Browsing by Author "Bharadwaj, Somnath"

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Characterizing Foreground for redshifted 21-cm radiation: 150 MHz GMRT observations
(2013-09-05) Ghosh, Abhik; Prasad, Jayanti; Bharadwaj, Somnath; Ali, Saiyad; Chengalur, Jayaram N.
Size of the longest filaments in the Universe
(2011-07-05) Bharadwaj, Somnath; Bhavsar, Suketu P.; Sheth, Jatush V.
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 −3o 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 the ﬁlament-void network. The reality of the 80 h−1 Mpc features in the −3o 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.
Using Gravitational Lensing to study damped Lyman - alpha clouds
(2000-03-29) Saini, Tarun Deep; Bharadwaj, Somnath; Sethi, Shiv K.
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 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 × 108 M⊙ ≤ MHI ≤ 2.5 × 1010 M⊙. The probability of detecting a cloud in- creases 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 fre- quencies. Observations at a detection threshold of 2 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 × 107 M⊙ ≤ MHI ≤ 109 M⊙. 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.