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# lec33 - Lecture 33 Galaxies Black Hole and The Hubble Law...

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Lecture 33 Galaxies: Black Hole and The Hubble Law Purdue University, Astronomy 364 1 Lecture 33

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Reminder The final exam will be on May 3 at 1-3pm in Rm 203 (PHYS). Details have been posted on the course web page. The exam will cover Chapters 17-22. Issues of conflicts or special accommodations must be resolved by this Friday (April 22). No accommodation will be made after that. Lecture 32 Purdue University, Astronomy 364 2
Elliptical Galaxies The potential energy of an elliptical galaxy can be written as U = α GM 2 r h M: total mass of all star in the galaxy r h: half-mass radius of the galaxy : density-profile dependent factor, ~0.4 The kinetic energy can be written as K = 1 2 M v 2 Virial Theorem: K = U 2 M = v 2 r h α G Lecture 33 3 Purdue University, Astronomy 364

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Velocity Dispersion Lecture 33 Purdue University, Astronomy 364 4 Unfortunately, the mean square velocity is not a measurable quantity for an elliptical galaxy. It can, however, be estimated from the velocity dispersion of the stars along the line of sight, i.e., radial velocity dispersion, which is measurable. Radial velocity dispersion is, however, a luminosity- weighted quantity, as opposed to mass-weighted. If we ignore the difference and assume that velocity dispersion is isotropic, we then have v 2 = 3 σ 2 The mass of an elliptical galaxy is then M = 3 σ 2 r h α G
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lec33 - Lecture 33 Galaxies Black Hole and The Hubble Law...

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