ICTS

Nuclear star clusters can be the missing link in the formation of massive black holes.  Million solar mass black holes can form within 100 Myr in the heaviest and most compact nuclear star clusters.  There are potential gravitational-wave signatures of the massive black-hole formation process. 

Interaction session between Prof. Joseph Silk and students

I will review recent progress in using optimal transport methods to reconstruct the initial positions of biased tracers such as galaxies from their present day, redshift-space distorted positions.  This is particularly useful for estimating the cosmic distance scale.

Traditional analysis of weak lensing data relies on 2-point statistics of galaxy shapes. However, for late-time density fields that are strongly non-Gaussian, such analyses are suboptimal. In this talk, I will discuss a field-level analysis pipeline for analyzing weak lensing datasets. Because it utilizes information from the entire field, such analysis is optimal at a given scale. I will talk about some recent progress and challenges in this effort. In particular, I will talk about our efforts to enable forward-modeled mass mapping on the curved sky — a step that is essential for analyzing current and future weak lensing data. I will further discuss the gain in cosmological information that is enabled by such a field-level analysis.

The Cosmological Principle is a key feature of the very successful Standard model of Cosmology. It stands on to major pillars, isotropy and homogeneity at large scales. These pillars have been questioned recently using quasars as tracers of the Large scale structure (LSS). To check the validity of the Cosmological Principle, we use the Million Quasars Catalogue 7.2 to probe the LSS for isotropy and homogeneity. To do this we use techniques based on Shannon Entropy. We use these techniques to measure the ( i ) homogeneity, and ( ii ) isotropy along radial and transverse directions. We discover that the difference between isotropic and homogeneous distribution and the LSS as traced by the catalogued quasars, using this method, is negligible (~ 0.001) beyond ~250Mpc/h. Thus, the Cosmological Principle remains validated at large scales.

Subaru HSC weak lensing of SDSS redMaPPer cluster satellite galaxies: Empirical upper limit on orphan fractions:(Kumar et al,2022, MNRAS) Gravitational lensing can directly estimate the matter distribution around objects. We measure the weak lensing signal around SDSS redMaPPer cluster satellite galaxies, induced on the shapes of galaxies background to them. We choose satellites where their central galaxy is defined with a probability $P {\rm cen}>0.95$ in the redshift range, $0.1\leq z\leq 0.33$. For shape measurements, we use galaxies from the Subaru Hyper Suprime-Cam (HSC) survey. In order to understand the effect of the various environmental processes on matter distribution (mainly dark matter), we bin our satellite galaxies by their distance from the cluster center. Then we compare the matter distribution around satellites to a sample of galaxies that do not reside in clusters but have colors and magnitudes similar to the satellites. We see hints of a difference in the mass of the subhalo of the satellite compared to the halo masses of galaxies in our control sample, especially for the innermost cluster-centric radial bin $0.1< r < 0.3$ $[h^{-1}{\rm Mpc}]$. We use this observed mass difference to put a first ever direct upper limit on the prevalence of orphan galaxies that have lost most of their dark matter, primarily due to multiple pericentric passages. However, these upper limits could be relaxed if there is substantial contamination in the satellite galaxy sample.

Weak gravitational lensing imprints a coherent distortion onto the observed shapes of distant galaxies. At the image level, this gravitational shear is degenerate with the intrinsic shape of galaxies, and the weak lensing signal-to-noise from an individual galaxy is of order 0.01. I will describe Kinematic Lensing, a new technique combining imaging and galaxy kinematics to measure weak lensing with signal-to-noise of order one per galaxy.

The Circum-Galactic Medium (CGM) is a massive gas reservoir containing the majority of the baryonic matter in galaxies. The CGM is arguably the next frontier of investigating gas physics in galactic halos as it bears crucial information about galactic evolution and cosmology. The CGM poses a potential solution to the galactic missing baryon problem. It also provides additional information on the astrophysical feedback processes such as supernovae and active galactic nuclei, pushing the gas out to large distances from the galactic center. However, the CGM is also the most poorly constrained component of a galaxy. A multi-wavelength analysis of the gas can help us better understand the distribution and dynamics of the CGM. The combination of ongoing and future sky surveys such as South Pole Telescope at millimeter, eROSITA in X-rays and Dark Energy Survey in optical will significantly improve the existing constraints on the CGM properties. Newly emerging tools such as kinetic Sunyaev-Zel'dovich effect and fast radio bursts will push our observational capabilities to lower mass galaxies where the impact of astrophysical feedback processes significantly alters the galactic eco-systems.

Galaxy clusters are rare and massive collapsed halos that have been successfully used to study cosmology, baryonic processes and the nature of dark matter, among other things. Galaxy cluster catalogs can be constructed from large area, multi-wavelength surveys. The most promising approach is to employ an intracluster medium signature (X-ray emission or thermal Sunyaev-Zel’dovich effect- SZE) to produce a candidate list and to then use deep, multi-band optical survey data to measure redshifts, remove contaminants and constrain the cluster halo mass using weak gravitational lensing. Numerical simulations and mock observations are additional crucial ingredients. The required X-ray and SZE surveys have existed now for several years, but ongoing cluster cosmological studies are only now seeing the full benefits of catalog cleaning and halo mass constraints from deep, multi-band optical survey data acquired with, e.g., the Dark Energy Camera (DECam) or the Hyper-Suprime- Camera (HSC). Current results include the largest cluster samples in SZE and X-ray yet, the best ever constraints on mass—observable relations and the promise of resolving the current S8 tension between structure formation based and primary CMB probes. The upcoming Euclid and Rubin datasets will offer tremendous advantages over the existing DECam and HSC datasets, but only if the challenges of improved calibration of shear and photometric redshifts can be met. In addition, significant improvements in the understanding of baryonic processes and their impact on cluster halos will be needed to reach the full potential of these upcoming, largest cosmological surveys.

I plan to give a brief review on how the hubble constant is measured from different techniques, and which assumptions each of these methods implicitly assume. 
In particular I will focus on a new method which is able to determine h from galaxy clustering data without assuming any sound-horizon scale physics. Such novel determination of h is key to rule out or confirm early dark energy models as a potential solutions of the low-high h measurements from Planck and Cepheids, respectively.

We present an efficient semi-numerical approach for modeling the reionization of the second electron of helium (HeII) while taking into account the spatial patchiness and the resultant impact of photoheating on the IGM. First, we model quasars as sources of ionization consistent with observed luminosity function data. We consider quasar properties such as their Spectral Energy Distribution described by the index \alpha^SED, a maximum cut-off magnitude (M^max) and a quasar lifetime (t q) . The resultant ionizing photon field is included in the Semi numerical Code for ReionIzation with PhoTon Conservation (SCRIPT) , (originally developed to model hydrogen reionization) after modifying it for HeII and incorporating inhomogenous HeII recombinations . The corresponding thermal history of the IGM is modelled analytically via a sub-grid prescription and is parameterized by the reionization temperature for HeII -- (T^re). The above procedure yields the HeII ionization history and the evolution of the equation of state of the IGM . We show that our simulations are consistent with observations for a reasonable set of values of our free parameters.

We analyse a $100$ MHz bandwidth (centering $444\,\rm{MHz}$) high resolution $(24.4\,\rm{kHz})$ uGMRT data aiming neutral hydrogen (\ion{H}{i}) $21$-cm intensity mapping (IM) in the redshift range $z=1.88-2.61$. We use the Cross TGE which grids the two cross-polarizations RR and LL separately and cross-correlates them to get an unbiased estimate of the multi-frequency angular power spectrum (MAPS) $C {\ell}(\nu a,\nu b)$. The measured $C {\ell}(\nu a,\nu b)$ is found to be foreground dominated. We identify the foreground eigenvectors using a Kosambi-Karhunen–Lo\`eve transform (KKLT). The foreground contribution in the MAPS is subtracted from the measured $C {\ell}(\nu a,\nu b)$ to obtain the residual $[C {\ell}(\nu a,\nu b)] {\rm{res}}$, which is used to constrain the $21$-cm signal. We still find some amount of residual foregrounds (or systematics) in $[C {\ell}(\nu a,\nu b)] {\rm{res}}$, which is, however, free from any negative systematics. We place upper limits on $\Delta^2(k)$ the mean-squared brightness temperature and $[\Omega {\ion{H}{i}}b {\ion{H}{i}}]^2$ the neutral hydrogen abundance parameter in the $k$-range $0.1<k
 

Halos are the densest regions of dark matter in the universe, the detailed structure, evolution and distribution can shed light on some of the most fundamental questions in Cosmology. In this talk I will focus on how the observed distribution of matter and galaxies within halos on different scales ranging from the inner scale radius to the splashback radius in the transition region, can be used to understand halo and galaxy co-evolution, the nature of gravity and dark matter.

In this talk I review the development of gravitational lensing in cosmology - a subject which is entering a "golden age" with the advent of Euclid and LSST.  I start with Newton, who, it seems, understood light deflection but didn't get cosmology right, and follow the trail to Einstein's calculation of light bending by the sun and its generalisation to light propagation in a lumpy universe.  I describe early exciting, but puzzling, applications to quasar-galaxy associations and galaxy-galaxy lensing and how, in particular, Tony Tyson's group's measurement of distortion of the "cosmic wallpaper" by galaxy clusters triggered the development of modern weak lensing. Finally, I shall discuss the insight that Raychauhuri's approach to light propagation brings to this subject.

Direct measurements of galaxy peculiar velocities, i.e., their motions with respect to us beyond that expected from just the expansion of the Universe, are undergoing a resurgence. Five years ago, individual surveys were limited to a few thousand galaxies and to within the z~0.05 Universe. With the newest state-of-the-art, the number of galaxies, and the cosmological volume they cover, will increase by more than an order of magnitude. In this talk I will present the motivation and mechanisms for carrying out these measurements, focusing on both how they can be combined with galaxy redshifts to provide the most precise tests of General Relativity on large scales, and how they can be used to create detailed cosmographic maps of hidden structures in the nearby Universe. As an example, I will discuss the construction and new results from our recently released Sloan Digital Sky Survey peculiar velocity catalogue. I will then finish the talk by highlighting the revolutionary results we expect from the next generation of peculiar velocity surveys being carried out with the Dark Energy Spectroscopic Instrument (DESI) and 4-Metre Multi-Object Spectroscopic Telescope (4MOST).

I will present my work in two parts. Firstly, I will explore the role of the cosmic web environment in establishing the rotation of haloes and galaxies using large cosmological simulations. The origin of rotation in celestial objects is still not fully understood, particularly in the context of galaxies and their dark matter haloes. I will show correlations between the spin-axis of haloes/galaxies with the orientation of the cosmic filaments they are growing in, and explore the spin transition from parallel to perpendicular as a function of halo or galaxy mass with respect to the spine of the host filament.

Secondly, I will present a neural network method for reconstructing the underlying 3D cosmological density and velocity fields from incomplete observed galaxy distributions, which can provide valuable information on cosmological parameters. I will show how neural network reconstructions are related to different conventional statistical estimators, and compare the performance of our reconstruction method with the traditional Wiener filter. I will highlight the advantages of the neural network approach, particularly in capturing non-linear features, and discuss the impact of neural networks on the future of the field with the huge expected inflow of data.

Model-independent constraints on modified gravity models hitherto exist mainly on linear scales. I present the first N-body simulations of modified gravity models based on a consistent parameterisation that is valid on all scales. I investigate the impact of a time-dependent modification of the gravitational force on the matter power spectrum and consequently on weak-lensing observables, with particular focus on the constraining power gained by including non-linear scales. I describe a fitting function validated against simulations for that can predict the non-linear matter power spectrum of the simulations for a wide range of parameters and discuss how this pipeline has been implemented for Euclid modified gravity forecasts. This paves the way for a full model-independent test of modified gravity using all of the data from such upcoming surveys.
 

In the Lambda-CDM model, Planck data provides a value of the Hubble constant H0 equal to 67.5 km/s/Mpc, which is around 5-sigma away from the locally measured value from Type Ia Supernovae, H0 equal to 73 km/s/Mpc. This Hubble tension is currently the biggest discrepancy in cosmology. Here we looked at the cosmological model that incorporates massive neutrino non-standard interaction mediated via a heavy scalar, which had previously shown a lot of promise in solving the Hubble tension in the strong interaction regime where the coupling strength is approximately 10^9 times the weak interaction coupling. However, with the latest Planck 2018 data, we found that the Planck high-multipole CMB polarization data disfavours such strong interactions. We also test the viability of Natural Inflation and Coleman-Weinberg Inflation in the presence of massive neutrino non-standard interactions, against CMB, BAO, and LSS data, with a particular focus on Planck and BICEP/Keck data. These inflationary models are ruled out at more than 2-sigma in the standard Lambda-CDM model with current cosmological data. But interestingly, we find that predictions from these inflationary models are allowed within 2-sigma when we include massive neutrino non-standard interactions.

This study utilizes the ELAIS N1 deep field with a 24-hour observation time and 120 MHz bandwidth to achieve a central background RMS noise of 233 μJy/beam and a resolution of approximately 11.5 arcseconds. The radio catalog contains 1232 sources with a flux density greater than 1.1 mJy, and their flux density accuracy and positional offset are cross-matched and compared to other radio catalogs to ensure precision. Additionally, the normalized source count derived from the radio catalog generated in this study agrees well with the source count in previous catalogs of various fields and the LOFAR catalog of the same field. Moreover, this study compares two techniques for estimating the statistical fluctuations in galactic diffuse synchrotron emission over a wide frequency range of 120-250 MHz using power spectrum estimators: Image-based and Visibility-based estimators. The Image-based estimator is found to be more effective for studying the diffuse emission in our galaxy at a lower frequency. This study provides valuable insights into the EoR/CD and demonstrates the improved proficiency of the uGMRT in band-2.
 

The halo assembly bias, a phenomenon referring to dependencies of the large-scale bias of a dark matter halo other than its mass, is a fundamental property of the standard cosmological model. By utilizing a constrained simulation that faithfully reproduces the observed structures larger than 2 Mpc in the local universe, for a sample of 634 massive clusters at z<0.12, we found their counterpart halos in the simulation and used the mass growth history of the matched halos to estimate the formation time of the observed clusters. This allowed us to construct a pair of early- and late-forming clusters, with a similar mass as measured via weak gravitational lensing, and large-scale biases differing at 3 sigma level, suggestive of the signature of assembly bias, which is further corroborated by the properties of cluster galaxies.  Our study paves a way to further detect assembly bias based on cluster samples constructed purely on observed quantities.

Our current understanding of  galaxy formation is based on the presence of an elusive matter component: the Cold Dark Matter (CDM). This simple  model has ben challenged many times in the past decade, mainly by galaxy observations on small scales: from the abundance of satellites, to the distribution of dark matter within galaxies, and more recently by the discovery of galaxies "without" dark matter.In my talk I will first revise all these claims with the help of cosmological numerical simulations of galaxy formation from the NIHAO project. I will then discuss whether there is indeed an observationally motivated need to abandon Cold Dark Matter and move beyond such a simple model. 

CMB measurements have been of tremendous utility for learning about cosmology. With current and upcoming experiments this will continue, as polarization measurements will provide a large amount of new information about the primary CMB, including the search for gravitational waves from inflation. At the same time, this new generation is starting to measure higher-order effects in the CMB, with the largest such one being the gravitational lensing of the CMB by lower redshift large scale structure. Other effects include fluctuations in the optical depth to Thomson scattering (“patchy tau”) as well as secondary fluctuations coming from moving electrons (the Sunyaev-Zeldovich effects). I will discuss these effects, using the South Pole Telescope as an example for what is possible with current and upcoming measurements.

One of the features directly predicted by the hierarchical galaxy formation are the stellar streams -- linear structures  produced by the disruption of accreted dwarf galaxies and stars clusters. The streams are valuable tracers of the Galactic potential as they approximately trace orbits in the potential. In last few years, the number of known streams have drastically increased. Using sophisticated statistical techniques we have been able to map these stream trajectories, motions and stellar densities. These analyses reveal a complicated picture where streams shows signs of many perturbations ranging from perturbations by low mass dark matter halos to large perturbation from the Magellanic Clouds and help us understand the distribution of dark matter in the Milky Way.

Formation of a galaxy and subsequent astrophysical processes are known to affect the dark matter content of its host halo. I will talk based on my recent publication. In that work, we study the response of the dark matter for haloes spanning over four orders of magnitude using hydrodynamical cosmological simulation suites IllustrisTNG and EAGLE. In particular, we characterize the change in spherically averaged distribution using quasi-adiabatic relaxation framework, producing simple fitting functions that accurately capture the response in dwarf scale to cluster scale haloes all the way from their virial boundary to inner region that is just one-hundredth of their virial radii. We show that commonly employed schemes, which consider the relative change in radius $r f/r i-1$ of a spherical dark matter shell to be a function of only the relative change in its total enclosed mass $M i/M f-1$, do not accurately describe the measured response of most haloes in IllustrisTNG and EAGLE. Instead, letting $r f/r i$ have an additional dependence on the halo-centric radius allows us to isolate the response of the dark matter halo that is relatively more universal across mass scales and also between IllustrisTNG and EAGLE. We also account for a previously unmodelled effect, likely driven by feedback-related outflows, in which shells having $r f/r i\simeq1$ (i.e., no relaxation) have $M i/M f$ significantly different from unity. Our results immediately apply to several semi-analytical tools for modelling galactic and large-scale structure (baryonification schemes, rotation curves, total mass profiles, etc.). We also study how this response depends on different halo and galaxy properties, finding that it is primarily related to the halo concentration and the current active star formation rate. We discuss how these results can be extended to build a deeper physical understanding of the connection between dark matter and baryons at small scales.

We estimate the secondary E and B mode polarisation of the cosmic microwave background (CMB) originating from the transverse peculiar velocity of free electrons during the reionisation era. Interestingly, apart from having a blackbody part, there is a y-type (SZ) distortion in the frequency spectrum. This makes it distinguishable from primordial polarisation as well as other secondary sources, such as gravitational lensing and paves a way to beat the cosmic variance. Furthermore, it is also differentiable from other y-type signals such as from the thermal SZ effect as it involves polarised radiation. We show that this signal is sensitive to the reionisation optical depth as well as duration. Future detection of this signal can serve as a new probe to constrain different cosmological parameters related to reionisation and structure formation.

The radiations from the first luminous sources drive the fluctuations in the 21-cm signal originating from the neutral hydrogen in the intergalactic medium (IGM) at Cosmic Dawn (CD). The astrophysical processes which are dominant in the IGM during this period are Lyα coupling, X-ray heating and photoionization. These processes together make the CD 21-cm signal highly non-Gaussian. The impact of these processes on the 21-cm signal and its non-Gaussianity vary depending on the properties of these first sources of light. One of the observable statistic that can be estimated from the radio interferometric observations of the CD and can quantify this non-Gaussianity present in the signal is the 21-cm bispectrum. In this talk, I will discuss how our detailed and comparative analysis of the power spectrum and bispectrum shows that the shape, sign and magnitude of the bispectrum combinedly provide the best measure of the signal fluctuations and its non-Gaussianity compared to the power spectrum. I will also show that it is important to study the sequence of sign changes along with the variations in the shape and magnitude of the bispectrum throughout the CD history to arrive at a robust conclusion about the dominant IGM processes at different cosmic times. In the final segment of my talk I will show how with the help of an Artificial Neural Network-based emulator of the 21-cm bispectrum and power spectrum, we have demonstrated that the bispectrum can provide tighter constraints on the CD parameters compared to the power spectrum.