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Publications (6 of 6) Show all publications
Georgiev, I., Gorce, A. & Mellema, G. (2024). Constraining cosmic reionization by combining the kinetic Sunyaev–Zel’dovich and the 21 cm power spectra . Monthly notices of the Royal Astronomical Society, 528(4), 7218-7235
Open this publication in new window or tab >>Constraining cosmic reionization by combining the kinetic Sunyaev–Zel’dovich and the 21 cm power spectra 
2024 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 528, no 4, p. 7218-7235Article in journal (Refereed) Published
Abstract [en]

During the Epoch of Reionization (EoR), the ultraviolet radiation from the first stars and galaxies ionized the neutral hydrogen of the intergalactic medium, which can emit radiation through its 21 cm hyperfine transition. Measuring the 21 cm power spectrum is a key science goal for the future Square Kilometre Array (SKA); however, observing and interpreting it is a challenging task. Another high-potential probe of the EoR is the patchy kinetic Sunyaev–Zel’dovich (pkSZ) effect, observed as a foreground to the cosmic microwave background temperature anisotropies on small scales. Despite recent promising measurements, placing constraints on reionization from pkSZ observations is a non-trivial task, subject to strong model dependence. We propose to alleviate the difficulties in observing and interpreting the 21 cm and pkSZ power spectra by combining them. With a simple yet effective parametric model that establishes a formal connection between them, we can jointly fit mock 21 cm and pkSZ data points. We confirm that these observables provide complementary information on reionization, leading to significantly improved constraints when combined. We demonstrate that with as few as two measurements of the 21 cm power spectrum with 100 h of observations with the SKA, as well as a single ℓ = 3000 pkSZ data point, we can reconstruct the reionization history of the universe and its morphology. We find that the reionization history (morphology) is better constrained with two 21 cm measurements at different redshifts (scales). Therefore, a combined analysis of the two probes will give access to tighter constraints on cosmic reionization even in the early stages of 21 cm detections.

Keywords
cosmological parameters – dark ages, reionization, first stars – observations
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-227985 (URN)10.1093/mnras/stae506 (DOI)001173224600001 ()2-s2.0-85186435479 (Scopus ID)
Funder
Swedish Research Council, 2020–04691_VR
Available from: 2024-04-05 Created: 2024-04-05 Last updated: 2024-04-24Bibliographically approved
He, Y., Giri, S. K., Sharma, R., Mtchedlidze, S. & Georgiev, I. (2024). Inverse Gertsenshtein effect as a probe of high-frequency gravitational waves. Journal of Cosmology and Astroparticle Physics (5), Article ID 051.
Open this publication in new window or tab >>Inverse Gertsenshtein effect as a probe of high-frequency gravitational waves
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2024 (English)In: Journal of Cosmology and Astroparticle Physics, E-ISSN 1475-7516, no 5, article id 051Article in journal (Refereed) Published
Abstract [en]

We apply the inverse Gertsenshtein effect, i.e., the graviton-photon conversion in the presence of a magnetic field, to constrain high-frequency gravitational waves (HFGWs). Using existing astrophysical measurements, we compute upper limits on the GW energy densities ΩGW at 16 different frequency bands. Given the observed magnetisation of galaxy clusters with field strength B ∼ μG correlated on (10) kpc scales, we estimate HFGW constraints in the (102) GHz regime to be ΩGW ≲ 1016 with the temperature measurements of the Atacama Cosmology Telescope (ACT). Similarly, we conservatively obtain ΩGW ≲ 1013 (1011) in the (102) MHz ((10) GHz) regime by assuming uniform magnetic field with strength B ∼ 0.1 nG and saturating the excess signal over the Cosmic Microwave Background (CMB) reported by radio telescopes such as the Experiment to Detect the Global EoR Signature (EDGES), LOw Frequency ARray (LOFAR), and Murchison Widefield Array (MWA), and the balloon-borne second generation Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE2) with graviton-induced photons. The upcoming Square Kilometer Array (SKA) can tighten these constraints by roughly 10 orders of magnitude, which will be a step closer to reaching the critical value of ΩGW = 1 or the Big Bang Nucleosynthesis (BBN) bound of ΩGW ≃ 1.2 × 10-6. We point to future improvement of the SKA forecast and estimate that proposed CMB measurement at the level of (100-2) nK, such as Primordial Inflation Explorer (PIXIE) and Voyage 2050, are needed to viably detect stochastic backgrounds of HFGWs.

Keywords
extragalactic magnetic fields, gravitational waves / experiments, primordial magnetic fields, Sunyaev-Zeldovich effect
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-231524 (URN)10.1088/1475-7516/2024/05/051 (DOI)001233579900003 ()2-s2.0-85192994621 (Scopus ID)
Available from: 2024-07-23 Created: 2024-07-23 Last updated: 2024-07-23Bibliographically approved
Georgiev, I. (2024). Studies of the intergalactic medium during the Epoch of Reionization: Understanding observational probes with simulations. (Doctoral dissertation). Stockholm: Department of Astronomy, Stockholm University
Open this publication in new window or tab >>Studies of the intergalactic medium during the Epoch of Reionization: Understanding observational probes with simulations
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The first billion years of the Universe is a unique era, marked by the formation of the first stars, galaxies, and accreting black holes, which release ionising radiation into the intergalactic medium (IGM). As a result, these luminous sources initiate a period during which the cold and dense IGM, primarily consisting of neutral hydrogen (HI), is heated and ionised. We refer to this era as the Epoch of Reionization (EoR). The EoR is a global phase transition that is not trivial to observe or model computationally. It is a multi-scale event that evolves with time and depends on the nature of the astrophysical processes that govern the formation of stars and galaxies, as well as the fundamental cosmology that defines the properties of the large-scale IGM. While various measurements of cosmic reionization exist, presently they are too few to constrain the entirety of the process. However, observations from the James Webb Space Telescope and the Square Kilometre Array (SKA), among others, will provide new insight into the process. Particularly, the SKA will observe the power spectrum (PS) of the 21 cm signal from the EoR, which originates from the hyperfine transition of neutral hydrogen atoms HI in the IGM that can emit 21 cm photons. 

In Paper I, we investigate the evolution of the 21 cm PS across the EoR by perturbing the signal and studying its composing terms. We highlight the importance higher-order terms play in shaping the PS on large scales and quantify its evolution. Crucially, we find a characteristic length scale within the 21 cm PS, determined by the mean free path ionising photons travel in the IGM (MFP). Hence, the 21 cm PS has two regimes. We show that the large-scale signal is a biased version of the cosmological density field, and the small-scale PS is determined by the astrophysics of reionization. In Paper II, we use the decomposition of the 21 cm PS and relate it to the PS of the free electron density field. Thus, we analytically connect the 21 cm observable to a probe of the free electron density field. Such a probe is the patchy kinetic Sunyaev-Zel'dovich effect (pkSZ), observed as a foreground to the primary cosmic microwave background temperature anisotropies on small scales. The pkSZ is an integrated probe sensitive to the duration of the EoR and the characteristic size of ionised bubbles. We construct a forecast study of both probes. We show that inferences from 21 cm PS from the SKA can be verified when combined with the pkSZ observation, as each data set is influenced by different systematics. In Paper III, we focus on the modelling of the MFP within large-scale simulations, focusing on the end of reionization (EndEoR). The MFP of ionising photons is inferred from quasar data and depends on several factors. In the post-EoR era, it depends on the distribution and evolution of Lyman Limit systems (LLS), small-scale absorbers that are typically not resolved in large-scale simulations. We investigate the assumptions needed to accurately model the LLS in simulations, and we study their impact on the observables at the EndEoR. We find that LLS modelling has a profound impact on the duration of the final stages of the EoR, the shape of the 21 cm PS as well as other observables of the ionised IGM inferred from quasar spectra, such as the Ultraviolet Background of ionising photons, the effective optical depth of Lyman alpha photons, and the MFP of ionising photons.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University, 2024. p. 80
Keywords
cosmology, theory, large-scale structure of Universe, reionization, first stars, first galaxies
National Category
Astronomy, Astrophysics and Cosmology Natural Sciences
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-228680 (URN)978-91-8014-811-5 (ISBN)978-91-8014-812-2 (ISBN)
Public defence
2024-06-14, FD5, floor 5, AlbaNova, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2024-05-22 Created: 2024-04-24 Last updated: 2024-05-20Bibliographically approved
Georgiev, I., Mellema, G., Giri, S. K. & Mondal, R. (2022). The large-scale 21-cm power spectrum from reionization. Monthly notices of the Royal Astronomical Society, 513(4), 5109-5124
Open this publication in new window or tab >>The large-scale 21-cm power spectrum from reionization
2022 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 513, no 4, p. 5109-5124Article in journal (Refereed) Published
Abstract [en]

Radio interferometers, such as the Low-Frequency Array and the future Square Kilometre Array, are attempting to measure the spherically averaged 21-cm power spectrum from the epoch of reionization. Understanding of the dominant physical processes which influence the power spectrum at each length-scale is therefore crucial for interpreting any future detection. We study a decomposition of the 21-cm power spectrum and quantify the evolution of its constituent terms for a set of numerical and semi-numerical simulations of a volume of (714 Mpc)3, focusing on large scales with k ≲ 0.3 Mpc−1. We find that after ∼10 per cent of the universe has been ionized, the 21-cm power spectrum follows the power spectrum of neutral hydrogen fluctuations, which itself beyond a certain scale follows the matter power spectrum. Hence the signal has a two-regime form where the large-scale signal is a biased version of the cosmological density field, and the small-scale power spectrum is determined by the astrophysics of reionization. We construct a bias parameter to investigate the relation between the large-scale 21-cm signal and the cosmological density field. We find that the transition scale between the scale-independent and scale-dependent bias regimes is directly related to the value of the mean free path of ionizing photons (λMFP), and is characterised by the empirical formula ktrans ≈ 2/λMFP. Furthermore, we show that the numerical implementation of the mean free path effect has a significant impact on the shape of this transition. Most notably, the transition is more gradual if the mean free path effect is implemented as an absorption process rather than as a barrier. 

Keywords
cosmology: theory – large-scale structure of Universe – dark ages, reionization, first stars
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-227983 (URN)10.1093/mnras/stac1230 (DOI)000799969700009 ()2-s2.0-85133139528 (Scopus ID)
Funder
Swedish Research Council, 2020-04691
Available from: 2024-04-05 Created: 2024-04-05 Last updated: 2024-05-06Bibliographically approved
He, Y., Giri, S. K., Sharma, R., Mtchedlidze, S. & Georgiev, I. Inverse Gertsenshtein effect as a probe of high-frequency gravitational waves.
Open this publication in new window or tab >>Inverse Gertsenshtein effect as a probe of high-frequency gravitational waves
Show others...
(English)In: Article in journal (Refereed) Accepted
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-228065 (URN)
Available from: 2024-04-09 Created: 2024-04-09 Last updated: 2024-04-12
Georgiev, I.The forest at EndEoR: The effect of Lyman Limit Systems on the End of Reionisation.
Open this publication in new window or tab >>The forest at EndEoR: The effect of Lyman Limit Systems on the End of Reionisation
(English)Manuscript (preprint) (Other academic)
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-228630 (URN)
Funder
Swedish Research Council, 2020-04691_VR
Available from: 2024-04-23 Created: 2024-04-23 Last updated: 2024-04-24
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-1950-5039

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