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Radio Observations of Galaxy NGC4911 in the Coma Cluster


Timothy McIntosh
Faculty Mentor: Michael Gregg Ph.D.


Introduction

    Radio Astronomy began with the realization that background noise heard in radio communications was extraterrestrial in origin. Working for AT&T Bell labs in New Jersey, Karl Jansky was assigned the task of finding the source of that radio noise and removing it. In his attempt to make radio waves a practical commercial carrier, he discovered in 1932 that among the noise from thunderstorms and other terrestrial sources, some radio noise originated beyond the earth and in the center of our galaxy. It wasn't until later that Physicists and Astronomers realized the significance of this discovery.

    Grote Reber, a radio engineer and active amateur radio operator, was the first man to spend a considerable amount of time developing radio telescopes and observing the skies at radio frequencies. Since his initial look upward in 1941 from his his backyard observatory in Illinois, radio telescopes and radio astronomers have evolved into a formidable tool of science.

    Radio Telescopes, now are constructed from vast combinations of antennas, and through the aid of computers, dump gigabytes of data into huge storage disks for later scientific evaluation. Radio astronomers now detect active galaxies that possibly contain black holes and can monitor the movement of hydrogen in distant galaxies as well as our own Milky Way.

    Since the development of modern astronomy, astronomers have investigated, cataloged and categorized the objects in the universe, trying to develop schemes to help us understand their nature and development. In the 1920's Edwin Hubble developed a very  important classification scheme after he found that spiral nebulae actually lie beyond our own group of stars, and are what we now know as Galaxies. Hubble developed a morphological classification scheme that is still used in various forms today. These classifications describe the shapes and structures of Galaxies, which give indications of their age, luminosity, size, distance, etc.. Working only in the optical wavelengths, Hubble, could only hypothesize about further differences in between Galaxies, but now with the aid of Radio astronomy, galaxies can further be divided  into active and inactive galaxies. Furthermore, Active galaxies can be subdivided into Seyfert Galaxies, N Galaxies, BL Lacertae ( or BL Lacs ), Radio Galaxies, Quasars, and QSOs ( Quasi-stellar Objects ) ( Shu 306 ).

    Although astronomy has advanced in leaps and bounds this century, watching extra galactic events is still a challenging task for astronomers and several things must be taken into account. First, the universe is expanding, so all objects in the sky appear to be accelerating away from everything else in the sky, including our own planet. Because of this universal expansion, a Doppler shift lowers the frequencies of any photons traveling to us. This red shift is distance dependent because each point in space is expanding, so the more points in space between objects the greater the rate of expansion is, and thus a larger acceleration is observed. Also, the objects may move relative to each other and cause further blue ( when the object comes toward he observer ) or red shifts. It is these shifts that allow astronomers to isolate and study very specific objects in the sky.

    Along with Doppler shifts the composition materials also affect the frequencies in which they emit, reflect and absorb photons. Neutral hydrogen can emit photons in a narrow 21 cm wavelength radio range, while stars often emit at many wavelengths and intensities. A material's emittance may result from various processes and in many different bandwidths. These can range from the very robust nuclear reaction emissions in stars to the exotic emissions from synchrotron radiation near black holes. They can range from recombination emissions to spin change emission, but all result from the interactions between sources of energy and matter.

    Galaxy clusters interest astronomers for the vast interactions of members and environments. Some large Galaxies become cannibalistic, swallowing up smaller Galaxies within the group. Others experience drag from the intergalactic medium ( IGM ) which lies between the cluster's galaxies. Along with stellar and galactic dances in the cluster, the IGM ( consisting of sparse high energy gases ) mixes and interacts with the neutral hydrogen and ionized hydrogen within the galaxies themselves, causing various reactions which we observe and study. 

    Galaxy NGC4911 is a large Sbc spiral galaxy  in the Coma cluster, with ~ 10^9 Solar masses ( Giovenelli & Haynes 1985 ). The Coma cluster is dominated by two giant spiral galaxies that lie in the cluster core, who are surrounded by an x-ray IGM field. NGC4911 is .25 degrees from the cluster center and has a recession velocity of ~8000 m/s. NGC4911 lies within a small out-crop of the x-ray IGM, and it is believed that this medium is exciting the leading arm of the galaxy, spurring star formation. Optical images show that the arm of the galaxy facing the cluster core and the direction of the galaxy's travel, is more luminous than the trailing arms. A Digital Sky Survey ( DSS ) image  shows the increased luminosity of the leading edges. This IGM is also thought to be blowing on the hydrogen gas within the galaxy itself, which would be revealed in neutral hydrogen ( HI ) studies as a concentration of HI in the trailing arm with little or none in the leading arm.  Previous studies at smaller observation depths by other research teams reveals that these thoughts are correct, and HI is being blown from the galaxy. Because of the levels of HI present, it is now believed that this is the Galaxy's first passage through the cluster core, because all the HI would be gone by now if the galaxy has been in the x-ray IGM before.

Materials and Methods

    Our observations of NGC4911 were collected on March 31, 1996 with the Very Large Array ( VLA ) in C configuration over a 9 hour integration. The data was calibrated and analyzed using the National Radio Astronomical Observatory ( NRAO ) Astronomical Imaging Processing System ( AIPS ) software bundle.

    The VLA is made up of 27 individual antennas at a site near Soccoro New Mexico, and is run by the NRAO. It is an interferometer, which utilizes the fact that phase shifted beams interfere with each other in very predictable ways. The VLA has 4 modes of operation. Each mode places the antennas in different configurations, called arrays, depending on what characteristics the observers value most. It can be placed in any of A, B, C, or D arrays, with A being the most compact, generating the highest resolution, and D being the largest, providing the greatest sensitivity. The D array is often used to study the detailed structure of objects, while the A array is used to observe faint objects.

    Since the VLA's antennas are separated ( often up to a 3 km ) in the shape of a Y, and because of the curvature and rotation of the earth and the fixed and flat plane of the sky, the radio beams strike each antenna at different times. This time delay causes the beams to be out of phase with each other, but since all the distances and therefore times differences are known, the phase differences can be calculated and corrected. These radio beams are collected with advanced detectors at the focal point of each antenna, and sent to one of the array's many correlators, where the signals are combined. The correlators combine the signals like beams passing through pairs of slits, and as it is well know, the diffraction pattern of a beam passing through two slits is a well defined Gaussian. The VLA has 27 active antennas, so the correlators combine each antenna into a double slit pairing with each the other antennas. As a result the telescope generates a vast collection of varying baseline double slit Gaussians. The data is stored in the Gaussian U, V plane and is sent to the central computers for calibration. This technique of interferometry allows the antennas to be separated at great distances, increasing the effective aperture of the telescope and photon collecting power.  Now, we have radio telescopes that can look at more than just one point in the sky and can resolve structure and faint objects equivalent to optical telescopes.

    Due to the complexity of the telescope, calibrating the U, V data and creating images from the VLA telescope requires a very powerful software program. The AIPS software package has been in development since the creation of the VLA in the 70's and represents 65 man years of effort since 1978. Rough estimates, place the package at well over 1 Million line of code long. With many hundreds of thousands of line of online help files. The AIPS group sites in Charlottesville and Soccoro have 5 full time scientists/ programmers and many other part time staff members involved in the AIPS development effort ( AIPS cookbook 30-March-1998 ).

    The observations of NGC4911 were carried out at 21 cm ( ~1.38 Ghz ) across 64 channels with bandwidth 3.125 Mhz, to look for neutral hydrogen. A calibrator ( 1331+305 ) was observed for 5 minutes every hour of the observation for flux and phase calibration purposes. A channel 0 continuum data set was created from the middle 75% of the channels, to find continuum sources. The spectral line data was clipped to 1 Jy to remove interference, and Hanning smoothing was applied in the bandpass calibration. We are in the process of removing the continuum from the Spectral line channels, for hydrogen detection. Initial detections have occurred when using the AIPS task uvsub to subtract the clean ( to 0.15 mJy )  components of the Channel 0 continuum data, and uvlin on the cube to subtract the residual continuum data.

    Observations detect neutral hydrogen at 21 cm because of spin-flip transitions. Spin-flip photons result from the electrons on hydrogen atoms reversing their spin to counter align with the proton's spin, forming a  lower energy state. Both proton and electron spin in one of two directions and act as small bar magnets. When the spins are aligned, the poles of these magnets oppose each other. Counter aligning the magnets puts the atom in a lower energy state which releases a photon with a 21 cm wavelength. Although this photon emission is rare, when there is a large quantity of neutral hydrogen the cumulative emission is large enough to detect. 

Results

    Continuum images show a vast field of radio sources. Many apparent double lobe structures are clearly discernible, and many faint point sources also surface in the image. The depth of the observation may yield unseen objects in these images. Optical overlays show strong support for the x-ray wind in galaxy NGC4921. We are currently cataloging these sources to reference with other radio source catalogues.

Continuum images of NGC4911, show little evidence of the x-ray wind. No traces of the extended Hydrogen were found in the regions of the optical detection.

    We have detected HI in NGC4911 after continuum subtractions. HI has been found in 30 channels and flux peaks of ~1.8 mJy found. Visual studies indicate a high rotational curve. No measurements have been made for this yet. Low level traces of extended structure have been found at levels just above the RMS noise. More studies need to be performed to determine conclusive support for the x-ray wind, yet visual inspections lead us to believe there is x-ray interaction. The fluxes and positions of HI within the galaxy have been overlaid to a DSS optical image. These overlays show higher peaks in the trailing regions of the galaxy, and low level structure to be extending away from the cluster core ( upper right of image ).

Discussion

    Optical evaluations of NGC4911, conclude that it near face-on with little z-axis motion, yet the detection of HI in a highly rotational curve through so many channels suggests that this galaxy is not face-on, and may be at an angle of up to 45 degrees. Unbalanced peaks on both sides of the rotational center support previous work demonstrating the existence of an x-ray wind. The peak on the trailing side of the rotational center is larger and has a greater area. Further studies will reinforce these hypothesis.

    The continuum images developed from the observation field yielded many more objects than anticipated. First radio detections of galaxies are very well possible through further cleaning and analysis. Continuum emissions develop from many types of events so studying the sources through these detections is very unlikely to result in any valuable information, yet the detection alone can be valuable by indicating the presence of emission material in the source.

Conclusion

    These radio observations of NGC4911, made in 1996, make this part of the sky that much more accessable to the astronomical theory builders. Several other groups of astronomers have looked at NGC4911 since then to a lesser depth, and since depth of an observation goes as the square root of observation time, we see no large advantage over these shorter observations. Adding to the views of the sky benefits scientists, if only to confirm working theories. So as a result, not a large amount of scientific knowledge has been collected through this observation, but these data sets have provided much more than neutral hydrogen detection and galactic discovery. They have helped to train me as an undergraduate student of physics, in the use of AIPS, the most valuable software package to radio Astronomy. It has furthered my knowledge of the science of astronomy, and there is one new field of Physics that has been met by this Undergraduate student. I have grown up from the hobby of amateur astronomy to the science of Astrophysics.
 

References

Finley Dave, "Early Radio Astronomy: The Ham Radio Connection." An Introduction to Radio
    Astronomy. http://www.nrao.edu/intro/ham.connection.html. Date accessed: August 16, 1998.

Kaufmann, William J III. Universe. 4th ed. W. H. Freeman and Company, New York. 1994.

Shu, Frank H. The Physical Universe: An introduction to Astronomy. University Science Books, Mill
    Valley, CA. 1982.

Sullivan, W. T. et al. VLA Observations of Unusual H I Distributions for Coma Cluster Spirals. 1996.

The National Radio Astronomy Observatory. Galactic and Extra Galactic Radio Astronomy. ed.
    Verschuur, Gerrit L. and Kellermann, Kennith I. Springer-Verlag New York inc. New York, 1974.

The National Radio Astronomy Observatory. AIPS Cookbook. Associated Universities Inc.
    Charlottesville, VA. 1998.

The National Radio Astronomy Observatory. An Introduction to the NRAO Very Large Array. ed.
    Hjellming, R. M. Associated Universities Inc. Soccoro, NM. 1982.


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