Lecture 10am 19 April 2005

Transcription

1 ASTR2050 Spring 2005 Lecture 10am 19 April 2005 Please turn in your homework now! In this class we will cover: Hubble s Law: Review of Friday s lab exercise Active galaxies Quasars Very distant objects: Gravitational lensing Exam #3 will be in class next Tuesday! 1

2 Hubble s Law See Friday (15 April) Studio and Kutner Section 18.3 The universe is undergoing uniform expansion. This is clear because the more distant a galaxy is from us, the faster it seems to recede from us. The extent to which the expansion is uniform is proven by a linear relationship between distance and velocity. This linear relationship is called Hubble s Law. The proportionality constant is the Hubble Constant H 0. 2

3 Today After some time After more time... and it doesn t matter what galaxy you re in! 3

4 Hubble Space Telescope Key Project W. Freedman, et al, Ap.J. 533(2001)47-72 H 0 = 72 ± 8 (km/sec)/mpc The hard part is getting good measurements of the distance to galaxies that are very far away. This group used many different measurement techniques to climb the distance ladder. 4

5 Active Galaxies A general class of abnormal galaxies with different types. Starburst Galaxies Radio Galaxies Seyfert Galaxies BL Lacertae objects 5

6 Starburst Galaxies: Intense Star Formation The Prototype: M82 Combined image with purple showing ionized hydrogen. True color image 6

7 Radio Galaxies Very large energy output, including strong radio signal. Spectrum typical of synchrotron radiation Our model for the core: Intensity (log scale) Normal galaxy Frequency Wavelength 7

8 Example: M87 also known as Virgo A Long exposure 40 kpc Short exposure jet 8

9 Extended Radio Galaxies Cygnus A: VLA Optical Hercules A: 3C465 9

10 3C279 Faster than light(?) Light years 10 See Kutner Figure This VLBA image shows a blob moving with apparent speed greater than c. However, the distance scale is found from the Hubble constant and optical redshift. This led some people to question Hubble s Law and the reliability of determining large distances.

11 Probable Resolution: Relativistic Effects O x θ y r Towards observer P Source moves with speed v from O to P Light from P has less distance to travel to observer than light from O so the apparent O P time is less. t app = t true x c = r v r cos! v app = y t app = or 11 r sin! c (r/v)(1 "cos!) v app = vsin! 1 "cos!

12 True v/c: Apparent v/c Emission angle! 12

13 Seyfert Galaxies Spiral galaxies with strong emission lines and a bright nucleus. Example: NGC 1566 Example: NGC See Kutner Figure 19.14

14 BL Lacertae Objects Strong variability, implying a small source size BL Lac Fuzzy 14

15 Quasars Fascinating mystery in the 1960 s but now we are pretty sure they are just active galaxies on steroids. Originally observed as strong radio sources with no optical counterpart. Finally, lunar occultation studies pinned down a few and associated them with stars. We now understand them as exceptionally bright active galaxies with exceptionally large redshifts. That means they are very, very luminous and very, very far away. 15 Sloan Survey: Z=4.16 QSO

16 A prototypical quasar: 3C273 3C273 Lab 16 Optical (HST) X-Ray (Chandra) Radio (Merlin) Optically star-like, a very strong radio source, a jet, and an enormous redshift!

17 HST Images of the host galaxies around the QSO s Spikes with fuzzy edges 17

18 Large Distances: Gravitational Lensing 18