VV785, AM0644-741, Graham's Ring Galaxy, The Lindsay-Shapley Ring

Below is a composite picture taken at an Mt John University Observatories in Tekapo, New Zealand (220k jpeg).


The above image is a combination of 75 minutes of 8 V-, R-, & I-band frames taken on an f/13.5, 24" B&C reflector from Mt. John Universities Observatory, located near Lake Tekapo in the central part of the South Island of New Zealand. This target was imaged during an October 1995 observing run after the program targets dropped below the bottom of the slit. The camera was a Compuscope 1600 (1.5kx1k) CCD purchased with money granted by the Lotteries Commission to a former Canterbury student, Tom Love.

Even in poor seeing conditions (~4"), structure in the rare, amazing ring is discernable in the aftermath of this galactic-scale collision. This 'ring galaxy' is at least 70 times more distant than our nearest big neighbor, the large spiral M31 in Andromeda.

This southern hemisphere ring galaxy, AM0644-741, has a doppler redshift of approximately 6500 km/s, which puts it somewhere between 200 & 400 million light-years distant. The field of view is approximately 5.9'x 3.9'. We see the ring obliquely and it's long axis is on the order of 100,000 light-years across.

The ring is a byproduct of a close interaction by the off-centre galaxy with another unidentified galaxy in the group. There is still some controversy whether the ring was caused by an expanding wave of star formation spreading outward through the dark material surrounding the core galaxy, or a gravitational effect produced by a nearly dead-center pass-through encounter of two galaxies. In the pass-through gravitational separation model, the outer region of stars in one of the galaxies (probably in this case, the bright off-centre one) is left behind as the galactic nucleus is shorn away by the gravitational "tug" from the encounter with the other galaxy.

Computer simulations show the torus of outer material left behind expanding slowly outward like a smoke-ring. Measurements of the thickness and diameter of the torus are useful data which can be combined with estimates of the encounter velocities, masses, and mass distributions when modelling galactic interactions.

Color Balanced 56k jpeg.

The integrated visual magnitude of AM0644-741 is in the neighborhood of 11, which puts the faintest objects visible in these pictures at around 18. Note also inside of the main ring a fainter second ring structure appears traceable over about 90 degrees of arc and may be part of a double-ring. Near the top of the bright, outer ring, a short linear-feature extending roughly perpendicular to the ring can be made out. In the nuclear area of the galaxy within the ring, the nucleus appears elongated, and a fainter 3rd star-like component is discernable.

There are many ways to attempt color balancing & correction, and the picture above is another example. The first image on this page was simply a default production from the Compuscope ProCntrl software supplied with the camera with no attempt to compensate for the different spectral responses. This second image is an attempt to adjust for that effect by using a background-level-equalization technique. For both pictures, the center of the color spectrum is redder than the human eye's center of spectral response by about 200nm, and response in the V-filter is less efficient than in either R- or I-band.

To build these color renditions, the green V-filter formed the "Blue" component of the image, R-filter served as "Green", and I-band as "Red". As a result, both images tend to 'undersample' hot (bluer) stars than cool (redder) ones. This second color jpeg shows a composition after the signal from the V-filter images was multiplied by a factor to bring the 'background sky' signal-strength up to the level of that of the R- & I-filter. This only partly compensates for the lack of color information in the blue portion of the spectrum.

What do the colors of the objects in the picture imply about what is going on in the galaxy? The 'yellowy' halo about the off-centre nucleus of the ring galaxy is likely composed of redder stars, while the color-neutral nucleus doesn't appear distinctly different in color than either of the ring 'impactor' candidates at bottom center and top right of the picture. The candidate at the top of the picture shows a faint diffuse bridge of material between it and the ring galaxy.

The color differences have implications about the evolutionary history of the stars and nebulae making up the ring galaxy. Overall, the galaxy appears color neutral and forms a 'baseline character' of the galaxy that the color imbalanced portions of the galaxy can be interpreted with respect to. Without spectra, it is chancey giving a confident interpretation of what is occuring, but a number of interesting questions arise in any case.

The faint inner halo of yellowy stars is discernable within the inner portion of the lower ring. Overall, the ring appears color neutral in the brighter areas with a bluish tint in the fainter region. Could the bluer areas be that of enhanced star formation with comparitively young stars? If so then why are the bright regions within the ring apparently color neutral? Where did the material in these bright knots come from?

Do the yellowy areas around the nucleus represent 'redder' stars, and if so, are they primarily a population of red giants or red dwarves? If they were giant stars at the end of the evolutionary sequence, then their formation could be the result of the galactic impact and their appearance is a brief phenomenon. Many galaxies have inner halos of stars that are made up largely of cooler dwarf stars and one might expect this to be the case for this island of stars as well. Note also that the candidate impactor galaxy in the upper right corner of the picture has a faint yellowy cast to its halo.

These images were taken during times when the prime targets of the observing were below the dome slit (can't take data on fun objects until work is done). In addition to the interesting pictures of the ring galaxy, we attempted to perform asteroid photometry.

Multiwavelength wavelength data can give a picture of the region around the galaxy, showing aspects of its general environment.

There have been many astronomers who have studied this and related ring galaxy systems. A very good source for information about collisional ring galaxy formation can be found in Appleton and Struck-Marcell's publication from Fundamentals of Cosmic Physics that is in html format on the web.

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