arXiv subjects

Tirthankar Roy Choudhury

Publications and source records attributed to Tirthankar Roy Choudhury.

83 records · Page 5Linked to original sources

arXiv2017

Warm dark matter constraints from high-$z$ Direct Collapse Black Holes using the JWST

We use a semi-analytic model, ${\it Delphi}$, that jointly tracks the dark matter and baryonic assembly of high-redshift ($z \simeq 4-20$) galaxies to gain insight on the number density of Direct Collapse Black Hole (DCBH) hosts in three different cosmologies: the standard Cold Dark Matter (CDM) model and two Warm Dark Matter (WDM) models with particle masses of 3.5 and 1.5 keV. Obtaining the Lyman-Werner (LW) luminosity of each galaxy from ${\it Delphi}$, we use a clustering bias analysis to identify all, pristine halos with a virial temperature $T_{vir}>=10^4$ K that are irradiated by a LW background above a critical value as, DCBH hosts. In good agreement with previous studies, we find the DCBH number density rises from $\sim10^{-6.1}$ to $\sim 10^{-3.5}\, \mathrm{cMpc^{-3}}$ from $z\simeq 17.5$ to $8$ in the CDM model using a critical LW background value of $30 J_{21}$ (where $J_{21}= 10^{-21} \, {\rm erg\, s^{-1}\, Hz^{-1} \, cm^{-2} \, sr^{-1}}$). We find that a combination of delayed structure formation and an accelerated assembly of galaxies results in a later metal-enrichment and an accelerated build-up of the LW background in the 1.5 keV WDM model, resulting in DCBH hosts persisting down to much lower redshifts ($z \simeq 5$) as compared to CDM where DCBH hosts only exist down to $z \simeq 8$. We end by showing how the expected colours in three different bands of the Near Infrared Camera (NIRCam) onboard the forthcoming James Webb Space Telescope (${\it JWST}$) can be used to hunt for potential $z \simeq 5-9$ DCBHs, allowing hints on the WDM particle mass.

astro-ph.CO↗

arXiv2017

Large 21 cm signals from AGN-dominated reionization

We present here predictions for the spatial distribution of 21 cm brightness temperature fluctuations from high-dynamic-range simulations for AGN-dominated reionization histories that have been tested against available Lyman-alpha and CMB data. We model AGN by extrapolating the observed M-sigma relation to high redshifts and assign them ionizing emissivities consistent with recent UV luminosity function measurements. We assess the observability of the predicted spatial 21 cm fluctuations by ongoing and upcoming experiments in the late stages of reionization in the limit in which the hydrogen 21 cm spin temperature is significantly larger than the CMB temperature. Our AGN-dominated reionization histories increase the variance of the 21 cm emission by a factor of up to ten compared to similar reionization histories dominated by faint galaxies, to values close to 100 mK^2 at scales accessible to experiments (k < 1 h/cMpc). This is lower than the sensitivity claimed to have been already reached by ongoing experiments by only a factor of about two or less. When reionization is dominated by AGN, the 21 cm power spectrum is enhanced on all scales due to the enhanced bias of the clustering of the more massive haloes and the peak in the large scale 21 cm power is strongly enhanced and moved to larger scales due to bigger characteristic bubble sizes. AGN dominated reionization should be easily detectable by LOFAR (and later HERA and SKA1) at their design sensitivity, assuming successful foreground subtraction and instrument calibration. Conversely, these could become the first non-trivial reionization scenarios to be ruled out by 21 cm experiments, thereby constraining the contribution of AGN to reionization.

astro-ph.CO↗

arXiv2017

VoIgt profile Parameter Estimation Routine (VIPER): H I photoionization rate at z<0.5

We have developed a parallel code called "VoIgt profile Parameter Estimation Routine (VIPER)" for automatically fitting the H I Ly-$α$ forest seen in the spectra of QSOs. We obtained the H I column density distribution function (CDDF) and line width ($b$) parameter distribution for $z < 0.45$ using spectra of 82 QSOs obtained using Cosmic Origins Spectrograph and VIPER. Consistency of these with the existing measurements in the literature validate our code. By comparing this CDDF with those obtained from hydrodynamical simulation, we constrain the H I photoionization rate ($Γ_{\rm HI}$) at $z < 0.45$ in four redshift bins. The VIPER, together with the Code for Ionization and Temperature Evolution (CITE) we have developed for GADGET-2, allows us to explore parameter space and perform $χ^2$ minimization to obtain $Γ_{\rm HI}$. We notice that the $b$ parameters from the simulations are smaller than what are derived from the observations. We show the observed $b$ parameter distribution and $b$ vs $\log {\rm N_{HI}}$ scatter can be reproduced in simulation by introducing sub-grid scale turbulence. However, it has very little influence on the derived $Γ_{\rm HI}$. The $Γ_{\rm HI}(z)$ obtained here, $(3.9 \pm 0.1) \times 10^{-14} \; (1+z)^{4.98 \pm 0.11} \;{\rm s^{-1}}$, is in good agreement with those derived by us using flux based statistics in the previous paper. These are consistent with the hydrogen ionizing ultra-violet (UV) background being dominated mainly by QSOs without needing any contribution from the non-standard sources of the UV photons.

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arXiv2017

Reionization and Galaxy Formation in Warm Dark Matter Cosmologies

We compare model results from a semi-analytic (merger-tree based) framework for high-redshift (z ~ 5-20) galaxy formation against reionization indicators, including the Planck electron scattering optical depth and the ionizing photon emissivity, to shed light on the reionization history and sources in Cold (CDM) and Warm Dark Matter (WDM; particle masses of $m_x = 1.5,3$ and 5 keV) cosmologies. This model includes all the key processes of star formation, supernova feedback, the merger/accretion/ejection driven evolution of gas and stellar mass and the effect of the ultra-violet background (UVB) in photo-evaporating the gas content of low-mass galaxies. We find that the delay in the start of reionization in light (1.5 keV) WDM models can be compensated by a steeper redshift evolution of the ionizing photon escape fraction and a faster mass assembly, resulting in reionization ending at comparable redshifts (z~5.5) in all the DM models considered. We find the bulk of the reionization photons come from galaxies with a halo mass $M_h < 10^9 M_\odot$ and a UV magnitude $ -15 < M_{UV} < -10$ in CDM. The progressive suppression of low-mass halos with decreasing $m_x$ leads to a shift in the reionization population to larger halo masses of $M_h > 10^9 M_\odot$ and $ -17 < M_{UV} < -13$ for 1.5 keV WDM. We find that current observations of the electron scattering optical depth and the Ultra-violet luminosity function are equally compatible with all the (cold and warm) DM models considered in this work. We propose that global indicators including the redshift evolution of the stellar mass density and the stellar mass-halo mass relation, observable with the James Webb Space Telescope, can be used to distinguish between CDM and WDM (1.5 keV) cosmologies.

astro-ph.CO↗

arXiv2016

Intergalactic Lyman continuum photon budget in the past 5 billion years

We constrain the H I photoionization rate $(Γ_{\rm HI})$ at $z \lesssim 0.45$ by comparing the flux probability distribution function and power spectrum of the Ly-$α$ forest data along 82 QSO sightlines obtained using Cosmic Origins Spectrograph with models generated from smoothed particle hydrodynamic simulations. We have developed a module named "Code for Ionization and Temperature Evolution (CITE)" for calculating the intergalactic medium (IGM) temperature evolution from high to low redshifts by post-processing the GADGET-2 simulation outputs. Our method, that produces results consistent with other simulations, is computationally less expensive thus allowing us to explore a large parameter space. It also allows rigorous estimation of the error covariance matrix for various statistical quantities of interest. We find that the best-fit $Γ_{\rm HI}(z)$ increases with $z$ and follows $(4 \pm 0.1) \times 10^{-14}\:(1+z)^{4.99 \pm 0.12}$ s$^{-1}$. At any given $z$ the typical uncertainties $ΔΓ_{\rm HI} / Γ_{\rm HI}$ are $\sim 25$ per cent which contains not only the statistical errors but also those arising from possible degeneracy with the thermal history of the IGM and cosmological parameters and uncertainties in fitting the QSO continuum. These values of $Γ_{\rm HI}$ favour the scenario where only QSOs contribute to the ionizing background at $z<2$. Our derived $3σ$ upper limit on average escape fraction is $0.008$, consistent with measurements of low-$z$ galaxies.

astro-ph.CO↗

arXiv2016

Models of the Cosmological 21 cm Signal from the Epoch of Reionization Calibrated with Lyman-alpha and CMB Data

We present here 21 cm predictions from high dynamic range simulations for a range of reionization histories that have been tested against available Lyman-alpha and CMB data. We assess the observability of the predicted spatial 21 cm fluctuations by ongoing and upcoming experiments in the late stages of reionization in the limit in which the hydrogen spin temperature is significantly larger than the CMB temperature. Models consistent with the available Lyman-alpha data and CMB measurement of the Thomson optical depth predict typical values of 10--20 mK^2 for the variance of the 21 cm brightness temperature at redshifts z=7--10 at scales accessible to ongoing and upcoming experiments (k < 1 h/cMpc). This is only a factor of a few below the sensitivity claimed to have been already reached by ongoing experiments in the signal rms value. Our different models for the reionization history make markedly different predictions for the redshift evolution and thus frequency dependence of the 21 cm power spectrum and should be easily discernible by LOFAR (and later HERA and SKA1) at their design sensitivity. Our simulations have sufficient resolution to assess the effect of high-density Lyman limit systems that can self-shield against ionizing radiation and stay 21 cm bright even if the hydrogen in their surroundings is highly ionized. Our simulations predict that including the effect of the self-shielded gas in highly ionized regions reduces the large scale 21 cm power by about 30%.

astro-ph.GA↗

arXiv2016

The redshift evolution of escape fraction of hydrogen ionizing photons from galaxies

Using our cosmological radiative transfer code, we study the implications of the updated QSO emissivity and star formation history for the escape fraction (f_esc) of hydrogen ionizing photons from galaxies. We estimate the f_esc that is required to reionize the Universe and to maintain the ionization state of the intergalactic medium in the post-reionization era. At z>5.5, we show that a constant f_esc of 0.14 to 0.22 is sufficient to reionize the Universe. At z<3.5, consistent with various observations, we find that f_esc can have values from 0 to 0.05. However, a steep rise in f_esc, of at least a factor of ~3, is required between z=3.5 to 5.5. It results from a rapidly decreasing QSO emissivity at z>3 together with a nearly constant measured H I photoionization rates at 3 4. In addition, a simple extrapolation of the contribution of such QSOs to high-z suggests that QSOs alone can reionize the Universe. This implies, at z>3.5, that either the properties of galaxies should evolve rapidly to increase the f_esc or most of the low-mass galaxies should host massive black holes and sustain accretion over a prolonged period. These results motivate a careful investigation of theoretical predictions of these alternate scenarios that can be distinguished using future observations. Moreover, it is also very important to revisit the measurements of H I photoionization rates that are crucial to the analysis presented here.

astro-ph.CO↗

arXiv2015

Lyman-alpha emitters gone missing: evidence for late reionization?

We combine high resolution hydrodynamical simulations with an intermediate resolution, dark matter only simulation and an analytical model for the growth of ionized regions to estimate the large scale distribution and redshift evolution of the visibility of Lyman-alpha emission in 6<=z<=8 galaxies. The inhomogeneous distribution of neutral hydrogen during the reionization process results in significant fluctuations in the Lyman-alpha transmissivity on large scales. The transmissivity depends not only on the ionized fraction of the intergalactic medium by volume and the amplitude of the local ionizing background, but is also rather sensitive to the evolution of the relative velocity shift of the Lyman-alpha emission line due to resonant scattering. We reproduce a decline in the space density of Lyman-alpha emitting galaxies as rapid as observed with a rather rapidly evolving neutral fraction between z=6-8, and a typical Lyman-alpha line velocity offset of 100 km/s redward of systemic at z=6 which decreases toward higher redshift. The new (02/2015) Planck results indicate such a recent end to reionization is no longer disfavoured by constraints from the cosmic microwave background.

astro-ph.CO↗

arXiv2012

Constraining large scale HI bias using redshifted 21-cm signal from the post-reionization epoch

In the absence of complex astrophysical processes that characterize the reionization era, the 21-cm emission from neutral hydrogen (HI) in the post-reionization epoch is believed to be an excellent tracer of the underlying dark matter distribution. Assuming a background cosmology, it is modelled through (i) a bias function b(k,z), which relates HI to the dark matter distribution and (ii) a mean neutral fraction (x_{HI}) which sets its amplitude. In this paper, we investigate the nature of large scale HI bias. The post-reionization HI is modelled using gravity only N-Body simulations and a suitable prescription for assigning gas to the dark matter halos. Using the simulated bias as the fiducial model for HI distribution at z\leq 4, we have generated a hypothetical data set for the 21-cm angular power spectrum (C_{l}) using a noise model based on parameters of an extended version of the GMRT. The binned C_{l} is assumed to be measured with SNR \gtrsim 4 in the range 400 \leq l \leq 8000 at a fiducial redshift z=2.5. We explore the possibility of constraining b(k) using the Principal Component Analysis (PCA) on this simulated data. Our analysis shows that in the range 0.2 < k < 2 Mpc^{-1}, the simulated data set cannot distinguish between models exhibiting different k dependences, provided 1 \lesssim b(k) \lesssim 2 which sets the 2-sigma limits. This justifies the use of linear bias model on large scales. The largely uncertain x_{HI} is treated as a free parameter resulting in degradation of the bias reconstruction. The given simulated data is found to constrain the fiducial x_{HI} with an accuracy of \sim 4% (2-sigma error). The method outlined here, could be successfully implemented on future observational data sets to constrain b(k,z) and x_{HI} and thereby enhance our understanding of the low redshift Universe.

astro-ph.CO↗

arXiv2010

Cross-correlation of the HI 21-cm Signal and Lyman-alpha Forest: A Probe Of Cosmology

Separating the cosmological redshifted 21-cm signal from foregrounds is a major challenge. We present the cross-correlation of the redshifted 21-cm emission from neutral hydrogen (HI) in the post-reionization era with the Ly-alpha forest as a new probe of the large scale matter distribution in the redshift range z=2 to 3 without the problem of foreground contamination. Though the 21-cm and the Ly-alpha forest signals originate from different astrophysical systems, they are both expected to trace the underlying dark matter distribution on large scales. The multi-frequency angular cross-correlation power spectrum estimator is found to be unaffected by the discrete quasar sampling, which only affects the noise in the estimate. We consider a hypothetical redshifted 21-cm observation in a single field of view 1.3 degrees (FWHM) centered at z=2.2 where the binned 21-cm angular power spectrum can be measured at an SNR of 3 sigma or better across the range 500 < l < 4000 . Keeping the parameters of the 21-cm observation fixed, we have estimated the SNR for the cross-correlation signal varying the quasar angular number density n of the Ly-alpha forest survey. Assuming that the spectra have SNR ~5 in pixels of length 44 km/s, we find that a 5 sigma detection of the cross-correlation signal is possible at 600 < l < 2000 with n=4 deg^{-2}. This value of n is well within the reach of upcoming Ly-alpha forest surveys. The cross-correlation signal will be a new, independent probe of the astrophysics of the diffuse IGM, the growth of structure and the expansion history of the Universe.

astro-ph.CO↗

arXiv2010

Probing intergalactic radiation fields during cosmic reionization through gamma-ray absorption

We discuss expectations for the absorption of high-energy gamma-rays by gamma-gamma pair production with intergalactic radiation fields (IRFs) at very high redshifts (z~5-20), and the prospects thereof for probing the cosmic reionization era. For the evolving IRF, a semi-analytical model incorporating both Population II and Population III stars is employed, which is consistent with a wide variety of existing high-z observations including QSO spectral measurements, WMAP Thomson depth constraints, near-IR source count limits, etc. We find that the UV IRF below the Lyman edge energy with intensities in the range of a few times 10^{-19} erg cm^{-2} s^{-1} Hz^{-1} sr^{-1} can cause appreciable attenuation above ~12 GeV at z~5, down to ~6-8 GeV at z>~8-10. This may be observable in the spectra of blazars or gamma-ray bursts by the Fermi Gamma-ray Space Telescope or next generation facilities such as the Cherenkov Telescope Array, Advanced Gamma-ray Imaging System or 5@5, providing invaluable insight into early star formation and cosmic reionization.

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