Giant Elliptical Galaxies Sometimes many galaxies collide and merge Large masses mean collisions will be high speed Gas will get heated very hot Giant galaxy becomes an elliptical Giant Elliptical Q. 96: Seeing The Past

Looking Out = Looking Back Galaxies in the Past Smaller than modern galaxies Irregulars are more common Why were they different? Galaxies collided a lot in the past Galaxies got bigger from mergers Light travels at one light-year per year If you look at very distant galaxies, you are seeing them as they were, not as they are 1 kly = 1000 years 1 Mly = 1 million years 1 Gly = 1 billion years You can see back almost to the beginning of the Universe

Galaxies Long Ago

Galaxies Long, Long Ago

What’s an Active Galaxy? Active Galaxies What’s an Active Galaxy? Most of the power of ordinary galaxies come from the stars Some galaxies have very bright sources right at the center Can be as bright as a galaxy, or even brighter Called Active Galactic Nuclei (AGN’s) They can be incredibly bright, up to 1015 LSun Their power comes out with different spectra Visible Radio Some of them vary their power in a day or less This proves they are very, very small! Infrared Ultraviolet X-rays

Fast  Small A large source will not – can not – change all at once Roar from a stadium crowd Light (and other EM radiation) travels at c If it changes in a time t it must be no larger than d = ct If it changes in a day, its size is no bigger than

Types of AGN’s Seyfert Galaxies Relatively dim as AGN’s go Spiral galaxies with lots of gas in plane of galaxy Radio Galaxies Produce enormous amounts of radio energy Often, the radio emission is mostly not from the nucleus Quasars Similar to Seyferts, but much brighter Radio Noisy Quasars Blazars / BL Lacartae Objects Visible and radio signal can vary in as little as an hour

Seyfert Galaxies

Core of a Seyfert Galaxy

Quasars Other Galaxies Quasar Foreground Star

Quasars

Centaurus A, a Radio Galaxy Visible Composite visible / radio

Radio Galaxy in Radio

Black Hole in M87 Galaxy in Radio Image released April 10, 2019 First image of black hole 6.5 billion solar masses

Radio Galaxy in Visible and Radio

What Causes an AGN? Black hole in center 104 – 1010 MSun Size of Earth up to size of Solar system Source of gas - a gas torus (doughnut) Gas getting dumped into the center An accretion disk of gas falling in Rotating very fast Friction makes it hot – X-rays Very efficient – 50% mass  energy Thin gas surrounding the center Heated by X-rays from the accretion disk Sometimes, jets shooting out

Jets and Lobes Magnetic fields trapped in gases can explode outwards Gases swept along Flung away from AGN very fast Beams light forward Beams radio waves mostly forward

Core of an Active Galaxy

Unified Picture of Active Galaxies Some are brighter, some are dimmer What you see depends on what angle you see it from Blazar Seyfert or Quasar or Radio Quasar Radio Galaxy

Active Galaxies Used to be More Common Active galaxies, especially bright ones, are rare now But common in the past Why? Active galaxies require fuel to be fed into the black hole Colliding galaxies allow gas to flow to center Galaxy collisions were much more common in the past

The Cosmic Distance Ladder Methods for Measuring Distance Radar Distances Parallax Spectroscopic Parallax Main Sequence Fitting Cepheid Variable Stars White Dwarf Supernovae Hubble’s Law Geometric Methods Standard Candles

Radar Distance 0 - few AU Earth Venus d 2d = ct, solve for d Radar distances We know what an AU is Effectively no error

nearest stars several ly away Parallax The farther apart you put your “two eyes”, the better you can judge distance The smaller p is, the farther away the star is. 1AU – 1000 ly d p in arc-seconds p (The distance 3.26 ly is also known as a parallax second) parsec nearest stars several ly away Q. 97 Which Distance Method For Galaxies?

Standard Candles Radar Distances Parallax Spectroscopic Parallax Main Sequence Fitting Cepheid Variable Stars White Dwarf Supernovae Hubble’s Law A standard candle is any object that is consistently the same luminosity Like 100 W light bulbs, or G2 main sequence stars How the technique works: Figure out how luminous your standard candles are If you know distance d and brightness B, you can figure this out from: L = 4d2B To find the distance to another of the same class: It should have the same luminosity L Measure its brightness B Deduce distance from: L = 4d2B

Spectroscopic Parallax Has nothing to do with parallax Works only on main sequence stars How it works: Observe the star – determine it’s brightness B Measure its spectral type from spectrum Deduce its luminosity from the Hertzsprung-Russell Diagram Find its distance from: L = 4d2B

Spectroscopic Parallax 10 ly – 200 kly Limitations: The main sequence is a band, not a line Because stars are different ages Causes significant error Main sequence stars are not the most luminous stars You can’t measure it if you can’t see it Limits maximum distance