Active Galaxies, Distant Galaxies, and The Expanding Universe

There are several kinds of active galaxy...

• most are variable, just look at their light curves:

  

• Seyfert galaxies: spiral galaxy with a bright nucleus, spectrum with emission lines, also radio/X-ray/IR source

  

• radio galaxies: giant/supergiant elliptical with double-lobed radio structure, jets

  

  

• BL Lacertae (BL Lac) objects: elliptical with bright nucleus, spectrum with featureless continuum

  

• quasars (quasi-stellar radio sources) are furthest galaxies known

  

  • intrinsically very bright, 100 trillion times the Sun, 1000x a galaxy!

  • what is their power source? an accreting supermassive black hole?
    • clues...

      • 2x variable over several weeks, month ---> brightness cannot vary faster than light travel time (if varies over one week, then diameter < 1 light-week) ---> all that light comes from small region!

      • 1000x light of galaxy in small volume ---> observed energy is many times larger than could be produced by stellar fusion ---> acceleration of infalling material ---> accretion disk about massive central object!

      • spectra have strong, broad emission lines ---> particles moving at high speeds ---> massive central object!

        

      • jets of hot, fast particles ---> black hole is particle accelerator due to intense magnetic, gravitational fields ---> particles escape from poles

      • for L = 10⁴⁷ erg/s, M_BH > 10⁹ solar masses (else radiation pressure too great); to produce this L, BH must swallow more than 10 solar masses/yr (could build one in 10⁶-10⁹ years)

    • quasars evolve with time?

      

    • and what about black hole mass vs. bulge mass relationship?

Distant Galaxies

• about 2000 galaxies, about 4 billion times fainter than can see by eye!

• ---> over whole sky, 40 billion galaxies! 10²⁰ stars!

• many are at least 12 billion light-years away ---> 90% look-back time ---> observe light soon after galaxy formed (a time machine!)

How do we measure distances to the furthest galaxies?

Note how the absorption lines associated with the element calcium in older stars shift to redder wavelengths as a galaxy's distance increases...

• how are redshifts made?
  • galaxies are stuck to points in the expanding space

  • as the Universe expands (stretches), those points move away from one another

  • light emitted from the galaxies gets stretched as it travels through the expanding space

    

  • stretching light increases its wavelength, so features at any wavelength in a galaxy's spectrum move to redder wavelengths

  • observed wavelength / emitted wavelength = 1 + z, where z is the redshift

• Hubble, after demonstrating that some ``nebulae'' are actually other galaxies, started measuring their distances and redshifts

  Edwin Hubble (again!)

  

  • Hubble's constant H₀ (conversion between redshift [or recession "velocity"] and distance), or recession ``velocity" = H₀ x distance

  • so from a redshift, you can estimate a distance! -- the larger the redshift, the greater the distance (distance = recession ``velocity'' / H₀)

  • immediate consequences of Hubble Law

    • Universe is expanding

      

    • there is no center to that expansion

    • Universe was denser in past

    • if expansion has been going on long enough, there was a time when distance between galaxies was zero ---> Universe has finite age

    • if galaxies do not accelerate or deccelerate, then age is t₀ = 1/H₀ = 1/70 km s^-1 Mpc^-1 = 14 Gyr (close to actual best estimates of age)

How do we measure distances to nearby galaxies (and calibrate the Hubble relation)? Extragalactic Distance Ladder...

• review: nearest stars (< 1 kpc): parallax, Cepheids (standard candle), novae (std candle), RR Lyrae (std candle), moving cluster method, main-sequence fitting

• Large Magellanic Cloud, Local Group, Virgo Cluster (< 20 Mpc): Cepheids and ...
  • supernova light echo method in SN1987a
    • ring of previously ejected material photoionized a week later by UV light from explosion

    • angular radius = theta, delta t = week (light travel time to physical radius R) = R/c = theta D/c, where D = LMC distance

    • therefore, D = 50 kpc (pretty close to other measures!)

      

• galaxies within < 100 Mpc: distances using locally calibrated measures from Virgo and closer
  • Tully-Fisher relation: luminosity proportional to v_c^a, where v_c is the asymptotic circular velocity at large radii on a flat rotation curve and "a" depends on bandpass

    

  • Faber-Jackson relation: similar relation except for ellipticals, except velocity dispersion (measured range of random velocities) used instead of circular velocity

  • globular clusters and planetary nebulae have well-defined luminosity distribution peaks (std candle)

  • also, surface brightness fluctuation method:

    

    • light from small angular area of galaxy produced by N stars

    • adjacent small area will have similar number of stars, but with Poisson fluctuations (variations) of N^1/2

    • relative fluctuations in surface brightness over many small regions will then go like 1/N^1/2

    • number of stars/unit angular area depends on distance (more stars in unit area if galaxy further away): N goes like D²

    • thus, measured relative fluctuations in surface brightness go like 1/D, where the proportionality constant is obtained from calibration of nearby galaxies

• galaxies out to ~1 Gpc: supernovae (type Ia), whose luminosities at time of maximum brightness are about the same (std candle) -- evidence that Universe is accelerating!!!

  

(thanks to Dan Maoz for above)

If you measure enough redshifts, you can make a map of the local Universe...

The Cosmological Principle

• homogeneity : uniform, evenly distributed on large scales

• isotropy: the universe looks the same in every direction, no observation could find edge or center

Does the Universe go on forever?

• Olber's paradox -- or, why is the sky dark at night?

  

  • in an infinite Universe filled with galaxies, every line of sight intercepts a galaxy

  • similar to an infinitely large forest filled with trees -- every line of sight eventually ends in tree, making it impossible to see the edge

  • resolution: age of galaxies and speed of light are finite

  • cannot see galaxies beyond about 12 billion light-years because they hadn't formed yet