Galaxies

With a touch of cosmology

Types of Galaxies

• Spiral

• Elliptical

• Irregular

Spiral Galaxies

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

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 107 MSun

– Giant ellipticals can be about 1013 MSun

Irregular Galaxies

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

5])[log(5 pcdMm

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 = H0d• H0 = 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 = H0d• H0 = 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 = H0d• H0 = 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 ~ 1012 LSun ~ 100 LMW

• 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 ~ 1011 – 1012 LSun

• Visible and radio emission

Radio Galaxies

• Extremely luminous radio sources– LRadio ~ 1013 LSun

– 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 109 MSun

• Radio emission comes from jets of material

• Visible light comes from super heated central area of accretion disk

• VIDEO