Astro 301/ Fall 2006(50405)Introduction to Astronomyhttp://www.as.utexas.edu/~sj/a301-fa06Instructor: Professor Shardha JogeeTAs: Bi-Qing For, Candace Gray, Irina MarinovaLecture 20: Tu Nov 14Types of GalaxiesLecture 20: Announcements1) Tue Nov 21 : Quiz 5 will be held and homework 4 will be given out in class2) Last call for extra creditBi-Qing will hold certification sessions for Painter Hall this Tue and Th. 3) Exam 3 will be held on Dec 7. There will be absolutely no make up exam under ANY circumstances.Recent and Upcoming topics in classGalaxies Galaxy Types How do we trace visible components in galaxies?How do we trace dark matter components in galaxies?Galaxy Interactions and Galaxy Mergers in the Present Day Is our own Galaxy Interacting? Starburst galaxies and active galaxiesHow did galaxies form and evolve over 13 billion years1) They have a disk component (shaped like a saucer). In the center of the disk, there is sometimes a spheroidal bulge (a 3-D `melon-shaped’ component). 2) They contain up to 1012stars and large amounts of gas, dust, ongoing star formation .3) Most spiral galaxies are barred, meaning that their disk contains an elongated stellar feature called a bar. Our Milky Way is a barred spiral. Bars carry gas from the disk to the center of a spiral galaxy, thus influencing its evolution.Spiral GalaxiesStrongly Barred spiral (SBbc) NGC 1300Unbarred spiral (SAab) NGC 4622Milky Way = a barred spiral galaxy, hosting our Sun and Solar systemFace-on view (Artist’s conception)Edge-on view (Artist’s conception)Edge-on view :Actual infrared image from COBE satelliteGiant elliptical M871)They are spheroidal systems (3-D, `water melon’ shape) and do not have extended disk components. Contain up to up to 1012stars. 2) They have a smooth appearance as they are mostly made of old stars, andhave little gas, dust, and recent star formation Elliptical GalaxiesWhat is a galaxy?A galaxy is made of visible components (stars, gas, dust) that emit light at differentwavelengths and of dark matter that emits no light at all .It contains a few times (108 to 1012) stars, which are orbiting a common center and are bound by the force of gravity exerted by the galaxy components.Galaxies exhibit larger diversity and can be classified according to different criteria. If we classify them according to their structure, sizes, total amounts of gas and star formation, we get the following types: Spiral galaxies Elliptical galaxies Irregular galaxies Dwarf galaxies Peculiar/Interacting galaxiesTypes of Galaxies1) They have irregular, peculiar morphologies in terms of gas, dust and star formation.2) They are low mass gas-rich systems.Typically contain up to a few x 109stars 3) Two of the three closest galaxy neighbors of the Milky Way, the LMC and SMC, are Irr galaxiesLMC; Irr; 30,000 ly across SMC; Irr ;18,000 ly acrossIrregular GalaxiesLeo I, dwarf elliptical1) They are much smaller than spirals or ellipticals, but may be comparable to Irr galaxies. Their optical radius is typically less than 15,000 lyr while that of spirals is greater than 50,000 lyr. 2) They typically contain up to a few x 108stars 3) They come in two types : dwarf ellipticals and dwarf irregulars Dwarf GalaxiesGalaxies which look peculiar and distorted. These distortions are often caused by interactions with other galaxies. Polar ring galaxyNGC 4650Ring galaxy AM 0644-741 50,000 ly acrossCartwheel galaxy Head-on collisionPeculiar/Interacting GalaxiesHow do we trace visible components (stars, dust, gas) in galaxies?Tracing visible components (stars, dust, gas) of a galaxy?Gamma rays 10-16m X rays 10-12m Ultraviolet 3 x 10-7mOptical 4 to 9 x 10-7m = Violet, blue, green, yellow, orange, redInfrared 10-6m to 10-4m Radio 10-3m to 10 m Visible components (stars, gas, dust) of galaxies emit electromagnetic radiationUltraviolet image Optical lightNear infrared imageStars of different mass traced by ultraviolet, optical, near-infrared continuum lightUltraviolet λ = 1 x 10-7m Very massive (M>10Mo) stars Optical λ = (3 to 7) x 10-7m Intermediate to low mass (5 to 0.8 Mo stars)Near infrared λ = 1x10-6m Lowest mass (~0.3 Mo) starTracing visible components (stars) of a galaxyHow do we know that light at these wavelengths trace these type of stars?The continuum emission of a source depends on surface temperatureWien’s lawThe continuum emission of a star or blackbody peaks at a wavelength λpeakthat depends inversely on its surface temperature Tλpeak= W/ T , where W = Wien’s constant = 2.9 x 10-3 m KRecall from last lectureKirchoff’s first lawAny hot solid, liquid or opaque gas emits light (as a continuum spectrum). In a hot object, atoms are moving randomly (vibrating). Vibrating electrons in the atoms emit vibrating electric fields, aka light!Wavelength of peak emission Surface Temperature of emitting source--------------------------------------------------------------------------------X rays 3 x 10-10m Ultraviolet 1 x 10-7mOptical blue= 3 x 10-7m Optical yellow=5 x 10-7m Optical red= 7 x 10-7m Near infrared 1x10-6m Mid-infrared 3x10-5m Far-infrared 1x10-4m Recall from last lecture We used Wien’s law to calculate the temperature of the source which emits most of its light at the wavelengths. λpeak= W/ TWavelength of peak emission Surface Temperature of emitting source--------------------------------------------------------------------------------X rays 3 x 10-10m 107K Ultraviolet 1 x 10-7m 30,000 KOptical blue= 3 x 10-7m 10,000 KOptical yellow=5 x 10-7m 6,000 KOptical red= 7 x 10-7m 4,300 KNear infrared 1x10-6m 3,000 KMid-infrared 3x10-5m 100 KFar-infrared 1x10-4m 30 K Recall from last lecture We used Wien’s law to calculate the temperature of the source which emits most of its light at the wavelengths. λpeak= W/ TUV trace sources at 30,000K. these are high mass (>8 M o) stars Blue, yellow, red light trace sources at 10,000 K, 6000 K, 4300 K these are
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