GalaxiesA2290-16 1GalaxiesRelativity and AstrophysicsLecture 16Terry HerterA2290-16 Galaxies 2Outline Discovering Galaxies Classifying Galaxies Hubble (morphological) classification Different types of galaxies Note: Most galaxy photos from Astronomy Picture of the Day (APOD) on the web – you can find them (and credits for images) by searching the name of the object.GalaxiesA2290-16 2A2290-16 Galaxies 3Galaxies A galaxy is a collection of stars, gas and dust along w/ associated starlight, magnetic fields and cosmic rays. Four broad categories based on morphology (appearance): E elliptical S spiral (normal & barred) S0 lenticular I irregularA2290-16 Galaxies 4Ellipticals Galaxies Range from spherical to highly flattened with designations E0 to E7 Contain old stars (Pop II) Very little gas and dust 1-200 kpc in diameter Mostly found in clusters of galaxies Average spectral type: K 106to 1013MsunNGC 4636 (E0/S01)NGC 3377 (E5-6) NGC 3115 (E7/S01)NGC 4406 (E3)NGC 4278 (E1)GalaxiesA2290-16 3M87Giant Elliptical GalaxyA2290-16 Galaxies 6Spiral Galaxies Flattened systems which have a thin disk Display spiral structure Divided into barred (SB) and unbarred (S) spirals Further subdivided into classes a, b, and c; e.g. SBb, Sc, ... where a large nuclear bulge & tightly wound spiral arms c small nuclear bulge & loosely wound spiral arms Young (Pop I) and old (Pop II) stars Copious amounts of gas and dust 5-50 kpc in diameter Found mostly in the “field” (outside clusters of galaxies) Average spectral type: A, F, G, K 109to 1011MsunM33 - ScGalaxiesA2290-16 4Sc: Whirlpool (M51) SABc: Southern Pinwheel (M83)Spiral Galaxy ImagesSBc: M109Sa: M65A2290-16 Galaxies 8Lenticulars (S0 Galaxies) Like spiral galaxies in shape and color but no spiral arms Flattened systems which are morphologically between ellipticals and spirals.M85NGC 5866GalaxiesA2290-16 5A2290-16 Galaxies 9Irregulars By definition, irregular in shape Mostly young stars (Pop I) Lots of gas and dust 1-10 kpc in diameter Found in the field (outside clusters) Average spectral type: A, F 108to 1010MsunM82A2290-16 Galaxies 10Hubble Tuning Fork – For classifying galaxiesEllipticalsE0E7S0Normal SpiralsBarred SpiralsSaSBaSbSBbScSBc The classification scheme is strictly morphological and does not necessarily imply an evolutionary sequence.GalaxiesA2290-16 6Thumbnail Version of Hubble Tuning ForkSource:M65, M77, M84, M101: IPAC Multiwavelength gallery; M49, M89, M91, M109: NOAO Image Gallery;M95: Nial R. Tanvir (through SEDS)M101, ScM77, SbM65, SaM49, E4M84, S0 (E1?)M89, E0M109, SBcM91, SBbM95, SBa/bA2290-16 Galaxies 12Other Types of Galaxies Dwarfs 106to 108stars Peculiar Exploding, Rings, Disrupted Seyfert Very Bright Nucleus Interacting Tidal Effects, Tails (pairs) QSO Collapsed Nuclei?GalaxiesA2290-16 7Dwarf GalaxiesLeo INGC 1313Interacting GalaxiesCartwheel GalaxyThe Antennae (NGC 4038 and NGC 4039)GalaxiesA2290-16 8Interacting GalaxiesArp 104 – NGC 5216 (top right) and NGC 5218Arp 273A2290-16 Galaxies 16The Local Group100 kpcLMCSMCMilky WayM33M31GalaxiesA2290-16 9SMC(Dwarf)50kpcNearby galaxies – SMC47 TucGlobularClusterLMC(Dwarf)50kpcNearby galaxies – LMCGalaxiesA2290-16 10M31(Sb)700kpcNearby galaxies – M31Great galaxy in AndromedaNGC205 (dE5) = M110M32 (cE2)=NGC 221A2290-16 Galaxies 20Notes on Classifying Galaxies Classifying of galaxies by their morphology is a bit problematic Appearance can depend on exposure time, wavelength, and angular resolution. Thus can change with redshift – even if no intrinsic evolution Classification Requirements Homogeneous data set Unique classification Flexible and unambiguous notation Unfortunately these requirements are often violated Can get ambiguities and inconsistencies Some examples of how galaxy morphology can change with exposure time and wavelength are given in the next slide.GalaxiesA2290-16 11A2290-16 Galaxies 21Cen A: Two Different ScalesThe image of the peculiar galaxy Cen A (NGC 5128) but with different image display scales. How would YOU classify this galaxy? The same effect could be achieved with different exposure times. [Image is IIIaJ, 4680A from the Digital Sky Survey (DSS) via NASA Extragalactic Database (NED).]Cen A: Two Different WavelengthsThe image of Cen A (NGC 5128) but with a two different wavelengths. Both images are from APOD (Astronomy Picture of the Day) Left image: antwrp.gsfc.nasa.gov/apod/ap030806.html Right image: antwrp.gsfc.nasa.gov/apod/ap040624.htmlGalaxiesA2290-16 12Centaurus A (Peculiar Galaxy)Nearest Radio GalaxyCentaurus A (Peculiar Galaxy)In the InfraredGalaxiesA2290-16 13HI + cont.8.0 um3.6 umB Color visualM81 at different wavelengths0.23 um (yellow), 0.16 um (blue)24 um70 um160 umM81: H-alpha emissionV image from NED H-alpha in blue, V in red + greenGalaxiesA2290-16 14M81: Blue lightColor enhanced image using B & V bands. Color mix is V=red, B=green, B/V=blue.Ratio of B to V imagesA2290-16 Galaxies 28Galaxy Classification Systems Hubble System (of Classification) Introduced in by Edwin Hubble in 1936 book, The Realm of the Nebulae Definitive expose by Alan Sandage in 1961, The Hubble Atlas of Galaxies A number of other classification systems exist De Vaucouleurs’ Extension of the Hubble System, 1959 Yerkes System (W. W. Morgon 1958, 1959, 1970) David Dunlap Observatory (DDO) System (S. van den Bergh 1960, 1976) Note – these system are based on visual
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