arXiv subjects

Aseem Paranjape

Publications and source records attributed to Aseem Paranjape.

At least 37 records · Page 2Linked to original sources

arXiv2023

Impact of tidal environment on galaxy clustering in GAMA

We constrain models of the galaxy distribution in the cosmic web using data from the Galaxy and Mass Assembly (GAMA) survey. We model the redshift-space behaviour of the 2-point correlation function (2pcf) and the recently proposed Voronoi volume function (VVF) -- which includes information beyond 2-point statistics. We extend the standard halo model using extra satellite degrees of freedom and two assembly bias parameters, $α_{\rm cen}$ and $α_{\rm sat}$, which respectively correlate the occupation numbers of central and satellite galaxies with their host halo's tidal environment. We measure $α_{\rm sat}=1.44^{+0.25}_{-0.43}$ and $α_{\rm cen}=-0.79^{+0.29}_{-0.11}$ using a combination of 2pcf and VVF measurements, representing a detection of assembly bias at the 3.3$σ$ (2.4$σ$) significance level for satellite (central) galaxies. This result remains robust to possible anisotropies in the halo-centric distribution of satellites as well as technicalities of estimating the data covariance. We show that the growth rate ($fσ_8$) deduced using models with assembly bias is about 7\% (i.e. $1.5σ$) lower than if assembly bias is ignored. When projected onto the $Ω_m$-$σ_8$ plane, the model constraints without assembly bias overlap with Planck expectations, while allowing assembly bias introduces significant tension with Planck, preferring either a lower $Ω_m$ or a lower $σ_8$. Finally, we find that the all-galaxy weak lensing signal is unaffected by assembly bias, but the central and satellite sub-populations individually show significantly different signals in the presence of assembly bias. Our results illustrate the importance of accurately modelling galaxy formation for cosmological inference from future surveys.

astro-ph.CO↗

arXiv2023

A fast method of reionization parameter space exploration using GPR trained SCRIPT

Efficient exploration of parameter spaces is crucial to extract physical information about the Epoch of Reionization from various observational probes. To this end, we propose a fast technique based on Gaussian Process Regression (GPR) training applied to a semi-numerical photon-conserving reionization model, SCRIPT. Our approach takes advantage of the numerical convergence properties of SCRIPT and constructs a training set based on low-cost, coarse-resolution simulations. A likelihood emulator is then trained using this set to produce results in approximately two orders of magnitude less computational time than a full MCMC run, while still generating reasonable 68% and 95% confidence contours. Furthermore, we conduct a forecasting study using simulated data to demonstrate the applicability of this technique. This method is particularly useful when full MCMC analysis is not feasible due to expensive likelihood computations.

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arXiv2023

Model-agnostic cosmological constraints from the baryon acoustic oscillation feature in redshift space

We develop a framework for self-consistently extracting cosmological information from the clustering of tracers in redshift space, $\textit{without}$ relying on model-dependent templates to describe the baryon acoustic oscillation (BAO) feature. Our approach uses the recently proposed Laguerre reconstruction technique for the BAO feature and its linear point $r_{\rm LP}$, and substantially extends it to simultaneously model the multipoles $\ell=0,2,4$ of the anisotropic galaxy 2-point correlation function (2pcf). The approach is `model-agnostic': it assumes that the non-linear growth of structure smears the BAO feature by an approximately Gaussian kernel with a smearing scale $σ_{\rm v}$, but does not assume any fiducial cosmology for describing the shape of the feature itself. Using mock observations for two realistic survey configurations assuming $Λ$ cold dark matter ($Λ$CDM), combined with Bayesian parameter inference, we show that the linear point $r_{\rm LP}$ and smearing scale $σ_{\rm v}$ can be accurately recovered by our method in both existing and upcoming surveys. The precision of the recovery of $r_{\rm LP}$ is always better than $1\%$, while $σ_{\rm v}$ can be recovered with $\lesssim10\%$ uncertainty provided the linear galaxy bias $b$ is separately constrained, e.g., using weak lensing observations. Our method is also sensitive to the linear growth rate $f$, albeit with larger uncertainties and systematic errors, especially for upcoming surveys such as DESI. We discuss how our model can be modified to improve the recovery of $f$, such that the resulting constraints on $\{f,σ_{\rm v},r_{\rm LP}\}$ can potentially be used as a test of cosmological models including and beyond $Λ$CDM.

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arXiv2023

A test of invariance of dark matter halo surface density using multiwavelength mock galaxy catalogues

A large number of observations have shown that the dark matter halo surface density, given by the product of halo core radius and core density is nearly constant for a diverse suite of galaxies. Although this invariance of the halo surface density is violated at galaxy cluster and group scales, it is still an open question on whether the aforementioned constancy on galactic scales can be explained within $Λ$CDM. For this purpose, we probe the variation of halo surface density as a function of mass using multi-wavelength mock galaxy catalogs from $Λ$CDM simulations, where the adiabatic contraction of dark matter halos in the presence of baryons has been taken into account. We find that these baryonified $Λ$CDM halos were best fitted with a generalized-NFW profile, and the halo surface density from these halos has a degeneracy with respect to both the halo mass and the virial concentration. We find that the correlation with mass when averaged over concentration is consistent with a constant halo surface density. However, a power-law dependence as a function of halo mass also cannot be ruled out.

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arXiv2023

Lognormal semi-numerical simulations of the Lyman-$α$ forest: comparison with full hydrodynamic simulations

Observations of the Lyman-$α$ (Ly$α$) forest in spectra of distant quasars enable us to probe the matter power spectrum at relatively small scales. With several upcoming surveys, it is expected that there will be a many-fold increase in the quantity and quality of data, and hence it is important to develop efficient simulations to forward model these data sets. One such semi-numerical method is based on the assumption that the baryonic densities in the intergalactic medium (IGM) follow a lognormal distribution. In this work, we test the robustness of the lognormal model of the Ly$α$ forest in recovering a set of IGM parameters by comparing with high-resolution Sherwood SPH simulations. We study the recovery of the parameters $T_0$ (temperature of the mean-density IGM), $γ$ (slope of the temperature-density relation) and $Γ_{12}$ (hydrogen photoionization rate) at $z \sim 2.5$ using a Markov Chain Monte Carlo (MCMC) technique for parameter estimation. Using three flux statistics, the probability distribution, the mean flux and the power spectrum, values of all three parameters, $T_0$, $γ$ and $Γ_{12}$ implied in the SPH simulations are recovered within $1 - σ$ ($\sim$ 9, 4 and 1% respectively) of the median (best-fit) values. We verify the validity of our results at different baryon smoothing filter, SNR, box size & resolution, and data seed and confirm that the lognormal model can be used as an efficient tool for modelling the Ly$α$ transmitted flux at $z \sim 2.5$.

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arXiv2023

The quasi-adiabatic relaxation of haloes in the IllustrisTNG and EAGLE cosmological simulations

The dark matter content of a gravitationally bound halo is known to be affected by the galaxy and gas it hosts. We characterise this response for haloes spanning over four orders of magnitude in mass in the hydrodynamical simulation suites IllustrisTNG and EAGLE. We present simple fitting functions in the spherically averaged quasi-adiabatic relaxation framework that accurately capture the dark matter response over the full range of halo mass and halo-centric distance we explore. We show that commonly employed schemes, which consider the relative change in radius $r_f/r_i-1$ of a spherical dark matter shell to be a function of only the relative change in its mass $M_i/M_f-1$, do not accurately describe the measured response of most haloes in IllustrisTNG and EAGLE. Rather, $r_f/r_i$ additionally explicitly depends upon halo-centric distance $r_f/R_{\rm vir}$ for haloes with virial radius $R_{\rm vir}$, being very similar between IllustrisTNG and EAGLE and across halo mass. We also account for a previously unmodelled effect, likely driven by feedback-related outflows, in which shells having $r_f/r_i\simeq1$ (i.e., no relaxation) have $M_i/M_f$ significantly different from unity. Our results are immediately applicable to a number of semi-analytical tools for modelling galactic and large-scale structure. We also study the dependence of this response on several halo and galaxy properties beyond total mass, finding that it is primarily related to halo concentration and star formation rate. We discuss possible extensions of these results to build a deeper physical understanding of the small-scale connection between dark matter and baryons.

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arXiv2022

Bayesian evidence comparison for distance scale estimates

Constraints on cosmological parameters are often distilled from sky surveys by fitting templates to summary statistics of the data that are motivated by a fiducial cosmological model. However, recent work has shown how to estimate the distance scale using templates that are more generic: the basis functions used are not explicitly tied to any one cosmological model. We describe a Bayesian framework for (i) determining how many basis functions to use and (ii) comparing one basis set with another. Our formulation provides intuition into how (a) one's degree of belief in different basis sets, (b) the fact that the choice of priors depends on basis set, and (c) the data set itself, together determine the derived constraints. We illustrate our framework using measurements in simulated datasets before applying it to real data.

astro-ph.CO↗

arXiv2022

The phenomenology of the external field effect in cold dark matter models

In general relativity (GR), the internal dynamics of a self-gravitating system under free-fall in an external gravitational field should not depend on the external field strength. Recent work has claimed a statistical detection of an `external field effect' (EFE) using galaxy rotation curve data. We show that large uncertainties in rotation curve analyses and inaccuracies in published simulation-based external field estimates compromise the significance of the claimed EFE detection. We further show analytically that a qualitatively similar statistical signal is, in fact, expected in a $Λ$-cold dark matter ($Λ$CDM) universe without any violation of the strong equivalence principle. Rather, such a signal arises simply because of the inherent correlations between galaxy clustering strength and intrinsic galaxy properties. We explicitly demonstrate the effect in a baryonified mock catalog of a $Λ$CDM universe. Although the detection of an EFE-like signal is not, by itself, evidence for physics beyond GR, our work shows that the $\textit{sign}$ of the EFE-like correlation between the external field strength and the shape of the radial acceleration relation can be used to probe new physics: e.g., in MOND, the predicted sign is opposite to that in our $Λ$CDM mocks.

astro-ph.CO↗

arXiv2022

A simulated annealing approach to parameter inference with expensive likelihoods

We present a new approach to parameter inference targeted on generic situations where the evaluation of the likelihood $\mathcal{L}$ (i.e., the probability to observe the data given a fixed model configuration) is numerically expensive. Inspired by ideas underlying simulated annealing, the method first evaluates $χ^2=-2\ln\mathcal{L}$ on a sparse sequence of Latin hypercubes of increasing density in parameter (eigen)space. The semi-stochastic choice of sampling points accounts for anisotropic gradients of $χ^2$ and rapidly zooms in on the minimum of $χ^2$. The sampled $χ^2$ values are then used to train an interpolator which is further used in a standard Markov Chain Monte Carlo (MCMC) algorithm to inexpensively explore the parameter space with high density, similarly to emulator-based approaches now popular in cosmological studies. Comparisons with example linear and non-linear problems show gains in the number of likelihood evaluations of factors of 10 to 100 or more, as compared to standard MCMC algorithms. As a specific implementation, we publicly release the code PICASA: Parameter Inference using Cobaya with Anisotropic Simulated Annealing, which combines the minimizer (of a user-defined $χ^2$) with Gaussian Process Regression for training the interpolator and a subsequent MCMC implementation using the COBAYA framework. Being agnostic to the nature of the observable data and the theoretical model, our implementation is potentially useful for a number of emerging problems in cosmology, astrophysics and beyond.

astro-ph.CO↗

arXiv2022

Mass accretion rates and multi-scale halo environment in cold and warm dark matter cosmologies

We study the evolving environment dependence of mass accretion by dark haloes in simulations of cold and warm dark matter (CDM and WDM) cosmologies. The latter allows us to probe the nature of halo growth at scales below the WDM half-mode mass, which form an extreme regime of nonlinear collisionless dynamics and offer an excellent test-bed for ideas relating to hierarchical growth. As environmental proxies, we use the local halo-centric matter density $δ$ and tidal anisotropy $α$, as well as large-scale halo bias $b_1$. Our analysis, while reproducing known trends for environment-dependent accretion in CDM, as well as the comparison between accretion in CDM and WDM, reveals several interesting new features. As expected from excursion set models, WDM haloes have higher specific accretion rates, dominated by the accretion of diffuse mass, as compared to CDM haloes. For low-mass WDM haloes, we find that the environment-dependence of both diffuse mass accretion as well as accretion by mergers is almost fully explained by $α$. For the other cases, $δ$ plays at least a comparable role. We detect, for the first time, a significant and evolving assembly bias due to diffuse mass accretion for low-mass CDM and WDM haloes (after excluding splashback objects), with a $z=0$ strength higher than with almost all known secondary variables and largely explained by $α$. Our results place constraints on semi-analytical merger tree algorithms, which in turn could affect the predictions of galaxy evolution models based on them.

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arXiv2021

The Life and Science of Thanu Padmanabhan

Thanu Padmanabhan was a renowned Indian theoretical physicist known for his research in general relativity, cosmology, and quantum gravity. In an extraordinary career spanning forty-two years, he published more than three hundred research articles, wrote ten highly successful technical and popular books, and mentored nearly thirty graduate students and post-doctoral fellows. He is best known for his deep work investigating gravitation as an emergent thermodynamic phenomenon. He was an outstanding teacher, and an indefatigable populariser of science, who travelled very widely to motivate and inspire young students. Paddy, as he was affectionately known, was also a close friend to his students and collaborators, treating them as part of his extended academic family. On September 17, 2021 Paddy passed away very unexpectedly, at the age of sixty-four and at the height of his research career, while serving as a Distinguished Professor at the Inter-University Centre for Astronomy and Astrophysics, Pune. His untimely demise has come as a shock to his family and friends and colleagues. In this article, several of them have come together to pay their tributes and share their fond memories of Paddy.

physics.hist-ph↗

arXiv2021

The radial acceleration relation in a $Λ$CDM universe

We study the radial acceleration relation (RAR) between the total ($a_{\rm tot}$) and baryonic ($a_{\rm bary}$) centripetal acceleration profiles of central galaxies in the cold dark matter (CDM) paradigm. We analytically show that the RAR is intimately connected with the physics of the quasi-adiabatic relaxation of dark matter in the presence of baryons in deep potential wells. This cleanly demonstrates how the mean RAR and its scatter emerge in the low-acceleration regime ($10^{-12}\,{\rm m\,s}^{-2}\lesssim a_{\rm bary}\lesssim10^{-10}\,{\rm m\,s}^{-2}$) from an interplay between baryonic feedback processes and the distribution of CDM in dark halos. Our framework allows us to go further and study both higher and lower accelerations in detail, using analytical approximations and a realistic mock catalog of $\sim342,000$ low-redshift central galaxies with $M_r\leq-19$. We show that, while the RAR in the baryon-dominated, high-acceleration regime ($a_{\rm bary}\gtrsim10^{-10}\,{\rm m\,s}^{-2}$) is very sensitive to details of the relaxation physics, a simple `baryonification' prescription matching the relaxation results of hydrodynamical CDM simulations is remarkably successful in reproducing the observed RAR without any tuning. And in the (currently unobserved) ultra-low-acceleration regime ($a_{\rm bary}\lesssim 10^{-12}\,{\rm m\,s}^{-2}$), the RAR is sensitive to the abundance of diffuse gas in the halo outskirts, with our default model predicting a distinctive break from a simple power-law-like relation for HI-deficient, diffuse gas-rich centrals. Our mocks also show that the RAR provides more robust, testable predictions of the $Λ$CDM paradigm at galactic scales, with implications for alternative gravity theories, than the baryonic Tully-Fisher relation.

astro-ph.GA↗

arXiv2021

The distribution of HI velocity profiles in a $Λ$CDM universe

We model the distribution of the observed profiles of 21 cm line emission from neutral hydrogen (HI) in central galaxies selected from a statistically representative mock catalog of the local Universe in the Lambda-cold dark matter framework. The distribution of these HI velocity profiles (specifically, their widths $W_{50}$) has been observationally constrained, but has not been systematically studied theoretically. Our model profiles derive from rotation curves of realistically baryonified haloes in an N-body simulation, including the quasi-adiabatic relaxation of the dark matter profile of each halo in response to its baryons. We study the predicted $W_{50}$ distribution using a realistic pipeline applied to noisy profiles extracted from our luminosity-complete mock catalog with an ALFALFA-like survey geometry and redshift selection. Our default mock is in good agreement with observed ALFALFA results for $W_{50}\gtrsim700$ km/s, being incomplete at lower widths due to the intrinsic threshold of $M_r\leq-19$. Variations around the default model show that the velocity width function at $W_{50}\gtrsim300$ km/s is most sensitive to a possible correlation between galaxy inclination and host concentration, followed by the physics of quasi-adiabatic relaxation. We also study the excess kurtosis of noiseless velocity profiles, obtaining a distribution which tightly correlates with $W_{50}$, with a shape and scatter that depend on the properties of the turbulent HI disk. Our results open the door towards using the shapes of HI velocity profiles as a novel statistical probe of the baryon-dark matter connection.

astro-ph.GA↗

arXiv2021

Multi-wavelength mock galaxy catalogs of the low-redshift Universe

We present a new suite of mock galaxy catalogs mimicking the low-redshift Universe, based on an updated halo occupation distribution (HOD) model and a scaling relation between optical properties and the neutral hydrogen (HI) content of galaxies. Our algorithm is constrained by observations of the luminosity function and luminosity- and colour-dependent clustering of SDSS galaxies, as well as the HI mass function and HI-dependent clustering of massive HI-selected galaxies in the ALFALFA survey. Mock central and satellite galaxies with realistic values of $r$-band luminosity, $g-r$ and $u-r$ colour, stellar mass and HI mass are populated in an $N$-body simulation, inheriting a number of properties of the density and tidal environment of their host halos. The host halo of each central galaxy is also `baryonified' with realistic spatial distributions of stars as well as hot and cold gas, along with the corresponding rotation curve. Our default HOD assumes that galaxy properties are a function of group halo mass alone, and can optionally include effects such as galactic conformity and colour-dependent galaxy assembly bias. The mocks predict the relation between the stellar mass and HI mass of massive HI galaxies, as well as the 2-point cross-correlation function of spatially co-located optical and HI-selected samples. They enable novel null tests for galaxy assembly bias, provide predictions for the HI velocity width function, and clarify the origin and universality of the radial acceleration relation in the $Λ$CDM framework.

astro-ph.GA↗

arXiv2021

Mock halo catalogs: assigning unresolved halo properties using correlations with local halo environment

Large-scale sky surveys require companion large volume simulated mock catalogs. To ensure precision cosmology studies are unbiased, the correlations in these mocks between galaxy properties and their large-scale environments must be realistic. Since galaxies are embedded in dark matter halos, an important first step is to include such correlations -- sometimes called assembly bias -- for dark matter halos. However, galaxy properties correlate with smaller scale physics in halos which large simulations struggle to resolve. We describe an algorithm which addresses and largely mitigates this problem. Our algorithm exploits the fact that halo assembly bias is unchanged as long as correlations between halo property $c$ and the intermediate-scale tidal environment $α$ are preserved. Therefore, knowledge of $α$ is sufficient to assign small-scale, otherwise unresolved properties to a halo in a way which preserves its large-scale assembly bias accurately. We demonstrate this explicitly for halo internal properties like formation history (concentration $c_{\rm 200b}$), shape $c/a$, dynamics $c_{v}/a_{v}$, velocity anisotropy $β$ and angular momentum (spin $λ$). Our algorithm increases a simulation's reach in halo mass and number density by an order of magnitude, with improvements in the bias signal as large as 45% for 30-particle halos, thus significantly reducing the cost of mocks for future weak lensing and redshift space distortion studies.

astro-ph.CO↗

arXiv2021

Analytical halo models of cosmic tidal fields

The non-linear cosmic web environment of dark matter haloes plays a major role in shaping their growth and evolution, and potentially also affects the galaxies that reside in them. We develop an analytical (halo model) formalism to describe the tidal field of anisotropic halo-centric density distributions, as characterised by the halo-centric tidal tensor $\langle T_{ij} \rangle(<R)$ spherically averaged on scale $R\sim4R_{\rm vir}$ for haloes of virial radius $R_{\rm vir}$. We focus on axisymmetric anisotropies, which allows us to explore simple and intuitive toy models of (sub)halo configurations that exemplify some of the most interesting anisotropies in the cosmic web. We build our models around the spherical Navarro-Frenk-White (NFW) profile after describing it as a Gaussian mixture, which leads to almost fully analytical expressions for the `tidal anisotropy' scalar $α(<4R_{\rm vir})$ extracted from the tidal tensor. Our axisymmetric examples include (i) a spherical halo at the axis of a cylindrical filament, (ii) an off-centred satellite in a spherical host halo and (iii) an axisymmetric halo. Using these, we demonstrate several interesting results. For example, the tidal tensor at the axis of a pure cylindrical filament gives $α^{\rm (fil)}(<R)=1/2$ exactly, for any $R$. Also, $α(<4R_{\rm vir,sat})$ for a satellite of radius $R_{\rm vir,sat}$ as a function of its host-centric distance is a sensitive probe of dynamical mass loss of the satellite in its host environment. Finally, we discuss a number of potentially interesting extensions and applications of our formalism that can deepen our understanding of the multi-scale phenomenology of the cosmic web.

astro-ph.CO↗

arXiv2020

Studying the Lyman-$α$ optical depth fluctuations at $z \sim 5.5$ using fast semi-numerical methods

We present a computationally efficient and fast semi-numerical technique for simulating the Lyman-$α$ (Ly$α$) absorption optical depth in presence of neutral hydrogen "islands" left over from reionization at redshifts $5 \lesssim z \lesssim 6$. The main inputs to the analysis are (i) a semi-numerical photon-conserving model of ionized regions during reionization (named SCRIPT) along with a prescription for simulating the shadowing by neutral islands and (ii) the fluctuating Gunn-Peterson approximation to model the Ly$α$ absorption. Our model is then used for simulating the large-scale fluctuations in the effective optical depth as observed along sight lines towards high$-z$ quasars. Our model is fully described by five parameters. By setting two of them to default values and varying the other three, we obtain the constraints on reionization history at $5 \lesssim z \lesssim 6$ as allowed by the data. We confirm that reionization is \emph{not} complete before $z \sim 5.6$ at $\gtrsim 2σ$ confidence, with the exact confidence limits depending on how the non-detections of the flux in the data are treated. We also confirm that the completion of reionization can be as late as $z \sim 5.2$. With further improvements in the model and with more sight lines at $z \sim 6$, we can take advantage of the computational efficiency of our analysis to obtain more stringent constraints on the ionization fraction at the tail-end of reionization.

astro-ph.CO↗

arXiv2020

Separate Universe calibration of the dependence of halo bias on cosmic web anisotropy

We use the Separate Universe technique to calibrate the dependence of linear and quadratic halo bias $b_1$ and $b_2$ on the local cosmic web environment of dark matter haloes. We do this by measuring the response of halo abundances at fixed mass and cosmic web tidal anisotropy $α$ to an infinite wavelength initial perturbation. We augment our measurements with an analytical framework developed in earlier work which exploits the near-Lognormal shape of the distribution of $α$ and results in very high precision calibrations. We present convenient fitting functions for the dependence of $b_1$ and $b_2$ on $α$ over a wide range of halo mass for redshifts $0\leq z\leq1$. Our calibration of $b_2(α)$ is the first demonstration to date of the dependence of non-linear bias on the local web environment. Motivated by previous results which showed that $α$ is the primary indicator of halo assembly bias for a number of halo properties beyond halo mass, we then extend our analytical framework to accommodate the dependence of $b_1$ and $b_2$ on any such secondary property which has, or can be monotonically transformed to have, a Gaussian distribution. We demonstrate this technique for the specific case of halo concentration, finding good agreement with previous results. Our calibrations will be useful for a variety of halo model analyses focusing on galaxy assembly bias, as well as analytical forecasts of the potential for using $α$ as a segregating variable in multi-tracer analyses.

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