RPA 21cm Observations

Observing HI using the RPA

These observations were made for Astro 203, Spring 2002. We used one of the ~10 ft. dishes at the RPA (Rapid Prototype Array, a test version of ATA) to look at the HI 21 cm line along the galactic plane. We observed galactic longitudes from 8 deg to 134 deg.

Data Analysis Overview

Things I've done so far:

Organize data by l.
Fit a 5th order polynomial to the baseline. The fit was done over a frequency range of ~1418-1422 MHz (bins 1500-2500 out of 4096 total), excluding the actual HI line.
Remove some RFI by hand.
Subtract off baseline, sum up the two polarizations, get data into IDL.
Make some neato plots (see below).
Convert reciever frequency to LSR frequency.
Convert that into velocity.
Re-do my neato plots so y axis is velocity.

Things I'm going to do:

Calibrate temperature axis based on hot/cold load tests.
Interpret data: Enclosed mass, dark matter, galactic structure ... ?

Software I used:

Polynomial fits - Done via C programs I wrote based on stuff in "Numerical Recipies". If you really want to see code, email me.
Doppler shift removal - Done in IDL. I wrote two routines, rpa_vel.pro and v_off.pro which only call standard GSFC astronomy routines.
Image processing - Also IDL. Again, if anyone wants to see it, email me.

A First Look

The aformentioned neato plots.

Here's an example of a baseline fit:

Here's the longitude-frequency-intensity plot:

More Detailed Analysis

Reference for this part is "Galactic Astronomy" by Binney and Merrifield.

Here's the same plot as above, with the y-axis converted to LSR velocity. Displaying l "backwards" seems to be the standard way of doing things. Included in this plot is the 10K contour through our data. (Compare to B&M figure 9.13)

Some of the quantities which are physically relevant for galactic dynamics are the boundaries of the emission line - ie, for each l, the fastest and slowest hydrogen. The plots below were made using the 10K countours of our data.

Here is the lower emission boundary. For simple circular rotation, this would be a piece of a sine curve. The amplitude of the sine curve tells us how fast our section of the disk is rotating. (Compare to B&M figure 9.14)

Here is a plot of the upper boundary, also known as the terminal velocity. The values on this curve (for l between 0 and 90) tell us the speed of circular rotation for every radius interior of ours. This shouldn't fall off below l=20 like it does here. To resolve the fast moving stuff near the center, we need better sensitivity than 10K. (Compare to B&M figure 9.16)

Coming soon... translate these plots into speeds, work on enclosed mass, etc?