The cosmological formation of massive galaxies Thorsten Naab MPA, Garching What regulates galaxy formation? Leiden, April, 22 nd

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 z2 Szomoru, Franx & van Dokkum 2012

Inside-out growth since z = 2 van Dokkum et al o Stacks of 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 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 of Pierre-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 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 for M31 & MW o Major mergers show a moderate increase in concentration o Rapid increase of Sersic index in minor mergers with dark matter o Major mergers mix dark matter into the center – relaxation (Boylan-Kolchin et al. 2008, Hilz et al. 2012, Hilz et al. 2013) o Minor 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 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 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 BCGs 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 BCGs 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 Moster et al 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 zoom-simulations Hirschmann et al. 2012, Oser et al ex-situ in-situ

The origin of stars in massive galaxies Naab et al 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 and more…) Feldmann et al Johansson et al. 2012

In-situ vs. accreted Lackner et al cosmological simulations Hirschmann et al using the Somerville et al. semi-analytical models Oser et al. 2010

The rapid size evolution of spheroids Oser et al Good agreement with observed strong size evolution for massive early-type galaxies proportional to (1+z) α, α=-1.22 (Franx et al. 2008), (Buitrago et al. 2008), (Williams et al. 2010)

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 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 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 enrichment with winds Hirschmann et al., 2013 metals metals & winds

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. 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 etc.. ConclusionsConclusions