galaxies
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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