arXiv subjects

Hamsa Padmanabhan

Publications and source records attributed to Hamsa Padmanabhan.

At least 19 recordsLinked to original sources

arXiv2026

Intensity mapping of Loeb-Rybicki haloes from scattering of galactic Lyman-$α$ emission by the diffuse intergalactic medium before reionization

We use the inferred evolution of Lyman-$α$ luminosities of galaxies in the redshift range of $z \sim 9-16$ from the James Webb Space Telescope (JWST) data to predict the power spectrum of Loeb-Rybicki haloes formed by the scattering of Lyman-$α$ photons from neutral hydrogen gas in the intergalactic medium expanding with the Hubble flow, until they Doppler shift out of resonance and escape towards the observer. This leads to the formation of the so-called Loeb-Rybicki intergalactic haloes, which are expected to be prominent even before the epoch of reionization. We find excellent prospects for a statistical detection of the intensity mapping signal from the clustering of these haloes, with current and future experiments such as the SPHEREx and CDIM. We also describe the detectability of the signal in cross-correlation with the 21-cm emission from the neutral hydrogen in the intergalactic medium at these redshifts. We find that the cross-correlation signal should be detectable at a significance of a few to a few tens of standard deviations out to $z \sim 13$ and marginally out to $z \sim 16$, using the above experiments in combination with the Square Kilometre Array (SKA)-LOW and its pathfinder, the Murchison Widefield Array (MWA).

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arXiv2025

Redshift space distortions in Lagrangian space and the linear large scale velocity field of dark matter

Untangling the connection between redshift space coordinates, a velocity measurement, and three dimensional real space coordinates, is a cosmological problem that is often modeled through a linear understanding of the velocity-position coupling. This linear information is better preserved in the Lagrangian space picture of the matter density field. Through Lagrangian space measurements, we can extract more information and make more accurate estimates of the linear growth rate of the universe. In this paper, we address the linear modelling of matter particle velocities through transfer functions, and in doing so examine to what degree the decrease in correlation with initial conditions may be contaminated by velocity-based nonlinearities. With a thorough analysis of the monopole-quadrupole ratio, we find the best-fit values for the Eulerian velocity dispersion, $σ_p = 378.3$ km/s for a Lorentzian finger-of-God damping factor and $σ_p = 254.6$ km/s for a Gaussian one. The covariance of the cosmological linear growth rate $f$, is estimated in the Eulerian and Lagrangian cases. Comparing Lagrangian and Eulerian, we find that the error in $f$ improves by a factor of 3, without the need for nonlinear velocity dispersion modelling.

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arXiv2024

An Independent Measure of the Kinematic Dipole from SDSS

We utilize the Sloan Digital Sky Survey (SDSS) extended Baryon Oscillation Spectroscopic Survey (eBOSS) and Baryon Oscillation Spectroscopic Survey (BOSS) catalogs with precise spectroscopic redshifts to estimate the kinematic redshift dipole caused by the proper motion of the Solar system. We find that the velocity extracted from the kinematic dipole is consistent with Cosmic Microwave Background inferred values. Although the small sky coverage and limited number density of the SDSS sources constrain us from obtaining precise and robust measurements, we leverage the redshift dipole method to estimate the kinematic dipole. The velocity measurements in this study are insensitive to intrinsic clustering, associated with the source count dipole. The kinematic dipole measured in this work and its consistency with CMB values do not guarantee isotropy at large scales. The anisotropy (excess dipole) measured with the NRAO VLA Sky Survey (NVSS) and the WISE Catalog (CatWISE) could be due to the intrinsic distribution of galaxies. The results in this work focus solely on the kinematic dipole term.

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arXiv2024

Cosmology with HI

Hydrogen, the most abundant element in the Universe, has traditionally been used to investigate astrophysical processes within and around our own Galaxy. In its chemically neutral, atomic form (known as HI in the astronomical literature), it has tremendous potential today as a tool for precision cosmology and testing theories of fundamental physics. Cosmological HI is accessed through two of its main spectral lines: the Lyman-$α$, with a rest wavelength of 1216 $Å$, in the ultraviolet and visible part of the spectrum, and the 21-cm, which manifests in the radio frequency band. A plethora of radio telescopes worldwide are focused on detecting the faint 21 cm signal from the dark ages and Cosmic Dawn, some of the earliest epochs of the Universe. This chapter will describe the formalism for doing cosmology with HI, the recent results from the facilities and their prospects for studying the evolution of the Universe.

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arXiv2024

Intergalactic Lyman-$α$ haloes before reionization are detectable with JWST

The $James \ Webb \ Space \ Telescope \ (JWST)$ recently reported a large population of UV luminous galaxies at high redshifts, $ z > 10$, as well as Lyman-$α$ emitting (LAE) galaxies out to $z \sim 11$. We use the observed UV luminosities along with a data-driven approach at lower redshifts to place constraints on the observability of the intergalactic Lyman-$α$ intensity, scattered in the form of Loeb-Rybicki haloes, during the pre-reionization and reionization epochs ($z \sim 9-16$). We forecast the sensitivity and resolution required to detect these intergalactic haloes, finding that individual haloes with LAE luminosities $> 10^{43}$ ergs/s are detectable at a few sigma level at $z \lesssim 9$, while stacking of $\sim 10$ haloes is expected to result in detections out to $z \sim 16$. Finding these haloes is expected to shed light on the neutral intergalactic hydrogen during cosmic reionization.

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arXiv2024

The HI intensity mapping power spectrum: insights from recent measurements

The first direct measurements of the HI intensity mapping power spectrum were recently made using the MeerKAT telescope. These measurements are on nonlinear scales, at redshifts 0.32 and 0.44. We develop a formalism for modelling small-scale power in redshift space, within the context of the mass-weighted HI halo model framework. This model is consistent with the latest findings from surveys on the HI-halo mass relation. In order to model nonlinear scales, we include the 1-halo, shot-noise and finger-of-god effects. Then we apply the model to the MeerKAT auto-correlation data, finding that the model provides a good fit to the data at redshift 0.32, but the data may indicate some evidence for an adjustment at $z \sim 0.44$. Such an adjustment can be achieved by an increase in the HI abundance or halo model bias.

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arXiv2024

Constraints on Supermassive Black Hole Binaries from JWST and NANOGrav

We use the recent statistics of dual active galactic nuclei (AGN) in the $James \ Webb \ Space \ Telescope$ (JWST) data at $z \sim 3.4$ to address two aspects of the feedback and evolution scenarios of supermassive black hole binaries (SMBHB). We find that the JWST data provide evidence for the members of a binary black hole being 'lit' at the same time, rather than independently -- a scenario which is consistent with gas-rich mergers being responsible for concurrent AGN activity. This conclusion is supported by the recent NANOGrav Pulsar Timing Array (PTA) measurements, whose upper limits on the stochastic gravitational wave strain amplitude lie below those expected from extrapolating the dual AGN fraction. The results indicate either a 'stalling' of the binaries at the separations probed by NANOGrav, or rapid gas-driven inspirals.

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arXiv2024

COMAP Early Science: VIII. A Joint Stacking Analysis with eBOSS Quasars

We present a new upper limit on the cosmic molecular gas density at $z=2.4-3.4$ obtained using the first year of observations from the CO Mapping Array Project (COMAP). COMAP data cubes are stacked on the 3D positions of 243 quasars selected from the Extended Baryon Oscillation Spectroscopic Survey (eBOSS) catalog, yielding a 95% upper limit for flux from CO(1-0) line emission of 0.129 Jy km/s. Depending on the balance of the emission between the quasar host and its environment, this value can be interpreted as an average CO line luminosity $L'_\mathrm{CO}$ of eBOSS quasars of $\leq 1.26\times10^{11}$ K km pc$^2$ s$^{-1}$, or an average molecular gas density $ρ_\mathrm{H_2}$ in regions of the universe containing a quasar of $\leq 1.52\times10^8$ M$_\odot$ cMpc$^{-3}$. The $L'_\mathrm{CO}$ upper limit falls among CO line luminosities obtained from individually-targeted quasars in the COMAP redshift range, and the $ρ_\mathrm{H_2}$ value is comparable to upper limits obtained from other Line Intensity Mapping (LIM) surveys and their joint analyses. Further, we forecast the values obtainable with the COMAP/eBOSS stack after the full 5-year COMAP Pathfinder survey. We predict that a detection is probable with this method, depending on the CO properties of the quasar sample. Based on the achieved sensitivity, we believe that this technique of stacking LIM data on the positions of traditional galaxy or quasar catalogs is extremely promising, both as a technique for investigating large galaxy catalogs efficiently at high redshift and as a technique for bolstering the sensitivity of LIM experiments, even with a fraction of their total expected survey data.

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arXiv2023

Constraining the AGN luminosity function from JWST with the X-ray Background

We predict the X-ray background (XRB) expected from the population of quasars detected by the JWST spectroscopic surveys over the redshift range $z \sim 4-7$. We find that the measured UV emissivities, in combination with a best-fitting quasar SED template, imply a $\sim 10$ times higher unresolved X-ray background than constrained by current experiments. We illustrate the difficulty of simultaneously matching the faint-end of the quasar luminosity function and the X-ray background constraints. We discuss possible origins and consequences of this discrepancy.

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arXiv2023

Alleviating the need for exponential evolution of JWST galaxies in 10$^{10} M_{\odot}$ haloes at $z > 10$ by a modified $Λ$CDM power spectrum

We infer the evolution of the UV luminosities of galaxies in haloes of masses $10^{10} - 10^{11} M_{\odot}$ in the redshift range of $z \sim 9-16$ from the recent JWST data. Within the standard $Λ$CDM cosmological model, it is found that the average luminosities in this halo mass range show an exponential evolution with redshift, in excess of that expected from astrophysical considerations including the evolution of UV luminosity from Population III galaxies. We find that an enhancement of power on scales $k \sim 1$ Mpc$^{-1}$, as captured by a cosmological transfer function modified from the $Λ$CDM form, is able to alleviate this effect and allow for a non-evolving UV luminosity as a function of redshift at $z > 10$, consistently with the corresponding findings for lower redshifts. We discuss the possible astrophysical and cosmological reasons for such an enhancement.

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arXiv2023

Unravelling the formation of the first supermassive black holes with the SKA pulsar timing array

Galaxy mergers at high redshifts trigger the activity of their central supermassive black holes, eventually also leading to their coalescence -- and a potential source of low-frequency gravitational waves detectable by the SKA Pulsar Timing Array (PTA). Two key parameters related to the fuelling of black holes are the Eddington ratio of quasar accretion $η_{\rm Edd}$, and the radiative efficiency of the accretion process, $ε$ (which affects the so-called active lifetime of the quasar, $t_{\rm QSO}$). We forecast the regime of detectability of gravitational wave events with SKA PTA, finding the associated binaries to have orbital periods on the order of weeks to years, observable through relativistic Doppler velocity boosting and/or optical variability of their light curves. Combining the SKA regime of detectability with the latest observational constraints on high-redshift black hole mass and luminosity functions, and theoretically motivated prescriptions for the merger rates of dark matter haloes, we forecast the number of active counterparts of SKA PTA events expected as a function of primary black hole mass at $z \gtrsim 6$. We find that the quasar counterpart of the most massive black holes will be ${uniquely \ localizable}$ within the SKA PTA error ellipse at $z \gtrsim 6$. We also forecast the number of expected counterparts as a function of the quasars' Eddington ratio and active lifetime. Our results show that SKA PTA detections can place robust constraints on the seeding and growth mechanisms of the first supermassive black holes.

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arXiv2023

Synergizing 21 cm and sub-millimetre surveys during reionization: new empirical insights

We use the latest results from Atacama Large Millimetre/submillimetre Array (ALMA) surveys targeting the ionized carbon [CII] 158 $μ$m and oxygen [OIII] 88 $μ$m lines, in combination with data-driven predictions for the evolution of neutral hydrogen (HI), to illustrate the prospects for intensity mapping cross-correlations between 21 cm and submillimetre surveys over $z \sim 5-7$. We work with a dataset including the ALPINE and REBELS surveys for [CII] over $z \sim 4.5-7$, and ALMA [OIII] detections over $z \sim 6-9$. The resultant evolution of the [CII] luminosity - halo mass relation is well described by a double power law at high redshifts, with the best-fitting parameters in good agreement with the results of simulations. The data favour secure detections of the auto-power spectrum of [CII] at all redshifts with an enhanced Fred Young Submillimetre Telescope (FYST)-like configuration. Such an experiment, along with the Murchinson Widefield Array (MWA) will be able to measure the 21 cm - [CII] cross-correlation power with a signal-to-noise ratio of a few tens to a few hundreds. We find that a balloon-borne experiment improving upon the Experiment for Cryogenic Large-Aperture Intensity Mapping (EXCLAIM) should be able to detect the 21 cm - [OIII] cross-correlation with the MWA and the Square Kilometre Array (SKA)-LOW out to $z \sim 7$. Our results have implications for constraining the evolution of luminous sources during the mid-to-end stages of reionization.

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arXiv2023

A new limit on intergalactic magnetic fields on sub-kpc scales from fast radio bursts

We use the measured scattering timescales of Fast Radio Bursts (FRBs) from the CHIME catalog to derive an upper limit on the magnetic field on sub-kpc scales in the intergalactic medium (IGM). A nonmagnetized, photoionized IGM is insufficient to explain the turbulent scattering at all redshifts, with a Warm-Hot component being marginally consistent with the data at $z \sim 1$. Accounting for the lower envelope of the temporal smearing distribution with a nonzero magnetic field leads to upper limits $B < 10-30$ nG on scales of 0.07-0.20 kpc in the IGM at $z \sim 1-2$. Our work introduces a novel technique to constrain small-scale magnetic fields in the IGM, in a regime unexplored by the Rotation and Dispersion Measures of FRBs.

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arXiv2023

The deconvolved distribution estimator: enhancing reionisation-era CO line-intensity mapping analyses with a cross-correlation analogue for one-point statistics

We present the deconvolved distribution estimator (DDE), an extension of the voxel intensity distribution (VID), in the context of future observations proposed as part of the CO Mapping Array Project (COMAP). The DDE exploits the fact that the observed VID is a convolution of correlated signal intensity distributions and uncorrelated noise or interloper intensity distributions. By deconvolving the individual VID of two observables away from their joint VID in a Fourier-space operation, the DDE suppresses sensitivity to interloper emission while maintaining sensitivity to correlated components. The DDE thus improves upon the VID by reducing the relative influence of uncorrelated noise and interloper biases, which is useful in the context of COMAP observations that observe different rotational transitions of CO from the same comoving volume in different observing frequency bands. Fisher forecasts suggest that the theoretical sensitivity in the DDE allows significant improvements in constraining power compared to either the cross power spectrum or the individual VID data, and matches the constraining power of the combination of all other one- and two-point summary statistics. Future work should further investigate the covariance and model-dependent behaviour of this novel one-point cross-correlation statistic.

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arXiv2022

Intensity mapping from the sky: synergizing the joint potential of [OIII] and [CII] surveys at reionization

We forecast the ability of future-generation experiments to detect the fine-structure lines of the carbon and oxygen ions, [CII] and [OIII] in intensity mapping (IM) from the Epoch of Reionization ($z \sim 6-8$). Combining the latest empirically derived constraints relating the luminosity of the [OIII] line to the ambient star-formation rate, and using them in conjunction with previously derived estimates for the abundance of [CII] in haloes, we predict the expected auto-correlation IM signal to be observed using new experiments based on the Fred Young Submillimetre Telescope (FYST) and the balloon-borne facility, Experiment for Cryogenic Large-Aperture Intensity Mapping (EXCLAIM) over $z \sim 5.3 - 7$. We describe how improvements to both the ground-based and balloon-based surveys in the future will enable a cross-correlation signal to be detected at $\sim$ 10-30 $σ$ over $z \sim 5.3 - 7$. Finally, we propose a space-based mission targeting the [OIII] 88 and 52 $μ$m lines along with the [CII] 158 $μ$m line, configured to enhance the signal-to-noise ratio of cross-correlation measurements. We find that such a configuration can achieve a high-significance detection (hundreds of $σ$) in both auto- and cross-correlation modes.

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arXiv2022

COMAP Early Science: IV. Power Spectrum Methodology and Results

We present the power spectrum methodology used for the first-season COMAP analysis, and assess the quality of the current data set. The main results are derived through the Feed-feed Pseudo-Cross-Spectrum (FPXS) method, which is a robust estimator with respect to both noise modeling errors and experimental systematics. We use effective transfer functions to take into account the effects of instrumental beam smoothing and various filter operations applied during the low-level data processing. The power spectra estimated in this way have allowed us to identify a systematic error associated with one of our two scanning strategies, believed to be due to residual ground or atmospheric contamination. We omit these data from our analysis and no longer use this scanning technique for observations. We present the power spectra from our first season of observing and demonstrate that the uncertainties are integrating as expected for uncorrelated noise, with any residual systematics suppressed to a level below the noise. Using the FPXS method, and combining data on scales $k=0.051-0.62 \,\mathrm{Mpc}^{-1}$ we estimate $P_\mathrm{CO}(k) = -2.7 \pm 1.7 \times 10^4μ\textrm{K}^2\mathrm{Mpc}^3$, the first direct 3D constraint on the clustering component of the CO(1-0) power spectrum in the literature.

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arXiv2022

Signatures of supermassive black hole binaries on maser systems

We illustrate a novel signature of black hole binaries from their effect on the kinematics of water maser emission in their environments. With the help of simulations, we establish the condition for clumps to mase based on their coherence lengths calibrated to those of the known maser galaxy NGC 4258. This is then used to identify masing clumps around a binary black hole system, and quantify the kinematic and spectral differences relative to the single black hole case. For some generic circumstances, blue-shifted masers around a binary black hole are found to preferentially follow the Keplerian rotation curve observed in the single black hole case. The redshifted ones, however, are found to visibly deviate from this relation, and also display more scatter with a tendency towards lower absolute values of the velocity along the line-of-sight. The spectrum of the masers as a function of line-of-sight velocity also shows a double peaked structure, reminiscent of recent observations of systems such as Mrk 1. Our results motivate future prospects for identifying binary black hole candidates with the help of water maser emissions.

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arXiv2022

COMAP Early Science: V. Constraints and Forecasts at $z \sim 3$

We present the current state of models for the $z\sim3$ carbon monoxide (CO) line-intensity signal targeted by the CO Mapping Array Project (COMAP) Pathfinder in the context of its early science results. Our fiducial model, relating dark matter halo properties to CO luminosities, informs parameter priors with empirical models of the galaxy-halo connection and previous CO(1-0) observations. The Pathfinder early science data spanning wavenumbers $k=0.051$-$0.62\,$Mpc$^{-1}$ represent the first direct 3D constraint on the clustering component of the CO(1-0) power spectrum. Our 95% upper limit on the redshift-space clustering amplitude $A_{\rm clust}\lesssim70\,μ$K$^2$ greatly improves on the indirect upper limit of $420\,μ$K$^2$ reported from the CO Power Spectrum Survey (COPSS) measurement at $k\sim1\,$Mpc$^{-1}$. The COMAP limit excludes a subset of models from previous literature, and constrains interpretation of the COPSS results, demonstrating the complementary nature of COMAP and interferometric CO surveys. Using line bias expectations from our priors, we also constrain the squared mean line intensity-bias product, $\langle{Tb}\rangle^2\lesssim50\,μ$K$^2$, and the cosmic molecular gas density, $ρ_\text{H2}<2.5\times10^8\,M_\odot\,$Mpc$^{-3}$ (95% upper limits). Based on early instrument performance and our current CO signal estimates, we forecast that the five-year Pathfinder campaign will detect the CO power spectrum with overall signal-to-noise of 9-17. Between then and now, we also expect to detect the CO-galaxy cross-spectrum using overlapping galaxy survey data, enabling enhanced inferences of cosmic star-formation and galaxy-evolution history.

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