arXiv subjects

Soumak Maitra

Publications and source records attributed to Soumak Maitra.

14 recordsLinked to original sources

arXiv2026

Forest without Trees is still Fruitful: Constraints on the thermal state of the neutral IGM at $z\approx5.6$ with the 21-cm forest power spectrum

Neutral regions of the intergalactic medium (IGM) during the Epoch of Reionization (EoR) remain largely unexplored due to the limitations of existing probes. Owing to discoveries of numerous high-redshift radio-bright sources, the 21-cm forest, a series of absorption features imprinted by the neutral IGM in the spectra of such sources, now offers an attractive probe of the thermal and ionization state of the predominantly neutral IGM at $z\gtrsim5.5$. We analyse archival upgraded Giant Metrewave Radio Telescope (uGMRT) observations of J352-15, the brightest known radio-loud quasar in the EoR ($z=5.82$), to measure the one-dimensional (1D) power spectrum of the 21-cm forest. By comparing the observed power spectrum with forward-modelled synthetic spectra generated from cosmological simulations spanning a wide range of ionization and X-ray pre-heating scenarios, we perform Bayesian inference even in the absence of a statistical detection. We also present an independent Murchison Widefield Array measurement, although its lower sensitivity prevents competitive constraints. Using uGMRT, we achieve a sensitivity of $3.62\,\rm mJy\,beam^{-1}$ per $6.1\,\rm kHz$ channel. While we do not detect the 21-cm forest statistically, the null detection jointly constrains the mean neutral hydrogen fraction, $\langle x_{\rm HI}\rangle$, and the mean temperature of the neutral IGM, $\langle T_{\rm HI}\rangle$. At the $68\%$ credible level, our analysis disfavours cold and substantially neutral IGM models at $z\approx5.6$, including models with $\langle T_{\rm HI}\rangle \lesssim 27\,\rm K$ for $\langle x_{\rm HI}\rangle=0.1$. These limits probe parameter space allowed by existing Ly$α$ and 21-cm observations, indicating substantial pre-heating of the neutral IGM above the adiabatic cooling floor. This demonstrates that the 21-cm forest has entered the regime of observationally informative statistics.

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arXiv2026

TORRCH: Tomographic reconstruction of the reionization of cosmic hydrogen with Ly$α$ emitters and non-Ly$α$-selected galaxies

Tomographic reconstruction of reionization is a long-sought goal. It would move the field beyond global summary statistics, such as the volume-averaged ionised fraction, to direct, field-level constraints on the ionization topology. With this in mind, we present TORRCH (TOmographic Reconstruction of the Reionization of Cosmic Hydrogen), a deep-learning framework that reconstructs the neutral-hydrogen fraction field during the epoch of reionization from the spatial distributions of Ly$α$ emitters (LAEs) and non-Ly$α$-selected galaxies (NLSGs) at luminosity limits comparable to current surveys. Using hydrodynamical simulations post-processed with radiative transfer, we train a deterministic 3D U-Net on mock surveys spanning diverse reionization scenarios and predict the neutral-fraction field. We find that TORRCH recovers the large-scale ionization morphology from synthetic data comparable to current surveys with high fidelity, and reproduces both the one-point distribution and the 2D power spectrum of projected neutral fractions. The predicted galaxy-IGM cross-correlation is also captured well, including the expected small-scale anti-correlation and its decline towards zero at large separations. Reconstruction quality depends on tracer completeness, with deep joint LAE+NLSG samples yielding the most accurate morphology, while LAE-only selections retain bubble-scale topology but with reduced fidelity. Robustness tests show that the method is stable to variations in ionization conditions between training and test data, and to realistic redshift uncertainties. Our results suggest that galaxy-based tomography can potentially deliver reliable reionization maps across realistic survey redshift windows.

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arXiv2025

The Lyman-$α$ emitter bispectrum as a probe of reionization morphology

Ly$α$ emitters (LAEs) have now been discovered out to redshift $z=13$, and are valuable probes of the reionization history at redshifts beyond the reach of other currently available tracers. Most inferences of the neutral hydrogen fraction from LAE observations rely on one-point and two-point statistics like the luminosity function and the power spectrum. We present here an analysis of the bispectrum of high-redshift LAEs and demonstrate its sensitivity to the Epoch of Reionization. We use the Sherwood-Relics suite of cosmological hydrodynamical simulations post-processed with the GPU-based radiative transfer code ATON-HE to generate realistic LAE mock catalogues for a wide range of reionization models, varying the ionization history and the source populations, including contributions of AGN to hydrogen reionization. We demonstrate that the bispectrum provides greater sensitivity than the power spectrum to both the timing and spatial morphology of reionization. Using reduced-$χ^2$ analysis we further show that the bispectrum also responds more strongly to variations in source population and AGN contribution, apparently more efficiently capturing morphological features missed by two-point statistics. The redshift evolution of the bispectrum reflects the increased clustering of ionizing sources at earlier epochs. The sensitivity of the bispectrum to peculiar velocities underscores the importance of velocity corrections in comparisons to observations. Our findings demonstrate that the LAE bispectrum is a powerful higher-order statistic for probing reionization through current and future LAE surveys using telescopes such as Subaru and JWST.

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arXiv2025

Efficient neutral-IGM inference from noisy 21-cm forest spectra with latent-space U-Net encoding and XGBoost

The 21-cm forest, comprising narrow absorption features imprinted on the radio spectra of high-redshift radio-loud quasars by intervening neutral hydrogen, offers a uniquely sensitive probe of the thermal state of the neutral intergalactic medium (IGM) during the epoch of reionization. Although over 30 such quasars are now known at $z > 5.5$, the signal remains elusive in practice, owing to instrumental noise, the intrinsic weakness of the absorption features, and the limited brightness of available background sources. Recent studies have focused on the one-dimensional transmission power spectrum as a statistical observable, but this approach also demands high signal-to-noise ratios. Here, we present a systematic comparison of five inference pipelines for recovering IGM parameters from mock 21-cm forest spectra at $z = 6$, incorporating realistic instrumental noise and telescope characteristics. We show that likelihood-free inference based on machine learning substantially outperforms traditional Bayesian methods. In particular, our most effective method dispenses with the power spectrum entirely: we use a convolutional U-Net to extract a latent-space encoding of the input spectrum and perform parameter regression using XGBoost. This approach yields accurate constraints on the IGM neutral fraction and X-ray heating efficiency even with a single 50-hour uGMRT sightline, which is an orders-of-magnitude improvement in integration time relative to existing techniques. We publicly release our code, training data, and models. Beyond the 21-cm forest, these results underscore the promise of hybrid deep learning and gradient-boosted inference techniques for extracting physical information from low-SNR data across astrophysics.

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arXiv2025

DeepCHART: Mapping the 3D dark matter density field from Ly$α$ forest surveys using deep learning

We present DeepCHART (Deep learning for Cosmological Heterogeneity and Astrophysical Reconstruction via Tomography), a deep learning framework designed to reconstruct the three-dimensional dark matter density field at redshift $z=2.5$ from Ly$α$ forest spectra. Leveraging a 3D variational autoencoder with a U-Net architecture, DeepCHART performs fast, likelihood-free inference, accurately capturing the non-linear gravitational dynamics and baryonic processes embedded in cosmological hydrodynamical simulations. When applied to joint datasets combining Ly$α$ forest absorption and coeval galaxy positions, the reconstruction quality improves further. For current surveys, such as Subaru/PFS, CLAMATO, and LATIS, with an average transverse sightline spacing of $d_\perp=2.4h^{-1}$cMpc, DeepCHART achieves high-fidelity reconstructions over the density range $0.4<Δ_{\rm DM}<15$, with a voxel-wise Pearson correlation coefficient of $ρ\simeq 0.77$. These reconstructions are obtained using Ly$α$ forest spectra with signal-to-noise ratios as low as 2 and instrumental resolution $R=2500$, matching Subaru/PFS specifications. For future high-density surveys enabled by instruments such as ELT/MOSAIC and WST/IFS with $d_\perp\simeq 1h^{-1}\mathrm{cMpc}$, the correlation improves to $ρ\simeq 0.90$ across a wider dynamic range ($0.25<Δ_{\rm DM}<40$). The framework reliably recovers the dark matter density PDF as well as the power spectrum, with only mild suppression at intermediate scales. In terms of cosmic web classification, DeepCHART successfully identifies 81% of voids, 75% of sheets, 63% of filaments, and 43% of nodes. We propose DeepCHART as a powerful and scalable framework for field-level cosmological inference, readily generalisable to other observables, and offering a robust, efficient means of maximising the scientific return of upcoming spectroscopic surveys.

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arXiv2024

Parameter estimation from Ly$α$ forest in Fourier space using Information Maximising Neural Network

We aim to present a robust parameter estimation with simulated Lya forest spectra from Sherwood-Relics simulations suite using Information Maximizing Neural Network(IMNN) to extract maximal information from Lya 1D-transmitted flux in Fourier space. We perform 1D estimations using IMNN for IGM thermal parameters $T_0$ & $γ$ at z=2-4 and cosmological parameters $σ_8$ & $n_s$ at z=3-4. We compare our results with estimates from power spectrum using posterior distribution from Markov Chain Monte Carlo(MCMC). We then check robustness of IMNN estimates against deviation in spectral noise levels,continuum uncertainties & instrumental smoothing effects. Using mock Lya forest sightlines from publicly available CAMELS project we also check the robustness of the trained IMNN on a different simulation. We also perform a 2D-parameter estimation for $T_0$ & HI photoionization rates $Γ_{HI}$. We obtain improved estimates of $T_0$ & $γ$ using IMNN over standard MCMC approach. These estimates are also more robust against SNR deviations at z=2 & 3. At z=4 the sensitivity to noise deviations is on par with MCMC estimates. The IMNN also provides $T_0$ and $γ$ estimates which are robust against continuum uncertainties by extracting continuum-independent small-scale information from Fourier domain. In case of $σ_8$ & $n_s$ IMNN performs on par with MCMC but still offers a significant speed boost in estimating parameters from a new dataset. The improved estimates with IMNN are seen for high instrumental-resolution(FWHM=6km/s). At medium or low resolutions IMNN performs similar to MCMC suggesting an improved extraction of small-scale information with IMNN. We also find that IMNN estimates are robust against the choice of simulation. By performing a 2D-parameter estimation for $T_0$ & $Γ_{HI}$ we also demonstrate how to take forward this approach observationally in the future.

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arXiv2023

Higher order clustering of Ly$α$ forest

Higher order clustering statistics of Ly$α$ forest provide a unique probe to study non-gaussianity in Intergalactic matter distribution up to high redshifts and from large to small scales. The author presents a brief review of his work studying the spatial clustering properties of Ly$α$ absorbers, with emphasis on 3-point statistics. The observational side of this involves redshift-space clustering of low-$z$ ($z<0.48$) and high-$z$ ($1.7<z<3.5$) Ly$α$ absorbers. This is complemented with astrophysical inferences drawn from N-body hydrodynamical simulations. We also use simulations to study 2-point and 3-point clustering statistics in the transverse direction using projected QSO triplet sightlines. Such studies will become possible observationally with upcoming surveys.

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arXiv2023

Role of ionizing background and galactic feedback on the redshift space clustering of OVI absorbers in hydrodynamical simulations

We explore the effect of ionizing UV background (UVB) on the redshift space clustering of low-$\textit{z}$ ($\textit{z} \leq 0.5$) OVI absorbers using Sherwood simulations incorporating "WIND" (i.e. outflows driven by stellar feedback) only and "AGN+WIND" feedbacks. These simulations show a positive clustering signals up to a scale of 3 Mpc. We find that the effect of feedback is restricted to small scales (i.e $\leq$ 2 Mpc or 200 $\textit{kms}^{-1} $ at $\textit{z}$ ~ 0.3) and "WIND" only simulations produce stronger clustering signal compared to simulations incorporating "AGN+WIND" feedbacks. How clustering signal is affected by the assumed UVB depends on the feedback processes assumed. For the simulations considered here the effect of UVB is confined to even smaller scales (i.e <1 Mpc or $\approx 100\textit{kms}^{-1}$ at $\textit{z}$ ~ 0.3). These scales are also affected by exclusion caused by line blending. Therefore, our study suggests clustering at intermediate scales (i.e 1-2 Mpc for simulations considered here) together with the observed column density distribution can be used to constrain the effect of feedback in simulations.

astro-ph.CO↗

arXiv2023

Role of ionizing background on the statistics of metal absorbers in hydrodynamical simulations

We study the statistical properties of O VI, C IV, and Ne VIII absorbers at low-$z$ (i.e., $z<0.5$) using Sherwood simulations with "WIND" only and "WIND+AGN" feedback and Massive black simulation that incorporates both "WIND" i.e. outflows driven by stellar feedback and AGN feedbacks. For each simulation, by considering a wide range of metagalactic ionizing UV background (UVB), we show the statistical properties such as distribution functions of column density ($N$), $b$-paramerer and velocity spread ($ΔV_{90}$), the relationship between $N$ and $b$-parameter and the fraction of Lya absorbers showing detectable metal lines as a function of $N$(H I) are influenced by the UVB used. This is because UVB changes the range in density, temperature, and metallicity of gas contributing to a given absorption line. For simulations considered here, we show the difference in some of the predicted distributions between different simulations is similar to the one obtained by varying the UVB for a given simulation. Most of the observed properties of O VI absorbers are roughly matched by Sherwood simulation with "WIND+AGN" feedback when using the UVB with a lower O VI ionization rate. However, this simulation fails to produce observed distributions of C IV and fraction of H I absorbers with detectable metals. Therefore, in order to constrain different feedback processes and/or UVBs, using observed properties of H I and metal ions, it is important to perform simultaneous analysis of various observable parameters.

astro-ph.GA↗

arXiv2023

Spectroscopy of QUBRICS quasar candidates: 1672 new redshifts and a Golden Sample for the Sandage Test of the Redshift Drift

The QUBRICS (QUasars as BRIght beacons for Cosmology in the Southern hemisphere) survey aims at constructing a sample of the brightest quasars with z>~2.5, observable with facilities in the Southern Hemisphere. QUBRICS makes use of the available optical and IR wide-field surveys in the South and of Machine Learning techniques to produce thousands of bright quasar candidates of which only a few hundred have been confirmed with follow-up spectroscopy. Taking advantage of the recent Gaia Data Release 3, which contains 220 million low-resolution spectra, and of a newly developed spectral energy distribution fitting technique, designed to combine the photometric information with the Gaia spectroscopy, it has been possible to measure 1672 new secure redshifts of QUBRICS candidates, with a typical uncertainty $σ_z = 0.02$. This significant progress of QUBRICS brings it closer to (one of) its primary goals: providing a sample of bright quasars at redshift 2.5 < z < 5 to perform the Sandage test of the cosmological redshift drift. A Golden Sample of seven quasars is presented that makes it possible to carry out this experiment in about 1500 hours of observation in 25 years, using the ANDES spectrograph at the 39m ELT, a significant improvement with respect to previous estimates.

astro-ph.CO↗

arXiv2021

Measurement of redshift-space two- and three-point correlation of Ly$α$ absorbers at $1.7< z <3.5$: Implications on evolution of the physical properties of IGM

We present redshift space two-point ($ξ$), three-point ($ζ$) and reduced three-point (Q) correlations of Ly$α$ absorbers (Voigt profile components having HI column density, $N_{\rm HI}>10^{13.5}$cm$^{-2}$) over three redshift bins spanning $1.7< z<3.5$ using high-resolution spectra of 292 quasars. We detect positive $ξ$ up to 8 $h^{-1}$ cMpc in all three redshift bins. The strongest detection of $ζ=1.81\pm 0.59$ (with Q$=0.68\pm 0.23$), is in $z=1.7-2.3$ bin at $1-2h^{-1}$ cMpc. The measured $ξ$ and $ζ$ values show an increasing trend with $N_{\rm HI}$, while Q remains relatively independent of $N_{\rm HI}$. We find $ξ$ and $ζ$ to evolve strongly with redshift. Using simulations, we find that $ξ$ and $ζ$ seen in real space may be strongly amplified by peculiar velocities in redshift space. Simulations suggest that while feedback, thermal and pressure smoothing effects influence the clustering of Ly$α$ absorbers at small scales, i.e $<0.5h^{-1}$ cMpc, the HI photo-ionization rate ($Γ_{\rm HI}$) has a strong influence at all scales. The strong redshift evolution of $ξ$ and $ζ$ (for a fixed $N_{\rm HI}$-cutoff) is driven by the redshift evolution of the relationship between $N_{\rm HI}$ and baryon overdensity. Our simulation using best-fitted $Γ_{\rm HI}(z)$ measurements produces consistent clustering signals with observations at $z\sim 2$ but under-predicts the clustering at higher redshifts. One possible remedy is to have higher values of $Γ_{\rm HI}$ at higher redshifts. Alternatively the discrepancy could be related to non-equilibrium and inhomogeneous conditions prevailing during HeII reionization not captured by our simulations.

astro-ph.CO↗

arXiv2021

Redshift space three-point correlation function of IGM at $z<0.48$

The Ly$α$ forest decomposed into Voigt profile components allow us to study clustering properties of the intergalactic medium and its dependence on various physical quantities. Here, we report the first detections of probability excess of low-z (i.e z<0.48) Ly$α$ absorber triplets over a scale of $r_\parallel\leq8$pMpc with a maximum amplitude of $8.76^{+1.96}_{-1.65}$ at a longitudinal separation of 1-2pMpc. We measure non-zero three-point correlation ($ζ=4.76^{+1.98}_{-1.67}$) only at this scale with reduced three-point correlation value of Q=$0.95^{+0.39}_{-0.38}$. The measured $ζ$ shows an increasing trend with increasing HI column density ($N_{HI}$) while Q does not show any $N_{HI}$ dependence. About 88% of the triplets contributing to $ζ$ (at z$\le0.2$) have nearby galaxies (whose distribution is known to be complete for 0.1L$_*$ at z<0.1 and for L$_*$ at z~0.25 and within 20' to the quasar sightlines) within a velocity separation of 500$kms^{-1}$ and a median impact parameter of 405pkpc. The measured impact parameters are consistent with appreciable number of triplets at z$\le0.2$ not originating from individual galaxies but tracing the underlying galaxy distribution. Frequency of occurrence of high-b absorbers in triplets (~85%) is a factor~3 higher than that found among the full sample (~32%). Using four different cosmological simulations, we quantify the effect of peculiar velocities, feedback effects and show that most of the observed trends are broadly reproduced. However, $ζ$ at small scales ($r_\parallel<1$pMpc) and b-dependence of $ζ$ in simulations are found inconsistent with the observations. This could either be related to the fact that none of these simulations reproduce the observed b-distribution and $N_{HI}$ distribution for $N_{HI}>10^{14}$cm$^{-2}$ self-consistently or to the widespread of signal-to-noise ratio in the observed data.

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arXiv2020

Three- and two-point spatial correlations of IGM at $z\sim 2$: Cloud based analysis using simulations

Ly$α$ forest absorption spectra decomposed into multiple Voigt profile components (clouds) allow us to study clustering of intergalactic medium (IGM) as a function of HI column density ($N_{\rm HI}$). Here, we explore the transverse three-point correlation ($ζ$) of these Ly$α$ clouds using mock triplet spectra obtained from hydrodynamical simulations at $z \sim 2$ on scales of 1-5 $h^{-1}$cMpc. We find $ζ$ to depend strongly on $N_{\rm HI}$ and scale and weakly on angle ($θ$) of the triplet configuration. We show that the "hierarchical ansatz" is applicable for scales $\ge~ 3h^{-1}$cMpc, and obtain a median reduced three-point correlation (Q) in the range 0.2-0.7. We show, $ζ$ is influenced strongly by the thermal and ionization state of the gas. As found in the case of galaxies, the influence of physical parameters on Q is weaker compared to that of $ζ$. We show difference in $ζ$ and Q between different simulations are minimized if we use appropriate $N_{\rm HI}$ cut-offs corresponding to a given baryon over-density ($Δ$) using the measured $N_{\rm HI}~vs~Δ$ relationship obtained from individual simulations. Additionally, we see the effect of pressure broadening on $ζ$ in a model with artificially boosted heating rates. However, for models with realistic thermal and ionization histories the effect of pressure broadening on $ζ$ is weak and sub-dominant compared to other local effects. We find strong redshift evolution shown by $ζ$, mainly originating from the redshift evolution of thermal and ionization state of the IGM. We discuss the observational requirements for the detection of three-point correlation, specifically, in small intervals of configuration parameters and redshift.

astro-ph.CO↗

arXiv2019

Three- and two-point spatial correlations of intergalactic medium at $z\sim 2$ using projected quasar triplets

We present analysis of two- and three-point correlation functions of Ly$α$ forest (at $2\le z\le 2.5$) using X-Shooter spectra of three background quasar triplets probing transverse separations of 0.5-1.6 pMpc. We present statistics based on transmitted flux and clouds identified using Voigt profile fitting. We show that the observed two-, three-point correlation functions and reduced three-point correlation (i.e Q) are well reproduced by our simulations. We assign probabilities for realising all the observed correlation properties simultaneously using our simulations. Our simulations suggest an increase in correlation amplitudes and Q with increasing $N_{\rm HI}$. We roughly see this trend in the observations too. We identify a concurrent gap of 17$\mathring{A}$ (i.e 14.2 $h^{-1}$cMpc, one of the longest reported) wide in one of the triplets. Such gap is realised only in 14.2% of our simulated sightlines and most of the time belongs to a void in the matter distribution. In the second triplet, we detect DLAs along all three sightlines (with spatial separations 0.64 to 1.6 pMpc) within a narrow redshift interval (i.e $Δz = 0.088$). Detection of a foreground quasar ($\sim$ 1 pMpc from the triplet sightlines) and excess partial Lyman Limit systems around these DLAs suggest that we may be probing a large over-dense region. We also report positive CIV- CIV correlations up to $\sim 500$ $km s^{-1}$ only in the longitudinal direction. Additionally, we conclude a positive CIV-Ly$α$ correlations for higher $N_{\rm HI}$ thresholds up to a scale of $\sim 1000$ $km s^{-1}$ both in transverse and longitudinal directions.

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