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ASTC22. Lecture L16 Spiral galaxies - observations

ASTC22. Lecture L16 Spiral galaxies - observations. 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?

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ASTC22. Lecture L16 Spiral galaxies - observations

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  1. ASTC22. Lecture L16 Spiral galaxies - observations 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 R25 and HI 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 Aperture synthesis at VLA Optical image, for comparison: (not to scale)

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

  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: IB = 25 mag/arcsec^2 as practically the lowest possible for measurement, thus defining the maximum extent of the visible light from a galaxy: R25 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.

  8. Why do we use surface brightness (mag/arcsec^2) so much in extragalactic photometry? (e.g., Freeedman law IB(0)=21.7 mag) Because it’s independent of the distance to the object! 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 A lamp We often measure the optical size of a galaxy as the radius R25 at which IB = 25 mag/arcsec^2 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)

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

  10. Diffuse HI gas ejected by supernovae?

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

  12. A polar ring galaxy: evidence of merger

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

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

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