1. Why do we need interferometry? Measuring the gas distribution and rotation in disk galaxies: radio observations with interferometer arrays and aperture synthesis Why is surface brightness independent of distance? What is the relation between R 25 and H I radius? Radial velocity observations: spider diagrams, evidence of mergers, polar rings

2. Diffraction limit

3. The angular resolution of a 2-element interferometer

4. VLA = Very Large Array, NM

5. A typical radio-map at 20cm Optical image, for comparison : (not to scale) Aperture synthesis at VLA

6. VLBA = Very Long Baseline Array Resolution ~0”.01 at wavelength 21cm

7. NGC 7331 - how far out can we trace the visible light from this galaxy? Until it is a tiny fraction of the sky brightness. Surface brightness of the moonless sky at B band is 22.7 mag/arcsec^2 and grows at I-band to 19.9 mag/arcsec^2 Therefore, we choose the following surface brightness in blue light: I B = 25 mag/arcsec^2 as practically the lowest possible for measurement, thus defining the maximum extent of the visible light from a galaxy: R 25 At radio wavelengths, the sky is much darker than in the visible, so we can see the atomic hydrogen further than the stars in any galaxy.

9. Why do we use surface brightness (mag/arcsec^2) so much in extragalactic photometry? ( e.g., Freeedman law I B ( 0 ) =21.7 mag ) Because it’s independent of the distance to the object! D=1 D=2 D=4 D=8 A=8 A=4 A=2 A=1 I=1 I =1 I=1 I=1 (try to see that all the lamps in the park seem equally bright, only gradually smaller, as long as they form extended images on the retina) L=1 (luminosity) R=1 (physical radius) D= distance Observed total flux = L/(4 pi D^2) A= angular diameter ~ 1/D I = surface brightness ~ L/(4 pi D^2)*A^2 ~D^0 We often measure the optical size of a galaxy as the radius R 25 at which I B = 25 mag/arcsec^2 A lamp

10. Gas is seen further than the stars in any given galaxy

11. Diffuse HI gas ejected by supernovae?

13. Evidence of past galaxy mergers How a slit spectrum of such a galaxy may look like: space VrVr

14. A polar ring galaxy: evidence of merger

15. Theoretical spider diagram for a simple rotation curve (axisymmetric disk assumed): set of isovelocity curves in spatially resolved Doppler maps

16. H I Sample spider diagram (can you see which side is approaching and which receding?)