arXiv subjects

Hamsa Padmanabhan

Publications and source records attributed to Hamsa Padmanabhan.

At least 37 records · Page 2Linked to original sources

arXiv2022

A Multi-messenger view of Cosmic Dawn: Conquering the Final Frontier

The epoch of Cosmic Dawn, when the first stars and galaxies were born, is widely considered the final frontier of observational cosmology today. Mapping the period between Cosmic Dawn and the present-day provides access to more than 90% of the baryonic (normal) matter in the Universe, and unlocks several thousand times more Fourier modes of information than available in today's cosmological surveys. We review the progress in modelling baryonic gas observations as tracers of the cosmological large-scale structure from Cosmic Dawn to the present day. We illustrate how the description of dark matter haloes can be extended to describe baryonic gas abundances and clustering. This innovative approach allows us to fully utilize our current knowledge of astrophysics to constrain cosmological parameters from future observations. Combined with the information content of multi-messenger probes, this will also elucidate the properties of the first supermassive black holes at Cosmic Dawn. We present a host of fascinating implications for constraining physics beyond the $Λ$CDM model, including tests of the theories of inflation and the cosmological principle, the effects of non-standard dark matter, and possible deviations from Einstein's general relativity on the largest scales.

astro-ph.CO↗

arXiv2022

Signatures of Population III supernovae at Cosmic Dawn: the case of GN-z11-flash

We illustrate the observability of the end stages of the earliest (Population III) stars at high redshifts $z \gtrsim 10$, using the recently observed transient, GN-z11-flash as an example. We find that the observed spectrum of this transient is consistent with its originating from a shock-breakout in a Population III supernova occurring in the GN-z11 galaxy at $z \sim 11$. The energetics of the explosion indicate a progenitor star of mass $\sim 300 M_{\odot}$ in that galaxy, with of order unity such events expected over an observing timescale of a few years. We forecast the expected number of such transients from $z > 10$ galaxies as a function of their host stellar mass and star formation rate. Our findings are important in the context of future searches to detect and identify the signatures of galaxies at Cosmic Dawn.

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arXiv2021

COMAP Early Science: III. CO Data Processing

We describe the first season COMAP analysis pipeline that converts raw detector readouts to calibrated sky maps. This pipeline implements four main steps: gain calibration, filtering, data selection, and map-making. Absolute gain calibration relies on a combination of instrumental and astrophysical sources, while relative gain calibration exploits real-time total-power variations. High efficiency filtering is achieved through spectroscopic common-mode rejection within and across receivers, resulting in nearly uncorrelated white noise within single-frequency channels. Consequently, near-optimal but biased maps are produced by binning the filtered time stream into pixelized maps; the corresponding signal bias transfer function is estimated through simulations. Data selection is performed automatically through a series of goodness-of-fit statistics, including $χ^2$ and multi-scale correlation tests. Applying this pipeline to the first-season COMAP data, we produce a dataset with very low levels of correlated noise. We find that one of our two scanning strategies (the Lissajous type) is sensitive to residual instrumental systematics. As a result, we no longer use this type of scan and exclude data taken this way from our Season 1 power spectrum estimates. We perform a careful analysis of our data processing and observing efficiencies and take account of planned improvements to estimate our future performance. Power spectrum results derived from the first-season COMAP maps are presented and discussed in companion papers.

astro-ph.IM↗

arXiv2021

COMAP Early Science: II. Pathfinder Instrument

Line intensity mapping (LIM) is a new technique for tracing the global properties of galaxies over cosmic time. Detection of the very faint signals from redshifted carbon monoxide (CO), a tracer of star formation, pushes the limits of what is feasible with a total-power instrument. The CO Mapping Project (COMAP) Pathfinder is a first-generation instrument aiming to prove the concept and develop the technology for future experiments, as well as delivering early science products. With 19 receiver channels in a hexagonal focal plane arrangement on a 10.4 m antenna, and an instantaneous 26-34 GHz frequency range with 2 MHz resolution, it is ideally suited to measuring CO($J$=1-0) from $z\sim3$. In this paper we discuss strategies for designing and building the Pathfinder and the challenges that were encountered. The design of the instrument prioritized LIM requirements over those of ancillary science. After a couple of years of operation, the instrument is well understood, and the first year of data is already yielding useful science results. Experience with this Pathfinder will drive the design of the next generations of experiments.

astro-ph.IM↗

arXiv2021

COMAP Early Science: I. Overview

The CO Mapping Array Project (COMAP) aims to use line intensity mapping of carbon monoxide (CO) to trace the distribution and global properties of galaxies over cosmic time, back to the Epoch of Reionization (EoR). To validate the technologies and techniques needed for this goal, a Pathfinder instrument has been constructed and fielded. Sensitive to CO(1-0) emission from $z=2.4$-$3.4$ and a fainter contribution from CO(2-1) at $z=6$-8, the Pathfinder is surveying $12$ deg$^2$ in a 5-year observing campaign to detect the CO signal from $z\sim3$. Using data from the first 13 months of observing, we estimate $P_\mathrm{CO}(k) = -2.7 \pm 1.7 \times 10^4μ\mathrm{K}^2 \mathrm{Mpc}^3$ on scales $k=0.051-0.62 \mathrm{Mpc}^{-1}$ - the first direct 3D constraint on the clustering component of the CO(1-0) power spectrum. Based on these observations alone, we obtain a constraint on the amplitude of the clustering component (the squared mean CO line temperature-bias product) of $\langle Tb\rangle^2<49$ $μ$K$^2$ - nearly an order-of-magnitude improvement on the previous best measurement. These constraints allow us to rule out two models from the literature. We forecast a detection of the power spectrum after 5 years with signal-to-noise ratio (S/N) 9-17. Cross-correlation with an overlapping galaxy survey will yield a detection of the CO-galaxy power spectrum with S/N of 19. We are also conducting a 30 GHz survey of the Galactic plane and present a preliminary map. Looking to the future of COMAP, we examine the prospects for future phases of the experiment to detect and characterize the CO signal from the EoR.

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arXiv2021

COMAP Early Science: VII. Prospects for CO Intensity Mapping at Reionization

We introduce COMAP-EoR, the next generation of the Carbon Monoxide Mapping Array Project aimed at extending CO intensity mapping to the Epoch of Reionization. COMAP-EoR supplements the existing 30 GHz COMAP Pathfinder with two additional 30 GHz instruments and a new 16 GHz receiver. This combination of frequencies will be able to simultaneously map CO(1--0) and CO(2--1) at reionization redshifts ($z\sim5-8$) in addition to providing a significant boost to the $z\sim3$ sensitivity of the Pathfinder. We examine a set of existing models of the EoR CO signal, and find power spectra spanning several orders of magnitude, highlighting our extreme ignorance about this period of cosmic history and the value of the COMAP-EoR measurement. We carry out the most detailed forecast to date of an intensity mapping cross-correlation, and find that five out of the six models we consider yield signal to noise ratios (S/N) $\gtrsim20$ for COMAP-EoR, with the brightest reaching a S/N above 400. We show that, for these models, COMAP-EoR can make a detailed measurement of the cosmic molecular gas history from $z\sim2-8$, as well as probe the population of faint, star-forming galaxies predicted by these models to be undetectable by traditional surveys. We show that, for the single model that does not predict numerous faint emitters, a COMAP-EoR-type measurement is required to rule out their existence. We briefly explore prospects for a third-generation Expanded Reionization Array (COMAP-ERA) capable of detecting the faintest models and characterizing the brightest signals in extreme detail.

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arXiv2021

Synergies between the COMAP CO Line Intensity Mapping mission and a Lyα galaxy survey: How to probe the early universe with voxel based analysis of observational data

Line Intensity Mapping (LIM) offers a novel avenue to observe and characterize our universe. LIM data of CO spectral lines are becoming available, such as those obtained by the CO Mapping Array Project (COMAP). COMAP data can be used to probe the molecular gas content of the universe from the last stages of the Epoch of Reionization (EoR) ($z < 8.0$) to $z \sim 2.5$. In this work, we examine the prospects for deriving voxel-level statistical constraints on high-redshift galaxies from COMAP data by considering the additional information available from observations of LAEs galaxies using the Visible Integral-Field Replicable Unit Spectrograph (VIRUS) on the Hobby-Eberly Telescope (HET). We post-process the IllustrisTNG300 galaxy-formation simulation with a set of prescriptions to consistently determine CO and Ly$α$ line luminosities. The different line prescriptions span the uncertainty in the CO line luminosity according to current observations by the VLA high-z CO surveys and set the Ly$α$ emission to be compatible with observational LAE luminosity functions. We produce mock observations for the two surveys over a $(300\, {\rm Mpc})^3$ volume. These are then used to formulate and test methodologies for data analysis and to predict COMAP constraints on CO emission. We use combinations of masking, stacking, voxel intensity distribution (VID), and other statistics. We find that in combination with VIRUS/HET, a voxel-level analysis of the COMAP Pathfinder survey can detect and characterize the CO signal from $z\sim3$ and improve current constraints on the $z\sim6$ signal, identify individual voxels with bright CO(1-0) emission at $z\sim3$ and probe the redshift evolution of the CO emission. This study illustrates the potential of synergies between LIM and galaxy surveys both to improve the significance of a detection and to aid the interpretation of noisy LIM data.

astro-ph.GA↗

arXiv2021

A new way to test the Cosmological Principle: measuring our peculiar velocity and the large scale anisotropy independently

We present a novel approach to disentangle two key contributions to the largest-scale anisotropy of the galaxy distribution: (i) the intrinsic dipole due to clustering and anisotropic geometry, and (ii) the kinematic dipole due to our peculiar velocity. Including the redshift and angular size of galaxies, in addition to their fluxes and positions allows us to measure both the direction and amplitude of our velocity independently of the intrinsic dipole of the source distribution. We find that this new approach applied to future galaxy surveys (LSST and Euclid) and a SKA radio continuum survey will allow to measure our velocity ($β= v/c$) with a relative error in the amplitude $σ(β)/β\sim (1.3 - 4.5)\%$ and in direction, $θ_β \sim 0.9^\circ - 3.9^\circ$, well beyond what can be achieved when analysing only the number count dipole. We also find that galaxy surveys are able to measure the intrinsic large-scale anisotropy with a relative uncertainty of $\lesssim 5\%$ (measurement error, not including cosmic variance). Our method enables two simultaneous tests of the Cosmological Principle: comparing the observations of our peculiar velocity with the CMB dipole, and testing for a significant intrinsic anisotropy on large scales which would indicate effects beyond the standard cosmological model.

astro-ph.CO↗

arXiv2021

A model of spectral line broadening in signal forecasts for line-intensity mapping experiments

Line-intensity mapping observations will find fluctuations of integrated line emission are attenuated by varying degrees at small scales due to the width of the line emission profiles. This attenuation may significantly impact estimates of astrophysical or cosmological quantities derived from measurements. We consider a theoretical treatment of the effect of line broadening on both the clustering and shot-noise components of the power spectrum of a generic line-intensity power spectrum using a halo model. We then consider possible simplifications to allow easier application in analysis, particularly in the context of inferences that require numerous, repeated, fast computations of model line-intensity signals across a large parameter space. For the CO Mapping Array Project (COMAP) and the CO(1-0) line-intensity field at $z\sim3$ serving as our primary case study, we expect a $\sim10\%$ attenuation of the spherically averaged power spectrum on average at relevant scales of $k\approx0.2$-$0.3$ Mpc$^{-1}$, compared to $\sim25\%$ for the interferometric Millimetre-wave Intensity Mapping Experiment (mmIME) targeting shot noise from CO lines at $z\sim1$-$5$ at scales of $k\gtrsim1$ Mpc$^{-1}$. We also consider the nature and amplitude of errors introduced by simplified treatments of line broadening, and find that while an approximation using a single effective velocity scale is sufficient for spherically-averaged power spectra, a more careful treatment is necessary when considering other statistics such as higher multipoles of the anisotropic power spectrum or the voxel intensity distribution.

astro-ph.CO↗

arXiv2021

Contribution of Flares from Tidal Disruption of Stars to high-redshift AGN

We explore the contributions of Tidal Disruption Events (TDEs) to the flares in Active Galactic Nuclei (AGN) at high redshifts. Using the latest data available from X-ray and optical observations of high-redshift galaxies, in combination with the evolution of their central supermassive black holes, we calculate the contribution of TDE to AGNs as a function of their luminosities. We find that at low redshifts ($z < 1$), a few percent of all AGN with bolometric luminosities $L_{\rm Bol} \lesssim 10^{44}$ erg s$^{-1}$ may be attributable to possible TDEs. However, this fraction can significantly increase at earlier cosmic times, including up to several tens of percent of the population of AGN at $z \gtrsim 3$. TDEs may comprise a significant fraction of the Compton-Thick (CT) AGN population at $z \gtrsim 3$. The above findings motivate further calibration with upcoming X-ray missions and spectroscopic surveys targeting TDE-AGN.

astro-ph.GA↗

arXiv2021

The effect of finite halo size on the clustering of neutral hydrogen

Post-reionisation 21cm intensity mapping experiments target the spectral line of neutral hydrogen (HI) resident in dark matter haloes. According to the halo model, these discrete haloes trace the continuous dark matter density field down to a certain scale, which is dependent on the halo physical size. The halo physical size defines an exclusion region which leaves imprints on the statistical properties of HI. We show how the effect of exclusion due to the finite halo size impacts the HI power spectrum, with the physical boundary of the host halo given by the splashback radius. Most importantly, we show that the white noise-like feature that appears in the zero-momentum limit of the power spectrum is exactly cancelled when the finite halo size is taken into consideration. This cancellation in fact applies to all tracers of dark matter density field, including galaxies. Furthermore, we show that the exclusion due to finite halo size leads to a sub-Poissonian noise signature on large scales, consistent with the results from N-body simulations

astro-ph.CO↗

arXiv2021

Beyond $Λ$CDM with HI intensity mapping: robustness of cosmological constraints in the presence of astrophysics

Mapping the unresolved intensity of the 21-cm emission of neutral hydrogen (HI) is now regarded as one the most promising tools for cosmological investigation in the coming decades. Here, we investigate, for the first time, extensions of the standard cosmological model, such as modified gravity and primordial non-Gaussianity, taking self-consistently into account the present constraints on the astrophysics of HI clustering in the treatment of the brightness temperature fluctuations. To understand the boundaries within which results thus obtained can be considered reliable, we examine the robustness of cosmological parameter estimation performed via studies of 21-cm intensity mapping, against our knowledge of the astrophysical processes leading to HI clustering. Modelling of astrophysical effects affects cosmological observables through the relation linking the overall HI mass in a bound object, to the mass of the underlying dark matter halo that hosts it. We quantify the biases in estimates of standard cosmological parameters and those describing modified gravity and primordial non-Gaussianity, that are obtained if one misconceives the slope of the relation between HI mass and halo mass, or the lower virial velocity cut-off for a dark matter halo to be able to host HI. Remarkably, we find that astrophysical uncertainties will not affect searches for primordial non-Gaussianity - one of the strongest science cases for HI intensity mapping - despite the signal being deeply linked to the HI bias.

astro-ph.CO↗

arXiv2021

Distinguishing AGN from starbursts as the origin of double peaked Lyman-Alpha Emitters in the reionization era

We discuss the possible origin of the double-peaked profiles recently observed in Lyman-Alpha Emitters (LAEs) at the epoch of reionization ($z \gtrsim 6.5$) from obscured active galactic nuclei (AGN). Combining the extent of the Lyman-$α$ near-zones estimated from the blue peak velocity offset in these galaxies, with the ionizing emissivity of quasars at $z \gtrsim 6$, we forecast the intrinsic UV and X-ray luminosities of the AGN needed to give rise to their double-peaked profiles. We also estimate the extent of the obscuration of the AGN by comparing their luminosities to those of similar quasar samples at these epochs. Future X-ray and radio observations, as well as those with the ${James \ Webb \ Space \ Telescope}$, will be valuable tools to test the AGN contribution to the intergalactic-scale ionization zones of high-redshift LAEs.

astro-ph.GA↗

arXiv2020

Intensity Mapping as a Probe of Axion Dark Matter

Intensity mapping (IM) of spectral lines has the potential to revolutionize cosmology by increasing the total number of observed modes by several orders of magnitude compared to the cosmic microwave background (CMB) anisotropies. In this paper, we consider IM of neutral hydrogen (HI) in the redshift range $0 \lesssim z \lesssim 3$ employing a halo model approach where HI is assumed to follow the distribution of dark matter (DM) halos. If a portion of the DM is composed of ultralight axions then the abundance of halos is changed compared to cold dark matter below the axion Jeans mass. With fixed total HI density, $Ω_{\rm HI}$, assumed to reside entirely in halos, this effect introduces a scale-independent increase in the HI power spectrum on scales above the axion Jeans scale, which our model predicts consistent with N-body simulations. Lighter axions introduce a scale-dependent feature even on linear scales due to its suppression of the matter power spectrum near the Jeans scale. We use the Fisher matrix formalism to forecast the ability of future HI surveys to constrain the axion fraction of DM and marginalize over astrophysical and model uncertainties. We find that a HIRAX-like survey is a very reliable IM survey configuration, being affected minimally by uncertainties due to non-linear scales, while the SKA1MID configuration is the most constraining as it is sensitive to non-linear scales. Including non-linear scales and combining a SKA1MID-like IM survey with the Simons Observatory CMB, the benchmark "fuzzy DM" model with $m_a = 10^{-22}\text{ eV}$ can be constrained at the 10% level. For lighter ULAs this limit improves below 1%, and allows the possibility to test the connection between axion models and the grand unification scale across a wide range of masses.

astro-ph.CO↗

arXiv2020

Constraining the host galaxy halos of massive black holes from LISA event rates

The coalescence of massive black hole binaries (with masses $10^4 - 10^7 M_{\odot}$) leads to gravitational wave emission that is detectable out to high redshifts ($z \sim 20$) with the forthcoming LISA observatory. We combine the theoretically derived merger rates for dark matter haloes at various redshifts, with an empirically motivated prescription that connects the mass of a dark matter halo and that of its central black hole. Using the expected constraints on the (chirp or reduced) masses of binary black holes, their mass ratios and redshift uncertainties, we forecast the measurement precision on the occupation fraction, normalization and slope of the black hole mass - halo mass relation at various redshifts, assuming a five-year LISA survey for three different confidence scenarios. We use the expected sizes of the LISA localization ellipses on the sky to estimate the number of electromagnetic counterparts to the gravitational wave sources which are detectable by future wide-field optical surveys, such as LSST.

astro-ph.CO↗

arXiv2020

New empirical constraints on the cosmological evolution of gas and stars in galaxies

We combine the latest observationally motivated constraints on stellar properties in dark matter haloes, along with data-driven predictions for the atomic (HI) and molecular (H$_2$) gas evolution in galaxies, to derive empirical relationships between the build-up of galactic components and their evolution over cosmic time. At high redshift ($z \gtrsim 4$), the frameworks imply that galaxies acquire their cold gas (both atomic and molecular) mostly by accretion, with the fraction of cold gas reaching about 20% of the cosmic baryon fraction. We infer a strong dependence of the star formation rate on the H$_2$ mass, suggesting a near-universal depletion timescale of 0.1-1 Gyr in Milky Way sized haloes (of masses $10^{12} \ M_{\odot}$ at $z = 0$). There is also evidence for a near-universality of the Kennicutt-Schmidt relation across redshifts, with very little dependence on stellar mass, if a constant conversion factor ($α_{\rm CO}$) of CO luminosity to molecular gas mass is assumed. Combining the atomic and molecular gas observations with the stellar build-up illustrates that galactic mass assembly in Milky-Way sized haloes proceeds from smooth accretion at high redshifts, towards becoming merger-dominated at late times ($z \lesssim 0.6$). Our results can be used to constrain numerical simulations of the dominant growth and accretion processes of galaxies over cosmic history.

astro-ph.GA↗

arXiv2020

Cross-correlating 21 cm and galaxy surveys: implications for cosmology and astrophysics

We forecast astrophysical and cosmological parameter constraints from synergies between 21 cm intensity mapping and wide field optical galaxy surveys (both spectroscopic and photometric) over $z \sim 0-3$. We focus on the following survey combinations in this work: (i) a CHIME-like and DESI-like survey in the northern hemisphere, (ii) an LSST-like and SKA I MID-like survey and (ii) a MeerKAT-like and DES-like survey in the southern hemisphere. We work with the $Λ$CDM cosmological model having parameters $\{h, Ω_m, n_s, Ω_b, σ_8\}$, parameters $v_{c,0}$ and $β$ representing the cutoff and slope of the HI-halo mass relation in the previously developed HI halo model framework, and a parameter $Q$ that represents the scale dependence of the optical galaxy bias. Using a Fisher forecasting framework, we explore (i) the effects of the HI and galaxy astrophysical uncertainties on the cosmological parameter constraints, assuming priors from the present knowledge of the astrophysics, (ii) the improvements on astrophysical constraints over their current priors in the three configurations considered, (ii) the tightening of the constraints on the parameters relative to the corresponding HI auto-correlation surveys alone.

astro-ph.CO↗

arXiv2020

It is Feasible to Directly Measure Black Hole Masses in the First Galaxies

In the local universe, black hole masses have been inferred from the observed increase in the velocities of stars at the centres of their host galaxies. So far, masses of supermassive black holes in the early universe have only been inferred indirectly, using relationships calibrated to their locally observed counterparts. Here, we use the latest observational constraints on the evolution of stellar masses in galaxies to predict, for the first time, that the region of influence of a central supermassive black hole at the epochs where the first galaxies were formed is $\mathit{directly \ resolvable}$ by current and upcoming telescopes. We show that the existence of the black hole can be inferred from observations of the gas or stellar disc out to $> 0.5$ kpc from the host halo at redshifts $z \gtrsim 6$. Such measurements will usher in a new era of discoveries unraveling the formation of the first supermassive black holes based on subarcsecond-scale spectroscopy with the $JWST$, $ALMA$, and the $SKA$. The measured mass distribution of black holes will allow forecasting of the future detection of gravitational waves from the earliest black hole mergers.

astro-ph.GA↗