1Hubble TypesHubble types are the z = 0 final state of bright galaxy evolutionEllipticals have old stellar populations, spirals have both old and youngcomponents while irregulars are dominated by young starsA significant amount of star formation must have occurred in the past for Es, but some young stars clearly exist in spirals Old starsYoung stars98% of all nearby bright galaxies can be placed into a Hubble typeChris ConselicePresent day galaxies• SDSS (Sloan Digital Sky Survey)–106targets, mostly galaxies, selected for spectroscopy–104deg2of sky–mr< 17.8 mag• Complete sample of 115,000 SDSS galaxies:Color Surface Brightness Form Luminosity ÎExponential r-1Disks (spirals)DeV r -1/4(ellipticals)Blanton et al. 2005, ApJ, 629, 143Blue RedRed sequence2Looking back from the present day• Keck DEEP Survey - 39,000 galaxiesAll galaxiesBlue galaxies (forming stars)Red galaxies (not forming stars)Redshift: 0.3 0.5 0.7 0.9 1.1= Log (Luminosity)Looking back from the present dayAll galaxiesBlue galaxies (forming stars)Red galaxies (not forming stars)Redshift: 0.3 0.5 0.7 0.9 1.1= Log (Luminosity)More luminousless luminous• Keck DEEP Survey - 39,000 galaxies3Keck DEEP SurveyResults so farAll galaxiesMore luminousless luminous3 Galaxy Evolution Scenarios“dry” = gas poor“quench” = shut off star formationLog mass ÎÍ Log mass The number of massive spheroidal galaxies has been growing over time.When did mass assembly occur relative to the quenching of star-formation?Absolute mag.Red = star formation doneBlue = still forming starsCo-moving density of Hubble Types with RedshiftCo-moving density drops from z ~ 1 to 1.5, even whenconsidering only bright galaxiesChris ConseliceAll galaxiesLuminousgalaxiesPeculiarsDisksEDid peculiar galaxies merge to form today’s massive E and S galaxies?4Are these high redshift peculiar galaxies mergers?Mergers:SuspiciousProbableDefinite No redshiftLeFevre et al., 2000, MNRAS, 311, 565• 285 galaxies.• HST images, CFHT spectra.• Merger fraction increases with redshiftConselice (2005) merger fractionsLow MassHigh MassLowLuminosityHighLuminosityΛCDMMerger Processes • Dynamical Friction - A slow accretion process.– Massive body moving through uniform distribution of stars• Pulls stars in behind it.• Creates high-density wake.• Gravitational pull from wake slows down massive body.• If massive body is in orbit in a galaxy, it will gradually spiral into the center.• Force on massive body isri= initial distance from center of galaxy.tc= time to spiral into center due to dynamical friction.rmax= max radius for capture within age of universe.vM(= 4 kpc for M31 and t = 13 Gyr)[CO eqn. 26.3]5The Antennae GalaxiesNGC 4038NGC 4039• Galaxy collisions –“impacts”.– Numerical simulations– Toomre & Toomre, 1972, ApJ 178, 623.• Tidal tails, etc.The Milky Way Meets Andromeda2.5 million ly awayApproaching at 500,000 km/hrÎ Collision in 3 billion yrsMovie lasts 1.3 billion yrs.Do collisions between spirals make ellipticals?6Globular Clusters as Merger Tracers• Galaxy collisions Îbursts of star formation.• Product includes globular clusters.• E galaxies have several different generations of globular clusters.Tracing the history of galaxy formationGiantelliptical galaxyX-ray sourcesÎ globular clustersFuture globular clusters Zepf & Co.Globular Clusters as Merger Tracers• > 60% of E-galaxies show bimodal or more complicated distributions of GC’s.• Need images in many colors to separate effects of age and metallicity.Metallicity ÎAge
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