arXiv subjects

Stefan Gottloeber

Publications and source records attributed to Stefan Gottloeber.

At least 37 records · Page 2Linked to original sources

arXiv2015

The Effect of Environment on Milky Way-mass galaxies in a Constrained Simulation of the Local Group

In this letter we present, for the first time, a study of star formation rate, gas fraction and galaxy morphology of a constrained simulation of the Milky Way (MW) and Andromeda (M31) galaxies, compared to other MW-mass galaxies. By combining with unconstrained simulations we cover a sufficient volume to compare these galaxies environmental densities ranging from the field to that of the Local Group (LG). This is particularly relevant as it has been shown that, quite generally, galaxy properties depend intimately upon their environment, most prominently when galaxies in clusters are compared to those in the field. For galaxies in loose groups such as the LG, however, environmental effects have been less clear. We consider the galaxy's environmental density in spheres of 1200 kpc (comoving) and find that whilst environment does not appear to directly affect morphology, there is a positive trend with star formation rates. This enhancement in star formation occurs systematically for galaxies in higher density environments, regardless whether they are part of the LG or in filaments. Our simulations suggest that the richer environment at Mpc-scales may help replenish the star-forming gas, allowing higher specific star formation rates in galaxies such as the MW.

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arXiv2014

The universal nature of subhalo accretion

We examine the angular infall pattern of subhaloes onto host haloes in the context of the large-scale structure. We find that this infall pattern is essentially driven by the shear tensor of the ambient velocity field. Dark matter subhaloes are found to be preferentially accreted along the principal axis of the shear tensor which corresponds to the direction of weakest collapse. We examine the dependence of this preferential infall on subhalo mass, host halo mass and redshift. Although strongest for the most massive hosts and the most massive subhaloes at high redshift, the preferential infall of subhaloes is effectively universal in the sense that its always aligned with the axis of weakest collapse of the velocity shear tensor. It is the same shear tensor that dictates the structure of the cosmic web and hence the shear field emerges as the key factor that governs the local anisotropic pattern of structure formation. Since the small (sub-Mpc) scale is strongly correlated with the mid-range (~10 Mpc) scale - a scale accessible by current surveys of peculiar velocities - it follows that findings presented here open a new window into the relation between the observed large scale structure unveiled by current surveys of peculiar velocities and the preferential infall direction of the Local Group. This may shed light on the unexpected alignments of dwarf galaxies seen in the Local Group

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arXiv2014

The distribution of gas in the Local Group from constrained cosmological simulations: the case for Andromeda and the Milky Way galaxies

We study the gas distribution in the Milky Way and Andromeda using a constrained cosmological simulation of the Local Group (LG) within the context of the CLUES (Constrained Local UniversE Simulations) project. We analyse the properties of gas in the simulated galaxies at $z=0$ for three different phases: `cold', `hot' and HI, and compare our results with observations. The amount of material in the hot halo ($M_{hot}\approx 4-5\times10^{10}\,$M$_{\odot}$), and the cold ($M_{cold}(r\lesssim10\,$kpc$)\approx10^{8}\,$M$_{\odot}$) and HI ($M_{HI}(r\lesssim50\,$kpc$)\approx 3-4\times10^8\,$M$_{\odot}$) components display a reasonable agreement with observations. We also compute the accretion/ejection rates together with the HI (radial and all-sky) covering fractions. The integrated HI accretion rate within $r=50\,$kpc gives $\sim$$0.2-0.3\,$M$_{\odot}\,$yr$^{-1}$, i.e. close to that obtained from high-velocity clouds in the Milky Way. We find that the global accretion rate is dominated by hot material, although ionized gas with $T\lesssim10^5\,$K can contribute significantly too. The $net$ accretion rates of $all$ material at the virial radii are $6-8\,$M$_{\odot}\,$yr$^{-1}$. At $z=0$, we find a significant gas excess between the two galaxies, as compared to any other direction, resulting from the overlap of their gaseous haloes. In our simulation, the gas excess first occurs at $z\sim1$, as a consequence of the kinematical evolution of the LG.

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arXiv2014

The Imprint of Reionization on the Star Formation Histories of Dwarf Galaxies

We explore the impact of cosmic reionization on nearby isolated dwarf galaxies using a compilation of SFHs estimated from deep HST data and a cosmological hydrodynamical simulation of the Local Group. The nearby dwarfs show a wide diversity of star formation histories; from ancient systems that have largely completed their star formation $\sim 10$ Gyr ago to young dwarfs that have formed the majority of their stars in the past $\sim 5$ Gyr to two-component systems characterized by the overlap of comparable numbers of old and young stars. Taken as an ensemble, star formation in nearby dwarfs dips to lower-than-average rates at intermediate times ($4<t$/Gyr $<8$), a feature that we trace in the simulation to the effects of cosmic reionization. Reionization heats the gas and drives it out of the shallow potential wells of low mass halos, affecting especially those below a sharp mass threshold that corresponds to a virial temperature of $\sim 2 \times 10^4 $ $\mathrm{K}$ at $z_{\rm reion}$. The loss of baryons leads to a sharp decline in the star forming activity of early-collapsing systems, which, compounded by feedback from early star formation, empties halos of gas and leaves behind systems where a single old stellar component prevails. In halos below the threshold at $z_{\rm reion}$, reionization heating leads to a delay in the onset of star formation that lasts until the halo grows massive enough to allow some of the remaining gas to cool and form stars. Young stellar components therefore dominate in dwarfs whose halos assemble late and thus form few stars before reionization. Two-component systems may be traced to late mergers of individual examples of the two aforementioned cases. The relative dearth of intermediate-age stars in nearby dwarfs might thus be the clearest signature yet identified of the imprint of cosmic reionization on the star formation history of dwarf galaxies.

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arXiv2013

Cosmic variance of the local Hubble flow in large-scale cosmological simulations

The increasing precision in the determination of the Hubble parameter has reached a per cent level at which large-scale cosmic flows induced by inhomogeneities of the matter distribution become non-negligible. Here we use large-scale cosmological N-body simulations to study statistical properties of the local Hubble parameter as measured by local observers. We show that the distribution of the local Hubble parameter depends not only on the scale of inhomogeneities, but also on how one defines the positions of observers in the cosmic web and what reference frame is used. Observers located in random dark matter haloes measure on average lower expansion rates than those at random positions in space or in the centres of cosmic voids, and this effect is stronger from the halo rest frames compared to the CMB rest frame. We compare the predictions for the local Hubble parameter with observational constraints based on type Ia supernovae (SNIa) and CMB observations. Due to cosmic variance, for observers located in random haloes we show that the Hubble constant determined from nearby SNIa may differ from that measured from the CMB by 0.8 per cent at 1sigma statistical significance. This scatter is too small to significantly alleviate a recently claimed discrepancy between current measurements assuming a flat LCDM model. However, for observers located in the centres of the largest voids permitted by the standard LCDM model, we find that Hubble constant measurements from SNIa would be biased high by 5 per cent, rendering this tension inexistent in this extreme case.

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arXiv2013

Dark Matter in the Local Universe

We review how dark matter is distributed in our local neighbourhood from an observational and theoretical perspective. We will start by describing first the dark matter halo of our own galaxy and in the Local Group. Then we proceed to describe the dark matter distribution in the more extended area known as the Local Universe. Depending on the nature of dark matter, numerical simulations predict different abundances of substructures in Local Group galaxies, in the number of void regions and in the abundance of low rotational velocity galaxies in the Local Universe. By comparing these predictions with the most recent observations, strong constrains on the physical properties of the dark matter particles can be derived. We devote particular attention to the results from the Constrained Local UniversE Simulations (CLUES) project, a special set of simulations whose initial conditions are constrained by observational data from the Local Universe. The resulting simulations are designed to reproduce the observed structures in the nearby universe. The CLUES provides a numerical laboratory for simulating the Local Group of galaxies and exploring the physics of galaxy formation in an environment designed to follow the observed Local Universe. It has come of age as the numerical analogue of Near-Field Cosmology.

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arXiv2013

Simulations of the Local Universe Constrained by Observational Peculiar Velocities

Peculiar velocities, obtained from direct distance measurements, are data of choice to achieve constrained simulations of the Local Universe reliable down to a scale of a few Megaparsecs. Unlike redshift surveys, peculiar velocities are direct tracers of the underlying gravitational field as they trace both baryonic and dark matter. This paper presents the first attempt to use solely observational peculiar velocities to constrain cosmological simulations of the nearby universe. In order to set up Initial Conditions, a Reverse Zel'dovich Approximation (RZA) is used to displace constraints from their positions at z=0 to their precursors' locations at higher redshifts. An additional new feature replaces original observed radial peculiar velocity vectors by their full 3D reconstructions provided by the Wiener-Filter (WF) estimator. Subsequently, the Constrained Realization of Gaussian fields technique (CR) is applied to build various realizations of the Initial Conditions. The WF/RZA/CR method is first tested on realistic mock catalogs built from a reference simulation similar to the Local Universe. These mocks include errors on peculiar velocities, on data-point positions and a large continuous zone devoid of data in order to mimic galactic extinction. Large scale structures are recovered with a typical accuracy of 5 Megaparsecs/h in position, the best realizations reaching a 2-3 Mpc/h precision, the limit imposed by the RZA linear theory. Then, the method is applied to the first observational radial peculiar velocity catalog of the project Cosmicflows. This paper is a proof of concept that the WF/RZA/CR method can be applied to observational peculiar velocities to successfully build constrained Initial Conditions.

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arXiv2013

Vector dark energy and high-z massive clusters

The detection of extremely massive clusters at $z>1$ such as SPT-CL J0546-5345, SPT-CL J2106-5844, and XMMU J2235.3-2557 has been considered by some authors as a challenge to the standard \LCDM$\;$cosmology. In fact,assuming Gaussian initial conditions, the theoretical expectation of detecting such objects is as low as $\leq 1%$. In this \textit{Letter} we discuss the probability of the existence of such objects in the light of the Vector Dark Energy (VDE) paradigm, showing by means of a series of $N$-body simulations that chances of detection are substantially enhanced in this non-standard framework.

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arXiv2013

N-body simulations with a cosmic vector for dark energy

We present the results of a series of cosmological $N$-body simulations of a Vector Dark Energy (VDE) model, performed using a suitably modified version of the publicly available \texttt{GADGET}-2 code. The setups of our simulations were calibrated pursuing a twofold aim: 1) to analyze the large scale distribution of massive objects and 2) to determine the properties of halo structure in this different ramework.We observe that structure formation is enhanced in VDE, since the mass function at high redshift is boosted up to a factor of ten with respect to \LCDM, possibly alleviating tensions with the observations of massive clusters at high redshifts and early reionization epoch. Significant differences can also be found for the value of the growth factor, that in VDE shows a completely different behaviour, and in the distribution of voids, which in this cosmology are on average smaller and less abundant. We further studied the structure of dark matter haloes more massive than $5\times10^{13}$\hMsun, finding that no substantial difference emerges when comparing spin parameter, shape, triaxiality and profiles of structures evolved under different cosmological pictures. Nevertheless, minor differences can be found in the concentration-mass relation and the two point correlation function; both showing different amplitudes and steeper slopes.Using an additional series of simulations of a \LCDM\ scenario with the same $Ω_M$ and $σ_8$ used in the VDE cosmology, we have been able to establish whether the modifications induced in the new cosmological picture were due to the particular nature of the dynamical dark energy or a straightforward consequence of the cosmological parameters.

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arXiv2013

Simulating Structure Formation of the Local Universe

In this work we present cosmological N-body simulations of the Local Universe with initial conditions constrained by the Two-Micron Redshift Survey (2MRS) within a cubic volume of 180 Mpc/h side-length centred at the Local Group. We use a self-consistent Bayesian based approach to explore the joint parameter space of primordial density fluctuations and peculiar velocity fields, which are compatible with the 2MRS galaxy distribution after cosmic evolution. This method (the KIGEN-code) includes the novel ALPT (Augmented Lagrangian Perturbation Theory) structure formation model which combines second order LPT (2LPT) on large scales with the spherical collapse model on small scales. Furthermore we describe coherent flows with 2LPT and include a dispersion term to model fingers-of-god (fogs). These implementations are crucial to avoid artificial filamentary structures, which appear when using a structure formation model with 2LPT and data with compressed fogs. We assume LCDM cosmology throughout our method. The recovered initial Gaussian fields are used to perform a set of 25 constrained simulations. Statistically this ensemble of simulations is in agreement with a reference set of 25 simulations based on randomly seeded Gaussian fluctuations in terms of matter statistics, power-spectra and mass functions. Considering the entire volume, we obtain correlation coefficients of about 98.3% for the comparison between the simulated density fields and the galaxy density field in log-space with Gaussian smoothing scales of r_S=3.5 Mpc/h (74% for r_S=1.4 Mpc/h). The cross power-spectra show correlations with the galaxy distribution which weaken towards smaller length scales until they vanish at scales of 2.2-3.0 Mpc/h. The simulations we present provide a fully nonlinear density and velocity field with a high level of correlation with the observed galaxy distribution at scales of a few Mpc.

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arXiv2013

Structure Finding in Cosmological Simulations: The State of Affairs

The ever increasing size and complexity of data coming from simulations of cosmic structure formation demands equally sophisticated tools for their analysis. During the past decade, the art of object finding in these simulations has hence developed into an important discipline itself. A multitude of codes based upon a huge variety of methods and techniques have been spawned yet the question remained as to whether or not they will provide the same (physical) information about the structures of interest. Here we summarize and extent previous work of the "halo finder comparison project": we investigate in detail the (possible) origin of any deviations across finders. To this extent we decipher and discuss differences in halo finding methods, clearly separating them from the disparity in definitions of halo properties. We observe that different codes not only find different numbers of objects leading to a scatter of up to 20 per cent in the halo mass and Vmax function, but also that the particulars of those objects that are identified by all finders differ. The strength of the variation, however, depends on the property studied, e.g. the scatter in position, bulk velocity, mass, and the peak value of the rotation curve is practically below a few per cent, whereas derived quantities such as spin and shape show larger deviations. Our study indicates that the prime contribution to differences in halo properties across codes stems from the distinct particle collection methods and -- to a minor extent -- the particular aspects of how the procedure for removing unbound particles is implemented. We close with a discussion of the relevance and implications of the scatter across different codes for other fields such as semi-analytical galaxy formation models, gravitational lensing, and observables in general.

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arXiv2013

Orbital anisotropy in cosmological haloes revisited

The velocity anisotropy of particles inside dark matter (DM) haloes is an important physical quantity, which is required for the accurate modelling of mass profiles of galaxies and clusters of galaxies. It is typically measured using the ratio of the radial-to-tangential velocity dispersions at a given distance from the halo centre. However, this measure is insufficient to describe the dynamics of realistic haloes, which are typically quite elongated. Studying the velocity distribution in massive DM haloes in cosmological simulations, we find that in the inner parts of the haloes the local velocity ellipsoids are strongly aligned with the major axis of the halo, the alignment being stronger for more relaxed haloes. In the outer regions of the haloes, the alignment becomes gradually weaker and the orientation is more random. These two distinct regions of different degree of the alignment coincide with two characteristic regimes of the DM density profile: shallower and steeper than ρr^{-2}. This alignment of the local velocity ellipsoids requires reinterpretation of features found in measurements based on the spherically averaged ratio of the radial-to-tangential velocity dispersions. In particular, we show that the velocity distribution in the central halo regions is highly anisotropic. For cluster-size haloes with mass 10^{14}-10^{15} h^-1 Msun, the velocity anisotropy along the major axis is nearly independent of radius and is equal to β=1-σ^2_{perp}/σ^2_{radial}=0.4, which is significantly larger than the previously estimated spherically averaged velocity anisotropy. The alignment of density and velocity anisotropies, and the radial trends may also have some implications for the mass modelling based on kinematical data of such objects as galaxy clusters or dwarf spheroidals, where the orbital anisotropy is a key element in an unbiased mass inference.

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arXiv2013

Cold versus Warm Dark Matter simulations of a galaxy group

The differences between cold (CDM) and warm (WDM) dark matter in the formation of a group of galaxies is examined by running two identical simulations where in the WDM case the initial power spectrum has been altered to mimic a 1keV dark matter particle. The CDM initial conditions were constrained to reproduce at z = 0 the correct local environment within which a "Local Group" (LG) of galaxies may form. Two significant differences between the two simulations are found. While in the CDM case a group of galaxies that resembles the real LG forms, the WDM run fails to reproduce a viable LG, instead forming a diffuse group which is still expanding at z = 0. This is surprising since, due to the suppression of small scale power in its power spectrum, WDM is naively expected to only affect the collapse of small haloes and not necessarily the dynamics on a scale of a group of galaxies. Furthermore the concentration of baryons in halo center's is greater in CDM than in WDM and the properties of the disks differ.

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arXiv2013

The clustering of galaxies at z~0.5 in the SDSS-III Data Release 9 BOSS-CMASS sample: a test for the LCDM cosmology

We present results on the clustering of 282,068 galaxies in the Baryon Oscillation Spectroscopic Survey (BOSS) sample of massive galaxies with redshifts 0.4 ~10%) in the correlation functions for scales less than ~1 Mpc/h and ~10-40 Mpc/h. A more realistic abundance matching model and better statistics from upcoming observations are needed to clarify the situation. We also estimate that about 12% of the "galaxies" in the abundance-matched sample are satellites inhabiting central haloes with mass M>~1e14 M_sun/h. Using the MultiDark simulation we also study the real space halo bias b(r) of the matched catalogue finding that b=2.00+/-0.07 at large scales, consistent with the one obtained using the measured BOSS projected correlation function. Furthermore, the linear large-scale bias depends on the number density n of the abundance-matched sample as b=-0.048-(0.594+/-0.02)*log(n/(h/Mpc)^3). Extrapolating these results to BAO scales we measure a scale-dependent damping of the acoustic signal produced by non-linear evolution that leads to ~2-4% dips at ~3 sigma level for wavenumbers k>~0.1 h/Mpc in the linear large-scale bias.

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arXiv2013

The kinematics of the Local Group in a cosmological context

Recent observations constrained the tangential velocity of M31 with respect to the Milky Way (MW) to be v_tan<34.4 km/s and the radial velocity to be in the range v_rad=-109+/- 4.4 km/s (van der Marel et al. 2012). In this study we use a large volume high resolution N-body cosmological simulation (Bolshoi) together with three constrained simulations to statistically study this kinematics in the context of the LCDM. The comparison of the ensembles of simulated pairs with the observed LG at the 1-sigma level in the uncertainties has been done with respect to the radial and tangential velocities, the reduced orbital energy (e_tot), angular momentum (l_orb) and the dimensionless spin parameter, lambda. Our main results are: (i) the preferred radial and tangential velocities for pairs in LCDM are v_rad=-80+/-20 km/s, v_tan=50+/-10 km/s, (ii) pairs around that region are 3 to 13 times more common than pairs within the observational values, (iii) 15%to 24% of LG-like pairs in LCDM have energy and angular momentum consistent with observations while (iv) 9% to 13% of pairs in the same sample show similar values in the inferred dimensionless spin parameter. It follows that within current observational uncertainties the quasi-conserved quantities that characterize the orbit of the LG, i.e. e_tot, r_orb and lambda, do not challenge the standard LCDM model, but the model is in tension with regard to the actual values of the radial and tangential velocities. This might hint to a problem of the LCDM model to reproduce the observed LG.

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arXiv2013

Size matters: the non-universal density profile of subhaloes in SPH simulations and implications for the Milky Way's dSphs

We use dark matter only and full hydrodynamical Constrained Local UniversE Simulations (CLUES) of the formation of the Local Group to study the density profile of subhaloes of the simulated Milky Way and Andromeda galaxies. We show that the Einasto model provides the best description of the subhaloes' density profile, as opposed to the more commonly used NFW profile or any generalisation of it. We further find that the Einasto shape parameter \nEin\ is strongly correlated with the total subhalo mass, pointing towards the notion of a non-universality of the subhaloes' density profile. We observe that the effect of mass loss due to tidal stripping, in both the dark matter only and the hydrodynamical run, is the reduction of the shape parameter \nEin\ between the infall and the present time. Assuming now that the dSphs of our Galaxy follow the Einasto profile and using the maximum and minimum values of \nEin\ from our hydrodynamical simulation as a gauge, we can improve the observational constraints on the \Rmax-\Vmax\ pairs obtained for the brightest satellite galaxies of the Milky Way. When considering only the subhaloes with $-13.2\lesssim M_V\lesssim-8.8$, i.e. the range of luminosity of the classical dwarfs, we find that all our simulated objects are consistent with the observed dSphs if their haloes follow the Einasto model with $1.6\lesssim n_{\rm E} \lesssim5.3$. The numerically motivated Einasto profile for the observed dSphs will alleviate the recently presented "massive failures" problem.

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arXiv2012

Reconstructing cosmological initial conditions from galaxy peculiar velocities. II. The effect of observational errors

The Reverse Zeldovich Approximation (RZA) is a reconstruction method which allows to estimate the cosmic displacement field from galaxy peculiar velocity data and to constrain initial conditions for cosmological simulations of the Local Universe. In this paper, we investigate the effect of different observational errors on the reconstruction quality of this method. For this, we build a set of mock catalogues from a cosmological simulation, varying different error sources like the galaxy distance measurement error (0 - 20%), the sparseness of the data points, and the maximum catalogue radius (3000 - 6000 km/s). We perform the RZA reconstruction of the initial conditions on these mock catalogues and compare with the actual initial conditions of the simulation. We also investigate the impact of the fact that only the radial part of the peculiar velocity is observationally accessible. We find that the sparseness of a dataset has the highest detrimental effect on RZA reconstruction quality. Observational distance errors also have a significant influence, but it is possible to compensate this relatively well with Wiener Filter reconstruction. We also investigate the effect of different object selection criteria and find that distance catalogues distributed randomly and homogeneously across the sky (such as spiral galaxies selected for the Tully-Fisher method) allow for a higher reconstruction quality than if when data is preferentially drawn from massive objects or dense environments (such as elliptical galaxies). We find that the error of estimating the initial conditions with RZA is always dominated by the inherent non-linearity of data observed at z=0 rather than by the combined effect of the observational errors. Even an extremely sparse dataset with high observational errors still leads to a good reconstruction of the initial conditions on a scale of about 5 Mpc/h.

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arXiv2012

Reconstructing cosmological initial conditions from galaxy peculiar velocities. I. Reverse Zeldovich Approximation

We propose a new method to recover the cosmological initial conditions of the presently observed galaxy distribution, which can serve to run constrained simulations of the Local Universe. Our method, the Reverse Zeldovich Approximation (RZA), can be applied to radial galaxy peculiar velocity data and extends the previously used Constrained Realizations (CR) method by adding a Lagrangian reconstruction step. The RZA method consists of applying the Zeldovich approximation in reverse to galaxy peculiar velocities to estimate the cosmic displacement field and the initial linear matter distribution from which the present-day Local Universe evolved.We test our method with a mock survey taken from a cosmological simulation. We show that the halo peculiar velocities at z = 0 are close to the linear prediction of the Zeldovich approximation, if a grouping is applied to the data to remove virial motions. We find that the addition of RZA to the CR method significantly improves the reconstruction of the initial conditions. The RZA is able to recover the correct initial positions of the velocity tracers with a median error of only 1.36 Mpc/h in our test simulation. For realistic sparse and noisy data, this median increases to 5 Mpc/h. This is a significant improvement over the previous approach of neglecting the displacement field, which introduces errors on a scale of 10 Mpc/h or even higher. Applying the RZA method to the upcoming high-quality observational peculiar velocity catalogues will generate much more precise constrained simulations of the Local Universe.

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