Download The formation and evolution of galaxies

The Formation and Evolution of
Galaxies
Michael Balogh
University of Waterloo
Outline
• Introduction: the scale of the
Universe
• Observations
 The local Universe (Sloan
Digital Sky Survey)
 The distant Universe: what
can distant galaxies tell us?
• Theory:
 How do galaxies form and
evolve?
• Summary
The scale of the Universe
The Milky Way
•~20 kpc = 65,000 light years
The Local Group
600 kpc = 2 million light years
Galaxy clusters: a different world
The Coma cluster: 90 Mpc (300 million light years) away
1 Mpc = 3 million light years
Home: the Local
Group
13.7 billion years after
the Big Bang
The Universe
The Coma cluster:
•300 million light years away
•Typical stellar lifetime is a few billion years
• So there has been little evolution
between the epoch of the Coma cluster
and today
The Hubble Ultra-deep field
• most distant galaxy, z=6
• 13 billion light years away
•<1 billion years after the Big Bang
The Cosmic Microwave
background
• relic radiation from the Big Bang
• z~1000
• < 1 million years after the Big Bang
Questions?
• How did the Milky Way form?
 Different components:
 Rotating disk of young stars
 Central bulge of intermediate-age stars
 Large halo of very old stars
• How did the Local Group form?
 ~30 nearby galaxies with different morphologies, sizes, luminosities, ages
 Need to explain the diverse properties of galaxies in the Local group.
• Why do cluster galaxies look so different?
 These are mostly old, elliptical galaxies. Why?
Two approaches:
1. The local Universe
• Try to reconstruct the history of galaxy evolution from the appearance of
galaxies around us
2. High-redshift galaxies
• Distant galaxies are seen at an earlier epoch, so can trace evolution
directly.
Star formation in the Milky Way
• In our own Galaxy we can
measure the ages of
individual stars (in some
cases)
• The Milky Way has been
constantly growing: the star
formation rate has been
approximately constant for
the last ~10 billion years.
Local Group galaxies
For some galaxies in the Local Group, it is possible to
measure the colours and magnitudes of individual stars
 Stellar evolution models can then be applied to determine the
star formation history of these galaxies
Local group
Galaxies in the Local Group have a wide range of ages. Some show only
old stars, others have only very young stars.
Leo A: young galaxy, with most
stars formed less than 2 billion
years ago
Leo I: intermediate aged galaxy,
with most stars formed ~3 billion
years ago
Ursa Minor: the only galaxy in
the Local Group with no young
stars.
Elliptical galaxies
• For more distant galaxies, we can’t resolve individual stars:
have to model the integrated colours, spectrum.
•These models show
elliptical galaxies
tend to be old
 Have formed most
of their stars at
least ~10 billion
years ago
• Star formation histories for spiral galaxies
tend to be more complicated
Can measure their current rates of star
formation (from UV flux, emission lines, or
dust emission)
The SDSS
• Sloan Digital Sky Survey: a great look at the local Universe
• Will measure accurate photometry for 100 million objects
• Redshifts (and star formation rates) for 1 million galaxies and 100,000 quasars.
 Out to z=0.2
 About 1 Gpc, or 2.7 billion light years
 The age of the Universe is ~13.7 billion years, so these galaxies are still relatively nearby
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Higher redshifts
HST Ultradeep field
 very deep, but small area and hard to get
redshifts
Keck DEEP2
 3.5 sq. degrees, R~24.5, z>0.75
 55,000 redshifts
CFHT-LS
 very wide, wide, and deep fields.
 Wide is 170 deg2 to r=26
COMBO-17
¾ square degree
5 broad + 12 medium
~ 25000 galaxies
%
λ [nm]
The epoch of galaxy evolution
• By measuring the star formation
rate per unit volume at different
distances, we can reconstruct the
star formation history of the
Universe.
• Galaxy formation appears to
have occurred mostly about 10
billion years ago. The global
star formation rate has been
steadily declining since.
• These results are sensitive to the
effects of dust extinction, and
assumptions about the shape of
the initial mass function.
Why Does Star Formation Stop?
Fossil history of local group
Galaxy formation: Theory
In the beginning …
In the beginning…
z ~ 1000
Perturbation Growth
WMAP
Growth of
fluctuations is due to
gravity, and depends on the
energy and matter content
of the Universe
The Composition of the Universe
• Total matter and energy density is equal to the critical density
needed to “close” the Universe.
• The matter density is 30% of this. The remaining 70% is an
unknown energy component (“dark energy”).
• Of all the matter in the Universe, only 16% of it is ordinary
matter (protons, electrons, neutrinos etc.). The rest is due to
dark matter, an unknown component that interacts only
through gravity.
• Only 5% of ordinary matter is visible!
 i.e. the stars and gas we see only make up <1% of all the matter and
energy in the Universe.
The
Thebaryon
baryonbudget:
budget:hot
stars
gas
Coma:
Coma cluster
XMM-Newton Observatory
The easy part: Dark matter
150 Mpc/h
3 Mpc/h
The hard part: baryons
Baryonic Physics
Invisible baryons: ~106 K
Radiative
cooling
Radiative
cooling
Mergers
Baryonic Physics
Radiative
cooling
Radiative
cooling
Why so few stars?
• Dense gas cools very effectively as
Predicted number
of galaxies
free electrons interact with each other.
• This is especially true at early times,
when the Universe was much denser.
Observed number
of galaxies
Simulation: dark matter
in the Local Group
• Overcooling leads to the formation
of hundreds more small galaxies than
are observed.
Supernova feedback?
• Explosive supernova events may be strong enough to eject matter and energy
into the gas surrounding galaxies.
M82 in Ha: Subaru Telescope
M82 in Xrays: Chandra Observatory
Evidence for Superwinds at high
redshift
• These winds may be even more effective at early times, when they are most
Blob1
needed.
• Still, there is probably not enough energy in supernova alone to keep the gas
sufficiently reheated
Bubble-like
structure
Conical
structure
200kpc
Lya at high redshift: Subaru
200kpc
What about active galaxies?
There is ~100 times more energy available in Active Galactic Nuclei
(accreting black holes) than in supernovae
Details of how this energy is coupled to the surrounding gas are still
uncertain
10 kpc
Perseus Cluster / Chandra
Perseus Cluster & 3C 84
Bubbles in the Intracluster Medium
• Simulations of bubbles in cluster
gas
• Effective at disrupting cooling in
the core, over the ~50 Myr lifetime
of the bubble
Summary
Some things we know:
 Galaxies formed most of their stars about 5-10
billion years ago
 Growth of galaxies is dominated by dark matter
 Birth of stars is regulated by very energetic events:
supernovae and black hole accretion
Some things we don’t know:
 Why has star formation all but stopped by today?
 Why do galaxies in rich clusters look different from
the Milky Way?
 How does nature maintain the precise balance
between the rapid cooling of gas and the injection
of energy through explosive events?
Future progress:
 Larger, more specialized telescopes to
observe smaller galaxies, at earlier times
 E.g. JWST, Thirty-Metre-Telescope
 Higher resolution simulations with better input
physics to understand the interplay between
feedback and cooling.