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Galaxy Detection within the RATS project
Michael Campbell
Zone of Avoidance Our solar system lies about two thirds, or 25,000 light years, away from the galactic centre of the Milky Way. Shown below is not the Milky Way but rather M77, a similar galaxy. Of course no external images of the Milky Way exist Z s we l Avoidance. As we lie within the Milky Way there exists the " aone ofie within it. The Zone of avoidance is the band around the sky in which very few extragalactic objects can be seen at because of the heavy absorption of light by dust in the plane of the milky way. The Galaxy Threshold Value After much trial and error it was discovered that the vast majority of object with a star class less than 0.35 were galaxies and the vast majority of objects above 0.35 were stars. I called this the Galaxy Threshold Value. Right are two histogram showing how the object number varied across the star classes. As suggested earlier this method of galaxy detection is far from perfect. Not every object of star class beneath 0.35 is a galaxy. Stars which are light years apart in the Z plane but occupy similar X and Y coordinates will overlap in the image and produce an irregular shape. SExtractor will identify this object in the image though it's stellar nature is apparent t Verifica n eye Catalogueo the humation . Immediately it is obvious how object number varies greatly from field to field. It was this that made some of the fields too dense to be of any practical use. The problem lies in the anomalous results recorded by S Extractor. All results of value 0.35 and below had to be checked by hand and removed from the catalogue if human error as with ever object the chance of misjudgement and the deletion of a galaxy increases. incorrect. This also introduced The catalogue is the list of object information, including celestial coordinates, that SExtractor produces after object detection.

The Great Attractor

Calculations prove that within the Zone of Avoidance, about 200250 million light years in the direction of the Centaurus constellation, there exists a localized concentration of mass around 5.4 x 10^16 times that of the Sun. This gravitational source is called "The Great Attractor". It has not been detected yet by visible wavelengths due to it lying within the Zone of Avoidance. RATS Armagh Observatory leads a project called RATS ­ short for Rapid Temporal Survey. This survey has collected many images of the sky at low galactic latitude (close to and within this Zone of Avoidance) and it was my goal to try and identify the galaxies in these images, or fields, and then test if any of these galaxies are located in clusters. Several fields of varying quality and density were chosen and I ran each one through a program called SExtractor. (One such field is shown on the left.) SExtractor identifies every light source, and hence every object, present in the image and classifies via a number of different methods. One of these methods is Star Class. A star is a point source and theoretically should appear as a perfect circle in the field, a galaxy however contains may stars and are of varying shapes, hence they do not appear as a perfect circle. S Extractor quantifies the sphericality of each object with 1 being a perfect circle and 0 anything but.


Individual Galaxy Identification

The program I used to view and edit the fields was a Graphical Astronomy and Image Analysis tool (GAIA for short). GAIA itself contains 17 different catalogues of objects from previous published Sky surveys carried out in various institutes. Each of the fields used had the celestial (space) coordinates embedded into it so GAIA knew the actual positions of the objects in my newly created catalogues. I decided, more so out of personal interest, to cross reference my catalogues with every published catalogue already present on GAIA and ascertain which galaxies had been previously recorded and which hadn't. This again involved a great deal of actual time as the only way to cross reference was to label every object in every catalogue and see which images within my catalogue where only labelled the once. I soon discovered that the majority of objects within my catalogues where not mentioned in any of these previous surveys. This may be due to two reasons. Either the lack of astronomical interest in surveying the Zone of Avoidance has left these relatively obvious galaxies unchecked, or the already quite flawed method of using Star Class and ergo sphericality to identify galaxies is extremely inaccurate and none of these bodies are actually galaxies.