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Elliptical Galaxies: Structure, Stellar Content, and Evolution

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Elliptical Galaxies: Structure, Stellar Content, and Evolution Thesis by Michael Andrew Pahre In Partial Ful¯llment of the Requirements for the Degree of Doctor of Philosophy California Institute of Technology Pasadena, California 1998 (Submitted February 3, 1998) ii c 1998 ° Michael Andrew Pahre All Rights Reserved iii Abstract A near{infrared imaging survey of 341 nearby early{type galaxies, combined with optical imaging and spectroscopic data from the literature, are used to construct the global scaling relations for this population of galaxies. These data demonstrate a number of important features of the early{type galaxy sequence: (1) the slope of the Fundamental Plane (FP) correlations systematically increases with wavelength; (2) the slope of these FP correlations deviates from the virial expectation at all wavelengths, implying a breakdown of either or both of the assumptions of constant mass{to{light ratio and homology; (3) the intrinsic scatter of the FP correlations is small but resolved at all wavelengths, implying a small cosmic scatter of early{type galaxy properties at any position in the galaxy sequence and contradicting any model in which various stellar populations parameters \conspire" with each other to keep the correlations thin at optical wavelengths; (4) there is no correla- tion among residuals of the metallicity{independent near{infrared FP and the metallicity{ sensitive Mg2{σ0 relation, implying that both age and metallicity variations contribute to the cosmic scatter of both correlations; and (5) the e®ective radii systematically decrease with increasing wavelength, fully consistent with the general presence of stellar populations gradients in early{type galaxies. A comprehensive and self{consistent model is described which simultaneously explains these and other global properties of the early{type galaxy sequence. This model demonstrates that age, metallicity, deviations from a dynamically homologous family, and populations gradients are all contributing to the form of the global scaling relations; the latter two e®ects are the least constrained by the observations and model, respectively. Additional constraints on the role of age in the global scaling relations is obtained by observing early{type galaxies at higher redshifts. This has been studied using three approaches: color evolution, evolution of the slope and intercept of the K{band FP cor- relations, and evolution of the absorption line strengths. The galaxies in each cluster are identi¯ed in a systematic way using two color (three bandpass) imaging|sampling ap- proximately the U, V , and I bandpasses in the rest frame|to eliminate late{type cluster member and general ¯eld interloper galaxies. This method is > 90% e®ective in identifying iv early{type galaxies at the target cluster redshift. Weak color evolution in rest{frame (U V ) has been detected in 26 rich clusters of ¡ 0 galaxies. The bluing trend in color is 0:05 0:03 mag at z = 0:2, 0:12 0:04 mag at z = 0:4, § § and 0:23 0:05 mag at z = 0:54. Using stellar population synthesis models from the » § literature, this color evolution is fully consistent with the galaxy population as a whole having formed at 1 < zf < 5. The FP correlations are studied for 128 galaxies in eight rich clusters at 0:1 < z < 0:6 using moderate dispersion spectroscopy (110 of the measurements are new) and imaging in the near{infrared K{band. These data more than quadruple the data in the literature which can be used to study the FP at high redshift. The near{infrared FP is constructed at high redshifts for the ¯rst time. The intercept of the FP on the surface brightness axis is observed to dim with redshift, as expected for the Tolman signal in an expanding world model. A small amount of luminosity evolution ¢K 2:5 log(1+z) mag is detected on top ¼ ¡ of the Tolman signal, which is consistent with the passive evolution of a stellar population that formed at high redshift. The slope of the near{infrared FP is observed to flatten with redshift, implying that the least luminous galaxies are evolving faster than the luminous galaxies. This is strong evidence that low luminosity ellipticals have a stellar content that is up to a factor of two times younger than high luminosity ellipticals, but age spreads much larger than this appear to be excluded, and age spreads somewhat smaller than this are still allowed depending on the choice of nearby galaxy sample used in the comparison. The absorption line strengths of the galaxies are observed to evolve slowly with redshift: the Mg and Fe indices weaken while the H¯ index strengthens. The latter e®ect in 2 h i G particular is another strong indication that the mean age of the stellar populations in early{type galaxies formed at redshifts 3 < zf < 5. All of these properties are moderately consistent with the models derived above based on the samples of nearby galaxies, implying that age is an important physical parameter underlying the global scaling relations for early{type galaxies. v Acknowledgments For a thesis like this one, in which a large quantity of data is collected from many sources, it is di±cult to fully acknowledge every person who has made a signi¯cant contribution along the way towards this ¯nal product. Professor George Djorgovski provided timely advice, wonderful encouragement, marvelous amounts of money for traveling to meetings and observing runs, tall back scratcher drinks on the coast of Hawaii, large quantities of powdered drugs from Colombia (the dark brown variety), observing time galore, and many a not{so{gentle shove to ¯nish some of the many projects we started. Above all, he showed patience in allowing me to run amok designing the various surveys reported here. Many thanks also go to that special woman for her moderating influence. I have spent many a night, cloudy and clear, shooting the breeze and sharing a mug of java with the night assistants of the Palomar and Las Campanas Observatories and the Observing Assistants of the Keck Observatories. The e±cient professionalism of the sta®s of these observatories has made the collection of all those ExaByte tapes proceed smoothly and resulted in beautiful data. The cooks of Palomar and Las Campanas Observatories are thanked for making even a cloudy run pleasurable. The directors of these observatories, and the Time Allocation Committees, have my highest gratitude for granting large blocks of observing time to pursue this thesis project. I pray that in the end they will ¯nd the product well worth the considerable investment. The year spent among the Santa Barbarians of Carnegie building a near{infrared camera was memorable due to their collective enthusiasm and remarkable scienti¯c insights. I now hold an even greater respect for the wide{ranging abilities possessed by the instrumentation teams at OCIW, and wish them great luck on their Magellanic expedition. It was a joy learning ¯rst{hand from Drs. Eric Persson, David Murphy, and Anand Sivaramakrishnan the right way to build an instrument. Professor Jeremy Mould deserves special appreciation for getting me started in infrared observing and surface brightness fluctuations, and for his super{human patience and persis- tence in our continued collaborations. Many of the things I know about elliptical galaxies stem directly from the untold numbers of conversations I have had over the years with Dr. vi Reinaldo de Carvalho. I have also learned a great deal through collaborations, or exten- sive discussions, with many other members of the Caltech astronomy community: Profes- sors Roger Blandford, Judy Cohen, Shri Kulkarni, Keith Matthews, Mark Metzger, Gerry Neugebauer, Tom Soifer, and Chuck Steidel; post{doctoral researchers John Blakeslee, Chris Clemens, Laura Ferrarese, and Ian Smail; and graduate students Chris Fassnacht, John Gizis, David Hogg, and James Larkin. Laura Ferrarese also read an early version of several chapters of this thesis and provided insightful comments. Many o±ce mates have shown patience with the ever{expanding masses of papers, folders, and tapes which occupy the surfaces of my desks. Martin Shepherd has proved to be a highly valuable source for all sorts of \nawk{ing." Finally, my thesis committee|George Djorgovski (chair), Roger Blandford, Andrew Lange, Eric Persson, Steve Shectman, and Chuck Steidel|have shown great en- couragement and o®ered excellent advice, comments, and six unique perspectives on this ambitious project. My family has shown great support and encouragement not only while I have spun deeper and faster into this black hole, but also all along my various routes which eventually led to Caltech. And Suzanne, the most complicated and fascinating person I have ever had the pleasure to know, to you I owe gratitude for your patience with my crazy work and travel habits, your unbridled enthusiasm for science and me, your excellent proofreading ability, and your constant love and a®ection. Ah yes, and many thanks to the support from Mieza Ekyii, Nyame Bekyereh, and Ko¯ Mieza. vii Contents Abstract iii Acknowledgments v 1 Introduction 1 1.1 The Standard Paradigm for Elliptical Galaxies . 1 1.2 The Fundamental Plane Correlations . 2 1.3 Outline of Thesis . 4 References . 5 2 Near{Infrared Photometry of Early{Type Galaxies in the Local Uni- verse: Global Photometric Parameters 7 2.1 Introduction . 8 2.2 Description of the Near{Infrared Imaging Survey . 11 2.3 Observations and Data Reduction . 17 2.3.1 Palomar 60{inch Telescope Data . 17 2.3.2 Las Campanas 1.0 m Swope Telescope Data . 18 2.3.3 Las Campanas 2.5 m du Pont Telescope Data . 19 2.3.4 Data Reduction . 19 2.3.5 Photometric Calibration . 20 2.4 Surface Photometry . 23 2.5 Seeing Corrections . 25 2.6 Measurement of Global Photometric Parameters .
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