Spectral Line Observations of Galaxies

 

Background: You are interested in using spectral line observations as a probe of the evolutionary state of star formation in galaxies.

Find the five nearest spiral galaxies for which there exists images of HI emission (or any particular lines of interest). For each
galaxy, overlay contours of HI emission on an optical image.

Software Needed: web browser, Topcat, Mirage

  1. Find a catalog of nearby galaxies:
    • Visit the NVO Registry and submit a keyword search with "nearby galaxies"
    • Ultimately, you would like a catalog that includes galaxy positions and distances. At the moment, it is not possible to search the registry based on the columns (but one day this should be possible). Instead, we just need to look at the headers of a few of the tables (based on their title and description) until we find one.
    • The first entry is the Nearby Galaxies Catalog from CDS, which sounds useful, but upon inspection we'll find that it doesn't include a distance. Hit "Next" until you see the record "Nearby Galaxies Catalogue" (ServiceType: CONE). Click on its view link to see the detailed info.
    • We don't really need a Cone Service at this point; however, the entry has a Reference URL we can consult. Enter the URL in your browser. This goes to a description of the table. This description tells us that this catalog has three tables, one of which contains positions and distances.
  2. Obtain a copy of the table.
    • We will use Vizier to access the catalog. Copy the table code, "VII/145", into your paste buffer. Now click on the Vizier link. In the form, paste the code into the input box under "Direct Access..." and click "Find Catalog". There we see buttons for the three tables; click on "VII/145/catalog". This gives us a search form that allows us to select rows from the table. Choose the following inputs:
      • Sort by r (distance)
      • Output layout: "XM - VOTable (DTD)"
      • R: "< 5"
    • The last constraint returns galaxies closer than 5 Mpc.
      Hit "Submit Query" button. This returns a VOTable which you should save to disk.
    • Meanwhile, in your browser, return to the page with the full
      catalog description (i.e. the "README" file).
  3. Load file into Topcat for filtering and display.
    • Start tomcat by typing "topcat". Choose "File-Load Table" from the menus. With the file chooser, pick the file you just saved. The table properties will appear in the main window.
    • To browse the header and select our view, choose "Table Views-Column Info". This shows a table describing the columns. To get a more compact view of our table, deselect the following columns: NBG, RA1950, DE1950, D25, d/D, BTbi, MB, recno. Note that the column "R" is our distance.
    • Now let's look at the row data. First, in the main window, note the display of the table properties. Choose "R" from the sort order menu. Now choose "Table Views-Table Data" to see the row data.
  4. Filter out "early-type" galaxies (e.g ellipticals).
    • Note from the README (Note 2) that the morphology type, column "HubCode", is given as a numerical code. Those greater than 0 are spiral types.
    • Now select "Table Views-Row Subsets". In the new pop-up window, choose "Subsets-New Subset". In the next pop-up, enter "spirals" (or whatever you like) as the Subset Name and "HubCode > 0" for the Expression. Hit cancel.
    • To view the subset, choose "spirals" in the Row Subset menu of the main window, under the table properties. Then choose "Table Views-Table Data".
  5. Find image observations for these a few nearby galaxies.
    • The table is order nearest to farthest (with a 5 Mpc cutoff). Galaxies with a _DEJ2000 &gt; 0 will definitely be visible from the northern hemisphere.
    • We will use DataScope to find images. Send your browser to http://heasarc.gsfc.nasa.gov/vo. Enter a galaxy name from catalog into "Object Position or Name". If DataScope indicates that it cannot match a name to a position, copy in the position; then hit "Send Request". Try "N 300."
    • Watch responses under the "images" section. You can select an optical image for seeing the location of the spiral arms. Good places for images of spectral line emission in the radio are the ADIL and NED. When these appear, click on the link to see summaries of matching images. For ADIL look at the "Description" column for an indication of the spectral line. For NED, look at the "Band" column.
  6. Download images of interest and load into Mirage or Aladin.