(2M, 2nd row, 3rd column) and kinematic twists (KT, lower right panel) are found in the family of fast rotators, while kinematically distinct cores (KDC, upper right panel) are a common feature in slow rotating galaxies. Some of those slow rotating galaxies even show no sign of ordered motion at all, and thus are called low-level velocity galaxies (LV, upper left panel of Fig. 1.9). Galaxies with counter rotating cores (CRC, 1st row, 3rd panel) occur in both families, as well as the 2σgalaxies (lower left panel). This last feature cannot be seen in the velocity maps but only in the velocity dispersion maps, where there are two off-center,but symmetric peaks. The majority of all early-type galaxies, neverthe-less, is still featureless (NF, second column in Fig. 1.9), and slow rotating early-type galaxies more often tend to possess features. Understanding the origin of those features will possibly provide insight into the different channels that are important for the formation of early-type galaxies.
While these results have proven that early-type galaxies are not at all featureless, dead systems, the current survey was limited to the innermost parts of the galaxies (the area within the projected effective radius). More kinematic features are likely to appear if larger radii are included in the study, as has been already shown for a small subset of the Atlas3Dby Foster et al. (2013) and Brodie et al.
(2014), who studied the kinematics in the outskirts of early-type galaxies using globular clusters (GCs) as tracer population (see Chap. 6 for more details). More extended studies are planned for the future, using instruments like MUSE to measure the kinematics. In addition, extremely deep optical imaging (Duc et al., 2015) with the aim to find remnants of (recent) merger events will also help to understand the complex mechanisms that lead to the formation of the most massive stellar structures in the Universe.
1.3. DYNAMICAL PROCESSES 21
their total massMas Ekin = 1
2Mhv2i, (1.9)
and the mean potential energy of the system can be calculated as Epot =−G M2
R , (1.10)
where R is a suitably defined “gravitational radius” andG is the gravitational constant. Using the virial theorem, this leads to a simple correlation between the system’s mean velocity, mass and radius (denoted asvvir,MvirandRvir, respectively):
1
2Mvirv2vir = 1 2
G M2vir
Rvir ; (1.11)
⇒ v2vir = G Mvir
Rvir , (1.12)
see also Mo et al. (2010), Binney & Tremaine (2008) and Goldstein et al. (2002) for more details. The virial mass and the virial radius are correlated as
Mvir= 4π
3 R3virρcrit∆ (1.13)
with ∆a variable that describes the amount of overdensity of the virialised halo as a fraction of the critical density of the Universe (see Eq. 1.5)ρcrit, and it depends on the applied cosmology. This is, of course, a rather crude approximation to the real states of galaxies, and the underlying equilibrium state to which galaxies evolve through their lifetimes is a multi-component problem and thus much more complex. Nevertheless, especially for galaxy clusters and globular clusters, the virial theorem is a good approximation to measure masses and radial extends. This is especially useful in the case of galaxy clusters, since the dark matter is the most dominant constituent in those structures. On the downside, most clusters in the Universe are not relaxed, especially in their outskirts, and thus those measurements are only approximations.
In practice, especially in observations,∆ = 200, that is an overdensity of 200 times the critical density of the Universe, is often used to approximate the virial mass and radius, and thus are denoted asM200andR200. As reported for example by Kravtsov (2013), there is also a close relation between the stellar half-mass radiusr1/2of a central galaxy (i.e., the radius which contains half the stellar mass of the galaxy) and theR200of the total halo, namelyr1/2≈0.015R200.
In such a spherical system which is in a relaxed equilibrium state, the time that a test mass (for example a stellar particle or a satellite galaxy) needs to orbit once through this system, the so called dynamical time, can be calculated as
tdyn=
s 3π
16Gρ¯ with ρ¯ = 3M
4πR3 (1.14)
the average density in the halo, as long as the test mass is small enough compared to the host system.
This is similar to the so-called free-fall timetff = tdyn/√
2, the time that a uniform, pressure free sphere needs to collapse into a point mass (Mo et al., 2010, p. 14). The free-fall time is often used to calculate the time which a particle that can redistribute its energy (for example a gas particle) needs to fall into the center of the halo (see also App. A.2 where this timescale is actually related to our results for the origin of the cold gas in spheroidal galaxies).
1.3.2 Disturbing the Equilibrium: Accretion and Merging
While the idea of a relaxed state of a collisionless system is useful to analytically solve equations of motions and calculate potentials, in reality most systems are not relaxed and far from an equilibrium state. In a standardΛCDM universe, structures grow hierarchical through merging processes, and especially at low redshifts this is the most important mechanism of significant mass growth. Even most disk galaxies show signs of structure accretion, for example in form of streams like the Sagittarius stream in our own galaxy, the Milky Way. All those processes significantly disturb the equilibrium.
The most important processes in those interaction scenarios are tidal stripping and galaxy merging.
Tidal strippingis a process which can happen to any two particle systems that pass each other in close proximity. In such a case, particles especially at the outskirts of those systems, which are less strongly bound, gain energy through the tidal forces between those systems, that is part of the orbital energy of the encounter is transformed into internal energy, increasing the binding energy between those two bodies and potentially causes the systems to merge eventually. However, this only happens in case that the speed of the encounter is high enough so that the deformations due to the tidal forces are lacking behind the encounter event, since otherwise the effects would cancel each other. Even in case that the systems do not finally merge, they exchange particles during the close passage due to the tidal forces, and those non-merging events are called fly-by events.
Depending on the mass ratio of the two merging particle systems, different kinds of structure can be caused by the tidal forces: If an intruder system is small compared to the main system, e.g., if a dwarf galaxy is accreted by a large galaxy, it is orbiting inside the dark matter halo of the host galaxy, while it feels the tidal forces from the host. As it is not a point mass, the particles of the satellite feel slightly different forces, depending on their internal binding energy. Those stars which are farther at the outskirts of the satellite feel the strongest forces, and the transfer of energy rips them away from the dwarf galaxy’s center, causing them to form leading or trailing tails in front or behind the orbit of the satellite inside the host halo potential. In time, the satellite gets more and more disrupted, building up a long, thin stream along the orbit of the satellite. The most well known example of such a tidal stream caused by the dispersion of a satellite galaxy in its host galaxies halo is the Sagittarius stream around the Milky Way (Newberg et al., 2002). In general, smaller systems need longer to be fully dispersed, and as such the remnants of the accretion of small satellites can be seen the longest in the stellar halo of the host galaxy.
If the two merging systems have nearly the same mass, both feel similarly strong tidal forces. In those cases, the outer, least bound particles of those systems feel a similar strong transfer of energy and are accelerated, i.e., ripped from the galaxies and form long tails. This mechanism is most efficient for encounters of dynamically cold systems, i.e., late-type galaxies, while there are hardly any tails in encounters of dynamically hot systems, i.e., early-type galaxies. The orbit of the encounter also plays an important role: if the orbital angular momentum and the disk angular momentum are aligned, the tidal tails are the most prominent, while they are the least prominent in case of a retrograde encounter (see also Toomre & Toomre 1972). One of the most famous examples of a galaxy pair with extended tidal tails are the well studied Antennae galaxies with tidal tails extending for more than 100 kpc, but there are many more galaxy pairs in the nearby Universe which exhibit more or less extended tidal tails, indicating that merging events between two (late-type) galaxies of similar mass are not an uncommon event.
If the orbital energy and angular momentum of two interacting systems is low enough, the systems will eventually merge. This event is calledgalaxy merging, and it is considered to be one of the most
1.3. DYNAMICAL PROCESSES 23
important building blocks of galaxy formation. The higher the orbital energy and angular momentum of the encounter, the longer it takes for the stellar systems to merge, that is to transfer the orbital energy into the internal energy of the newly formed system. If the orbital energy of an encounter between two galaxies is negative, the orbit is called “elliptical” (or bound), if the orbital energy of the encounter is positive, the orbit is called “hyperbolical” (or unbound). Systems which collide on a bound orbit will always merge eventually. Interacting systems on an originally hyperbolical orbit will most likely not be able to merge, however, since the tidal interactions during the encounter (the first passage) already transfer orbital energy into internal energy of the systems, this might deplete enough orbital energy and enable a successive merging event, as long as the angular momentum of the orbit is not too large.
As the outermost areas of the galaxies merge first, and the stellar parts of the galaxies usually live at the centers of extended dark matter halos, the dark matter parts are the first ones to merge. As the baryonic parts of the galaxies live at the deepest parts of the potential, they survive as individual objects much longer than their surrounding dark matter halos, and therefore it is possible for two galaxies to be still in the process of merging while they are already moving around in a common, large dark halo. This is, for example, the case in clusters of galaxies, but also already in compact groups (Remus, 2009, and references therein). While the likelihood for a merging event in a group environment is high due to the enhanced galaxy density, in a cluster environment, on the contrary, it is very unlikely for all galaxies but the BCG to undergo a merging event: since the velocity dispersion of the cluster is much higher (σ≈1000 km/s) than the internal velocities of the satellite galaxies on their orbits in the cluster, a merger event is nearly impossible as their encounter speed is too high to bind them (Mo et al., 2010).
In a merger event between two galaxies of similar mass, the properties of the resulting galaxy depend on the properties of the progenitor galaxies and the orbit of the encounter. Merger events between two galaxies where one is much more massive than the other one, however, usually only slightly change the morphology of the main progenitor galaxy, as their contribution to the mass is low and many of their stars are ripped away while they are still in the outer parts of the main progenitor (however, since in-situ star formation does basically not occur in the outer halos of galaxies, those accreted stars from small satellite mergers are most likely the main building blocks of the stellar outer halos of galaxies). Mergers with a (stellar) mass ratio between 1:1 and 3:1 are usually called “major merger”, while mergers with (stellar) mass ratios larger than 3:1 are called “minor merger”. While one single minor merger event can hardly change the morphology of a galaxy significantly, several minor merger events in a row can, similar to a major merger event, alter the appearance of a galaxy significantly (see for example Oser et al., 2010; Mo et al., 2010, and references therein).
In general, smaller systems need longer to be fully dispersed, and as such the remnants of the accretion of small satellites can be seen the longest in the stellar halo of the host galaxy. The remnants of such encounters, like tidal streams of shells, are a powerful tool to analyse the potential of the host galaxy and gather information about the mass accretion events of the host galaxy. Throughout this thesis, the impact of merger events on the host galaxies’ dark matter and stellar halos will be discussed in more detail, especially in the light of information which is encoded in the outskirts of the halos about the formation history of a galaxy. For more details on simulations of merger events see Chap. 2, for more details on the theory of dynamical interactions during galaxy encounters see Binney
& Tremaine (2008), Mo et al. (2010) and references therein.
1.3.3 Relaxing the System: Phase Mixing, Violent Relaxation and Dynamical Friction There are several processes that lead to a relaxation of a collisionless system once it is distorted (some of those processes also continue even when the system is relaxed). The most important ones of those relaxation processes are phase mixing, violent relaxation, and dynamical friction. Phase mixingis a process that occurs in every galaxy whether it is relaxed or not. It basically describes the fact that two particles that at a given time have nearly the same orbit with similar velocities inside a common potential, will separate with time due to the slight differences in their phase-space characteristics without changing their energy contents (Binney & Tremaine, 2008, pp. 379). Thus, stars that are formed inside the same molecular cloud will redistribute over the whole galaxy in time. Similarly, the tidal streams formed from a satellite galaxy orbiting in a galaxy potential will lose their coherence with time, until they finally are broadly distributed inside the stellar halo of the host galaxy. However, even if the phase mixing has seemingly dissolved information about the origin of the stars inside a satellite, in phase space the information is still present (as long as a system did not undergo violent relaxation). Therefore, this could be used to estimate the origin of stars inside a galaxy, but since phase space information is extremely difficult to gain observationally, it is only rarely used. Mixing usually occurs on timescales similar to the dynamical time (see Eq. 1.14), but can also be much longer.
Violent relaxation, on the other hand, only occurs in systems where the gravitational potential changes. If a potential is changed, for example due to a merger event, the energies of the orbits of the (collisionless) particles change accordingly until a new equilibrium state is reached, and the system
“reorders” itself. This is discussed in detail by Lynden-Bell (1967), who also showed that violent relaxation usually takes place on timescales similar to the free-fall time, and is thus a relatively fast process (explaining the choice of the name). However, the end state of a relaxed system in a cos-mological context is not fully understood yet, and the statistically approach used so far has several difficulties (Mo et al., 2010, pp. 254). Nevertheless, this is an extremely interesting problem, espe-cially in the light of new observations of galaxies which are strong lenses, where a direct measurement of the total density profiles is possible, as will be discussed later on. The nature of this relaxed state is one of the key questions this thesis tries to address, and as such it will be in the main focus of chapters 3 and 4.
One additional process which is important in re-ordering a collisionless system is dynamical friction. This process occurs in all systems with collisionless particles of different masses. If a particle with a higher mass is moving through a cloud of particles with lower masses, part of its energy and momentum are transfered to the particles with lower mass, thus slowing down the more massive particle while slightly speeding up the lower mass particles. Basically, the gravitational force of the field of less massive particles is dragging at the more massive particle while it is moving with a higher velocity than its surrounding particles, slowing it down. As a consequence, the orbit of the massive particle decays as its orbital energy is decreasing. This leads to a segregation of masses, with more massive particles orbiting farther inside the potential than less massive particles. The time that a particle (or a bound system of particles like a satellite galaxy) of mass MSat needs to orbit from its initial orbit at the outskirts into the center of the potential of the host halo of mass MHostdue to dynamical friction is (for the approximation of a circular orbit) given as
tdf ≈ 1.17 ln(MHost/MSat)
MHost MSat
! 1
10H(z), (1.15)
where 101H(z) ≈ rHost/vcircis used as an approximation (see Mo et al., 2010, pp. 557 and references therein). Thus, dynamical friction is faster the more massive the infalling particle/system is. This
1.3. DYNAMICAL PROCESSES 25
estimate of the dynamical friction timescales can change depending on the properties (e.g., mass loss, presence of a gaseous component) and the orbit of the system. However, the fact that it is generally faster for more massive systems is always valid.
1.3.4 Involving the Gas: The Impact of Dissipation
So far we have only considered collisionless systems, that is systems which only interact through gravity. However, most galaxies also contain a gas component, which usually splits up in a cold gas disk (mostly in late-type galaxies, but they can also appear in early-type galaxies as discussed before) and a hot, spheroidal halo surrounding the whole galaxy. In contrast to the collisionless parts of a galaxy, the gas can redistribute energy much more efficient, and thus especially redistribute the angular momentum, transporting it from the outskirts of the galaxies to their inner parts (see for example Teklu et al., 2015b, and references therein). This ability to redistribute energy enables the gas to cool and settle in a disk perpendicular to the major axis of the angular momentum vector on circular orbits, therefore causing the existence of disk galaxies. While this is an important process already during the undisturbed, secular evolution of a galaxy, where the star formation is driven by the clumping of the gas in the central parts of the galaxies in those disks, it also significantly alters the outcome of merging events.
If cold gas is present during a merger event, it already collides during the first encounter, leading to enhanced star formation on very short timescales, so called star burst events (e.g., Mihos & Hernquist, 1996), as well as enhanced activity of the central black hole (enhanced AGN activity). This is also observed in case of the Antennae system and other ongoing merger events. Even on large scales, that is galaxy cluster mergers, where the hot component that does not form stars is the dominant gas component, the gas behaves different from the collisionless components, as can be seen in case of the Bullet cluster: While the collisionless components of the merging clusters are far apart from each others and have not merged yet, X-ray observations show that the hot gas component leaves a clear strong signal in between those two components. This indicates that the Bullet cluster already had its first encounter, which caused the hot halos of both clusters to collide and settle in the common center of mass of those two merging clusters, while the collisionless components take longer to redistribute the orbital energy and build up a new, massive cluster structure. This again shows that the same processes that are important on galaxy scales, are also important on the largest scales where the dark matter component is even more dominant than on galaxy scales.
Commonly, in case of galaxy encounters we distinguish between two different kinds of merger events: dry merger are merger events in which only very little or no cold gas is involved, while merger events with a large amount of cold gas involved are calledwet merger. In the major merger scenario, dry merger events usually lead to the formation of a spheroidal galaxy. Even in case of a cold gas fraction of 20% in the progenitor galaxies, i.e., the amount of cold gas found in typical present-day massive spiral galaxies, the final galaxy after the merger event resembles a spheroidal, albeit the remaining cold gas settles in a small gas disk at its centers. In a wet major merger, however, the resulting galaxy after the merger event might as well be a disk galaxy with a strong bulge at its center (Springel & Hernquist, 2005; Schlachtberger, 2014).
In the minor merger scenario, the effects of the gas are similar to the major merger scenario, especially if the mass ratios are close to the major merger case. One special case should, nevertheless, be mentioned at this point: Observationally it is known that late-type galaxies are also surrounded by dwarf galaxies, as it is the case for the Milky Way and Andromeda. Therefore, merging events are very
likely, and as mentioned before they do not influence the morphology of the main galaxy drastically as long as the merger events are not too many and the mass ratios between the infalling satellite and the main galaxy are large (10:1 or more). Still, the accretion of a satellite onto the disk transports energy from the encounter to the disk, and thus leads to a heating of the disk, which basically means that the stellar disk is getting thicker with each merger and effectively is destroyed (Purcell et al., 2009).
Therefore, the likelihood to find spiral galaxies with thin disks like the Milky Way or Andromeda should be low. The reason why spiral galaxies with thin disks still exist despite the high fraction of minor merger events in the hierarchical Universe is the presence of the gas: Due to its ability to redistribute energy, the gas component in the disk can absorb most of the kinetic energy brought in by the merger event and dissipate it, and even might regrow a disk after the merger event if enough gas is present (Moster et al., 2010). Thus, the gas stabilizes the disk, and therefore, as long as a galaxy can replenish its gas disk, minor merger events of mass ratios 10:1 or larger can effectively not destroy the disk.
1.3.5 The Galaxy Cluster Environment
If a galaxy enters a very dense environment, i.e., a galaxy cluster, there are further processes that can significantly alter the appearance of a galaxy and disturb its internal secular evolution processes.
As mentioned before in Sec. 1.2.2, the number of spheroidal galaxies in the dense environments is enhanced compared to the field. However, the likelihood for a merger event of two random galaxies inside a cluster potential is very low, because the velocities of galaxies in the cluster potential are so large that encounters between two galaxies do not lead to a capture and successive merging. However, if such a high speed encounter between two galaxies is happening, orbital energy is still transferred from the encounter to the internal energy of the galaxies. This effectively results in a heating of the galaxies, which means the particles within the galaxies become less bound, especially in the outskirts, and eventually even get stripped from the galaxy and feed the intra-cluster light component. Every high-speed encounter lessens the binding energy of the particles, puffing up the galaxies and enabling particle stripping. This process is calledgalaxy harassment. The less compact an object, the more sensitive are its outskirts to harassment processes, and thus harassment could transform a disk galaxy into a small compact spheroidal by ripping away the outer areas and only leaving the central bulge component behind.
Another important process in cluster environments isram-pressure stripping, where first the hot halo and then even the cold gas component are stripped from a (disk) galaxy while it moves inside the cluster environment due to the pressure caused by the hot gas halo of the cluster (see Sec. 1.2.2 for more details on observations of ram pressure stripping inside the Virgo cluster). Jaff´e et al. (2015) have shown that one orbital period inside a galaxy cluster halo can be sufficient to strip a galaxy offits gas, nevertheless, the presence of molecular gas might actually lower the efficiency of ram-pressure stripping since the gas of the hot halo is too thin to strip offmolecular gas from a galaxy effectively.
However, there is an additional process which is efficient in a cluster environment, which is called strangulation: Inside the cluster environment, galaxies (but the BCG) cannot accrete new gas from its environment, i.e., they cannot replenish their cold gas once star formation has drained the galaxy.
Both processes together lead to a very fast and efficient shut offof the star formation, and thus to a change of the appearance, from a blue to a red spiral. Once the massive stars have died, the galaxy loses its spiral appearance and morphologically transforms into a spheroidal.
Those processes all apply to all galaxies in a cluster environment but the BCG. As the BCGs are