NormalGals - 2/9/10 Normal Galaxies Chapter 17 Galaxy...

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2/9/10 1 Normal Galaxies Chapter 17 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. Hubble Tuning Fork diagram (Hubble 1936) Ellipticals Lenticular (S0) Spiral and Barred Spiral Irregular
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2/9/10 2 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. MW is between a Sb and Sc 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 Spiral Galaxies Comprise about 2/3rds of bright galaxies Grand Design Spiral - well defined spiral structure Flocculent - less organized spiral design Spirals clearly contain much gas and dust Most starlight is from young, blue stars - ongoing star formation Sizes - radius = 10 to 30 kpc Masses - M = 10 7 to 10 11 M sun
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2/9/10 3 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) Types of Elliptical Galaxies Giant Ellipticals - few Mpc across, 1 trillion stars Dwarf Ellipticals (dE) - as small as 1 kpc across, 1 million stars Ellipticals: • contain little or no gas (only 1% of mass in ISM) • no current star formation • mostly older, redder stars - like the halo of our galaxy • Unlike halo stars, metal abundances of these stars are high (about twice solar metallicity) NGC 205 Leo I dSph Dwarf Spheroidal - Classification dSph Very low stellar density and surface brightness S0 Galaxies (Lenticulars) •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 but some contain dust and gas •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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2/9/10 4 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: smooth but with dust lanes and gas filaments (e.g. M82) - explosive M82-Irr II NGC 4485-Irr II Irr I Limitations of the Hubble Classification Scheme 1. Only includes massive galaxies (doesn’t include dwarf spheroidals, dwarf irregulars, blue compact dwarfs) 2. Three different parameters for classifying spirals is unsatisfactory because the parameters are not perfectly correlated.
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This note was uploaded on 02/13/2012 for the course AST 3019 taught by Professor Sarajedini during the Spring '08 term at University of Florida.

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NormalGals - 2/9/10 Normal Galaxies Chapter 17 Galaxy...

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