Evolution of Surfae Density - Dissipationless merging and the evolution of early-type galaxies

Nextwetake aloserlookatthe surfae densitiesof the mergerremnants. In the rst

and third panel of the left olumn in Fig. 7.3, the surfae density of the equal-mass

mergers grows at all radii, i.e. the lines are shifted more or less parallel to higher

densities. This piture is the same for both major merger histories of one- (rst left)

and two- (third left) omponent models. Correspondingly, the mass assembles at all

radii, whih is depited in the small panels beyond the respetive surfae densities.

This evolution senario is ontrary to the observations of van Dokkum et al. (2010)

(Fig. 2.2, Chapter 2), whih show, that the ompat early-type galaxies grow

inside-out, i.e. the entral densities stay onstant and most of the mass assembles at larger

radii, buildingup an extended envelope of stars.

The seond olumn depits the surfae densities and mass assembly of the minor

mergers with an initial mass ratio of 1:5. For the bulge only models (top) with a

diuse satellite(Sat 1:5in Fig. 7.1),the surfaedensitystays nearly onstantout toa

radiusof

r ≈ 1 − 2

^kp^and ^inreases ^mainlyⁱⁿ^the ^outer^parts. ^This^behavior ^an ^also

be seen for the orresponding mass assembly. The solid lines in the last two panels

of the seond olumn show that the same senario is even more eient using

two-omponent models. Due to the massive dark matter halo, most of the bulge partiles

get stripped atlargerradiiand theentralsurfaedensity stays unaeted. Therefore,

itjustinreases atradii

r > 2 − 3

^kp^and^most^of^the ^size^growth^is^due^to^the ^build^up

ofamassivestellarenvelope. Thedottedlineinthesepanelsdepitthe fourremnants,

where the satellites are more ompat (Sat 1:5 in Fig. 7.1) and lie on the

z ∼ 2

Figure 7.3: First row of panels: Surfae densities for the bulge only models. For the

equal-mass mergers (left), they inrease at all radii and for ve generations (from blak

to red)of minor mergers with an initial mass ratio of 1:5 (middle) they grow more in the

outer regions. For an initial mass ratio of 1:10 (right), we an see the same behavior,

i.e. after the seond (blue), forth (green) and sixth (red) generation the surfae density

slightly inreases at largeradii and stays onstant in the enter. The panels of the seond

row show the mass assembly aording to the surfae densities of the top panels. Third

row: Surfae densities ofthe orrespondingtwo-omponent models. Again,forequal-mass

mergers (left), the surfae density gets shifted parallel to higher values, but for a higher

initial mass ratio of 1:5 (1:10), we an learly see, that the entral surfae densities stay

onstant and a lot of partiles assemble at radii larger than

r > 2

^kp ⁽

r > 4

^kp), ^whih

is very similar to the inside-out growth senario of van Dokkum et al. (2010) (see also

Fig. 2.2 in Chapter 2). Regarding the aordingmass assembly of the bulge+halomodels

(bottomrow),it is even moreobvious, thatthe galaxies growinside-out. Thedotted lines

for the 1:5 minor mergers indiate, that the aretion of a more ompat satellite (Sat

1:5)yieldsthesameresults. Butinthe1:10senariowithompatsatellites, morematerial

goes further towards the enter and less material assembles at large radii. Nevertheless,

the entral density alsostays onstant (for

r < 2

⁾^and ^most^of ^the ^areted^partiles ^build

relation of Williams et al. (2010). Obviously, as the sale radii are very similar, the

results stay the same.

The six generations of minor mergers with an initialmass ratio of 1:10 are shown

in the last olumn of Fig. 7.3. In the ase of bulge only models (top), the surfae

density grows predominantly at largerradii, similar to the previous senario, but now

thesatelliteiseven lessboundomparedtothe1:5aseandtherefore,itgetsdestroyed

rapidly,even withoutadarkmatterhalo. In theaseoftwo-omponentminormergers

of diuse satellites(solidlines, thirdand forthpanel)this eet beomesenhaned,as

the satelliterst orbits through the massive dark matter halo, before it gets loser to

thehost'senter. Thenallthe materialgetsstrippedatverylargeradiiandthesurfae

density stays onstantout to aradius of

r = 5

^kp. ^Regarding ^the ^mass ^assembly, ^this

is even moreobvious, asthe entralmass stays onstant out to aradius of

r ∼ 10

^kp.

Therefore,thissenarioseemstobeveryeient,astheoutersurfaedensityinreases

signiantly, although the total amount of added mass is 40% less than for the 1:5

hierarhy, where the initial host mass gets doubled. However, this evolution senario

might be too extreme ompared to observations (Fig. 2.2) and we an rule out the

very diuse satellitesat aredshift of

z ∼ 2

This piture hanges, if we use the more bound, ompat satellite (Sat 1:10),

depited with the dotted lines in the last two panels. As the sale length of this

satellite is two times smaller, it is muh more bound and resists the drag fore of the

hostpotentialforalongertime. Consequently,morematerialgetsloser tothe entral

regions, the remnant's surfae densities grow outside a radius of

r > 2

^kp ^and ^more

mass assembles at smaller radii. Regarding the aording mass assembly (last panel),

it grows predominantly outside a radius of

5

^kp, ^whih ^is ^also ^more ^onsistent ^with

the observed evolution.

In Fig. 7.4 we show the evolution of a full set (10 generations) of two-omponent

minor mergers with a ompat satellite (Sat 1:10, Fig. 7.1), but due to muh lower

omputation time, we took radial orbits. Comparing the surfae densities of this

se-quene(solidlines,Fig. 7.4)withthefourgenerationswithangularmomentum(dotted

lines, Fig. 7.3and 7.4),they evolvenearly thesame. The surfae densities(toppanel)

stay onstantout toa radiusof

r ≈ 2

^kp^and ^the ^high^size ^growth ^of ^a^fator ^of

≈ 4.5

(see setion 7.3) is driven by building up an extended envelope of luminous material.

The mass assembly alsolooks very promising, as mostof the partilesarete at radii

largerthan

5

^kp. ^The^results^of ^this^senario^are ^very ^similar^to^the ^1:5^minor^mergers

of two-omponentmodels, whihniely resembletheobservations (vanDokkum etal.,

2010).

Altogetherthat means,that the mass assembly inminormergersstrongly depends

onthe eet of dynamial fritionand tidal stripping, whih of ourse are muh more

eient for the two-omponent models, where the dark matter strips the partiles of

the in-fallingsatellites at exatly the right regions of the initial host galaxy. As

on-sequene, nearly no material aretes in the entral regions and therefore the entral

surfae density staysonstant. Lookingatthearetion of diusesatellitesinthe 1:10

senario, they seem tobe tooextreme and lose their materialat toolarge radii.

How-Figure 7.4: Toppanel: Surfaedensities alongthemajoraxisfor10generationsof

head-onminormergerswith theompatbulge+halosatelliteSat1:10(Fig. 7.1). Thedierent

olors give the generation and the dotted lines highlight the ompat minor mergers with

angular momentum of the last panel in Fig. 7.3. Bottom panel: Assembled mass plotted

againsttheradius,asinFig. 7.3. Asourperientri distanesareverysmall,bothsenarios

show a very similar evolution, i.e. the more ompat satellites go further to the enter,

omparedto the lessboundsatellites (Sat 1:10, Fig. 7.1),and the hostassembles mass at

radii

r > 5

^kp.

ever, fortheotherbulge+halomodelsourresultsareingoodagreementtoobservations,

where the ompat early-type galaxies make up the entral ores of today's elliptial

(Hopkins etal.,2009a;Bezansonetal.,2009;vanDokkum etal.,2010;Szomoru etal.,

2011). Without a dark matter halo, the eet of dynamial frition is muh weaker

and the minor mergers of bulgeonly systems giveless promising results.

Im Dokument Dissipationless merging and the evolution of early-type galaxies (Seite 104-108)