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 B:       A quick look at the Virgo Cluster


The Virgo Cluster is the nearest cluster of galaxies, at a distance of 16.7 Mpc. A large portion of the cluster is included in the region covered by α.40. Here, your task is to consider a few issues associated with observing Virgo.

3.B.1     The Virgo cluster


Use this link to the NASA Extragalatic Database (NED) to find out some of the basic information about the Virgo that you will need. Record here the parameters given in NED.

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 V-band  
Galactic extinction in I-band  


Why do you think the distance given in NED is different from 16.7 Mpc, the commonly accepted value?


Because the Virgo cluster is so close to us, it extends over a very large region of the sky and its galaxies can be large in angular extent. To review the structure of the cluster, we can use the "Arecibo General Catalog", the private database of galaxies maintained by Martha and Riccardo at Cornell which is made available to members of the ALFALFA team with the usual caveat that you get what you pay for: be sure to understand the limitations of the AGC and read about its conditions of use.

We have used the AGC to create two CSV files for use in this activity: The files contain the galaxy AGC number, RA, Dec, Vhelio and the projected separation of the galaxy from the cluster center in arcminutes. You can use these files to explore the structure of the cluster.


3.B.2     The structure of the Virgo cluster on the sky


An important compilation of galaxies in the Virgo cluster was presented by Binggeli, Sandage and Tammann 1986, Astron. J. 90, 1681. Their Virgo Cluster Catalog consists of 2096 galaxies which they believed, based on examination of high quality photographic plates, are likely cluster members. Note that their investigation of the cluster was based on many fewer redshifts than are available today. To the right is the sky distribution of possible cluster members as appeared in their paper. (If you click on the image, a larger version will be displayed.)

Using the Virgo AGC/α.40 data files (linked above), plot (on the same graph) the sky distribution of galaxies in the (a) optical sample and (b) the α.40 galaxies. Be sure to plot the distribution so that east is to the left and west is the right; why do we do that? Consider the result: what do you notice and how can you explain what you see? How does your diagram compare to that seen in Fig 2. of Binggeli+ (1986)?

Clusters of galaxies have a typical radius of 2 Mpc. At the distance of 16.7 Mpc, what is the angular extent of 2 Mpc?



3.B.3     The distribution of heliocentric velocity with projected separation


Using the Virgo AGC/α.40 data files (linked above), plot (on the same graph) the variation in heliocentric velocities (y-axis) with the projected separation from the cluster center (x-axis) for the (a) optical sample and (b) the α.40 galaxies. Consider the result: what do you notice and how can you explain what you see?

Why do some galaxies have negative heliocentric velocities?

Can you use Hubble's Law to derive distances in the Virgo region?

Rather than simply using Hubble's law to derive distances, we use a model of the local velocity field to estimate distances to galaxies, and the α.40 catalog includes the distance assumed in the derivation of HI masses. What is a "peculiar velocity"? What is a "flow model"? What is the "triple valued region" around the Virgo cluster? How does our distance calculation routine work, that is, how do we assign the distance to a galaxy?   Hint: Look at the description of the entry for distance in the α.40 catalog presented in the Haynes+ (2011) paper.

In what favorite ALFALFA team movie does the physicist hero say: "You know, there's such a very thin dividing line between 'inspiration' and 'obsession', that sometimes it's very hard to decide which side we're really on!"?


3.B.4     The velocity distribution of galaxies in the Virgo cluster


Using the Virgo AGC/α.40 data files (linked above), plot (on the same graph) the distribution of heliocentric velocities (using the histogram option) for the (a) optical sample and (b) the α.40 galaxies. Consider the result: what do you notice and how can you explain what you see?

Take a careful look at the histogram for the optical sample. Why do there seem to be more galaxies at higher velocities than at lower ones, i.e. the histogram is skewed and not a perfectly symmetric Gaussian function?

What character in a famous film said: "Well, the universe is everything, and if it's expanding, some day it will break apart and that will be the end of everything."




3.B.5     The properties of local α.40 galaxies in Virgo and outside Virgo


Let's take a look at the HI properties of galaxies detected by ALFALFA in the Virgo cluster in comparison with ones at similar distances but outside Virgo. Here are two useful files:

In both files, we have calculated the optical luminosity for each galaxy (given in logarithmic units as logL) and the "gas fraction" (gas2L = log MH - log L). Note that we do this for both the V-band and the i-band. What are they? In what ways might we expect them to give different results? Why?

Using these files, plot (on the same graph) the optical luminosity (x axis, in log units) versus the HI mass (y axis, in log units) for (a) the Virgo α.40 galaxies and (a) the non-Virgo ones.

Additionally, examine the histograms of the gas fraction parameter.

Do this for both filter bands you are given, V and i.

Consider the results: what do you notice and how can you explain what you see? Feel free to make any other plots that might be interesting/useful.



3.B.6     Make a cool image of NGC 4438 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 r-band data for NGC 4438, near the center of the Virgo cluster. 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: Thu Jan 10 08:33:44 EST 2013 by Martha