Try to finish this hunt by 7pm on Wednesday evening

UAT13.01 Scavenger Hunt #3:
Introduction to how we can use ALFALFA in combination with other datasets to explore the local galaxy population

This scavenger hunt will provide an introduction to the ALFALFA and SDSS catalogs, deriving galaxy properties from observational data, what the ALFALFA galaxy population is like compared to an optically selected one and investigating the large scale structure. As an example of the kind of science the UAT groups project is doing, we will take a quick look at the galaxy population in ALFALFA, in the SDSS database and in the vicinity of the nearby "poor" cluster known as MKW 11.

In a departure from previous Scavenger Hunts, each team will be assigned a separate project dealing with the use of ALFALFA data to learn about galaxies in the local universe. We will then reassemble Wednesday evening when team will have 10 minutes to present their results.

Team D:       Galaxies in the MKW 11 cluster

The Undergraduate ALFALFA Team groups project is a joint effort to explore how local galaxy environment impact the evolution of galaxies. In this activity, you will examine galaxies contained in the vicinity of the nearby "poor" cluster known as MKW 11. One of the other teams 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. Your task will assume that we have already done that, and we will provide you with the data you need to explore the galaxy population near the cluster itself.

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.."

Your task is to construct a CMD 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.

Click for a larger view.
Figure 3 from Gavazzi+ (2010)

3.D.0     The astronomical rainbow
Remind yourself of the meaning of the terms magnitude, color index, extinction and other relevant quantities.

3.D.1     Intro to colors and magnitudes from the SDSS
First, you need to investigate what magnitudes are given in the SDSS. In fact, determining a magnitude for a galaxy is not trivial. See, for example, the SDSS photometry page.

3.D.2     The cluster MKW 11

Use this link to the NASA Extragalatic Database (NED) to find out some of the basic information about MKW 11 that you will need. Notice that it has numerous other names include the "NGC 5171 group". Another team will tell us more about the cluster itself; you need only to record some basic information about the cluster for your team's further use:

Recessional velocity in heliocentric rest frame  
Recessional velocity in CMB rest frame  
Distance to the cluster (quoted in NED)  
Galactic latitude  
Galactic extinction in V-band  
Galactic extinction in g-band  
Galactic extinction in i-band  

3.D.3     SDSS data for galaxies in the region around the cluster MKW 11

Find here a file containing the data you need. It was generated by searching the SDSS 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, ask. 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 ask. See if you can figure out what you've got and what you need before you ask us for help.

3.D.4     The CMD of the galaxies in the region of cluster MKW 11

Using the data in the file provided and TOPCAT, 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) 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 in 3.D.2. Assume the redshift is heliocentric.

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). How many points (a few? a lot? any particular ones?) get lost when you restrict the axis scales?

Compare the results from obtained for the galaxies in the region of MKW 11 with the general ones found by Gavazzi+ (2010). Which galaxies are included in both plots? In only one or the other, but not both? Can you explain the results?

3.D.5     Gas and stars in the MKW 11 galaxies

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?

3.D.6     Make an image of the core of the MKW 11 cluster using Montage

Note: this may take a little while to run, because system use at IPAC is unpredictable. We suggest you not wait until the last minute to start it.

Note also: Someone in your group will have already be registered to use this facility, or else one of you will have to setup up an account; find someone in your group to be the registered user and then prNote: during UAT13, we are having some trouble access the Montage Web service. If that is the case, you can download the resulting FITS file here, keeping in mind that it is a fairly large file (155 MB).


Montage is a Virtual Observatory tool that allows you to make images up to 1 degree in size from selected public databases via a web-based interface. Read about the full capabilities of the service here.

Use the Montage web service to create a 0.5 degree image of the SDSS g-band data for MKW 11. How many original SDSS images go into making up this mosaic? How long does it take for the job to run (seconds, minutes, hours, days)? What kinds of output are produced? Figure out a way to show us the "result page".

Note: during UAT13, we are having some trouble access the Montage Web service. If that is the case, you can download the resulting FITS file here, keeping in mind that it is a fairly large file (155 MB).

Team C is going to be looking at the i-band image of the 1 deg region of the cluster; it will be interesting to compare results with them.

If someone in your group has access to ds9, you can examine the FITS image.

Assignments given to the other teams:

This page created by and for the members of the ALFALFA Survey Undergraduate team

Last modified: Wed Jan 9 13:09:18 EST 2013 by Martha