Search

We present a Wide-band tapered gridded estimator (TGE), which incorporates baseline migration and variation of the primary beam pattern for neutral hydrogen (⁠|$H\, {\small I}$|⁠) 21-cm intensity mapping (IM) with large frequency bandwidth radio-interferometric observations. Here we have analysed |$394-494 \, {\rm MHz}$| (z  = 1.9–2.6) uGMRT data to estimate the Multifrequency Angular Power Spectrum (MAPS) C ℓ(Δν) from which we have removed the foregrounds using the polynomial fitting (PF) and Gaussian Process Regression (GPR) methods developed in our earlier work. Using the residual C ℓ(Δν) to estimate the mean-squared 21-cm brightness temperature fluctuation Δ2(k), we find that this is consistent with 0 ± 2σ in several k bins. The resulting 2σ upper limit |$\Delta ^2(k) \lt (4.68)^2 \, \rm {mK^2}$| at |$k=0.219\, \rm {Mpc^{-1}}$| is nearly 15 times tighter than earlier limits obtained from a smaller bandwidth (⁠|$24.4 \, {\rm MHz}$|⁠) of the same data. The 2σ upper limit |$[\Omega _{H\, {\small I}} b_{H\, {\small I}}] \lt 1.01 \times 10^{-2}$| is within an order of magnitude of the value expected from independent estimates of the |$H\, {\small I}$| mass density |$\Omega _{H\, {\small I}}$| and the |$H\, {\small I}$| bias |$b_{H\, {\small I}}$|⁠. The techniques used here can be applied to other telescopes and frequencies, including |$\sim 150 \, {\rm MHz}$| Epoch of Reionization observations. [ABSTRACT FROM AUTHOR]

Neutral hydrogen (⁠|${\rm H\, \small {I}}$|⁠) 21-cm intensity mapping (IM) is a promising probe of the large-scale structures in the Universe. However, a few orders of magnitude brighter foregrounds obscure the IM signal. Here, we use the tapered gridded estimator to estimate the multifrequency angular power spectrum C ℓ(Δν) from a |$24.4\hbox{-} \rm {MHz}$| bandwidth upgraded Giant Metrewave Radio Telescope Band 3 data at |$432.8\ \rm {MHz}$|⁠. In C ℓ(Δν) foregrounds remain correlated across the entire Δν range, whereas the 21-cm signal is localized within Δν ≤ [Δν] (typically, 0.5–1 MHz). Assuming the range Δν > [Δν] to have minimal 21-cm signal, we use C ℓ(Δν) in this range to model the foregrounds. This foreground model is extrapolated to Δν ≤ [Δν], and subtracted from the measured C ℓ(Δν). The residual [ C ℓ(Δν)]res in the range Δν ≤ [Δν] is used to constrain the 21-cm signal, compensating for the signal loss from foreground subtraction. [ C ℓ(Δν)]res is found to be noise-dominated without any trace of foregrounds. Using [ C ℓ(Δν)]res, we constrain the 21-cm brightness temperature fluctuations Δ2(k), and obtain the 2σ upper limit |$\Delta _{\rm UL}^2(k)\le (18.07)^2\ \rm {mK^2}$| at |$k=0.247\ \rm {Mpc}^{-1}$|⁠. We further obtain the 2σ upper limit |$[\Omega _{{\rm H\, \small {I}}}b_{{\rm H\, \small {I}}}]_{\rm UL}\le 0.022$|⁠ , where |$\Omega _{{\rm H\, \small {I}}}$| and |$b_{{\rm H\, \small {I}}}$| are the comoving |${\rm H\, \small {I}}$| density and bias parameters, respectively. Although the upper limit is nearly 10 times larger than the expected 21-cm signal, it is 3 times tighter over previous works using foreground avoidance on the same data. [ABSTRACT FROM AUTHOR]

Neutral hydrogen (H  i) 21-cm intensity mapping (IM) offers an efficient technique for mapping the large-scale structures in the Universe. We introduce the 'Cross' Tapered Gridded Estimator (Cross TGE), which cross-correlates two cross-polarizations (RR and LL) to estimate the multifrequency angular power spectrum C ℓ(Δ ν). We expect this to mitigate several effects like noise bias, calibration errors, etc. which affect the 'Total' TGE that combines the two polarizations. Here, we apply the Cross TGE on |$24.4 \text{-} \rm {MHz}$| -bandwidth uGMRT (upgraded Giant Metrewave Radio Telescope) Band 3 data centred at |$432.8 \, \rm {MHz}$| aiming H  i IM at z  = 2.28. The measured C ℓ(Δ ν) is modelled to yield maximum likelihood estimates of the foregrounds and the spherical power spectrum P (k) in several k bins. Considering the mean squared brightness temperature fluctuations, we report a 2σ upper limit |$\Delta _{\mathrm{ UL}}^{2}(k) \le (58.67)^{2} \, {\rm mK}^{2}$| at |$k=0.804 \, {\rm Mpc}^{-1}$|⁠ , which is a factor of 5.2 improvement on our previous estimate based on the Total TGE. Assuming that the H  i traces the underlying matter distribution, we have modelled C ℓ(Δ ν) to simultaneously estimate the foregrounds and |$[\Omega _{\rm{ H}\, {\small {I}}} b_{\rm{ H}\, {\small {I}}}]$|⁠ , where |$\Omega _{\rm{ H}\, {\small {I}}}$| and |$b_{\rm{ H}\, {\small {I}}}$| are the H  i density and linear bias parameters, respectively. We obtain a best-fitting value of |$[\Omega _{\rm{ H}\, {\small {I}}}b_{\rm{ H}\, {\small {I}}}]^2 = 7.51\times 10^{-4} \pm 1.47\times 10^{-3}$| that is consistent with noise. Although the 2σ upper limit |$[\Omega _{\rm{ H}\, {\small {I}}}b_{\rm{ H}\, {\small {I}}}]_{\mathrm{ UL}} \le 0.061$| is ∼50 times larger than the expected value, this is a considerable improvement over earlier works at this redshift. [ABSTRACT FROM AUTHOR]

The post-reionization (z ≤ 6) neutral hydrogen (H  i) 21-cm intensity mapping signal holds the potential to probe the large-scale structures, study the expansion history, and constrain various cosmological parameters. Here, we apply the Tapered Gridded Estimator (TGE) to estimate P (k ⊥, k ∥) the power spectrum of the |$z = 2.28\, (432.8\, {\rm MHz})$| redshifted 21-cm signal using a |$24.4\, {\rm MHz}$| sub-band drawn from uGMRT Band 3 observations of European Large-Area ISO Survey-North 1 (ELAIS-N1). The TGE allows us to taper the sky response, which suppresses the foreground contribution from sources in the periphery of the telescope's field of view. We apply the TGE on the measured visibility data to estimate the multifrequency angular power spectrum (MAPS) C ℓ(Δν) from which we determine P (k ⊥, k ∥) using maximum likelihood that naturally overcomes the issue of missing frequency channels (55 per cent here). The entire methodology is validated using simulations. For the data, using the foreground avoidance technique, we obtain a |$2\, \sigma$| upper limit of |$\Delta ^2(k) \le (133.97)^2 \, {\rm mK}^{2}$| for the 21-cm brightness temperature fluctuation at |$k = 0.347 \, \textrm {Mpc}^{-1}$|⁠. This corresponds to |$[\Omega _{\rm H\, {\small I}~}b_{\rm H\, {\small I}~}] \le 0.23$|⁠ , where |$\Omega _{\rm H\, {\small I}~}$| and |$b_{\rm H\, {\small I}~}$|⁠ , respectively, denote the cosmic H  i  mass density and the H  i  bias parameter. A previous work has analysed |$8 \, {\rm MHz}$| of the same data at z  = 2.19, and reported |$\Delta ^{2}(k) \le (61.49)^{2} \, {\rm mK}^{2}$| and |$[\Omega _{\rm H\, {\small I}~} b_{\rm H\, {\small I}~}] \le 0.11$| at |$k=1 \, {\rm Mpc}^{-1}$|⁠. The upper limits presented here are still orders of magnitude larger than the expected signal corresponding to |$\Omega _{\rm H\, {\small I}~} \sim 10^{-3}$| and |$b_{\rm H\, {\small I}~} \sim 2$|⁠. [ABSTRACT FROM AUTHOR]

Characterizing the diffuse Galactic synchrotron emission at arcminute angular scales is needed to reliably remove foregrounds in cosmological 21-cm measurements. The study of this emission is also interesting in its own right. Here, we quantify the fluctuations of the diffuse Galactic synchrotron emission using visibility data for two of the fields observed by the TIFR GMRT Sky Survey. We have used the 2D Tapered Gridded Estimator to estimate the angular power spectrum (Cl) from the visibilities. We find that the sky signal, after subtracting the point sources, is likely dominated by the diffuse Galactic synchrotron radiation across the angular multipole range 240 ≤ l ≤ 500. We present a power-law fit, Cl=Ax(1000l)β, to the measured Cl over this l range. We find that (A, β) have values (356 ± 109 mK², 2.8 ± 0.3) and (54 ± 26 mK², 2.2 ± 0.4) in the two fields. For the second field, however, there is indication of a significant residual point source contribution and for this field we interpret the measured Cl as an upper limit for the diffuse Galactic synchrotron emission. While in both fields the slopes are consistent with earlier measurements, the second field appears to have an amplitude that is considerably smaller compared to similar measurements in other parts of the sky. [ABSTRACT FROM AUTHOR]

We present an improved visibility-based tapered gridded estimator (TGE) for the power spectrum of the diffuse sky signal. The visibilities are gridded to reduce the total computation time for the calculation, and tapered through a convolution to suppress the contribution from the outer regions of the telescope's field of view. The TGE also internally estimates the noise bias, and subtracts this out to give an unbiased estimate of the power spectrum. An earlier version of the 2D TGE for the angular power spectrum Cl is improved and then extended to obtain the 3D TGE for the power spectrum P(k) of the 21-cm brightness temperature fluctuations. Analytic formulas are also presented for predicting the variance of the binned power spectrum. The estimator and its variance predictions are validated using simulations of 150-MHz Giant Metrewave Radio Telescope (GMRT) observations. We find that the estimator accurately recovers the input model for the 1D spherical power spectrum P(k) and the 2D cylindrical power spectrum P(k⊥, k||), and that the predicted variance is in reasonably good agreement with the simulations. [ABSTRACT FROM AUTHOR]

It is important to correctly subtract point sources from radio-interferometric data in order to measure the power spectrum of diffuse radiation like the Galactic synchrotron or the Epoch of Reionization 21-cm signal. It is computationally very expensive and challenging to image a very large area and accurately subtract all the point sources from the image. The problem is particularly severe at the sidelobes and the outer parts of the main lobe where the antenna response is highly frequency dependent and the calibration also differs from that of the phase centre. Here, we show that it is possible to overcome this problem by tapering the sky response. Using simulated 150MHz observations, we demonstrate that it is possible to suppress the contribution due to point sources from the outer parts by using the Tapered Gridded Estimator to measure the angular power spectrum Cl of the sky signal. We also show from the simulation that this method can self-consistently compute the noise bias and accurately subtract it to provide an unbiased estimation of Cl. [ABSTRACT FROM AUTHOR]

We present two estimators to quantify the angular power spectrum of the sky signal directly from the visibilities measured in radio interferometric observations. This is relevant for both the foregrounds and the cosmological 21-cm signal buried therein. The discussion here is restricted to the Galactic synchrotron radiation, the most dominant foreground component after point source removal. Our theoretical analysis is validated using simulations at 150 MHz, mainly for GMRT and also briefly for Low-Frequency Array. The Bare Estimator uses pairwise correlations of the measured visibilities, while the Tapered Gridded Estimator uses the visibilities after gridding in the uv plane. The former is very precise, but computationally expensive for large data. The latter has a lower precision, but takes less computation time which is proportional to the data volume. The latter also allows tapering of the sky response leading to sidelobe suppression, an useful ingredient for foreground removal. Both estimators avoid the positive bias that arises due to the system noise. We consider amplitude and phase errors of the gain, and the w-term as possible sources of errors. We find that the estimated angular power spectrum is exponentially sensitive to the variance of the phase errors but insensitive to amplitude errors. The statistical uncertainties of the estimators are affected by both amplitude and phase errors. The w-term does not have a significant effect at the angular scales of our interest. We propose the Tapered Gridded Estimator as an effective tool to observationally quantify both foregrounds and the cosmological 21-cm signal. [ABSTRACT FROM AUTHOR]

ABSTRACT Foreground removal is a major challenge for detecting the redshifted 21 cm neutral hydrogen (H i) signal from the Epoch of Reionization. We have used 150 MHz Giant Metrewave Radio Telescope observations to characterize the statistical properties of the foregrounds in four different fields of view. The measured multifrequency angular power spectrum Cℓ(Δν) is found to have values in the range 104-2 × 104 mK2 across 700 ≤ ℓ ≤ 2 × 104 and Δν ≤ 2.5 MHz, which is consistent with model predictions where point sources are the most dominant foreground component. The measured Cℓ(Δν) does not show a smooth Δν dependence, which poses a severe difficulty for foreground removal using polynomial fitting. The observational data were used to assess point source subtraction. Considering the brightest source (∼1 Jy) in each field, we find that the residual artefacts are less than 1.5 per cent in the most sensitive field (FIELD I). Considering all the sources in the fields, we find that the bulk of the image is free of artefacts, the artefacts being localized to the vicinity of the brightest sources. We have used FIELD I, which has an rms noise of 1.3 mJy beam−1, to study the properties of the radio source population to a limiting flux of 9 mJy. The differential source count is well fitted with a single power law of slope −1.6. We find there is no evidence for flattening of the source counts towards lower flux densities which suggests that source population is dominated by the classical radio-loud active galactic nucleus. The diffuse Galactic emission is revealed after the point sources are subtracted out from FIELD I. We find Cℓ ∝ ℓ−2.34 for 253 ≤ ℓ ≤ 800 which is characteristic of the Galactic synchrotron radiation measured at higher frequencies and larger angular scales. We estimate the fluctuations in the Galactic synchrotron emission to be [ABSTRACT FROM AUTHOR]

ABSTRACT Foreground removal is a challenge for 21-cm tomography of the high-redshift Universe. We use archival Giant Metrewave Radio Telescope (GMRT) data (obtained for completely different astronomical goals) to estimate the foregrounds at a redshift of ∼1. The statistic we use is the cross power spectrum between two frequencies separated by Δν at the angular multipole ℓ, or equivalently the multi-frequency angular power spectrum Cℓ(Δν). An earlier measurement of Cℓ(Δν) using these data had revealed the presence of oscillatory patterns along Δν, which turned out to be a severe impediment for foreground removal. Using the same data, in this paper we show that it is possible to considerably reduce these oscillations by suppressing the sidelobe response of the primary antenna elements. The suppression works best at the angular multipoles ℓ for which there is a dense sampling of the u− v plane. For three angular multipoles ℓ= 1405, 1602 and 1876, this sidelobe suppression along with a low order polynomial fitting completely results in residuals of (≤ 0.02 mK2), consistent with the noise at the 3σ level. Since the polynomial fitting is done after estimation of the power spectrum it can be ensured that the estimation of the H i signal is not biased. The corresponding 99 per cent upper limit on the H i signal is , where is the mean neutral fraction and b is the bias. [ABSTRACT FROM AUTHOR]

The redshifted 21-cm signal from neutral hydrogen (H ) is an important future probe of the high-redshift Universe. We have analysed Giant Metrewave Radio Telescope (GMRT) observations towards detecting this signal from . The multi-frequency angular power spectrum is used to characterize the statistical properties of the background radiation across angular scales arcsec to 10 arcmin, and a frequency bandwidth of with resolution . The measured which ranges from 7 to is dominated by foregrounds, the expected H signal is several orders of magnitude smaller and detecting this is a big challenge. The foregrounds, believed to originate from continuum sources, is expected to vary smoothly with whereas the H signal decorrelates within , and this holds the promise of separating the two. For each ℓ, we use the interval to fit a fourth-order polynomial which is subtracted from the measured to remove any smoothly varying component across the entire bandwidth . The residual , we find, has an oscillatory pattern with amplitude and period, respectively, and at the smallest ℓ value of 1476, and the amplitude and period decreasing with increasing ℓ. Applying a suitably chosen high pass filter, we are able to remove the residual oscillatory pattern for where the residual is now consistent with zero at the 3σ noise level. Based on this we conclude that we have successfully removed the foregrounds at and the residuals are consistent with noise. We use this to place an upper limit on the H signal whose amplitude is determined by , where and b are the H neutral fraction and the H bias, respectively. A value of greater than 7.95 would have been detected in our observation, and is therefore ruled out at the 3σ level. For comparison, studies of quasar absorption spectra indicate which is times smaller than our upper limit. We have not succeeded in completely removing the residual oscillatory pattern, whose cause is presently unknown to us, for the larger ℓ values. [ABSTRACT FROM AUTHOR]

Foreground subtraction is the biggest challenge for future redshifted 21-cm observations to probe reionization. We use a short Giant Meter Wave Radio Telescope (GMRT) observation at 153 MHz to characterize the statistical properties of the background radiation across ∼1° to subarcmin angular scales, and across a frequency band of 5 MHz with 62.5 kHz resolution. The statistic we use is the visibility correlation function, or equivalently the angular power spectrum . We present the results obtained from using relatively unsophisticated, conventional data calibration procedures. We find that even fairly simple-minded calibration allows one to estimate the visibility correlation function at a given frequency . From our observations, we find that is consistent with foreground model predictions at all angular scales except the largest ones probed by our observations where the model predictions are somewhat in excess. On the other hand, the visibility correlation between different frequencies seems to be much more sensitive to calibration errors. We find a rapid decline in , in contrast with the prediction of less than 1 per cent variation across 2.5 MHz. In this case, however, it seems likely that a substantial part of the discrepancy may be due to limitations of data reduction procedures. [ABSTRACT FROM AUTHOR]

The clustering pattern in high-redshift H i maps is expected to be anisotropic for 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 H i distribution at the epoch when the radiation originated. We use simple models for the H i distribution at the epoch of reionization and the post-reionization era to investigate exactly what we hope to learn from future observations of the anisotropy pattern in H i maps. We find that such observations will probably tell us more about the H i distribution than about the background cosmological model. Assuming that reionization can be described by spherical, ionized bubbles all of the same size with their centres 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 H i in the post-reionization era traces the dark matter with a possible bias, we find that changing the bias and changing the background cosmology have 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. [ABSTRACT FROM AUTHOR]

Redshifted 21-cm radiation originating from the cosmological distribution of neutral hydrogen (H i) appears as background radiation in low-frequency radio observations. The angular and frequency domain fluctuations in this radiation carry information concerning cosmological structure formation. We propose that correlations between visibilities measured at different baselines and frequencies in radio-interferometric observations be used to quantify the statistical properties of these fluctuations. This has an inherent advantage over other statistical estimators in that it deals directly with the visibilities which are the primary quantities measured in radio-interferometric observations. Also, the visibility correlation has a very simple relation with the power spectrum. We present estimates of the expected signal for nearly the entire post-recombination era, from the dark ages to the present epoch. The epoch of reionization, where H ihas a patchy distribution, has a distinct signature where the signal is determined by the size of the discrete ionized regions. The signal at other epochs, where H ifollows the dark matter, is determined largely by the power spectrum of dark matter fluctuations. The signal is strongest for baselines where the antenna separations are within a few hundred times the wavelength of observation, and an optimal strategy would preferentially sample these baselines. In the frequency domain, for most baselines the visibilities at two different frequencies are uncorrelated beyond, a signature which, in principle, would allow the H isignal to be easily distinguished from the continuum sources of contamination. [ABSTRACT FROM AUTHOR]

Loeb and Zaldarriaga have recently proposed that observations of the cosmic microwave background radiation (CMBR) brightness temperature fluctuations produced by H i 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 H i optical depth, both of which are caused by density perturbations. For the spin temperature, we investigate in detail its evolution in the presence of H i 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 per cent or more in the brightness temperature fluctuations. [ABSTRACT FROM AUTHOR]