The Orbital Structure of Galaxies and Dark Matter Halos in N-Body Simulations

We examine in this work two different formation mechanism of galaxies in N-body simulations. Under the assumption that particles in a spherical dark matter halo move on circular orbits we can predict the amount of contraction of the dark matter halo during the slow formation of the disk with an adiabatic approximation. We find in N-body simulations that the adiabatic approximation is valid for all realistic mass ratios between dark matter halos and disk galaxies and that deviations from circular orbits cannot play a decisive role. In the second part we focus on the formation of ellipticals through mergers of disk galaxies. We classify the complex orbital structure in a sample of 150 collisions. The classification is correlated with shape parameters of an elliptical galaxy, such as its triaxiality or the ratios of its principal axes. We are deriving a global occupation probability for self-consistent triaxial galaxies which are in agreement with theoretical expectations. Furthermore we find that the isophotal structure of the merger remnants cannot be explained by one orbit class alone, but by a superposition of classes. The dichotomy of observed isophotal shape in boxy and disky elliptical galaxies, cannot be completely explained by the dominance of box-like, respectivley disk-like orbits in those galaxies. Current simulations cannot reproduce observed correlation between the h_3 parameter and the mean velocity. We identify a central box orbit component as the reason for this discrepancy, which are overproduce in dissipationless simulations. The z-tube component follows the correlation very well. It follows also the observed correlation between the h_3 parameter and v/sigma_0. We conclude that only one dynamical component is necessary to explain the observed correlations, which looks like a puffy disk with high velocity dispersion

The Validity of the Adiabatic Contraction Approximation for Dark Matter Halos

We use high resolution numerical simulations to investigate the adiabatic contraction of dark matter halos with a Hernquist density profile. We test the response of the halos to the growth of additional axisymmetric disk potentials with various central concentrations and the spherically symmetric potential of a softened point mass. Adding the potentials on timescales that are long compared to the dynamical time scale of the halo, the contracted halos have density profiles that are in excellent agreement with analytical predictions based on the conservation of the adiabatic invariant $M(r)r$. This is surprising as this quantity is strictly conserved only for particles on circular orbits and in spherically symmetric potentials. If the same potentials are added on timescales that are short compared to the dynamical timescale, the result depends strongly on the adopted potential. The adiabatic approximation still works for disk potentials. It does, however, fail for the central potential.Comment: 7 pages, 3 figures, 1 table. Added reference. Accepted for publication in ApJ

2-Dimensional Kinematics of Simulated Disc Merger Remnants

We present a two-dimensional kinematic analysis for a sample of simulated binary disc merger remnants with mass ratios 1:1 and 3:1. For the progenitor discs we used pure stellar models as well as models with 10% of their mass in gas. A multitude of phenomena also observed in real galaxies are found in the simulations. These include misaligned rotation, embedded discs, gas rings, counter-rotating cores and kinematic misaligned discs. Using the 2D maps we illustrate projection effects and the change in properties of a merger remnant when gas is included in the merger. We find that kinematic peculiar subsystems are preferably formed in equal mass mergers. Equal-mass collisionless remnants can show almost no rotation, regular rotation or strong kinematic misalignment. The inclusion of gas makes the remnants appear more round(1:1) and axisymmetric(3:1). Counter-Rotating Cores (CRCs) are almost exclusively formed in equal-mass mergers with a dissipational component. 3:1 remnants show a much more regular structure. We quantify these properties by applying the kinemetric methods recently developed by Krajnovi\'c et al. This work will help to understand observations of elliptical galaxies with 2D field spectrographs, like SAURON.Comment: accepted for publication in MNRAS, discussion substantially enlarged, conclusion unchange

SAURON's Challenge for the Major Merger Scenario of Elliptical Galaxy Formation

The intrinsic anisotropy delta and flattening epsilon of simulated merger remnants is compared with elliptical galaxies that have been observed by the SAURON collaboration, and that were analysed using axisymmetric Schwarzschild models. Collisionless binary mergers of stellar disks and disk mergers with an additional isothermal gas component, neglecting star formation cannot reproduce the observed trend delta = 0.55 epsilon (SAURON relationship). An excellent fit of the SAURON relationship for flattened ellipticals with epsilon >= 0.25 is however found for merger simulations of disks with gas fractions >= 20%, including star formation and stellar energy feedback. Massive black hole feedback does not strongly affect this result. Subsequent dry merging of merger remnants however does not generate the slowly-rotating SAURON ellipticals which are characterized by low ellipticities epsilon < 0.25 and low anisotropies. This indicates that at least some ellipticals on the red galaxy sequence did not form by binary mergers of disks or early-type galaxies. We show that stellar spheroids resulting from multiple, hierarchical mergers of star-bursting subunits in a cosmological context are in excellent agreement with the low ellipticities and anisotropies of the slowly rotating SAURON ellipticals and their observed trend of delta with epsilon. The numerical simulations indicate that the SAURON relation might be a result of strong violent relaxation and phase mixing of multiple, kinematically cold stellar subunits with the angular momentum of the system determining its location on the relation.Comment: 13 pages, 3 figures, submitted to Ap

2-Dimensional Kinematics of Simulated Disc Merger Remnants

ABSTRACT We present a two-dimensional kinematic analysis for a sample of simulated binary disc merger remnants with mass ratios 1:1 and 3:1. For the progenitor discs we used pure stellar models as well as models with 10% of their mass in gas. A multitude of phenomena also observed in real galaxies are found in the simulations. These include misaligned rotation, embedded discs, gas rings, counter-rotating cores and kinematic misaligned discs. Using the 2D maps we illustrate projection effects and the change in properties of a merger remnant when gas is included in the merger. We find that kinematic peculiar subsystems are preferably formed in equal mass mergers. Equal-mass collisionless remnants can show almost no rotation, regular rotation or strong kinematic misalignment. The inclusion of gas makes the remnants appear more round(1:1) and axisymmetric(3:1). Counter-Rotating Cores (CRCs) are almost exclusively formed in equal-mass mergers with a dissipational component. 3:1 remnants show a much more regular structure. We quantify these properties by applying the kinemetric methods recently developed by Krajnović et al. This work will help to understand observations of elliptical galaxies with integral field spectrographs, like SAURON

Specific Angular Momentum of Disc Merger Remnants and the λR\lambda_R-Parameter

We use two-dimensional kinematic maps of simulated binary disc mergers to investigate the $\lambda_R$-parameter, which is a luminosity weighted measure of projected angular momentum per unit mass. This parameter was introduced to subdivide the SAURON sample of early type galaxies in so called fast $\lambda_R > 0.1$ and slow rotators $\lambda_R < 0.1$. Tests on merger remnants reveal that $\lambda_R$ is a robust indicator of the true angular momentum content in elliptical galaxies. We find the same range of $\lambda_R$ values in our merger remnants as in the SAURON galaxies. The merger mass ratio is decisive in creating a slow or a fast rotator in a single binary merger, the former being created mostly in an equal mass merger. Slow rotators have a $\lambda_R$ which does not vary with projection. The confusion rate with face-on fast rotators is very small. Merger with low gas fractions form slow rotators with smaller ellipticities and are in much better agreement with the SAURON slow rotators. Remergers of merger remnants are slow rotators but tend to have too high ellipticities. Fast rotators maintain the angular momentum content from the progenitor disc galaxy if merger mass ratio is high. Some SAURON galaxies have values of $\lambda_R$ as high as our progenitor disc galaxies.Comment: 12 pages, 11Figures, submitted to MNRA

The influence of gas on the structure of merger remnants

We present a large set of merger simulations of early-type disc galaxies with mass ratios of 1:1 and 3:1 and 10% of the total disc mass in gas. In contrast to the collisionless case equal-mass mergers with gas do not result in very boxy remnants which is caused by the suppression of box orbits and the change of the projected shape of minor-axis tube orbits in the more axisymmetric remnants. The isophotal shape of 3:1 remnants and the global kinematic properties of 1:1 and 3:1 remnants are only weakly affected by the presence of gas. 1:1 remnants are slowly rotating whereas 3:1 remnants are fast rotating and discy. The shape of the stellar LOSVD is strongly influenced by gas. The LOSVDs of collisionless remnants have broad leading wings while their gaseous counterparts show steep leading wings, more consistent with observations of elliptical galaxies. We show that this change is also caused by the suppressed populating of box orbits and it is amplified by the formation of extended gas discs in the merger remnants. If elliptical galaxies have formed from mergers our results indicate that massive, slowly rotating boxy elliptical galaxies can not have formed from dissipative mergers of discs. Pure stellar (dry) mergers are the more likely candidates. On the other hand lower mass, fast rotating and discy ellipticals can have formed from dissipative (wet) mergers of early-type discs. So far, only unequal-mass disc mergers with gas can successfully explain their observed substructure. This is consistent with the revised morphological classification scheme of increasing importance of gas dissipation when moving from boxy ellipticals to discy ellipticals and then to spiral galaxies, proposed by Kormendy & Bender (abbreviated).Comment: accepted for publication by MNRA

Relaxation and Stripping: The Evolution of Sizes, Dispersions and Dark Matter Fractions in Major and Minor Mergers of Elliptical Galaxies

We revisit collisionless major and minor mergers of spheroidal galaxies in the context of the size evolution of elliptical galaxies. The simulations are performed as a series of mergers with mass-ratios of 1:1 and 1:10 for models representing pure bulges as well as bulges embedded in dark matter halos. For major and minor mergers, respectively, we identify and analyze two different processes, violent relaxation and stripping, leading to size evolution and a change of the dark matter fraction within the observable effective radius. Violent relaxation - which is the dominant mixing process for major mergers but less important for minor mergers - scatters relatively more dark matter particles than bulge particles to small. Stripping in minor mergers assembles stellar satellite particles at large radii in halo dominated regions of the massive host. This strongly increases the size of the bulge into regions with higher dark matter fractions leaving the inner host structure almost unchanged. A factor of two mass increase by minor mergers increases the dark matter fraction by 20 per cent. We present analytic corrections to simple one-component virial estimates for the evolution of the gravitational radii. If such a two-component system grows by minor mergers alone its size growth, $r_{\mathrm{e}} \propto M^\alpha$, reaches values of $\alpha \approx 2.4$, significantly exceeding the simple theoretical limit of $\alpha = 2$. For major mergers the sizes grow with $\alpha \lesssim 1$. Our results indicate that minor mergers of galaxies embedded in massive dark matter halos provide a potential mechanism for explaining the rapid size growth and the build-up of massive elliptical systems predicting significant dark matter fractions and radially biased velocity dispersions at large radii (abbreviated)Comment: accepted for publication in MNRA