Non-linear redshift distortions: the two-point correlation function     

Somnath Bharadwaj 《Monthly notices of the Royal Astronomical Society》2001,327(2):577-587

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 normalized     ,     cold dark matter (CDM) power spectrum, our results for ξ ₄ deviate from the linear predictions by a factor of two on the scale of ∼10  h ⁻¹ Mpc. The deviations from the linear predictions depend separately on f (Ω) and 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     .
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.  相似文献   

Testing homogeneity on large scales in the Sloan Digital Sky Survey Data Release One     

Jaswant Yadav  Somnath Bharadwaj  Biswajit Pandey  T. R. Seshadri 《Monthly notices of the Royal Astronomical Society》2005,364(2):601-606

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A two-dimensional analysis of percolation and filamentarity in the Sloan Digital Sky Survey Data Release One     

Biswajit Pandey  Somnath Bharadwaj 《Monthly notices of the Royal Astronomical Society》2005,357(3):1068-1076

We have used the largest cluster statistics and the average filamentarity to quantify respectively the connectivity and the shapes of the patterns seen in the galaxy distribution in two volume-limited subsamples extracted from the equatorial strips of the Sloan Digital Sky Survey (SDSS) Data Release One (DR1). The data was projected on to the equatorial plane and analysed in two dimensions (2D). Comparing the results with Poisson point distributions at various levels of smoothing we find evidence for a network-like topology with filaments being the dominant patterns in the galaxy distribution. With increasing smoothing, a transition from many individual filamentary structures to an interconnected network is found to occur at a filling factor of 0.5–0.6. We have tested the possibility that the connectivity and the morphology of the patterns in the galaxy distribution may be luminosity dependent and find significant evidence for a luminosity–morphology relation, the brighter galaxies exhibiting lower levels of connectivity and filamentarity compared to the fainter ones. Using a statistical technique, Shuffle, we show that the filamentarity in both the SDSS strips is statistically significant up to 80  h ⁻¹ Mpc but not beyond. Larger filaments, though identified, are not statistically significant. Our findings reaffirm earlier work establishing the filaments to be the largest known statistically significant coherent structures in the Universe.  相似文献   

Prospects for Detecting the 326.5 MHz Redshifted 21-cm HI Signal with the Ooty Radio Telescope (ORT)     

Sk. Saiyad Ali  Somnath Bharadwaj 《Journal of Astrophysics and Astronomy》2014,35(2):157-182

Observations of the redshifted 21-cm HI fluctuations promise to be an important probe of the post-reionization era (z≤ 6). In this paper we calculate the expected signal and foregrounds for the upgraded Ooty Radio Telescope (ORT) which operates at frequency ν _o = 326.5 MHz which corresponds to redshift z = 3.35. Assuming that the visibilities contain only the HI signal and system noise, we show that a 3 σ detection of the HI signal (～1 mK) is possible at angular scales 11^′ to 3^° with ≈1000 h of observation. Foreground removal is one of the major challenges for a statistical detection of the redshifted 21 cm HI signal. We assess the contribution of different foregrounds and find that the 326.5 MHz sky is dominated by the extragalactic point sources at the angular scales of our interest. The expected total foregrounds are 10⁴?10⁵ times higher than the HI signal.  相似文献   

Using HI to probe large scale structures at z∼3     

Somnath Bharadwaj  Biman B. Nath  Shiv K. Sethi 《Journal of Astrophysics and Astronomy》2001,22(1):21-34

The redshifted 1420 MHz emission from the HI in unresolved damped Lyman-α clouds 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 power spectrum. Existing estimates of the HI density atz−3 imply a mean brightness temperature of 1 mK at 320 MHz. The cross-correlation between the temperature fluctuation across different frequencies and sight lines is predicted to vary from 10⁻⁷ K² to 10⁻⁸ K² 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.  相似文献   

HI fluctuations at large redshifts: I-visibility correlation     

Somnath Bharadwaj  Shiv K. Sethi 《Journal of Astrophysics and Astronomy》2001,22(4):293-307

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 forz ≲ 4.5. TheHI 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 〈V_v(U) V_v′(U)〉 at two frequenciesi andv′ of the redshiftedHI emission for an interferometric observation. In particular we give numerical results for the two GMRT channels centered aroundν = 325 MHz andν = 610 MHz from density inhomogeneity and peculiar velocity of the HI distribution. The visibility correlation is- 10^-10-10^-9 Jy². 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. On leave from Harish-Chandra Research Institute, Chhatnag Road, Jhusi, Allahabad 211 019, India.  相似文献   

Contrasting parageneses in the manganese silicate-carbonate rocks from Parseoni,Sausar Group,India and their interpretation     

Somnath Dasgupta  Pulak Sengupta  M. Fukuoka  Supriya Roy 《Contributions to Mineralogy and Petrology》1993,114(4):533-538

Mn silicate-carbonate rocks at Parseoni occur as conformable lenses within metapelites and calc-silicate rocks of the Precambrian Sausar Group, India. The host rocks are estimated to have been metamorphosed at uppermost P-T conditions of 500–550°C and 3–4 kbar. The Mn-rich rocks contain appreciable Fe, reflected in the occurrence of magnetite₍₁₎ (MnO 1%), magnetite₍₂₎ (MnO 15%) and magnetite₍₃₎ (MnO 10%). Two contrasting associations of pyroxmangite, with and without tephroite, developed in the Mn silicate-carbonate rocks under isothermal-isobaric conditions. The former assemblage formed in relatively Fe-rich bulk compositions and equilibrated with a metamorphic fluid having a low X _{CO 2} (<0.2), and the latter equilibrated with a CO₂-rich fluid. Rhodochrosite+magnetite₍₁₎+quartz protoliths produced the observed mineral assemblages on metamorphism. Partitioning of major elements between coexisting phases is somewhat variable. Fe shows preference for tephroite over pyroxmangite at the ambient physical conditions of metamorphism. Oxygen fugacity during metamorphism was monitored at or near the QFM buffer in tephroite bearing domains, and the fluid composition was buffered by mineral reactions in respective domains. As compared to other metamorphosed Mn deposits of the Sausar Group, the Mn silicate-carbonate rocks at Parseoni were, therefore, metamorphosed at much lower f _{O 2} through complex mineral-fluid interactions.  相似文献   

Prospects of Measuring the Angular Power Spectrum of the Diffuse Galactic Synchrotron Emission with SKA1 Low     

Sk. Saiyad Ali  Somnath Bharadwaj  Samir Choudhuri  Abhik Ghosh  Nirupam Roy 《Journal of Astrophysics and Astronomy》2016,37(4):35

The Diffuse Galactic Syncrotron Emission (DGSE) is the most important diffuse foreground component for future cosmological 21-cm observations. The DGSE is also an important probe of the cosmic ray electron and magnetic field distributions in the turbulent interstellar medium (ISM) of our galaxy. In this paper we briefly review the Tapered Gridded Estimator (TGE) which can be used to quantify the angular power spectrum C _? of the sky signal directly from the visibilities measured in radio-interferometric observations. The salient features of the TGE are: (1) it deals with the gridded data which makes it computationally very fast, (2) it avoids a positive noise bias which normally arises from the system noise inherent to the visibility data, and (3) it allows us to taper the sky response and thereby suppresses the contribution from unsubtracted point sources in the outer parts and the side lobes of the antenna beam pattern. We also summarize earlier work where the TGE was used to measure the C _? of the DGSE using 150 MHz GMRT data. Earlier measurements of C _? are restricted to \(\ell \le \ell _{\max } \sim 10^{3}\) for the DGSE, the signal at the larger ? values is dominated by the residual point sources after source subtraction. The higher sensitivity of the upcoming SKA1 Low will allow the point sources to be subtracted to a fainter level than possible with existing telescopes. We predict that it will be possible to measure the C _? of the DGSE to larger values of \(\ell _{\max }\) with SKA1 Low. Our results show that it should be possible to achieve \(\ell _{\max }\sim 10^{4}\) and ～10⁵ with 2 minutes and 10 hours of observations respectively.  相似文献   

Deriving Crop Phenology Metrics and Their Trends Using Times Series NOAA-AVHRR NDVI Data     

Vinay Kumar Sehgal  Surabhi Jain  Pramod Kumar Aggarwal  Somnath Jha 《Journal of the Indian Society of Remote Sensing》2011,39(3):373-381

In this study, an attempt has been made to derive the spatial patterns of temporal trends in phenology metrics and productivity of crops grown, at disaggregated level in Indo-Gangetic Plains of India (IGP), which are helpful in understanding the impact of climatic, ecological and socio-economic drivers. The NOAA-AVHRR NDVI PAL dataset from 1981 to 2001 was stacked as per the crop year and subjected to Savitzky-Golay filtering. For crop pixels, maximum and minimum values of normalized difference vegetation index (NDVI), their time of occurrence and total duration of kharif (June-October) and rabi (November–April) crop seasons were derived for each crop year and later subjected to pixel-wise regression with time to derive the rate and direction of change. The maximum NDVI value showed increasing trends across IGP during both kharif and rabi seasons indicating a general increase in productivity of crops. The trends in time of occurrence of peak NDVI during kharif dominated with rice showed that the maximum vegetative growth stage was happening early with time during study period across most of Punjab, North Haryana, Parts of Central and East Uttar Pradesh and some parts of Bihar and West Bengal. Only central parts of Haryana showed a delay in occurrence of maximum vegetative stage with time. During rabi, no significant trends in occurrence of peak NDVI were observed in most of Punjab and Haryana except in South Punjab and North Haryana where early occurrence of peak NDVI with time was observed. Most parts of Central and Eastern Uttar Pradesh, North Bihar and West Bengal showed a delay in occurrence of peak NDVI with time. In general, the rice dominating system was showing an increase in duration with time in Punjab, Haryana, Western Uttar Pradesh, Central Uttar Pradesh and South Bihar whereas in some parts of North Bihar and West Bengal a decrease in the duration with time was also observed. During rabi season, except Punjab, the wheat dominating system was showing a decreasing trend in crop duration with time.  相似文献   

Timescales of crustal melting in the Higher Himalayan Crystallines (Sikkim, Eastern Himalaya) inferred from trace element-constrained monazite and zircon chronology   总被引：11，自引：0，他引：11  

Daniela Rubatto  Sumit Chakraborty  Somnath Dasgupta 《Contributions to Mineralogy and Petrology》2013,165(2):349-372

The petrology and timing of crustal melting has been investigated in the migmatites of the Higher Himalayan Crystalline (HHC) exposed in Sikkim, India. The metapelites underwent pervasive partial melting through hydrous as well as dehydration melting reactions involving muscovite and biotite to produce a main assemblage of quartz, K-feldspar, plagioclase, biotite, garnet ± sillimanite. Peak metamorphic conditions were 8–9 kbar and ~800 °C. Monazite and zircon crystals in several migmatites collected along a N–S transect show multiple growth domains. The domains were analyzed by microbeam techniques for age (SHRIMP) and trace element composition (LA-ICP-MS) to relate ages to conditions of formation. Monazite preserves the best record of metamorphism with domains that have different zoning pattern, composition and age. Zircon was generally less reactive than monazite, with metamorphic growth zones preserved in only a few samples. The growth of accessory minerals in the presence of melt was episodic in the interval between 31 and 17 Ma, but widespread and diachronous across samples. Systematic variations in the chemical composition of the dated mineral zones (HREE content and negative Eu anomaly) are related to the variation in garnet and K-feldspar abundances, respectively, and thus to metamorphic reactions and P–T stages. In turn, this allows prograde versus decompressional and retrograde melt production to be timed. A hierarchy of timescales characterizes melting which occurred over a period of ~15 Ma (31–17 Ma): a given block within this region traversed the field of melting in 5–7 Ma, whereas individual melting reactions lasted for time durations below, or approaching, the resolution of microbeam dating techniques (~0.6 Ma). An older ~36 Ma high-grade event is recorded in an allocthonous relict related to mafic lenses. We identify two sections of the HHC in Sikkim that traversed similar P–T conditions at different times, separated by a tectonic discontinuity. The higher structural levels reached melting and peak conditions later (~26–23 Ma) than the lower structural levels (~31–27 Ma). Diachronicity across the HHC cannot be reconciled with channel flow models in their simplest form, as it requires two similar high-grade sections to move independently during collision.  相似文献