ASTR 200 : Lecture 26. Gravitational Lensing

Transcription

1 ASTR 200 : Lecture 26 Gravitational Lensing 1

2 Announcements HW 7 due Thursday Nov 15. No office hours today No office hour next Monday (UBC closed) No class on Monday Nov 12 (UBC closed for holiday in lieu of Remembrance day. Homework Graphs 2

3 If mass produces acceleration, light could be lensed GMm GM Remember that m a= a= 2 2 r r and so in Newtonian physics, acceleration is independent of mass of the object being accelerated (Galileo with cannonballs and grapes) Even in the 18th century, based on Newton's belief that light was made of particles, people realized that if light skimmed past the Sun, there would be an angular deflection, with an angle a of 0.85 The effect would be to make the star appear to shift away from the Sun Of course, the issue is that it is difficult to see background stars behind the Sun... 3

4 But, during a solar eclipse... -But in 19th century there was no way to make the measurement. - Couldn't look through a telescope and see 1 deviation The bending falls off if the light path is further from the Sun 4

5 General Relativity made a DIFFERENT prediction GR predicts a deflection angle a 1 4 GM b c2 where b is the impact parameter the distance between the photon and the center of the mass at closest approach along the path (see diagram) For the Sun, a skimming path is deflected by ~1.7'', which is twice the Newtonian expectation This was a 1915 prediction of GR that was different than Newtonian physics, and so testing it (which had now become possible due to the invention of photographic plates) was the subject of an expedition to a solar eclipse in Africa in b

6 GR : 1 Newtonian : 0 6 This was the only detection of gravitational lensing for a long time...

7 The general approach: The lens equation 7

8 The general approach: The lens equation (Must have dimensions of length squared) 8 DS

9 The Einstein radius Suppose source precisely behind the lens. r=0 and then r'=re and the source appears moved by re. Because configuration symmetric, we see a ring of re angular radius 4 GM D LS θe = DS = c 2 DL Ds θe 9 DS

10 An Einstein Ring 10 The central 'lensing' elliptical galaxy is lensing a background galaxy - Requires nearly perfect alignment of the background object, thus rare

11 A collection of Einstein rings 11

12 x Amplification in Lensing Lensing magnifies/amplifies the brightness of background sources because more total light is delivered to the observer Imagine placing the background object at various places along a line (above) with a given y=b. As a function of x, you get various brightenings (right) 12 `horizontal' x

13 Multiple images in gravitational lensing In the realistic case an object is lensed into a finite number of images, with different amplifications and offsets The number of images, their amplification, and their position actually probes the mass distribution of the lensing distribution So, this allows one to: (1) see, via amplification, very distant sources (2) probe mass (of lens) using lensing physics (independent of dynamics) 13

14 Gravitational lensing by clusters of galaxies These arcs are commonly seen when looking through massive galaxy clusters 14

15 Gravitational lensing by clusters of galaxies One can model the amount and distribution of mass in the cluster using the image shapes This probe has no dependence on velocities, etc 15

16 Other types of lensing What we have discussed today is all what is called 'strong' gravitational lensing We have also discussed : Micro Lensing, where there is no resolved image of the lensed object, there is only amplification 16 (review on next slide) And there is also 'weak lensing' (subsequent slides)

17 brighter 17 Microlensing Previously discussed when taking about the halo Can also be used to detect exoplanets around the (invisible) lens (see prior lecture). Here the lens is not seen, and there is only 1 image However, one uses the fact that the geometry of the lens versus source is CHANGING due to relative motion, and one is detecting the changing amplification of the background source in intensity This has been used to study populations statistics in the Milky Way halo

18 Weak lensing When light passes a through a low-mass system, a circular cross section has its shape only very slightly affected The lens may be essentially invisible If there were only one lensed object, almost nothing can be learned from such weak lensing 18

19 Weak lensing, on a large number of background sources This is done statistically using the observed shape of large numbers of galaxies affected by some foreground mass distribution The are correlations between galaxies on the sky due to the presence of mass in the foreground. This allows detection of mass even if the lens emits no light! 19

20 Abell 502 : The Bullet Cluster 20 A massive cluster which is actually two clusters post a collision

21 Abell 502 : The Bullet Cluster The Chandra X-ray telescope image shows that the two clusters have passed through each other, leaving especially hot stripped gas between them 21

22 Abell 502 : can be studies by weak lensing However, most of the mass will be in dark matter. This can be detected via its weak lensing on background galaxies. Where does it say the dark matter is? (see next slide) 22

23 Abell 502 : Dark matter stays with the galaxies The weak lensing reconstructed mass distribution (contours) shows that the dark matter `stays with the galaxies' and is not distributed like the hot gas This shows that the dark matter interacts only gravitationally, and not like the gas 23

24 Moral: lensing is kewl The Cheshire Cat lens. Will disappear over the next Gyr as the two elliptical lens galaxies merge 24