Shiv K. Sethi

Foregrounds in wide-field redshifted 21 cm power spectra

Detection of 21 cm emission of H I from the epoch of reionization, at redshifts > z 6, is limited primarily by foreground emission. We investigate the signatures of wide-field measurements and an all-sky foreground model using the delay spectrum technique that maps the measurements to foreground object locations through signal delays between antenna pairs. We demonstrate interferometric measurements are inherently sensitive to all scales, including the largest angular scales, owing to the nature of wide-field measurements. These wide-field effects are generic to all observations but antenna shapes impact their amplitudes substantially. A dish-shaped antenna yields the most desirable features from a foreground contamination viewpoint, relative to a dipole or a phased array. Comparing data from recent Murchison Widefield Array observations, we demonstrate that the foreground signatures that have the largest impact on the H I signal arise from power received far away from the primary field of view. We identify diffuse emission near the horizon as a significant contributing factor, even on wide antenna spacings that usually represent structures on small scales. For signals entering through the primary field of view, compact emission dominates the foreground contamination. These two mechanisms imprint a characteristic pitchfork signature on the “foreground wedge” in Fourier delay space. Based on these results, we propose that selective down-weighting of data based on antenna spacing and time can mitigate foreground contamination substantially by a factor of ∼100 with negligible loss of sensitivity.

Photoelectric heating for dust grains at high redshifts

Lyman-α absorption systems at z ∼ 3 with NH I≥ 3 × 1014 cm-2 have been found to be enriched with a mean metallicity of Z/ Z⊙∼ 10-2.5, and a large scatter in the metallicity. It is reasonable to assume that the process of initial enrichment of the intergalactic medium (IGM) at z ≥ 3 also produced dust grains. We explore the implications of the presence of dust grains in the IGM at high redshift, in particular, the contribution of photoelectric emission from grains by hard background photons to the net heating rate of the IGM. We show that (i) the charge on dust particles and the characteristics of photoemitted electrons differ substantially from those in the interstellar medium (ISM) in several respects: (a) grains are exposed to and charged by photons beyond the Lyman limit, and (b) because of this, the photoelectrons have typical energy of tens of eV. We also show that: (ii) silicates are more efficient heating agents than graphites; (iii) small grains contribute mostly to the net heating; (iv) at densities typical of the IGM at z ∼ 3 and for Ly α absorbers, dust heating can be comparable to or exceed photoionization heating within an order of magnitude; and (v) this increases the temperature of overdense regions, compared to the case of no dust heating, by a factor of ∼ 2. We discuss the implications of this extra heating source in Ly α absorbing systems.

The Low-Frequency Environment of the Murchison Widefield Array: Radio-Frequency Interference Analysis and Mitigation

This is the Accepted Manuscript version of the following article: A. R. Offringa, et al., “The low-frequency environment of the Murchison Widefield Array: radio-frequency interference analysis and mitigation”, Publications of the Astronomical Society of Australia, Vol. 32, March 2015. The final published version is available at: https://doi.org/10.1017/pasa.2015.7

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 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 〈Vv(U) Vv′(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.

Dissipative fluid in Brans-Dicke theory and late time acceleration

We investigate the possibility of having a late time accelerated expansion phase for the universe. We use a dissipative fluid in Brans-Dicke (BD) theory for this purpose. The model does not involve any potential for the BD scalar field. We obtain the best fit values for the different parameters in our model by comparing our model predictions with SNIa data and also with the data from the ultracompact radio sources.

H i signal from re-ionization epoch

We investigate the all-sky signal in redshifted atomic hydrogen (H I) line from the re-ionization epoch. We model the phase of re-ionization as multiple point sources that carve out spherical Stromgren spheres. We study ionization histories compatible with Wilkinson Microwave Anisotropy Probe (WMAP) observation. The Lyman a and soft X-ray emission from these sources are taken into account for studying the H i signal. H i can be observed in both emission and absorption depending on the ratio of Lyman α to ionizing flux and the spectrum of the radiation in soft X-ray. We also compute the signal from pre-reionization epoch and show that within the uncertainty in cosmological parameters, it is fairly robust. The main features of H i signal can be summarized as follows. (i) The pre-ionized H I can be seen in absorption for v ≃ 10-40 MHz; the maximum signal strength is ≃70-100 mK. (ii) A sharp absorption feature of width?5 MHz might be observed in the frequency range ≃50-100 MHz, depending on the re-ionization history. The strength of the signal is proportional to the ratio of the Lyman a and the hydrogen-ionizing flux and the spectral index of the radiation field in soft X-ray. (iii) At larger frequencies, HI is seen in emission with peak frequency between 60 and 100 MHz, depending on the ionization history of the Universe; the peak strength of this signal is ≃50 mK. From Fisher matrix analysis, we compute the precision with which the parameters of the model can be estimated from a future experiment as follows: (i) the pre-reionization signal can constrain a region in the Ω b h 2 -Ω m h 2 plane (ii) H I observed in emission can be used to give precise,?1 per cent, measurement of the evolution of the ionization fraction in the Universe and (iii) the transition region from absorption to emission can be used as a probe of the spectrum of ionizing sources; in particular, the Hi signal in this regime can give a reasonably precise measurement of the fraction of the Universe heated by soft X-ray photons.

Using HI to probe large scale structures at z∼3

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.

Parametrizing Epoch of Reionization foregrounds: a deep survey of low-frequency point-source spectra with the Murchison Widefield Array

Experiments that pursue detection of signals from the Epoch of Reionization (EoR) are relying on spectral smoothness of source spectra at low frequencies. This article empirically explores the effect of foreground spectra on EoR experiments by measuring high-resolution full-polarization spectra for the 586 brightest unresolved sources in one of the MWA EoR fields using 45 h of observation. A novel peeling scheme is used to subtract 2500 sources from the visibilities with ionospheric and beam corrections, resulting in the deepest, confusion-limited MWA image so far. The resulting spectra are found to be affected by instrumental effects, which limit the constraints that can be set on source-intrinsic spectral structure. The sensitivity and power-spectrum of the spectra are analysed, and it is found that the spectra of residuals are dominated by PSF sidelobes from nearby undeconvolved sources. We release a catalogue describing the spectral parameters for each measured source.

The importance of wide-field foreground removal for 21 cm cosmology: a demonstration with early MWA epoch of reionization observations

In this paper we present observations, simulations, and analysis demonstrating the direct connection between the location of foreground emission on the sky and its location in cosmological power spectra from interferometric redshifted 21 cm experiments. We begin with a heuristic formalism for understanding the mapping of sky coordinates into the cylindrically averaged power spectra measurements used by 21 cm experiments, with a focus on the effects of the instrument beam response and the associated sidelobes. We then demonstrate this mapping by analyzing power spectra with both simulated and observed data from the Murchison Widefield Array. We find that removing a foreground model which includes sources in both the main field-of-view and the first sidelobes reduces the contamination in high k_parallel modes by several percent relative to a model which only includes sources in the main field-of-view, with the completeness of the foreground model setting the principal limitation on the amount of power removed. While small, a percent-level amount of foreground power is in itself more than enough to prevent recovery of any EoR signal from these modes. This result demonstrates that foreground subtraction for redshifted 21 cm experiments is truly a wide-field problem, and algorithms and simulations must extend beyond the main instrument field-of-view to potentially recover the full 21 cm power spectrum.

First Season MWA EoR Power Spectrum Results at Redshift 7

The Murchison Widefield Array (MWA) has collected hundreds of hours of Epoch of Reionization (EoR) data and now faces the challenge of overcoming foreground and systematic contamination to reduce the data to a cosmological measurement. We introduce several novel analysis techniques such as cable reflection calibration, hyper-resolution gridding kernels, diffuse foreground model subtraction, and quality control methods. Each change to the analysis pipeline is tested against a two dimensional power spectrum figure of merit to demonstrate improvement. We incorporate the new techniques into a deep integration of 32 hours of MWA data. This data set is used to place a systematic-limited upper limit on the cosmological power spectrum of