Transcript Watching Galaxies Form Near the Beginning of Time

Watching Galaxies Form
Near the Beginning of Time
The Cosmic Time Machine
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,
Astronomical telescopes can’t
help viewing the past.
Distance and time are always mixed
in astronomical observations.
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
z = shift/initial wavelength
Redshift versus lookback time
WMAP cosmology:
Hubble constant 71 km/s/Mpc
Flat spacetime
W(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
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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
Tracing Galaxy Evolution
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Galaxy types in clusters (Butcher-Oemler)
Stellar populations from spectra
Size/luminosity
Hubble types
Star-formation rate (UV,far-IR, radio, Ha)
Nuclear activity (quasar density,luminosity)
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
300 nm 450 nm
606 nm 814 nm
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.
Submillimeter-bright Galaxies
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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
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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
ERO
Subgalactic objects
Radio galaxy
Quasars
The early Universe could be crowded
(a group at z=2.4)
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
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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)
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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