Weak lensing provides more strong evidence

Following on from the recent news that 70,000 galaxies had been examined to check weak gravitational lensing effects operate as expected within at least the 3.5 billion light years over which the survey’s objects were spread, another similar experiment has cropped up. This time 446,000 galaxies were used and the weak lensing effect became a probe of the mass density of the universe. In one area at least.

Gravitational lensing is a prediction of general relativity. Any mass creates a distortion in space time (the old ball on a rubber sheet analogy). This distortion gives us gravity (in relativity), but it also alters the path of light flowing through the area, like a concave lens. In strong lensing effects, the light is deflected far from the lensing object. In weak lensing, the masses involved are lower and instead of spreading the light far and wide, the lenses merely smear the shape a little.

Using galaxies in the Hubble space telescope’s COSMOS (Cosmic Evolution) survey, astronomers used the weak lensing between the galaxies and ourselves to reconstruct where the universe had lots of mass and where the mass was less prevalent. As most gravitationally active matter is in a form not normally detected – dark matter – it is only through observing the gravitational effects in deep space that we can determine how much stuff there is out there.

For 194,000 of the galaxies, further observations were carried out. The Subaru telescope in Hawaii was used to determine the redshifts, giving the distances to each of the galaxies. Knowing the amount of matter between us and them and the distances implied by the redshifts, a column density of matter and dark matter could be produced. That then was compared to the Millennium Survey, the most accurate of these kinds of surveys done before, to help with statistical errors.

The result is not only a map of dark matter concentrations, but a map of how the concentrations evolved over several billion years.

Image credit: NASA/ESA/P Simon (University of Bonn)/T Schrabback (Leiden University)

Image credit: NASA/ESA/P Simon (University of Bonn)/T Schrabback (Leiden University)

However, these were not the only eyes on galaxies in recent times. A survey has been carried out to check suspicions that other surveys using the wavelength 121.6nm, otherwise known as Hydrogen Lyman Alpha, missed the signatures of distant galaxies. It turned out they did – with ninety percent of the galaxies captured by the new survey missing in Lyman Alpha surveys.

Hydrogen Lyman Alpha is light emitted when the electron in a hydrogen atom falls from the first excited state to the ground state. Hydrogen Balmer Alpha, otherwise known simply as Hydrogen Alpha for short, is light emitted when the electron falls from the second excited state to the first excited state (the alpha means the electron falls by one excited state – beta means a two level drop and so on – Balmer means it ends up on the first excited state – Lyman means the ground state and there are other names for series where the electron ends up in other energy levels). Hydrogen alpha is red light at 656.3nm. Lyman alpha is more heavily scattered by dust, meaning that the brightness decreases with distance faster than it does for Hydrogen alpha. Ninety percent of the galaxies were bright enough in hydrogen alpha to be detected by the new survey, but not bright enough in Lyman alpha to be detected in surveys using that light.

The team used the European Southern Observatory’s Very Large Telescope to first conduct a survey in Lyman Alpha and then to rescan the area in Hydrogen Alpha.


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