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.Comment: 16 pages including 7 figures, published in JAp

The Effect of w−termw-term on Visibility Correlation and Power Spectrum Estimation

Visibility-visibility correlation has been proposed as a technique for the estimation of power spectrum, and used extensively for small field of view observations, where the effect of $w-term$ is usually ignored. We consider power spectrum estimation from the large field of view observations, where the $w-term$ can have a significant effect. Our investigation shows that a nonzero $w$ manifests itself as a modification of the primary aperture function of the instrument. Using a gaussian primary beam, we show that the modified aperture is an oscillating function with a gaussian envelope. We show that the two visibility correlation reproduces the power spectrum beyond a certain baseline given by the width, $U_{w}$ of the modified aperture. Further, for a given interferometer, the maximum $U_{w}$ remains independent of the frequencies of observation. This suggests that, the incorporation of large field of view in radio interferometric observation has a greater effect for larger observing wavelengths.Comment: 9 pages, 4 figures, 2 table

Dylan Thomas’s 18 Poems: The Poet’s Articulate Voice

In 18 Poems, Dylan Thomas’s search for rhymes around the poles is really a quest for the significant voice of poetry. At one level, the poem articulates the poet’s craving for home and the assurance that this resemblance of a home provides. But it reveals a deeper concern, a quest for and commitment to human reality; and for Thomas, reality is now identified with the paradoxical poetry of Yeats in contrast to Auden’s intellectual art. Linda M. Shires holds that “what is remarkable is the originality and intensity with which” his themes such as birth and death, process and decay, are introduced. To Walford Davies, Thomas’s early poetry, while offering “the reader only an impenetrable enigma” is “difficult and obscure in an individual way”. John Ackerman explains that the paradoxical attitude of Thomas in 18 Poems “occasions much of the obscurity … the images, however, are usually grouped by a sturdy advancing rhythm”. In the study of Thomas’s 18 Poems, the critics whose focal point is more on obscurity and musical setting hardly discuss his search for poetic image. Hence, this paper, adopting a figurative study, strives to unfold the meaning of the poet’s dramatic language suggestive of the Yeatsian articulate voice that contradicts the Word-centric articulate silence of Auden

What will anisotropies in the clustering pattern in redshifted 21 cm maps tell us?

The clustering pattern in high redshift HI maps is expected to be anisotropic due to two distinct reasons, the Alcock-Paczynski effect and the peculiar velocities, both of which are sensitive to the cosmological parameters. The signal is also expected to be sensitive to the details of the HI distribution at the epoch when the radiation originated. We use simple models for the HI distribution at the epoch of reionizaation and the post-reionization era to investigate exactly what we hope to learn from future observations of the anisotropy pattern in HI maps. We find that such observations will probably tell us more about the HI distribution than about the background cosmological model. Assuming that reionization can be described by spherical, ionized bubbles all of the same size with their centers possibly being biased with respect to the dark matter, we find that the anisotropy pattern at small angles is expected to have a bump at the characteristic angular size of the individual bubbles whereas the large scale anisotropy pattern will reflect the size and the bias of the bubbles. The anisotropy also depends on the background cosmological parameters, but the dependence is much weaker. Under the assumption that the HI in the post-reionization era traces the dark matter with a possible bias, we find that changing the bias and changing the background cosmology has similar effects on the anisotropy pattern. Combining observations of the anisotropy with independent estimates of the bias, possibly from the bi-spectrum, may allow these observations to constrain cosmological parameters.Comment: Minor changes, Accepted to MNRA