arXiv subjects

Bhaskar Arya

Publications and source records attributed to Bhaskar Arya.

8 recordsLinked to original sources

arXiv2026

Fast modelling of ionization balance in the intergalactic medium: I- implications for the IGM metallicity

Ionization balance in the intergalactic medium (IGM) is central to the interpretation of quasar absorption spectra, linking observed ionic columns to the underlying gas density, temperature, metallicity, and ionizing radiation field. Because ionization, recombination, and cooling timescales can be comparable to the timescales over which the ultraviolet background (UVB) and gas thermodynamic state evolve, ion populations may retain a strong memory of their past history. To this end, we present a fast, metals-inclusive, zero-dimensional framework for modeling the redshift evolution of the IGM. The model follows the coupled thermal and ionization evolution of a Lagrangian gas parcel in a redshift-dependent UVB, solving stiff, time-dependent rate equations for H, He, and 107 metal ions while self-consistently evolving the temperature through photoheating and standard cooling processes. We validate the framework against full three-dimensional hydrodynamical non-equilibrium calculations and find that it reproduces the thermal and ionization histories of the IGM with good accuracy over a wide redshift range, including the heating associated with $\rm He_{\,\rm II}$ reionization. As an application, we predict the cosmic $\rm C_{\,\rm IV}$ density parameter, $\Omega_{\rm CIV}$, and use it to infer the origin of metal ions in the IGM and the corresponding metallicities from observational measurements, obtaining values broadly consistent with literature constraints. The framework is well suited for rapid parameter studies of how reionization timing, UVB spectral hardness, self-shielding, and UVB inhomogeneity shape the thermal and ionization history of the IGM and the resulting metal-line observables.

astro-ph.CO↗

arXiv2025

A trailing lognormal approximation of the Lyman-$\alpha$ forest: comparison with full hydrodynamic simulations at $2.2\leq z\leq 2.7$

Lyman-$\alpha$(Ly$\alpha$) forest in the spectra of distant quasars encodes the information of the underlying cosmic density field at smallest scales. The modelling of the upcoming large and high-fidelity forest data using cosmological hydrodynamical simulations is computationally challenging and therefore, requires accurate semi-analytical techniques. One such approach is based on the assumption that baryonic density fields in the intergalactic medium (IGM) follow lognormal distribution. Keeping this in mind, we extend our earlier work to improve the lognormal model of the Ly$\alpha$ forest in recovering the parameters characterizing IGM state, particularly the hydrogen photoionization rate ($\Gamma_{12}$), between $2.2 \leq z \leq 2.7$, by simulating the model spectra at a slightly lower redshift than the Sherwood smooth particle hydrodynamical simulations (SPH) data. The recovery of thermal parameters, namely, the mean-density IGM temperature ($T_0$) and the slope of the temperature-density relation ($\gamma$) is also alleviated. These parameters are estimated through a Markov Chain Monte Carlo (MCMC) technique, using the mean and power spectrum of the transmitted flux. We find that the usual lognormal distribution of IGM densities tend to over-predict the number of Ly$\alpha$ absorbers seen in SPH simulation. A lognormal model simulated at a lower redshift than SPH data can address this limitation to a certain extent. We show that with such a "trailing" model of lognormal distribution, values of $\Gamma_{12}$ are recovered at $\lesssim 1-\sigma$. We argue that this model can be useful for constraining cosmological parameters.

astro-ph.CO↗

arXiv2024

Playground of Lognormal Seminumerical Simulations of~the~Lyman~$\alpha$ Forest: Thermal History of the Intergalactic Medium

This study aims to test a potential application of lognormal seminumerical simulations to recover the thermal parameters and Jeans length. This could be suitable for generating large number of synthetic spectra with various input data and parameters, and thus ideal for interpreting the high-quality data obtained from QSO absorption spectra surveys. We use a seminumerical approach to simulate absorption spectra of quasars at redshifts $ 3 \leq z \leq 5$. These synthetic spectra are compared with the 1D flux power spectra and using the Markov Chain Monte Carlo analysis method we determine the temperature at mean density, slope of the temperature-density relation and Jeans length. Our best-fit model is also compared with the evolution of the temperature of the intergalactic medium from various UVB models. We show that the lognormal simulations can effectively recover thermal parameters and Jeans length. Besides, by comparing the synthetic flux power spectra with observations from Baryon Oscillation Spectroscopy Survey we found, that such an approach can be also used for the cosmological parameter inference.

astro-ph.CO↗

arXiv2023

Covariance matrices for the Lyman-$\alpha$ forest using the lognormal approximation

We investigate the nature of correlations in the small-scale flux statistics of the Lyman-$\alpha$ (Ly$\alpha$) forest across redshift bins. Understanding these correlations is important for unbiased cosmological and astrophysical parameter inference using the Ly$\alpha$ forest. We focus on the 1-dimensional flux power spectrum (FPS) and mean flux ($\bar F$) simulated using the semi-numerical lognormal model we developed in earlier work. The lognormal model can capture the effects of long wavelength modes with relative ease as compared to full smoothed particle hydrodynamical (SPH) simulations that are limited by box volume. For a single redshift bin of size $\Delta z\simeq 0.1$, we show that the lognormal model predicts positive cross-correlations between $k$-bins in the FPS, and a negative correlation for $\bar F\times$ FPS, in qualitative agreement with SPH simulations and theoretical expectations. For measurements across two neighbouring redshift bins of width $\Delta z$ each (obtained by 'splitting' skewers of length $2\Delta z$ in half), the lognormal model predicts an anti-correlation for FPS $\times$ FPS and a positive correlation for $\bar F\times$ FPS, caused by long wavelength modes. This is in contrast to SPH simulations which predict a negligible magnitude for cross-redshift correlations derived from such `split' skewers, and we discuss possible reasons for this difference. Finally, we perform a preliminary test of the impact of neglecting long wavelength modes on parameter inference, finding that whereas the correlation structure of neighbouring redshift bins has relatively little impact, the absence of long wavelength modes in the model can lead to $\gtrsim2-\sigma$ biases in the inference of astrophysical parameters. Our results motivate a more careful treatment of long wavelength modes in analyses that rely on the small scale Ly$\alpha$ forest for parameter inference.

astro-ph.CO↗

arXiv2023

A modified lognormal approximation of the Lyman-$\alpha$ forest: comparison with full hydrodynamic simulations at $2\leq z\leq 2.7$

Observations of the Lyman-$\alpha$ forest in distant quasar spectra with upcoming surveys are expected to provide significantly larger and higher-quality datasets. To interpret these datasets, it is imperative to develop efficient simulations. One such approach is based on the assumption that baryonic densities in the intergalactic medium (IGM) follow a lognormal distribution. We extend our earlier work to assess the robustness of the lognormal model of the Lyman-$\alpha$ forest in recovering the parameters characterizing IGM state, namely, the mean-density IGM temperature ($T_0$), the slope of the temperature-density relation ($\gamma$), and the hydrogen photoionization rate ($\Gamma_{12}$), by comparing with high-resolution Sherwood SPH simulations across the redshift range $2 \leq z \leq 2.7$. These parameters are estimated through a Markov Chain Monte Carlo technique, using the mean and power spectrum of the transmitted flux. We find that the usual lognormal distribution of IGM densities cannot recover the parameters of the SPH simulations. This limitation arises from the fact that the SPH baryonic density distribution cannot be described by a simple lognormal form. To address this, we extend the model by scaling the linear density contrast by a parameter $\nu$. While the resulting baryonic density is still lognormal, the additional parameter gives us extra freedom in setting the variance of density fluctuations. With this extension, values of $T_0$ and $\gamma$ implied in the SPH simulations are recovered at $\sim 1-\sigma$ ($\lesssim$ 10%) of the median (best-fit) values for most redshifts bins. However, this extended lognormal model cannot recover $\Gamma_{12}$ reliably, with the best-fit value discrepant by $\gtrsim 3-\sigma$ for $z > 2.2$. Despite this limitation in the recovery of $\Gamma_{12}$, we argue that the model remains useful for constraining cosmological parameters.

astro-ph.CO↗

arXiv2022

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

Observations of the Lyman-$\alpha$ (Ly$\alpha$) 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$\alpha$ 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), $\gamma$ (slope of the temperature-density relation) and $\Gamma_{12}$ (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, $T_0$, $\gamma$ and $\Gamma_{12}$ implied in the SPH simulations are recovered within $1 - \sigma$ ($\sim$ 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$\alpha$ transmitted flux at $z \sim 2.5$.

astro-ph.CO↗

arXiv2018

Evolution of axis ratios from phase space dynamics of triaxial collapse

We investigate the evolution of axis ratios of triaxial haloes using the phase space description of triaxial collapse. In this formulation, the evolution of the triaxial ellipsoid is described in terms of the dynamics of eigenvalues of three important tensors: the Hessian of the gravitational potential, the tensor of velocity derivatives and the deformation tensor. The eigenvalues of the deformation tensor are directly related to the parameters that describe triaxiality, namely, the minor to major and intermediate to major axes ratios ($s$ and $q$) and the triaxiality parameter $T$. Using the phase space equations, we evolve the eigenvalues and examine the evolution of the PDF (probability distribution function) of the axes ratios as a function of mass scale and redshift for Gaussian initial conditions. We find that the ellipticity and prolateness increase with decreasing mass scale and decreasing redshift. These trends agree with previous analytic studies but differ from numerical simulations. However, the PDF of the scaled parameter ${\tilde q} = (q-s)/(1-s)$ follows a universal distribution over two decades in mass range and redshifts which is in qualitative agreement with the universality for conditional PDF reported in simulations. We further show using the phase space dynamics that, in fact, ${\tilde q}$ is a phase space invariant and is conserved individually for each halo. These results, demonstrate that the phase space analysis is a useful tool that provides a different perspective on the evolution of perturbations and can be applied to more sophisticated models in the future.

astro-ph.CO↗

arXiv2016

Quenching parameter in a holographic thermal QCD

We have calculated the quenching parameter, $\hat{q}$ in a model-independent way using the gauge-gravity duality. In earlier calculations, the geometry in the gravity side at finite temperature was usually taken as the pure AdS blackhole metric for which the dual gauge theory becomes conformally invariant unlike QCD. Therefore we use a metric which incorporates the fundamental quarks by embedding the coincident D7 branes in the Klebanov-Tseytlin background and a finite temperature is switched on by inserting a black hole into the background, known as OKS-BH metric. Further inclusion of an additional UV cap to the metric prepares the dual gauge theory to run similar to thermal QCD. Moreover $\hat{q}$ is usually defined in the literature from the Glauber-model perturbative QCD evaluation of the Wilson loop, which has no reasons to hold if the coupling is large and is thus against the main idea of gauge-gravity duality. Thus we use an appropriate definition of $\hat{q}$: $\hat{q} L^- = 1/L^2$, where $L$ is the separation for which the Wilson loop is equal to some specific value. The above two refinements cause $\hat{q}$ to vary with the temperature as $T^4$ always and to depend linearly on the light-cone time $L^-$ with an additional ($1/L^-$) correction term in the short-distance limit whereas in the long-distance limit, it depends only linearly on $L^-$ with no correction term. These observations agree with other holographic calculations directly or indirectly.

hep-th↗