arXiv subjects

Cristiano Porciani

Publications and source records attributed to Cristiano Porciani.

At least 37 records · Page 2Linked to original sources

arXiv2021

The large-scale monopole of the power spectrum in a Euclid-like survey: wide-angle effects, lensing, and the `finger of the observer'

Radial redshift-space distortions due to peculiar velocities and other light-cone effects shape the maps we build of the Universe. We address the open question of their impact onto the monopole moment of the galaxy power spectrum, $P_0(k)$. Specifically, we use an upgraded numerical implementation of the LIGER method to generate $140$ mock galaxy density fields for a full Euclid-like survey and we measure $P_0(k)$ in each of them utilising a standard estimator. We compare the spectra obtained by turning on and off different effects. Our results show that wide-angle effects due to radial peculiar velocities generate excess power above the level expected within the plane-parallel approximation. They are detectable with a signal-to-noise ratio of 2.7 for $k<0.02\,h$ Mpc$^{-1}$. Weak-lensing magnification also produces additional power on large scales which, if the current favourite model for the luminosity function of H$α$ emitters turns out to be realistic, can only be detected with a signal-to-noise ratio of 1.3 at best. Finally, we demonstrate that measuring $P_0(k)$ in the standard of rest of the observer generates an additive component reflecting the kinematic dipole overdensity caused by the peculiar velocity. This component is characterised by a damped oscillatory pattern on large scales. We show that this `finger of the observer' effect is detectable in some redshift bins and suggest that its measurement could possibly open new research directions in connection with the determination of the cosmological parameters, the properties of the galaxy population under study, and the dipole itself.

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arXiv2021

Cosmological parameters from the likelihood analysis of the galaxy power spectrum and bispectrum in real space

We present a joint likelihood analysis of the halo power spectrum and bispectrum in real space. We take advantage of a large set of numerical simulations and of an even larger set of halo mock catalogs to provide a robust estimate of the covariance properties. We derive constraints on bias and cosmological parameters assuming a theoretical model from perturbation theory at one-loop for the power spectrum and tree-level for the bispectrum. By means of the Deviance Information Criterion, we select a reference bias model dependent on seven parameters that can describe the data up to $k_{\rm max,P}=0.3\, h \, {\rm Mpc}^{-1}$ for the power spectrum and $k_{\rm max,B}=0.09\, h \, {\rm Mpc}^{-1}$ for the bispectrum at redshift $z=1$. This model is able to accurately recover three selected cosmological parameters even for the rather extreme total simulation volume of $1000\, h^{-3} \, {\rm Gpc}^3$. With the same tools, we study how relations among bias parameters can improve the fit while reducing the parameter space. In addition, we compare common approximations to the covariance matrix against the full covariance estimated from the mocks, and quantify the (non-negligible) effect of ignoring the cross-covariance between the two statistics. Finally, we explore different selection criteria for the triangular configurations to include in the analysis, showing that excluding nearly equilateral triangles rather than simply imposing a fixed maximum $k_{\rm max,B}$ on all triangle sides can lead to a better exploitation of the information contained in the bispectrum.

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arXiv2021

The reach of next-to-leading-order perturbation theory for the matter bispectrum

We provide a comparison between the matter bispectrum derived with different flavours of perturbation theory at next-to-leading order and measurements from an unprecedentedly large suite of $N$-body simulations. We use the $χ^2$ goodness-of-fit test to determine the range of accuracy of the models as a function of the volume covered by subsets of the simulations. We find that models based on the effective-field-theory (EFT) approach have the largest reach, standard perturbation theory has the shortest, and `classical' resummed schemes lie in between. The gain from EFT, however, is less than in previous studies. We show that the estimated range of accuracy of the EFT predictions is heavily influenced by the procedure adopted to fit the amplitude of the counterterms. For the volumes probed by galaxy redshift surveys, our results indicate that it is advantageous to set three counterterms of the EFT bispectrum to zero and measure the fourth from the power spectrum. We also find that large fluctuations in the estimated reach occur between different realisations. We conclude that it is difficult to unequivocally define a range of accuracy for the models containing free parameters. Finally, we approximately account for systematic effects introduced by the $N$-body technique either in terms of a scale- and shape-dependent bias or by boosting the statistical error bars of the measurements (as routinely done in the literature). We find that the latter approach artificially inflates the reach of EFT models due to the presence of tunable parameters.

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arXiv2021

The HI-halo mass relation at redshift $z \sim 1$ from the Minkowski functionals of 21 cm intensity maps

The mean and the scatter of the HI content of a dark-matter halo as a function of the halo mass are useful statistics that can be used to test models of structure and galaxy formation. We investigate the possibility of constraining this HI-halo mass relation (HIHMR) from intensity maps of the redshifted 21 cm line. In particular, we use the geometry and topology of the brightness-temperature isocontours in a single frequency channel as quantified by the Minkowski functionals. First, we generate mock maps from a large N-body simulation considering the impact of thermal noise and foreground removal. We then use the Fisher information formalism to forecast constraints on a parametric model for the HIHMR. We consider a 20,000 deg$^2$ survey (originally proposed for dark-energy science) conducted with the Square Kilometre Array Phase 1 (SKA-1) MID observatory operating in single-dish mode. For a channel bandwidth of 2 MHz, we show that an integration time of a few$\,\times\,10^4$ s per pointing is sufficient to image the smoothed HI distribution at redshift $z \simeq 1$ and to measure the HIHMR in a nearly optimal way from the Minkowski functionals. Tighter constraints on some of the parameters can be obtained by using also an independent measurement of the mean HI density. Combining the results from different frequency channels provides exquisite constraints on the evolution of the HIHMR, especially in the central frequency range of the data cube.

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arXiv2020

Towards cosmological constraints from the compressed modal bispectrum: a robust comparison of real-space bispectrum estimators

Higher-order clustering statistics, like the galaxy bispectrum, can add complementary cosmological information to what is accessible with two-point statistics, like the power spectrum. While the standard way of measuring the bispectrum involves estimating a bispectrum value in a large number of Fourier triangle bins, the compressed modal bispectrum approximates the bispectrum as a linear combination of basis functions and estimates the expansion coefficients on the chosen basis. In this work, we compare the two estimators by using parallel pipelines to analyze the real-space halo bispectrum measured in a suite of $N$-body simulations corresponding to a total volume of $\sim 1{,}000 \,h^{-3}\,{\rm Gpc}^3$, with covariance matrices estimated from 10,000 mock halo catalogs. We find that the modal bispectrum yields constraints that are consistent and competitive with the standard bispectrum analysis: for the halo bias and shot noise parameters within the tree-level halo bispectrum model up to $k_{\rm max} \approx 0.06 \, (0.10) \,h\,{\rm Mpc}^{-1}$, only 6 (10) modal expansion coefficients are necessary to obtain constraints equivalent to the standard bispectrum estimator using $\sim$ 20 to 1,600 triangle bins, depending on the bin width. For this work, we have implemented a modal estimator pipeline using Markov Chain Monte Carlo simulations for the first time, and we discuss in detail how the parameter posteriors and modal expansion are robust to, or sensitive to, several user settings within the modal bispectrum pipeline. The combination of the highly efficient compression that is achieved and the large number of mock catalogs available allows us to quantify how our modal bispectrum constraints depend on the number of mocks that are used to estimate covariance matrices and the functional form of the likelihood.

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arXiv2020

A comparison of $\text{H}_2$ formation models at high redshift

Modelling the molecular gas that is routinely detected through CO observations of high-redshift galaxies constitutes a major challenge for ab initio simulations of galaxy formation. We carry out a suite of cosmological hydrodynamic simulations to compare three approximate methods that have been used in the literature to track the formation and evolution of the simplest and most abundant molecule, H$_2$. Namely, we consider: i) a semi-empirical procedure that associates H$_2$ to dark-matter haloes based on a series of scaling relations inferred from observations, ii) a model that assumes chemical equilibrium between the H$_2$ formation and destruction rates, and iii) a model that fully solves the out-of-equilibrium rate equations and accounts for the unresolved structure of molecular clouds. We study the impact of finite spatial resolution and show that robust H$_2$ masses at redshift $z\approx 4$ can only be obtained for galaxies that are sufficiently metal enriched in which H$_2$ formation is fast. This corresponds to H$_2$ reservoirs with masses $M_{\mathrm{H_2}}\gtrsim 6\times 10^9 \mathrm{M}_\odot$. In this range, equilibrium and non-equilibrium models predict similar molecular masses (but different galaxy morphologies) while the semi-empirical method produces less H$_2$. The star formation rates as well as the stellar and H$_2$ masses of the simulated galaxies are in line with those observed in actual galaxies at similar redshifts that are not massive starbursts. The H$_2$ mass functions extracted from the simulations at $z\approx 4$ agree well with recent observations that only sample the high-mass end. However, our results indicate that most molecular material at high $z$ lies yet undetected in reservoirs with $10^9<M_{\mathrm H_2}<10^{10} \mathrm{M}_\odot$.

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arXiv2020

The $n$-point streaming model: how velocities shape correlation functions in redshift space

Starting from first principles, we derive the fundamental equations that relate the $n$-point correlation functions in real and redshift space. Our result generalises the so-called `streaming model' to higher-order statistics: the full $n$-point correlation in redshift-space is obtained as an integral of its real-space counterpart times the joint probability density of $n-1$ relative line-of-sight peculiar velocities. Equations for the connected $n$-point correlation functions are obtained by recursively applying the generalised streaming model for decreasing $n$. Our results are exact within the distant-observer approximation and completely independent of the nature of the tracers for which the correlations are evaluated. Focusing on 3-point statistics, we use an $N$-body simulation to study the joint probability density function of the relative line-of-sight velocities of pairs of particles in a triplet. On large scales, we find that this distribution is approximately Gaussian and that its moments can be accurately computed with standard perturbation theory. We use this information to formulate a phenomenological 3-point Gaussian streaming model. A practical implementation is obtained by using perturbation theory at leading order to approximate several statistics in real space. In spite of this simplification, the resulting predictions for the matter 3-point correlation function in redshift space are in rather good agreement with measurements performed in the simulation. We discuss the limitations of the simplified model and suggest a number of possible improvements. Our results find direct applications in the analysis of galaxy clustering but also set the basis for studying 3-point statistics with future peculiar-velocity surveys and experiments based on the kinetic Sunyaev-Zel'dovich effect.

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arXiv2020

Toward a robust inference method for the galaxy bispectrum: likelihood function and model selection

The forthcoming generation of galaxy redshift surveys will sample the large-scale structure of the Universe over unprecedented volumes with high-density tracers. This advancement will make robust measurements of three-point clustering statistics possible. In preparation for this improvement, we investigate how several methodological choices can influence inferences based on the bispectrum about galaxy bias and shot noise. We first measure the real-space bispectrum of dark-matter haloes extracted from 298 N-body simulations covering a volume of approximately $1000 h^{-3} \mathrm{Gpc}^3$. We then fit a series of theoretical models based on tree-level perturbation theory to the numerical data. To achieve this, we estimate the covariance matrix of the measurement errors by using 10,000 mock catalogues generated with the Pinocchio code. We study how the model constraints are influenced by the binning strategy for the bispectrum configurations and by the form of the likelihood function. We also use Bayesian model-selection techniques to single out the optimal theoretical description of our data. We find that a three-parameter bias model combined with Poissonian shot noise is necessary to model the halo bispectrum up to scales of $k_\mathrm{max}\lesssim 0.08 h \mathrm{Mpc}^{-1}$, although fitting formulae that relate the bias parameters can be helpful to reduce the freedom of the model without compromising accuracy. Our data clearly disfavour local Eulerian and local Lagrangian bias models and do not require corrections to Poissonian shot noise. We anticipate that model-selection diagnostics will be particularly useful to extend the analysis to smaller scales as, in this case, the number of model parameters will grow significantly large.

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arXiv2020

Forecasts for Next Generation tSZ Surveys: the Impact of a Cosmology-Dependent Selection Function

The thermal Sunyaev-Zel'dovich (tSZ) effect is one of the primary tools for finding and characterizing galaxy clusters. Several ground-based experiments are either underway or are being planned for mapping wide areas of the sky at $\sim 150$ GHz with large-aperture telescopes. We present cosmological forecasts for a 'straw man' tSZ survey that will observe a sky area between $200$ and $10^4$ deg$^2$ to an rms noise level between 2.8 and 20.2 $μ$K-arcmin. The probes we consider are the cluster number counts (as a function of the integrated Compton-$Y$ parameter and redshift) and their angular clustering (as a function of redshift). At fixed observing time, we find that wider surveys constrain cosmology slightly better than deeper ones due to their increased ability to detect rare high-mass clusters. In all cases, we notice that adding the clustering information does not practically improve the constraints derived from the number counts. We compare forecasts obtained by sampling the posterior distribution with the Markov-chain-Monte-Carlo method against those derived using the Fisher-matrix formalism. We find that the latter produces slightly optimistic constraints where errors are underestimated at the 10 per cent level. Most importantly, we use an analytic method to estimate the selection function of the survey and account for its response to variations of the cosmological parameters in the likelihood function. Our analysis demonstrates that neglecting this effect (as routinely done in the literature) yields artificially tighter constraints by a factor of 2.2 and 1.7 for $σ_8$ and $Ω_\mathrm{M}$, respectively.

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arXiv2019

Renormalisation of linear halo bias in N-body simulations

The interpretation of redshift surveys requires modeling the relationship between large-scale fluctuations in the observed number density of tracers, $δ_\mathrm{h}$, and the underlying matter density, $δ$. Bias models often express $δ_\mathrm{h}$ as a truncated series of integro-differential operators acting on $δ$, each weighted by a bias parameter. Due to the presence of `composite operators' (obtained by multiplying fields evaluated at the same spatial location), the linear bias parameter measured from clustering statistics does not coincide with that appearing in the bias expansion. This issue can be cured by re-writing the expansion in terms of `renormalised' operators. After providing a pedagogical and comprehensive review of bias renormalisation in perturbation theory, we generalize the concept to non-perturbative dynamics and successfully apply it to dark-matter haloes extracted from a large suite of N-body simulations. When comparing numerical and perturbative results, we highlight the effect of the window function employed to smooth the random fields. We then measure the bias parameters as a function of halo mass by fitting a non-perturbative bias model (both before and after applying renormalisation) to the cross spectrum $P_{δ_\mathrm{h}δ}(k)$. Finally, we employ Bayesian model selection to determine the optimal operator set to describe $P_{δ_\mathrm{h}δ}(k)$ for $k<0.2\,h$ Mpc$^{-1}$ at redshift $z=0$. We find that it includes $δ, \nabla^2δ, δ^2$ and the square of the traceless tidal tensor, $s^2$. Considering higher-order terms (in $δ$) leads to overfitting as they cannot be precisely constrained by our data. We also notice that next-to-leading-order perturbative solutions are inaccurate for $k\gtrsim 0.1\,h$ Mpc$^{-1}$.

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arXiv2019

The Goldilocks problem of the quasar contribution to reionization

The detection of an unexpectedly large number of faint candidate quasars (QSO) at $z \gtrsim 4$ has motivated Madau and Haardt to investigate a reionization scenario in which active galactic nuclei dominate the photon budget at all times. Their analytical study reveals that this picture is in agreement with the evolution of the HI volume fraction in the intergalactic medium (IGM) and the optical depth to Thomson scattering of the cosmic microwave background. We employ a suite of hydrodynamical simulations post-processed with a radiative transfer code to further investigate the properties of such a reionization history. In particular, we generate synthetic absorption-line spectra for HI and HeII that we compare with observational data. Although we confirm the analytical results mentioned above, we also find that the evolution of the IGM temperature and of the HeII optical depth are at odds with current observational constraints. Nevertheless, the QSO-dominated scenario presents the attractive feature that it naturally generates an inhomogeneous IGM and thus produces an extended tail at high values in the distribution function of the HI effective optical depth, in agreement with recent observations. We find tentative evidence that considering some QSO emission at redshift $z \gtrsim 5$ could reconcile such observations with numerical simulations that have so far failed to reproduce this feature. Finally, we show that collecting more HeII-absorption spectra at $z \gtrsim 3$ and studying their distinctive characteristics will be key to precisely constraining the QSO contribution to reionization.

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arXiv2018

Cosmological information in the redshift-space bispectrum

We use the Fisher-matrix formalism to investigate whether the galaxy bispectrum in redshift space, $B$, contains additional cosmological information with respect to the power spectrum, $P$. We focus on a $Euclid$-like survey and consider cosmological models dominated by dark energy and cold dark matter with Gaussian primordial perturbations. After discussing the phenomenology of redshift-space distortions for the bispectrum, we derive an expression for the cross-covariance between $B$ and $P$ at leading order in perturbation theory. Our equation generalizes previous results that did not consider binning in the orientation of wavevector triangles with respect to the line of sight. By considering Fourier modes with wavenumber $k<0.15 \,h$ Mpc$^{-1}$, we find that $B$ and $P$ set similar constraints on the cosmological parameters. Generally, error bars moderately improve when the two probes are combined together. For instance, the joint 68.3 per cent credible region for the parameters that describe a dynamical dark-energy equation of state shrinks by a factor of 2.6 with respect to only using the power spectrum. Regrettably, this improvement is cancelled out when the clustering analysis is combined with priors based on current studies of the cosmic microwave background. In this case, combining $B$ and $P$ does not give any appreciable benefit other than allowing a precise determination of galaxy bias. Finally, we discuss how results depend on the binning strategy for the clustering statistics as well as on the maximum wavenumber. We also show that only considering the bispectrum monopole leads to a significant loss of information.

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arXiv2018

On the streaming model for redshift-space distortions

The streaming model describes the mapping between real and redshift space for 2-point clustering statistics. Its key element is the probability density function (PDF) of line-of-sight pairwise peculiar velocities. Following a kinetic-theory approach, we derive the fundamental equations of the streaming model for ordered and unordered pairs. In the first case, we recover the classic equation while we demonstrate that modifications are necessary for unordered pairs. We then discuss several statistical properties of the pairwise velocities for DM particles and haloes by using a suite of high-resolution $N$-body simulations. We test the often used Gaussian ansatz for the PDF of pairwise velocities and discuss its limitations. Finally, we introduce a mixture of Gaussians which is known in statistics as the generalised hyperbolic distribution and show that it provides an accurate fit to the PDF. Once inserted in the streaming equation, the fit yields an excellent description of redshift-space correlations at all scales that vastly outperforms the Gaussian and exponential approximations. Using a principal-component analysis, we reduce the complexity of our model for large redshift-space separations. Our results increase the robustness of studies of anisotropic galaxy clustering and are useful for extending them towards smaller scales in order to test theories of gravity and interacting dark-energy models.

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arXiv2018

Forecasts on dark energy from the X-ray cluster survey with eROSITA: constraints from counts and clustering

We forecast the potential of the forthcoming X-ray galaxy-cluster survey with eROSITA to constrain dark-energy models. We focus on spatially-flat cosmological scenarios with either constant or time-dependent dark-energy equation-of-state parameters. Fisher information is extracted from the number density and spatial clustering of a photon-count-limited sample of clusters of galaxies up to z~2. We consider different scenarios for the availability of (i) X-ray follow-up observations, (ii) photometric and spectroscopic redshifts, and (iii) accurate knowledge of the observable -- mass relation down to the scale of galaxy groups. With about 125,000 clusters (detected with more than 50 photons and with mass M500c > $10^{13} h^{-1}$ Msun) from an average all-sky exposure of 1.6 ks, eROSITA will give marginalized, one-dimensional, 1-$σ$ errors of $Δσ_8 = \pm~0.008$ ($\sim$1 per cent), $ΔΩ_{\rm M} = \pm~0.006$ (2.2 per cent), $Δw_0 = \pm~0.07$ (7 per cent), and $Δw_a = \pm~0.25$ (optimistic scenario) in combination with (and largely improving upon) current constraints from various cosmological probes (cosmic microwave background, BAOs, Type Ia SNe). Our findings correspond to a dark-energy figure of merit in the range of $116-162$ (after the four years of all-sky survey), making eROSITA one of the first Stage IV experiments to come on line according to the classification of the Dark Energy Task Force. To secure improved mass calibrations and to include high-redshift clusters (z > 0.5) as well as objects at the group-mass scale (M500c < 5 $\times~ 10^{13} h^{-1}$ Msun) will be vital to reach such accuracies.

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arXiv2018

On the radial acceleration relation of $Λ$CDM satellite galaxies

The radial acceleration measured in bright galaxies tightly correlates with that generated by the observed distribution of baryons, a phenomenon known as the radial acceleration relation (RAR). Dwarf spheroidal satellite galaxies have been recently found to depart from the extrapolation of the RAR measured for more massive objects but with a substantially larger scatter. If confirmed by new data, this result provides a powerful test of the theory of gravity at low accelerations that requires robust theoretical predictions. By using high-resolution hydrodynamical simulations, we show that, within the standard model of cosmology ($Λ$CDM), satellite galaxies are expected to follow the same RAR as brighter systems but with a much larger scatter which does not correlate with the physical properties of the galaxies. In the simulations, the RAR evolves mildly with redshift. Moreover, the acceleration due to the gravitational field of the host has no effect on the RAR. This is in contrast with the External Field Effect in Modified Newtonian Dynamics (MOND) which causes galaxies in strong external fields to deviate from the RAR. This difference between $Λ$CDM and MOND offers a possible way to discriminate between them.

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arXiv2017

ZOMG III: The effect of Halo Assembly on the Satellite Population

We use zoom hydrodynamical simulations to investigate the properties of satellites within galaxy-sized dark-matter haloes with different assembly histories. We consider two classes of haloes at redshift $z=0$: `stalled' haloes that assembled at $z>1$ and `accreting' ones that are still forming nowadays. Previously, we showed that the stalled haloes are embedded within thick filaments of the cosmic web while the accreting ones lie where multiple thin filaments converge. We find that satellites in the two classes have both similar and different properties. Their mass spectra, radial count profiles, baryonic and stellar content, and the amount of material they shed are indistinguishable. However, the mass fraction locked in satellites is substantially larger for the accreting haloes as they experience more mergers at late times. The largest difference is found in the satellite kinematics. Substructures fall towards the accreting haloes along quasi-radial trajectories whereas an important tangential velocity component is developed, before accretion, while orbiting the filament that surrounds the stalled haloes. Thus, the velocity anisotropy parameter of the satellites ($β$) is positive for the accreting haloes and negative for the stalled ones. This signature enables us to tentatively categorize the Milky Way halo as stalled based on a recent measurement of $β$. Half of our haloes contain clusters of satellites with aligned orbital angular momenta corresponding to flattened structures in space. These features are not driven by baryonic physics and are only found in haloes hosting grand-design spiral galaxies, independently of their assembly history.

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arXiv2017

ZOMG-I. How the cosmic web inhibits halo growth and generates assembly bias

The clustering of dark matter haloes with fixed mass depends on their formation history, an effect known as assembly bias. We use zoom N -body simulations to investigate the origin of this phenomenon. For each halo at redshift z=0, we determine the time in which the physical volume containing its final mass becomes stable. We consider five examples for which this happens at z~1.5 and two that do not stabilize by z=0. The zoom simulations show that early-collapsing haloes do not grow in mass at z=0 while late-forming ones show a net inflow. The reason is that 'accreting' haloes are located at the nodes of a network of thin filaments feeding them. Conversely, each 'stalled' halo lies within a prominent filament that is thicker than the halo size. Infalling material from the surroundings becomes part of the filament while matter within it recedes from the halo. We conclude that assembly bias originates from quenching halo growth due to tidal forces following the formation of non-linear structures in the cosmic web, as previously conjectured in the literature. Also the internal dynamics of the haloes change: the velocity anisotropy profile is biased towards radial (tangential) orbits in accreting (stalled) haloes. Our findings reveal the cause of the yet unexplained dependence of halo clustering on the anisotropy. Finally, we extend the excursion-set theory to account for these effects. A simple criterion based on the ellipticity of the linear tidal field combined with the spherical collapse model provides excellent predictions for both classes of haloes.

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arXiv2017

ZOMG II: Does the halo assembly history influence central galaxies and gas accretion?

The growth-rate and the internal dynamics of galaxy-sized dark-matter haloes depend on their location within the cosmic web. Haloes that sit at the nodes grow in mass till the present time and are dominated by radial orbits. Conversely, haloes embedded in prominent filaments do not change much in size and are dominated by tangential orbits. Using zoom hydrodynamical simulations including star formation and feedback, we study how gas accretes onto these different classes of objects that, for simplicity, we dub 'accreting' and 'stalled' haloes. We find that all haloes get a fresh supply of newly accreted gas in their inner regions, although this slowly decreases with time, in particular for the stalled haloes. The inflow of new gas is always higher than (but comparable with) that of recycled material. Overall, the cold-gas fraction increases (decreases) with time for the accreting (stalled) haloes. In all cases, a stellar disc and a bulge form at the centre of the simulated haloes. The total stellar mass is in excellent agreement with expectations based on the abundance-matching technique. Many properties of the central galaxies do not seem to correlate with the large-scale environment in which the haloes reside. However, there are two notable exceptions that characterise stalled haloes with respect to their accreting counterparts: i) the galaxy disc contains much older stellar populations; ii) its vertical scale-height is larger by a factor of two or more. This thickening is likely due to the heating of the long-lived discs by mergers and close flybys.

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