Andreas Faltenbacher - Academia.edu (original) (raw)

Papers by Andreas Faltenbacher

Light from `point sources' such as supernovae is observed with a beam width of order of the s... more Light from `point sources' such as supernovae is observed with a beam width of order of the sources' size - typically less than 1 AU. Such a beam probes matter and curvature distributions that are very different from coarse-grained representations in N-body simulations or perturbation theory, which are smoothed on scales much larger than 1 AU. The beam typically travels through unclustered dark matter and hydrogen with a mean density much less than the cosmic mean, and through dark matter halos and hydrogen clouds. Using N-body simulations, as well as a Press-Schechter approach, we quantify the density probability distribution as a function of beam width and show that, even for Gpc-length beams of 500 kpc diameter, most lines of sight are significantly under-dense. From this we argue that modelling the probability distribution for AU-diameter beams is absolutely critical. Standard analyses predict a huge variance for such tiny beam sizes, and nonlinear corrections appear to ...

The Astrophysical Journal, 2012

Tracing the cosmic evolution of the Baryonic Acoustic Oscillation (BAO) scale with galaxy two poi... more Tracing the cosmic evolution of the Baryonic Acoustic Oscillation (BAO) scale with galaxy two point correlation functions is currently the most promising approach to detect dark energy at early times. A number of ongoing and future experiments will measure the BAO peak with unprecedented accuracy. We show based on a set of N-Body simulations that the matter distribution is anisotropic out to ∼ 150 h −1 Mpc, far beyond the BAO scale of ∼ 100 h −1 Mpc, and discuss implications for the measurement of the BAO. To that purpose we use alignment correlation functions, i.e., cross correlation functions between high density peaks and the overall matter distribution measured along the orientation of the peaks and perpendicular to it. The correlation function measured along (perpendicular to) the orientation of high density peaks is enhanced (reduced) by a factor of 2 compared to the conventional correlation function and the location of the BAO peak shifts towards smaller (larger) scales if measured along (perpendicular to) the orientation of the high density peaks. Similar effects are expected to shape observed galaxy correlation functions at BAO scales.

Monthly Notices of the Royal Astronomical Society, 2010

We examine the effects of environment on the dynamical structure of satellite systems based on th... more We examine the effects of environment on the dynamical structure of satellite systems based on the Millennium II Simulation. Satellite haloes are defined as subhaloes within the virial radius of a host halo. The satellite sample is restricted to those subhaloes which showed a maximum circular velocity above 30 km s −1 at the time of accretion. Host halo masses range from 10 11 to 10 14 h −1 M. We compute the satellites' average accretion redshift, z acc , velocity dispersion, σ , and velocity anisotropy parameter, β, utilizing stacked satellite samples of equal-mass hosts at similar background densities. The main results are as follows. (1) On an average, satellites within hosts in high-density environments are accreted earlier (z ≈ 0.1) compared to their counterparts at low densities. For host masses above 5 × 10 13 h −1 M this trend weakens, and may reverse for higher host masses. (2) The velocity dispersion of satellites in low-density environments follows that of the host, i.e. no velocity bias is observed for host haloes at low densities independent of host mass. However, for low-mass hosts in high-density environments the velocity dispersion of the satellites can be up to ∼30 per cent larger than that of the host halo, i.e. the satellites are dynamically hotter than their host haloes. (3) The anisotropy parameter depends on host mass and environment. Satellites of massive hosts show more radially biased velocity distributions. Moreover in low-density environments, satellites have more radially biased velocities (β 0.1) as compared to their counterparts in highdensity environments. We believe that our approach allows us to predict a similar behaviour for observed satellite galaxy systems.

Based on the Millennium Simulation we examine assembly bias for the halo properties: shape, triax... more Based on the Millennium Simulation we examine assembly bias for the halo properties: shape, triaxiality, concentration, spin, shape of the velocity ellipsoid and velocity anisotropy. For consistency we determine all these properties using the same set of particles, namely all gravitationally self-bound particles belonging to the most massive sub-structure of a given friends-of-friends halo. We confirm that near-spherical and high-spin halos show enhanced clustering. The opposite is true for strongly aspherical and low-spin halos. Further, below the typical collapse mass, M*, more concentrated halos show stronger clustering whereas less concentrated halos are less clustered which is reversed for masses above M*. Going beyond earlier work we show that: (1) oblate halos are more strongly clustered than prolate ones; (2) the dependence of clustering on the shape of the velocity ellipsoid coincides with that of the real-space shape, although the signal is stronger; (3) halos with weak ve...

Simulation (MS), we investigate the alignment between galaxies and large-scale struc-ture. For th... more Simulation (MS), we investigate the alignment between galaxies and large-scale struc-ture. For this purpose, we develop two new statistical tools, namely the alignment cor-relation function and the cos(2θ)-statistic. The former is a two-dimensional extension of the traditional two-point correlation function and the latter is related to the ellipticity correlation function used for cosmic shear measurements. Both are based on the cross correlation between a sample of galaxies with orientations and a reference sample which represents the large-scale structure. We apply the new statistics to the SDSS galaxy cat-alog. The alignment correlation function reveals an overabundance of reference galaxies along the major axes of red, luminous (L L∗) galaxies out to projected separations of 60 h−1Mpc. The signal increases with central galaxy luminosity. No alignment signal is detected for blue galaxies. The cos(2θ)-statistic yields very similar results. Starting from a MS semi-analytic galaxy c...

On the basis of a large scale ’adiabatic’, namely non-radiative and non-dissipative, cosmological... more On the basis of a large scale ’adiabatic’, namely non-radiative and non-dissipative, cosmological smooth particle hydrodynamic simulation we compare the entropy profiles of the gas and the dark matter (DM) in galaxy clusters. We employ the quantity Kg = 3kBTgρ −2/3 g /(µmp) = σ2 gρ −2/3 g as measure for the entropy of the intra-cluster gas. By analogy the DM entropy is defined as KDM = σ2 DMρ−2/3 DM (σ2DM is the 3D velocity dispersion of the DM). The DM entropy is related to the DM phase space density by KDM ∝ Q −2/3 DM. In accord with other studies the radial DM phase space density profile follows a power law behaviour, QDM ∝ r−1.82, which corresponds to KDM ∝ r1.21. The simulated intra-cluster gas has a flat entropy core within (0.8 ± 0.4)Rs, where Rs is the NFW scale radius. The outer profile follows the DM behaviour, Kg ∝ r1.21, in close agreement with X-ray observations. Upon scaling the DM and gas densities by their mean cosmological values we find that outside the entropy cor...

Abstract. In this talk, we present our recent study of galaxy mergers in a high-resolution cosmol... more Abstract. In this talk, we present our recent study of galaxy mergers in a high-resolution cosmological hydro/N-body simulation with star formation, and compare the measured merger timescales with theoretical predictions based on the Chandrasekhar formula. In contrast to Navarro et al., our numerical results indicate, that the commonly used equation for the merger timescale given by Lacey and Cole, systematically underestimates the merger timescales for minor mergers and overestimates those for major mergers. This behavior is partly explained by the poor performance of their expression for the Coulomb logarithm, ln(/Wpri//Wsat). The two alternative forms ln(l +/Wpri//Wsat) and l/2In[l + [m^^/msnt) ] for the Coulomb logarithm can account for the mass dependence of merger timescale successfiilly, but both of them underestimate the merger time scale by a factor 2. Since ln(l + /Wpn/msat) represents the mass dependence slightly better we adopt this expression for the Coulomb logarithm. ...

We study motions of galaxies in galaxy clusters formed in the concordance ΛCDM cosmology. We use ... more We study motions of galaxies in galaxy clusters formed in the concordance ΛCDM cosmology. We use high-resolution cosmological simulations that follow dynamics of dark matter and gas and include various physical processes critical for galaxy formation: gas cooling, heating and star formation. Analysing motions of galaxies and the properties of intracluster gas in the sample of eight simulated clusters at z = 0, we study velocity dispersion profiles of the dark matter, gas, and galaxies. We measure the mean velocity of galaxy motions and gas sound speed as a function of radius and calculate the average Mach number of galaxy motions. The simulations show that galaxies, on average, move supersonically with the average Mach number of ≈ 1.4, approximately independent of the cluster-centric radius. The supersonic motions of galaxies may potentially provide an important source of heating for the intracluster gas by driving weak shocks and via dynamical friction, although these heating proce...

The formation and structure of dark matter (DM) halos is studied by means of constrained realizat... more 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 h −1 M ⊙ 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 Rs 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 ...

The NGC 5044 galaxy group is dominated by a luminous elliptical galaxy which is surrounded by ∼ 1... more The NGC 5044 galaxy group is dominated by a luminous elliptical galaxy which 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 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 halos 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 disrup...

Within the cosmic framework clusters of galaxies are relatively young objects. Many of them have ... more Within the cosmic framework clusters of galaxies are relatively young objects. Many of them have recently experienced major mergers. Here we investigate an equal mass merging event at z ≈ 0.6 resulting in a dark matter haloe of ∼ 2.2 × 10 14 h −1 M ⊙ at z = 0. The merging process is covered by 270 outputs of a high resolution cosmological N-body simulation performed with the ART (adaptive refinement tree) code. Some 2 Gyrs elapse between the first peri-centre passage of the progenitor cores and their final coalescence. During that phase the cores experience six peri-centre passages with minimal distances declining from ∼ 30 to ∼ 2 h −1 kpc. The time intervals between the peri-centre passages continuously decrease from 9 to 1 × 10 8 yrs. We follow the mean density, the velocity dispersion and the entropy of the two progenitors within a set of

Based on the Millennium Simulation we examine assembly bias for the halo properties: shape, triax... more Based on the Millennium Simulation we examine assembly bias for the halo properties: shape, triaxiality, concentration, spin, shape of the velocity ellipsoid and velocity anisotropy. For consistency we determine all these properties using the same set of particles, namely all gravitationally self–bound particles belonging to the most massive sub–structure of a given friends–of–friends halo. We confirm that near–spherical and high–spin halos show enhanced clustering. The opposite is true for strongly aspherical and low–spin halos. Further, below the typical collapse mass, M∗, more concentrated halos show stronger clustering whereas less concentrated halos are less clustered which is reversed for masses above M∗. Going beyond earlier work we show that: (1) oblate halos are more strongly clustered than prolate ones; (2) the dependence of clustering on the shape of the velocity ellipsoid coincides with that of the real–space shape, although the signal is stronger; (3) halos with weak ve...

Proceedings of 3rd Annual Conference on High Energy Astrophysics in Southern Africa — PoS(HEASA2015), Feb 24, 2016

This work investigates the impact of the extragalactic background light fluctuations on very high... more This work investigates the impact of the extragalactic background light fluctuations on very high energy γ-ray spectra from distant Quasars. We calculate the extragalactic background light spectral energy distribution using a model that builds upon those proposed by Razzaque et al. (2009ApJ.697.483R) and Finke et al. (2010ApJ.712.238F). This model implements the fluctuations in the extragalactic background light based on the fluctuations in the star formation rate density, since both phenomena can reasonably be expected to be correlated. The fluctuations in the star formation rate are derived from the semi-analytical galaxy catalogue of Guo et al. (2013MNRAS.428.1351G). The determination of the mean, the lower and the upper limits for the scatter of the star formation rate density allows us to compute the corresponding limits on the extragalactic background light spectrum and ultimately the impact of these fluctuations on the γ-ray optical depth. The model predicts variations of up to 10% between upper and lower limits for the γ-ray opacity in the energy range less than 100 GeV for nearby sources. At higher energies the impact is smaller, 5%, but still significant even for redshifts 5.

Proceedings of MeerKAT Science: On the Pathway to the SKA — PoS(MeerKAT2016)

The cosmic evolution of galaxies' neutral atomic gas content is a major science driver for the Sq... more The cosmic evolution of galaxies' neutral atomic gas content is a major science driver for the Square Kilometre Array (SKA), as well as for its South African (MeerKAT) and Australian (ASKAP) precursors. Among the H I large survey programs (LSPs) planned for ASKAP and MeerKAT, the deepest and narrowest tier of the "wedding cake" will be defined by the combined L-band+UHF-band Looking At the Distant Universe with the MeerKAT Array (LADUMA) survey, which will probe H I in emission within a single "cosmic vuvuzela" that extends to z = 1.4, when the universe was only a third of its present age. Through a combination of individual and stacked detections (the latter relying on extensive multi-wavelength studies of the survey's target field), LADUMA will study the redshift evolution of the baryonic Tully-Fisher relation and the cosmic H I density, the variation of the H I mass function with redshift and environment, and the connection between H I content and galaxies' stellar properties (mass, age, etc.). The survey will also build a sample of OH megamaser detections that can be used to trace the cosmic merger history. This proceedings contribution provides a brief introduction to the survey, its scientific aims, and its technical implementation, deferring a more complete discussion for a future article after the implications of a recent review of MeerKAT LSP project plans are fully worked out.

Proceedings of 4th Annual Conference on High Energy Astrophysics in Southern Africa — PoS(HEASA 2016)

Based on the Marenostrum-MultiDark SImulation of galaxy Clusters (MUSIC-2) we develop semi-analyt... more Based on the Marenostrum-MultiDark SImulation of galaxy Clusters (MUSIC-2) we develop semi-analytical models which provide multi-wavelength emission maps generated by dark matter (DM) annihilation processes in galaxy clusters and their sub-halos. We focus on radio and gamma-ray emission maps from neutralino DM annihilation processes testing two different neutralino masses, M χ , 35 GeV and 60 GeV along with two different models of magnetic fields. A comparison of the radio flux densities from our DM annihilation model with the observed diffuse radio emission from the Coma cluster shows that they are of the same order of magnitude. We determine the DM densities with a Smoothed Particle Hydrodynamics (SPH) kernel. This enables us to integrate the DM annihilation signal along any given line-of-sight through the volume of the cluster. In particular it allows us to investigate the contribution of sub-halos to the DM annihilation signal with very high resolution. Zooming in on a subset of high mass-to-light ratio DM sub-halos, i.e. DM sub-halos with very low baryon content, we demonstrate that such targets can generate prominent annihilation signals. The radial distribution of high mass-to-light ratio (M/L) DM sub-halos is more strongly peaked at ≈ R 200crit compared to the distribution of all sub-halos which may suggest that the search for DM annihilation signals from sub-halos in clusters is most promising at R 200crit. The radio flux densities from DM sub-halos are well within the sensitivity limit of Square Kilometer Array (SKA) with an integration time of 1000 hours and unlike clusters their gamma-ray spectrum is seen to be dominated by pion decay over a wide range of gamma-ray energies. Our model makes clear predictions for future radio and gamma-ray observations of the DM annihilation signal in clusters and their sub-halos.

Clusters of galaxies have been extensively studied in all available wave bands. They belong to th... more Clusters of galaxies have been extensively studied in all available wave bands. They belong to the most massive and eye-catching structures in the universe and are thought to closely resemble the cosmic composite of 85

Astrophys J, 2009

Using constrained simulations of the local universe for generic cold dark matter (CDM) and for 1 ... more Using constrained simulations of the local universe for generic cold dark matter (CDM) and for 1 keV warm dark matter (WDM), we investigate the difference in the abundance of dark matter halos in the local environment. We find that the mass function (MF) within 20 h -1 Mpc of the Local Group is ~2 times larger than the universal MF in the 109-1013 h -1 M sun mass range. Imposing the field of view of the ongoing H I blind survey Arecibo Legacy Fast ALFA (ALFALFA) in our simulations, we predict that the velocity function (VF) in the Virgo-direction region (VdR) exceeds the universal VF by a factor of 3. Furthermore, employing a scheme to translate the halo VF into a galaxy VF, we compare the simulation results with a sample of galaxies from the early catalog release of ALFALFA. We find that our simulations are able to reproduce the VF in the 80-300 km s-1 velocity range, having a value ~10 times larger than the universal VF in the VdR. In the low-velocity regime, 35-80 km s-1, the WDM simulation reproduces the observed flattening of the VF. In contrast, the simulation with CDM predicts a steep rise in the VF toward lower velocities; for V max = 35 km s-1, it forecasts ~10 times more sources than the ones observed. If confirmed by the complete ALFALFA survey, our results indicate a potential problem for the CDM paradigm or for the conventional assumptions about energetic feedback in dwarf galaxies.

The Astrophysical Journal, 2009

ABSTRACT We study the correlation between galaxy clustering and their intrinsic properties. We fi... more ABSTRACT We study the correlation between galaxy clustering and their intrinsic properties. We find in high density regions an enhancement of the spin parameter as well as an over-abundance of old objects.

Light from `point sources' such as supernovae is observed with a beam width of order of the s... more Light from `point sources' such as supernovae is observed with a beam width of order of the sources' size - typically less than 1 AU. Such a beam probes matter and curvature distributions that are very different from coarse-grained representations in N-body simulations or perturbation theory, which are smoothed on scales much larger than 1 AU. The beam typically travels through unclustered dark matter and hydrogen with a mean density much less than the cosmic mean, and through dark matter halos and hydrogen clouds. Using N-body simulations, as well as a Press-Schechter approach, we quantify the density probability distribution as a function of beam width and show that, even for Gpc-length beams of 500 kpc diameter, most lines of sight are significantly under-dense. From this we argue that modelling the probability distribution for AU-diameter beams is absolutely critical. Standard analyses predict a huge variance for such tiny beam sizes, and nonlinear corrections appear to ...

The Astrophysical Journal, 2012

Tracing the cosmic evolution of the Baryonic Acoustic Oscillation (BAO) scale with galaxy two poi... more Tracing the cosmic evolution of the Baryonic Acoustic Oscillation (BAO) scale with galaxy two point correlation functions is currently the most promising approach to detect dark energy at early times. A number of ongoing and future experiments will measure the BAO peak with unprecedented accuracy. We show based on a set of N-Body simulations that the matter distribution is anisotropic out to ∼ 150 h −1 Mpc, far beyond the BAO scale of ∼ 100 h −1 Mpc, and discuss implications for the measurement of the BAO. To that purpose we use alignment correlation functions, i.e., cross correlation functions between high density peaks and the overall matter distribution measured along the orientation of the peaks and perpendicular to it. The correlation function measured along (perpendicular to) the orientation of high density peaks is enhanced (reduced) by a factor of 2 compared to the conventional correlation function and the location of the BAO peak shifts towards smaller (larger) scales if measured along (perpendicular to) the orientation of the high density peaks. Similar effects are expected to shape observed galaxy correlation functions at BAO scales.

Monthly Notices of the Royal Astronomical Society, 2010

We examine the effects of environment on the dynamical structure of satellite systems based on th... more We examine the effects of environment on the dynamical structure of satellite systems based on the Millennium II Simulation. Satellite haloes are defined as subhaloes within the virial radius of a host halo. The satellite sample is restricted to those subhaloes which showed a maximum circular velocity above 30 km s −1 at the time of accretion. Host halo masses range from 10 11 to 10 14 h −1 M. We compute the satellites' average accretion redshift, z acc , velocity dispersion, σ , and velocity anisotropy parameter, β, utilizing stacked satellite samples of equal-mass hosts at similar background densities. The main results are as follows. (1) On an average, satellites within hosts in high-density environments are accreted earlier (z ≈ 0.1) compared to their counterparts at low densities. For host masses above 5 × 10 13 h −1 M this trend weakens, and may reverse for higher host masses. (2) The velocity dispersion of satellites in low-density environments follows that of the host, i.e. no velocity bias is observed for host haloes at low densities independent of host mass. However, for low-mass hosts in high-density environments the velocity dispersion of the satellites can be up to ∼30 per cent larger than that of the host halo, i.e. the satellites are dynamically hotter than their host haloes. (3) The anisotropy parameter depends on host mass and environment. Satellites of massive hosts show more radially biased velocity distributions. Moreover in low-density environments, satellites have more radially biased velocities (β 0.1) as compared to their counterparts in highdensity environments. We believe that our approach allows us to predict a similar behaviour for observed satellite galaxy systems.

Based on the Millennium Simulation we examine assembly bias for the halo properties: shape, triax... more Based on the Millennium Simulation we examine assembly bias for the halo properties: shape, triaxiality, concentration, spin, shape of the velocity ellipsoid and velocity anisotropy. For consistency we determine all these properties using the same set of particles, namely all gravitationally self-bound particles belonging to the most massive sub-structure of a given friends-of-friends halo. We confirm that near-spherical and high-spin halos show enhanced clustering. The opposite is true for strongly aspherical and low-spin halos. Further, below the typical collapse mass, M*, more concentrated halos show stronger clustering whereas less concentrated halos are less clustered which is reversed for masses above M*. Going beyond earlier work we show that: (1) oblate halos are more strongly clustered than prolate ones; (2) the dependence of clustering on the shape of the velocity ellipsoid coincides with that of the real-space shape, although the signal is stronger; (3) halos with weak ve...

Simulation (MS), we investigate the alignment between galaxies and large-scale struc-ture. For th... more Simulation (MS), we investigate the alignment between galaxies and large-scale struc-ture. For this purpose, we develop two new statistical tools, namely the alignment cor-relation function and the cos(2θ)-statistic. The former is a two-dimensional extension of the traditional two-point correlation function and the latter is related to the ellipticity correlation function used for cosmic shear measurements. Both are based on the cross correlation between a sample of galaxies with orientations and a reference sample which represents the large-scale structure. We apply the new statistics to the SDSS galaxy cat-alog. The alignment correlation function reveals an overabundance of reference galaxies along the major axes of red, luminous (L L∗) galaxies out to projected separations of 60 h−1Mpc. The signal increases with central galaxy luminosity. No alignment signal is detected for blue galaxies. The cos(2θ)-statistic yields very similar results. Starting from a MS semi-analytic galaxy c...

On the basis of a large scale ’adiabatic’, namely non-radiative and non-dissipative, cosmological... more On the basis of a large scale ’adiabatic’, namely non-radiative and non-dissipative, cosmological smooth particle hydrodynamic simulation we compare the entropy profiles of the gas and the dark matter (DM) in galaxy clusters. We employ the quantity Kg = 3kBTgρ −2/3 g /(µmp) = σ2 gρ −2/3 g as measure for the entropy of the intra-cluster gas. By analogy the DM entropy is defined as KDM = σ2 DMρ−2/3 DM (σ2DM is the 3D velocity dispersion of the DM). The DM entropy is related to the DM phase space density by KDM ∝ Q −2/3 DM. In accord with other studies the radial DM phase space density profile follows a power law behaviour, QDM ∝ r−1.82, which corresponds to KDM ∝ r1.21. The simulated intra-cluster gas has a flat entropy core within (0.8 ± 0.4)Rs, where Rs is the NFW scale radius. The outer profile follows the DM behaviour, Kg ∝ r1.21, in close agreement with X-ray observations. Upon scaling the DM and gas densities by their mean cosmological values we find that outside the entropy cor...

Abstract. In this talk, we present our recent study of galaxy mergers in a high-resolution cosmol... more Abstract. In this talk, we present our recent study of galaxy mergers in a high-resolution cosmological hydro/N-body simulation with star formation, and compare the measured merger timescales with theoretical predictions based on the Chandrasekhar formula. In contrast to Navarro et al., our numerical results indicate, that the commonly used equation for the merger timescale given by Lacey and Cole, systematically underestimates the merger timescales for minor mergers and overestimates those for major mergers. This behavior is partly explained by the poor performance of their expression for the Coulomb logarithm, ln(/Wpri//Wsat). The two alternative forms ln(l +/Wpri//Wsat) and l/2In[l + [m^^/msnt) ] for the Coulomb logarithm can account for the mass dependence of merger timescale successfiilly, but both of them underestimate the merger time scale by a factor 2. Since ln(l + /Wpn/msat) represents the mass dependence slightly better we adopt this expression for the Coulomb logarithm. ...

We study motions of galaxies in galaxy clusters formed in the concordance ΛCDM cosmology. We use ... more We study motions of galaxies in galaxy clusters formed in the concordance ΛCDM cosmology. We use high-resolution cosmological simulations that follow dynamics of dark matter and gas and include various physical processes critical for galaxy formation: gas cooling, heating and star formation. Analysing motions of galaxies and the properties of intracluster gas in the sample of eight simulated clusters at z = 0, we study velocity dispersion profiles of the dark matter, gas, and galaxies. We measure the mean velocity of galaxy motions and gas sound speed as a function of radius and calculate the average Mach number of galaxy motions. The simulations show that galaxies, on average, move supersonically with the average Mach number of ≈ 1.4, approximately independent of the cluster-centric radius. The supersonic motions of galaxies may potentially provide an important source of heating for the intracluster gas by driving weak shocks and via dynamical friction, although these heating proce...

The formation and structure of dark matter (DM) halos is studied by means of constrained realizat... more 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 h −1 M ⊙ 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 Rs 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 ...

The NGC 5044 galaxy group is dominated by a luminous elliptical galaxy which is surrounded by ∼ 1... more The NGC 5044 galaxy group is dominated by a luminous elliptical galaxy which 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 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 halos 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 disrup...

Within the cosmic framework clusters of galaxies are relatively young objects. Many of them have ... more Within the cosmic framework clusters of galaxies are relatively young objects. Many of them have recently experienced major mergers. Here we investigate an equal mass merging event at z ≈ 0.6 resulting in a dark matter haloe of ∼ 2.2 × 10 14 h −1 M ⊙ at z = 0. The merging process is covered by 270 outputs of a high resolution cosmological N-body simulation performed with the ART (adaptive refinement tree) code. Some 2 Gyrs elapse between the first peri-centre passage of the progenitor cores and their final coalescence. During that phase the cores experience six peri-centre passages with minimal distances declining from ∼ 30 to ∼ 2 h −1 kpc. The time intervals between the peri-centre passages continuously decrease from 9 to 1 × 10 8 yrs. We follow the mean density, the velocity dispersion and the entropy of the two progenitors within a set of

Based on the Millennium Simulation we examine assembly bias for the halo properties: shape, triax... more Based on the Millennium Simulation we examine assembly bias for the halo properties: shape, triaxiality, concentration, spin, shape of the velocity ellipsoid and velocity anisotropy. For consistency we determine all these properties using the same set of particles, namely all gravitationally self–bound particles belonging to the most massive sub–structure of a given friends–of–friends halo. We confirm that near–spherical and high–spin halos show enhanced clustering. The opposite is true for strongly aspherical and low–spin halos. Further, below the typical collapse mass, M∗, more concentrated halos show stronger clustering whereas less concentrated halos are less clustered which is reversed for masses above M∗. Going beyond earlier work we show that: (1) oblate halos are more strongly clustered than prolate ones; (2) the dependence of clustering on the shape of the velocity ellipsoid coincides with that of the real–space shape, although the signal is stronger; (3) halos with weak ve...

Proceedings of 3rd Annual Conference on High Energy Astrophysics in Southern Africa — PoS(HEASA2015), Feb 24, 2016

This work investigates the impact of the extragalactic background light fluctuations on very high... more This work investigates the impact of the extragalactic background light fluctuations on very high energy γ-ray spectra from distant Quasars. We calculate the extragalactic background light spectral energy distribution using a model that builds upon those proposed by Razzaque et al. (2009ApJ.697.483R) and Finke et al. (2010ApJ.712.238F). This model implements the fluctuations in the extragalactic background light based on the fluctuations in the star formation rate density, since both phenomena can reasonably be expected to be correlated. The fluctuations in the star formation rate are derived from the semi-analytical galaxy catalogue of Guo et al. (2013MNRAS.428.1351G). The determination of the mean, the lower and the upper limits for the scatter of the star formation rate density allows us to compute the corresponding limits on the extragalactic background light spectrum and ultimately the impact of these fluctuations on the γ-ray optical depth. The model predicts variations of up to 10% between upper and lower limits for the γ-ray opacity in the energy range less than 100 GeV for nearby sources. At higher energies the impact is smaller, 5%, but still significant even for redshifts 5.

Proceedings of MeerKAT Science: On the Pathway to the SKA — PoS(MeerKAT2016)

The cosmic evolution of galaxies' neutral atomic gas content is a major science driver for the Sq... more The cosmic evolution of galaxies' neutral atomic gas content is a major science driver for the Square Kilometre Array (SKA), as well as for its South African (MeerKAT) and Australian (ASKAP) precursors. Among the H I large survey programs (LSPs) planned for ASKAP and MeerKAT, the deepest and narrowest tier of the "wedding cake" will be defined by the combined L-band+UHF-band Looking At the Distant Universe with the MeerKAT Array (LADUMA) survey, which will probe H I in emission within a single "cosmic vuvuzela" that extends to z = 1.4, when the universe was only a third of its present age. Through a combination of individual and stacked detections (the latter relying on extensive multi-wavelength studies of the survey's target field), LADUMA will study the redshift evolution of the baryonic Tully-Fisher relation and the cosmic H I density, the variation of the H I mass function with redshift and environment, and the connection between H I content and galaxies' stellar properties (mass, age, etc.). The survey will also build a sample of OH megamaser detections that can be used to trace the cosmic merger history. This proceedings contribution provides a brief introduction to the survey, its scientific aims, and its technical implementation, deferring a more complete discussion for a future article after the implications of a recent review of MeerKAT LSP project plans are fully worked out.

Proceedings of 4th Annual Conference on High Energy Astrophysics in Southern Africa — PoS(HEASA 2016)

Based on the Marenostrum-MultiDark SImulation of galaxy Clusters (MUSIC-2) we develop semi-analyt... more Based on the Marenostrum-MultiDark SImulation of galaxy Clusters (MUSIC-2) we develop semi-analytical models which provide multi-wavelength emission maps generated by dark matter (DM) annihilation processes in galaxy clusters and their sub-halos. We focus on radio and gamma-ray emission maps from neutralino DM annihilation processes testing two different neutralino masses, M χ , 35 GeV and 60 GeV along with two different models of magnetic fields. A comparison of the radio flux densities from our DM annihilation model with the observed diffuse radio emission from the Coma cluster shows that they are of the same order of magnitude. We determine the DM densities with a Smoothed Particle Hydrodynamics (SPH) kernel. This enables us to integrate the DM annihilation signal along any given line-of-sight through the volume of the cluster. In particular it allows us to investigate the contribution of sub-halos to the DM annihilation signal with very high resolution. Zooming in on a subset of high mass-to-light ratio DM sub-halos, i.e. DM sub-halos with very low baryon content, we demonstrate that such targets can generate prominent annihilation signals. The radial distribution of high mass-to-light ratio (M/L) DM sub-halos is more strongly peaked at ≈ R 200crit compared to the distribution of all sub-halos which may suggest that the search for DM annihilation signals from sub-halos in clusters is most promising at R 200crit. The radio flux densities from DM sub-halos are well within the sensitivity limit of Square Kilometer Array (SKA) with an integration time of 1000 hours and unlike clusters their gamma-ray spectrum is seen to be dominated by pion decay over a wide range of gamma-ray energies. Our model makes clear predictions for future radio and gamma-ray observations of the DM annihilation signal in clusters and their sub-halos.

Clusters of galaxies have been extensively studied in all available wave bands. They belong to th... more Clusters of galaxies have been extensively studied in all available wave bands. They belong to the most massive and eye-catching structures in the universe and are thought to closely resemble the cosmic composite of 85

Astrophys J, 2009

Using constrained simulations of the local universe for generic cold dark matter (CDM) and for 1 ... more Using constrained simulations of the local universe for generic cold dark matter (CDM) and for 1 keV warm dark matter (WDM), we investigate the difference in the abundance of dark matter halos in the local environment. We find that the mass function (MF) within 20 h -1 Mpc of the Local Group is ~2 times larger than the universal MF in the 109-1013 h -1 M sun mass range. Imposing the field of view of the ongoing H I blind survey Arecibo Legacy Fast ALFA (ALFALFA) in our simulations, we predict that the velocity function (VF) in the Virgo-direction region (VdR) exceeds the universal VF by a factor of 3. Furthermore, employing a scheme to translate the halo VF into a galaxy VF, we compare the simulation results with a sample of galaxies from the early catalog release of ALFALFA. We find that our simulations are able to reproduce the VF in the 80-300 km s-1 velocity range, having a value ~10 times larger than the universal VF in the VdR. In the low-velocity regime, 35-80 km s-1, the WDM simulation reproduces the observed flattening of the VF. In contrast, the simulation with CDM predicts a steep rise in the VF toward lower velocities; for V max = 35 km s-1, it forecasts ~10 times more sources than the ones observed. If confirmed by the complete ALFALFA survey, our results indicate a potential problem for the CDM paradigm or for the conventional assumptions about energetic feedback in dwarf galaxies.

The Astrophysical Journal, 2009

ABSTRACT We study the correlation between galaxy clustering and their intrinsic properties. We fi... more ABSTRACT We study the correlation between galaxy clustering and their intrinsic properties. We find in high density regions an enhancement of the spin parameter as well as an over-abundance of old objects.