arXiv subjects

Apashanka Das

Publications and source records attributed to Apashanka Das.

13 recordsLinked to original sources

arXiv2026

Emergent gravity from Michel flow with position dependent adiabatic index

Spherically symmetric, general relativistic Bondi accretion is known as the Michel flow. The stationary integral transonic solutions for the Michel flow has been constructed for multi-component accretion described by an equation of state where the adiabatic index varies with the radial distance along which the streamlines are studied, and the corresponding phase portrait spanned by such radial distance and the flow Mach number has been obtained. Borrowing the techniques used in the dynamical systems theory, the nature of the transonic points of the aforementioned flow has been classified. The steady state flow has been perturbed to study the stability of the stationary solutions, and it has been found that such flows are stable under the (linear) radial perturbation. As a consequence of the stability analysis, the corresponding acoustic space time embedded within the accreting matter has been obtained, and the horizon of the metric of such sonic space time has been identified by constructing the causal structure with the help of the Carter-Penrose diagrams. In this way, the accreting black hole systems in the general relativistic set up has been investigated from various different perspectives - from its astrophysical aspects, from the dynamical systems point of view, as well as within the realm of the classical analogue gravity phenomena.

astro-ph.HE↗

arXiv2026

Emergent gravity from nonlinear perturbation of spherical accretion with variable adiabatic index

The main aim of the present work is to demonstrate that the analogue gravity phenomena are not an artifact of linear perturbation, rather gravity-like effects emerge through the non linear higher order perturbation of transonic fluid as well. To establish that fact, a spherically accreting astrophysical system has been considered where the hydrodynamic accretion with a relativistic, multi-component equation of state with position dependent adiabatic index onto compact astrophysical objects has been considered. By extending the acoustic metric formalism beyond the linear regime, it has been shown that the aforementioned perturbations satisfy a covariant wave equation in an effective acoustic spacetime with non-linear corrections, making the analogue geometry dynamical. As a consequence, the acoustic horizon can shift (inward or outward), depending on the relative amplitudes of density, temperature, and mass accretion-rate fluctuations. This provides a more realistic framework to investigate the dynamics of the non-linear analogue spacetime in astrophysically relevant accretion flows.

gr-qc↗

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↗

arXiv2025

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↗

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↗

arXiv2024

The roles of environment and interactions on the evolution of red and blue galaxies in the EAGLE simulation

We study the evolution of the red and blue galaxies from $z=3$ to $z=0$ using the EAGLE simulation. The galaxies in the blue cloud and the red sequence are separated at each redshift using a scheme based on Otsu's method. Our analysis shows that the two populations have small differences in the local density and the clustering strength until $z=2$, after which the red galaxies preferentially occupy the denser regions and exhibit a significantly stronger clustering than the blue galaxies. The significant disparities in cold gas mass and specific star formation rate (sSFR) observed before $z=2$ suggest that factors beyond environmental influences may also contribute to the observed dichotomy. Interacting galaxy pairs at a given separation exhibit a higher SFR at increasing redshifts, which may be linked to the rising gas fractions at higher redshift. As redshift decreases, the SFR decreases across all separations, suggesting a gradual depletion of the cold gas reservoir. At pair separations $<50$ kpc, an anomalous increase in the SFR among paired galaxies in isolation around $z \sim 2$ suggests that environmental effects begin to dominate at this redshift, thereby increasing the rate of galaxy interactions and the occurrence of starburst galaxies. We observe a substantial decrease in the blue fraction in paired galaxies starting from $z=1$ to the present. However, the decrease in the blue fraction in paired galaxies with their second nearest neighbour at a distance greater than 500 kpc continues until $z=0.5$, after which the blue fraction begins to increase.

astro-ph.GA↗

arXiv2023

Galaxy interactions in filaments and sheets: insights from EAGLE simulations

We study the colour and star formation rates of paired galaxies in filaments and sheets using the EAGLE simulations. We find that the major pairs with pair separation $<50$ kpc are bluer and more star forming in filamentary environments compared to those hosted in sheet-like environments. This trend reverses beyond a pair separation of $\sim 50$ kpc. The interacting pairs with larger separations ($>50$ kpc) in filaments are on average redder and low-star forming compared to those embedded in sheets. The galaxies in filaments and sheets may have different stellar mass and cold gas mass distributions. Using a KS test, we find that for paired galaxies with pair separation $<50$ kpc, there are no significant differences in these properties in sheets and filaments. The filaments transport gas towards the cluster of galaxies. Some earlier studies find preferential alignment of galaxy pairs with filament axis. Such alignment of galaxy pairs may lead to different gas accretion efficiency in galaxies residing in filaments and sheets. We propose that the enhancement of star formation rate at smaller pair separation in filaments is caused by the alignment of galaxy pairs. A recent study with the SDSS data (Das et al., 2023) reports the same findings. The confirmation of these results by the EAGLE simulations suggests that the hydrodynamical simulations are powerful theoretical tools for studying the galaxy formation and evolution in the cosmic web.

astro-ph.GA↗

arXiv2023

Do minor interactions trigger star formation in galaxy pairs?

We analyze the galaxy pairs in a set of volume limited samples from the SDSS to study the effects of minor interactions on the star formation rate (SFR) and colour of galaxies. We carefully design control samples of the isolated galaxies by matching the stellar mass and redshift of the minor pairs. The SFR distributions and colour distributions in the minor pairs differ from their controls at $>99\%$ significance level. We also simultaneously match the control galaxies in stellar mass, redshift and local density to assess the role of the environment. The null hypothesis can be rejected at $>99\%$ confidence level even after matching the environment. Our analysis shows a quenching in the minor pairs where the degree of quenching decreases with the increasing pair separation and plateaus beyond 50 kpc. We also prepare a sample of minor pairs with $H_α$ line information. We calculate the SFR of these galaxies using the $H_α$ line and repeat our analysis. We observe a quenching in the $H_α$ sample too. We find that the majority of the minor pairs are quiescent systems that could be quenched due to minor interactions. Combining data from the Galaxy Zoo and Galaxy Zoo2, we find that only $\sim 1\%$ galaxies have a dominant bulge, $4\%-7\%$ galaxies host a bar, and $5\%-10\%$ galaxies show the AGN activity in minor pairs. This indicates that the presence of bulge, bar and AGN activity plays an insignificant role in quenching the galaxies in minor pairs. The more massive companion satisfies the criteria for mass quenching in most of the minor pairs. We propose that the stripping and starvation likely caused the quenching in the less massive companion at a later stage of evolution.

astro-ph.GA↗

arXiv2022

Galaxy interactions in filaments and sheets: effects of the large-scale structures versus the local density

The major interactions are known to trigger star formation in galaxies and alter their colour. We study the major interactions in filaments and sheets using the SDSS data to understand the influence of large-scale environments on the galaxy interactions. We identify the galaxies in filaments and sheets using the local dimension and also find the major pairs residing in these environments. The star formation rate and colour of the interacting galaxies as a function of pair separation are separately analyzed in filaments and sheets. The analysis is repeated for three volume limited samples covering different magnitude ranges. The major pairs residing in the filaments show a significantly higher star formation rate (SFR) and bluer colour than those residing in the sheets up to the projected pair separation of $\sim 50$ kpc. We observe a complete reversal of this behaviour for both the SFR and colour of the galaxy pairs having a projected separation larger than 50 kpc. Some earlier studies report that the galaxy pairs align with the filament axis. Such alignment inside filaments indicates anisotropic accretion that may cause these differences. We do not observe these trends in the brighter galaxy samples. The pairs in filaments and sheets from the brighter galaxy samples trace relatively denser regions in these environments. The absence of these trends in the brighter samples may be explained by the dominant effect of the local density over the effects of the large-scale environment.

astro-ph.GA↗

arXiv2022

On the origin of red spirals: Does assembly bias play a role?

The formation of the red spirals is a puzzling issue in the standard picture of galaxy formation and evolution. Most studies attribute the colour of the red spirals to different environmental effects. We analyze a volume limited sample from the SDSS to study the roles of small-scale and large-scale environments on the colour of spiral galaxies. We compare the star formation rate, stellar age and stellar mass distributions of the red and blue spirals and find statistically significant differences between them at $99.9\%$ confidence level. The red spirals inhabit significantly denser regions than the blue spirals, explaining some of the observed differences in their physical properties. However, the differences persist in all types of environments, indicating that the local density alone is not sufficient to explain the origin of the red spirals. Using an information theoretic framework, we find a small but non-zero mutual information between the colour of spiral galaxies and their large-scale environment that are statistically significant ($99.9\%$ confidence level) throughout the entire length scale probed. Such correlations between the colour and the large-scale environment of spiral galaxies may result from the assembly bias. Thus both the local environment and the assembly bias may play essential roles in forming the red spirals. The spiral galaxies may have different assembly history across all types of environments. We propose a picture where the differences in the assembly history may produce spiral galaxies with different cold gas content. Such a difference would make some spirals more susceptible to quenching. In all environments, the spirals with high cold gas content could delay the quenching and maintain a blue colour, whereas the spirals with low cold gas fractions would be easily quenched and become red.

astro-ph.GA↗

arXiv2021

Galaxy interactions in different environments: An analysis of galaxy pairs from the SDSS

We analyze the galaxy pairs in a volume limited sample ($M_r \leq -21$) from the SDSS to study the effects of galaxy interactions on the star formation rate and colour of galaxies in different environments. We study the star formation rate and colour of the paired galaxies as a function of projected separation and compare the results with their control samples matched in stellar mass, redshift and local density. We find that the major interactions significantly enhance the star formation rate in paired galaxies and turn them bluer with decreasing pair separation within $30$ kpc. The impact of tidal interactions on star formation rate and colour are more significant in the heavier members of the major pairs. The star formation enhancement in major pairs is significantly higher at the low-density environments, where the influence can extend up to $\sim 100$ kpc. Contrarily, the major pairs at high-density environments show suppression in their star formation. Depending on the embedding environments, the major interactions in the intrinsically brighter galaxy pairs can thus enhance or quench star formation. We find that the minor pairs at both low-density and high-density environments are significantly less star-forming and redder than their control galaxies. It indicates that the minor interactions in intrinsically brighter galaxy pairs always suppress the star formation irrespective of their environment. The lighter members in these minor pairs show a greater susceptibility to suppressed star formation. Our results imply that both the major and minor interactions can contribute to the observed bimodality. We conclude that the galaxy evolution is determined by a complex interplay between the galaxy properties, galaxy interactions, and environment.

astro-ph.GA↗

arXiv2021

Green valley galaxies in the cosmic web: internal versus environmental quenching

We analyze the SDSS data to classify the galaxies based on their colour using a fuzzy set-theoretic method and quantify their environments using the local dimension. We find that the fraction of the green galaxies does not depend on the environment and $10\%-20\%$ of the galaxies at each environment are in the green valley depending on the stellar mass range chosen. Approximately $10\%$ of the green galaxies at each environment host an AGN. Combining data from the Galaxy Zoo, we find that $\sim 95\%$ of the green galaxies are spirals and $\sim 5\%$ are ellipticals at each environment. Only $\sim 8\%$ of green galaxies exhibit signs of interactions and mergers, $\sim 1\%$ have dominant bulge, and $\sim 6\%$ host a bar. We show that the stellar mass distributions for the red and green galaxies are quite similar at each environment. Our analysis suggests that the majority of the green galaxies must curtail their star formation using physical mechanism(s) other than interactions, mergers, and those driven by bulge, bar and AGN activity. We speculate that these are the massive galaxies that have grown only via smooth accretion and suppressed the star formation primarily through mass driven quenching. Using a Kolmogorov-Smirnov test, we do not find any statistically significant difference between the properties of green galaxies in different environments. We conclude that the environmental factors play a minor role and the internal processes play the dominant role in quenching star formation in the green valley galaxies.

astro-ph.GA↗

arXiv2018

Time Evolution of Density Parameters for Matter and Dark Energy and their Interaction Term in Brans-Dicke Gravity

In the framework of Brans-Dicke (BD) theory, the first part of the present study determines the time dependence of BD parameter, energy density and equation of state (EoS) parameter of the cosmic fluid in a universe expanding with acceleration, preceded by a phase of deceleration. For this purpose, a scale factor has been chosen such that the deceleration parameter, obtained from it, shows a signature flip with time. Considering the dark energy to be responsible for the entire pressure, the time evolution of energy parameters for matter and dark energy and the EoS parameter for dark energy have been determined. An effective interaction term, between matter and dark energy, has been proposed and calculated. Its negative value at the present time indicates conversion of matter into dark energy. Using this term, the time dependence of the rates of change of matter and dark energy has been determined. It is found that the nature of dependence of the scalar field upon the scale factor plays a very important role in governing the time evolution of the cosmological quantities studied here. The present study provides us with a simple way to determine the time evolution of dark energy for a homogeneous and isotropic universe of zero spatial curvature, without involving any self-interaction potential or cosmological constant in the formulation.

gr-qc↗