Tracing the “cosmic” evolution does not tell us how single galaxies evolve….. (ARAA again)

Changing Paradigms of Galaxy Formation Classical ( ): galaxies evolve in isolation present-day properties governed by SF history ellipticals: prompt conversion of gas  stars spirals: gradual consumption of gas, continuous SF Dark matter-based (1985-): g. instability governs merging of DM halos low mass halos collapse first (bottom up formation) mergers transform morphologies (ellipticals form late) dense environments evolve faster (clusters older than field) Importance of feedback and other processes (1995-) evolution of morphology-density relation  environment assembly history as a function of mass  downsizing

Hubble’s sequence: basically a run of spheroid --> disk Split is not fundamental. If only Hubble had Photoshop and a Bubble-jet color printer…

“..describes a true order among the galaxies, not one imposed by the classifier” (Sandage 2005) Distinguishes dynamically distinct structures: spirals & S0s – rotating stellar disks spheroids – ellipsoidal/triaxial systems with anisotropic dispersions There exist physical variables that change along the sequence: * gas content/integrated color  ratio of current to past average star formation rate * inner structures  bulge/disk ratio Hubble’s Morphological Sequence

Monolithic Collapse Hierarchical Assembly How Did Galaxies Form? Nature Nurture Short course:

Hierarchical Assembly DM fractionates prior to recombination (halos) which grow whilst baryons locked to radiation After recombination gas cools into halos which continue to merge hierarchically `Morphology’ is directly linked to mergers: disks form first and those that merge form ellipticals Baugh et al MNRAS 283, 1361 (1996)

Spiral, S0EllipticalSpiral, Irregular Spirals are the building blocks, even though N-body simulations can’t make a big, rotating disk galaxy. Transient galaxy morphology

Numerical simulations suggest product of equal mass encounter may resemble spheroidal Toomre, A 1977 Yale Conference `Evolution of Galaxies’ ed. Tinsley & Larson Barnes & Hernquist 1996 Ap J 471, 115  Mass ratio used as the basis of defining morphological transformations in semi- analytic models Mergers are a key feature

From the 1970’s through the ‘80’s (the “dark matter-based era”) these two simple ideas formed the basis for thinking about the morphology of galaxies and where it came from: Elliptical galaxies are merged spirals S0 galaxies are “stripped” spirals. Despite their popularity, it was clear through this period that these ideas really don’t work to explain basic morphology: Problem #1 (which is enough): Most S0 galaxies (~90%) are in low-density environments -- they have never seen the inside of a cluster. Hot gas, ram pressure, harassment? -- only for the few, not the many.

. Problem #1: The stars in ellipticals are old, especially massive systems L > 4L* -- 90% of stars formed before z=2. Even when it is said that there is a young population, it’s just a “frosting” accounting for only a few percent of the stars (the ages are luminosity weighted) Problem #2: alpha-element enhancement requires prompt star formation, then very little or nothing (type-II SN) -- see Tantalo & Chiosi 2003 MNRAS 353, 917) Elliptical in the making? Not likely.

Ashman & Zepf 1992 How about this one? Maybe. Mostly, they are spheroids already. How about this one? No way.

VISUAL VERSUS AUTOMATED ALGORITHMS

MORPHOLOGIES HST breakthrough

Morphological Evolution: HST z = 0z > 1

Clusters of galaxies: cl z=0.55 Smail et al (MORPHS collaboration)

Dressler et al. 1999

55 nearby clusters from Dressler 1980’s sample – (plot from Dressler et al. 1997) MORPHOLOGY-DENSITY RELATION projected surface density (log) Fraction of galaxies ESpirals S0

Dressler et al Z=0 Z= Redshift N S0 /N E E Spirals S0

Fasano et al. 2000

MORPHOLOGIES OF DISTANT CLUSTER GALAXIES Lots of spirals, many disturbed (Dressler et al. 1994, Couch et al. 1994, Wirth et al. 1994, Dressler et al. 1997, Oemler et al. 1997, Couch et al BUT, many of them are red and passive, Poggianti et al. 1999) Low S0 fraction in clusters at z=0.4. Already as many ellipticals as at z=0. Spirals evolving into S0s? (Dressler et al. 1997, Fasano et al. 2000, Kodama & Smail 2001) Capability to recognize S0s questioned (Andreon 1998, Fabricant et al. 2000) -- “Diplomatic” evolution of early-type fraction (van Dokkum et al. 2000, Lubin et al )

Lubin et al. 03

Desai et al. in prep. EDisCS: Galaxy morphologies with HST Redshift Sp+Irr % E+S0 % S0 % E %

Morphology-density at z~1 Postman et al projected density f_Sp+Irr f_S0 f_E f_E+S0

Three paths from spiral to S0 1.Some spirals simply exhaust their gas, particularly those dominated by large spheroids (bulges). Outside triggers or intervention may not be necessary. 2.Mergers of gas-rich galaxies may lead to another spiral, but it may more likely leave an S0 after a starburst. 3.Strong interactions and accretions may speed a spiral’s evolution by stimulating star formation (exhausting the gas). Although most common in rich clusters, this kind of action probably happens in groups of galaxies -- as galaxy density increases, refueling by intergalactic gas may be cut off. Gas is heating up as the universe ages.

Desai et al Morphological fractions and redshift

Desai et al. 2007

Morphological fractions and velocity dispersion – at high z

Morphological fractions and L X Postman et al S0 % E % E+S0 % z = LXLX