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 A:       The population of galaxies detected by ALFALFA


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 α.40 population 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.A.0     The astronomical rainbow
Remind yourself of the meaning of the terms magnitude, color index, extinction and other relevant quantities.


3.A.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.A.2     SDSS data for the α.40 galaxies
Find here the CSV file containing useful information extracted from the population of galaxies included in both the 2011 public ALFALFA survey catalog (α.40 Haynes+ 2011) and the SDSS DR7 database. You can use this file to explore some interesting characteristics of the population of galaxies detected by ALFALFA. 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. For example, in practice, to calculate colors, you need to correct the observed apparent magnitudes at both bands for Galaxtic and internal extinction; those corrections are given in the table. See if you can figure out what you've got and what you need before you ask us for help.


3.A.3     The CMD for the α.40 galaxies
Using the data provided in the file, use TOPCAT to construct the CMD for the α.40 galaxies. In order to compare what you find with Figure 3 of Gavazzi+ (2010) be sure to generate the plot with the same filters (that is, (g-i) versus MI), scaling and orientation they use. You can use the arithmetic capability of TOPCAT to apply the corrections you need. What are they? Especially: what is the K-correction? Do we need to worry about it?

We suggest that you make a first plot with free scaling and a second using the scaling x-axis (-14, -24), y-axis (-1.0, 3.0). 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 α.40 CMD with that presented by Gavazzi+ (2010). Explain the differences in terms of the galaxy populations included in the two datasets.


3.A.4     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. To make the previous graph, you calculated the i-band absolute magnitude. Use the arithmetic functions in to calculate the associated luminosity (logL_i) and the "gas fraction" (gas2L = log MH - log L_i) for each α.40 galaxy which also has SDSS photometry. To calculate the i-band luminosity, assume that the absolute magnitude of the Sun at i-band is +4.58. Save your work by exporting the new table in TOPCAT.

Using your results, plot the 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?

Note: If you are pressed for time and don't have time to convert the absolute magnitude to luminosity and compute the gas fraction yourself, Martha (for a price of course...) might give you a file containing the values already calculated....


3.A.5     Make a cool image of NGC 3166 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 proceed.

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 NGC 3169, an interesting ALFALFA galaxy which you can read more about in Lee-Waddell+ 2012. 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).

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 09:48:07 EST 2013 by Martha