaxies in each; this is a great time to learn how to use a spreadsheet.

     1. • Note: Do not use the z value listed in the box on the top-right of the Navigate page. This is a magnitude, not a redshift. I know this is confusing, please feel free to send your concerns to the Queen of Astronomy.

        • If one of your galaxies has a very different redshift from the rest, it's likely a foreground or background galaxy. You should discard it and find another member.

        • Some spectra in SDSS have already converted the redshift to velocity--saving you a step! These spectra have "cz=" and units of km/s in the header instead of "z=" and no units.

  3. Calculate the velocity (in km/s) that corresponds to the average redshift you found for each cluster (just like you did in the Redshift lab, with c=3.00*105 km/s).

  4. Find a modern value of the Hubble constant, H0 in (km/s)/Mpc from a source you can cite (remember that you cannot cite a search engine). Make sure it's a modern value; Hubble's original value was very wrong for a couple reasons, but partly because the necessary measurements can really only be taken from space--that's why the Hubble Space Telescope was built.

  5. Divide each cluster's recessional velocity by the Hubble constant to find its distance in megaparsecs (Mpc).

  6. Finally, for most distant cluster only, convert the distance you found to light years (remember that Google is a great tool for unit conversion: 2 Mpc in light years). The distance in light years is also how long it took that cluster's light to reach us--we call that number the cluster's lookback time. It's literally the number of years into the past we're looking when we study that cluster. (see the beginning of Section 16.3 of your text if that doesn't make sense.)

  7. Create a page showing data for each Abell cluster, presented so it's easy to see each cluster's average redshift and distance (e.g. Abel 1064: z=0.034, d=250 Mpc) and a short paragraph stating:

     1. • The Hubble constant you found, H0, with (a real) citation.

        • The distance to the furthest cluster you measured, in megaparsecs.

        • The lookback time to that cluster in years (see step 6). To put that number in context, use Smithsonian.com's Interactive Timeline of Earth to answer the question, "What was Earth like when the light from these galaxies started its journey?"

You're welcome to use any tool that can arrange text on a page, but please submit something Canvas can open (Microsoft Word, Powerpoint, or a pdf).

Suggested Rubric