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Galaxy Morphology The Tuning Fork that Blossomed into a Lemon

Galaxy Morphology The Tuning Fork that Blossomed into a Lemon. Lance Simms MASS Talk 9/8/08. Hubble’s Tuning Fork. Tuning Fork Diagram used by Hubble from 1925-1935 Irregular class was later added to right hand side Hubble originally thought evolution was from left to right.

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Galaxy Morphology The Tuning Fork that Blossomed into a Lemon

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  1. Galaxy MorphologyThe Tuning Fork that Blossomed into a Lemon Lance Simms MASS Talk 9/8/08

  2. Hubble’s Tuning Fork • Tuning Fork Diagram used by Hubble from 1925-1935 • Irregular class was later added to right hand side • Hubble originally thought evolution was from left to right Irregulars would fall over here Lenticulars S0 galaxies with large central bulge No spiral arms, gas, or dust Flattened disc of stars Ellipticals – En n=10(1-b/a) b: semi-minor axis a: semi-major axis Bulge/Disc Ratio Loose Arms Gas and Dust

  3. Lemon Classification of Vaucouleurs A=‘Normal’ B=‘Barred’ Image: Mod. Phys Rev, G. De Vucouleurs, Large-Scale Structure and Direction of Rotation in Galaxies

  4. Rotational Velocity Curves • Differential rotation can be observed through spectra • Useful for Spiral Galaxies that are viewed edge-on • Difficult to use for Ellipticals • Overall shift in spectral lines gives velocity and with Hubble Law, approximate distance away Away from us Towards us N II-658.53 nm (in rest frame) Note: Galaxy should be edge-on Image for illustrative purposes Hα-656.28 nm (in rest frame)

  5. Velocity Dispersions Increasing dispersion • Profile width gives velocity dispersion σ • Spectral fitting methods vary • Mass is obtained via the virial theorem • Very useful for elliptical galaxies Virial Theorem K – Kinetic Energy U – Potential Energy α – Constant that depends on distribution of mass within galaxy

  6. Irregular Galaxies • Small percentage of known galaxies are irregulars (~3%) • Galaxies that do not show spiral or elliptical structure • No nuclear bulge • No spiral arms • Divided into two main types • Irr-I : some structure • Irr-II : chaotic mess • Some are Starburst Galaxies • Very high rate of star formation IC 1613 – Cetus Mass range: 108 −1010 solar masses Size range: 1 − >10 kiloparsecs Magnitudes: −13 to −20 in B bandpass Composition: Varied Young Stars HII regions Color: Varied, toward blue IC 10 - Cassiopea

  7. Spiral Galaxies • We think about 66% of galaxies are spirals • Most have active Star Formation (SF) occurring in spiral arms • Appearance depends on angle relative to our line of sight • Consist of 4 Distinct Components • 4 MAIN COMPONENTS OF SPIRAL • 1)Flattened, rotating disc of stars and gas • − Arms are in plane of disc • 2) Central bulge with mainly old stars • − Brightest component of galaxy • 3) Nearly spherical halo of stars • − Globular Clusters • − Dark Matter • 4) Supermassive black hole at center 1 2 4 3

  8. Spiral Galaxies: A Slice of the Lemon A – Normal spiral -- no bar r – internal ring around nucleus -- spiral arms begin on ring s – no internal ring -- spiral arms begin directly at nucleus B – Barred Spiral

  9. Spiral Galaxies Mass range: 109 −1012 solar masses Size range: 5 − >100 kiloparsecs Magnitudes: −16 to −23 in B bandpass Composition: Young and Old Stars • Active Star Formation (SF) occurring in spiral arms is very bright in UV • Young stars emit towards UV • Several types shown below

  10. Spiral Galaxies Our Spiral – The Milky Way • Mapping the Milky Way • In past, mostly done with 2 methods: • Mapping HI regions with radio observations • - 21 cm line measurements • Mapping HII regions via Hα emission lines • - HII regions trace active star formation • Old data showed that there were 4 arms • New data from Spitzer indicates that there • are only 2 major spiral arms: • -Scutum and Perseus Arms 10,000 ly Our Sun

  11. Elliptical Galaxies • Ellipticals appear to have very little gas or dust • Approximately 10% of known galaxies are elliptical • Stars orbit the galaxy center in all different planes • Circular orbital velocity measurements do not work very well • Sometimes a preferred direction of very slow rotation • Luminosity decreases quickly from center so measurements are always made within 10 kpc. • Detailed kinematic observations ( σ(r) and Vsys(r) ) only exist for some 10s of galaxies • Usually limited to σo and Vsys at center Before 1977 Theorists thought they understood ellipticals well in 1970s = axially symmetric isothermal ensembles = increasingly flattened the more rapidly they rotate about center After 1977 Observations proved them wrong = Spectroscopic data (stellar absorption lines) showed that ellipticals do not rotate globally = Not isothermal = Velocity dispersion is anisotropic = Now strong evidence that they are triaxial ellipsoids M32 http://www.astr.ua.edu/

  12. Elliptical Galaxies Mass range: 107 −1013 solar masses Size range: 0.1 − >100 kiloparsecs Smallest: Dwarf Ellipticals Composition: Mostly old, red stars Color: Towards the red end Luminosity Profiles: Hubble’s Law (1930) I is intensity emitted per unit area at r from center a is core radius; Io is intensity per unit area at center De Vaucouleurs’s Law (1948) reis radius containing half of total luminosity Ieis intensity at a distance refrom center M87 –Largest Galaxy in Virgo Cluster

  13. Dwarf Spheroidal Galaxies • Low luminosity galaxies • More spherical than elliptical • Companions to Milky Way or other galaxies such as M31 • Little or no gas or dust • No recent star formation • Approximately spheroidal in shape NGC 147 – Dwarf Spheroidal in Local Group Spheroids: A spheroid is basically an ellipsoid with to of its axes equal Saturn is an oblate spheroid, flattened near equator Equation in 3-d: Oblate Spheroid

  14. Globular Clusters • Large, gravitationally bound groups of stars • 10,000 – 1,000,000 stars • Not galaxies; considered a part of our galaxy • Orbit center of our galaxy in elliptical orbits • Some orbits are highly extended • Some contain “Tidal Tails” • Highly concentrated in Galactic Longitude (337°) NGC 5466 Tidal Tails When globulars pass by bulge of Milky Way, gravity is strong enough to rip stars away Trail of stars left behind is called a Tidal Tail

  15. Dwarf Spheroidal or Globular Cluster? • Distinction between globulars (GCs) and Dwarf Spheroidal Galaxies (dSphs) is ambiguous • Globular clusters are generally more compact, but some dwarf galaxies are also • Small galaxies have about same mass as globulars • Galaxies are more “isolated”, but there are intergalactic ‘tramp’ globulars • Color Magnitude Diagrams (CMD) look similar • As of 2003, there were • ~150 GCs • ~9 dSphs • Now, there are ~20 dSphs

  16. Globulars and DSphs • There is significant overlap in • Mass iii) Luminosity iii) Size • Mass-to-light ratio iv) Spread in Metallicity • Apparently, ellipticity may be a distinguishing factor • only 20 galaxies in plot, 1.4 data points per plot point Taken from van den Bergh

  17. Dwarf Spheroidal or Globular? • Carina Low Surface Brightness (LSB) dSph

  18. Dwarf Spheroidal or Globular? NGC 288 Globular Cluster

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