Lecture2 - Galaxy Classification In 1924, Edwin Hubble...

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Galaxy Classification In 1924, Edwin Hubble divided galaxies into different “classes” based on their appearance. Why begin here? •Hubble classification serves as the basic  language of the field. •The morphological sequence reflects a  fundamental physical and evolutionary  sequence, which offers important clues to  galactic structure, formation and evolution.
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Hubble Tuning Fork diagram   (Hubble 1936) Ellipticals Lenticular (S0) Spiral and Barred Spiral Irregular
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Spiral Galaxies •Disk + spiral arms + bulge (usually) •Subtype a b c defined by 3 criteria: •Bulge/disk luminosity ratio •Sa: B/D>1   Sc: B/D<0.2 •Spiral pitch angle •Sa: tightly wound arms  Sc: loosely wound arms •Degree of resolution into knots, HII regions, etc.
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Barred Spiral Galaxies •Contain a linear feature of nearly uniform brightness centered on nucleus •Subclasses follow those of spirals with subtypes a b and c
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Elliptical Galaxies •Smooth structure and symmetric, elliptical contours •Subtype E0 - E7 defined by flattening •En where n = 10(a-b)/a where a and b are the projected major and minor axes  (doesn’t tell what the 3-D shape is)
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Lenticulars or S0 Galaxies •Smooth, central brightness concentration (bulge similar to E) surrounded by a  large region of less steeply declining brightness (similar to a disk) •No spiral arm structure •Originally thought to be transition objects between Sa and E but typical S0 is  1-2 mags fainter than typical Sa, E    (van den Bergh 1998)
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Irregular Galaxies •No morphological symmetry •Lots of young, blue stars and interstellar material •Smaller than most spirals and elliptical galaxies •Two major subtypes: •Irr I: spiral-like but without defined arms, show bright knots with O,B stars •Irr II: unsymmetrical with dust lanes and gas filaments (e.g. M82) - explosive M82-Irr II NGC 4485-Irr II Irr I
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Decreasing Bulge/Disk Decreasing stellar age Increasing fractional gas content Increasing ongoing star formation Limitations of the Hubble Classification Scheme 1. Only includes massive galaxies (doesn’t include dwarf spheroidals,  dwarf irregulars, blue compact dwarfs) 1. Three different parameters for classifying spirals is unsatisfactory  because the parameters are not perfectly correlated. 1.
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Lecture2 - Galaxy Classification In 1924, Edwin Hubble...

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