arXiv subjects

Somnath Bharadwaj

Publications and source records attributed to Somnath Bharadwaj.

144 records · Page 8Linked to original sources

arXiv2004

Using the filaments in the LCRS to test the LambdaCDM model

It has recently been established that the filaments seen in the Las Campanas Redshift Survey (LCRS) are statistically significant at scales as large as 70 to 80 Mpc/h in the $-3^{\circ}$ slice, and 50 to 70 Mpc/h in the five other LCRS slices. The ability to produce such filamentary features is an important test of any model for structure formation. We have tested the LCDM model with a featureless, scale invariant primordial power spectrum by quantitatively comparing the filamentarity in simulated LCRS slices with the actual data. The filamentarity in an unbiased LCDM model, we find, is less than the LCRS. Introducing a bias b=1.15, the model is in rough consistency with the data, though in two of the slices the filamentarity falls below the data at a low level of statistical significance. The filamentarity is very sensitive to the bias parameter and a high value b=1.5, which enhances filamentarity at small scales and suppresses it at large scales, is ruled out. A bump in the power spectrum at k~0.05 Mpc/h is found to have no noticeable effect on the filamentarity.

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arXiv2004

The CMBR fluctuations from HI perturbations prior to reionization

Loeb and Zaldarriaga (2003) have recently proposed that observations of the CMBR brightness temperature fluctuations produced by the HI inhomogeneities prior to reionization hold the promise of probing the primordial power spectrum to a hitherto unprecedented level of accuracy. This requires a precise quantification of the relation between density perturbations and brightness temperature fluctuations. Brightness temperature fluctuations arise from two sources (1.) fluctuations in the spin temperature, and (2.) fluctuations in the HI optical depth, both of which are caused by density perturbations. For the spin temperature, we investigate in detail its evolution in the presence of HI fluctuations. For the optical depth, we find that it is affected by density perturbations both directly and through peculiar velocities which move the absorption features around in frequency. The latter effect, which has not been included in earlier studies, is similar to the redshift space distortion seen in galaxy surveys and this can cause changes of 50% or more in the birghtness temperature fluctuations.

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arXiv2004

HI Fluctuations at Large Redshifts: III - Simulating the Signal Expected at GMRT

We simulate the distribution of neutral hydrogen (HI) at the redshifts z=1.3 and 3.4 using a cosmological N-body simulation along with a prescription for assigning HI masses to the particles. The HI is distributed in clouds whose properties are consistent with those of the damped Lyman-αabsorption systems (DLAs) seen in quasar spectra. The clustering properties of these clouds are identical to those of the dark matter. We use this to simulate the redshifted HI emission expected at 610 MHz and 325 MHz, two of the observing bands a the GMRT. These are used to predict the correlations expected between the complex visibilities measured at different baselines and frequencies in radio-interferometric observations with the GMRT. The visibility correlations directly probe the power spectrum of HI fluctuations at the epoch when the HI emission originated, and this holds the possibility of using HI observations to study large-scale structures at high z.

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arXiv2003

The Size of the Longest Filaments in the Universe

We analyze the filamentarity in the Las Campanas redshift survey (LCRS) and determine the length scale at which filaments are statistically significant. The largest length-scale at which filaments are statistically significant, real objects, is between 70 to $80 h^{-1}$Mpc, for the LCRS $-3^o$ slice. Filamentary features longer than $80 h^{-1}$Mpc, though identified, are not statistically significant; they arise from chance alignments. For the five other LCRS slices, filaments of lengths $50 h^{-1}$Mpc to $70 h^{-1}$Mpc are statistically significant, but not beyond. These results indicate that while individual filaments up to $80 h^{-1}$Mpc are statistically significant, the impression of structure on larger scales is a visual effect. On scales larger than $80 h^{-1}$Mpc the filaments interconnect by statistical chance to form the filament-void network. The reality of the $80 h^{-1}$Mpc features in the $-3^o$ slice make them the longest coherent features in the LCRS. While filaments are a natural outcome of gravitational instability, any numerical model attempting to describe the formation of large scale structure in the universe must produce coherent structures on scales that match these observations.

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arXiv2003

HI Fluctuations at Large Redshifts: II - the Signal Expected for GMRT

For the GMRT, we calculate the expected signal from redshifted HI emission at two frequency bands centered at 610 and 325 MHz. The study focuses on the visibility-visibility cross-correlations, proposed earlier as the optimal statistical estimator for detecting and analyzing this signal. These correlations directly probe the power spectrum of density fluctuations at the redshift where the radiation originated, and thereby provide a method for studying the large scale structures at large redshifts. We present detailed estimates of the correlations expected between the visibilities measured at different baselines and frequencies. Analytic fitting formulas representing the salient features of the expected signal are also provided. These will be useful in planning observations and deciding an optimal strategy for detecting this signal.

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arXiv2003

Modelling galaxy halos using dark matter with pressure

We investigate whether a dark matter with substantial amounts of pressure, comparable in magnitude to the energy density, could be a viable candidate for the constituent of dark matter halos. We find that galaxy halos models, consistent with observations of flat rotation curves, are possible for a variety of equations of state with anisotropic pressures. It turns out that the gravitational bending of light rays passing through such halos is very sensitive to the pressure. We propose that combined observations of rotation curves and gravitational lensing can be used to determine the equation of state of the dark matter. Alternatively, if the equation of state is known from other observations, rotation curves and gravitational lensing can together be used to test General Relativity on galaxy scales.

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arXiv2002

HI Fluctuations at Large Redshifts: I--Visibility correlation

We investigate the possibility of probing the large scale structure in the universe at large redshifts by studying fluctuations in the redshifted 1420 MHz emission from the neutral hydrogen (HI) at early epochs. The neutral hydrogen content of the universe is known from absorption studies for z<4.5. The HI distribution is expected to be inhomogeneous in the gravitational instability picture and this inhomogeneity leads to anisotropy in the redshifted HI emission. The best hope of detecting this anisotropy is by using a large low-frequency interferometric instrument like the Giant Meter-Wave Radio Telescope (GMRT). We calculate the visibility correlation function at two frequencies nu and nu' of the redshifted HI emission for an interferometric observation. In particular we give numerical results for the two GMRT channels centered around nu =325 and 610 MHz from density inhomogeneity and peculiar velocity of the HI distribution. The visibility correlation is ~10^-9 to 10^-10 Jy^2. We calculate the signal-to-noise for detecting the correlation signal in the presence of system noise and show that the GMRT might detect the signal for integration times ~ 100 hrs. We argue that the measurement of visibility correlation allows optimal use of the uncorrelated nature of the system noise across baselines and frequency channels.

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arXiv2001

Nonlinear redshift distortions: The two-point correlation function

We consider a situation where the density and peculiar velocities in real space are linear, and we calculate ξ_s the two-point correlation function in redshift space, incorporating all non-linear effects which arise as a consequence of the map from real to redshift space. Our result is non-perturbative and it includes the effects of possible multi-streaming in redshift space. We find that the deviations from the predictions of the linear redshift distortion analysis increase for the higher spherical harmonics of ξ_s. While the deviations are insignificant for the monopole ξ_0, the hexadecapole ξ_4 exhibits large deviations from the linear predictions. For a COBE normalised Γ=0.25, h=0.5 CDM power spectrum our results for ξ_4 deviate from the linear predictions by a factor of two at the scales ~10 h^{-1} Mpc. The deviations from the linear predictions depend separately on Ωand b. This holds the possibility of removing the degeneracy that exists between these two parameters in the linear analysis of redshift surveys which yields only β=Ω^0.6/b. We also show that the commonly used phenomenological model where the non-linear redshift two-point correlation function is calculated by convolving the linear redshift correlation function with an isotropic pair velocity distribution function is a limiting case of our result.

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arXiv2001

Using Gravitational Lensing to study HI clouds at high redshift

We investigate the possibility of detecting HI emission from gravitationally lensed HI clouds (akin to damped Lyman-$α$ clouds) at high redshift by carrying out deep radio observations in the fields of known cluster lenses. Such observations will be possible with present radio telescopes only if the lens substantially magnifies the flux of the HI emission. While at present this holds the only possibility of detecting the HI emission from such clouds, it has the disadvantage of being restricted to clouds that lie very close to the caustics of the lens. We find that observations at a detection threshold of 50 micro Jy at 320 MHz (possible with the GMRT) have a greater than 20% probability of detecting an HI cloud in the field of a cluster, provided the clouds have HI masses in the range 5 X 10^8 M_{\odot} < M_{HI} < 2.5 X 10^{10} M_{\odot}. The probability of detecting a cloud increases if they have larger HI masses, except in the cases where the number of HI clouds in the cluster field becomes very small. The probability of a detection at 610 MHz and 233 MHz is comparable to that at 320 MHz, though a definitive statement is difficult owing to uncertainties in the HI content at the redshifts corresponding to these frequencies. Observations at a detection threshold of 2 micro Jy (possible in the future with the SKA) are expected to detect a few HI clouds in the field of every cluster provided the clouds have HI masses in the range 2 X 10^7 M_{\odot} < M_{HI} < 10^9 M_{\odot}. Even if such observations do not result in the detection of HI clouds, they will be able to put useful constraints on the HI content of the clouds.

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arXiv2001

Using HI to probe large scale structures at z ~ 3

The redshifted 1420 MHz emission from the HI in unresolved damped Lyman-αclouds at high z will appear as a background radiation in low frequency radio observations. This holds the possibility of a new tool for studying the universe at high-z, using the mean brightness temperature to probe the HI content and its fluctuations to probe the power spectrum. Existing estimates of the HI density at z~3 imply a mean brightness temperature of 1 mK at 320 Mhz. The cross-correlation between the temperature fluctuations across different frequencies and sight lines is predicted to vary from 10^{-7} K^2 to 10^{-8} K^2 over intervals corresponding to spatial scales from 10 Mpc to 40 Mpc for some of the currently favoured cosmological models. Comparing this with the expected sensitivity of the GMRT, we find that this can be detected with \~10 hrs of integration, provided we can distinguish it from the galactic and extragalactic foregrounds which will swamp this signal. We discuss a strategy based on the very distinct spectral properties of the foregrounds as against the HI emission, possibly allowing the removal of the foregrounds from the observed maps.

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arXiv1999

Evidence for Filamentarity in the Las Campanas Redshift Survey

We apply Shapefinders, statistical measures of `shape' constructed from two dimensional partial Minkowski functionals, to study the degree of filamentarity in the Las Campanas Redshift Survey (LCRS). In two dimensions, three Minkowski functionals characterise the morphology of an object, they are: its perimeter (L), area (S), and genus. Out of L and S a single dimensionless Shapefinder Statistic, F can be constructed (0 <=F <=1). F acquires extreme values on a circle (F = 0) and a filament (F = 1). Using F, we quantify the extent of filamentarity in the LCRS by comparing our results with a Poisson distribution with similar geometrical properties and having the same selection function as the survey. Our results unambiguously demonstrate that the LCRS displays a high degree of filamentarity both in the Northern and Southern galactic sections a result that is in general agreement with the visual appearance of the catalogue. It is well known that gravitational clustering from Gaussian initial conditions gives rise to the development of non-Gaussianity reflected in the formation of a network-like filamentary structure on supercluster scales. Consequently the fact that the smoothed LCRS catalogue shows properties consistent with those of a Gaussian random field (Colley 1997) whereas the unsmoothed catalogue demonstrates the presence of filamentarity lends strong support to the conjecture that the large scale clustering of galaxies is driven by gravitational instability.

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arXiv1999

Nature of Clustering in the Las Campanas Redshift Survey

We have carried out a multi-fractal analysis of the distribution of galaxies in the three Northern slices of the Las Campanas Redshift Survey. Our method takes into account the selection effects and the complicated geometry of the survey. In this analysis we have studied the scaling properties of the distribution of galaxies on length scales from $20 h^{-1} Mpc$ to $200 h^{-1} Mpc$. Our main results are: (1) The distribution of galaxies exhibits a multi-fractal scaling behaviour over the scales $20 h^{-1} Mpc$ to $80 h^{-1} Mpc$, and, (2) the distribution is homogeneous on the scales $80h^{-1} Mpc$ to $200 h^{-1} Mpc$. We conclude that the universe is homogeneous at large scales and the transition to homogeneity occurs somewhere in the range $80 h^{-1} Mpc$ to $100 h^{-1} Mpc$.

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arXiv1998

Radial Redshift Space Distortions

The radial component of the peculiar velocities of galaxies cause displacements in their positions in redshift space. We study the effect of the peculiar velocities on the linear redshift space two point correlation function. Our analysis takes into account the radial nature of the redshift space distortions and it highlights the limitations of the plane parallel approximation. We consider the problem of determining the value of βand the real space two point correlation function from the linear redshift space two point correlation function. The inversion method proposed here takes into account the radial nature of the redshift space distortions and can be applied to magnitude limited redshift surveys that have only partial sky coverage.

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arXiv1997

Skewness in the Cosmic Microwave Background Anisotropy from Inflationary Gravity Wave Background

In the context of inflationary scenarios, the observed large angle anisotropy of the Cosmic Microwave Background (CMB) temperature is believed to probe the primordial metric perturbations from inflation. Although the perturbations from inflation are expected to be gaussian random fields, there remains the possibility that nonlinear processes at later epochs induce ``secondary'' non-gaussian features in the corresponding CMB anisotropy maps. The non-gaussianity induced by nonlinear gravitational instability of scalar (density) perturbations has been investigated in existing literature. In this paper, we highlight another source of non-gaussianity arising out of higher order scattering of CMB photons off the metric perturbations. We provide a simple and elegant formalism for deriving the CMB temperature fluctuations arising due to the Sachs-Wolfe effect beyond the linear order. In particular, we derive the expression for the second order CMB temperature fluctuations. The multiple scattering effect pointed out in this paper leads to the possibility that tensor metric perturbation, i.e., gravity waves (GW) which do not exhibit gravitational instability can still contribute to the skewness in the CMB anisotropy maps. We find that in a flat $Ω=1$ universe, the skewness in CMB contributed by gravity waves via multiple scattering effect is comparable to that from the gravitational instability of scalar perturbations for equal contribution of the gravity waves and scalar perturbations to the total rms CMB anisotropy. The secondary skewness is found to be smaller than the cosmic variance leading to the conclusion that inflationary scenarios do predict that the observed CMB anisotropy should be statistically consistent with a gaussian random distribution.

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arXiv1997

Decaying Neutrinos and Large Scale Structure Formation

We study the growth of density perturbations in a universe with unstable dark matter particles. The mass (m_ν) range 30 eV < m_ν< 10 keV with lifetimes ($t_d$) in the range 10^7 sec < t_d < 10^16 sec are considered. We calculate the COBE normalized matter power spectrum for these models. We find that it is possible to construct models consistent with observations for masses $m_ν> 50 eV by adjusting t_d so as to keep the quantity (m_ν(keV))^2 t_d(yr) constant at a value around 100. For m_ν< 1 keV the power spectrum has extra power at small scales which could result in an early epoch of galaxy formation. We do not find any value of t_d which gives a viable model in the mass range m_ν< 50 eV. We also consider the implications of radiatively decaying neutrinos---models in which a small fraction B << 1 of neutrinos decay into photons-which could possibly ionize the intergalactic medium (IGM) at high redshift. We show that the parameter space of decaying particles which satisfies the IGM observations does not give viable models of structure formation.

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arXiv1996

Self-similarity and the Pair Velocity Dispersion

We have considered linear two point correlations of the form $1/{x^γ}$ which are known to have a self-similar behaviour in a $Ω=1$ universe. We investigate under what conditions the non-linear corrections, calculated using the Zel'dovich approximation, have the same self-similar behaviour. We find that the scaling properties of the non-linear corrections are decided by the spatial behaviour of the linear pair velocity dispersion and it is only for the cases where this quantity keeps on increasing as a power law (i.e. for $γ< 2$) do the non-linear corrections have the same self-similar behaviour as the linear correlations. For $(γ> 2)$ we find that the pair velocity dispersion reaches a constant value and the self-similarity is broken by the non-linear corrections. We find that the scaling properties calculated using the Zel'dovich approximation are very similar to those obtained at the lowest order of non-linearity in gravitational dynamics and we propose that the scaling properties of the non-linear corrections in perturbative gravitational dynamics also are decided by the spatial behaviour of the linear pair velocity dispersion.

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arXiv1996

The Evolution of Correlation Functions in the Zel'dovich Approximation and its Implications for the Validity of Perturbation Theory

We investigate whether it is possible to study perturbatively the transition in cosmological clustering between a single streamed flow to a multi streamed flow. We do this by considereing a system whose dynamics is governed by the Zel'dovich approximation (ZA) and calculating the evolution of the two point correlation function using two methods: 1.Distribution functions 2.Hydrodynamic equations without pressure and vorticity. The latter method breaks down once multistreaming occurs whereas the former does not. We find that the two methods give the same results to all orders in a perturbative expansion of the two point correlation function. We thus conclude that we cannot study the transition from a single stream flow to a multi-stream flow in a perturbative expansion. We expect this conclusion to hold even if we use the full gravitational dynamics (GD) instead of ZA. We use ZA to look at the evolution of the two point correlation function at large spatial separations and we find that until the onset of multi-streaming the evolution can be described by a diffusion process where the linear evolution at large scales gets modified by the rearrangement of matter on small scales. We compare these results with the lowest order nonlinear results from GD. We find that the difference is only in the numerical value of the diffusion coefficient and we interpret this physically. We also use ZA to study the induced three point correlation function. At the lowest order we find that, as in the case of GD, the three point correlation does not necessarily have the hierarchical form. We also find that at large separations the effect of the higher order terms for the three point correlatin function is very similar to that for the the two point correlation and in this case too the evolution can be be described in terms of

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arXiv1995

Perturbative Growth of Cosmological Clustering II: The Two Point Correlation

We use the BBGKY hierarchy equations to calculate, perturbatively, the lowest order nonlinear correction to the two point correlation and the pair velocity for Gaussian initial conditions in a critical density matter dominated cosmological model. We compare our results with the results obtained using the hydrodynamic equations which neglect pressure and we find that the two match, indicating that thare are no effects of multistreaming at this order of perturbation. We analytically study the effect of small scales on the large scales by calculating the nonlinear correction for a Dirac delta function initial two point correlation. We find that the induced two point correlation has a $x^{-6}$ behaviour at large separations. We have considered a class of initial conditions where the initial power spectrum at small $k$ has the form $k^n$ with $0 < n \le 3$ and have numerically calculated the nonlinear correction to the two point correlation, its average over a sphere and the pair velocity over a large dynamical range. We find that at small separations the effect of the nonlinear term is to enhance the clustering whereas at intermediate scales it can act to either increase or decrease the clustering. At large scales we find a simple formula which gives a very good fit for the nonlinear correction in terms of the initial function. This formula explicitly exhibits the influence of small scales on large scales and because of this coupling the perturbative treatment breaks down at large scales much before one would expect it to if the nonlinearity were local in real space. We physically interpret this formula in terms of a simple diffusion process. We have also investigated the case $n=0$ and we find that it differs from the other cases in certain respects. We investigate a recently proposed scaling property of

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