Mapping Nearby Space & The Local Group

The goal of this project is to produce visuzalizations, 3 Dimensional if possible) of the distribution of galaxies in nearby space. There are two parts of this project, the first is the study of just the Local Group of galaxies, and the second is a somewhat larger volume --- still local, however, to enable us to see how the Local Group fits into our view of the very nearby Universe. This volume studied should be a sphere or cube of diameter 400 million light years (about 13 Megaparsecs) which means we need to collect all the galaxies within about 7 Megaparsecs of the Milky Way. Ideally we would code the software/data used for this project so as to be able to extend the plots arbitrarily as additional data sets are created.

Input dataset: lowz.dat a file containing all the low redshift galaxies (and perhaps other things!) in the CfA Redshift catalog.

  • (a) Galaxies within 10 Mpc

    You should:

  • 1. look at all these galaxies --- see utilities directory for how
  • 2. classify them and eliminate non galaxies if any (let me know what you find!)
  • 3. Figure out how to place them in a plot/graphic/display with symbols that show their morphological type and also how luminous they are.
  • 4. remember the difference between apparent magnitude and absolute magnitude = log luminosity!
  • Software for simple mapping and visualization has been produced by several different organizations and companies; one well known commercial variant is Mathematica (Wolfram & Co., there is a student license available through Harvard ITS), and a public software package called ``Partiview'' has been written by Stuart Levy of the National Center for Supercomputing Applications (NCSA) and is distributed by the American Museum of Natural History & the Hayden Planetarium as part of the Digital Universe Project. Partiview and its documentation are available at:

  • Partiview (
  • and is available in versions for Windows, Mac OS-X and Linux.

    Some examples of local maps include:

    A 2-D plot (very crude) looking down on the LG, missing lots!)

  • Cornell Astro plot
  • And quasi 3-D plots of the Local Group can be found at:

  • LG 1
  • LG 2
  • 2-D sky plots of the local supercluster:

    that last was produced by the map program in your directory,

    Many of these are just for the Local Group. There are a couple of books and print atlases, Fairall's "Large Scale Strcture" (perhaps too complicated) and Tully & Fisher's "Nearby Galaxy Atlas" that are worth looking at.

    Larger scale maps (beyond just the local supercluster) can be found at:


    Some of these have plots that might be of interest for figuring out what you might want to produce.

    Here are additional links to data for the Local Group:

    Tables of local group members:

  • (a) Local.Group.dat an early version of M. Irwin's list, or
  • (b) Mateo's LG List
  • (c) Mike Irwin's List
  • (c) LPL List
  • Images of individual galaxies can be found either just by Googleing them and using Google's image search (works pretty well for named galaxies) or by going to the NASA/IPAC extragalactic database:

  • NED
  • By definition, the Local Group is getting larger all the time --- not necessarily in size but definitely in numbers of galaxies. In the last few years, several new members have been discovered and should be added to the plots. If you can find them, we'll add them to the list of members for the course.

    The Mass and Luminosity of the Local Group

    After you have identified all the known members of the Local Group, determine the mass and the integrated luminosity of the group --- and thus its Mass-to-Light ratio. Remember that the Locl Group is actually only those galaxies at the center of the larger, "Nearby Universe" map you have made above. Most LG galaxies are inside a radius of 1 Megaparsec from the Milky Way. See the tables of Local Group members given above. The mass of a group or cluster can be calculated two ways, one by applying the Virial estimator and one by applying the Projected Mass estimator. I've written a more complete description of these two estimators and of the Virial Theorem from which the first is derived which can be viewed in the following power-point file: Mass Estimators. Both give you general formulae to use for either of these calculations, if you can determine the group center, the separations of the galaxies from the center and the velocities of the galaxies with respect to the center.

    The Virial Mass Estimator , after corrections for projection effects both in the projected radius (to go from 2-D to 3-D, and in the radial velocity (to go from 1-D to 3-D), is:

  •               MVT = (3&pi N)/(2G)   (&sum vi2)/ &sum i < j (1/Rij)
  • where Rij is the separation between the ith and jth particle (the limitation i < j is meant to eliminate double counting) and vi is the velocity if the ith galaxy w.r.t. the average cluster velocity, VC. I.e. vi = vk - VC, where vk is the velocity of the kth galaxy in the sample.

    The average cluster velocity VC can be defined in several ways, the most common is to just average all the velocities of known cluster members, the second most common is to weight the velocities in the average by the luminosities of the galaxies. Here N is the number of galaxies in the sample (in the cluster).

  •               VC = (&sum vk) / N
  • and
  •               VC(Luminosity weighted) = (&sum vk Lk) / (&sum Lk)
  • where Lk is the Luminosity of the kth galaxy, and is usually expressed in terms of Solar Luminosities.

    Note that the luminosity of a galaxy (or anything else for that matter) is usually calculated from its absolute magnitude and will depend on the bandpass or color it has been measured in, e.g. blue or visial or whatever. The absolute magnitude, M, is calculated from the apparent magnitude, m, (but for this Local Group Exercise it is already given) and the distance, D, to the object.

  •              M = m - 25 - 5 log(D)
  • For this exercise, again, use only objects in the Local Group table with measured distances.

    Now magnitudes are logarithmic quantities so do not "add" --- luminosities add. Absolute magnitudes can be converted to luminosities in solar units using the known absolute magnitude of the Sun in the appropriate bandpass. For blue, visual or infrared (B, V or K) magnitudes, the absolute magnitude of the Sun is:

                 BSun = +5.48

                 VSun = +4.83

                 KSun = +3.28

    so, for V band (visual) magnitiudes, the Luminosity of any galaxy is given by

  •              LV (Suns) = 10(VSun - MV)/2.5 = 10(4.83 - MV)/2.5
  • As an example, the absolute magnitude of the Milky Way is given in the table as MV = -20.6. Thus its luminosity in Solar units is 1.5 x 1010 LSun.

    If you calculate both the weighted and unweighted mean "velocity" of the Local Group, how different are those numbers?

    The mass you calculate will depend on the units used for the constants and the parameters. You need to be consistent. If G is given in the cgs system, i.e. G = 6.67259x10-8 cm3 gram-1 s-2, then v and R should be expressed in those same units, cm/s and cm, and the output mass will then be in grams. Ditto for the MKS or SI units based on meters/kilograms/seconds. To convert to Solar masses, remember that the mass of the Sun = 1.989x1033 grams = 1.989x1030 kg. It has also been pointed out by a group of students that you can work in "natural" units where G = 4.3x10-9 km2 Mpc MSun-1 s-2 and directly use separations in Megaparsecs and velocities in km/s rather than converting all units to either cgs or mks! Then your estimated mass will be in Solar masses.

    The Projected Mass Estimator of Heisler, Bahcall & Tremaine is:

  •               MPM = fp/GN   (&sum vi2 Ri,c)
  • where Ri,c is the projected separation between the galaxy and the cluster center, and vi is as above. The projection factor, fp, depends on the average orbital eccentricity of the system,

  •             fp = 32/&pi    for primarily radial orbits
  •              = 16/&pi    for isotropic orbits (the usual choice)
  •              =   8/&pi    for primarily circular orbits
  • How different are the PM and Virial estimates of the Local Group's mass?

    Finally, a key number in extragalactic astronomy and cosmology is the Mass-to-Light ratio (M/L) for a given object or system of objects. What, in Solar Units again, is the M/L of the Local Group?

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    Copyright John P. Huchra <> 2009