SH #3: Understanding Galaxies         Group Project

Team C:       The structure of the Virgo Cluster and its galaxy population

As we have mentioned befpre, the Virgo Cluster is the nearest cluster of galaxies, at a distance of about 16.7 Mpc. A large portion of the cluster is included in the region covered by the completed ALFALFA survey α.40 and it is wholly included in the SDSS (and will be in the next ALFALFA data release). Your task here is to consider some of the important issues related to observing Virgo. We will provide you with all the data you need, but you'll have to try to understand some of what we have done to get the data ready to use. Then, you can use TOPCAT to explore the galaxy distribution and what it reveals about the cluster. Keep in mind that the ultimate goal is for the team to present your results to the other participants tomorrow evening. You will need to explain everything that you have done, all the assumptions you have made and what uncertainties remain.

Note: Team B is also looking into some aspects of the Virgo cluster, notably how we can measure the "peculiar velocities" of galaxies to detect the perturbations on the Hubble expansion caused by galaxies being locally attracted to the cluster overdensity.

Facts about the Virgo cluster

Use the NASA Extragalactic Database (NED) to find out some of the basic information about the Virgo that you will need. Record here the parameters given in NED.

R.A.(J2000)  
Dec.(J2000)  
Heliocentric radial velocity  
Recessional velocity in CMB rest frame  
Distance to the cluster (quoted in NED)  
Angular size corresponding to 2 Mpc  
Galactic latitude  
Galactic extinction in V-band  
Galactic extinction in I-band  


  •   Why do you think the distance given in NED is different from 16.7 Mpc, the commonly accepted value?


  •   What does the value of the Galactic extinction suggest about the location of Virgo relative to the plane of the Milky Way?


    Getting ready to look at Virgo

    Because the Virgo cluster is so close to us, it extends over a very large region of the sky and its galaxies can be large in angular extent. To review the structure of the cluster, we can use the "Arecibo General Catalog", the private database of galaxies maintained by the ExtraGalactic Group (EGG) at Cornell which is made available to members of the ALFALFA team with the usual caveat that "you get what you pay for".

    It is now standard practice in astronomy to provide data for use by others in standard formats such as "CSV", so we have done that. We have used the AGC to create two CSV files for use in this activity which you can use if you want to explore on your own; otherwise, try to understand what the files contain. The files contain the galaxy AGC number, RA, Dec, Vhelio and the projected separation of the galaxy from the Virgo Cluster center in arcminutes. You can use these files to explore the structure of the cluster as seen be the two surveys.


  •   Why did we restrict the heliocentric velocity to be less than 3000 km/s?

  •   What is the recessional velocity corresponding to a distance of Virgo?

  •   What angular scale will you adopt for the radius of the Virgo cluster?


    The structure of the Virgo cluster on the sky

    An important compilation of galaxies in the Virgo cluster was presented by Binggeli, Sandage and Tammann 1985, Astron. J. 90, 1681. Their Virgo Cluster Catalog (VCC) consists of 2096 galaxies which they believed, based on examination of high quality photographic plates, are likely cluster members. Note that their investigation of the cluster was based on many fewer redshifts than are available today. To the right is the sky distribution of possible cluster members as appeared in their paper.

  •   Using the Virgo AGC/α.40 data files (linked above), plot (on the same graph) the sky distribution of galaxies in the (a) optical sample and (b) the α.40 galaxies. Assign red symbols to the optical sample and blue ones to the α.40 galaxies. Keep in mind that the α.40 sample is limited to the Dec = +15 deg strip of grids so it stops at +16 degrees.


    • From Figure 2 of Binggeli, Sandage & Tammann 1985
  •   Be sure to plot the distribution so that east is to the left and west is the right; why do we do that?

  •   Consider the result: what do you notice and how can you explain what you see? How does our diagram compare to that seen in Fig 2. of Binggeli, Sandage & Tammann (1985)?

    The distribution of heliocentric velocity with projected separation

    Using the Virgo AGC/α.40 data files (linked above), plot (on the same graph) the variation in heliocentric velocities (y-axis) with the projected separation from the cluster center (x-axis) for the (a) optical sample and (b) the α.40 galaxies. Again, plot the optical sample with red symbols and the α.40 galaxies as blue ones.

  •   Consider the result: what do you notice and how can you explain what you see?

  •   Why can't we use Hubble's Law to derive distances in the Virgo region?

  •   Why do some galaxies have negative heliocentric velocities?

  •   What is a "peculiar velocity"?


    Rather than simply using Hubble's law to derive distances, we use a model of the local velocity field to estimate distances to galaxies, and the α.40 catalog includes the distance assumed in the derivation of HI masses. This model was derived by my former PhD student Karen Masters, now at the Institute for Gravitation and Cosmology at the University of Portsmouth (UK). Team B is going to report more about peculiar velocities and the flow in the Local Supercluster as part of their project.

  •   Suppose a galaxy lies directly in front of the Virgo cluster (i.e., between the Local Group and the Virgo cluster) at a distance (from the Milky Way) of 12 Mpc. What recessional velocity would it have if Virgo were not there? Would you expect its observed recessional velocity to be higher or lower than that? Why?


    The velocity distribution of galaxies in the Virgo cluster

    Using the Virgo AGC/α.40 data files (linked above), plot (on the same graph) the distribution of heliocentric velocities (using the histogram option) for the (a) optical sample and (b) the α.40 galaxies. Again, using different color codings, red and blue.

  •   Consider the result: what do you notice and how can you explain what you see?


  •   Take a careful look at the histogram for the optical sample. Why do there seem to be more galaxies at higher velocities than at lower ones, i.e. the histogram is skewed and not a perfectly smooth and symmetric (Gaussian) function?


    The properties of local α.40 galaxies in Virgo and outside Virgo

    Let's take a look at the HI properties of galaxies detected by ALFALFA in the Virgo cluster in comparison with ones at similar distances but outside Virgo. Here are two useful files:


    In both files, we have calculated the optical luminosity for each galaxy (given in logarithmic units as logL) and the "gas fraction" (gas2L = log MH - log L). Note that we do this for both the V-band and the i-band. What are they? In what ways might we expect them to give different results? Why?

    Using these files, plot (on the same graph) the optical luminosity (x axis, in log units) versus the HI mass (y axis, in log units) for (a) the Virgo α.40 galaxies and (a) the non-Virgo ones. Assign red symbols to the Virgo α.40 galaxies and blue ones to the non-cluster galaxies. Keep in mind though, that galaxies in both of these samples have been detected by ALFALFA and therefore contain some interstellar hydrogen. Additionally, examine the histograms of the gas fraction parameter.

    You could do this for either filter band in the files, but first use the V-band plot.


  •   What do you notice and how can you explain what you see?


    Make a similar plot, investigating how the gas fraction (gas2L_V) varies with luminosity.


  •   What do you notice and how can you explain what you see?

    Feel free to make any other plots that might be interesting/useful.


    Other things to consider

  •   Using the information you have and making reasonable assumptions, how could you estimate the mass of the Virgo cluster?

  •   Look up some more information about galaxies in the Virgo cluster and find some interesting images of its notable members.



    Last modified: Sat Jun 11 10:33:28 EDT 2016 by martha