SH #3: Understanding Galaxies         Group Project

Team D:       Galaxies in the MKW 11 cluster

In this activity, you will examine the population of galaxies found in the vicinity of the nearby "poor" cluster known as MKW 11. Note that Team C will be looking at the structure of the cluster and its relationship to other clusters and superclusters and, especially, how we figure out whether a galaxy is a member of a cluster. Team B will be looking at the Virgo cluster. Your task is to explore the galaxy population near the cluster itself and then to compare the MKW 11 population with a more widely distributed set of galaxies in the Coma-Abell 1367 supercluster.

Your first task will be to construct a color-magnitude for the galaxies in the region of the poor cluster MKW 11 using colors and magnitudes derived from the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7). Luckily for you, we will provide you with the data you need, but before you begin, you should understand a bit about the SDSS photometric dataset.

The astronomical rainbow particularly as viewed by the SDSS.

  •   Remind yourself of the meaning of the terms magnitude, color index, extinction and other relevant quantities.


  •   To the right is the diagram showing the wavelength response of the five SDSS filters. Which one is g? which is i?


  •   In a very famous movie, what would be the (g-i) color index of Mary Kate Danaher's hair?


  •   Explain why higher values of the (g-i) color index indicate redder galaxies.


    		•
    Credit: SDSS

    The cluster MKW 11

    The E-S0 galaxy NGC 5171 lies at the rough center of a bunch of galaxies of similar redshift; take a look at the galaxy via the SDSS Navigator link to it. Use the scale adjustment to examine the surrounding field, containg the structure known as the MKW 11 cluster which is the target of your investigation.

  •   Use the NASA Extragalatic Database (NED) to find out some of the basic facts about MKW 11. Notice that it has numerous other names.

    R.A.(J2000)  
    Dec.(J2000)  
    Recessional velocity in heliocentric rest frame  
    Recessional velocity in CMB rest frame  
    Distance to the cluster (quoted in NED)  
    Galactic latitude  
    Galactic extinction in g-band  
    Galactic extinction in i-band  


    Using the information and links in NED, learn what you can about the cluster. Here are some questions to get you started.

  •   What is a "poor" cluster?


  •   What does "MKW" stand for?


    A color magnitude diagram of SDSS galaxies in the Coma supercluster

    Before you begin examining the galaxies in the cluster MKW 11, we need to take a look at a larger sample of galaxies at roughly the same distance which we can use for comparison.

    A common way to explore the properties of a population of galaxies involves constructing its color-magnitude diagram (CMD). A nice example which we will use for reference is the work of ALFALFA team member Peppo Gavazzi and his co-workers who constructed the CMD of a large sample of galaxies within 420 square degrees of sky covering the Coma supercluster and its member groups and clusters of galaxies as presented in Gavazzi+ (2010), A&A 517, 73 and shown here to the right. Taken from Fig. 3 of that paper, the figure shows the "g-i color versus i-band absolute magnitude relation of all galaxies in the C[oma]S[upercluster] coded according to Hubble type: red = early- type galaxies (dE-E-S0-S0a); blue = disk galaxies (Sbc-Im-BCD); green = bulge galaxies (Sa-Sb)... Contours of equal density are given. The continuum line g-i = -0.0585 *(Mi + 16) + 0.78 represents the empirical separation between the red-sequence and the remaining galaxies. The dashed line illustrates the effect of the limiting magnitude r=17.77 of the spectroscopic SDSS database, combined with the color of the faintest E galaxies g-r ~0.70 mag.."

  •   Be sure you understand this diagram and what it shows.

    		•
    Figure 3 from Gavazzi+ (2010)

    The SDSS data for galaxies in the region around the cluster MKW 11

    We have compiled a file of useful parameters which you can use to construct a similar CMD for galaxies in MKW 11. You can find the file here in CSV format or ASCII text format.

    The file was generated by searching the SDSS DR7 database for all galaxies with Hα redshifts within the volume: 198. <= RA <= 207., 7. <= Decl <= 16. and z <= 0.061. Examine the contents of the file before you start working with it and be sure that you understand what all the columns mean; if you don't understand any of them, look at the SDSS website. Note that (1) there is a lot more here than you actually need because, we've done you a big favor in generating the file for you and (2) we are deliberately not telling you what everything is so you'll have to think about the contents. See if you can figure out what you've got and what you need.


    Preparing the data for use: from observed properties to useful quantities

    Use the data in the file provided and TOPCAT to construct the CMD of all the galaxies in the region of MKW 11. You can use the arithmetic capability of TOPCAT to apply the corrections you need.

    In order to compare what you find with Figure 3 of Gavazzi+ (2010) (shown above) be sure to: (a) calculate distances from CMB velocities and using Ho = 73 km/s/Mpc; (b) correct observed magnitudes for galactic extinction; and (c) generate the plot with the same scaling and orientation they use. Here, you can ignore the correction for internal extinction but make the minor corrections for Galactic extinction ignoring the difference between g and V. For the average corrections, use the answers you obtained before. Assume the redshift is heliocentric.

    For this exercise, we can use average corrections, based on the position of the cluster, RA, Dec = 202.3837, 11.7892. In particular, for all galaxies, we can assume the same CMB correction given by what is given for the cluster. We need to apply minor corrections for extinction. Use TOPCAT itself to make the new columns. Below is what we have done; be sure you understand all the details:
            vhelio = z * 299796.
            vcmb = vhelio + (7138. - 6850.)
            distance=vcmb/73. (to match the CMB in Gavazzi, Fumagalli et al.
            m - M = 5 log D -5+ A, where A is the extinction in mags.
    Extinction at g and i are: 0.113 and 0.058 respectively;
    According to SDSS, use petroMag for luminosity but modelmag for colors (a minor detail)
            corrpetromagI=petroMag_i - 0.058
            corrmodelmagI=modelMag_i - 0.058
            corrmodelmagg=modelMag_g - 0.113
    M = m + 5 - 5 log D *10^6     (remember D in pc here!)
    AbsImag = corrmag + 5 - 30 - 5* log10(distance) so,
            AbsImag = corrpetromagI - 25. - 5*log10(distance)
            gminusi = corrmodelMag_g - corrmodelMag_i

  •   Save a new copy of the file, with the columns you have added.


    The Malmquist bias and the effect of small volume

    We always have to worry about the limitations of our data!

  •   Make a "Spanhauer diagram" of the galaxies in the SDSS sample, that is a plot of the distribution of the absolute magnitude of the galaxies versus their distance from us. Be sure the vertical scale is inverted (-24 at the top, -10 at the bottom); why do we plot it that way?


  •   What does this diagram tell you about the large scale structure in the direction of MKW 11?


  •   What does this diagram tell you about "sample bias"? Keep this bias in mind!


    The color-magnitude diagram of galaxies in the MKW 11 region

    Make the color-magnitude diagram. We suggest that you make a first plot with free scaling but then, to compare with the Gavazzi CMD set the axes to be: x-axis (-16, -23.5), y-axis (0.0, 1.5).

  •   Which galaxies are included in both plots? In only one or the other, but not both? Can you explain the results?



    Gas and stars in the MKW 11 galaxies

    Next, let's take a look at the properties of the stars and gas in galaxies found in the MKW 11 cluster by examining the relationship between the optical luminosity and HI gas mass in the galaxies found both in the α.40 catalog and the SDSS. Note that your previous plots included all the galaxies in a large region around the MKW 11 cluster, most of which are not actually cluster members. (Team C in fact will be examining cluster membership). If we actually want to examine the difference between cluster members and non-cluster members for an individual cluster, we run into problems of small number statistics. This problem is especially important because, in rich clusters, galaxies are known to be HI-deficient so that their HI is not detectable by ALFALFA.

    In the end, we'll need to combine results from many groups and clusters to look for statistically significant trends. (Remember the famous ALFALFA quote: "If it were easy, it would already have been done.").

    Here let us take a quick look at how we might proceed... and also see what the small number statistics issue does for us. We have conveniently constructed several files for you to use. They all contain galaxies which are both in α.40 catalog and the SDSS and include properties derived from both datasets. Whereas before, you calculated the i-band absolute magnitude, here, we also use the associated luminosity (logL) and the "gas fraction" (gas2L = log MH - log L) for each α.40 galaxy which also has SDSS photometry. To calculate the i-band luminosity, we assume that the absolute magnitude of the Sun at i-band is +4.58. Note that the files also contain the V-band luminosity and gas fraction parameter.

    Use the three files (in the order above) to superpose the three sets on two graphs, one of optical luminosity (x axis, in log units) versus the HI mass (y axis, in log units) and, separately, the optical luminosity (x axis, in log units) versus the gas fraction parameter.

  •   Consider the results: what do you notice about the scaling with optical luminosity and how can you explain what you see?



    X-ray emission in MKW 11

    The hot intracluster gas in rich clusters is detected by its X-ray emission. Recent work by Burenin et al 2007 presents a catalog of measurements of the X-ray luminosities of low redshift clusters including MKW 11 made with the Roentgen Satellite (ROSAT).

  •   X-ray astronomers like to quote energy bands instead of wavelengths. What wavelengths correspond to the 0.5-2 keV energy band discussed by Burenin et al 2007?

  •   What is the temperature of the gas that is detected? How did you get your answer?   ( Hint: what simple law can you use to guess-timate the temperature?)

    In fact, in many clusters, there is more baryonic mass contained in the hot intracluster gas than in all the stars and gas in all the galaxies in the cluster!

    Next, let's compare the distribution of galaxies to the distribution of the hot X-ray gas. To the right is an optical image with contours of the ROSAT X-ray emission superposed.

    Use TOPCAT to plot the sky distribution, over the same region, of galaxies in our 2 Mpc sample with heliocentric radial velocities in the range 5850 to 7850 km/s. Plot the galaxies with optical velocities and ones with HI 21 cm velocities with different symbols so that you can see any differences in their distributions.


    Credit: GEMS Team

  •   Revisit the SDSS Navigator link (centered on NGC 5171) to orient yourself on the extent of the X-ray emission compared to the scales we have been examining (6 degrees, 2 Mpc). What are your impressions of the X-ray emission relative to the galaxy distribution?


  •   What is the linear extent (in kpc or Mpc) of the X-ray emission evident in this image?


  •   How does the X-ray luminosity of MKW 11 compare to that of the Coma cluster?


    Other things to consider

  •   Consult with Team B about their investigation of the population of galaxies in the Virgo cluster and with Team C about their investigations of the large scale structure in the MKW 11 region.

  •   Look up some more information about galaxies in the MKW 11 cluster and find some interesting images of its notable members.

  •   What other issues/questions are raised in your mind?


    Last modified: Mon May 15 10:34:33 EDT 2017 by martha