The epoch of Cosmic Dawn, where the first stars and galaxies were born, signals the start of the second major phase transition of nearly all the normal matter in the universe, known as Cosmic Reionization. The evolution of the temperature of the intergalactic medium (IGM) closely depends on the timing and characteristics of reionization. I showed that tight constraints could be placed on the temperature evolution of the intergalactic medium, which is directly related to the contribution of quasars to reionization. On the observational front, the SKA will be able to image ionized bubbles at cosmic dawn -- for which collaborators and I developed an analytical formalism -- and the European Southern Observatory (ESO)'s facilities will provide constraints on the galaxies responsible for the ionization.

Recently, some extremely luminous galaxies have been detected towards the epoch of reionization, some of which show evidence for so-called 'double-peaked' line profiles in the Lyman-alpha line. This is rare among high-redshift galaxies, as the increasing neutral fraction in the intergalactic medium almost always leads to the resonant absorption of the blue wing of the line at these epochs. In our recent paper, we pointed out a novel scenario for the ionization zones around these galaxies being created by obscured quasars, whose intrinsic Lyman-alpha line is blocked by the effect of obscuration. I gave a talk on this at the UK National Astronomical Meeting (NAM) 2022, which is available here.

The origin and growth of the earliest black holes in the Universe remains an unsolved enigma in cosmology. Observations have revealed the presence of billion-solar-mass supermassive black holes (SMBHs) just hundreds of millions of years after the birth of the Universe, which pose a strong challenge to theoretical models of their formation. Two main parameters governing the fuelling and growth of SMBHs are (i) their Eddington ratio and (ii) their active lifetime of accretion, the latter of which is related to its so-called radiative efficiency. In recent work, we pointed out a direct, observable signature of the prevalence of massive black holes in the centres of the first galaxies during the period of reionization.

Intermediate-mass black holes (IMBHs), with masses between a hundred and a million solar masses, are often referred to as the “missing-link” in the formation scenarios of SMBHs. An important fuelling mechanism for IMBHs are the so-called tidal disruption events (TDEs), in which a star, passing too close to the black hole gets ripped apart by its gravity. So far, about a hundred TDEs have been observed, typically accompanied by flares in the luminosity of the central active black holes. TDEs could lead to about a 50% increase in the mass of an IMBH over cosmological timescales.

We used the latest quasar luminosity functions and analytical data-driven models of the tidal disruption phenomenon, to find that TDEs could be a salient source fuelling IMBH at early times. They would also contribute significantly to the so-called “Changing-Look” phenomenon in the earliest quasars, which causes the disappearance and re-appearance of the quasars’ broad emission lines over a period of time. My talk summarising this at the “What Drives the Growth of Black Holes?” Conference at Reykjavik, Iceland, 26-30 September 2022, is here.

We showed that the properties of these earliest sources of reionization can be constrained through their gravitational wave emission detectable by the next-generation Laser Interferometer Space Antenna (LISA) instrument and the Square Kilometre Array Pulsar Timing Array (PTA). We find that SKA should be sensitive to SMBHBs with masses greater than a billion solar masses and separations 1-50 milliparsec, with a minimum mass ratio of ~ 0.005-0.25, fairly independently of redshift. We use the measured abundances of quasars at early times to deduce that for SMBHs of 10 billion solar masses, a unique quasar should be localisable by electromagnetic follow-up to the SKA detection. Importantly, the PTA detections will be able to place robust constraints on the Eddington ratio and the active lifetime of quasars. My talk on this topic at the Swiss SKA Days, held at Lugano over October 3-4, 2022 is here.

There are very good prospects for investigating reionization using molecular lines. The CO molecular spectrum has a 'ladder' of states, and hence the CO 1-0 line from the epochs probed by the CO Mapping Array Project (COMAP) contains a contribution from the CO 2-1 line at redshifts 6-8, the mid to late stages of reionization. One of my chief modelling aims with COMAP is to probe large-scale structure during the Epoch of Reionization. The subsequently planned phases of COMAP (COMAP-EoR and COMAP-ERA) involve cross-correlating the signal in two frequency bands to isolate the signal coming from the Epoch of Reionization.

I am a member of the modelling team involved in this effort, whose latest forecasts predict a detection of the Reionization signal at high significance for the COMAP-EoR survey over redshifts 6-8. It allows us to place very tight constraints on the cosmic molecular gas density where there is a significant contribution from faint galaxies that would otherwise be missed by current and future galaxy surveys, re-iterating the unique ability of line intensity mapping to constrain the properties of the earliest galaxies.

In a recent paper, I have extended my data-driven halo model approach for CO to the [C II] line emission, and provided forecasts for constraining early star formation history with various instrumental configurations, including the ALMA. With the MWA collaboration, I envision novel project efforts synergizing future 21 cm surveys with observations of individual high-redshift galaxies from ALMA (selected in CO and [CII]), and the forthcoming AtLAST survey which I recently joined -- a proposed ambitious successor to the ALMA.

I recently co-led the Cosmic Dawn and Reionization chapter for the Fundamental Physics with the Square Kilometre Array (SKA) white paper, which explores the ways in which the SKA will revolutionize our understanding of four key areas of fundamental physics.