Andreas Faltenbacher

Heating Cooling Flows with Weak Shock Waves

The discovery of extended, approximately spherical weak shock waves in the hot intercluster gas in Perseus and Virgo has precipitated the notion that these waves may be the primary heating process that explains why so little gas cools to low temperatures. This type of heating has received additional support from recent gasdynamical models. We show here that outwardly propagating, dissipating waves deposit most of their energy near the center of the cluster atmosphere. Consequently, if the gas is heated by (intermittent) weak shocks for several Gyr, the gas within 30-50 kpc is heated to temperatures that far exceed observed values. This heating can be avoided if dissipating shocks are sufficiently infrequent or weak so as not to be the primary source of global heating. Local PV and viscous heating associated with newly formed X-ray cavities are likely to be small, which is consistent with the low gas temperatures generally observed near the centers of groups and clusters where the cavities are located.

Velocity distributions in clusters of galaxies

We employ a high-resolution dissipationless N-body simulation of a galaxy cluster to investigate the impact of subhalo selection on the resulting velocity distributions. Applying a lower limit on the present bound mass of subhaloes leads to high subhalo velocity dispersions compared to the diffuse dark matter (positive velocity bias) and to a considerable deviation from a Gaussian velocity distribution (kurtosis ∼ -0.6). However, if subhaloes are required to exceed a minimal mass before accretion on to the host, the velocity bias becomes negligible and the velocity distribution is close to Gaussian (kurtosis ∼ -0.15). Recently, it has been shown that the latter criterion results in subhalo samples that agree well with the observed number-density profiles of galaxies in clusters. Therefore, we argue that the velocity distributions of galaxies in clusters are essentially unbiased. The comparison of the galaxy velocity distribution and the sound speed, derived from scaling relations of X-ray observations, results in an average Mach number of 1.24. Altogether 65 per cent of the galaxies move supersonically and 8 per cent have Mach numbers larger than 2 with respect to the intracluster gas.

Imprints of mass accretion on properties of galaxy clusters

A large-scale smoothed particle hydrodynamics (SPH) plus N-body simulation (GADGET) of the concordance A cold dark matter (CDM) universe is used to investigate orientation and angular momentum of galaxy clusters at z = 0 in connection with their recent accretion histories. The basic cluster sample comprises the 3000 most massive friends-of-friends (FoF) haloes found in the 500 h -1 Mpc simulation box. Two disjoint subsamples are constructed, using the mass ratio of the two most massive progenitors at z = 0.5 m 2 /m 1 [m 1 ≥ m 2 ), namely a recent major merger sample and a steady accretion mode sample. The mass of clusters in the merger sample is on average ∼43 per cent larger than the mass of the two progenitors (m + m 2 ), whereas in the steady accretion mode sample a smaller increase of ∼25 per cent is found. The separation vector connecting the two most massive progenitor haloes at z = 0.5 is strongly correlated with the orientation of the cluster at z = 0. The angular momentum of the clusters in the recent major merger sample tends to be parallel to orbital angular momentum of the two progenitors, whereas the angular momentum of the steady accretion mode sample is mainly determined by the angular momentum of the most massive progenitor. The long-range correlations for the major and the minor principal axes of cluster pairs extend to distances of ∼100 h -1 Mpc. Weak angular momentum correlations are found for distances ≤20 h -1 Mpc. Within these ranges, the major axes tend to be aligned with the connecting line of the cluster pairs, whereas minor axes and angular momenta tend to be perpendicular to this line. A separate analysis of the two subsamples reveals that the long-range correlations are independent of the mass accretion mode. Thus, orientation and angular momentum of galaxy clusters is mainly determined by the accretion along the filaments independently of the particular accretion mode.

Spatial distribution of galactic halos and their merger histories

We use a novel statistical tool, the mark correlation functions (MCFs), to study clustering of galaxy- size halos as a function of their properties and environment in a high-resolution numerical simulation of the CDM cosmology. We applied MCFs using several types of continuous and discrete marks: maximum circular velocity of halos, merger mark indicating whether halos experienced or not a major merger in their evolution history (the marks for halo with mergers are further split according to the epoch of the last major merger), and a stripping mark indicating whether the halo underwent a tidal stripping (i.e., mass loss). We nd that halos which experienced a relatively early ( z> 1) major merger or mass loss (due to tidal stripping) in their evolution histories are over-abundant in halo pairs with separations <3 h 1 Mpc. This result can be interpreted as spatial segregation of halos with dierent merger histories, qualitatively similar to the morphological segregation in the observed galaxy distribution. In addition, we nd that at z = 0 the mean circular velocity of halos in pairs of halos with separations <10 h 1 Mpc is larger than the mean circular velocity vcirc of the parent halo sample. This mean circular velocity enhancement increases steadily during the evolution of halos from z =3t oz = 0, and indicates that the luminosity dependence of galaxy clustering may be due to the mass segregation of galactic dark matter halos. The analysis presented in this paper demonstrate that MCFs provide powerful, yet algorithmically simple, quantitative measures of segregation in the spatial distribution of objects with respect to their various properties (marks). This should make MCFs very useful for analysis of spatial clustering and segregation in current and future large redshift surveys.

Evolution of Characteristic Quantities for Dark Matter Halo Density Profiles

We have investigated the effect of an assembly history on the evolution of galactic dark matter (DM) halos of 1012 h-1 M☉ using constrained realizations of random Gaussian fields. Five different realizations of a DM halo with distinct merging histories were constructed and have been evolved using collisionless high-resolution N-body simulations. Our main results are as follows: A halo evolves via a sequence of quiescent phases of a slow mass accretion intermitted by violent episodes of major mergers. In the quiescent phases, the density is well fitted by an NFW profile, the inner scale radius Rs and the mass enclosed within it remain constant, and the virial radius (Rvir) grows linearly with the expansion parameter a. Within each quiescent phase the concentration parameter (c) scales as a, and the mass accretion history (Mvir) is well described by the Tasitsiomi et al. fitting formula. In the violent phases the halos are not in a virial dynamical equilibrium and both Rs and Rvir grow discontinuously. The violent episodes drive the halos from one NFW dynamical equilibrium to another. The final structure of a halo, including c, depends on the degree of violence of the major mergers and the number of violent events. Next, we find a distinct difference between the behavior of various NFW parameters taken as averages over an ensemble of halos and those of individual halos. Moreover, the simple scaling relations c-Mvir do not apply to the entire evolution of individual halos, and therefore we have the common notion that late-forming halos are less concentrated than early-forming ones. The entire evolution of the halo cannot be fitted by single analytical expressions.

Constrained Cosmological Simulations of Dark Matter Halos

The formation and structure of dark matter (DM) halos is studied by means of constrained realizations of Gaussian fields using N-body simulations. A series of experiments of the formation of a 10^{12} Msun halo is designed to study the dependence of the density profile on its merging history. We confirm that the halo growth consists of violent and quiescent phases, with the density well approximated by the Navarro-Frenk-White (NFW) profile during the latter phases. We find that (1) the NFW scale radius R_s stays constant during the quiescent phase and grows abruptly during the violent one. In contrast, the virial radius grows linearly during the quiescent and abruptly during the violent phases. (2) The central density stays unchanged during the quiescent phase while dropping abruptly during the violent phase. (3) The value of \rs reflects the violent merging history of the halo, and depends on the number of violent events and their fractional magnitudes, independent of the time and order of these events. It does not reflect the formation time of the halo. (4) The fractional change in R_s is a nonlinear function of the fractional absorbed kinetic energy within R_s in a violent event.

On the dynamics of the satellite galaxies in NGC 5044

The NGC 5044 galaxy group is dominated by a luminous elliptical galaxy that is surrounded by ∼160 dwarf satellites. The projected number density profile of this dwarf population deviates within ∼1/3 of the virial radius from a projected Navarro, Frenk and White (NFW) profile, which is assumed to approximate the underlying total matter distribution. By means of a semi-analytic model, we demonstrate that the interplay between gravitation, dynamical friction and tidal mass loss and destruction can explain the observed number density profile. We use only two parameters in our models: the total to stellar mass fraction of the satellite haloes and the disruption efficiency. The disruption efficiency is expressed by a minimum radius. If the tidal radius of a galaxy (halo) falls below this radius, it is assumed to become unobservable. The preferred parameters are an initial total to stellar mass fraction of ∼20 and a disruption radius of 4 kpc. In that model, about 20 per cent of all the satellites are totally disrupted on their orbits within the group environment. Dynamical friction is less important in shaping the inner slope of the number density profile because the reduction in mass by tidal forces lowers the impact of the friction term. The main destruction mechanism is tide. In the preferred model, the total B-band luminosity of all disrupted galaxies is about twice the observed luminosity of the central elliptical galaxy, indicating that a significant fraction of stars are scattered into the intragroup medium. Dwarf galaxy satellites closer to the centre of the NGC 5044 group may exhibit optical evidence of partial tidal disruption. If dynamical friction forces the satellite to merge with the central elliptical, the angular momentum of the satellite tends to be removed at the apocentre passage. Afterwards, the satellite drops radially towards the centre.

Large scatter in X-ray emission among elliptical galaxies : Correlations with mass of host HALO

Optically similar elliptical galaxies have an enormous range of X-ray luminosities. We show that this range can be attributed to large variations in the dark halo mass Mvir determined from X-ray observations. The K-band luminosity of ellipticals varies with virial mass, LK ∝ M, but with considerable scatter, probably due to the stochastic incidence of massive satellite galaxies that merge by dynamical friction to form group-centered ellipticals. Both the observed X-ray luminosity LX ∝ M and LX/LK ∝ M are sufficiently sensitive to the virial mass to explain the wide variation observed in LX among galaxies of similar LK. The central galaxy supernova energy per particle of diffuse gas increases dramatically with decreasing virial mass, and elliptical galaxies with the lowest X-ray luminosities (and Mvir) are easily explained by supernova-driven outflows.

Baryonically closed galaxy groups

Elliptical galaxies and their groups having the largest LX/LB lie close to the locus LX = 4.3 × 1043(LB/1011 LB, ☉)1.75 expected for closed systems having baryon fractions equal to the cosmic mean value, fb ≈ 0.16. The estimated baryon fractions for several of these galaxies/groups are also close to fb = 0.16 when the gas density is extrapolated to the virial radius. Evidently they are the least massive baryonically closed systems. Gas retention in these groups implies that nongravitational heating cannot exceed about 1 keV per particle, consistent with the heating required to produce the deviation of groups from the LX-T correlation for more massive clusters. Isolated galaxies/groups with X-ray luminosities significantly lower than baryonically closed groups may have undermassive dark halos, overactive central AGNs, or higher star formation efficiencies. The virial mass and hot gas temperatures of nearly or completely closed groups correlate with the group X-ray luminosities and the optical luminosities of the group-centered elliptical galaxy, i.e., Mvir ∝ L, an expected consequence of their merging history. The ratio of halo mass to the mass of the central galaxy for X-ray-luminous galaxies/groups is Mvir/M* ~ 80.

Simulations of the Local Universe

One of the greatest challenges of modern astrophysics is understanding how galaxies, such as our Milky Way, form within the framework of the Big Bang cosmology. The current theory of structure formation, the extension of the Big Bang model called the Cold Dark Matter (CDM) scenario, predicts that galaxies form within extended massive dark matter halos built from smaller pieces that collided and merged, resulting in the hierarchy of galaxies, groups, and clusters observed today. The entire sequence of events is thought to be seeded by quantum fluctuations in the very early Universe and governed by mysterious “dark matter” which constitutes about 85% of all matter in the universe. Although the accurate properties of galaxies depend on complicated baryonic processes (radiative cooling, formation and evolution of stars, etc.) operating on small scales, we expect that overall spatial distribution of “dark matter” halos is closely related to the observed galaxy distribution. Here we present numerical simulations designed to study the formation, evolution and present day properties of such dark matter halos in different cosmological environments.