arXiv subjects

Biswajit Pandey

Publications and source records attributed to Biswajit Pandey.

At least 19 recordsLinked to original sources

arXiv2026

Triggering and quenching in the shadow of AGN: How does AGN proximity affect star formation in the EAGLE simulation?

Active galactic nuclei (AGN) inject vast amounts of energy into their surroundings and are widely recognised as key drivers of galaxy evolution through feedback processes. Although the effects of AGN feedback within host galaxies are well studied, the extent to which AGN influence star formation in neighbouring galaxies remains an open question. In this work, we use the EAGLE cosmological hydrodynamical simulation to investigate how AGN proximity modulates star formation in nearby star-forming galaxies (SFGs) on scales up to 2 Mpc. We employ a carefully constructed control sample matched in stellar mass and local density to isolate the environmental impact of AGN, and quantify deviations in star formation using an SFR offset relative to matched controls. Our analysis reveals a clear, non-uniform response to AGN proximity: low-mass galaxies exhibit systematically different star formation responses than their high-mass counterparts, with gas-rich systems frequently showing mild star formation enhancement, consistent with triggering via AGN-driven compression or turbulence, while gas-poor systems preferentially exhibit suppressed star formation, indicative of thermal heating inhibiting gas cooling. Cold gas availability is found to be more strongly associated with this modulation than stellar mass alone. These signatures are strongest within $\sim 1$ Mpc of AGN hosts but persist out to 2 Mpc, demonstrating that AGN feedback operates well beyond the host halo. Together, our results establish AGN as non-local regulators of galaxy evolution, capable of shaping star formation in neighbouring galaxies through extended feedback processes that link black hole accretion physics to the large-scale cosmic environment.

astro-ph.GA↗

arXiv2026

Galaxy evolution in the cosmic web: the relative impact of nodes and filaments in the EAGLE simulation

Galaxies evolve within the intricate geometry of the cosmic web, yet the distinct roles of its primary components - nodes and filaments remain incompletely understood. Using the EAGLE cosmological hydrodynamical simulation, we investigate how distances to filament spines and cluster-scale nodes jointly and independently influence galaxy evolution. Galaxies are classified into red, green, and blue populations through a fully data-driven entropic thresholding technique, and the nodes and filaments are identified using DisPerSE. We find that red galaxies preferentially reside near filament cores and nodes, whereas blue and green galaxies dominate the outskirts. This spatial segregation reveals two characteristic transition scales: a node-related crossover at $\sim 2.5~\mathrm{Mpc}$ and a filament-related crossover at $\sim 0.75~\mathrm{Mpc}$. To further quantify environmental influence, we adopt an information-theoretic approach and measure the normalised mutual information between dominant mass component and galaxy colour across the $d_{\mathrm{f}}$-$d_{\mathrm{n}}$ plane. The mass-colour coupling increases with distance from nodes at fixed filament proximity, indicating a weakening of this relation in cluster-scale environments and a stronger coupling within filamentary regions. This behaviour is strongly mass dependent, with low-mass galaxies exhibiting a more pronounced environmental modulation than high-mass systems. These results support a scale-dependent view of galaxy evolution across the cosmic web, highlighting the distinct and complementary influence of nodes and filaments.

astro-ph.GA↗

arXiv2026

Stellar Halo Memory: A New Observable for Galactic Archaeology

The Galactic stellar halo preserves a fossil record of the Milky Way's assembly history, yet there is currently no unified framework for quantifying how much of this information survives in the present-day Galaxy. We introduce an information-theoretic framework that establishes stellar halo memory as a directly measurable observable for Galactic archaeology. Using APOGEE Data Release 17 halo stars, we quantify the statistically significant information shared among stellar dynamical and chemical observables through the excess mutual information, enabling a physically motivated decomposition into dynamical, chemical, and cross-memory reservoirs. We construct cumulative radial memory profiles together with two dimensionless diagnostics, the memory dominance ratio and the coupling efficiency, to investigate how the surviving assembly memory is partitioned throughout the Galactic halo. We find that the different memory observables exhibit distinct radial evolution in the inner halo but all evolve toward a common statistically significant residual memory state beyond $\sim 20$ kpc. Comparison with randomized realizations demonstrates that the measured memories exceed the corresponding null expectation by large factors, confirming that they represent genuine astrophysical structure rather than statistical fluctuations. The surviving assembly memory is consistently dominated by dynamical correlations while retaining a finite chemo-dynamical coupling across the halo. These results suggest that phase mixing redistributes, rather than completely erases, the memory imprinted during galaxy formation. Our study establishes stellar halo memory as a new observable for Galactic archaeology and provides a unified statistical framework for investigating galaxy assembly in both observations and cosmological simulations.

astro-ph.GA↗

arXiv2026

From metallicity distributions to mutual information: A new perspective on stellar halo assembly

The metallicity structure of stellar halos encodes the fossil record of galaxy assembly, tracing the chemical evolution and dynamical imprint of past mergers. Using five Milky Way-mass halos from the Aquarius simulations, we introduce an information-theoretic framework to quantify spatial-chemical correlations through the mutual information (MI) between angular position and metallicity. We divide stars in each halo into high- and low-metallicity populations based on their median metallicity and examine their metallicity distribution functions (MDFs), spatial anisotropies, and angular-metallicity couplings as a function of galactocentric radius. The MDFs exhibit remarkable diversity, ranging from single-peaked distributions dominated by one or two massive progenitors to broad or bimodal forms shaped by multiple accretion events, revealing the stochastic nature of halo assembly. The low-metallicity stars, primarily contributed by disrupted satellites, display higher spatial anisotropy and stronger angular clustering than their metal-rich counterparts. After removing bound satellites, anisotropy decreases significantly, yet high-metallicity stars remain marginally more anisotropic, reflecting the lingering debris of massive, centrally deposited progenitors. The mutual information between angular position and metallicity increases with radius before saturating in the outskirts, with the difference between the data and randomized controls confined mainly to the inner halo signifying residual spatial-chemical coupling driven by incomplete phase mixing. Our results demonstrate that information-theoretic diagnostics provide a powerful and intuitive way to quantify the chemical complexity of stellar halos and offer a promising route to compare simulations with forthcoming high-dimensional Galactic survey data.

astro-ph.GA↗

arXiv2026

Entropic backreaction from cosmic structure formation: a thermodynamic approach to the late-time cosmological tensions

High-precision cosmological observations have revealed persistent tensions within the standard $Λ$CDM paradigm, most notably the discrepancy in the Hubble constant and the lower than predicted amplitude of late-time matter clustering quantified by $S_8$. We propose a unified thermodynamic framework in which entropic backreaction generated during cosmic structure formation modifies both the background expansion history and the growth of matter perturbations. As gravitational instability drives the growth of cosmic structures, the configuration entropy associated with the matter distribution decreases through the nonlinear redistribution of gravitational binding energy. The resulting entropic energy density contributes a late-time backreaction that enhances the cosmic expansion rate without altering early-Universe physics or the CMB sound horizon. Simultaneously, the same irreversible entropy dissipation process induces a dissipative correction within the cosmic velocity flow, suppressing the efficiency of coherent gravitational clustering at late times. The framework operates entirely within standard General Relativity: the Einstein field equations, Poisson equation, and gravitational coupling remain unmodified, and no new propagating degrees of freedom or fifth forces are introduced. Entropic backreaction therefore provides a thermodynamically motivated, theoretically conservative, and observationally testable mechanism that may simultaneously alleviate the major late-time cosmological tensions.

astro-ph.CO↗

arXiv2026

A Multi-parameter Fuzzy Set Framework for Classifying Red, Blue, and Green Valley Galaxies

We present a data-driven fuzzy set framework for classifying galaxies into the red sequence, blue cloud, and green-valley populations using multiple observables from the Sloan Digital Sky Survey (SDSS DR18). Unlike traditional methods based on hard boundaries in colour or stellar mass, our approach assigns continuous membership degrees using sigmoidal functions derived from bimodal galaxy properties, including $(u-r)$ colour, specific star formation rate (sSFR), and $D4000$. Membership functions are constructed via Gaussian mixture modeling and combined using a conservative fuzzy minimum operator. Applying this method to a volume-limited sample of 88,579 galaxies, we compare with the empirical classification of \citet{schawinski14}. The fuzzy approach reduces contamination in the red and green-valley populations and yields more physically consistent distributions of star formation and morphology. Red galaxies show a unimodal low-sSFR distribution, while green-valley galaxies exhibit clearer signatures of morphological evolution. We also examine the dependence of active galactic nucleus (AGN) fraction on stellar mass and find no significant differences between methods, indicating robust global AGN trends. However, clustering analysis reveals subtle differences: fuzzy-classified red galaxies show enhanced large-scale clustering, suggesting a stronger association with highly biased dark matter halos. These results demonstrate that fuzzy classification provides a flexible, physically motivated alternative to hard-cut methods, enabling a more accurate and interpretable view of galaxy populations and their evolution.

astro-ph.GA↗

arXiv2026

The size and shape dependence of the SDSS galaxy bispectrum

We have measured the spherically averaged bispectrum of the SDSS main galaxy sample, considering a volume-limited $[296.75\, \rm Mpc]^3$ data cube with mean galaxy number density $0.63 \times 10^{-3} \, {\rm Mpc}^{-3}$ and median redshift $0.102$. Our analysis considers $\sim 1.37 \times 10^{8}$ triangles, for which we have measured the binned bispectrum and analysed its dependence on the size and shape of the triangle. It spans wavenumbers $k_1=(0.075-0.434)\,{\rm Mpc}^{-1}$ for equilateral triangles, and a smaller range of $k_1$ (the largest side) for triangles of other shapes. For all shapes, we find that the measured bispectrum is well modelled by a power law $A\,\big(k_1/1\mpci\big)^{n}$, where the best-fit values of $A$ and $n$ vary with the shape. We have also analysed mock galaxy samples constructed from $Λ$CDM N-body simulations by applying a simple Eulerian bias prescription where the galaxies reside in regions where the smoothed density field exceeds a threshold. We find that the bispectrum from the mock samples with bias $b_1=1.2$ is in good agreement with the SDSS results. We further divided our galaxy sample into red and blue classes and studied the nature of the bispectrum for each category. The red galaxies exhibit higher bispectrum amplitude $A$ than the blue galaxies for all possible triangle configurations. Red galaxies are old, and their larger bispectra indicate non-linear evolutionary interactions within their environments over time, resulting in their distribution being highly clustered and more biased than younger blue galaxies.

astro-ph.CO↗

arXiv2026

Galaxy quenching across the Cosmic Web: disentangling mass and environment with SDSS DR18

We investigate the influence of large-scale cosmic web environments on galaxy quenching using a volume-limited, stellar mass-matched galaxy sample from SDSS DR18. Galaxies are classified as residing in sheets, filaments, or clusters based on the eigenvalues of the tidal tensor derived from the smoothed density field. The quenched fraction increases with stellar mass and is highest in clusters, intermediate in filaments, and lowest in sheets, reflecting the increasing efficiency of environmental quenching with density. A flattening of the quenched fraction beyond $\log_{10}(M_\star/M_\odot) \sim 10.6$ across all environments signals a transition from environment-driven to mass-driven quenching. In contrast, the bulge fraction continues to rise beyond this threshold, indicating a decoupling between star formation suppression and morphological transformation. At the high-mass end ($\log_{10}(M_\star/M_\odot) \gtrsim 11.5$), both quenched and bulge fractions bifurcate, increasing in clusters but declining in sheets, suggesting a divergent evolutionary pathway where massive galaxies in sheets retain cold gas and disk-like morphologies, potentially sustaining or rejuvenating star formation. The AGN fraction also increases with stellar mass and is somewhat higher in sheets than in clusters, indicating enhanced AGN activity in low-density, gas-rich environments. The high-mass trends are independently corroborated by our analysis of specific star formation rate, $(u-r)$ colour, concentration index, and D4000 in the stellar mass-density plane, which show that massive galaxies in sheets remain bluer, younger, more star-forming, and structurally less evolved than their cluster counterparts. Our results highlight the cosmic web as an active driver of galaxy evolution.

astro-ph.GA↗

arXiv2026

Probing cosmic isotropy with Gamma-ray bursts: A dipole and quadrupole analysis of BATSE and Fermi GBM data

The cosmological principle, asserting large-scale homogeneity and isotropy, underpins the standard model of cosmology. Testing its validity using independent astronomical probes remains crucial for understanding the global structure of the Universe. We investigate the angular distribution of Gamma-Ray Bursts (GRBs) using two of the most comprehensive all-sky datasets available, the BATSE (CGRO) and Fermi GBM catalogs, to test the isotropy of the GRB sky at large angular scales. We perform spherical harmonic decomposition of the GRB sky maps and estimate the dipole and quadrupole amplitudes. Statistical significance is evaluated by comparing the observed multipole amplitudes against distributions derived from 500 Monte Carlo realizations of isotropic skies. Our results show that the observed dipole amplitudes for both BATSE and Fermi GBM datasets lie within the $1σ$ region of their respective null distributions. However, the quadrupole amplitude in the raw, uncorrected BATSE and Fermi GBM skies appears elevated at $3.7σ$ and $3.0σ$, respectively. After incorporating the BATSE sky exposure function, this apparent quadrupole anisotropy vanishes, indicating that instrumental non-uniformities fully account for the signal in that case. Owing to the absence of a publicly available full-sky exposure model for Fermi GBM, the Fermi analysis is restricted to the raw sky distribution. Our method's reliability is validated through controlled simulations, which show it can detect the injected dipoles in BATSE-sized isotropic skies. These findings reinforce the statistical isotropy of the GRB sky and underscore the importance of accurate exposure corrections in cosmological anisotropy analyses.

astro-ph.HE↗

arXiv2026

An information-theoretic bound on cosmic coherence in finite-volume simulations

We quantify the physical memory of the cosmic density field using mutual information between $N$-body snapshots at different redshifts, removing a random baseline to isolate gravitational correlations. The shared mutual information rises with scale, peaks near $\simeq L/8$ (where $L$ is the simulation box size), and declines thereafter. This behaviour is robust to box size and discretization, and identifies the largest coherently retained modes unaffected by missing long-wavelength power, establishing a finite-volume limit on the coherence of cosmic structure with direct implications for homogeneity studies.

astro-ph.CO↗

arXiv2025

Spatial and velocity anisotropies of stellar halos across cosmic web environments: Insights from IllustrisTNG simulation

The role of large-scale environment in shaping the structural and kinematic properties of stellar halos remains an open question. We investigate whether the cosmic web environments affect the spatial and velocity anisotropies of stellar halos in Milky Way-mass galaxies. Using high-resolution data from the TNG50 simulation, we analyze 29 stellar halos from each environment and quantify their spatial and kinematic anisotropies as a function of halo-centric radius. We find that stellar halos across all environments generally exhibit increasing spatial anisotropy with radius, with fluctuations corresponding to bound substructures. The velocity anisotropy profiles show radially dominated orbits on average, but also display significant local variation, including tangentially dominated regions. However, no statistically significant differences are observed in the mean spatial or velocity anisotropy profiles across environments, for either the total stellar halo population or for the in situ and ex situ components individually. The large scatter within each environment suggests that the formation of stellar halos is primarily driven by stochastic, small-scale processes such as satellite merger histories, rather than the large-scale geometry of the cosmic web. Our results imply that, at fixed halo mass, the influence of cosmic web environment on the structure of stellar halo is weak or highly non-deterministic. Possible environmental effects may be more prominent at higher masses where accretion is more anisotropic. Exploring this regime will require simulations with both larger volume and higher resolution.

astro-ph.GA↗

arXiv2025

Separating halo and disk stars in galaxies with Fuzzy Set Theory

Disk and halo stars are generally classified using several conventional methods, such as the Toomre diagram, sharp cuts in metallicity ([Fe/H]), vertical distance ($\left|Z\right|$) from the Galactic plane, or thresholds on the orbital circularity parameter ($ε$). However, all these methods rely on hard selection cuts, which either contaminate samples when relaxed or exclude genuine members when applied too strictly, leading to uncertain and biased classifications. We develop a flexible and reliable approach to classify disk and halo stars in galaxies by applying fuzzy set theory, which can overcome the limitations of traditional hard-cut selection methods. As a case study, we analyze one of the Milky Way/M31-like galaxies in the TNG50 catalogue. We consider multiple stellar properties as fuzzy variables and characterize their variations between disk and halo stars to construct the respective membership functions. These functions are then combined to assign each star a membership degree corresponding to its galactic component. Our fuzzy set approach provides a more realistic distinction between the disk and the halo stars. This method effectively reduces contamination and recovers genuine members that are often excluded by rigid selection criteria. The fuzzy set theory framework offers a robust alternative to conventional hard-cut methods, enabling more accurate and physically meaningful separation of stellar populations in galaxies.

astro-ph.GA↗

arXiv2025

Tracing correlations between galaxy properties across the Cosmic Web: An IllustrisTNG-based study

We explore the impact of cosmic web environments on galaxy properties such as $(u-r)\,$colour, stellar mass, star formation rate, and stellar metallicity, using a stellar mass-matched sample of simulated galaxies from the IllustrisTNG simulation. We use Normalized Mutual Information (NMI) to quantify correlations among galaxy properties and apply Student's t-test to assess the statistical significance of their differences across cosmic web environments. In every case, the null hypothesis is rejected at $> 99.99\%$ confidence, providing strong evidence that correlations among galaxy properties are strongly dependent on cosmic web environments.

astro-ph.CO↗

arXiv2025

Exploring the evolution of red and blue galaxies in different cosmic web environments using IllustrisTNG simulation

We analyze the evolution of red and blue galaxies in different cosmic web environments from redshift $z=3$ to $z=0$ using the IllustrisTNG simulation. We use Otsu's method to classify the red or blue galaxies at each redshift and determine their geometric environments from the eigenvalues of the deformation tensor. Our analysis shows that initially, blue galaxies are more common in clusters followed by filaments, sheets and voids. However, this trend reverses at lower redshifts, with red fractions rising earlier in denser environments. At $z<1$, most massive galaxies ($\log(\frac{M_{*}}{M_{\odot}})>10.5$) are quenched across all environments. In contrast, low-mass galaxies ($\log(\frac{M_{*}}{M_{\odot}})<10.5$) are more influenced by their environment, with clusters hosting the highest red galaxy fractions at low redshifts. We observe a slower mass growth for low-mass galaxies in clusters at $z<1$. Filaments show relative red fractions (RRF) comparable to clusters at low masses, but host nearly $60\%$ of low-mass blue galaxies, representing a diverse galaxy population. It implies that less intense environmental quenching in filaments allows galaxies to experience a broader range of evolutionary stages. Despite being the densest environment, clusters display the highest relative blue fraction (RBF) for high-mass galaxies, likely due to interactions or mergers that can temporarily rejuvenate star formation in some of them. The $(u-r)$ colour distribution transitions from unimodal to bimodal by redshift $z=2$ across all environments. At $z<1$, clusters exhibit the highest median colour and lowest median specific star formation rate (sSFR), with stellar mass being the primary driver of colour evolution in massive galaxies. Our study suggests that stellar mass governs quenching in high-mass galaxies, while a complex interplay of mass and environment shapes the evolution of low-mass galaxies.

astro-ph.GA↗

arXiv2025

Revealing a transitional epoch of large-scale cosmic anisotropy in the quasar distribution

The Cosmological Principle posits that the Universe is isotropic on the largest scales. While widely supported, this foundational assumption remains testable. We analyse the angular distribution of over one million quasars from the Gaia-unWISE catalogue using Renyi entropy, a multiscale statistical measure sensitive to higher-order clustering. Dividing the sample into three redshift bins, we find that both the low- and high-redshift distributions are statistically consistent with isotropy. However, at intermediate redshift ($1 \leq z < 2.2$), we detect a statistically significant and scale-dependent anisotropy that persists under stringent masking, suggesting a physical origin. We interpret this as evidence for a transitional epoch in cosmic history, during which large-scale structures such as superclusters became prominent before their growth was gradually damped by the onset of accelerated expansion. These findings position Renyi entropy as a powerful probe of cosmic evolution and highlight the potential thermodynamic links between structure formation, entropy dissipation, and the emergence of large-scale isotropy.

astro-ph.CO↗

arXiv2025

Impact of cosmic web on galaxy properties and their correlations: Insights from Principal Component Analysis

We use Principal Component Analysis (PCA) to analyze a volume-limited sample from the SDSS and explore how cosmic web environments affect the interrelations between various galaxy properties, such as $(u-r)$ colour, stellar mass, specific star formation rate, metallicity, morphology, and $D4000$. Our analysis reveals that the first three principal components (PC1, PC2 and PC3) account for approximately $85\%$ of the data variance. We classify galaxies into different cosmic web environments based on the eigenvalues of the deformation tensor and compare PC1, PC2, PC3 across these environments. PC1 is dominated by colour, sSFR, D4000, and morphology. It displays clear bimodality across all cosmic web environments, with sheets and clusters showing distinct preferences for negative and positive PC1 values, respectively. This variation reflects the strong role of environmental processes in regulating star formation. PC2 and PC3, respectively show positively and negatively skewed unimodal distributions in all environments. PC2 is primarily influenced by metallicity whereas PC3 is dominated by stellar mass. It indicates that metallicity evolves gradually and is less sensitive to environmental extremes. PC3 likely captures residual variation in stellar mass. A Kolmogorov-Smirnov (KS) test confirms that the distributions of PC1, PC2 and PC3 differ significantly across environments, with a confidence level exceeding $99.99\%$. Furthermore, we calculate the normalized mutual information (NMI) between the principal components and individual galaxy properties within different cosmic web environments. A two-tailed t-test reveals that for each relationship and each pair of environments, the null hypothesis is rejected with a confidence level $>99.99\%$. Our analysis confirms that cosmic web environments play a significant role in shaping the correlations between galaxy properties.

astro-ph.GA↗

arXiv2025

Clustering and physical properties of AGN and Star-Forming Galaxies at fixed stellar mass: does assembly bias have a role in AGN activity?

We analyze a volume-limited sample from the Sloan Digital Sky Survey (SDSS) to compare the spatial clustering and physical properties of active galactic nuclei (AGN) and star-forming galaxies (SFG) at fixed stellar mass. We find no statistically significant difference in clustering strength or local density between AGN and SFG. However, after matching their stellar mass distributions, we detect statistically significant differences (at a confidence level $>99.99\%$) in colour, star formation rate (SFR), $4000Å$ break measurements (D$4000$), and morphology. These differences persist across both low- and high-density environments, suggesting that AGN are not driven by environmental factors. The development of favourable conditions for AGN activity within a galaxy may depend on the diverse evolutionary histories of galaxies. Our results imply that AGN activity may arise stochastically, modulated by the complex assembly history of galaxies.

astro-ph.GA↗

arXiv2025

The long road to the Green Valley: Tracing the evolution of the Green Valley galaxies in the EAGLE simulation

We study the evolution of the progenitors of the present-day Green Valley (GV) galaxies across redshift $z=10-0$ using data from the EAGLE simulations. We identify the present-day green valley galaxies using entropic thresholding and track the evolution of the physical properties of their progenitors up to $z=10$. Our study identifies three distinct phases in their evolution: (i) an early growth phase ($z=10-6$), where progenitors are gas-rich, efficiently form stars, and experience AGN feedback regulating star formation in massive galaxies, (ii) a transition phase ($z=6-2$), marked by frequent interactions and mergers in higher-density environments, driving starbursts, depleting gas reservoirs, and strengthening correlations between cold gas and halo properties, and (iii) a quenching phase ($z=2-0$), dominated by environmental and mass-dependent processes that suppress star formation and deplete cold gas. Our analysis shows that at $z<1$, environmental factors and cold gas depletion dominate quenching, with tighter correlations between stellar mass, SFR, and cold gas content. The interplay between mass and environmental density during this period drives diverse and distinct evolutionary pathways. Our analysis shows that majority of the main progenitor branches of the present-day GV galaxies entered the green valley at $z<1$. We also find that a small fraction ($\sim 5\%$) of the main progenitor branches had already crossed the green valley and joined the red sequence by $z=0.1$, indicating that some galaxies may undergo late-time rejuvenation, that allows them to reenter the green valley by the present day. Our findings provide a comprehensive view of the mechanisms shaping the GV population across cosmic time.

astro-ph.GA↗