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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. Hubble Tuning Fork diagram (Hubble 1936) Ellipticals Lenticular (S0) Spiral and Barred Spiral Irregular Types of Galaxies = Spirals Spirals are classified according to how tightly or loosely wound the arms are. The brightness of the central nucleus is correlated to the tightness of the arm. The galaxies M 104 (below) and M 51 (right) respectively show tightly and loosely wounds. Notice the effects of dust in both galaxies. M31 - The Great Spiral Galaxy in Andromeda This is the nearest galaxy to the Milky Way which is 2.5 million light years away. The Nuclear Bulge Young stars have formed along the foreground spiral arm. M31’s two satellite galaxies M32 and NGC 205, both dwarf elliptical galaxies, are in the bottom center and upper right. The Outer Disk of M31 Central Region of a Spiral Galaxy Spiral Galaxies •Disk + spiral arms + bulge (usually) Barred Spiral Galaxies The spiral galaxies M 91 (left) and M 109 (right) have bars across their nuclei from which spiral arms unwind. Galaxies rotate such that the spiral arms trail behind in the rotation. The Milky Way is thought to be a barred spiral galaxy. Barred Spiral Galaxies •Contain a linear feature of nearly uniform brightness centered on nucleus Types of Galaxies: Ellipticals Elliptical galaxies have no spiral arms, little dust and contain very few old stars. The elliptical galaxies M 32 (below) and M 110 (right) show varying degrees of ellipticity. Elliptical Galaxies •Smooth structure and symmetric, elliptical contours Types of Galaxies: Irregulars -Irregular galaxies lack any specific form and contain stars, gas and dust generally associated with a young galaxy. -The irregular galaxy at right is the Large Magellanic Cloud, a satellite of the Milky Way located about 180,000 light years from the sun. - The bright reddish feature in the upper right is the “Tarantula Nebula” a region of star formation in the LMC. (NOAO/AURA Photo) Dwarf Irregular Galaxy in Sagittarius Hubble Space Telescope Image Irregular Galaxies NGC 4485-Irr II M82-Irr II Irr I •No symmetry •Lots of young, blue stars and interstellar material •Smaller than most spirals and elliptical galaxies •Two major subtypes: •spiral-like but without defined arms, show bright knots with O,B stars •unsymmetrical with dust lanes and gas filaments, explosive Schematic Diagram of Revised Hubble Classification Cross section of diagram No Bar Ring shaped Spiral shaped Bar Revised Classification System Properties of Galaxies Property Spirals Ellipticals Irregulars Mass/M of Sun 109 to 4x1011 105 to 1013 108 to 3x1010 Luminosity/L of Sun 108 to 2x1010 3x105 to 1011 107 to 3x109 3x103 to 7x105 3x103 to 3x104 Diameter (light years) 16x103 to 8x105 %-age of galaxies 77% 20% 3% National Optical Astronomy Observatory images From this table, you should take note of which galaxies are the most and least massive, most and least luminous, and largest and smallest in size. Rotation of Galaxies – The Missing Mass Problem The Doppler Effect permits us to measure the speed of material orbiting around the center of a galaxy. If matter were really concentrated in this fashion, we would see “rotation curves” following the “expected” path in the diagram at right. Rotation curves tend to remain high as far out as they can be measured. Observed Expected Distance from galaxy center This implies the existence of massive halos of dark matter in galaxies. The nature of the material comprising this dark matter is completely unknown at present, making this one of the greatest problems of contemporary astronomy. Clusters of Galaxies Rather than occurring individually in space, galaxies are grouped in clusters ranging in size from a few dozens to thousands of galaxies. The Coma Cluster, shown at right, is 300 million light years from the Milky Way and contains more than 1,000 (and possibly as many as 10,000) galaxies. The Milky Way is a member of a small cluster called the Local Group which contains about 40 galaxies. The largest member of the Local Group is M 31, with the Milky Way coming in second in size. (NOAO/AURA Photo) Gravitational Lensing in Abell 2218 Cluster As predicted by Einstein’s General Theory of Relativity, a compact intervening object is bending and distorting light from individual members of this cluster so that we see a halo effect. Hubble Space Telescope Image Galaxies in Collision In this close encounter between two spiral galaxies, their arms are dramatically warped and massive star formation is triggered when the hydrogen gas clouds in the two collide. It is believed the Milky Way may have “cannibalized” small galaxies in the past through collision. Hubble Space Telescope Image The Disrupted Galaxy NGC 5128 Active Galaxies The galaxy NGC 7742 is an otherwise normal spiral galaxy except for its extraordinarily bright nucleus that outshines the rest of the galaxy. Such galaxies, i.e. spirals with extremely bright nuclei, form a class of active galaxies known as Seyfert galaxies. Hubble Space Telescope Image Active Galaxies The elliptical galaxy M87, shown below in a wide-field ground-based image, has a very bright, point-like nucleus from which a jet of material emanates. The jet is seen in great detail from an HST image at right. Hubble Space Telescope Image Active Galaxies NGC 4139 Mkn 205 This image shows the spiral galaxy NGC 4319 and the quasar Markarian 205. The distance to NGC is 80 million light years, which Mkn 205 is 14 times farther away at a distance of 1 billion light year. The very distant quasar is nearly as bright as the much closer galaxy. The extraordinary brightness of quasars, which is a blending of the term quasi-stellar radio source, indicates that some incredibly powerful mechanism must be producing enormous amounts of energy from a small volume of space. Hubble Space Telescope Image A Lensed Quasar An intervening galaxy between us and this distant quasar is causing light from the quasar to be bent along curved paths that give rise to an Einstein cross, a phenomenon predicted by Einstein’s General Theory of Relativity. National Optical Astronomy Observatories Image Active Galaxies = The Central Engine Narrow-line region (NLR) scattering region Broad-line region (BLR) Supermassive black hole +accretion disk torus radio jet Diagram from Mike Crenshaw