The History of Light:
How Stars Formed in Galaxies
Kai Noeske
European Space Agency/
Space Telescope Science Institute
Hubble Science Briefing, 1 Mar 2012
What is a Galaxy?
2
The Milky Way
100 Billion Stars like our sun
3
The Milky Way
www.atlasoftheuniverse.com
4
Meet the Neighbors.
M31 (“Andromeda Galaxy”),
our close neighbor
and similar to the Milky Way
M51 (“Whirlpool Galaxy”)
M104 (“Sombrero Galaxy”)
5
6
Stars are not evenly distributed in the
universe.
Stars are born and live in galaxies.
Most galaxies have billions of stars.
There are billions of galaxies in the known
universe.
Did they always look the same?
7
A long time ago in galaxies far, far away:
The HST Ultra Deep Field
8
A long time ago in galaxies far, far away:
The HST Ultra Deep Field
9
A long time ago in galaxies far, far away:
The HST Ultra Deep Field
10
Two immediate results:
I. Galaxies formed at some point in the distant
past
II. Galaxies evolved with time
Where do the Stars and Galaxies
come from?
11
Timeline
(very rough)
●
●
Most galaxies have
very old stars
Most galaxies
started forming their
stars some 10-13
Billion years ago,
shortly after the
beginning of the
Universe
12
3.2%
22%
74%
13
The Cosmic Microwave Background:
a baby photo of the Universe when it was just
300,000 years old
It reveals tiny irregularities;
the density of matter varied by parts in a million
14
Dark Matter is more abundant,
and dominates gravity.
To understand how gravity created
structure (galaxies) from the early
homogeneous Universe, we need to
simulate Dark Matter.
Outcome depends strongly on the
structure/geometry of the Universe and
the content of Dark Matter
15
Supercomputer simulations of Dark Matter:
gravity grows the initial density perturbations,
structure forms
From
http://cosmicweb.uchicago.edu/filaments.html
Quic kTime™ and a
Y UV 420 c odec dec ompr es sor
are needed to s ee t his pic tur e.
choose a “rotating box” version such as
http://cosmicweb.uchicago.edu/images/mov/s02_0.gif
16
Gravity grows a “Cosmic Web” of Dark Matter - voids,
filaments, clusters of clumps that host galaxies
Simulation: A.Kravtsov
17
Gravity grows a “Cosmic Web” of Dark Matter - voids,
filaments, clusters of clumps that host galaxies
Galaxies form from overdense regions
Simulation: A.Kravtsov
18
Luminous matter, formation of gas disk and stars:
Luminous matter (gas!) is viscous,
and heated as it falls into dark matter
halos; then heat is radiated away gas cools - contracts
angular momentum is conserved >spin-up of rotation (“figure skater”) fast rotating disk
energy in turbulent/random motions
(perpendicular to disk) is dissipated
(viscosity->friction->heating ->heat is
radiated away)
-> motions perpendicular to
ordered rotation disappear
->cold, dense gas disk -> STARS
19
Recap: From Dark Matter to Stars
1) The Universe contains mostly
Dark Matter
2) Tiny irregularities in the Dark
Matter density in the early
Universe grew rapidly through
gravity
3) Gas fell into the resulting Dark
Matter clumps/”halos” (galaxies)
and formed cold, dense gas disks
4) Stars are born and live in galaxies
because they need cold, dense
gas to form
20
Hierarchical galaxy formation;
disks merge to disk bulges and Ellipticals
In a “hierarchical” scenario, smaller
structures form first, and later merge into
bigger ones:
-Galaxies merge to form larger ones
-Mergers of roughly equal-sized galaxies
often (not always) turn Spirals into
Ellipticals
Bertola
et al. Halo;
Blue: Dark
matter
yellow: gas; red: stars
21
Galaxy interactions/mergers:
Observations and
Numerical simulations
http://www.youtube.com/watch?v=agqLEbOFT2A&feature=youtu.be
Credits: Patrik Jonsson, Greg Novak & Joel Primack, University of California, Santa Cruz
22
QuickTime™ and a
decompressor
are needed to see this picture.
23
QuickTime™ and a
decompressor
are needed to see this picture.
24
QuickTime™ and a
decompressor
are needed to see this picture.
25
QuickTime™ and a
decompressor
are needed to see this picture.
26
II. How did we learn about galaxy
formation?
27
New Sky Surveys at many
Wavelengths
28
Multiwavelength surveys: combined efforts to get the whole picture.
A new era of astronomy:
big collaborations, huge databases
29
Multiwavelength surveys: combined efforts to get the whole picture.
A new era of astronomy:
big collaborations, huge databases
HST (visual,
near infrared)
SPITZER
(infrared)
Dust, star form.,
black holes...
GALEX (UV)
Chandra (X-ray)
star formation
XMM (X-ray)
DEEP2 (KECK,DEIMOS)
VLA (radio)
(gas, mass,
black holes,
star formation)
Redshift, dynamics, ...
30
Astronomers can look back in time:
light from very distant galaxies took billions of years to reach us.
Looking far is looking back
even more
distant galaxy
distant galaxy
nearby galaxy
Text
Long (billions
of years)
time light travels
to reach us
Short (millions
of years)
31
Astronomers can look back in time:
light from very distant galaxies took billions of years to reach us.
Looking far is looking back
even more
distant galaxy
distant galaxy
nearby galaxy
Text
Long (billions
of years)
time light travels
to reach us
Short (millions
of years)
32
Astronomers can look back in time:
light from very distant galaxies took billions of years to reach us.
Looking far is looking back
even more
distant galaxy
distant galaxy
nearby galaxy
Text
Long (billions
of years)
time light travels
to reach us
Short (millions
of years)
33
Astronomers can look back in time:
light from very distant galaxies took billions of years to reach us.
Looking far is looking back
even more
distant galaxy
distant galaxy
nearby galaxy
Text
Long (billions
of years)
time light travels
to reach us
Short (millions
of years)
34
Large telescopes on the ground:
Spectroscopy gives each galaxy a “time stamp”
35
DEIMOS spectrograph on the Keck II telescope
Built by Sandra Faber & team, UC Santa Cruz
Can observe spectra of hundreds of distant galaxies
at a time
36
Overlapping
slitmask layout
37
120 spectra of distant galaxies
emission lines
of ionized gas
wavelength
The emission lines are at longer wavelengths than
measured in the lab: They are “redshifted”.
This is because distant galaxies move away from us
(“Doppler effect”, expansion of the Universe).
wavelength
The redshift (=velocity) measures the distance and
how far we look back in time
38
For galaxies in the early
universe, the infrared matters:
39
UV
Visible Light
Infrared
spectral flux
Nearby Galaxy
(not redshifted)
young stars
(starbirth)
older stars
Distant Galaxy
(redshifted)
young stars
(starbirth)
older stars
wavelength
For distant galaxies, light from young stars (UV) and older stars
(visible) is redshifted to long wavelengths (Infrared)
40
The Spitzer Space Telescope provided
infrared data: pierce through the dust,
measure star formation rates
Spitzer Extended Deep Survey
Reduction: M. Ashby
41
Hubble & JWST
Probe the Early Universe
HST: currently the most sensitive telescope in the short-wavelength infrared (nearinfrared): Can observe redshifted UV (star formation) from the most distant galaxies
JWST (launch: 2018) will be more sensitive, and reach longer infrared wavelengths:
will reach even further back in time, and observe redshifted visible & infrared light in
earliest galaxies
42
JWST will have much improved sensitivity to faint distant galaxies:
First Stars & Galaxies
Small galaxies across cosmic time
...
HST Ultra Deep Field
JWST Ultra Deep Field
Simulation
43
Star formation in galaxies over the last 10 billion years
Space Density of Star Formation
10 Billion yrs ago
Space Density of Star Formation
now
Hopkins & Beacom 2006
now
10 Billion yrs ago
Big Galaxies
Small Galaxies
Heavens et al. 2004
44
Star formation in galaxies over the last 10 billion years
Space Density of Star Formation
10 Billion yrs ago
Space Density of Star Formation
now
Hopkins & Beacom 2006
now
10 Billion yrs ago
Big Galaxies
Small Galaxies
Heavens et al. 2004
Co-moving star formation rate (SFR)
density declined by ~x10
Galaxy star formation histories are massdependent:
massive galaxies formed bulk of stars
quickly and early, less massive galaxies
formed on longer timescales (“Downsizing”)
45
Star formation in galaxies over the last 10 billion years
Space Density of Star Formation
10 Billion yrs ago
Space Density of Star Formation
now
now
10 Billion yrs ago
Big Galaxies
Small Galaxies
Heavens et al. 2004
Hopkins & Beacom 2006
Co-moving star formation rate (SFR)
density declined by ~x10
Galaxy star formation histories are massdependent:
massive galaxies formed bulk of stars
quickly and early, less massive galaxies
formed on longer timescales (“Downsizing”)
Reason for declining star
formation:
Galaxies run out of gas!
46
today
billions of years ago
rapid star
birth & gas
consumption
big galaxies
(image: Driver 1998)
small galaxies 47
today
billions of years ago
rapid star
birth & gas
consumption
big galaxies
slow star birth
& gas
consumption
(image: Driver 1998)
small galaxies 48
today
billions of years ago
rapid star
birth & gas
consumption
big galaxies
slow star birth
& gas
consumption
(image: Driver 1998)
small galaxies 49
today
billions of years ago
rapid star
birth & gas
consumption
big galaxies
slow star birth
& gas
consumption
(image: Driver 1998)
small galaxies 50
http://hubblesite.org
http://candels.ucolick.org
http://aegis.ucolick.org
http://www.atlasoftheuniverse.com/
noeske@stsci.edu
51
Questions?
52