arXiv subjects

A. Faltenbacher

Publications and source records attributed to A. Faltenbacher.

At least 19 recordsLinked to original sources

arXiv2018

Detailed modelling of the EBL along VHE γ-ray paths

Interactions between the extragalactic background light (EBL) and very high energy γ rays (VHE; E > 10 GeV) from cosmological sources alter their γ-ray spectrum. The stronger absorption of harder γ rays causes a steepening of the observed γ-ray spectrum at the high energy tail which can be expressed by an increase of the power index. The effect provides a link between high energy astrophysics and the evolution of galaxies. In this work we develop a new hybrid EBL model by augmenting our previous analytic model with information from semi-analytic galaxy catalogues. The model allows us to study the γ-ray opacities of individual γ-ray paths through the (simulated) universe and evaluate the effect of local fluctuations in the EBL intensity along each path. We confirm an order of magnitude fluctuations in the local EBL (based on the cumulative light from model galaxy distributions within spheres of 50 $h^{-1}$Mpc). However, the effect of these fluctuations on the VHE γ-ray opacity is insignificant due to the overall very small contribution of the local EBL to the total EBL. We also investigate the effect which a galaxy crossing the line of sight of the γ-ray source may have on the VHE spectrum. We find that galaxies with stellar masses of M$ > 10^{11}$ M$_{\odot}$ could have significant effect on the γ-ray absorption. It is unlikely that the observed variation in the spectral index is caused by the proximity of single galaxies to the γ-ray paths as these are extremely rare.

astro-ph.GA↗

arXiv2017

Cosmic Web Type Dependence of Halo Clustering

We use the Millennium simulation to show that halo clustering varies significantly with cosmic web type. Halos are classified as node, filament, sheet and void halos based on the eigenvalue decomposition of the velocity shear tensor. The velocity field is sampled by the peculiar velocities of a fixed number of neighbouring halos and spatial derivatives are computed using a kernel borrowed from smoothed particle hydrodynamics. The classification scheme is used to examine the clustering of halos as a function of web type for halos with masses larger than $10^{11}$. We find that node halos show positive bias, filament halos show negligible bias, and void and sheet halos are anti-biased independent of halo mass. Our findings suggest that the mass dependence of halo clustering is rooted in the composition of web types as a function of halo mass. The substantial fraction of node type halos for halo masses $\gtrsim 2\times10^{13}\,h^{-1}\rm M_\odot$ leads to positive bias. Filament type halos prevail at intermediate masses, $10^{12} - 10^{13}\,h^{-1}\rm M_\odot$, resulting in unbiased clustering. The large contribution of sheet type halos at low halo masses $\lesssim 10^{12}\,h^{-1}\rm M_\odot$ generates anti-biasing.

astro-ph.CO↗

arXiv2017

Effects of Spatial Fluctuations in the Extra Galactic Background Light on Hard Gamma Ray Spectra

This study investigates the impact of the fluctuations in the extra galactic background light (EBL) on the attenuation of the hard $γ$-ray spectra of distant blazars. EBL fluctuations occur on the scales up to 100 Mpc and are caused by clustering of galaxies. The EBL photons interact with high energy $γ$-rays via the electron-positron pair production mechanism: $γ+ γ' \rightarrow e^+ + e^-$. The attenuation of $γ$-rays depends on their energy and the density of the intervening EBL photon field. Using a simple model for the evolution of the mean EBL photon density, we implement an analytical description of the EBL fluctuations. We find that the amplitudes of the EBL energy density can vary by $\pm 1\%$ as a function of environment. The EBL fluctuations lead to mild alterations of the optical depth or equivalently the transmissivity for $γ$-rays from distant blazars. Our model predicts maximum changes of $\pm 10\%$ in the $γ$-ray transmissivity. However, this translates into marginal differences in the power law slopes of currently observed $γ$-ray spectra. The slopes of deabsorbed $γ$-ray spectra differ by not more than $\pm 1\%$ if EBL fluctuations are included.

astro-ph.HE↗

arXiv2015

Lagrangian Methods Of Cosmic Web Classification

The cosmic web defines the large scale distribution of matter we see in the Universe today. Classifying the cosmic web into voids, sheets, filaments and nodes allows one to explore structure formation and the role environmental factors have on halo and galaxy properties. While existing studies of cosmic web classification concentrate on grid based methods, this work explores a Lagrangian approach where the V-web algorithm proposed by Hoffman et al. (2012) is implemented with techniques borrowed from smoothed particle hydrodynamics. The Lagrangian approach allows one to classify individual objects (e.g. particles or halos) based on properties of their nearest neighbours in an adaptive manner. It can be applied directly to a halo sample which dramatically reduces computational cost and potentially allows an application of this classification scheme to observed galaxy samples. Finally, the Lagrangian nature admits a straight forward inclusion of the Hubble flow negating the necessity of a visually defined threshold value which is commonly employed by grid based classification methods.

astro-ph.CO↗

arXiv2013

The environmental dependence of neutral hydrogen in the GIMIC simulations

We use the Galaxies-Intergalactic Medium Interaction Calculation (GIMIC) cosmological hydrodynamic simulation at z=0 to study the distribution and environmental dependence of neutral hydrogen (HI) gas in the outskirts of simulated galaxies. This gas can currently be probed directly in, for example, Ly$α$ absorption via the observation of background quasars. Radio facilities, such as the Square Kilometre Array, will provide a complementary probe of the diffuse HI in emission and will constrain the physics underpinning the complex interplay between accretion and feedback mechanisms which affect the intergalactic medium. We extract a sample of 488 galaxies from a re-simulation of the average cosmic density GIMIC region. We estimate the neutral hydrogen content of these galaxies and the surrounding intergalactic medium within which they reside. We investigate the average HI radial profiles by stacking the individual profiles according to both mass and environment. We find high HI column densities at large impact parameters in group environments and markedly lower HI densities for non-group galaxies. We suggest that these results likely arise from the combined effects of ram pressure stripping and tidal interactions present in group environments.

astro-ph.CO↗

arXiv2013

Detection of the large scale alignment of massive galaxies at z~0.6

We report on the detection of the alignment between galaxies and large-scale structure at z~0.6 based on the CMASS galaxy sample from the Baryon Oscillation Spectroscopy Survey data release 9. We use two statistics to quantify the alignment signal: 1) the alignment two-point correlation function which probes the dependence of galaxy clustering at a given separation in redshift space on the projected angle (theta_p) between the orientation of galaxies and the line connecting to other galaxies, and 2) the cos(2theta)-statistic which estimates the average of cos(2theta_p) for all correlated pairs at given separation. We find significant alignment signal out to about 70 Mpc/h in both statistics. Applications of the same statistics to dark matter halos of mass above 10^12 M_sun/h in a large cosmological simulation show similar scale-dependent alignment signals to the observation, but with higher amplitudes at all scales probed. We show that this discrepancy may be partially explained by a misalignment angle between central galaxies and their host halos, though detailed modeling is needed in order to better understand the link between the orientations of galaxies and host halos. In addition, we find systematic trends of the alignment statistics with the stellar mass of the CMASS galaxies, in the sense that more massive galaxies are more strongly aligned with the large-scale structure.

astro-ph.CO↗

arXiv2012

Anisotropy in the matter distribution beyond the baryonic acoustic oscillation scale

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 Mpc/h, far beyond the BAO scale of ~100M pc/h, 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.

astro-ph.CO↗

arXiv2011

Clustering of HI galaxies in HIPASS and ALFALFA

We investigate the clustering of HI-selected galaxies in the ALFALFA survey and compare results with those obtained for HIPASS. Measurements of the angular correlation function and the inferred 3D-clustering are compared with results from direct spatial-correlation measurements. We are able to measure clustering on smaller angular scales and for galaxies with lower HI masses than was previously possible. We calculate the expected clustering of dark matter using the redshift distributions of HIPASS and ALFALFA and show that the ALFALFA sample is somewhat more anti-biased with respect to dark matter than the HIPASS sample.

astro-ph.CO↗

arXiv2010

The halo mass function conditioned on density from the Millennium Simulation: insights into missing baryons and galaxy mass functions

The baryon content of high-density regions in the universe is relevant to two critical unanswered questions: the workings of nurture effects on galaxies and the whereabouts of the missing baryons. In this paper, we analyze the distribution of dark matter and semianalytical galaxies in the Millennium Simulation to investigate these problems. Applying the same density field reconstruction schemes as used for the overall matter distribution to the matter locked in halos we study the mass contribution of halos to the total mass budget at various background field densities, i.e., the conditional halo mass function. In this context, we present a simple fitting formula for the cumulative mass function accurate to ~ 5% for halo masses between 10^{10} and 10^{15}Msol/h. We find that in dense environments the halo mass function becomes top heavy and present corresponding fitting formulae for different redshifts. We demonstrate that the major fraction of matter in high-density fields is associated with galaxy groups. Since current X-ray surveys are able to nearly recover the universal baryon fraction within groups, our results indicate that the major part of the so-far undetected warm-hot intergalactic medium resides in low-density regions at low temperatures. Similarly, we show that the differences in galaxy mass functions with environment seen in observed and simulated data stem predominantly from differences in the mass distribution of halos. In particular, the hump in the galaxy mass function is associated with the central group galaxies, and the bimodality observed in the galaxy mass function is therefore interpreted as that of central galaxies versus satellites.

astro-ph.CO↗

arXiv2009

The velocity function in the local environment from LCDM and LWDM constrained simulations

Using constrained simulations of the local Universe for generic cold dark matter and for 1keV warm dark matter, we investigate the difference in the abundance of dark matter halos in the local environment. We find that the mass function within 20 Mpc/h of the Local Group is ~2 times larger than the universal mass function in the 10^9-10^13 M_odot/h mass range. Imposing the field of view of the on-going HI blind survey ALFALFA in our simulations, we predict that the velocity function in the Virgo-direction region exceeds the universal velocity function by a factor of 3. Furthermore, employing a scheme to translate the halo velocity function into a galaxy velocity function, 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 velocity function in the 80-300 km/s velocity range, having a value ~10 times larger than the universal velocity function in the Virgo-direction region. In the low velocity regime, 35-80 km/s, the warm dark matter simulation reproduces the observed flattening of the velocity function. On the contrary, the simulation with cold dark matter predicts a steep rise in the velocity function towards lower velocities; for V_max=35 km/s, 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 cold dark matter paradigm or for the conventional assumptions about energetic feedback in dwarf galaxies.

astro-ph.CO↗

arXiv2009

The velocity--shape alignment of clusters and the kinetic Sunyaev--Zeldovich effect

We use the Millennium simulation to probe the correlation between cluster velocities and their shapes and the consequences for measurements of the kinetic Sunyaev-Zeldovich (kSZ) effect. Halos are generally prolate ellipsoids with orientations that are correlated with those of nearby halos. We measure the mean streaming velocities of halos along the lines that separate them, demonstrating that the peculiar velocities and the long axes of halos tend to be somewhat aligned, especially for the most massive halos. Since the kSZ effect is proportional to the line-of-sight velocity and the optical depth of the cluster, the alignment results in a strong enhancement of the kSZ signature in clusters moving along the line of sight. This effect has not been taken into account in many analyses of kSZ signatures.

astro-ph.CO↗

arXiv2009

Alignment between galaxies and large-scale structure

Based on the Sloan Digital Sky Survey DR6 (SDSS) and Millennium Simulation (MS) we investigate the alignment between galaxies and large-scale structure. For this purpose we develop two new statistical tools, namely the alignment correlation function and the cos(2theta)-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. Applied to the SDSS galaxy catalog 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 Mpc/h. No alignment signal is detected for blue galaxies. The cos(2θ)-statistic yields very similar results. Starting from a MS semi-analytic galaxy catalog we assign an orientation to each red, luminous and central galaxy, based on the central region of the host halo. Alternatively, we use the orientation of the host halo itself. We find a mean projected misalignment between a halo and its central region of ~25 deg. Agreement with the SDSS results is good if the central orientations are used. Using the halo orientations overestimates the observed alignment by more than a factor of 2. The large volume of the MS allows to generate two-dimensional maps of the alignment correlation function which show the reference galaxy distribution to be flattened parallel to the orientations of red luminous galaxies with axis ratios of ~0.5 and ~0.75 for halo and central orientations,respectively. These ratios are almost independent of scale out to 60 Mpc/h.

astro-ph↗

arXiv2007

Spatial and kinematic alignments between central and satellite halos

Based on a cosmological N-body simulation we analyze spatial and kinematic alignments of satellite halos within six times the virial radius of group size host halos (Rvir). We measure three different types of spatial alignment: halo alignment between the orientation of the group central substructure (GCS) and the distribution of its satellites, radial alignment between the orientation of a satellite and the direction towards its GCS, and direct alignment between the orientation of the GCS and that of its satellites. In analogy we use the directions of satellite velocities and probe three further types of alignment: the radial velocity alignment between the satellite velocity and connecting line between satellite and GCS, the halo velocity alignment between the orientation of the GCS and satellite velocities and the auto velocity alignment between the satellites orientations and their velocities. We find that satellites are preferentially located along the major axis of the GCS within at least 6 Rvir (the range probed here). Furthermore, satellites preferentially point towards the GCS. The most pronounced signal is detected on small scales but a detectable signal extends out to 6 Rvir. The direct alignment signal is weaker, however a systematic trend is visible at distances < 2 Rvir. All velocity alignments are highly significant on small scales. Our results suggest that the halo alignment reflects the filamentary large scale structure which extends far beyond the virial radii of the groups. In contrast, the main contribution to the radial alignment arises from the adjustment of the satellite orientations in the group tidal field. The projected data reveal good agreement with recent results derived from large galaxy surveys. (abridged)

astro-ph↗

arXiv2007

A fitting formula for the merger timescale of galaxies in hierarchical clustering

We study galaxy mergers using 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 (m_pri/m_sat). The two alternative forms \ln (1+m_pri/m_sat) and 1/2\ln [1+(m_pri/m_sat)^2] for the Coulomb logarithm can account for the mass dependence of merger timescale successfully, but both of them underestimate the merger time scale by a factor 2. Since \ln (1+m_pri/m_sat) represents the mass dependence slightly better we adopt this expression for the Coulomb logarithm. Furthermore, we find that the dependence of the merger timescale on the circularity parameter εis much weaker than the widely adopted power-law ε^{0.78}, whereas 0.94*ε^{0.60}+0.60 provides a good match to the data. Based on these findings, we present an accurate and convenient fitting formula for the merger timescale of galaxies in cold dark matter models.

astro-ph↗

arXiv2007

Three Different Types of Galaxy Alignment within Dark Matter Halos

Using a large galaxy group catalogue based on the Sloan Digital Sky Survey Data Release 4 we measure three different types of intrinsic galaxy alignment within groups: halo alignment between the orientation of the brightest group galaxies (BGG) and the distribution of its satellite galaxies, radial alignment between the orientation of a satellite galaxy and the direction towards its BGG, and direct alignment between the orientation of the BGG and that of its satellites. In agreement with previous studies we find that satellite galaxies are preferentially located along the major axis. In addition, on scales r < 0.7 Rvir we find that red satellites are preferentially aligned radially with the direction to the BGG. The orientations of blue satellites, however, are perfectly consistent with being isotropic. Finally, on scales r < 0.1 \Rvir, we find a weak but significant indication for direct alignment between satellites and BGGs. We briefly discuss the implications for weak lensing measurements.

astro-ph↗

arXiv2007

Entropy of gas and dark matter in galaxy clusters

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. The quantity K_g = T_g ρ_g^{-2/3} provides a measure for the entropy of the intra-cluster gas. By analogy with the thermodynamic variables of the gas the velocity dispersion of the DM is associated with a formal temperature and thereby K_DM = σ_DM^2 ρ_DM^{-2/3} is defined. This DM entropy is related to the DM phase space density by K_DM \propto Q_DM^{-2/3}. In accord with other studies the DM phase space density follows a power law behaviour, Q_DM \propto r^{-1.82}, which corresponds to K_DM \propto r^{1.21}. The simulated intra-cluster gas has a flat entropy core within (0.8 \pm 0.4) R_s, where R_s is the NFW scale radius. The outer profile follows the DM behaviour, K_g \propto r^{1.21}, 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 core a constant ratio of K_g / K_{DM} = 0.71 \pm 0.18 prevails. By extending the definition of the gas temperature to include also the bulk kinetic energy the ratio of the DM and gas extended entropy is found to be unity for r > 0.8 R_s. The constant ratio of the gas thermal entropy to that of the DM implies that observations of the intra-cluster gas can provide an almost direct probe of the DM.

astro-ph↗

arXiv2006

Oscillatory relaxation of a merging galaxy cluster

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 times 10^{14} Msol/h 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 kpc/h. The time intervals between the peri-centre passages continuously decrease from 9 to 1 times 10^8 yrs. We follow the mean density, the velocity dispersion and the entropy of the two progenitors within a set of fixed proper radii (25, 50, 100, 250, 500, 1000 kpc/h). During the peri-centre passages we find sharp peaks of the mean densities within these radii, which exceed the sum of the corresponding progenitor densities. In addition to the intermixing of the merging haloes, the densities increase due to contraction caused by the momentary deepening of the potential well. At the end of the oscillatory relaxation phase the material originating from the less concentrated of the two equal mass progenitors is deposited at larger radii and shows a slightly more radially anisotropic velocity dispersion compared to the material coming from the more concentrated progenitor. Every peri-centre passage is accompanied by a substantial drop of the central potential well. We briefly discuss the possibility that AGN outbursts are triggered by the periodically changing potential.

astro-ph↗

arXiv2006

Large scatter in X-ray emission among elliptical galaxies: correlations with mass of host halo

The enormous range of X-ray luminosities among elliptical galaxies with similar optical luminosities can be attributed to large scatter and systematic variations in the dark halo mass determined from X-ray observations. The mean halo mass decreases sharply with both the X-ray and K-band luminosities. Smaller halos contain less diffuse hot gas and have lower X-ray luminosities. In addition the hot gas in low mass halos is more easily depleted by supernova-driven outflows, further lowering the X-ray luminosity.

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