Jag är postdoktor vid Wenner-Gren vid Stockholms universitet och arbetar vid Institutionen för astronomi och Oskar Klein-centret.
Epoch of Reionization (EoR), Large-Scale Structure of the Universe, Cosmological N -body Simulations, Simulations of EoR 21-cm signal, Statistics of non-Gaussian cosmological signal, Dark Matter, Cosmic Microwave Background (CMB), Low-Frequency Radio Interferometry
I urval från Stockholms universitets publikationsdatabas
Interpreting LOFAR 21-cm signal upper limits at z ≈ 9.1 in the context of high-z galaxy and reionization observations
2021. Bradley Greig (et al.). Monthly notices of the Royal Astronomical Society 501 (1), 1-13Artikel
Using the latest upper limits on the 21-cm power spectrum at z approximate to 9.1 from the Low Frequency Array (LOFAR), we explore the regions of parameter space which are inconsistent with the data. We use 21CMMC, a Monte Carlo Markov chain sampler of 21CMFAST which directly forward models the three dimensional (3D) cosmic 21-cm signal in a fully Bayesian framework. We use the astrophysical parametrization from 21CMFAST, which includes mass-dependent star formation rates and ionizing escape fractions as well as soft-band X-ray luminosities to place limits on the properties of the high-z galaxies. Further, we connect the disfavoured regions of parameter space with existing observational constraints on the Epoch of Reionization such as ultra-violet (UV) luminosity functions, background UV photoionization rate, intergalactic medium (IGM) neutral fraction, and the electron scattering optical depth. We find that all models exceeding the 21-cm signal limits set by LOFAR at z approximate to 9.1 are excluded at greater than or similar to 2 sigma by other probes. Finally, we place limits on the IGM spin temperature from LOFAR, disfavouring at 95 per cent confidence spin temperatures below similar to 2.6 K across an IGM neutral fraction range of 0.15 less than or similar to (x) over bar (HI) less than or similar to 0.6. Note, these limits are only obtained from 141 h of data in a single redshift bin. With tighter upper limits, across multiple redshift bins expected in the near future from LOFAR, more viable models will be ruled out. Our approach demonstrates the potential of forward modelling tools such as 21CMMC in combining 21-cm observations with other high-z probes to constrain the astrophysics of galaxies.
Redshifted 21-cm bispectrum – I. Impact of the redshift space distortions on the signal from the Epoch of Reionization
2020. Suman Majumdar (et al.). Monthly notices of the Royal Astronomical Society 499 (4), 5090-5106Artikel
The bispectrum can quantify the non-Gussianity present in the redshifted 21-cm signal produced by the neutral hydrogen (HI) during the Epoch of Reionization (EoR). Motivated by this, we perform a comprehensive study of the EoR 21-cm bispectrum using simulated signals. Given a model of reionization, we demonstrate the behaviour of the bispectrum for all unique triangles in k space. For ease of identification of the unique triangles we parametrize the k-triangle space with two parameters, namely the ratio of the two arms of the triangle (n = k2/k1) and the cosine of the angle between them (cos θ). Furthermore, for the first time we quantify the impact of the redshift space distortions (RSD) on the spherically averaged EoR 21-cm bispectrum in the entire unique triangle space. We find that the real space signal bispectra for small and intermediate k1-triangles (k1≤0.6Mpc−1) is negative in most of the unique triangle space. It takes a positive sign for squeezed, stretched, and linear k1-triangles, specifically for large k1 values (k1≥0.6Mpc−1). The RSD affects both the sign and magnitude of the bispectra significantly. It changes (increases/decreases) the magnitude of the bispectra by 50−100 per cent without changing its sign (mostly) during the entire period of the EoR for small and intermediate k1-triangles. For larger k1-triangles, RSD affects the magnitude by 100−200 per cent and also flips the sign from negative to positive. We conclude that it is important to take into account the impact of RSD for a correct interpretation of the EoR 21-cm bispectra.
Tight constraints on the excess radio background at z=9.1 from LOFAR
2020. Rajesh Mondal (et al.). Monthly notices of the Royal Astronomical Society 498 (3), 4178-4191Artikel
The ARCADE2 and LWA1 experiments have claimed an excess over the cosmic microwave background (CMB) at low radio frequencies. If the cosmological high-redshift contribution to this radio background is between 0.1 per cent and 22 per cent of the CMB at 1.42 GHz, it could explain the tentative EDGES low-band detection of the anomalously deep absorption in the 21-cm signal of neutral hydrogen. We use the upper limit on the 21-cm signal from the Epoch of Reionization (z = 9.1) based on 141 h of observations with LOFAR to evaluate the contribution of the high-redshift Universe to the detected radio background. Marginalizing over astrophysical properties of star-forming haloes, we find (at 95 per cent CL) that the cosmological radio background can be at most 9.6 per cent of the CMB at 1.42 GHz. This limit rules out strong contribution of the high-redshift Universe to the ARCADE2 and LWA1 measurements. Even though LOFAR places limit on the extra radio background, excess of 0.1-9.6 per cent over the CMB (at 1.42 GHz) is still allowed and could explain the EDGES low-band detection. We also constrain the thermal and ionization state of the gas at z = 9.1, and put limits on the properties of the first star-forming objects. We find that, in agreement with the limits from EDGES high-band data, LOFAR data constrain scenarios with inefficient X-ray sources, and cases where the Universe was ionized by stars in massive haloes only.
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