Galaxy formation: lecture 4 - Astronomy at Durham University
Transcript of Galaxy formation: lecture 4 - Astronomy at Durham University
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Galaxy formation: lecture 4
Carlton Baugh Institute for Computational Cosmology
Durham University ICTP Summer School on Cosmology
Trieste 2012
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Lecture 4
• Bias: How do galaxies trace the dark
matter?
• Outstanding problems in galaxy formation
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Two-degree Field Galaxy Redshift Survey
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Sloan Digital Sky Survey
faint bright
Late type Early-type
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Biased galaxy formation
Kaiser 1984 – originally derived to explain clustering of clusters
Clusters associated with high peaks in density field
Galaxies may trace DM distribution in a complicated way
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Galaxy clustering
vs
dark matter
clustering
• Galaxy correlation
function ~ power
law over 3-4
decades in r
• DM correlation
function not a power
law
• Scale dependent
bias
Jenkins et al. 1998
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Associate galaxies with DM haloes
instead of DM or peaks
• First “Halo Occupation
Distribution” model
• Scale dependent bias
• No low mass cut off
• No split between
centrals and satellites
Jing, Mo & Boerner 1998
“biased” CDM
Las Campanas
Redshift survey
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Predict connection between different
galaxy samples and dark matter
H-a selection H-band selection
Orsi et al. 2009 z=1
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Bias for different galaxy samples
Angulo et al. 2008
Driven by prediction for N(M) by following baryonic physics
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Galaxy clustering in SAMs
• Models that match LF
give robust predictions for
correlation function
• Can recover power-law
simply by predicting
number of galaxies per
halo
Benson et al. 2000
Kauffmann et al. 1999a, b
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Galaxy clustering from gas dynamics
Pearce et al. 1999
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Explaining the form of the
correlation function
Benson et al. 2000
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Halo Occupation Distribution
Break down galaxy clustering into contributions from pairs within
same DM halo (1-halo term) and in different haloes (2-halo term)
David Weinberg
centrals
satellites
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Zehvai et al. 2004, 2005; review by Sheth & Cooray 2002
1-halo
2-halo
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Han-Seek Kim et al. 2009
Models
predict
HOD
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How robust are the predictions
from different SAMs?
Contreras et al. 2012
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Predict clustering for different selections:
e.g. cold gas mass
Mass of host DM halo
Gala
xy c
old
ga
s m
ass
Universal baryon fraction in cold gas in one object within halo
Han-Seek Kim et al. 2011
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Predict HOD for cold gas samples
Han-Seek Kim et al. 2011
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Associate galaxies with sub-haloes?
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Avoiding “overmerging” of DM haloes
• Should we
compare
galaxies with
haloes or
subhaloes?
• Early
simulations
lacked mass &
force resolution
to follow
subhalos
Klypin et al. 1999
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Hierarchies of substructure
Springel et al 2008
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Matching
sub-haloes to
“galaxies”
• Put cut on
subhalo circular
velocity
• Associate
subhaloes with
galaxies
• Early version of
SHAM
Colin et al. 1999
Klypin et al. 1999
Kravtsov et al. 2004
x (
r )
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SHAM – sub-halo abundance matching
• Assume a monotonic relation between (sub)halo mass and galaxy luminosity
(Vale & Ostriker 2004; 2006; 2008)
KEY ASSUMPTIONS:
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Which halo mass to assign?
central
satellite
Use mass of substructure at infall for satellite
Assign all galaxies mass of host halo: Main subhalo
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SHAM – sub-halo abundance matching
• Assume a monotonic relation between (sub)halo mass and galaxy luminosity
(Vale & Ostriker 2004; 2006; 2008)
• For central galaxy, use host halo mass
• For satellite galaxies, use sub-halo mass at time of accretion (Kravtsov et al 2004; Nagai & Kravtsov 2005)
KEY ASSUMPTIONS:
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SHAM in action
• Use SDSS stellar mass function
• Use Millennium simulations (sub)halo mass functions
• Need to resolve subhalos
• Guo et al. 2010
MI
MII
(sub)halo mass function
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Which galaxies are in which halos?
• Match SDSS obs. stellar mass function to Millennium subhalos using SHAM
• Peak in M*/Mhalo
• Guo et al. 2010
Stel
lar
mas
s
(sub)halo mass
Stel
lar
mas
s/h
alo
mas
s
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Testing SHAM with simulations
SPH simulations Simha et al. 2011
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Stellar fraction in gas simulations
Guo et al. 2010
SPH runs tend to convert too many baryons to stars
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SHAM in SAM Stellar mass vs host halo mass
Host halo mass
Stel
lar
mas
s centrals
satellites
Bower et al. 2006 z=0
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SHAM in SAM: Stellar mass vs (sub)halo mass
centrals
satellites
Bower et al. 2006 z=0
(sub)halo mass
Stel
lar
mas
s
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SHAM in SAM: Stellar mass vs (sub)halo mass
centrals
satellites
all galaxies
Medians, 10-90 percentile limits
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http://galaxy-catalogue.dur.ac.uk:8080/Millennium/
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Some outstanding problems in galaxy formation
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Is substructure a problem for CDM?
Moore et al 1999
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Particle physics solution?
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Astrophysical solution?
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Navarro, Frenk & White 1997
The assembly of DM Haloes
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Navarro, Frenk & White 1997
A universal density profile?
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Slide from Andrew Pontzen)
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Multiple episodes of inflow and outflow Shake up inner part of DM halo, softening cuspy core
Pontzen & Governato 2012 Governato et al. 2012
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Forming realistic disks in CDM
• For a long time simulations failed to produce disks with the observed scale lengths
• Insufficient resolution?
• Insufficient sub-grid physics (feedback)
• Weil et al. • Sales et al. • Governato et al
20004, 2007 • Zavala et al 2008
Bulge dominated Disk dominated
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Forming realistic disks in CDM
Weak feedback Strong feedback
Zavala et al. 2008
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Massive galaxies at z>0
Gemini Deep Deep Survey Glazebrook et al. 2004
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Evolution of the stellar
mass function
Bower et al. 2006
Interpretation relies on choice of IMF Do analyses take into account scatter in M/L?
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The Tully-Fisher relation
Halo vcirc
Galaxy vcirc at
Half-mass radius
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Summary
Efstathiou 2003