Photo by Serge Bruneil

1

• Photo by Serge Bruneil
• http//antwrp.gsfc.nasa.gov/apod/ap051004.h
  tml

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The Galactic Plane at 1420MHz

• Shawn Price and Elissa Thorn
• ST562 Radio Astronomy for Teachers
• Summer 2006

3
HI emission
Zeilik Gregory Figure 15-2, p
294

• 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.

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Galactic Motion
Zeilik Gregory Figure 148, p
280

• 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.

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SRT Observing Procedure

• Establish observing times - two sessions spaced
  12 hours apart to maximize coverage of galactic
  plane
• Describe search pattern - 7.5 degree spacing
  along galactic plane within telescope limits
• Create automated observing script - 25x 4kHz bins
  centered on 1420.4 MHz, integrating at each point
  for 3 minutes
• Execute script - SRT.cmd
• Collect resulting data files (.rad) into a
  dedicated directory for each observing session to
  maintain data integrity

6
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.

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Graph of data for all observed longitudes
(337.5-240)
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Graph of data for all observed longitudes
(337.5-240)
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Local Standard of Rest (LSR)
Zeilik Gregory Figure 19-9, p
387
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Expected Velocity vs. Longitude
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Turning our frequency data into velocities

• Doppler Shift
• LSR Correction
• Equation
• Vrad (?o ?) / ?o c VLSR
• (1420.4 MHz - ?)/1420.4 MHz3105km/s
  )-VLSR

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Observed Velocity vs. Longitude
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Published Data
Zeilik Gregory Figure 19-8, p
387
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Our Data
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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

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Refinements

• 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.
• 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.

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Extensions

• 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.
• 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.
• 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).

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Obtaining a Galactic Rotation Curve
Zeilik Gregory Figure 20-2, p
394
19

• Illustration by R. Hurt http//antwrp.
  gsfc.nasa.gov/apod/ap050825.html

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Rotation Curves for Simple Models

• Keplers Law rapid drop in V as D increases
  (1/D2)
• Solid body rotation linear increase in V as D
  increases

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Observed Galactic Rotation Curve
Zeilik Gregory Figure 19-10, p
388

• The Milky Way exhibits differential rotation (not
  solid body), but not Keplerian!
• There must be a lot of mass beyond the Suns
  orbit. (Dark matter!)

22

• http//antwrp.gsfc.nasa.gov/apod/ap050104.ht
  ml

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Helpful Resources

• MIT. "Measurement of Galactic Rotation Curve,"
  lab exercise. MIT Haystack Observatory.
  lthttp//www.haystack.mit.edu/edu/undergrad/srt/SRT
  20Projects/index.htmlgt.
• Murphy, Ed. "VLSR Calculator V1.0." Johns Hopkins
  University. lthttp//fuse.pha.jhu.edu/support/tools
  /vlsr.htmlgt.
• NASA/IPAC. "Coordinate Transformation Galactic
  Extinction Calculator." Jet Propulsion Lab.,
  Calif. Institute of Tech. lthttp//nedwww.ipac.calt
  ech.edu/forms/calculator.htmlgt.
• Nemiroff, Robert and Jerry Bonnell. "Astronomy
  Picture of the Day." Jay Norris. NASA.
• lthttp//antwrp.gsfc.nasa.gov/apod/astropix.htmlgt.
  
• NEROC Haystack Obs. Undergrad. Research
  Initiative. "Small Radio Telescope Operator's
  Manual." MIT Haystack Observatory.
  lthttp//www.haystack.mit.edu/edu/undergrad/srt/SRT
  20Software/SRTManual.pdfgt.
• Zeilik, Michael and Stephen A. Gregory.
  Introductory Astronomy and Astrophysics. 4th ed.
  Fort Worth Saunders College Publishing, 1998.