QSO Absorption Lines and Cosmological Simulations:
The Quest to Understand Galaxies
Chris Churchill
New Mexico State
- Galaxies form in the cosmic web
- They accrete IGM gas, form stars, and deposit energy/metals back into IGM
- Extended metal enriched “halos” are observed from z=0 to z=4
June 20, 1011
Observational data of these “halos” are underutilized for constraining galaxy
formation physics in cosmological simulations…
-how to do it?
Getting acquainted with the universe…
The universe is about 13.6 billion years old
The universe is expanding and its geometric curvature is “flat”
The universe comprises (add to 100%)
70% dark energy
26% dark matter
4% baryonic matter (normal stuff)
dark matter is 85% of all matter
baryonic matter is 15% of all matter
How do we know there is dark energy?
About 6 billion yrs ago, the expansion rate of the universe changed from
deceleration to acceleration; dark energy acts like a “negative pressure” and it
began to dominate later in the life of the universe
How do we know there is dark matter?
If we add up all the mass in stars within any random galaxy and measure the
velocities of the stars, we would deduce that the galaxies should fly apart
(become unbound); star must contribute only 10% to the galaxy’s mass
How do we know any of this?
“Astronomers are a breed of historian with lots of extra hubris.” – Sandra Faber
“Studying the history of the universe is something like trying to piece together
the history of the earth’s climate by placing dixie cups out in the rain and then
studying the few water drops collected” – Paul Hodge
Astronomers are handicapped experimental physicists, called “observers”.
We collect light in huge light buckets, try and organize the light by color,
intensity, etc, apply the laws of atomic and gravitational physics- and…
ka-chow(!) … we have a little understanding of 4% of the the universe
… AND, FOLKS… WHAT A 4% IT IS!
Tips of the ice bergs
Galactic Pathology is a transient train wreck phenomena…
How do galaxies actually form…
These galaxies started their lives as gaseous halos before any stars ever form!
The gas (4% of the universe) gravitationally follows the formation of dark matter
halos (85% of matter), which formed due to localized gravitational instabilities as
the universe expands
These instabilities were seeded by quantum fluctuations created in the first
fraction of a second of the Big Bang; the distribution of galaxies in the universe
and the fluctuations in the cosmic microwave background provide the mass
spectrum of the fluctuations (we know it well!)
All sky CMB fluctuations
Cosmic web distribution of galaxies
Building galaxies the only way we know how…
We put the dark matter fluctuation spectrum in as the initial condition in a
cosmological simulations code, with which we model gravitational forces, star
formation, stellar feedback from supernovae explosions, and the build up of
chemical species heavier than helium, and model the expansion of the universe
and… let ‘er rip… we also put in the “trace” baryonic (normal) matter and let it
follow the dark matter and we get- galaxies
… and yes, we get out disk/spiral galaxies that rotate and elliptical galaxies, and
al the train wrecks you could ask for
… and we can track the baryonic gas that is otherwise invisible in that it does not
radiate enough light for us to detect the gaseous structures
SO.. HERE IS A LITTLE DIDDY DESCRIBING OUR CURRENT VIEW OF THE LARGE
SCALE STRCUTURE OF MATTER N THE UNIVERSE AND ON GALAXIES…
Click inside the box to start movie
Just as simulations suggest, galaxies are observed to be surrounded by lots of gas
Images alone do NOT provide
detailed physics required to fully
exploit the simulations (or improve
them!)
Also, galaxies that are farther away
appear dimmer and we become
photon starved (crummy data)
We need an observational technique
that yields the physics (chemical
composition, temperatures,
kinematics, and ionization
conditions) AND is equally sensitive
for both close and far galaxies…
A Spectrum is Worth a Thousand Pictures…
As powerful as simulations are they are far from complete and
require observational constraints and direct comparison testing with
the real universe…
… since light interacts with matter (the 4%!) in very specific ways, the
pattern of light intensity as a function of wavelength (spectrum) can
be decoded and then atomic physics can be applied to deduce the
very highly detailed physical state of the matter
Absorption lines!
Star spectra, galaxy spectra, QSO spectra all sport them.
When a continuous spectrum passes through a “cloud” of gas, almost all the light
passes through unimpeded…. but…
each type of atom/ion in the gas cloud interacts with photons of precisely well
defined wavelengths that are specific to that atom/ion- the atoms absorb the
light energy and become excited or ionized
There is a unique pattern of absorption lines for each atom/ion!
We are the FBI and absorption lines are atomic fingerprints
Astronomer’s Periodic Table
H, He, metals!
Neutral hydrogen (HI) has very strong
absorption, called Lyman a (Lya)
But not all atoms are neutral… they
get ionized by high energy photons!
1x ionized Mg (MgII) acts like neutral
Na (sodium like)
3x ionized C (CIV) acts like neutral Li
(lithium like)
5x ionized O (OVI) acts like neutral Li
(also lithium like)
These are special because of fine
structure splitting that results in
doublets
-> pairs of absorption line!
We do require a background light source with a continuous spectrum
Thank The Maker for the physics
of giant black holes!
QSO
In the past, due to galaxy
mergings, giant black holes
formed and found plenty of
“food” in the centers of train
wrecks as they relaxed and settled
down
We call them QSOs… short for quasi-stellar object… they beam with the power of
1,000,000,000,000,000,000 Suns blasting x-rays, UV, visible, and IR light in a smooth
continuous spectrum
The line of sight to a QSO provides a core sample through the universe, recording the
finger prints of every ion in every gas cloud it pierces!
Telling Cosmic Time: Cosmological Redshifting of Light
Not a Doppler effect; a stretching!
Photon wavelengths are “comoving” with space
(a) At some time in the past a photon is emitted (or
absorbed) with wavelength l0
(b) At later time the universe has expanded and the
photon has “stretched” in step with it
The present-day observed wavelength of
a photon emitted (absorbed) in the past
is
l = l0 (1+z)
z = redshift
Redshift is a measure of cosmic time
The Business of QSO Absorption Lines: gaseous core samples of the universe
QSO
Very powerful for constraining cosmic
structure growth and galaxy/IGM,
chemical, and dynamical evolution.
Hydrogen absorption is ubiquitous (IGM, filaments, galaxy ISM and halos)
Metal absorption tracks galactic structures (metal enriched by past stars!)
Their metal line doublets are easy to spot in spectra!
MgII 2786,2803 - low ionization; traces moderately dense gas with T~30,000 K
CIV 1548,1550 - intermediate ionization; traces gas with T~30,000-100,000 K
OVI 1031,1038 - high ionization; traces hot gas with 100,000<T<300,000 K
Schematic of QSO Absorption Lines: Intrinsic Emission Lines
l = l0 (1+z)
1215.7(1+3.000) = 4863
1549.8(1+3.000) = 6199
Schematic of QSO Absorption Lines: Cosmic Web HI (Lya) absorption lines
l = l0 (1+z)
1215.7(1+2.907) = 4571
1215.7(1+2.455) = 4203
1215.7(1+2.049) = 3712
1215.7(1+1.262) = 2750
Schematic of QSO Absorption Lines: Galaxy/Halo metal lines (MgII, CIV)
The dixie cup twins!
Keck Twins
10-meter Mirrors
Inside the dome…
The Keck I in parked position
before a night of observing.
The spectrograph “room”
mounted on the west azimuth
platform
The High Resolution Echelle Spectrograph (HIRES/Keck)
2-Dimensional Echelle Image
Dark features are absorption lines – atomic fingerprints
1-Dimensional Echelle Spectrum of a QSO at z=2.406
<- ultraviolet
<- visible ->
infrared ->
Note that almost every feature is redshifted! Lya and CIV emission originate in the far UV!
The Level of Detail- is… just simply awesome!
Resolution = l/Dl = c/Dv = 45,000!
Dv=6.5 km/s
A typical sample of the
Lya forest lines at z=2
The “velocity” interval
sampled per pixel is
2 km/s!
High Resolution yields numbers of clouds in a halo, and their line of sight velocities!
Simple model for interpreting complex absorption profiles
- Individual clouds provide projected velocity along line of sight (Doppler formula)
Blended Line
Morphology;
Asymmetric
Velocity
Resolved Line
Morphology;
Symmetric
Velocity
Observed spectra contain a mixture of both models
Converting to velocity in the galaxy rest frame
When studying absorption from galaxies, we prefer to view the absorption in the
rest-frame velocity of the galaxy
v = c (l-lz0)/lz0
lz0 = l0 (1+ zgal)
Galaxy Halo A
Galaxy Halo B
neutral
hydrogen
(Note the larger scale)
low ionization
magnesium
mid ionization
carbon
high ionization
oxygen
The Q0002+051 Field
z=0.2981
z=0.5915
z=0.8514
D=18.2
z=0.2981
D=41.7
QSO
z=0.5915
D=25.5
Field by field, galaxy by galaxy, we
build a consensus of the
relationship between galaxies and
their extended halos
z=0.8514
Our Sample: arranged by impact
parameter
2796
2803
How do we try to make
sense of these data?
Observationally: compare relative kinematics
Does the gas co-rotate with the disk?
It should if it is coupled to the galaxy angular
momentum
We line a slit along the galaxy and take its
spectrum too… we use emission lines and the
Doppler effect to measure the galaxy rotation
Usually the gas cannot be fully coupled to the
galaxy (blue curves give predictions)
Observationally: compare relative kinematics
Sometimes higher ionization gas kinematics
different than low ionization kinematics!
Note CIV is moving away from us compared to
the galaxy, while MgII is moving toward us
But we do not know where the gas is located
spatially compared to the galaxy… so
USE SIMULATIONS….
stars
1000 kpc
z = 2.3
z = 1.3
z = 0.2
density cm-3
temp K
Z solar
Temperature Evolution from z=3 to z=1
z=3
z=1
At z=3 (~11 billion yrs ago), hot T=100,000 K
gas expanding outward over 200 kpc radius
while filaments from the cosmic web/IGM
are infalling with T=10,000 (not yet heated)
By z=1 (~7 billion yrs ago), the gas is spread
over 600 radius kpc (volume of Local Group!)
and has cooled in substructures of T=1000 K ,
others (the majority) of T= 10,000, and warm
to hot substructures of 100,000 K; note the
filaments have been disrupted; also appearing
(white, near galaxy) is shock heated (T=10
million K) X-ray gas
Material is outflowing due to star forming
activity (supernova winds)
Metal Enrichment Evolution from z=3 to z=1
z=3
z=1
At z=3 (~11 billion yrs ago), metals are
differentiated somewhat uniformly;
- blue halo is entrained IGM metal poor gas
- yellow is ~1/100th solar
- red is ~1/10th solar
- white is nearly solar
By z=1 (~7 billion yrs ago), the vast majority of
the halo gas is 1/10th solar over a volume of
the local group
-Upper infalling filament enriched in situ by
dwarf galaxies
- By present day (z=0), however, much of this
enriched material falls back in toward the
galaxy, leaving much patchier halos
Cosmic Structure Evolution in 10 Mpc Box
Track metals, account for ionization balance,
and present regions where CII, CIV and OVI are
dominant ionization stages of C and O
credit: Ben Oppenhiemer
Examining CII and CIV Absorption
Evolution in a Single Halo
Place line of sight (LOS) through simulation,
compute absorption spectrum at each time
step and animate…
Intensity at vel (km/s)
+30 kpc
CIV
CII
LOS vel (km/s) at x
60 kpc
LOS position x, kpc
-30 kpc
credit: Matias Steinmetzr
A Example Detailed Study…
view from the sky
side view in plane of
Example showing 2 “QSOs” lines of sight (LOS) through a z=0.8 simulated galaxy
These two LOS pass through fairly low density structures of a well evolved galaxy
Putting it altogether…
How we get spatial and kinematic information
Summary/Conclusions
1. QSO absorption lines provide a powerful method for probing the role of gas in galaxy
evolution. The sensitivity remains constant with redshift and an otherwise invisible
component of the universe is revealed in all its physical detail.
2. The method itself is limited; only line of sight velocity information can be directly
observed; we need simulations to interpret observations and obtain 3D spatial,
temporal, and contextual/cosmic environmental information.
3. Cosmological simulations need QSO absorption line observations to test that they in
fact correctly model both large and small scale gas hydrodynamics in the cosmological
setting; these simulations include the physics of star formation, supernovae winds,
and stellar feedback- all brand new physics being explored only now.
1. We find that the extended gaseous “halos” discovered via QSO absorption line studies
are complex entities coupled to both the small parsec scale physics within galaxies
and the large scale physics of galaxy formation in the cosmic web setting.
2. There is much work to do; we give the simulations a “B-” at this time… (I could give
another hour lecture on the shortcomings based upon statistical and quantitative
tests); over the next years this line of research holds great promise for developing a
new era marked by a comprehensive understanding of how galaxies form and evolve.
Thank you Rose City Astronomers!