The cosmological formation of massive galaxies

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

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 ~ 4o r ~ Ma , ≥ 2, a no significant star formation

Szomoru et al. 2012

What are the implications for massive galaxy evolution?

o No ‘monolithic collapse’ at high redshift followed by passive evolution – galaxies would be too small and too red today

o No formation of massive present day elliptical galaxies by just ‘binary mergers of disk galaxies’ – small/large sizes cannot be explained

o Dissipative early formation – high phase space densities

o 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

o Evolution by a common process in hierarchical cosmologies:

‘minor’ mergers – major mergers of massive galaxies are ‘rare’ and stochastic

o Additional processes? – rapid/slow mass-loss (stars;AGN;M/L…)

Late assembly of outer stellar halos in progress

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)

Courtesy ofPierre-Alain Duc

Inside-out growth since z = 2

Hilz et al. 2012, 2013

o Major mergers result in moderate redistribution of stars (White 1978/1979/1980)

o Minor mergers result in significant inside-out growth (Villumsen 1983)

Inside-out growth since z = 2

Hilz et al. 2013

Minor mergers easily increase the Sersic index by depositing stars at large radii - a process promoted by the presence of dark matter

Relaxation and Stripping – minor mergers promote rapid structural evolution

Hilz et al. 2012, 2013, see e.g. Boylan-Kolchin et al.2008, Libeskind et al. 2011 for M31 & MW

oMajor mergers show a moderate increase in

concentration

oRapid increase of Sersic index in minor mergers

with dark matter

oMajor mergers mix dark matter into the center – relaxation (Boylan-Kolchin et

al. 2008, Hilz et al. 2012, Hilz et al. 2013)

oMinor mergers increase galaxy sizes enclosing

more dark matter – stripping (Hilz et al. 2012, Hilz et al. 2013)

Inside-out growth since z = 2

Hilz et al. 2013

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

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’

Isolated speroid with outflow timescales 0 – 80 Myrs (Ragone-Figuera & Granato 2012)

Cosmological zoom simulation of BCG with AGN (Martizzi et al. 2012)

Binary disk galaxy merger simulation with AGN (Choi et al. 2012, Choi et al. in prep. , see Hopkins et al. 2010 for a discussion)

The collisionless assembly of central cluster galaxies…

Laporte, White, Naab & Gao 2013

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)

Central cluster galaxies…

Laporte, White, Naab & Gao 2013

Which two BCG’s had the most major mergers?

Cosmological predictions from models

o 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

o Galaxies lower than Milky Way mass grow by in-situ star formation only – galaxy mergers are unimportant

o Most massive galaxies grow by mergers at all epochs (see also De Lucia & Blaizot 2007)

o 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

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)

Behroozi et al. 2012

Moster et al. 2013

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

oCosmological simulations are the ultimate way to understand this process

oCompact high-redshift galaxies form naturally

(e.g. Joung et al. 2009, Naab et al. 2009, Sommer-Larsen et al. 2010)

oTypical contribution of mergers (> 1:4) in

massive galaxies since z=2 is 30% - 40%

oExtract dark matter and galaxy merger histories for zoom-simulations

Hirschmann et al. 2012, Oser et al. 2012

ex-situ

in-situ

The origin of stars in massive galaxies

Naab et al. 2007

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

Johansson et al. 2012

In-situ vs. accreted

Lackner et al. 2012 cosmological simulations

Hirschmann et al. 2012 using the Somerville et al. semi-analytical models

Oser et al. 2010

The rapid size evolution of spheroids

Oser et al. 2012

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)

Size evolution … and some consequences

oMore massive galaxies had more accretion

oIn-situ stars are the core and accreted stars build the outer envelope

oMass-size relation is driven by accretionOser et al. 2010

Formation and assembly of stars

Oser et al. 2010, 2012

In cosmological simulations stars at large radii form early and are accreted late in minor (mass-weighted mean of 1:5) mergers

Central dark matter fractions

The average central dark matter fractions agree with estimates from lensing and dynamical modeling - see SLACS

Barnabe et al. 2011

Assessing the global impact of feedback

oTest the effects of metal cooling, enrichment and feedback on the formation and evolution of galaxies in spatially resolved simulations

oFull analysis of the evolution of central galaxies and satellites

oComparison of simulations with and without metal enrichment and metal enrichment with winds

Hirschmann et al., 2013

metals metals & winds

SFR and metals

o Feedback delays the onset of early star formationo Drives low mass galaxies to higher present day star formation rateso 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. 2012)

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

o The relative importance of accretion of gas and stars determines the galaxy properties

o At low redshifts massive galaxies grow only by stellar mergers – following the cosmological assembly of the dark matter halos

o Major mergers with and without gas are rare but have dramatic effects on mass growth, morphology and dynamical properties

o The effect of stellar major mergers on the structural evolution is less dramatic

o Minor mergers are very frequent and seem to be the main driver for the structural evolution of massive galaxies

o 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

o 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..

Conclusions