Try to finish this hunt by 5:15pm on Tuesday afternoon

Useful links:
  • Using Arecibo for ALFALFA
  • A2010 observer's page
  • Instructions for Level I processing of ALFALFA drift data

    Rules of the scavenger hunts: You may consult any source anywhere but please be sure to indicate where you got your information. And watch out for bad websites!

    UAT13.01 Scavenger Hunt #2:   Introduction to LBW spectra and TOPCAT

    This scavenger hunt will provide an introduction to the spectra we obtain from the L-band wide observations and also get you to use the TOPCAT, the Tool for OPerations on Catalogues And Tables.

    A2010 observers' favorite quote
    "A hypothesis or theory is clear, decisive, and positive, but it is believed by no one but the [person] who created it. Experimental findings, on the other hand, are messy, inexact things, which are believed by everyone except the [person] who did the work."
    -- Harlow Shapley, "Through Rugged Ways to the Stars"

    Another favorite ALFALFA quote is "Real astronomers need more than a cell phone to conduct a survey."

    2.0   Temperatures, temperatures, temperatures...

    Radio astronomers are always talking about temperatures. But, what are they talking about?

        a.   What do we mean by "system temperature"?

        b.   What is the temperature in the LBW dewar?

        c.   How are the LBW amplifiers kept that cold? (i.e. what refrigerant is used)

        d.   What do 21 cm HI line astronomers mean when they talk about the "spin temperature"?

        e.   What is the "brightness temperature" of a radio source?

        f.   What is the "antenna temperature" of a radio source?

        g.   What do astronomers mean by the "system gain"?

        h.   What is the flux density (in Janskies) of the Sun at 1.4 GHz? How does that compare with the system temperature of LBW?   (Hint: Think about how bright it would appear if we pointed LBW at it.)

        i.   What is the typical temperature and density of a diffuse HI cloud in the Milky Way?

        j.   What is the typical temperature and density of a molecular H2 cloud in the Milky Way?

        k.   What is the typical temperature and density of an ionized HII cloud in the Milky Way?

        l.   What is the temperature of the Cosmic Microwave Background radiation?

        m.   What is the temperature in Ithaca, NY right now?

        n.   How hot was it in the famous futuristic 1967 film starring Julie Christie and Oskar Werner?

    2.1   ALFALFA followup: Why bother?...

    Now that we have completed the ALFALFA drift scan observations, we are conducting targeted observations of interesting ALFALFA detections or possible detections. We call this program "Harvesting ALFALFA".

        a.   How long does it take a point source to drift across an ALFA beam?   Hmmm... seems like we've thought about this one somewhere else..

        b.   We say that the "effective" integration time for ALFALFA is about 40 seconds per beam. How do we get the answer "40 seconds", i.e. what considerations apply?

        c.   The AGES program maps small areas (~10-20 square degrees each vs. 7000 for ALFALFA) using ALFA with an effective integration time of 300 seconds. For the same spectral resolution, how much more sensitive (lower rms) is AGES than ALFALFA?   Hint: use simple scaling!

        d.   We might imagine that if we observe for a really long time, the noise in the spectrum would continue to decrease. In practice, this is not the case. Why not?

        e.   What rms noise (in mJy) will we expect our 3-minute ON-OFF observations to give us, at a velocity resolution of 10 km/s? How does that compare to ALFALFA?

        f.   The ALFALFA bandpass covers 1335-1435 MHz. What spectral line has a rest frequency of about 1424 MHz? If we wanted (deliberately) to observe emission from that line close to its rest frequency, where (in the sky) might we look?

        g.   Suppose we observe a source with a frequency that, if the line is HI, is -1515 km/s. Why don't we think this is an HI source? If it is an OH megamaser, what is its redshift?

    2.2   The dreaded RFI: ugly GPS stuff!

    Here is FLAGBB display of one 10-minute drift scan observations from the main ALFALFA survey. The horizontal axis shows frequency and the vertical axis shows the successive 1-sec records. The blue scale reflects the intensity at each frequency pixel in the map, with white meaning higher flux.

    Three bright spots are flagged in the center of the frame. They arise from the Nuclear Detection (NUDET) system aboard the Global Positioning System (GPS) satellites. Refer to this figure in answering the following question. Click here for a larger view of the screen.

    a.   What is the approximate frequency of the NUDET transmissions?

    b.   What is the radial velocity of a galaxy whose HI line emission will be observed at that same frequency?

    c.   How many GPS satellites are there?

    d.   What is the orbital period of a GPS satellite?

    e.   How could you find out how many GPS satellites there are above Puerto Rico now? You don't have to do this; just tell us how you would go about it.

    f.   In the movie "Some Like It Hot", who (what actor) utters the famous closing words "Well, nobody's perfect."?

    2.3   Enigmatic ALFALFA sources

    Let's look at some of the spectra taken during last spring's LBW followup observations, under AO program A2669. You can click on each spectrum to view a larger version.

  • To the right is the LBW spectrum of the galaxy AGC 749542, a code 4 detection in ALFALFA.

          a.   What is a "code 4" detection?

          b.   Based on your examination of the spectrum alone, what is the approximate recessional velocity of this galaxy?

          c.   Using only your answer to b., make a "back of the envelope" calculation of the distance to the galaxy. What more would you need to know to get a more accurate answer?

  • To the right is a spectrum of the galaxy AGC 198363, a code 4 detection in ALFALFA observed in the standard doppler tracking mode.

          d.   Explain what the two features evident in the spectrum are likely to be.

  • For our doppler tracking LBW observations, we actually obtain four different spectra of each target and in the end wind up with two different final ones.

          e.   Explain the differences among first the four, and then, the two. Be quantitative where relevant (e.g. bandwidth, number of channels, velocity coverage, frequency resolution, velocity resolution etc.). Remind yourself how the spectral resolutions of these spectra compare to that of the main ALFALFA survey.

  • To the right are the two different final spectra of AGC 198305.

          f.   How do the two spectra correspond to the final spectra referred to in the previous question?

          g.   Based on your examination of the spectra, what is the recessional velocity of this object?   Please, no cheating by consulting other sources!

    For fun, take a look at the SDSS-DS9 image of this object.

    Click here to see a zoomed in version of the 2nd one.

  • To the right is the LBW spectrum of the galaxy AGC 174577 which was easily detected by ALFALFA and which was observed by A2669 as a calibrator (i.e., to check that everything was working and giving the right velocity and flux scales).

          h.   What is unusual about the y-axis scale? Why is it that way?

  •       i.   What actor delivered the line: "My Mama always said, 'Life was like a box of chocolates; you never know what you're gonna get.'"?


    2.4   Introduction to TOPCAT: the Tool for OPerations on Catalogues And Tables, an interactive java graphical program which has been developed by astronomers at the Virtual Astronomical Observatory.

        a.   Do you have access to the UAT groups Google sites page?.   Hint: Find it at: If you do, go there and log in. If not, send email to Becky or David's gmail account. David's is They can add you to the site. When you reach the site, look the bar on the left. Under **Programs** you will see UAT TOPCAT Docs, click there. If you have not visited this site recently (or at all), briefly review the documents there; refer to them as needed as we proceed.

        b.   Start up TOPCAT on your machine.   If you have not installed TOPCAT on your laptop or local machine, follow the steps in **Installing and Getting Started** for your system. Java web start is generally the easiest, or Mac disk images, but TOPCAT is also easy to install on Windows machines.

        c.   In the latter part of the **Installing and getting started** on the UAT wiki, read section on the 2.1 **Loading data from a CSV file**. What does "CSV" stands for?

        d.   Go to the public ALFALFA catalog release page. If you have not already committed it to perfect memory, peruse the α.40 paper. You will find detailed information on the content and format of the three data files in the header of the ASCII versions of the data files. What does "ASCII" stand for?

        e.   Right-click (or whatever) to download the .csv file associated with Table 1 of the α.40 paper to you computer using the **CSV** link. Be sure you know where it is located on your disk. Then, using TOPCAT, load the .csv file into a table. How do you let TOPCAT know that it is a .csv file?

        f.   Review the column headers in the Table within TOPCAT. They should match the descriptions in the header of its ASCII file. At what resolution is the rms calculated?

        g.   Make a plot of distance (x-axis) versus log Msun for the α.40 database. What is log Msun?

        h.   Why are the points distributed so that they spread out in the vertical direction at nearer distances but are more confined in log Msun as the distance increases?

        g.   There appears to be a gap in the distribution near the right at a distance of about 220 Mpc. What causes that gap?

        h.   There is a less obvious gap at a distance of about 80 Mpc. What causes that gap?

        i.   There appears to be a vertical line of points at a distance of about 17 Mpc. What causes the apparent line?

    There are lots of other things you can do with TOPCAT. We encourage you to learn how to exploit its capability!

    2.5   (Important) questions about the Arecibo staff

        a.   Which Arecibo staff member spent 8 years with the Klamath Indians in Oregon (rustling cattle, so the rumor goes) before moving to Puerto Rico?

        b.   Who is perfecting a new drink called the "avocalada"?

        c.   What was the traditional sumptuous delicacy made by Martha and savored by Willy during late night observing runs over the Christmas and New Year's holidays?

        d.   Who played Carl Sagan wandering around the observatory during the filming of the Cosmos series, when Carl did not make the trip to Arecibo?

    Last updated Mon Jan 13 21:31:43 AST 2013 by martha