Freshman Seminar Project

The primary goal of this project is to examine the properties of galaxies from the database of a large, homogeneous galaxy catalog. We will use J. Huchra's redshift catalog galaxies selected from the 2MASS (2 Micron All-Sky Survey) catalog. The catalog here is a flux (magnitude) limited survey of galaxies detected in the 2MASS survey downto 11.25 magnitude. For now, this catalog is only available to his seminar and should not be disseminated further without the express permission of the instructor. (Copyright JPH 2009.)

The input catalog above contains the coordinates, Right Ascension (RA) and Declination (Dec) of the galaxies, their apparent magnitude in the near infrared K band (Mag) , their Heliocentric radial veloxity, V_h, in km/s, the error in that number, the source of the velocity, the morphological type of the galaxy numerically coded as in Gerard deVaucouleurs Reference Catalog of Bright galaxies, the major and minor axis apparent diameters in arc minutes (D1 & D2), the infrared colors, J-K and H-K, the galaxy's axial ratio, b/a = D2/D1 a measure of its inclination on the sky, and a measure of the density of its surroundings relative to the mean density of galaxies in the catalog.

You should examine some of the properties of galaxies by plotting various interesting properties of galaxies against others. Plots should be made for:

  • 1. Apparent magnitude versus redshift
  • 2. Apparent diameter versus redshift
  • 3. Absolute size versus absolute luminosity or magnitude
  • 4. Surface brightness versus absolute magnitude
  • 5. Average color ((J-K), (H-K)) versus type
  • 6. Color versus inclination (versus type as well)
  • 7. Color vs type versus density
  • 8. Color versus color (J-H) versus (H-K)
  • 9. Average color ((J-K), (H-K)) versus redshift
  • 10. Average color versus morphological type.
  • Plus any other combinations you might think are useful. Note a galaxy's inclination is usially given (estimated) by the axial ratio, b/a, or D2/D1 if the minor and major axis diameters have been measured. What do you think the colors of galaxies might mean? Young star formation makes a galaxy blue, dust extinction makes a galaxy red. The colors above are infrared colors so will be less influenced by both dust and star formation, but if we had optical/infrared colors we would be likely to see the full glory of both dust and star formation in the properties above.

    Input datasets:

  • 2mass.11.25.dat
  • You should:

  • 1. look at any outlying galaxies on the digitized Palomar sky survey --- see utilities directory for how to extract images using 'getimage' or go to NED, the NASA/IPAC Extragalactic Database.
  • 2. Fit relations to the properties using whatever you are confortable with (IDL, Excel, Mathematica, etc ...)

    And see if you can interpret the results. For example: What might drive the color versus redshift relation? Why is there a relation betwen absolute size and absolute luminosity?

    For these "absolute" quantities, remember you can use the redshift to estimate the distance --- R(Mpc) ~ (v_heliocentric)/(Hubble Constant), and the absolute magnitude, M, and absolute diameter, D, are

  •              M = m - 25 - 5 log(R)
  • and

  •              D = R tan(theta)
  • where theta is the angular size of the object in radians. The data table gives each galaxy's major (D1) and minor (D2) axis diameters in arc minutes, so theta in radians = theta in arc minutes / (60'/degree) / (57.296 degree/radian).

    Two hints: Beware of outliers on the plots -- they could be due to errors in the catalog or programming proble,s. Also, check your units. Absolute sizes should end up being fractions of Megaparsecs, and probably should be converted to kiloparsecs. If you want to convert Absolute magnitudes to Luminosities, you can use the following conversion:

    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 of Local Group ggalaxies (see the LG Project) as MV = -20.6. Thus its luminosity in Solar units is 1.5 x 1010 LSun.


    Summarize what you have learned by looking at these properties of galaxies. What is correlated, what is not, and why? Can you compare the properties of different galaxy types?

    Back to the Future

    FAS Seminar Link

    Faculty Seminar Link

    FAS 21x. Link

    Copyright John P. Huchra <> 2009