Right after the Big Bang, the Universe was filled with a thick fog of neutral hydrogen gas that blocked visible light, plunging it into a long "dark age." This darkness ended when the first galaxies and stars emerged and began emitting powerful radiation that ionized the surrounding hydrogen — a process known as reionization. However, it has remained unclear which galaxies played the most important roles in driving this dramatic transition.

In our work, we used detailed semi-analytic models to track how ionized regions around galaxies grew over time, and specifically focus on identifying the dark matter haloes that host violent bursty galaxies that were responsible for reionization. We found that at early times, small but brighter galaxies living in low-mass haloes were the first to ignite and start reionization. These galaxies were very numerous and produced most of the initial ionizing photons that began to carve out transparent bubbles in the foggy early Universe.

As reionization progressed, the intense radiation began to suppress star formation in these smaller haloes, making it increasingly difficult for them to keep contributing. At this stage, larger galaxies hosted in more massive haloes, which are less affected by this feedback, gradually took over and became the main drivers of reionization. Our findings show that by the time reionization was nearly complete, most of the ionizing photons came from these more massive haloes.

Overall, our study suggests that reionization was not simply the achievement of a few massive galaxies but rather a collective effort. Many small galaxies lit the first sparks, and larger galaxies carried the process to completion. This richer and more dynamic picture helps us better understand how the first structures shaped the Universe and provides valuable guidance for interpreting observations from powerful new telescopes like the James Webb Space Telescope (JWST).

Almost every galaxy hosts a supermassive black hole (SMBH) at its centre, even our own Milky Way. These monsters are typically more than a million times more massive than the common stellar black holes. They are known to grow as they feed on the stars and gas of a galaxy over its lifetime of more than a billion years. But recently a number of such monsters have been detected in the newly formed galaxies when the Universe was young, about only 400 million years old compared to its current age of 14 billion years. Existence of the monster black holes early in the Universe is a mystery and an open question.

Our research paper explaining the origin of supermassive black holes in the newly formed galaxies at cosmic dawn, has recently appeared in the monthly notices of royal astronomical society (MNRAS). See the media report on this research. 

The study finds that the growth of the central black hole occurs in two phases. In the initial first phase, the seed black hole at the centre grows at a slow pace as per the standard feeding or accretion rate which was proposed in the seminal paper by the British-Austrian physicist Hermann Bondi. However, we have found that, subsequently there is a crucial second phase, in which the evolving dark matter halo of a galaxy takes control of the accretion, as it soon modifies the Bondi accretion rate, and increases it multi-folds, leading to a quick growth of the seed black hole into a monster, with a mass comfortably more than tens of millions solar masses, that agree with the recently detected SMBHs at cosmic dawn by the James Webb Space Telescope (JWST) launched last year by NASA. 

The second rapid phase is the key, and the dark matter of a galaxy is the reason for it. Dark matter is everywhere and it is a reality of our Universe; in fact visible galaxies also have an underlying dark character. Each galaxy at a fundamental level is a spherical blob of dark matter known as the halo, which is `invisible’, that engulfs the `visible’ stars and gas. Dark matter is in fact the omnipresent base fabric in which the visible galaxies and their stars develop, and emerge as colourful designs that we see in the night sky. 

These results have now revealed that dark matter is also responsible for the emergence of monster supermassive black holes, that too quite early on in the Universe at the cosmic dawn which is a major focus of research worldwide driving mega international projects such as the square kilometre array (SKA) and the JWST.  

The maximum evolution in a star occurs towards the end of its lifetime when it takes off from the so called 'main sequence'. The main sequence is the trend that we see when plotting stars on HR diagram with their temperature on x-axis and the luminosity on y-axis. Most of the stars of any given star-cluster would lie on the main sequence. A star evolves very slowly when it is on the main sequence in the HR diagram. The slow evolution can also be termed as the secular evolution, that is because of the virial equilibrium combined with the fact that the loss of energy to cooling and radiation is compensated by the amount of energy generated due to nuclear fusion.

The galaxies might be following a similar mechanism as they evolve, although not implicitly clear. We investigate this with our model of galaxy formation. We show that, indeed, the galaxies also evolve secularly as they also obey the virial equilibrium. Analogous to a star, the decrease in the binding energy due to accretion is compensated by the energy generation due to star-formation and supernovae.  Using this argument, and the known accretion rates from cosmological simulations as input, we predict the star formation rates in galactic haloes of different masses as they evolve from the time of their birth to the present day.  Link to the paper: https://arxiv.org/abs/1906.10135 

How rapidly the Universe was reionized? Whether it was instantaneous, or if it was a more extended process? The answer to these questions depends on which galaxies were the main sources of reionization.

One has to understand that galaxies are not all alike. We observe galaxies like our own 'Milky Way', which is a spiral galaxy with a rather low star formation rate. On the other hand are the galaxies like the M82, that is an active dwarf starburst galaxy as it is aggressively forming stars. Further, not only the galaxies differ in the behavior of star formation; they also differ in shapes, mass, sizes etc.

The galaxies at the same epoch are diverse, and then there are subtle variations when we compare similar galaxies from two different epochs.  Out of this whole diversity of galaxies, one would be keen on knowing which subset of galaxies provided the ionizing photons vital for reionization.

One of the main parameters to explore the diversity  of  galaxies at an epoch is the galactic mass. Let's consider the two possibilities, that the reionization was carried out by low mass galaxies or alternatively it was carried out by high mass galaxies. The two scenarios would have distinguishable consequences as the duration of reionization will be significantly different in these two cases.  That is the key idea in this study of ours

Link to the paper:  https://arxiv.org/abs/1712.06619

The sun is one of the millions of stars in the Milky Way, and we remember from school textbooks that it is a blob of hydrogen which is constantly burning (undergoing nuclear fusion) to produce helium, as a  result generating enormous amount of energy and light. 

An addition to this high-school knowledge is that, helium is heavier than hydrogen (It sits at number 2 position in the periodic table compared to number 1 spot of hydrogen; higher the number heavier the element). But the sun also contains other heavier elements such as carbon, oxygen..... and yes even iron. How do we know this? 

Each element has a signature emission. When we detect the spectrum of light from the sun, we see the imprints of all those heavier elements. Where do these heavier elements come from? We know that helium is being produced as the sun is burning and depleting its reservoir of hydrogen. But what about, carbon, oxygen, iron? Where do they come from?

When we see millions of other stars in our Galaxy, they are not all alike. They differ from the sun, and from each other, when we compare the relative abundance of their constituents. Some stars (called CEMP stars) have more carbon relative to iron when compared to the same ratio in the sun. This points to the central question: what is the origin of the heavier elements in the sun, and in other stars; and why the abundance of these elements differ from star to star? We have tried to answer these questions in the following study of ours

Link to the paper: https://arxiv.org/abs/1611.03868