The Cosmic Time Machine Astronomical telescopes can’t help viewing the past. Distance and time are always mixed in astronomical observations. The finite speed of light means that we always see things after they have happened–a delay of 8 minutes for the Sun and about 12 billion years for the most distant galaxies we can observe. In other words,
Expansion of the Universe Features in the spectra of galaxies are essentially always observed at wavelengths longer than the corresponding features in laboratories on Earth (the “redshift”). The cosmological redshift is not exactly a Doppler shift, but is linked to the expansion of space as light propagates as well as the gravitational field of the universe. Hubble found a redshift-distance relation that could be interpreted as a uniform expansion. Friedmann had shown that such an expansion was a solution to Einstein’s equations.
Lookback time Observable quantity is redshift z Need distance scale and cosmology to derive lookback time to a given redshift Distance has multiple definitions in an expanding Universe
Redshift versus lookback time WMAP cosmology: Hubble constant 71 km/s/Mpc Flat spacetime (matter)=0.27
Galaxy Evolution - Cosmic History Galaxies change with redshift, reflecting development in their stellar content, gas content, and dynamical structure. These changes are most pronounced at large redshift and short wavelengths. New techniques allow us to approach the time when galaxies took shape and the first stars were formed.
Stellar spectra – the fossil record Chemistry of stellar surface reflects initial chemical makeup until late in its lifespan Stars’ orbits change only very slowly over time Makeup and motions of stars preserve a detailed record of our galaxy’s history Early stars formed with low heavy-element abundances and in a nearly spherical system
Galaxies Today Spiral, elliptical, irregular Stellar and gas content linked to morphology Dwarf galaxies most common Dark matter dominates overall dynamics Disk and bulge components; differ in motions and stellar properties
The early galaxy bestiary Lyman-break galaxies Extremely red objects (EROs) - the oldest young galaxies and dusty environments Star-forming subgalactic objects Submillimeter galaxies Quasars and radio galaxies Absorption-line systems These often occur in combination (groupings)
Lyman-Break Galaxies (LBGs) Galaxy spectra show a cutoff at 912 A due to absorption by neutral hydrogen This allows a straightforward multicolor selection (blue in two bands, missing shortward of that) Thousands of galaxies at z>2.7 have now been found in this way
The Lyman Break The Lyman break from satellite UV observations of a star-forming region in the nearby spiral M33 The brightest LBG in the Hubble Deep Field, a clumpy galaxy at z=3.21. 300 nm 450 nm 606 nm 814 nm
Submillimeter-bright Galaxies Found at z=2-3 Most powerful early star-forming sites? Key on dust emission, not stars Clump with other high-redshift objects Many have buried quasar cores Background: ionized-gas plume in submm galaxy ELAIS N2 850.4 at z=2.4, from NASA Infrared Telescope Facility, April 2003)
Extremely Red Objects (EROs) May be either intrinsically red or reddened by dust absorption; both kinds exist A way to seek the oldest galaxies at a particular redshift, a sensitive probe of when galaxy formation begn in earnest
Subgalactic Clumps Small size, blue color, Lyman a emission Active star formation, low metallicity Evidence for global winds escaping systems Exist in groupings with bright galaxies/AGN Are these the early units predicted by hierarchical schemes (and fitting dark-matter simulations)?
Blue subgalactic objects versus nearby spiral M101 at the same ultraviolet emitted wavelength Size: 1 kpc~3000 light-yr Many are double Comparable UV luminosity to bright galaxies now
The early Universe could be crowded (a group at z=2.4) Subgalactic objects Quasars Radio galaxy ERO
Quasars in the Early Universe Trace supermassive black holes and their growth by accretion Black holes today are ubiquitous in bright galaxies Quasars now seen to 0.5 Gyr after beginning, very common 10 Gyr ago Surrounding gas heavily processed by supernovae, even at highest redshifts
Heavy elements in high-redshift quasars HST composite, courtesy W. Zheng)
Ingredients of a cosmic history Gravitational collapse and gas infall Star formation (a feedback process) Heavy-element production Winds and the intergalactic medium Growth of supermassive black holes The first stars – a breed apart
The First Stars (Population III) Formed of pure hydrogen/helium Very massive (80-300 solar masses) Hot, short-lived Energetic supernova explosions Enriched surrounding gas, disrupted parental gas clouds Enrichment led to “normal” star formation Enriched intergalactic gas as well