Photo by Serge Bruneilhttp://antwrp.gsfc.nasa.gov/apod/ap051004.html

The Galactic Plane at 1420MHz

Shawn Price and Elissa Thorn

ST562 Radio Astronomy for Teachers

Summer 2006

HI emission

1420 MHz is the emission line for the hyperfine transition of neutral hydrogen shown above. We chose this observing band for two primary reasons: the SRT can easily observe at this wavelength, and there are large numbers of neutral hydrogen atoms in the clouds of interstellar material in our galaxy.

Zeilik & Gregory

Figure 15-2, p 294

Galactic Motion

• Shifted spectral lines imply motion of the source and/or observer.

• Mapping the motion of points in our galaxy provides clues to large scale motion.

• Points along the galactic disk should show the most pronounced velocities.

Zeilik & Gregory

Figure 148, p 280

SRT Observing Procedure

1. Establish observing times - two sessions spaced 12 hours apart to maximize coverage of galactic plane

2. Describe search pattern - 7.5 degree spacing along galactic plane within telescope limits

3. Create automated observing script - 25x 4kHz bins centered on 1420.4 MHz, integrating at each point for 3 minutes

4. Execute script - SRT.cmd5. Collect resulting data files (*.rad) into a dedicated

directory for each observing session to maintain data integrity

Data Reduction

We began by graphing intensity vs. frequency for each point observed. We then placed a linear trend on the graph to approximate the noise in the signal. The second graph shows the observed data with this trend line subtracted.

Gal 135 0

0

200

400

600

800

1000

1200

1419.8 1420 1420.2 1420.4 1420.6 1420.8 1421

Freq (MHz)

Inte

nsi

ty

Gal 135 0

14500

15000

15500

16000

16500

17000

17500

1419.8 1420 1420.2 1420.4 1420.6 1420.8 1421

Freq (MHz)

Inte

nsi

ty

-500

0

500

1000

1500

2000

Counts (-Trend)

1420 1420 1420 1420 1421 1421 1421

337.5

352.5

7.5

22.5

37.5

52.5

67.5

82.5

97.5

112.5

127.5

142.5

157.5

172.5

187.5

202.5

217.5

232.5

Frequency (MHz)

Galactic Longitude

Summary

Graph of data for all observed longitudes (337.5-240)

-500

0

500

1000

1500

2000

Counts (-Trend)

1420 1420 1420 1420 1420 1421 1421 1421 1421

337.5

0

22.5

45

67.5

90

112.5

135

157.5

180

202.5

225

Frequency (MHz)

Galactic Longitude

Summary

Graph of data for all observed longitudes (337.5-240)

Local Standard of Rest (LSR)

Zeilik & Gregory

Figure 19-9, p 387

Expected Velocity vs. Longitude-40

-20

020

40

0 60 120 180 240 300 360

Galactic Longitude

Vel

oci

ty (

km/s

)

Turning our frequency data into velocities:

• Doppler Shift

• LSR Correction

• Equation:

Vrad = {[(νo – ν) / νo] * c} – VLSR

= {[(1420.4 MHz - ν)/1420.4 MHz]*3*105km/s )-VLSR

Observed Velocity vs. Longitude

-60.00

0.00

60.00

0 60 120 180 240 300 360

Galactic Longitude

Ve

loc

ity

(k

m/s

))

Published Data

Zeilik & Gregory

Figure 19-8, p 387

Our Data

-60.00

0.00

60.00

0 60 120 180 240 300 360

Galactic Longitude

Vel

oci

ty (

km/s

))

Conclusions

• Some galactic structure was evident in our data

• The structure observed, albeit rough, resembles that noted in published sources

• The SRT is indeed an adequate instrument for basic exploration of HI emission phenomena

Refinements

1. We see evidence of relatively fine scale structure in the data we collected; it would be productive to explore this using a more closely spaced search pattern and finer frequency gradations across a wider frequency range.

2. It would be interesting to augment our data with that from similar observations at other latitudes, allowing us to fill in areas of the galactic plane that cannot be observed from our site.

Extensions

1. All points covered by our observations were on the galactic plane. It would be interesting to expand the observations to include points at latitudes other than zero, allowing us to further explore the structure we observed in two dimensions.

2. Comparing our HI data to data collected at other wavelengths might allow the structure we observed at 1420 MHz to be connected to features observed in infrared, optical, etc.

3. Galactic rotation curve information could be calculated from our data (although we would need to repeat our work with refinement #1 in place in order to get reasonable results).

Obtaining a Galactic Rotation Curve

Gal 135 0

0

200

400

600

800

1000

1200

1419.8 1420 1420.2 1420.4 1420.6 1420.8 1421

Freq (MHz)

Inte

nsi

ty

Zeilik & Gregory

Figure 20-2, p 394

Illustration by R. Hurthttp://antwrp.gsfc.nasa.gov/apod/ap050825.html

Rotation Curves for Simple Models

• Kepler’s Law: rapid drop in V as D increases (1/D2)

• Solid body rotation: linear increase in V as D increases

Keplerian

D

V

Solid Body

D

V

Observed Galactic Rotation Curve

• The Milky Way exhibits differential rotation (not solid body), but not Keplerian!

• There must be a lot of mass beyond the Sun’s orbit. (Dark matter!)

Zeilik & Gregory

Figure 19-10, p 388

http://antwrp.gsfc.nasa.gov/apod/ap050104.html

Helpful Resources

MIT. "Measurement of Galactic Rotation Curve," lab exercise. MIT Haystack Observatory. <http://www.haystack.mit.edu/edu/undergrad/srt/SRT%20Projects/index.html>.

Murphy, Ed. "VLSR Calculator V1.0." Johns Hopkins University. <http://fuse.pha.jhu.edu/support/tools/vlsr.html>.

NASA/IPAC. "Coordinate Transformation & Galactic Extinction Calculator." Jet Propulsion Lab., Calif. Institute of Tech. <http://nedwww.ipac.caltech.edu/forms/calculator.html>.

Nemiroff, Robert and Jerry Bonnell. "Astronomy Picture of the Day." Jay Norris. NASA. <http://antwrp.gsfc.nasa.gov/apod/astropix.html>.

NEROC Haystack Obs. Undergrad. Research Initiative. "Small Radio Telescope Operator's Manual." MIT Haystack Observatory. <http://www.haystack.mit.edu/edu/undergrad/srt/SRT%20Software/SRTManual.pdf>.

Zeilik, Michael and Stephen A. Gregory. Introductory Astronomy and Astrophysics. 4th ed. Fort Worth: Saunders College Publishing, 1998.