Lecture 35 – Extragalactic Astronomy Summary so far DistanceWhat you encounter ~1 MpcGalaxies of local group ~ few MpcNeighbor small groups 20 MpcVirgo cluster, p601 Every 50 – 100 MpcOther rich clusters 1000 MpcVoids, filaments, walls > 1000 MpcStill more

More general formula for the redshift Reason: If d = 1000 Mpc, then v = H 0 d gives v = 70,000 km/sec = 0.25 c. More distant galaxies have v > c? What’s up? Figure 24.3

An interlude…Radio Astronomy…will make sense later “To remind you of what you already know….” Radio astronomy = study of universe with EM waves having wavelength from about 1 millimeter to 50 meters Comparison: optical astronomy studies wavelengths between 4E-07 to 7E-07 meters

The Radio Sky Radio sources – many associated with giant elliptical galaxies

Radio Galaxies, e.g. 3C296 Radio galaxies “shine” by a mechanism called synchrotron radiation

What is synchrotron radiation? Accelerated electrons radiate EM waves DEMO

History of Radio Astronomy, circa 1960 Some of the brightest radio sources, 3C48, 3C273, did not seem to be associated with galaxies, but with star-like objects

Spectra of these sources showed highly redshifted lines great distances Z = (w-w 0 )/w 0 = 0.16, 0.48, 1.7, 4.42 !! Quasar = Quasi-Stellar Radio Sources

With Hubble Space Telescope, we have imaged Quasars

Summary of Quasar Characteristics Clearly are a brilliant, energetic phenomenon in centers of galaxies Quasars are very distant. We see them as they were long ago Let’s look at the distribution of quasar redshifts

Question: what does this mean? 7236 quasars

The slide with no name