- Lorentz Center

The cosmological formation of massive
Thorsten Naab
MPA, Garching
What regulates galaxy formation?
Leiden, April, 22nd
How do massive galaxies get their gas and stars?
o Gas accretion involves dissipation – energy can be radiated away
o Gas forms disks which can form stars - eventually high phase space densities
can be reached, in particular in mergers or the (early) assembly of low angular
momentum gas
o Disk galaxies are built from the accretion of higher and higher angular
momentum gas
o Accretion of stars is dissipationless (almost energy conserving) – energy is
only transferred by dynamical friction
o The evolution of multi-component systems (stars, halo, gas) is complicated
Compact massive ellipticals at z≈2
Szomoru, Franx & van Dokkum 2012
Inside-out growth since z = 2
van Dokkum et al. 2010
Szomoru et al. 2012
o Stacks of 70-80 galaxies at different redshifts (van Dokkum et al. 2010) and direct
comparison to Virgo ellipticals (Szomoru et al. 2012) indicate inside-out growth of
ellipticals since z ≈ 2 (see also Patel et al. 2013)
o Mass increase by a factor of ~ 2 , Size increase by a factor of ~ 4
o r ~ Ma  a   no significant star formation
What are the implications for massive galaxy evolution?
No ‘monolithic collapse’ at high redshift followed by passive evolution –
galaxies would be too small and too red today
No formation of massive present day elliptical galaxies by just ‘binary
mergers of disk galaxies’ – small/large sizes cannot be explained
Dissipative early formation – high phase space densities
Size growth and mass growth is not dominated by star formation, unlike
for disk galaxies – average stellar populations are old and leave little
room for new stars born late
Evolution by a common process in hierarchical cosmologies: ‘minor’
mergers – major mergers of massive galaxies are ‘rare’ and stochastic
Additional processes? – rapid/slow mass-loss (stars;AGN;M/L…)
Late assembly of outer stellar halos in progress
Courtesy of
Pierre-Alain Duc
o The current assembly of the outer halos of elliptical galaxies can be observed with deep
imaging (Duc et al. 2012)
o This process is very important for galaxy clusters (see e.g. Laporte et al. 2013)
Inside-out growth since z = 2
o Major mergers result in moderate redistribution of stars
(White 1978/1979/1980)
o Minor mergers result in significant inside-out growth
(Villumsen 1983)
Hilz et al. 2012, 2013
Inside-out growth since z = 2
Minor mergers easily increase the Sersic index by depositing stars at large
radii - a process promoted by the presence of dark matter
Hilz et al. 2013
Relaxation and Stripping – minor mergers promote rapid
structural evolution
o Major mergers show a
moderate increase in
o Rapid increase of Sersic index
in minor mergers with
o Major mergers mix dark matter
into the center – relaxation
(Boylan-Kolchin et al. 2008, Hilz et al.
2012, Hilz et al.
o Minor mergers increase
galaxy sizes enclosing more
dark matter – stripping (Hilz et
al. 2012, Hilz et al. 2013)
Hilz et al. 2012, 2013, see e.g. Boylan-Kolchin et al.2008, Libeskind et al. 2011 for M31 & MW
Inside-out growth since z = 2
o Isolated 1:1 (mm) and 10:1 (acc) mergers of spheroidal galaxies
without (1C) and with (2C) dark matter
o Only minor mergers with dark matter result in inside-out growth
Hilz et al. 2013
The size evolution ‘problem’
Major and minor mergers might not be sufficient to explain the observed
size growth - in particular at 1 < z < 2 - and the small scatter in the scaling
relations (Newman at al. 2011, Nipoti et al. 2012)
Different conclusion by Oogi & Habe 2012, Hilz et al. 2013, Bedorf &
Portegies Zwart 2013 – size growth is sufficient
Is an additional process necessary?
Rapid outflow - ‘puffing up’
Binary disk galaxy
merger simulation with
AGN (Choi et al. 2012, Choi et
al. in prep. , see Hopkins et al.
2010 for a discussion)
Isolated speroid with
outflow timescales
0 – 80 Myrs (Ragone-Figuera
& Granato 2012)
Cosmological zoom
simulation of BCG with
AGN (Martizzi et al. 2012)
The collisionless assembly of central cluster galaxies…
High resolution dark matter simulations (Phoenix) of cluster assembly with a weighting
scheme to attach a stellar component at z =2 following observed size and theoretical
abundance contraints.
Assembly of BCG’s and cluster galaxies can be understood by collisionless mergers of z=2
progenitors without significant star formation (see also e.g. Bullock et al., Cooper et al 2013)
Laporte, White, Naab & Gao 2013
Central cluster galaxies…
Which two BCG’s had the most major mergers?
Laporte, White, Naab & Gao 2013
Cosmological predictions from models
Analysis of semi-analytical
models by Guo & White 2008
indicate that minor mergers
contribute more to galaxy
growth than major mergers –
except for high masses
Galaxies lower than Milky Way
mass grow by in-situ star
formation only – galaxy
mergers are unimportant
Most massive galaxies grow
by mergers at all epochs (see
also De Lucia & Blaizot 2007)
In-situ star formation becomes
more important at high redshift
Independent constraints from abundance matching
o Abundance matching techniques - rank order dark matter halos by mass and match observed
galaxy mass functions (Vale & Ostriker 2004, 2006; Conroy et al. 2006, Moster et al. 2010,
2013; Behroozi et al. 2010, 2013; Guo et al. 2010; CLF approach: van den Bosch et al. 2003;
Yang et al. 2012, 2013)
o Models by Moster et al. including orphans and a proper treatment of subhalos (Moster et al.
2010, Moster, Naab & White 2013)
Independent constraints on in-situ vs. accreted
Behroozi et al. 2012
Yang et al. 2013
Moster et al. 2013
Constraints from abundance matching show similar trends – at Milky Way mass major mergers
are NOT relevant (Moster, Naab & White 2012; Behroozi, Wechsler & Conroy 2012, Yang et al. 2013)
Global insights into galaxy assembly
o Galaxy formation is detached from halo formation - in different ways
at different halo masses
o Massive galaxies form ‘earlier’ than their halos, low mass galaxies
form ‘later’ than their halos (see also Conroy et al., Behroozi et al. 2010)
The complex cosmological assembly histories
o Cosmological simulations
are the ultimate way to
understand this process
o Compact high-redshift
galaxies form naturally (e.g.
Joung et al. 2009, Naab et al. 2009,
Sommer-Larsen et al. 2010)
o Typical contribution of
mergers (> 1:4) in massive
galaxies since z=2 is 30% 40%
o Extract dark matter and
galaxy merger histories for
Hirschmann et al. 2012, Oser et al. 2012
The origin of stars in massive galaxies
A significant fraction of stars in massive galaxies is accreted
(e.g. White & Rees 1978, Cole et al. 2000, Abadi et al. 2006, de Lucia et al. 2006, Cooper et al. 2010, Guo et al. 2011,
Laporte et al. 2012 and more…)
Feldmann et al. 2010
Naab et al. 2007
Johansson et al. 2012
In-situ vs. accreted
Oser et al. 2010
Hirschmann et al. 2012 using the
Somerville et al. semi-analytical models
Lackner et al. 2012 cosmological
The rapid size evolution of spheroids
Good agreement with observed strong size evolution for massive
early-type galaxies proportional to (1+z)α, α=-1.22 (Franx et al. 2008) ,
1.48 (Buitrago et al. 2008) , -1.17 (Williams et al. 2010)
Oser et al. 2012
Size evolution … and some consequences
o More massive galaxies had more accretion
o In-situ stars are the core and accreted stars build the outer envelope
o Mass-size relation is driven by accretion
Oser et al. 2010
Formation and assembly of stars
In cosmological simulations stars at large
radii form early and are accreted late in
minor (mass-weighted mean of 1:5)
Oser et al. 2010, 2012
Central dark matter fractions
The average central dark
matter fractions agree with
estimates from lensing and
dynamical modeling - see
Barnabe et al. 2011
Assessing the global impact of feedback
metals & winds
o Test the effects of metal cooling, enrichment and feedback on the formation and
evolution of galaxies in spatially resolved simulations
o Full analysis of the evolution of central galaxies and satellites
o Comparison of simulations with and without metal enrichment and metal
with winds
Hirschmann et al., 2013
SFR and metals
o Feedback delays the onset of early star formation
o Drives low mass galaxies to higher present day star formation rates
o Nice work by Haas et al. 2012/2013, Dave et al. 2013, Oppenheimer
& Dave 2006/ 2008/ 2010 etc., Kannan et al. 2013, Stinson et al. 2013,
and more
Accretion origin of population gradients
Higher fraction of in-situ vs. accreted in simulations with strong
Feedback from SN (Hirschmann et al. 2013)
The global impact of black hole feedback
o Black hole feedback reduces the in-situ star formation in massive
galaxies (Sijacki et al. 2006, 2007; Teyssier et al. 2010, Booth & Schaye 2009, 2010,
2011, 2012; Puchwein et al. 2010, 2012; Sijacki et al., Teyssier et al. 2012, Martizzi et al.
Puchwein et al. 2012
‘Heuristic’ feedback models
o Combination of momentum-driving and energy-driving scaling for low
mass galaxies motivated by Hopkins et al.
o Star formation in high-mass galaxies is artificially ‘quenched’ by heating
the gas component
Dave et al. 2013
The relative importance of accretion of gas and stars determines the galaxy properties
At low redshifts massive galaxies grow only by stellar mergers – following the cosmological
assembly of the dark matter halos
Major mergers with and without gas are rare but have dramatic effects on mass growth,
morphology and dynamical properties
The effect of stellar major mergers on the structural evolution is less dramatic
Minor mergers are very frequent and seem to be the main driver for the structural evolution
of massive galaxies
Minor mergers built the (massive) stellar halos of elliptical galaxies from old stars formed in
other galaxies, drive the evolution in dark matter fraction and physically link the stellar
component of the galaxies to their dark matter halos
Size growth, the concurrent increase in dark matter fraction, downsizing, profile shape
changes are a natural result of the hierarchical assembly of massive galaxies in modern
cosmologies – some of these conclusions made long ago from SAMs starting with Kauffmann et al.
1993, Khochfar & Silk 2006, Guo et al. 2011, Porter et al. 2012 etc..

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