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The Cores of Galaxies in the Coma Cluster

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The Cores of Galaxies in the Coma Cluster The cores of galaxies in the Coma cluster Rob ert Minchin April Contents Intro duction Historical Background This pro ject Data Reduction Image pro cessing The images ray removal Cosmic Ellipse tting Galphot ellipt IRAF ellipse Checking the ellipse ts Dusty galaxies Data calibration Other data Analysis of data Graphs from the ellipse ts Isophote shap es Images Power laws Fitting p ower laws Information from the t Graphs from the p owerlaw ts analysis of p owerlaw ts Statistical Results The Results Dierences b etween the two regions vs M v Central region Halo region r vs b r r vs M b lim v Break radius vs Eective radius Comparison with previous results M vs v r vs b Diskiness Dening a Core Conclusions vs M v M v vs r b r r vs M b lim v Diskiness Summary A Errors in the ellipses Notes on Galaxies B C Dieren tials of the Nuker Law programmes D Computer Recip es routines D Numerical D My routines D Pro jectc D Funcsc List of Tables Semima jor axis ts Average radius ts Other galaxy data List of Figures Fab er et al results for r vs M b v Data from ellipsetting to NGC Nukerlaw t to NGC Data from the averageradius t Distribution of Nukerlaw t to NGC Plots of against M for the central and halo regions v and m r vs r v b e comparison to Fab er et als data cos terms for disky galaxies Nukerlaw t to NGC Abstract I have analyzed HST images of galaxies selected from the central and halo regions of the Coma cluster A nukerlaw t to the proles has b een found and the relationships b etween the parameters of the nukerlaw and b etween the total luminosity and isophote shap e has b een investigated with half of the galaxies in the sample Problems were encountered with dust in the galaxies suering some degree of dust p ollution Where this was severe enough to aect the prole the galaxy was excluded from the data set The dierences b etween the central and halo regions have b een investigated It is predicted that coretyp e galaxies will form preferrentially in the central region This predicted relationship is not proven to exist although there is some evidence for it Due to a dierence in magnitudes b etween the two samples it is necessary for a relationship b etween and luminosity to exist in order to show preferrential core formation in the central region It is not p ossible to say that this relationship is denately seen in the data Comparison of the data with previous results shows that the lack of cores seen in lessluminous galaxies could b e an eect of resolution This would create an articial trend of with luminosity and an apparent dichotomy as only the steep outer p owerlaw would b e resolved in galaxies with smaller cores giving a large value of This trend and the values of higher than could b e a M galaxy selection eect Disky galaxies are seen to b e able to form cores and a bright v is seen with no resolved core Chapter Intro duction Historical Background Originally cores were dened as the areas of constant surface brightness in the centre of the King mo del This was shown by King to b e a reasonable t to the central regions of elliptical galaxies Later CCD imaging showed that the King mo del did not describ e adaquately the surface brightness proles of elliptical galaxies the central regions were not to the limit of resolution areas of constant surface brightness The regions now known as cores are regions of low logarithmic slop e sep erated from the steep er outer logarithmic slop e by a sharp break The inner logarithmic slop e do es not go to zero as predicted by the King mo del Lauer et al at the limit of resolution Imaging of the cores of elliptical galaxies with the Hubble Space Telescop e has allowed them to b e examined at previously unattainable resolutions The galaxies app ear to divide into two typ es Jae et al Larger galaxies generally have proles with well dened cores describ ed by a double p owerlaw whilst smaller galaxies can b e describ ed by a single p owerlaw and have no well Kormendy et al dened core Bosch et al It is known that larger galaxies are triaxial systems supp orted by random motions and there fore having b oxy isophotes while smaller galaxies are rotationally supp orted with disky isophotes Davies et al This division into two sorts of elliptical galaxies app ears to correlate well with the division into those with and without unresolved cores Kormendy et al Jae et al Lauer et al tted a parameter double p owerlaw to the inner proles of galaxies This er law ts b oth coretyp e and p owerlaw galaxies well law known as the Nuk r r b I r I b r r b The parameters tted by this law are the inner logarithmic slop e the outer logarithmic the sharpness of the break b etween the two slop es the radius of the break r and slop e b the intensity at the break I In terms of dening a core is the most imp ortant as it shows b the slop e of the surfacebrightness prole as it tends towards zero It can b e seen that for r r b I r r r I r r and for r b The results of tting this law to a numb er of elliptical galaxies and to the bulges of spiral galaxies are given in Fab er et al This classies galaxies as having cores if and if the break is wellresolved The claim is also made that there is a divide b etween the coretyp e galaxies and the p owerlaw galaxies which have The coretyp e galaxies are large triaxial systems with M and the p owerlaw galaxies are small rotationally supp orted systems v with M v Fab er et al nd that there is a clear division b etween the core and p owerlaw proles see g from Fab er et al which can b e seen on the graph of r vs M and also as a gap in the b v distribution of b etween and Figure also shows the division found into b oxy and disky galaxies Figure Fab er et al results for r vs M b v This pro ject Bender Burstein and Fab er prop osed that the controlling factor in the evolution of a galaxy is the amount of gas present during formation and mergers If there is a large amount of gas present then most of the energy is dissipated and the galaxy forms a small rotationally supp orted disky system Larger galaxies are formed later by mergers of these smaller galaxies when most of the gas has formed into stars These stellar mergers do not dissipate energy and so form triaxial systems continuum GS continuum Fab er et al puts forward This theorem is called the gasstellar this theorem as explaining why disky galaxies show p owerlaws as the gas in dissipative mergers is carried towards the centre of the galaxies The Coma cluster provides a go o d test area for this theory as the central region consists of a dynamically relaxed system where the galaxies have interacted frequently and have little gas and the halo region consists of relatively gasrich galaxies which have not yet visited the core and have had little chance to interact The cluster is rich enough to b e able to form a statistically signicant cluster has shown that it is sample of b oth regions and pro jection of HST data from the Virgo p ossible to distinguish the two kinds of proles at the.
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