Radio Observations of Galaxy NGC4911 in the Coma Cluster
Faculty Mentor: Michael Gregg Ph.D.
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
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.
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.
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 ).
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
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.
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
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