The Milky Way and Its Neighbors Lance Simms MASS Talk 9/8/08
Hubble’s Tuning Fork Tuning Fork Diagram used by Hubble from 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 Ellipticals – En n=10(1-b/a) b: semi-minor axis a: semi-major axis Bulge/Disc Ratio Loose Arms Gas and Dust
Lemon Classification of Vaucouleurs Image: Mod. Phys Rev, G. De Vucouleurs, Large-Scale Structure and Direction of Rotation in Galaxies
A Note About Velocities Position of spectral lines gives systemic velocity of the galaxy, V sys Profile width gives velocity dispersion σ – In general we say larger σ indicates larger mass
A Note About Velocities Profile width gives velocity dispersion σ – Spectral fitting methods vary Mass is obtained via the virial theorem Increasing dispersion
Spiral Galaxies
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 V sys (r) ) only exist for some 10s of galaxies Usually limited to σ o and V sys at center M32 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
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