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The Milky Way and Its Neighbors

The Milky Way and Its Neighbors. 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. Ellipticals – E n n=10(1-b/a)

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The Milky Way and Its Neighbors

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  1. The Milky Way and Its Neighbors 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 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 Image: Mod. Phys Rev, G. De Vucouleurs, Large-Scale Structure and Direction of Rotation in Galaxies

  4. A Note About Velocities • Position of spectral lines gives systemic velocity of the galaxy, Vsys • Profile width gives velocity dispersion σ • In general we say larger σ indicates larger mass

  5. A Note About Velocities Increasing dispersion • Profile width gives velocity dispersion σ • Spectral fitting methods vary • Mass is obtained via the virial theorem

  6. Spiral Galaxies

  7. Elliptical Galaxies • Ellipticals appear to have very little gas or dust • 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 σoand 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/

  8. Elliptical or Globular Cluster? • 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

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