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Introduction to Observational Cosmology
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The basic pillars of our cosmological picture
(i.e. we are starting with the answer first)
1. Averaged over sufficiently large scales, the universe is
nearly homogeneous and isotropic (=cosmological principle)
2. The universe, i.e. space itself, is expanding so that the
distance D between any pairs of widely separated points
increases as dD/dt~D (=Hubble law)
3. (?) the universe expanded from a very dense, hot initial
state (=big bang)
4. The expansion of the universe is determined by its
mass/energy content and the laws of General Relativity
5. On “small scales” (<10-100 Mpc) a great deal of structure
has formed through gravitational self-organization.
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Historical Milestones of
Cosmology
• 1920s : Galaxies (=“nebulae”) are distinct entities of stars
like our Milky Way  Universe // Milky Way
Note: Nearly co-identical with Einstein's General Relativity
(1916)
 DAnd  0.5 Mpc
Distances in the Universe
First measure of characteristic distance to other galaxies:
Öpik (1922) measured the rotation speed of the Andromeda
galaxy and determined the distance at which its mass to
light ratio equals the Milky Way's: (M/L)And  (M/L)* in MW
 R  v  / G and R    D
with M ~
obs
And
2
circ
 DAnd  0.5 Mpc
L
lD
Hubble (1929) measured distances to galaxies using
Cepheids (M31, M33) and brightest stars, obtaining similar
distances.
2
And
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Expansion of the Universe
• 1929 and 1931 Hubble
(and Humason)
• Fainter (and hence more distant) galaxies recede from the
Milky Way at a rate proportionate to their (estimated)
distance.
10,000 km/s
1000 km/s
Mean apparent magnitude
• Big problem: what is the proportionality constant, the
distance scale? (Hubble got it wrong by a factor of 8)
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Redshift
•
Observational result:
The measured redshift (often falsely interpreted as
"recession velocity") of an object
z
em  0
(0 = rest wavelength)
0
is proportional to the (independently) measured distance D
of an object (but with some scatter!!)
•
Longstanding problem:
We need the absolute distance measure to get the slope of
the relation v = H0 · D ; the proportionality constant is called
"Hubble constant".
More appropriate qualitative interpretation:
The universe/space has expanded by a factor of (1+z)
between emission and observation of the light.
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Quasars
• In 1963 Maarten Schmidt (+Jesse Greenstein) found that a
radio source was actually a galaxy nucleus at z @ 0.16
that was by far the most distant object at the time
• 1965 Quasars with redshifts z>2 found!
 cosmological look-back!
Vatican 2003 Lecture 20 HWR
Cosmic Microwave Background (CMB)
• “Afterglow” of the
big-bang
• Discovered by Penzias
and Wilson (1965)
 glimpse of the universe
in its infancy
<T> = 2.73°K
Small temperature
fluctuations arising from
early weak fluctuations
in the matter
distribution
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Distance Measurements
• Measuring distances is one of the most
fundamental tasks of astronomy, needed to:
–
–
–
–
Convert angular sizes to physical sizes
Convert apparent flux to absolute luminosity
Determine relative geometry of different ojects
Determine look-back time
• Measuring astronomical distances is difficult,
there are three types of methods:
– Geometric methods
– Standard Candles
– Direct physical estimates (gravitational lensing, CMB)
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Geometric Distance Measurements
• Parallax measurements
i.e. Earth’s motion reflex
• State of the art: Hipparchos (ESA Mission in the 1990s)
Parallax accuracy of 0.9 milli-arcseconds  distances to 1 kpc
Application
Calibrate nearby (~ 100pc)
stars of the appropriate
metallicity
 match to globular
cluster main-sequence
 Distance to globular
clusters (to 5%), which
contain cepheids
Reid 1997
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Standard Candles
Approach:
• Select objects whose intrinsic luminosity can be estimated,
either from physical first principles, from empirical
calibrations of nearby examples or can be inferred from
another distance-independent observable.
• Instrinsic luminosity + apparent flux  distance (modulus)
Examples:
• Cepheids: luminosity estimate from their pulsation period
• Spiral Galaxies: luminosity estimate from their disk rotation
curve
• Type-Ia Supernovae (SNIa): luminosity estimate from their
light-curve shape
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Cepheid Distances
• E.g. HST Key-Project to measure Hubble constant, H0
(Freedman, Kennicutt,Mould, et al.)
lightcurves
• Compare Cepheid brightness in M81 to LMC
and local Milky Way Cepheids  DM81=3.63+-0.34
• This way we can measure distances to
Galaxies with where Supernovae exploded.
Note: for nearby (<50Mpc) galaxies distance and
redshift are correlated with considerable scatter
 Measuring H0 is not easy
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Supernova Type Ia Distances
• SN Ia: white dwarf stars near the Chandrasekhar mass limit
(1.4 Msun), where Carbon and Oxygen burn explosively.
• Most luminous variety of Supernova. Can be seen to z>1!
Perlmutter etal
2002

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SN Ia as Pseudo-Standard Candles
Phillips, Hamuy, Ries, Kirshner and others ~1996
– Intrinsically bright SN Ia decline slower
 SN Ia: H0=67+-5 km/s/Mpc
(Current estimate (all methods): H0=70+-3)
Note:
with correction
- still needs Cepheid calibration
- Galaxy velocities differ from the local
mean by ~300 km/s  systematic uncertainty in H0
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Distant Supernovae
• The distance modulus M-m
to a certain redshift z
depends on the expansion
history, not just the current
expansion rate.
• Type-Ia Supernovae can be
seen to great distances: z>1
 probes of the expansion
history.
• 1998: expansion of the
Universe is accelerating (!?)
•
•
Riess etal 1998, AJ, 116, 1009
Perlmutter et al. 1999, ApJ, 517, 565
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Surveying Galaxies
Goal:
• We want to draw up a comprehensive picture of the galaxy
population in either the nearby, present-day universe, or in
the distant, earlier universe.
• Such a “picture” includes:
– the frequency of galaxies as a function of their
•
•
•
•
•
Luminosity
Structure (Size, bulge-to-disc ratio, etc..)
Dynamical mass
Star formation rate
Color (or spectral energy distribution) of the stellar population
– the correlation of these parameters with each other.
• Tully-Fisher relation, fundamental plane, star formation – color
– The connection of individual galaxy properties with their
“environment” (e.g. cluster or field, etc..)
Ideally: we would like to know “everything” about all galaxies
in a given, large volume ………
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Observables in Galaxy (or Star) Surveys
Observables in large surveys:
– Flux in a given aperture for several bands (e.g. IR, optical X-ray)
– Spectra (for D/<> ~0.5) and redshift (Distance +- 4Mpc)
– Some morphology/structure information.
Observational constraints in galaxy/cosmology surveys
– For compact or unresolved objects surveys are flux-limited
(separate flux limit in each observational band).
– For extended objects surveys are surface brightness limited (i.e.
limited by the contrast to the background)
– For imaging surveys, the observed bands correspond to different
rest-frame wavelengths.
– As galaxies have different intrinsic spectral energy distributions,
some objects will be above the survey limit in one band, but not the
other.
How to deal with these difficulties?
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Some Consequences
• Impracticability of nearby,
volume limited samples:
– Given a flux limit flim, any
object of luminosity L, can be
seen only within an volume
V=dW*(L/(4pflim,))3/2
– In practice, it means one only
needs to know the maximum
volume within which the
object would have in the
survey
From the 2DFRS
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K-Corrections
L(1+z) 

K(z)=-2.5log (1+z)
L 

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Galaxy Surveys:
the present-day luminosity function
•
Luminosity function:
– basic, long-standing statistic
to describe the galaxy
population: described the
space density of galaxies at a
given luminosity (or absolute

 L   L 
 L 
d 
   *
 e
 L*  L*
 L*

 L 


 L* 
 L 
d

 L*
magnitude):
– Often parameterized as a
“Schechter function” (1976),
to be a power-law at the faint
end and an exponential at the
brightest end, with a
characteristic luminosity L*
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The LF from SDSS
(+2MASS and 2DFRS)
Sloan Digital Sky Survey:
Five-color digital map of the
Northern sky (8000 sq.
deg) with
• photometry on
>10,000,000 galaxies
spectra (redshifts) for
nearly 1,000,000 galaxies.
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How the Sloan Digital Sky Survey (SDSS) works
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www.sdss.org
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SDSS Luminosity Function
(5% of Data)
Blanton et al 2001
 bright
about 1 L* galaxy / 100 Mpc3 ; M* (now)=-20.8 in r-band (Note: by convention H0=100)
Schechter function is a good representation
LF (= abundance distribution) depends on the intrinsic color of the galaxy
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The Galaxy Mass (in Stars) Function
• Bell et al. 2003:
use galaxy colors to
estimate (M/L)stars
and hence Mstars
Early type == concentrated light
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Compare this to the local group
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The local size distribution of galaxies
•
Shen et al 2003 from SDSS
Note: Size Specific Angular Momentum
Vatican 2003 Lecture 20 HWR
Present-day color distribution of galaxies
(Strateva etal 2001, Hogg etal 2002)
Vatican 2003 Lecture 20 HWR
The Population of Galaxies at Earlier Epochs
• Basic look-back approach to galaxy
evolution:
– Select earlier epoch (=redshift
slice) and identify galaxies.
– Measure their properties and
compare to “now”
• Milestones of such surveys:
– Canada-French-Redshift Survey
(CFRS): Lilly, LeFevre etal. 1990’s
– LDSS and Auto-Fib Survey: Ellis,
Broadhurst et al 1990’s
– Hubble Deep Field: Williams et al.,
Madau etal.
• Basic Data:
– Redshifts, luminosities,
SFRs(?),SEDs
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COMBO-17 in practice
30´x30´
Vatican 2003 Lecture 20 HWR
COMBO-17
Technique
Eff.
[%]
Wavelength [nm]
Wolf, etal. 2001, A&A, 681;
Wolf, Meisenheimer, Roeser, 2001, A&A, 660
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Fit
SED and z
simultaneously
Wolf et al 2001
dz~0.015
works well to z~1
Vatican 2003 Lecture 20 HWR
Redshift Histograms in COMBO-17 Fields
>25,000 redshifts
(Wolf et al. 2002)
Fields: 30´x30´each
Existing comparable
surveys:
CFRS,CNOC2,Keck
Incl. GOODS !
<1000 redshifts
(Wm/WL)=(0.3/0.7)
Vatican 2003 Lecture 20 HWR
Recent Evolution of the Galaxy Population
using “COMBO-17” as an example
• Sample: 30,000 galaxies to
mr~24 and to z~1.2
• How did the luminosity
function of galaxies evolve
over the last 8x109 years?
– This depends very much on
their SED-type (~color),
assumed to be non-evolving!!
– Red galaxies used to be
much more rare!
- Luminous, blue galaxies used
to be much more common
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Global averages
What Light from what type of galaxies?
Starbursts
Sbc-S.B.
Sa-Sbc
E-Sa
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COMBO-17: How did galaxy colors
change since z~1 (Bell etal 2003)
.
Vatican 2003 Lecture 20 HWR
The (Red) Color-Magnitude
Relation
• The CMR zero-point
evolution is consistent
with passive aging of
ancient stellar populations.
• But: the total mass in star
in the “red-and-dead” is
now twice as high as it was
at z~1.
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GEMS: Key to “internal structure”
(Galaxy Evolution from Morphology and SEDs)
•
Large HST program: 125+50 orbits PI: H.-W. Rix
to image
“extended-Chandra-Deep-Field-South”
–
–
–
–
10,000 redshifts from COMBO-17
9x9 ACS tiles  150 x HDF
V and z
Limit: mz~27.5
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GEMS 58
1.5% of
total
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COMBO-17 (~0.7”) vs.
HST/ACS
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GEMS: Color vs Morphology
•
What kinds of galaxies do
we select?
Bulge-Dominated
Disk-Dominated
Interacting/Peculiar
Irregulars/Clumpy
.
Vatican 2003 Lecture 20 HWR
Summary
• Hubble constant (=distance scale) now known to 5-10%
• Expansion history can now be mapped with SN Ia to z>1.
• New generation of surveys (e.g. SDSS) now can present the
properties of the present day galaxy population as a function
of e.g. luminosity, size, color, (environment), etc..
• Properties of the galaxy population can now be mapped
(samples of >10,000) to z~1 and beyond:
– disappearance of luminous blue galaxies
– Red galaxies are becoming redder, etc..
Vatican 2003 Lecture 20 HWR