Astronomy 210 Class Work

Astronomy 210 Class Work (Feb. 14)

HI line observations

In this exercise, you will estimate the masses (dynamical and HI gas) and distances of several galaxies observed as part of the Arecibo Legacy Fast ALFA (ALFALFA) survey. Observations from this survey are available at the ALFALFA Archive, part of the Cornell Digital HI Archive.

1. For each of the galaxies below, examine the optical image and the spectrum, including the HI line emission. The link will take you to the ALFALFA archive page for the object. You will see two optical images on top (from the Digital Sky Survey, DSS, and the SDSS) and the spectrum below. Information about the galaxy is given in a column at right. For each galaxy, write down a brief description of the galaxy, including an approximate Hubble type, and a description of the HI line spectrum. Also click on the SDSS image to connect to the SDSS website, and examine the optical spectrum if available. How does the shape and width of the HI line spectrum depend on the galaxy?
UGC 7849
UGC 8159
UGC 8408
UGC 8573
AGC 220690
AGC 7249

2. Here are two more unusual examples. For each, write a short description of the galaxy and its HI spectrum. What is odd about each? Speculate about the cause of the oddity.
UGC 7365
AGC 226056 (To read further about this object, check out ALFALFA observations of Virgo HI21.)

3. You will now calculate properties of UGC 7849 based on the HI information and information from other databases. (You will also have to remember or look up several formulae that we have discussed.)
(a) Click on the ALFALFA link and write down the central velocity of the line (cz), the width of the line, the flux of the line. Also note the cataloged distance and HI mass.
(b) Find the distance based on cz. Compare to the cataloged distance.
(c) Find the HI mass. (Review your notes and refer to these pages.)
(d) Make a rough estimate of the dynamical mass:

Find an approximate value for v_flat, using the observed width of the line and your understanding of the rotation curve.
This value must be corrected for inclination of the galaxy. We will use optical sizes to estimate the inclination. Click on the 'Additional HI archive entry' on the ALFALFA archive page for the galaxy (the last link under Resources at right). This will take you to the Cornell University Digital HI archive of earlier observations of this galaxy. At left is a table of measured values. Write down the values for a and b, the optical major and minor axes of the galaxy, respectively. (These values are isophotal.) Calculate the inclination using the approximation i = cos^-1 (b/a). Using this value, correct your velocity. (You will show in your homework that the corrected velocity is v = v_obs / sin i.)
Now find the approximate size of the galaxy. You have the angular size of the major axis. Convert this to a distance in parsecs, using the known distance of the galaxy. This number is the optical diameter of the galaxy. You need a radius for the dynamical mass equation. Estimate an appropriate radius, recalling that the HI disk typically extends 2-3 times beyond the optical disk.
Now calculate the dynamical mass. You may use G=4.43x10^-3 if you keep the units appropriate. What fraction of the dynamical mass in the galaxy is made up of HI gas?

(e) Finally, we will very roughly estimate the stellar mass of the galaxy:

First we need a measure of the luminosity of the galaxy. Go back to the archive page. Click on the SDSS link and write down the r magnitude of the galaxy. Using the known distance, calculate the absolute r magnitude.
Using this value and a value of +4.68 for the absolute r magnitude of the Sun, find the luminosity of the galaxy in solar units.
Use a standard mass-to-light ratio to estimate the luminous mass.
Compare the luminous, gas, and dynamical masses of the galaxy.

Complete all work for Tuesday, Feb 19.

Last updated February 14, 2008