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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
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
• Standard candles – objects that have the same
absolute magnitude
– Cepheids, SNIa
• Hubble Law – use distance dependence of Universal
expansion rate
• 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
• Best way to
determine distance
to stars within
about 1,000 lyr
MS fitting
• Use parallax to calibrate
• Convert apparent
magnitudes to absolute
• m  M  5 log( d [ pc])  5
– d in parsecs
• Only good to distances
in the Milky Way
• Evolved massive stars
that have internal
• Obey a periodluminosity relation
• Can measure distances
up to a few million lyrs
• All SN Ia have the same
• Use Cepheids to
calibrate supernovae
• We can see SN to
billions of light years
Hubble Law
• Velocity of distant
objects increases with
• Clear correlation
between velocity and
• Line fit to data is Hubble
• v = H0d
• H0 = 22 km/s/Mlyr
Hubble Law
• Velocity of distant
objects increases with
• Clear correlation
between velocity and
• Line fit to data is Hubble
• v = H0d
• H0 = 22 km/s/Mlyr
Hubble Law
• Velocity of distant
objects increases with
• Clear correlation
between velocity and
• Line fit to data is Hubble
• v = H0d
Cosmological redshift makes
• H0 = 22 km/s/Mlyr
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
Giant Elliptical Galaxies
• Located at the centers
of galaxy clusters
• Always the most
massive object in
• 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
• 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
• Visible light comes from super heated
central area of accretion disk