Cosmic reionization history and dark matter scenarios

Summary plot is Fig. 8: constraints on escape fraction and UV limiting magnitude for cold and non-cold dark matter. This version matches the published oneInternational audienceWe perform an analysis of the cosmic reionization in the standard cold dark matter (CDM) paradigm and in alternative dark matter scenarios. Building upon the work of Corasaniti et al. (2017), we predict the reionization history for CDM, for warm dark matter (WDM), late-forming dark matter (LFDM) and ultra-light axion dark matter (ADM) models which reproduce state-of-art measurements of the galaxy luminosity function at very high-redshifts $6\le z\le 10$. To this purpose we adopt a reionization model parametrized in terms of the limiting UV-magnitude of galaxies contributing to the reionization $M_{\rm lim}$ and the average effective escape fraction of UV photons reaching the intergalactic medium $\tilde{f}$. For each DM model we compute the redshift evolution of the Thomson scattering optical depth $\tau_e(z)$ and the comoving ionization fraction $Q_{\rm HII}(z)$. We find the different DM models to have similar reionization histories. Differences with respect to the CDM case increase at fainter limiting UV-magnitudes and are degenerate with the effect of varying the reionization model parameters. Using Planck's determination of the integrated optical depth in combination with measurements of the neutral hydrogen fraction at different redshifts, we infer constraints on $\tilde{f}$ and $M_{\rm lim}$. The results are largely independent of the assumed DM scenario, in particular for $M_{\rm lim}\gtrsim -13$ we obtain that the effective escape fraction lies in the range $0.07\lesssim \tilde{f}\lesssim 0.15$ at $2\sigma$

Cosmic neutral hydrogen as tracer of the large scale structure of the Universe

The nature of the most abundant components of the Universe, dark energy and dark matter, is still to be uncovered. The study of the matter distribution in the Universe and of its clustering properties may reveal key insight on the origin of these phenomena. However, matter is not directly observable, but can be mapped through tracers. In this study we consider neutral hydrogen, which pervades space from the time of recombination up to present day. It has a characteristic line emission at around 21cm that being redshifted gives a measure of comic distance and makes it possible to reconstruct the three-dimensional density field over a wide range of redshift and scales. In the upcoming years a new generation of experiments will map the distribution of neutral hydrogen from unresolved galaxies up to the high-redshift Universe, by employing a technique called intensity mapping. In this thesis we assess the potential of the 21cm intensity maps for constraining cosmological models. We analyse competitive and realistic dark energy and dark matter models with state-of-the-art numerical simulations and show how different cosmologies produce distinctive and detectable effects in the 21cm signal. We compute radio telescope forecasts showing how these models will be distinguishable in an unprecedented way. The drawback of 21cm observations is the intrinsic weakness of the signal compared to the expected large foregrounds. Motivated by this, we study the cross-correlation signal between 21cm intensity maps and the transmitted Lyman-alpha forest flux, representing a powerful way to extract more information, by isolating instrumental and astrophysical systematics

Matter around Kerr black holes in scalar-tensor theories: scalarization and superradiant instability

In electrovacuum stationary, asymptotically flat black holes in scalar-tensor theories of gravity are described by the Kerr-Newman family of solutions, just as in general relativity. We show that there exist two mechanisms which can render Kerr black holes unstable when matter is present in the vicinity of the black hole, as this induces an effective mass for the scalar. The first mechanism is a tachyonic instability that appears when the effective mass squared is negative, triggering the development of scalar hair --- a black hole version of "spontaneous scalarization". The second instability is associated with superradiance and is present when the effective mass squared is positive and when the black hole spin exceeds a certain threshold. The second mechanism is also responsible for a resonant effect in the superradiant scattering of scalar waves, with amplification factors as large as 10^5 or more.Comment: 11 pages, 3 figures. v2: published versio

Particle ejection during mergers of dark matter halos

Dark matter halos are built from accretion and merging. During merging some of the dark matter particles may be ejected with velocities higher than the escape velocity. We use both N-body simulations and single-particle smooth-field simulations to demonstrate that rapid changes to the mean field potential are responsible for such ejection, and in particular that dynamical friction plays no significant role in it. Studying a range of minor mergers, we find that typically between 5-15% of the particles from the smaller of the two merging structures are ejected. We also find that the ejected particles originate essentially from the small halo, and more specifically are particles in the small halo which pass later through the region in which the merging occurs.Comment: 18 pages, 12 figures. Accepted for publication in JCA

Black holes with surrounding matter in scalar-tensor theories

We uncover two mechanisms that can render Kerr black holes unstable in scalar-tensor gravity, both associated to the presence of matter in the vicinity of the black hole and the fact that this introduces an effective mass for the scalar. Our results highlight the importance of understanding the structure of spacetime in realistic, astrophysical black holes in scalar-tensor theories.Comment: 5 pages. Published version. arXiv admin note: substantial text overlap with arXiv:1305.693

The foreground transfer function for HI intensity mapping signal reconstruction: MeerKLASS and precision cosmology applications

Blind cleaning methods are currently the preferred strategy for handling foreground contamination in single-dish HI intensity mapping surveys. Despite the increasing sophistication of blind techniques, some signal loss will be inevitable across all scales. Constructing a corrective transfer function using mock signal injection into the contaminated data has been a practice relied on for HI intensity mapping experiments. However, assessing whether this approach is viable for future intensity mapping surveys where precision cosmology is the aim, remains unexplored. In this work, using simulations, we validate for the first time the use of a foreground transfer function to reconstruct power spectra of foreground-cleaned low-redshift intensity maps and look to expose any limitations. We reveal that even when aggressive foreground cleaning is required, which causes ${>}\,50\%$ negative bias on the largest scales, the power spectrum can be reconstructed using a transfer function to within sub-percent accuracy. We specifically outline the recipe for constructing an unbiased transfer function, highlighting the pitfalls if one deviates from this recipe, and also correctly identify how a transfer function should be applied in an auto-correlation power spectrum. We validate a method that utilises the transfer function variance for error estimation in foreground-cleaned power spectra. Finally, we demonstrate how incorrect fiducial parameter assumptions (up to ${\pm}100\%$ bias) in the generation of mocks, used in the construction of the transfer function, do not significantly bias signal reconstruction or parameter inference (inducing ${<}\,5\%$ bias in recovered values).Comment: 25 pages, 20 figures. See Figure 4 for the main demonstration of the transfer function's performance for reconstructing signal loss from foreground cleaning. Submitted to MNRAS for publicatio

SKAO HI intensity mapping: blind foreground subtraction challenge

Neutral Hydrogen Intensity Mapping (H I IM) surveys will be a powerful new probe of cosmology. However, strong astrophysical foregrounds contaminate the signal and their coupling with instrumental systematics further increases the data cleaning complexity. In this work, we simulate a realistic single-dish HI IM survey of a 5000 deg2 patch in the 950–1400 MHz range, with both the MID telescope of the SKA Observatory (SKAO) and MeerKAT, its precursor. We include a state-of-the-art HI simulation and explore different foreground models and instrumental effects such as non-homogeneous thermal noise and beam side lobes. We perform the first Blind Foreground Subtraction Challenge for HI IM on these synthetic data cubes, aiming to characterize the performance of available foreground cleaning methods with no prior knowledge of the sky components and noise level. Nine foreground cleaning pipelines joined the challenge, based on statistical source separation algorithms, blind polynomial fitting, and an astrophysical-informed parametric fit to foregrounds. We devise metrics to compare the pipeline performances quantitatively. In general, they can recover the input maps’ two-point statistics within 20 per cent in the range of scales least affected by the telescope beam. However, spurious artefacts appear in the cleaned maps due to interactions between the foreground structure and the beam side lobes. We conclude that it is fundamental to develop accurate beam deconvolution algorithms and test data post-processing steps carefully before cleaning. This study was performed as part of SKAO preparatory work by the HI IM Focus Group of the SKA Cosmology Science Working Group

Mapping Cosmic Dawn and Reionization: Challenges and Synergies

Cosmic dawn and the Epoch of Reionization (EoR) are among the least explored observational eras in cosmology: a time at which the first galaxies and supermassive black holes formed and reionized the cold, neutral Universe of the post-recombination era. With current instruments, only a handful of the brightest galaxies and quasars from that time are detectable as individual objects, due to their extreme distances. Fortunately, a multitude of multi-wavelength intensity mapping measurements, ranging from the redshifted 21 cm background in the radio to the unresolved X-ray background, contain a plethora of synergistic information about this elusive era. The coming decade will likely see direct detections of inhomogenous reionization with CMB and 21 cm observations, and a slew of other probes covering overlapping areas and complementary physical processes will provide crucial additional information and cross-validation. To maximize scientific discovery and return on investment, coordinated survey planning and joint data analysis should be a high priority, closely coupled to computational models and theoretical predictions.Comment: 5 pages, 1 figure, submitted to the Astro2020 Decadal Survey Science White Paper cal

Cosmology with the Highly Redshifted 21cm Line

In addition to being a probe of Cosmic Dawn and Epoch of Reionization astrophysics, the 21cm line at $z>6$ is also a powerful way to constrain cosmology. Its power derives from several unique capabilities. First, the 21cm line is sensitive to energy injections into the intergalactic medium at high redshifts. It also increases the number of measurable modes compared to existing cosmological probes by orders of magnitude. Many of these modes are on smaller scales than are accessible via the CMB, and moreover have the advantage of being firmly in the linear regime (making them easy to model theoretically). Finally, the 21cm line provides access to redshifts prior to the formation of luminous objects. Together, these features of 21cm cosmology at $z>6$ provide multiple pathways toward precise cosmological constraints. These include the "marginalizing out" of astrophysical effects, the utilization of redshift space distortions, the breaking of CMB degeneracies, the identification of signatures of relative velocities between baryons and dark matter, and the discovery of unexpected signs of physics beyond the $\Lambda$CDM paradigm at high redshifts.Comment: Science white paper submitted to Decadal 2020 surve