Galaxies With a touch of cosmology

Types of Galaxies Spiral Elliptical Irregular

Spiral Galaxies

Disk component – where the spiral arms are – Interstellar medium – Star formation Spheroidal component – Bulge – central part of galaxy – Halo – where the oldest stars are located Make up 75-80% of the largest galaxies in the Universe

Features of Spiral Galaxies Rings Bars Spiral Arm Type – Grand design – well defines spiral arms – Flocculent – patchy and discontinuous arms – Lenticular – disk with no arms Bulge size

Elliptical Galaxies

Only have spheroidal component – Sphericity (definitely not a word) varies Little to no star formation – Composed mainly of low mass stars Huge range in masses – Dwarf ellipticals can be about 10 7 M Sun – Giant ellipticals can be about M Sun

Irregular Galaxies

Catch-all for everything that is not either a spiral or elliptical galaxy Two basic types: – Type I: closely related to spiral galaxies, but their structure is less organized – Type II: structure is highly chaotic and typically are gravitationally interacting with another galaxy Lots of star formation More common are large distances

Hubble Classification

Determining Distance

Distance Ladder RADAR – bounce radio waves off objects and measure travel time Parallax – measure apparent movement of object due to Earth’s orbit MS fitting – convert apparent magnitudes of cluster stars into absolute magnitudes using theoretical models Standard candles – objects that have the same absolute magnitude – Cepheids, SNIa Hubble Law – use distance dependence of Universal expansion rate

RADAR Radio waves are bounced off of Venus, and with Keper’s Laws and a little geometry, the length of one AU can be determined Crucial for using the parallax method

Parallax Best way to determine distance to stars within about 1,000 lyr

MS fitting Use parallax to calibrate Convert apparent magnitudes to absolute – d in parsecs Only good to distances in the Milky Way

Cepheids Evolved massive stars that have internal instabilities Obey a period- luminosity relation Can measure distances up to a few million lyrs

SN Ia All SN Ia have the same luminosity Use Cepheids to calibrate supernovae We can see SN to billions of light years

Hubble Law Velocity of distant objects increases with distance Clear correlation between velocity and distance Line fit to data is Hubble Law v = H 0 d H 0 = 22 km/s/Mlyr

Hubble Law Velocity of distant objects increases with distance Clear correlation between velocity and distance Line fit to data is Hubble Law v = H 0 d H 0 = 22 km/s/Mlyr

Hubble Law Velocity of distant objects increases with distance Clear correlation between velocity and distance Line fit to data is Hubble Law v = H 0 d H 0 = 22 km/s/Mlyr Cosmological redshift makes distant objects appear redder than they are

Galaxy Surveys

Galaxy Formation Start with collapse of protogalactic cloud Type of galaxy depends on: – Protogalactic spin – faster spinning clouds make spiral galaxies – Protogalactic density High density clouds cool efficiently  fast star formation  ellipticals Low density clouds cool inefficiently  slow star formation  spirals VIDEO

Giant Elliptical Galaxies Located at the centers of galaxy clusters Always the most massive object in cluster Likely the product of several galaxy mergers Collisions between galaxies would results in lots of star formation – Starburst galaxies Star formation would consume all gas, so none is left

Active Galactic Nuclei

Quasars Look like stars through a telescope Extremely distant Have strong visible and radio emission Extremely luminous – L ~ L Sun ~ 100 L MW Bipolar jets

Other AGN Less Luminous versions of quasars Some AGN change their luminosity in only a few hours – Light emitting region can be no more that a few light-hours across L ~ – L Sun Visible and radio emission

Radio Galaxies Extremely luminous radio sources – L Radio ~ L Sun – Little to no visible light radiated Observations show radio galaxies and quasars are likely the same type of object view from a different angle – Quasars: face on view of accretion disk gives visible light – Radio galaxies: edge on view of accretion disk blocks visible light

Power Source Accretion disk around Supermassive Black Hole – Up to 10 9 M Sun Radio emission comes from jets of material Visible light comes from super heated central area of accretion disk VIDEO