Measurement of Galactic Rotation and Evidence for Dark Matter

The University of Michigan at Ann Arbor, August 1–3, 2015

(One set-up)

Host and Mentor

Photo of Dr. Akerlof

Carl Akerlof obtained his B.A. in Physics at Yale University followed by a PhD in elementary particle physics at Cornell University in 1967. In 1966, he took a job as a post-doc in physics at the University of Michigan that transformed to a teaching position and tenure in 1972. His research interests originally centered on experimental high energy physics but the increasing importance of astrophysics led to experiments that discovered TeV radiation from Active Galactic Nuclei. This was followed by an intrigue with the mysterious nature of gamma-ray bursts and his development of the first cameras to record the prompt optical emission from these very distant objects. His teaching efforts for the past seven years have centered on the advanced undergraduate lab course for which he has introduced a number of new experiments to better reflect the broad scope of physics in the 21st Century.

Carl W. Akerlof, Professor of Physics, University of Michigan, Randall Laboratory of Physics, 450 Church Street, Ann Arbor, MI 48109-1040. Email: cakerlof@umich.edu. Telephone: 734-764-9278.

Photo of Dr. Hughes

Philip Hughes received his doctorate from the University of Sussex, spent two years with the Radio Astronomy group in the Cavendish Laboratory in Cambridge, and moved to Ann Arbor in 1983 as a postdoc, ultimately becoming a Research Scientist and Adjunct Professor. His research has been on jets from active galactic nuclei, using relativistic hydrodynamic simulations and radiative transfer calculations to understand the internal structure and stability of these nearly light-speed flows that can penetrate far into the intracluster plasma. In teaching, he has used the Small Radio Telescope at every opportunity over the last fifteen years: from an introductory course for astronomy majors, to an advanced undergraduate/graduate course on Galactic structure. The experiment is sufficiently flexible that he has been able to use it to good effect in the Michigan Math-Science Scholars summer enrichment program for high school students. He has found that, at every level, students are engaged by the possibility of measuring the Galactic rotation curve, and thus the mass of our dark matter halo, from a brightly-lit college campus.

Philip A. Hughes, Lecturer, University of Michigan, Department of Astronomy, 311 West Hall, 1085 South University Avenue, Ann Arbor, MI 48109-1107. Email: phughes@umich.edu. Telephone: 734-615-6108.

At the present time, the two most outstanding mysteries of cosmology and elementary particle physics are the nature of Dark Matter and Dark Energy. One of the best pieces of evidence for the former is the anomalous shape of the galactic rotation curve for our own Milky Way. This can be measured relatively simply by observing the H I hyperfine transition of neutral atomic hydrogen at 1420.4 MHz (21.1 cm wavelength). Starting in 1998, a radio telescope for this purpose was developed by Alan Rogers at MIT—Haystack. It has recently undergone a second iteration in design and although the instrument is not sold commercially, the instructions for construction and operation are freely available from the MIT Haystack Web site. This “Immersion” workshop will describe our experience at the University of Michigan with building and operating the new Small Radio Telescope II (SRT-II). The total cost for this instrument is somewhat dependent on the environment in which it will be located but the investment will be similar in scale to the purchase of a new pulsed NMR experiment.

Picture of the SRT

During the workshop, the following topics will be covered:

  1. Overall description and tour of the 2.3-m diameter SRT-II instrument on the roof of Angell Hall
  2. Discussion of the atomic physics of the 21 cm line and hydrogen clouds in the Milky Way
  3. Design and construction of the antenna dish and rotator system
  4. Design, construction and performance of the electronic receiver system
  5. Discussion of the FFT techniques to perform a spectral analysis of the 20 MHz ADC data stream
  6. Discussion and operation of the MIT Haystack data acquisition and analysis program
  7. Demonstration of receiver noise calibration
  8. Measurement of the telescope angular response function using the Sun as a point source
  9. Spectral measurements of the 21 cm line from various galactic longitudes in the Galactic disk
  10. Reconstruction of the Milky Way rotation curve and its relevance to Dark Matter

Unlike optical astronomy, radio observations can be performed without interruption during daylight hours. At the L-band frequency of the 21 cm line, atmospheric clouds are effectively transparent so local weather is also not an issue. This is not a simple experiment to implement but the intellectual opportunities to explore a subject on the cutting edge of modern science are substantial.

Picture of the SRT

Participants are urged to bring a laptop, notebook and sunscreen.

The cost of the commercially available components required for this experiment is about $9000, spread out over more than 100 items. That includes the antenna dish, motorized computer controlled mount and the receiver electronics with a 20 MHz A/D board. However, this does not include the column on which the antenna must be mounted as well as the ancillary costs that are highly dependent on the site at which the telescope is installed. In addition, there are a number of custom fabricated parts which were made in our departmental machine shop to our own drawings. The cost to duplicate these is probably around $6000. These include a counterweighted mount structure to attach the dish to the antenna elevation shaft, modifications to various electronic enclosures and base plates as well as the antenna feed structure. For more detail, any interested participant should contact Carl Akerlof.

Please note that the Jonathan F. Reichert Foundation has just established a grant program (ALPhA webpage; Foundation website) to help purchase apparatus used in Laboratory Immersions. Limitations and exlusions apply, but generally speaking the foundation may support up to 40% of the cost of the required equipment.