Where were the first stars formed?

An international team of researchers, led by researchers from Tel Aviv University, found a new method for discovering the first stars from the early period when the age of the universe was only one percent of its current age * One of the most fascinating fields in astronomy is the study of the era of the formation of the first stars, when the universe began to take its modern form

The researchers showed with the help of a modern computer model that the speed difference between gas and dark matter causes the first stars to form a pattern of large concentrations next to starless regions. The study was published in Nature magazine on June 20, 2012

An international team of researchers including Prof. Renan Barkana from Tel Aviv University (team leader) and Anastasia Fialkov (Tel Aviv University), Eli Weisbel (Harvard University), and Prof. Kris Hirata and Dmitri Tasliakovich (California Institute of Technology), found a new method to discover the The first stars from the early period when the age of the universe was only one percent of its current age. Using a state-of-the-art computer model, the researchers showed that the speed difference between gas and dark matter causes the first stars to form a pattern of large concentrations next to starless regions. This discovery allows astronomers to go out and look for the radio waves with a wavelength of 21 cm emitted by the primordial hydrogen, when it was heated by the first stars when the universe was only 200 million years old.

The formation of stars is part of our cosmic history. Astronomers today know that long before the stars appeared, the early universe was filled with hot, uniform gas. On the other hand, today's complex universe contains stars and galaxies. One of the most fascinating fields in astronomy is the study of the era of the formation of the first stars, when the universe began to take its modern form. The most distant galaxy discovered so far was formed when the universe was 800 million years old, and it will be very difficult to directly see much earlier galaxies. Since the universe was full of hydrogen atoms at these times, the most promising way to observe the era of the formation of the first stars is with the help of the radiation of hydrogen at a wavelength of 21 cm (which falls in the field of radio waves). This radiation is also difficult to measure, because one has to deal with the strong radiation, at the same wavelength, of our galaxy and nearby galaxies. The hope is that the cosmic signal will not be uniform but will change from place to place, and then it will be easier to separate it from the local radiation which is relatively uniform.

Indeed, large disturbances in the distribution of the first stars are expected, so that some regions will be full of stars (divided into mini-galaxies that are very small compared to today's galaxies such as the Milky Way), while other regions will be almost empty. The reason for this can be understood from a simple analogy: suppose we want to find on the map all the peaks above 5000 meters. These peaks (about 200 in number) are not uniformly distributed on the Earth, but are all found in several concentrations on the great mountain ranges. The point is that on a large mountain range, each hill becomes a high peak, while a similar hill located in a valley is just a hill. Similarly, the first galaxies are concentrated in regions where the average density of matter is high throughout the region. The high density increases the gravitational force in the entire region and helps create concentrations of dark matter, which the gas then falls into and forms stars.

To this basic idea was recently added the distinction that dark matter and ordinary matter (the gas) moved at different speeds in the early universe. In the last two years, the effect of these speed differences has been investigated with the help of mathematical models and numerical simulations. In our article we created for the first time a simulation of the three-dimensional distribution of the first stars and showed that the speed differences greatly increase the disturbances (fluctuations) on large scales. Specifically, in the era of the first heating of the hydrogen between the galaxies (by X-ray radiation from the galaxies), we will observe large disturbances on a scale that corresponds to 400 million light years in the universe today. These disturbances are seen on an angular scale of 2/3 of a degree (for comparison, we see the Sun and the Moon at a size of about 1/2 of a degree), so it will be relatively easy to measure them (because there is no need for telescopes with high angular separation). Therefore, the pattern we predict opens up the possibility of discovering with the help of radio waves the early stars that were formed when the age of the universe was about 180 million years (1.3% of the age of the universe today). The signal we are predicting also includes an observational signature such that its measurement will clearly indicate the existence of small mini-galaxies at that time and the effect of the speed differences on them. This is a great opportunity and we expect a considerable observational effort in this direction.

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