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Tirthankar Roy Choudhury

Publications and source records attributed to Tirthankar Roy Choudhury.

At least 73 records · Page 4Linked to original sources
arXiv2019

Constraints on dark matter annihilation in dwarf spheroidal galaxies from low frequency radio observations

We present the first observational limits on the predicted synchrotron signals from particle Dark Matter annihilation models in dwarf spheroidal galaxies at radio frequencies below 1 GHz. We use a combination of survey data from the Murchison Widefield Array (MWA) and the Giant Metre-wave Radio Telescope (GMRT) to search for diffuse radio emission from 14 dwarf spheroidal galaxies. For in-situ magnetic fields of 1 $μG$ and any plausible value for the diffusion coefficient, our limits do not constrain any Dark Matter models. However, for stronger magnetic fields our data might provide constraints comparable to existing limits from gamma-ray and cosmic ray observations. Predictions for the sensitivity of the upgraded MWA show that models with Dark Matter particle mass up to $\sim$ 1.6 TeV (1 TeV) may be constrained for magnetic field of 2 $μG$ (1 $μG$). While much deeper limits from the future low frequency Square Kilometre Array (SKA) will challenge the LHC in searches for Dark Matter particles, the MWA provides a valuable first step toward the SKA at low frequencies.

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arXiv2019

First study of reionization in tilted flat and untilted non-flat dynamical dark energy inflation models

We examine the effects of dark energy dynamics and spatial curvature on cosmic reionization by studying reionization in tilted spatially-flat and untilted non-flat XCDM and $ϕ$CDM dynamical dark energy inflation models that best fit the Planck 2015 cosmic microwave background (CMB) anisotropy and a large compilation of non-CMB data. We carry out a detailed statistical study, based on a principal component analysis and a Markov chain Monte Carlo analysis of a compilation of lower-redshift reionization data, to estimate the uncertainties in the cosmological model reionization histories. We find that, irrespective of the nature of dark energy, there are significant differences between the reionization histories of the spatially-flat and non-flat models. Although both the flat and non-flat models can accurately match the low-redshift ($z\lesssim6$) reionization observations, there is a clear discrepancy between high-redshift ($z>7$) Lyman-$α$ emitter data and the predictions from non-flat models. This is solely due to the fact that the non-flat models have a significantly larger electron scattering optical depth, $τ_{\rm el}$, compared to the flat models, which requires an extended and much earlier reionization scenario supported by more high-redshift ionizing sources in the non-flat models. Non-flat models also require strong redshift evolution in the photon escape fraction, that can become unrealistically high ($\gtrsim1$) at some redshifts. However, $τ_{\rm el}$ is about 0.9-$σ$ lower in the tilted flat $Λ$CDM model when the new Planck 2018 data are used and this reduction will partially alleviate the tension between the non-flat model predictions and the data.

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arXiv2019

Detailed study of the ELAIS N1 field with the uGMRT - I. Characterizing the 325 MHz foreground for redshifted 21 cm observations

In this first paper of the series, we present initial results of newly upgraded Giant Meterwave Radio Telescope (uGMRT) observation of European Large-Area ISO Survey-North 1 (ELAIS-N1) at 325 MHz with 32 MHz bandwidth. Precise measurement of fluctuations in Galactic and extragalactic foreground emission as a function of frequency as well as angular scale is necessary for detecting redshifted 21-cm signal of neutral hydrogen from Cosmic Dawn, Epoch of Reionization (EoR) and post-reionization epoch. Here, for the first time we have statistically quantified the Galactic and extragalactic foreground sources in the ELAIS-N1 field in the form of angular power spectrum using the newly developed Tapered Gridded Estimator (TGE). We have calibrated the data with and without direction-dependent calibration techniques. We have demonstrated the effectiveness of TGE against the direction dependent effects by using higher tapering of field of view (FoV). We have found that diffuse Galactic synchrotron emission (DGSE) dominates the sky, after point source subtraction, across the angular multipole range $ 1115 \leqslant \mathcal{\ell} \leqslant 5083 $ and $ 1565 \leqslant \mathcal{\ell} \leqslant 4754 $ for direction-dependent and -independent calibrated visibilities respectively. The statistical fluctuations in DGSE has been quantified as a power law of the form $\mathcal{C}_{\mathcal{\ell}}= A \mathcal{\ell}^{-β} $. The best fitted values of (A, $β$) are ($ 62 \pm 6$ $mK^{2}$, $2.55 \pm 0.3 $) and ($ 48 \pm 4$ $mK^{2}$, $2.28 \pm 0.4 $ ) for the two different calibration approaches. For both the cases, the power law index is consistent with the previous measurements of DGSE in other parts of sky.

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arXiv2019

Ruling out 3 keV warm dark matter using 21 cm-EDGES data

Weakly interacting cold dark matter (CDM) particles, which are otherwise extremely successful in explaining various cosmological observations, exhibit a number of problems on small scales. One possible way of solving these problems is to invoke (so-called) warm dark matter (WDM) particles with masses $m_x \sim$ keV. Since the formation of structure is delayed in such WDM models, it is natural to expect that they can be constrained using observations related to the first stars, e.g., the 21 cm signal from cosmic dawn. In this work, we use a detailed galaxy formation model, Delphi, to calculate the 21 cm signal at high-redshifts and compare this to the recent EDGES observations. We find that while CDM and 5 keV WDM models can obtain a 21 cm signal within the observed redshift range, reproducing the amplitude of the observations requires the introduction of an excess radio background. On the other hand, WDM models with $m_x \leq 3$ keV can be ruled out since they are unable to match either the redshift range or the amplitude of the EDGES signal, irrespective of the parameters used. Comparable to values obtained from the low-redshift Lyman Alpha forest, our results extend constraints on the WDM particle to an era inaccessible by any other means; additional forthcoming 21 cm data from the era of cosmic dawn will be crucial in refining such constraints.

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arXiv2019

Is patchy reionization an obstacle in detecting the primordial gravitational wave signal?

The large-scale CMB B-mode polarization is the direct probe to the low frequency primordial gravitational wave signal. However, unambiguous measurement of this signal requires a precise understanding of the possible contamination. One such potential contamination arises from the patchiness in the spatial distribution of free electrons during the epoch of reionization. We estimate the B-mode power spectrum due to patchy reionization using a combination of \emph{photon-conserving} semi-numerical simulation and analytical calculation, and compare its amplitude with the primordial B-mode signal. For a reionization history which is in agreement with several latest observations, we find that a stronger secondary B-mode polarization signal is produced when the reionization is driven by the sources in massive halos and its amplitude can be comparable to the recombination bump for tensor to scalar ratio $(r) \lesssim 5 \times 10^{-4}$. If contamination from patchy reionization is neglected in the analysis of B-mode polarization data, then for the models of reionization considered in this analysis, we find a maximum bias of about $30\%$ in the value of $r=\,10^{-3}$ when spatial modes between $\ell \in [50, 200]$ are used with a delensing efficiency of $50\%$. The inferred bias from patchy reionization is not a severe issue for the upcoming ground-based CMB experiment Simons Observatory, but can be a potential source of confusion for proposed CMB experiments which target to detect the value of $r< 10^{-3}$. However, this obstacle can be removed by utilizing the difference in the shape of the power spectrum from the primordial signal.

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arXiv2019

The hierarchical assembly of galaxies and black holes in the first billion years: predictions for the era of gravitational wave astronomy

In this work we include black hole (BH) seeding, growth and feedback into our semi-analytic galaxy formation model, Delphi. Our model now fully tracks the, accretion- and merger-driven, hierarchical assembly of the dark matter halo, baryonic and BH masses of high-redshift ($z>5$) galaxies. We use a minimal set of mass- and $z$-independent free parameters associated with star formation and BH growth (and feedback) and include suppressed BH growth in low-mass galaxies to explore a number of physical scenarios including: (i) two types of BH seeds (stellar and those from Direct Collapse BH; DCBH); (ii) the impact of reionization feedback; and (iii) the impact of instantaneous versus delayed galaxy mergers on the baryonic growth. While both reionization feedback and delayed galaxy mergers have no sensible impact on the evolving ultra-violet luminosity function, the latter limits the maximum BH masses achieved at these high-$z$. We then use this model, baselined against all available high-$z$ galaxy and BH data-sets, to predict the LISA detectability of merger events at $z > 5$. As expected, the merger rate is dominated by stellar BH mergers for all scenarios and our model predicts an expected upper limit of about 20 mergers in the case of instantaneous merging and no reionization feedback over the 4-year mission duration. Including the impact of delayed mergers and reionization feedback reduces this to about 12 events over the same observational time-scale.

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arXiv2019

Can SKA--Phase 1 go much beyond the LHC in supersymmetry search?

We study the potential of the Square Kilometre Array in the first phase (SKA1) in detecting dark matter annihilation signals from dwarf spheroidals in the form of diffuse radio synchrotron. Taking the minimal supersymmetric standard model as illustration, we show that it is possible to detect such signals for dark matter masses about an order of magnitude beyond the reach of the Large Hadron Collider, with about 100 hours of observation with the SKA1.

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arXiv2018

Probing the fluctuating Ultra-violet background using the Hubble Frontier Fields

In recent years, the rise in the number of Lyman Break Galaxies detected at high redshifts z >= 6 has opened up the possibility of understanding early galaxy formation physics in great detail. In particular, the faint-end slope (alpha) of the Ultra-violet luminosity function (UV LF) of these galaxies is a potential probe of feedback effects that suppress star formation in low mass haloes. In this work, we propose a proof-of-concept calculation for constraining the fluctuating UV background during reionization by constraining alpha in different volumes of the Universe. Because of patchy reionization, different volumes will experience different amount of photo-heating which should lead to a scatter in the measured alpha. Our approach is based on a simple model of the UV LF that is a scaled version of the halo mass function combined with an exponential suppression in the galaxy luminosity at the faint-end because of UV feedback. Although current data is not sufficient to constrain alpha in different fields, we expect that, in the near future, observations of the six lensed Hubble Frontier Fields with the James Webb Space Telescope (JWST) will offer an ideal test of our concept.

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arXiv2018

First study of reionization in the Planck 2015 normalized closed $Λ$CDM inflation model

We study reionization in two non-flat $Λ$CDM inflation models that best fit the Planck 2015 cosmic microwave background anisotropy observations, ignoring or in conjunction with baryon acoustic oscillation distance measurements. We implement a principal component analysis (PCA) to estimate the uncertainties in the reionization history from a joint quasar-CMB dataset. A thorough Markov Chain Monte Carlo analysis is done over the parameter space of PCA modes for both non-flat $Λ$CDM inflation models as well as the original Planck 2016 tilted, spatially-flat $Λ$CDM inflation model. Although both flat and non-flat models can closely match the low-redshift ($z\lesssim6$) observations, we notice a possible tension between high-redshift ($z\sim8$) Lyman-$α$ emitter data and the non-flat models. This is solely due to the fact that the closed models have a relatively higher reionization optical depth compared to the flat one, which in turn demands more high-redshift ionizing sources and favors an extended reionization starting as early as $z\approx14$. We conclude that as opposed to flat-cosmology, for the non-flat cosmology models (i) the escape fraction needs steep redshift evolution and even unrealistically high values at some redshifts and (ii) most of the physical parameters require to have non-monotonic redshift evolution, especially apparent when Lyman-$α$ emitter data is included in the analysis.

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arXiv2018

Unbiased constraints on reionization model parameters in presence of the foreground wedge

A possible way to disentangle the redshifted 21 cm signal of neutral hydrogen (HI) in the Epoch of Reionization (EoR) from the much larger astrophysical foregrounds is to restrict the analysis to a wedge-shaped region in the Fourier space. The foregrounds are confined to this wedge because of their smooth spectral properties which allows one to estimate the HI power spectrum in the foreground-free portion, known as the reionization window. The estimate of the spherically averaged power spectrum in the window, however, differs from the true value because of the line of sight anisotropies in the signal. The difference can be estimated and taken into account for a given reion- ization model by expanding the power spectrum in terms of the Legendre polynomials. This corrected power spectrum, called as the clustering wedges, can then be used for comparing the model predictions with the observations. In this work, we carefully ex- amine whether the clustering wedges are appropriate for such model comparisons and whether they provide truly unbiased constraints on the EoR parameters. We use the excursion-set based semi-numerical simulations, coupled with MCMC-based statistical methods, for our analysis. We find that the clustering wedges not only yield faithful constraints, the statistical uncertainties on the parameter values too are comparable to those obtained using the data without the presence of any foregrounds. We also find that the clustering wedges are helpful in breaking the degeneracies between different EoR model parameters.

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arXiv2018

Measuring the reionization 21 cm fluctuations using clustering wedges

One of the main challenges in probing the reionization epoch using the redshifted 21 cm line is that the magnitude of the signal is several orders smaller than the astrophysical foregrounds. One of the methods to deal with the problem is to avoid a wedge-shaped region in the Fourier $k_{\perp} - k_{\parallel}$ space which contains the signal from the spectrally smooth foregrounds. However, measuring the spherically averaged power spectrum using only modes outside this wedge (i.e., in the reionization window), leads to a bias. We provide a prescription, based on expanding the power spectrum in terms of the shifted Legendre polynomials, which can be used to compute the angular moments of the power spectrum in the reionization window. The prescription requires computation of the monopole, quadrupole and hexadecapole moments of the power spectrum using the theoretical model under consideration and also the knowledge of the effective extent of the foreground wedge in the $k_{\perp} - k_{\parallel}$ plane. One can then calculate the theoretical power spectrum in the window which can be directly compared with observations. The analysis should have implications for avoiding any bias in the parameter constraints using 21 cm power spectrum data.

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arXiv2018

Lyman-alpha emitters gone missing: the different evolution of the bright and faint populations

We model the transmission of the Lyman-alpha line through the circum- and intergalactic media around dark matter haloes expected to host Lyman-alpha emitters (LAEs) at z > 5.7, using the high-dynamic-range Sherwood simulations. We find very different CGM environments around more massive haloes (~10^11 M_sun) compared to less massive haloes (~10^9 M_sun) at these redshifts, which can contribute to a different evolution of the Lyman-alpha transmission from LAEs within these haloes. Additionally we confirm that part of the differential evolution could result from bright LAEs being more likely to reside in larger ionized regions. We conclude that a combination of the CGM environment and the IGM ionization structure is likely to be responsible for the differential evolution of the bright and faint ends of the LAE luminosity function at z > 6. More generally, we confirm the suggestion that the self-shielded neutral gas in the outskirts of the host halo can strongly attenuate the Lyman-alpha emission from high redshift galaxies. We find that this has a stronger effect on the more massive haloes hosting brighter LAEs. The faint-end of the LAE luminosity function is thus a more reliable probe of the average ionization state of the IGM. Comparing our model for LAEs with a range of observational data we find that the favoured reionization histories are our previously advocated `Late' and `Very Late' reionization histories, in which reionization finishes rather rapidly at around z ~ 6.

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arXiv2018

Halo models of HI selected galaxies

Modelling the distribution of neutral hydrogen (HI) in dark matter halos is important for studying galaxy evolution in the cosmological context. We use a novel approach to infer the HI-dark matter connection at the massive end ($m_{\rm HI} > 10^{9.8} M_{\odot}$) from radio HI emission surveys, using optical properties of low-redshift galaxies as an intermediary. In particular, we use a previously calibrated optical HOD describing the luminosity- and colour-dependent clustering of SDSS galaxies and describe the HI content using a statistical scaling relation between the optical properties and HI mass. This allows us to compute the abundance and clustering properties of HI-selected galaxies and compare with data from the ALFALFA survey. We apply an MCMC-based statistical analysis to constrain the free parameters related to the scaling relation. The resulting best-fit scaling relation identifies massive HI galaxies primarily with optically faint blue centrals, consistent with expectations from galaxy formation models. We compare the HI-stellar mass relation predicted by our model with independent observations from matched HI-optical galaxy samples, finding reasonable agreement. As a further application, we make some preliminary forecasts for future observations of HI and optical galaxies in the expected overlap volume of SKA and Euclid/LSST.

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arXiv2018

Dark energy constraints from the 21~cm intensity mapping surveys with SKA1

Understanding the nature of dark energy is one of the most outstanding problems in cosmology at present. In last twenty years, cosmological observations related to SNIa, Cosmic Microwave Background Radiation, Baryon Acoustic Oscillations etc, have put stringent constraints on the the dark energy evolution, still there is enough uncertainty in our knowledge about dark energy that demands new generation of cosmological observations. Post-reionization neutral hydrogen 21 cm intensity mapping surveys are one of the most promising future cosmological observations that have the potential to map the cosmological evolution from dark ages till present time with unprecedented accuracy and Square Kilometer Array (SKA) is one of the most sensitive instruments to measure the post-reionization 21 cm signal. In this work, we study the future dark energy constraints using post-reionization 21 cm intensity mapping power spectra with SKA1-mid specifications. We use three different parametrizations for dark energy equation of state (EoS) including the widely used CPL one. To generate simulated data, we use to two fiducial models: the concordance $Λ$CDM and the best fit CPL model for Planck+SNIa+BAO+HST. Our study shows that SKA1-mid alone has the potential to reach the present accuracy for combined Planck+SNIa+BAO+HST to constrain the dark energy behaviour. Whether dark energy is phantom or non-phantom or whether it exhibits phantom crossing, we may potentially address such questions with SKA1-mid. We also show that it is crucial to choose the correct parametrization for dark energy equation of state as some parametrizations are better than others to constrain the dark energy behaviour. Specifically, as observed in this study, the widely used CPL parametrization may not give the best constraint for dark energy behaviour.

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arXiv2018

Dark matter, extra-terrestrial gamma-rays and the MSSM: a viability study

We fit the $γ$-ray excess from the galactic centre (GC) in terms of parameters of the minimal supersymmetric standard model (MSSM). Consistency with other $γ$-ray observation, such as those from dwarf spheroidal galaxies, is also ensured, in addition to the constraints from direct dark matter search. Furthermore, we expect the contribution to the relic density from the MSSM dark mater candidate, namely, the lightest neutralino, should not go below the stipulated value; otherwise it will amount to going beyond the MSSM by including some additional dark matter source. After a detailed scan of the parameter space in terms of four representative types of particle spectra, we identify the ones that are best fit to the observed data. However, these two are somewhat unsatisfactory in terms of $χ^2_{min}$ as well as $p$-values. In some case(s), the unacceptability of low-$χ^2_{min}$ regions due to direct search constraint is responsible for this. In others, the observed shape of the $γ$-ray spectrum makes the fits unsatisfactory. The imposed lower limit on relic density, too, has a role to play all along. On the whole, the conclusion is that the MSSM is not a very satisfactory fit for the GC $γ$-ray compounded with other cosmological observations and direct search limits.

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arXiv2017

Do astrophysical data disfavour the minimal supersymmetric standard model?

If the minimal supersymmetric standard model (MSSM) is the only new physics around the TeV-scale, it has to account for the entire dark matter relic density, or else it will cease to be `minimal'. We use this expectation to obtain the best quantitative explanation of the galactic centre $γ$-ray excess. The $γ$-ray data/flux limits from other astrophysical sources are also taken into account, together with all laboratory constraints. The lower limit on the relic density, together with the latest direct dark matter search constraints and the shape of the galactic centre $γ$-ray spectrum, makes the MSSM fits appear rather poor. A comparison with similar fits of the Higgs boson mass from indirect and direct search results makes one suspicious that the MSSM is not a good explanation of data related to dark matter.

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arXiv2017

Efficient adiabatic hydrodynamical simulations of the high-redshift intergalactic medium

We present a post-processing tool for GADGET-2 adiabatic simulations to model various observed properties of the Ly$α$ forest at $2.5 \leq z \leq 4$ that enables an efficient parameter estimation. In particular, we model the thermal and ionization histories that are not computed self-consistently by default in GADGET-2. We capture the effect of pressure smoothing by running GADGET-2 at an elevated temperature floor and using an appropriate smoothing kernel. We validate our procedure by comparing different statistics derived from our method with those derived using self-consistent simulations with GADGET-3. These statistics are: line of sight density field power spectrum, flux probability distribution function, flux power spectrum, wavelet statistics, curvature statistics, HI column density (${\rm N_{HI}}$) distribution function, linewidth ($b$) distribution and $b$ versus $\log {\rm N_{HI}}$ scatter. For the temperature floor of $10^4$ K and typical signal-to-noise of 25, the results agree well within 20 percent of the self-consistent GADGET-3 simulation. However, this difference is smaller than the expected $1σ$ sample variance for an absorption path length of $\sim 5.35$ at $z=3$. Moreover for a given cosmology, we gain a factor of $\sim N$ in computing time for modelling the intergalactic medium under $N \gg 1$ different thermal histories. In addition, our method allows us to simulate the non-equilibrium evolution of thermal and ionization state of the gas and include heating due to non-standard sources like cosmic rays and high energy $γ$-rays from Blazars.

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