Chandra space telescope image of an X-ray binary system containing a neutron star. [Credit: X-ray: NASA/CXC/Univ. of Wisconsin-Madison/S.Heinz et al; Optical: DSS; Radio: CSIRO/ATNF/ATCA]

Chandra space telescope image of an X-ray binary system containing a neutron star. [Credit: X-ray: NASA/CXC/Univ. of Wisconsin-Madison/S.Heinz et al; Optical: DSS; Radio: CSIRO/ATNF/ATCA]

About 380,000 years after the Big Bang, the Universe cooled off enough for stable atoms to form out of the primordial plasma. However, sometime in the billion years or so after that, something happened to heat the gas up again, returning it to plasma form. Though we know reionization (as it is called) happened, that epoch in the history of the cosmos is hard to study, so we don’t know exactly when and how the reheating happened. If a new proposed model is correct, though, ionization happened close to the end of that era, and was driven by binary systems containing a black hole or neutron star.

One new model, proposed by Anastasia Fialkov, Rennan Barkana, and Eli Visbal, suggests that energetic X-rays could have heated the primoridal gas to the point that reionization happened relatively rapidly. That’s in contrast with other hypotheses, which predict a more gradual reionization process. The X-rays in the new model were emitted by systems that include neutron stars or black holes. The nicest feature of the new proposal is that it predicts a unique pattern in light emission from the primordial gas, which could conceivably be measured by current radio telescopes. [Read more….]

Ionizing the Universe with black holes and neutron stars

Green Peas were all my joy, galaxies were my delight

Most galaxies are somewhat red or blue in appearance, depending on the populations of stars that comprise them. However, citizen scientists working with the GalaxyZoo project identified a previously unknown type of galaxy: Green Peas, so named because they are small and green. The color comes from ionized oxygen, a particular form of emission that only happens under unusual conditions. A new study shows that Green Peas could resemble a kind of early galaxy responsible for reionization: the breakdown of atoms due to aggressive star formation when the Universe was young.

A new paper by A. E. Jaskot and M. S. Oey argues that galaxies much like the Green Peas could be responsible for the reionizing radiation. They analyzed the light emissions from the galaxies, and determined that their gas is thinner than in typical star-forming galaxies, which could allow more ultraviolet light into intergalactic space. The researchers also found signs in a few Green Peas of extremely massive stars, the ones most responsible for ionizing radiation. [Read more…]

We have a lot of reasons to be interested in the earliest stars that formed in our Universe. Particularly, these stars were the first to fuse hydrogen and helium into (nearly) all the heavier elements that exist today, including the carbon, oxygen, iron, calcium, and the like that make up life as we know it. However, not only are these stars too far away to observe directly, much their light is hidden by foreground sources, including our Milky Way. A new indirect observation may have solved that problem, though:

Now a study using the Fermi Gamma-ray Space Telescope has used the light emanating from supermassive black holes known as blazars to measure the diffuse light produced by reionization. When high energy gamma rays interacted with the ultraviolet photons produced by early stars, they were converted to particle/antiparticle pairs, and this creates a dropoff at a specific point in the blazar spectrum. This absorption was evident in a sample of 150 blazars, and the data can help constrain models of the very first stars in the Universe. [Read more…]

Early stars stole gamma rays from blazars