Summary & Disussion

stripping, it enhanes the eet and due to the nally high eetive radii, the dark

matterfrationalsogrowsbynearlyafatorof2after10generationsofminormergers.

•

^{Violent relaxation does not eet the overall dierential energy} distributions of the host galaxy.

•

^{Duetodynamialfritionandtidalstrippingthestellarpartilesdevelopa}

promi-nent bumpat low bindingenergies.

•

^{The veloity dispersionofthe bulgeonly}(one-omponent)models donot hange their shape,keep theirinitialHernquist proleand stay isotropioverthe whole

radial range.

•

^For two-omponent aretions the nal oalesene of the bulges always is on radial orbits, the stellar veloities beome radially anisotropi at radii

approxi-mately larger than the spherialhalf-mass radius

•

^{Using diuse} satellites, the mean square speeds of the remnants derease with eahsubsequentgeneration,whihisonlylimitedbythehighamountofmass-loss

and onsequently the gravitational radii inrease muh less than expeted.

•

^{The head-on minor mergers of ompat} one-omponent models evolve nearly homologous, i.e. the observable values like the line-of-sight veloity dispersion

and the eetiveradius evolve very lose to those of the whole system.

•

^{In all other minor merger sequenes, we observe a dramati break of homology,}

asthe remnantsbuild up anextended envelope ofstars, whilethe entral

ong-uration stays onstant.

•

^{Therefore the eetive radii inrease rapidly up toafator of}

4.5

^{, whihismuh}

loser to virialexpetations.

•

^{The rapidsize growthresultsina signiantinrease of thedarkmatterfration}

within the spherialhalf-mass radius up toa fator of

∼ 1.8

^.

•

^{Due to the inreasing dark matter fration, the eetive} line-of-sight veloity dispersions donot derease but stay onstant.

One important question whih has to be solved for elliptial galaxies is, how the

ompat early-types at a redshift

z ∼ 2

^{grow with time. As their stellar} distribution is already red without signiant star formation, we used dry mergers to explain this

evolution. van Dokkum et al. (2010) nds a size-mass relation of

r e α M ^2.04

^{, whih}

indiatesa size inrease of a fator of4 asthe galaxy'smass gets doubled sine

z ∼ 2

^.

The resultingrelationof our minor mergersenarios of two-omponent models iseven

higher

r e α M ^>2.04

^{up to a exponent of}

2.4

^{, whih shows that} dissipationless minor mergers are a good way to solve this problem. However, Nipotiet al. (2009a) tried a

similarapproahandndamuhlowersizeinreaseintheirsimulations(

r _e ∝ M ^1.09

^).

One reason forthis bigdisrepany is,that they alulated the exponent of thestellar

mass by averagingoverall theirmergerhierarhies. As they havemore major mergers

than minor mergers, this of ourse lowers the size inrease signiantly. Additionally,

they use a steeper slope for the stellar density prole of the host and the satellite

galaxies, where the size inrease an not be that eient, as the areted material

is more onentrated in the satellite's enter ompared to our setup. Finally, their

satellites are even more ompat than our satellites whih lie on an extrapolation of

the

z = 2

^{mass-size relationof Williamsetal. (2010).}

Furthermore we nd that the dark matter frations for our idealized simulations

agreewellwithpreviouswork,wherethe darkmatterfrationinreasesindrymergers.

This hanges the ratio of dynamial and stellar mass and might, e.g. help to explain

the tiltof the fundamentalplane (Boylan-Kolhin etal.,2005). Ofourse, that isjust

one possibility to explain the tilt and Grillo & Gobat (2010) suggest that it depends

moreon

M ∗ /L

^{, butit isnotlear yethowstrongthe single} ontributions are. We also agree with Nipoti etal. (2009a),that the inrease of the dark matterfration is more

eientfor minormergersandforthis senarioisdominatedbythe rapidsize growth.

ButinontrasttoNipotietal.(2009a),wend thattheentraldarkmatterfrationof

equal-massmergersillustratesa'real'hangeaused byviolent relaxationand mixing.

Looking at the veloities at dierent radii, our minor merger results are not able

to explain reent observations of very high veloity dispersions at high redshift (van

Dokkum et al., 2009; van de Sande et al., 2011). Our results indiate, that we get a

derease of the mean square speeds of the total system, but the observed line-of-sight

veloity dispersionhardly hanges. This indiates, that simple dissipationlessmergers

arenotabletodereasetheveryhighLOSVDofsomeompatearlytypegalaxies(van

Dokkumet al.,2009). Thisproblemmightbesolved,if we inludesome gas andAGN

feedbak oruse more realisti galaxymodels, whih have dierent orbital properties.

Butaltogether ourwork shows thatdissipationlessdrymergersareable toinrease

the size of aompat earlytypegalaxy. As welieeven abovethe observed preditions

asmallamountofgas,whihisknown tolowerthesizegrowth (Covingtonetal.,2011;

Hopkins et al.,2008), would perhaps not be enoughto destroy this senario.

CHAPTER 7

SIZE AND PROFILE SHAPE

EVOLUTION OF MASSIVE

QUIESCENT GALAXIES

In this hapter, we fous on the evolution of the density struture and the

size evolution of ompat early-type galaxies and try to understand the

im-portane of dark matter. We know that the sizes and mass distributions of

ompat, quiesent, massive galaxies evolve rapidly from

z ∼ 2 − 3

^{to the}

present. Many of the

∼ 10 ¹¹

^{systems athigh redshift havesizes of}

∼

^{1kp and}

surfae brightness proleswith Sersi indies

<

^{4. At}

z = 0

^{elliptialgalaxies}

above

2 · 10 ¹¹

^{solar masses are morethan afator of 4larger, indiatinga size}

evolution of r

∝ M ^α

^with

α ≥ 2

^{. They also have surfae brigtness proles}

with

n ser ≥ 8

^{. Within a hierarhial galaxy formation senariothis evolution}

an be explained under two assumptions. The galaxies predominantly grow

by mergerswith lower massgalaxies and the galaxies have tobe embedded in

massive dark matter halos so that stars of merging satellites are stripped at

large radii inreasing the prole shapeparameter. We drawthese onlusions

fromidealizedsimulationsof thegrowth ofompat spheroidalgalaxies -with

and without dark matter - by repeated ollisionlessmergers with mass ratios

of 1:1, 1:5, and 1:10. In simulations withoutdark matter the sizes evolve less

than the orresponding bulge+halo senarios. If the galaxies are embedded

in darkmatter halos the stars of the lower mass satellites are more eiently

stripped at large radii resultingin a signiantly faster size inrease than

ex-petedfromvirialestimates. Repeated1:5mergersgive

α = 2.3

^andafteronly

twomergergenerations theSersiindex has alreadyinreased to

n ser > 8

^{. For}

an assumed mass inrease of the observed galaxies of a fator of two sine z

=2 we onlude that the presene of a massive dark matter halo around the

galaxies duringtheir minor mergerdriven assembly is neessary to explain

si-multaneously their large present day sizes, r

>

^{4 kp and high Sersi indies,}

n _ser > 6

^.

7.1 Introdution

Intheurrentlyfavoredosmologial

Λ

^{CDMmodel,theuniverseonsistsof}

24%

^matter

and

76%

^{darkenergy (}

Λ

^{),where only}

4%

^{of the total matter is inbaryoni form(e.g.}

Spergel etal. (2007)). The other 96%onsist of old darkmatter,whihhas not been

deteteddiretly,buthasbeenmostsuessfullyappliedtoexplainmanyobservational

aveats like the rotation urves of spiral galaxies. On large sales the

Λ

^{CDM model,}

shows verygoodagreementwithobservationsoftheosmimirowavebakgroundand

the large sale struture of galaxies. In the ontext of the

Λ

^{CDM model, struture in}

the universe forms bottom up (White & Rees, 1978; Davis et al., 1985), where the

rst objets ollapse at high redshifts due to utuations in the bakground density

eld. Theserstobjetsmergeandbuildup thedarkmatterhalosoftoday'sobserved

galaxies.

The baryons assemble in the potential wells of these dark matter halos and form

stars whihbuildthe observablepartsof theuniverse. Thebrightestandmost massive

objets are elliptial galaxies, whih form ata redshift of

z ∝ 2 − 3

^{in gas-rih major}

diskmergers(Davisetal.,1985;Bournaudetal.,2011)anddue togiantoldgas ows,

diretly feeding the entral galaxy (Kere² et al., 2005; Naab et al., 2007, 2009; Joung

et al., 2009;Dekel etal.,2009; Kere² etal.,2009; Oser etal.,2010). Theirsubsequent

evolutionisnotfullyunderstoodyet,astheseelliptialsareafator

∼

^{4-5smallerthan}

their ounterparts in the present day universe. On the other hand, they are already

quiesent, without star formation, and are only a fator of

∼

^{2 less massive (Daddi}

et al., 2005; Trujilloet al., 2006; Longhettiet al., 2007; Toft et al., 2007; Zirm et al.,

2007;Trujillo etal.,2007;Zirm et al.,2007; Buitrago et al., 2008; vanDokkum et al.,

2008; Cimatti et al., 2008; Franx et al., 2008; Sarao et al., 2009; Damjanov et al.,

2009; Bezanson et al., 2009). In addition, they have very dierent surfae brightness

proles. In partiular, the ompat, high redshift elliptials always have steep power

lawusps intheirenter whereas themore extended presentday elliptialshaveored

proles. This means, that tting a Sersi prole to elliptial galaxies either results

in entral extra light, where the entral surfae brightness is above the tted prole

or in ase of ore elliptials the prole predits more light than the galaxy has. The

Sersi indies,whih area measurementof theprole'survatureare

∼

^{4forthe uspy}

galaxies and

∼

^{8-10 for the ore} elliptials. Furthermore, reent observations of strong gravitationallensinginthe SLACS sample(Koopmansetal.,2006;Boltonetal.,2008;

Gavazzietal.,2007,2008;Auger etal.,2009,2010)haverevealedaninreasingentral

dark matterfration with stellar mass (Barnabè etal., 2011).

Inthishapter,weinvestigatetheevolutionofelliptialgalaxieswiththeaidof

high-resolution N-body simulations of idealized one- and two-omponent galaxy models.

Withdierentinitialmassratios andadierenthoie ofmergerorbitswe explorethe

eetoffrequentdissipationlessgalaxymergers. Insetion7.2wegiveashortoverview

of the galaxy properties and the simulation parameters. In Setion 7.3we present the

eieny ofdry mergersfor the size growthof ompat galaxies and inSetion7.4we

lookatthe evolutionofthe surfaedensitiesand themass assemblyinmultiplemerger

generations. InSetion7.5weonvert oursurfaedensitiestoviablesurfaebrightness

prolesand explore the evolutionof the Sersi proleand in Setions 7.6we illustrate

the hange of dark matterfrations. Finally wedraw our onlusions inSetion 7.7.

Im Dokument Dissipationless merging and the evolution of early-type galaxies (Seite 94-100)