Union UAT Workshop June 2015         Group Project

The populations of galaxies detected by ALFALFA and by SDSS

In this exercise, you will explore the differences in the populations of galaxies detected by the two surveys: ALFALFA and the SDSS.

We will provide you with all the data you need. We hope that you have already had a chance to play with TOPCAT. If you have not used TOPCAT before, check out the links on the UAT Groups Google Site and/or find someone who has used it before.


1. Datasets

1.1 The ALFALFA α.40 dataset:

The ALFALFA catalog data are available either as part of the publication in the Astronomical Journal or from our own data products page. 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.

  •   Download the file in CSV format or ASCII text format and then use TOPCAT to generate the plots.


    Distance bias:

    First, let's examine how the HI masses of galaxies detected by ALFALFA differ as we move from the very nearby galaxies to more distant ones. Use TOPCAT to make the plot of Distance (horizontal axis) versus log HI mass. This type of plot is called a Spanhauer diagram.

  •   What are your impressions of this plot?

  •   There are two "gaps" (fewer than expected) at distances of about 85 Mpc and 230 Mpc. For the HI 21 cm line, what frequencies do these gaps correspond to? Any ideas on what causes the gaps?

  •   There appears to be a vertical line-up of (more than expected) points at a distance of 17 Mpc. What might cause this?

  •   Why does the distribution of points appear as it does? Why is it important to keep this diagram in mind as we explore galaxies detected by ALFALFA?


    1.2 SDSS data for the α.40 galaxies
    This CSV with SDSS data file contains useful information extracted from the population of galaxies included in both the 2011 public ALFALFA survey catalog (α.40 Haynes et al (2011)) and the SDSS DR7 database.

    Examine the contents of the file before and see if you can figure out what the table contains. Parameters include distance, apparent magnitudes, absolute magnitude, luminosity, and color. (We will not use all of these parameters, but we want you to try to understand more than the minimum). A longer version of this exercise, which includes calculation of some of the magnitudes you will use, is provided here; we encourage you to run through this on your own.

  • How does the sky distribution of galaxies from the α.40 (HIpos) compare to that of the SDSS data (OCpos)?


    1.3 Combining ALFALFA and SDSS Data


    Using TOPCAT to match two catalogs

    Next we will use TOPCAT to match entries in the two catalogs. Crossmatch the two catalogs by AGC number in TOPCAT. To do this, use the "Joins" function with the "Pair Match" option (alternatively hit the double-match icon in the main window). Use the 'Exact Value' Algorithm and pick the AGC number for each table in TOPCAT.

  • How many matches do you get? How does this compare to the numbers in each catalog?

    Going back to the original catalogs, make a crossmatch using the "Sky" algorithm. Use the HI positions for the α.40 catalog.

  • Now how many matches do you get? Why don't the positions match exactly? How large must "Max Error" be to get on the order of the same number of matches that you found when you cross matched by AGC number? Explain your answer.

  • Try matching again, this time using the optical positions from the HI catalog. How many matches do you get?

    2.0 ALFALFA-SDSS Scaling Relations

    2.1 Gas and stars in the α.40 galaxies

    Now let's look at how the gas and stars are connected in the α.40 galaxy population. Note that the SDSS file contains parameters for both V-band and i-band, but let's stick to the i-band for this exercise. Plot the i-band optical luminosity, log_LI, on the x axis versus the HI mass, logM_HI, on the y axis. We suggest you scale the axes as log_LI=[6,11.5] and logM_HI = [6,11.5].

    Make a second plot using the optical luminosity and the gas fraction parameter. You will need to calculate the "gas fraction" (gas2L = logM_HI - log_LI) for each α.40 galaxy which also has SDSS photometry. Use TOPCAT to do this, by adding a column and typing in the formula. Then plot the i-band optical luminosity, log_LI, on the x axis versus the gas fraction, logM_HI, on the y axis.

    Consider the results:
    2.2 Scaling Relations

    The plots you made above are called "scaling relations" because they illustrate how one property of a galaxy scales (is related to) another. The optical luminosity that you plotted may also be converted to an estimate of the stellar mass in the galaxy (i.e., how much mass produces the light) using stellar evolution and population models. Plots illustrating the scaling relations between gas, stars, and star formation can be found in Figure 2 of Huang et al. 2012, for example.

    If you are finished, you may check out the GalaxyZoo and Color Magnitude Activity

    Last modified: Mon Jun 22 07:29:06 EDT 2015 by becky