arXiv subjects

J. S. Bagla

Publications and source records attributed to J. S. Bagla.

At least 19 recordsLinked to original sources

arXiv2026

Joint Optical-HI mock catalogues and prospects for upcoming HI surveys

Atomic hydrogen (HI) regulates star formation as cold gas fuels star formation. It represents a key phase in the baryon cycle. Redshifted $21$ cm line emission serves as a key tracer for investigating HI in the interstellar medium (ISM) and circumgalactic medium (CGM) in distant galaxies, and enables the study of galaxy evolution. Direct detections of HI are currently limited to $z \leq 0.4$ due to the inherently weak $21$ cm emission line. Ongoing and upcoming large radio surveys aim to detect $21$ cm emission from galaxies up to $z \gtrsim 1$. We present a novel approach for creating optical-HI joint mock catalogues for upcoming SKA precursor surveys: MIGHTEE-HI, LADUMA and WALLABY. Incorporation of optical properties along with HI makes these mock catalogues a powerful tool for making predictions for surveys and provides a benchmark for exploring the HI science (e.g., conditional HIMF and optical-to-HI scaling relations) expected from these surveys. As a case study, we show the use of the joint catalogues for predicting the expected outcome of stacking detection for average HI mass in galaxies that are below the threshold for direct detection. We show that combining stacking observations with only the number of direct detections puts a strong constraint on the HI mass function. We also show that this can be done separately for parts of the color-magnitude diagram for galaxies, enabling a fine grained study of galaxy evolution. This may be used to set priors for the full determination of the HI mass function.

astro-ph.GA↗

arXiv2026

Universality of Halo Shape and its Morphological Evolution across Cosmic Time

We investigate the evolution of dark matter halo shapes in cosmological N-body simulations both in scale free Einstein-De Sitter (EdS) and $Λ$CDM cosmologies. We compute the axis ratios ($q=b/a,s=c/a$) of well resolved central halos using the shape tensor. These halos are identified using two different halo finding algorithms, SUBFIND and ROCKSTAR. We find that at fixed mass, halos become more spherical with decreasing redshift. The distribution $P(q,s)$ along with their median values ($q$ and $s$) shows self-similar behaviour as a function of mass scaled by the non-linear mass, $(M/M_{nl})$ across power-law spectral indices for scale free EdS models. However the median $q$ and $s$ show a tighter self-similar evolution as a function of peak height $ν=δ_c/σ(M,z)$. We find that the median $q(ν)$ and $s(ν)$ are consistent with an evolution along a universal curve described by $y=α-δ\tanh \left[ ω\left(\log_{10}(ν) - μ\right)\right]$ across the spectral indices ranging from $n=-1.0$ to $n=-2.2$. Our results hold for both SUBFIND and ROCKSTAR, although there are some differences between them. The universality of the evolution of median $q(ν)$ and $s(ν)$ also holds for the $Λ$CDM runs, although with a different behaviour at small $ν$ compared to the scale free models. The width of the distributions of $P(q)$ and $P(s)$ in both, scale-free and $Λ$CDM, classes of simulations can be reduced further by classifying halos as oblate, triaxial and prolate, each of which also follows a universal behaviour. Although oblate halos are relatively rare at all redshifts, their fraction increases over time at the expense of the other two populations.

astro-ph.CO↗

arXiv2025

Gravitational lensing of 21 cm HI signal: detection prospects at z ~ 1 with uGMRT in galaxy cluster lenses

The atomic hydrogen HI content of galaxies is intimately related to star formation and galaxy evolution through the baryon cycle, which involves processes such as accretion, feedback, outflows, and gas recycling. While probing the HI gas over cosmic time has improved our understanding, direct HI detection with the redshifted 21 cm line is essentially limited to $z\lesssim 0.42$. Detections beyond this redshift are based on stacking to obtain average HI mass of galaxy populations. Gravitational lensing by the cluster lenses enhances the HI signal and can extend the redshift limit further. In this work, we describe simulations of HI lensing in cluster lenses. We explore the feasibility of detecting strongly lensed HI emission from background galaxies using known $50$ cluster lenses within the uGMRT sky coverage. We demonstrate that certain clusters offer a strong likelihood of HI detection. We also investigate how strong lensing distorts the HI spectral lines. The shape of the HI signal in these lensing models provides useful information and can be used in optimising signal extraction in blind and targeted HI surveys. We find that blind detection of HI signal from galaxies in the redshift range up to $1.58$ requires more than a few hundred hours of observations of individual clusters with the uGMRT. Detecting HI emission in galaxies with strong optical lensing seems promising, with a $5σ$ detection potential in less than 50 hours for Abell 370 and 75 hours for Abell 1703 using the uGMRT.

astro-ph.CO↗

arXiv2025

On the origin of transient features in cosmological N-Body Simulations

We study the effect of gravitational clustering at small scales on larger scales by studying mode coupling between virialised halos. We build on the calculation by Peebles (1974) where it was shown that a virialised halo does not contribute any mode coupling terms at small wave numbers $k$. Using a perturbative expansion in wave number, we show that this effect is small and arises from the deviation of halo shapes from spherical and also on tidal interactions between halos. We connect this with the impact of finite mass resolution of cosmological N-Body simulations on the evolution of perturbations at early times. This difference between the expected evolution and the evolution obtained in cosmological N-Body simulations can be quantified using such an estimate. We also explore the impact of a finite shortest scale up to which the desired power spectrum is realised in simulations. Several simulation studies have shown that this effect is small in comparison with the effect of perturbations at large scales on smaller scales. It is nevertheless important to study these effects and develop a general approach for estimating their magnitude. This is especially relevant in the present era of precision cosmology. We provide basic estimates of the magnitude of these effects and their power spectrum dependence. We find that the impact of small scale cutoff in the initial power spectrum and discreteness increases with $(n+3)$, with $n$ being the index of the power spectrum. In general, we recommend that cosmological simulation data should be used only if the scale of non-linearity, defined as the scale where the linearly extrapolated {\it rms} amplitude of fluctuations is unity, is larger than the average inter-particle separation.

astro-ph.CO↗

arXiv2023

Upcoming SKA precursor surveys and sensitivity to HI mass function

We describe a simulation for the distribution of galaxies focusing on the atomic Hydrogen content. We aim to make predictions for surveys of galaxies using the redshifted 21 cm line emission. We take the expected distribution of HI masses, circular velocities, sizes of galaxies and orientations into account for this simulation. We use the sensitivity of ASKAP and MeeKAT radio telescopes to estimate the number of detections of HI galaxies in upcoming surveys. We validate our simulation with earlier estimates carried out by using some of these considerations. We show that unlike earlier simulations that take some of the factors into account, the predicted number of galaxies and their distribution across masses changes significantly when all of these are accounted for. We describe our predictions for the MIGHTEE-HI and WALLABY surveys for blind detection of galaxies using the redshifted 21 cm radiation. We study the dependence of the predicted number of detections on the HI mass function. We also describe our future plans for improving the simulation.

astro-ph.CO↗

arXiv2022

Gravitational Lensing of Core Collapse Supernova Gravitational Wave Signals

We discuss the prospects of gravitational lensing of gravitational waves (GWs) coming from core-collapse supernovae (CCSN). As the CCSN GW signal can only be detected from within our own Galaxy and the local group by current and upcoming ground-based GW detectors, we focus on microlensing. We introduce a new technique based on analysis of the power spectrum and association of peaks of the power spectrum with the peaks of the amplification factor to identify lensed signals. We validate our method by applying it on the CCSN-like mock signals lensed by a point mass lens. We find that the lensed and unlensed signal can be differentiated using the association of peaks by more than one sigma for lens masses M$_{\rm L} {>} 150{\rm M}_{\odot}$. We also study the correlation integral between the power spectra and corresponding amplification factor. This statistical approach is able to differentiate between unlensed and lensed signals for lenses as small as M$_{\rm L} {\sim} 15{\rm M}_{\odot}$. Further, we demonstrate that this method can be used to estimate the mass of a lens in case the signal is lensed. The power spectrum based analysis is general and can be applied to any broad band signal and is especially useful for incoherent signals.

gr-qc↗

arXiv2021

Exotic Image Formation in Strong Gravitational Lensing by Clusters of Galaxies -- III: Statistics with HUDF

We study the image formation near point singularities (swallowtail and umbilics) in the simulated strongly lensed images of Hubble Ultra Deep Field (HUDF) by the Hubble Frontier Fields (HFF) clusters. In this work, we only consider nearly half of the brightest (a total of 5271) sources in the HUDF region. For every HFF cluster, we constructed 11 realizations of strongly lensed HUDF with an arbitrary translation of the cluster centre within the central region of HUDF and an arbitrary rotation. In each of these realizations, we visually identify the characteristic/exotic image formation corresponding to the different point singularities. We find that our current results are consistent with our earlier results based on different approaches. We also study time delay in these exotic image formations and compare it with typical five-image geometries. We find that the typical time delay in exotic image formations is an order of magnitude smaller than the typical time delay in a generic five-image geometry.

astro-ph.CO↗

arXiv2021

Exotic Image Formation in Strong Gravitational Lensing by Clusters of Galaxies -- II: Uncertainties

Due to the finite amount of observational data, the best-fit parameters corresponding to the reconstructed cluster mass have uncertainties. In turn, these uncertainties affect the inferences made from these mass models. Following our earlier work, we have studied the effect of such uncertainties on the singularity maps in simulated and actual galaxy clusters. The mass models for both simulated and real clusters have been constructed using grale. The final best-fit mass models created using grale give the simplest singularity maps and a lower limit on the number of point singularities that a lens has to offer. The simple nature of these singularity maps also puts a lower limit on the number of three image (tangential and radial) arcs that a cluster lens has. Hence, we estimate the number of galaxy sources giving rise to the three image arcs, which can be observed with the James Webb Space Telescope (JWST). We find that we expect to observe at least 20-30 tangential and 5-10 radial three-image arcs in the Hubble Frontier Fields cluster lenses with the JWST.

astro-ph.CO↗

arXiv2021

Non-linear spherical collapse in tachyon models, and a comparison of collapse in tachyon and quintessence models of dark energy

We study evolution of perturbations in dark matter and dark energy for spherical collapse. We study Tachyon models of dark energy using the approach outlined in Rajvanshi and Bagla (2018). We work with models that are allowed by current observations. We find that as with Quintessence models allowed by observations, dark energy perturbations do not affect evolution of perturbations in dark matter in a significant manner. Perturbations in dark energy remain small for such models. In order to compare different classes of dark energy models, we use reconstruction of potentials and study dark matter and dark energy perturbations for same expansion history in the two models. We find that dark matter perturbations carry no imprint of the class of dark energy models for the same expansion history: this is significant in that we can work with any convenient model to study clustering of dark matter. We find that the evolution of dark energy perturbations carries an imprint of the class of models in that these grow differently in Tachyon models and Quintessence models. However, the difference between these diminishes for (1+w)<<1 and hence prospects for differentiating between models using characteristics of perturbations are limited in our Universe.

astro-ph.CO↗

arXiv2021

Tachyonic vs Quintessence dark energy: linear perturbations and CMB data

We use linear perturbation theory to study perturbations in dynamical dark energy models. We compare quintessence and tachyonic dark energy models with identical background evolution. We write the corresponding equations for different models in a form that makes it easier to see that the two models are very hard to distinguish in the linear regime, especially for models with $(1 + w) \ll 1$. We use Cosmic Microwave Background data and parametric representations for the two models to illustrate that they cannot be distinguished for the same background evolution with existing observations. Further, we constrain tachyonic models with the Planck data. We do this analysis for exponential and inverse square potentials and find that the intrinsic parameters of the potentials remain very weakly constrained. In particular, this is true in the regime allowed by low redshift observations.

astro-ph.CO↗

arXiv2021

Exotic Image Formation in Strong Gravitational Lensing by Clusters of Galaxies. I: Cross-Section

In a recent paper we have discussed the higher order singularities in gravitational lensing. We have shown that a singularity map, comprising of $A_3$-lines and unstable (point) singularities ($A_4$ and $D_4$), is a compact representation of high magnification regions corresponding to a given lens model for all possible source redshifts. It marks all the optimal locations for deep surveys in the lens plane. Here we present singularity maps for ten different cluster lenses selected from the \textit{Hubble Frontier fields} (HFF) and the \textit{Reionization Lensing Cluster Survey} (RELICS) surveys. We have identified regions in the lens plane with a high magnification for sources up to redshift ten. To determine the dependence of unstable (point) singularities on lens mass model reconstruction techniques, we compared singularity maps corresponding to the different mass models (provided by various groups in the HFF survey) for each cluster lens. We find that the non-parametric (free-form) method of lens mass reconstruction yields the least number of point singularities. In contrast, mass models reconstructed by various groups using a parametric approach have a significantly larger number of point singularities. We also estimate the number of galaxies lying near these unstable (point) singularities, which can be observed with the \textit{James Webb Space Telescope} (JWST). We find that we expect to get at least one hyperbolic umbilic and one swallowtail image formation for a source at $z > 1$ for every five clusters with JWST. These numbers are much higher than earlier estimates.

astro-ph.CO↗

arXiv2020

Study of galaxies on large-scale filaments in simulations

We use data from the Evolution and Assembly of GaLaxies in their Environment (EAGLE) cosmological simulation to study properties of galaxies in the cosmic web. Galaxies become more redder and form stars at a lower rate relative to their counterparts further away from the cylindrical axis of the large-scale filaments. These trends are particularly strong for galaxies with $M_*/M_{\odot}\lesssim10^{10}$. We also find that at distances $<0.5$ Mpc from the spine of the filaments, the median gas and stellar mass fraction in filament galaxies rises sharply with decreasing distance from the spine of the filament. These results, together with matching trends in the SFR/$M_*$ and the $g-r$ colour of filament galaxies suggest that (i) the intrafilamentary gas condenses into the filament galaxies thus fuelling star formation in them, and (ii) increased number density of galaxies closer to the central axis of the filament enhances the rate of gravitational interactions among filament galaxies closer to the spine.

astro-ph.GA↗

arXiv2020

Erratum: Nonlinear spherical perturbations in quintessence models of dark energy

We reported results of our study on non-linear spherical perturbations in quintessence models of dark energy. In the process of some follow up studies we discovered that a scaling factor in the code used for numerical calculations that should have been set to unity was set to a large value. Thus the scale of perturbations was much larger than intended, and for the larger scales the amplitude of dark matter perturbations was much higher than realistic. We provide corrected results here in this erratum. We find that there is no change in the perturbations for dark matter. The amplitude of perturbations in dark energy is much smaller than presented in the paper. Same holds true for spatial variation in the equation of state parameter.

astro-ph.CO↗

arXiv2019

Finding Singularities in Gravitational Lensing

The number of strong lens systems is expected to increase significantly in ongoing and upcoming surveys. With an increase in the total number of such systems we expect to discover many configurations that correspond to unstable caustics. In such cases, the instability can be used to our advantage for constraining the lens model. We have implemented algorithms for detection of different types of singularities in gravitational lensing. We test our approach on a variety of lens models and then go on to apply it to the inferred mass distribution for Abell 697 as an example application. We propose to represent lenses using A3-lines and singular points (A4 and D4) in the image plane. We propose this as a compact representation of complex lens systems that can capture all the details in a single snapshot.

astro-ph.CO↗

arXiv2019

Gravitational Waves from merging binaries

We discuss gravitational waves from merging binaries using a Newtonian approach with some inputs from the Post-Newtonian formalism. We show that it is possible to understand the key features of the signal using fundamental physics and also demonstrate that an approximate calculation gives us the correct order of magnitude estimate of the parameters describing the merging binary system. We build on this analysis to understand the range for different types of sources for given detector sensitivity. We also consider known binary pulsar systems and discuss the expected gravitational wave signal from these.

physics.pop-ph↗

arXiv2019

Reconstruction of Dynamical Dark Energy Potentials: Quintessence, Tachyon and interacting models

Dynamical models for dark energy are an alternative to the cosmological constant. It is important to investigate properties of perturbations in these models and go beyond the smooth FRLW cosmology. This allows us to distinguish different dark energy models with the same expansion history. For this, one often needs the potential for a particular expansion history. We study how such potentials can be reconstructed obtaining closed formulae for potential or reducing the problem to quadrature. We consider three classes of models here: tachyons, quintessence, and, interacting dark energy. We present results for constant w and the CPL parameterization. The method given here can be generalized to any arbitrary form of w(z).

astro-ph.CO↗

arXiv2019

Ram pressure stripping: An analytical approach

We use an analytical approach to study ram pressure stripping with simple models for discs and halo gas distribution to study the phenomena in cluster, group and galaxy halos. We also study variations with galaxy properties and redshift. In each case we model the worst case scenario (i.e., maximum effect due to ram pressure). We show that there is little variation in the worst case scenario with redshift. We find that gas discs in galaxies with a higher spin parameter get stripped sooner than galaxies with a smaller spin parameter. Galaxies in cluster halos get stripped of gas more effciently as compared to group and galaxy halos: this is due to the higher infall speed and a higher density of gas in the ICM due to a greater retention of baryons. We comment on the limitations of our model and situations where a signiffcant amount of gas may be retained in galaxy disc and also give an illustration for the same. Lastly, we discuss implications for star formation in galaxies as these fall into halos.

astro-ph.GA↗

arXiv2019

Angular clustering of point sources at 150 MHz in the TGSS survey

We study the angular clustering of point sources in The GMRT (Giant Meter Wave Telescope) Sky Survey (TGSS). The survey at 150 MHz with delta > -53.5 degrees has a sky coverage of 3.6 pi steradians, i.e., 90% of the whole sky. We created subsamples by applying different total flux thresholds limit (S >> 5 sigma) for good completeness and measured the angular correlation function omega(theta) of point sources at large scales ( >= 1 degree). We find that the amplitude of angular clustering is higher for brighter subsamples, this indicates that higher threshold flux samples are hosted by massive halos and cluster strongly: this conclusions is based on the assumption that the redshift distribution of sources does not change with flux and this is supported by models of radio sources. We compare our results with other low-frequency studies of clustering of point sources and verify that the amplitude of clustering varies with the flux limit. We quantify this variation as a power law dependence of the amplitude of correlation function with the flux limit. This dependence can be used to estimate foreground contamination due to clustering of point sources for low frequency HI intensity mapping surveys for studying the epoch of reionisation.

astro-ph.GA↗