arXiv subjects

S. Sankar

Publications and source records attributed to S. Sankar.

10 recordsLinked to original sources

arXiv2026

A cosmic collision in the making: JWST/NIRISS reveals a merging and maturing protocluster core at z~3 around a red quasar

We report the discovery of ``The Step'', a dense protocluster at z=3 revealed by JWST/NIRISS Wide-Field Slitless Spectroscopy (WFSS) of the field around the luminous quasar and starburst SDSSJ165202.64+172852.3. The protocluster contains at least 18 member galaxies and exhibits two substructures in the 3D position-velocity space - notably a distinctive step-like distribution, from which the nickname derives. We propose that the Step may be undergoing a merger of two haloes, which could also explain the large velocity span (700-850km/s) between galaxies along the densest line of sight. This sightline contains seven galaxies within 70 projected kpc from the quasar, consistent with a remarkably dense protocluster core. We derive a galaxy overdensity of a factor >100 in the core and >15 within 1 Mpc$^2$, making it one of the densest structures known at this redshift, and a very massive halo of $13.1 < log M_{\rm halo}/ M_\odot < 13.8$, which suggests the structure could potentially evolve into a massive, Coma-like cluster at z=0. Compared to the field, the star-forming galaxies in the Step show slightly elevated levels of star formation. Yet, we find two quiescent galaxies with strong D4000 breaks and no evidence for ongoing star formation. Compared to other z=3 protoclusters, the Step exhibits similar or slightly suppressed star-forming activity. We also identify two new AGN candidates and find tentative evidence for an enhanced AGN fraction (10-20%) compared to the field. The protocluster constitutes one of the emerging handful of maturing structures at 2 < z < 4 that host both star-forming and quenched galaxies. This redshift regime may thus be a key transitional epoch for cluster studies, where overdensities transition from being the most active sites of star formation in the early Universe towards shaping the massive passive ellipticals seen in the cores of today's clusters.

astro-ph.GA↗

arXiv2026

MAUVE: Cold neutral gas in the outflow of NGC 4383 and evidence for a fountain flow

We present a multiphase study of the star-formation-driven outflow in the Virgo galaxy NGC 4383, combining ALMA CO(2-1) data with deep MeerKAT HI imaging and MUSE spectroscopy obtained as part of the Multiphase Astrophysics to Unveil the Virgo Environment (MAUVE) program. Our previous work revealed a spectacular ionised outflow, but the effect of the outflow on the cold phase remained unclear. Our analysis shows that potentially outflowing molecular gas is detected only within the inner 1 kpc above the disc, where CO clouds exhibit disturbed kinematics and spatial correspondence with the ionisation cone. At larger heights, the CO surface brightness rapidly drops, indicating that the molecular phase contributes little to the mass of outflowing gas. In contrast, the HI distribution shows plumes a few kiloparsecs above the disc that are aligned with the ionised cone, and complex kinematics suggestive of parts of the atomic phase being entrained in the outflow. However, the extended and warped HI disc associated with NGC 4383 complicates the unambiguous identification of outflowing atomic gas and, most importantly, the quantification of outflowing mass and loading factor. Independent support for a cold component in the outflow comes from dust extinction features associated with the outflow and coincident with HI plumes. Despite significant uncertainties in the estimate of the mass of cold gas associated with the outflow, these results suggest that the atomic phase likely dominates the cold outflow above 1 kpc. The observed cold gas velocities remain below the velocities of the ionised phase, suggesting that NGC 4383 does not host a large-scale escaping wind but more likely a galactic fountain, in which feedback redistributes material within the halo and regulates ongoing and future star formation.

astro-ph.GA↗

arXiv2025

Reentrant superconductivity and superconductor-to-insulator transition in a naturally occurring Josephson junction array tuned by RF power

Superconductivity, characterized by dissipationless current flow with flux expulsion or quantization, is usually muted when the magnetic field or the temperature is sufficiently high. However, in rare instances, superconductivity can reappear upon increasing the temperature or magnetic field, a phenomenon known as reentrant superconductivity. It usually emerges from competing orders in strongly correlated materials. Here we demonstrate reentrant superconductivity as a function of both temperature and magnetic field, tuned by radio frequency (RF) power in a relatively simple system: granular aluminum (grAl), which exhibits the properties of a naturally occurring Josephson junction array. At low temperatures, giant Shapiro steps emerge, exhibiting characteristics of a single Josephson junction. Coherent phase locking across the array's multiple junctions amplifies the quantized voltage, enabling tunability at radio frequencies, as observed in artificially designed Josephson arrays. We show that our system can be tuned from a coherent superconducting (stiff-phase) to an insulating (phase-fluctuating) state using RF power. We propose that the RF power modulates the Josephson coupling energy, $E_J$. Remarkably, at elevated temperatures, the screening of the electron charge suppresses the charging energy, causing superconductivity to reappear. This many-body effect cannot be described within a single junction framework and involves many-body correlations. Our system can therefore be tuned to observe both the single-junction regime and many-body correlation effects, serving as a quantum simulator for complex phenomena in condensed matter physics.

cond-mat.supr-con↗

arXiv2024

Blow-up of stochastic semilinear parabolic equations driven by L\'evy noise

The blow-up phenomena of stochastic semilinear parabolic equations with additive as well as linear multiplicative L\'evy noises are investigated in this work. By suitably modifying the concavity method in the stochastic context, we establish the blow-up phenomena of such systems defined on bounded domains.

math.PR↗

arXiv2023

Global existence and non-existence of weak solutions for non-local stochastic semilinear reaction-diffusion equations driven by a fractional noise

In the present paper, we study the existence and blow-up behavior to the following stochastic non-local reaction-diffusion equation: \begin{equation*} \left\{ \begin{aligned} du(t,x)&=\left[(\Delta+\gamma) u(t,x)+\int_{D}u^{q}(t,y)dy -ku^{p}(t,x)+\delta u^{m}(t,x)\int_{D}u^{n}(t,y)dy \right]dt &\quad+\eta u(t,x)dB^{H}(t), u(t,x)&=0, \ \ t>0, \ \ x\in \partial D, u(0,x)&=f(x) \geq 0, \ \ x\in D, \end{aligned} \right. \end{equation*} where $D\subset \mathbb{R}^{d}\ (d \geq 1)$ is a bounded domain with smooth boundary $\partial D$. Here, $k>0, \gamma, \delta, \eta \geq 0$ and $p,q,n>1,\ m\geq 0$ with $m+n \geq q\geq p$. The initial data $f $ is a non-negative bounded measurable function in class $C^{2}$ which is not identically zero. Here, $\left\{ B^{H}(t) \right\}_{t \geq 0} $ is a one-dimensional fractional Brownian motion with Hurst parameter $\frac{1}{2} \leq H<1$ defined on a filtered probability space $\left( \Omega, \mathcal{F}, (\mathcal{F}_{t})_{t \geq 0}, \mathbb{P} \right)$. First, we estimate a lower bound for the finite-time blow-up and by choosing a suitable initial data, we obtain the upper bound for the finite-time blow-up of the above equation. Next, we provide a sufficient condition for the global existence of a weak solution of the above equation. Further, we obtain the bounds for the probability of blow-up solution.

math.PR↗

arXiv2023

First results from the JWST Early Release Science Program Q3D: Benchmark Comparison of Optical and Mid-IR Tracers of a Dusty, Ionized Red Quasar Wind at z=0.435

The [OIII] 5007 A emission line is the most common tracer of warm, ionized outflows in active galactic nuclei across cosmic time. JWST newly allows us to use mid-infrared spectral features at both high spatial and spectral resolution to probe these same winds. Here we present a comparison of ground-based, seeing-limited [OIII] and space-based, diffraction-limited [SIV] 10.51 micron maps of the powerful, kiloparsec-scale outflow in the Type 1 red quasar SDSS J110648.32+480712.3. The JWST data are from the Mid-InfraRed Instrument (MIRI). There is a close match in resolution between the datasets (0."6), in ionization potential of the O$^{+2}$ and S$^{+3}$ ions (35 eV), and in line sensitivity (1e-17 to 2e-17 erg/s/cm$^2$/arcsec$^2$). The [OIII] and [SIV] line shapes match in velocity and linewidth over much of the 20 kpc outflowing nebula, and [SIV] is the brightest line in the rest-frame 3.5-19.5 micron range, demonstrating its usefulness as a mid-IR probe of quasar outflows. [OIII] is nevertheless intriniscally brighter and provides better contrast with the point-source continuum, which is strong in the mid-IR. There is a strong anticorrelation of [OIII]/[SIV] with average velocity, which is consistent with a scenario of differential obscuration between the approaching (blueshifted) and receding (redshifted) sides of the flow. The dust in the wind may also obscure the central quasar, consistent with models that attribute red quasar extinction to dusty winds.

astro-ph.GA↗

arXiv2022

Existence of global and explosive mild solutions of fractional reaction-diffusion system of semilinear SPDEs with fractional noise

In this paper, we investigate the existence and finite-time blow-up for the solution of a reaction-diffusion system of semilinear stochastic partial differential equations (SPDEs) subjected to a two-dimensional fractional Brownian motion given by \begin{eqnarray*} du_{1}(t,x)&=&\left[ \Delta_{\alpha}u_{1}(t,x)+\gamma_{1}u_{1}(t,x)+u^{1+\beta_{1}}_{2}(t,x) \right]dt &\qquad \ \ +k_{11}u_{1}(t,x)dB^{H}_{1}(t)+k_{12}u_{1}(t,x)dB^{H}_{2}(t), du_{2}(t,x)&=&\left[ \Delta_{\alpha}u_{2}(t,x)+\gamma_{2}u_{2}(t,x)+u^{1+\beta_{2}}_{1}(t,x) \right]dt &\qquad \ \ +k_{21}u_{2}(t,x)dB^{H}_{1}(t)+k_{22}u_{2}(t,x)dB^{H}_{2}(t), \end{eqnarray*} for $x \in \mathbb{R}^{d},\ t \geq 0$, along with \begin{equation*} \begin{array}{ll} u_{i}(0,x)=f_{i}(x), &x \in \mathbb{R}^{d}, \nonumber \end{array} \end{equation*} where $\Delta_{\alpha}$ is the fractional power $-(-\Delta)^{\frac{\alpha}{2}}$ of the Laplacian, $0<\alpha \leq 2$ and $\beta_{i}>0,\ \gamma_{i}>0$ and $k_{ij}\geq 0, i,j=1,2$ are constants. We provide sufficient conditions for the existence of a global weak solution. Under the assumption that $\beta_{1}\geq \beta_{2}>0$ with Hurst index $ 1/2 \leq H < 1,$ we obtain the blow-up times for an associated system of random partial differential equations in terms of an integral representation of exponential functions of Brownian motions. Moreover, we provide lower and upper bounds for the finite-time blow-up of the above system of SPDEs and obtain the upper bounds for the probability of non-explosive solutions to our considered system.

math.AP↗

arXiv2022

Blow-up estimates for a system of semilinear SPDEs driven by mixed fractional Brownian motions

In this paper, we obtain the existence and finite-time blow-up for the solution to a system of semilinear stochastic partial differential equations driven by a combination of Brownian and fractional Brownian motions. Under suitable assumptions, lower and upper bounds for the finite-time blow-up solution are obtained. We provide sufficient conditions for the existence of a global weak solution to the system. Further, a lower bound for the probability of the finite-time blow-up solution of the considered system is provided by using Malliavin calculus.

math.PR↗

arXiv2022

Lower and upper bounds for the explosion times of a system of semilinear SPDEs

In this paper, we obtain lower and upper bounds for the blow-up times to a system of semilinear stochastic partial differential equations. Under suitable assumptions, lower and upper bounds of explosion times are obtained by using explicit solutions of an associated system of random partial differential equations and a formula due to Yor. We also provide an estimate for the probability of the finite-time blow-up. With a suitable choice of parameters, the impact of the noise on the solution is investigated. The above-obtained results are also extended for semilinear SPDEs forced by two dimensional Brownian motions.

math.AP↗

arXiv2013

Magneto-transport in copper-doped noncentrosymmetric BiTeI

BiTeI exhibits large Rashba spin splitting due to its noncentrosymmetric crystal structure. The study of chemical doping effect is important in order to either tune the Fermi level or refine the crystal quality. Here, we report the magneto-transport measurement in high quality BiTeI single crystals with different copper dopings. We found that a small amount of copper doping improves the crystal quality significantly, which is supported by the transport data showing higher Hall mobility and larger amplitude in Shubnikov-de Haas oscillation at low temperature. Two distinct frequencies in Shubnikov-de Haas oscillation were observed giving extremal Fermi surface areas of $A_{S} = 9.1 \times 10^{12}$ cm$^{-2}$ and $A_{L} = 3.47\times 10^{14}$ cm$^{-2}$ with corresponding cyclotron masses $m_S^*$ = 0.0353 $m_e$ and $m_L^*$ = 0.178 $m_e$, respectively. Those results are further compared with relativistic band structure calculations using three reported Te and I refined or calculated positions. Our analysis infers the crucial role of Bi-Te bond length in the observed large bulk Rashba-type spin splitting effect in BiTeI.

cond-mat.mtrl-sci↗