Previous studies have been made on the relationship between local density and morphological type (Dressler 1980), showing that a higher density environment will have a greater percentage of elliptical galaxies than a lower density environment. Four different sections of the Coma Cluster were observed in the V filter to a limiting magnitude of about 22 mag/arcsec2 over the course of two non-photometric nights. Galaxies were distinguised from stars by looking at the FWHMs of various objects in each image. The FWHMs of stars are significantly lower than those of galaxies. Galaxies were classified into two types - elliptical and spiral. The morphology density relation was determined by examining the number of galaxies in each field, and then looking at the fractional part of each of the morphological types in each field. The data does appear to show a correlation between local density and morphological type.
An explanation for the relationship between galaxy morphology and cluster type (regular or irregular) is still being investigated. The traditional model explained by Peebles (1970) holds that the collapse and virialization of the cluster affects galaxy morphology. In Peebles' model, a cluster undergoes a period of free-fall collapse, until the density is sufficiently high that violent relaxation (virialization) occurs. When virialization occurs, spiral galaxies are swept of their gas to form the S0 galaxies which are seen at the centers of regular clusters.
This hypothesis of virialization affecting galaxy morphology has been investigated by Dressler (1980), and no conclusive link has been found between the morphologies of galaxies within a certain cluster type. Dressler has instead proposed that the link between morphology lies within the local density of the region of the cluster that a galaxy inhabits. Dressler has proposed that the more dense a region of the cluster is, the more E and S0 galaxies will be present, and the less dense a region is, the more Sp galaxies will be present. The purpose of this work is to show that local density does indeed affect the morphologies of the galaxies in a specific region.
Data were taken in the V filter on two non-photometric nights, 2004 May 16 and 2004 May 22, with the SBIG ST-8 CCD and the 0.25 meter GOT. Four different regions of the Coma Cluster were imaged over the two nights, with approximate coordinates (J2000) of: (1) RA=12:59:40, Dec=28:01:00, (2) RA=13:02:50, Dec=27:55:30, (3) RA=12:58:40, Dec=28:10:00, (4) unknown. Region (1) is the center of the cluster, region (2) is approximately 35 arsec directly to the West of the cluster center, and region (3) is approximately 22 arcsec NE of the cluster center. Region 4 is unknown because the coordinates of the frame cannot be found. The CAT on the telescope listed that the coordinates were RA=12:52.0, Dec=27:19, but when the image was compared with an image of those coordinates, it was clear that the CAT did not give the exactly correct coordinates.
Two 600s exposures were taken in each region, with the exception of region (1), as the image aborted after 429s. The images in each region were co-added to give total times of 1200s (and 1029s in the case of Region 1). On the first night, 10 flats in the V filter, 22 zeros (0.11s exposures), and 3x600s darks were taken. On the second night, 9 flats in V, 20 zeros, and 4x600s darks were taken.
The CCD camera was kept at a temperature of -14 degrees Celsius the first night, and -7 degrees Celsius the second night. The readnoise of the ST-8 CCD camera is 11.8 electrons RMS, the gain is 2.9 electrons/ADU, and the plate scale is 0.7 arcsec/pixel.
Data reduction was done with the Image Reduction Analysis Facility (IRAF). In both nights (but separate for each night), the zero frames were combined and then used to zero-correct the flat-field and dark images. The flat-field images were then dark corrected using the zero corrected dark. The 429s object frame was also dark and zero corrected using the zero corrected dark. The 600s object frames were dark and zero corrected using only the 600s darks. After careful examination, the flat fields were combined, and the object frames were flat field corrected.
Cosmic ray removal was not preformed on the object frames because the xzap function removed more than only cosmic rays from the image. The object images were registered using xregister task in IRAF, and then co-added for an increased signal to noise ratio.
Galaxies were distinguished from stars by looking at the point spread functions of various stars and galaxies in each of the fields. The FWHM for stars was about 8 pixels in Regions 1 and 2, and about 6 pixels in Region 3. Objects in Regions 1 and 2 which had FWHMs of greater than 12 pixels were taken to be galaxies. Objects in Region 3 which had FWHMs of greater than 9 pixles were considered galaxies. The cutoff point for Regions 1 and 2 is differnt from Region 3 because the data were taken on different nights with different seeing conditions. The fourth region was not used because all of the objects in that image appeared to be stars, all having FWHM of about 6 pixels.
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Figure 1. Histogram of FWHMs of all measured objects in Region 1. Objects that are stars have FWHMs of 9 pixles or less. Objects that are galaxies have FWHMs of greater than 12 pixles. |
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Figure 2. Histogram of FWHMs of all measured objects in Region 2. Objects that are stars have FWHMs of 9 pixles or less. Objects that are galaxies have FWHMs of greater than 12 pixles. |
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Figure 3. Histogram of FWHMs of all measured objects in Region 3. Objects that are stars have FWHMs of 6 pixles or less. Objects that are galaxies have FWHMs of greater than 9 pixles. |
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Figure 4. Histogram of FWHMs of all measured objects in Region 1. Objects that are stars have FWHMs of 6 pixles or less. All but one of the objects in this region have FWHMs of less than the stated 6 pixels. |
In each of the three co-added images that were used, the galaxy morphologies were visually identified. Only two morphology distinctions were used, namely Elliptical or Spiral, and only the brightest galaxies in each field were looked at. Ellipticals were identified by their diffuse edges, lack of structure, and generally round shape. Spirals were identified by their bright central bulge, quickly tapering edges, internal structure, and elongation (if they were seen nearly edge on).
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Figure 5. Elliptical galaxies in Region 1. |
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Figure 6. Spiral galaxy in Region 1. |
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Figure 7. Elliptical galaxies in Region 2. |
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Figure 8. Spiral galaxy in Region 2. |
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Figure 9. Elliptical galaxies in Region 3. |
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Figure 10. Spiral galaxies in Region 3. |
Dressler (1980) held that the local density of a region of a cluster affected the morphologies of the galaxies within that local region. By looking at the fraction of ellipticals/spirals to the total number of galaxies, and log of the number density of galaxies in those same-sized regions, the relationship between local density and galaxy morphology becomes evident.
Local galaxy density was calculated by counting the number of galaxies in each field of a given area whose morphology was able to be identified. Then each galaxy in that field was identified as either a spiral or an elliptical.
Figure 11.
Graph of the Morphology Density Relation |
Figure 12. Graph of the Morphology Radius Relation |
The results found by Dressler (1980) show that as the local density of a region increases, the fractional number of elliptical galaxies increases, and the fractional number of spiral galaxies decreases.
From this data, it appears that the local density of a cluster has a more fundamental relationship to galaxy morphology than does the radius from the center of the cluster of a given area. From the M-D graph, the relationship between galaxy density and galaxy morphology is evident, i.e. that with increasing density the fractional number of elliptical galaxies increases and the fractional number of spiral galaxies decreases. The M-R graph appears to show no relationship between raduis from cluster center and galaxy morphology. This result might be enhanced with greater coverage of the Coma Cluster.