The Cosmic Horseshoe Is Not the Lucky Beacon That Astronomers Had Hoped For

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A UC Riverside-lead team of astronomers used a new approach by using the gravitationally lensed galaxy to try to measure the escaping fraction of photons.

An interesting galaxy has been circled in this NASA/ESA Hubble Space Telescope image. The galaxy — one of a group of galaxies called Luminous Red Galaxies — has an unusually large mass, containing about ten times the mass of the Milky Way. However, it’s actually the blue horseshoe shape that circumscribes the red galaxy that is the real prize in this image. This blue horseshoe is a distant galaxy that has been magnified and warped into a nearly complete ring by the strong gravitational pull of the massive foreground Luminous Red Galaxy. To see such a so-called Einstein Ring required the fortunate alignment of the foreground and background galaxies, making this object’s nickname “the Cosmic Horseshoe” particularly apt. The Cosmic Horseshoe is one of the best examples of an Einstein Ring. It also gives us a tantalising view of the early Universe: the blue galaxy’s redshift — a measure of how the wavelength of its light has been stretched by the expansion of the cosmos — is approximately 2.4. This means we see it as it was about 3 billion years after the Big Bang. The Universe is now 13.7 billion years old. Astronomers first discovered the Cosmic Horseshoe in 2007 using data from the Sloan Digital Sky Survey. But this Hubble image, taken with the Wide Field Camera 3, offers a much more detailed view of this fascinating object. This picture was created from images taken in visible and infrared light on Hubble’s Wide Field Camera 3. The field of view is approximately 2.6 arcminutes wide.


Around 380,000 years after the Big Bang, electrons and protons bound together to form hydrogen atoms for the first time. They make up more than 90% of the atoms in the universe, and can very efficiently absorb high-energy photons and become ionized. However, there were very few energetic sources to ionize these atoms in the early universe. One billion years after the Big Bang, the material between the galaxies was reionized (transparent). The main energy source of the reionization is widely believed to be massive stars formed within early galaxies. These stars had a short lifespan and were usually born in the midst of dense gas clouds, which made it very hard for ionizing photons to escape their host galaxies.

Previous studies suggested that about 20 percent of these ionizing photons need to escape the dense-gas environment of their host galaxies to significantly contribute to the reionization of the material between galaxies. Unfortunately, a direct detection of these ionizing photons is very challenging and previous efforts have not been very successful. Therefore, the mechanisms leading to their escape are poorly understood.

This has led many astrophysicists to use indirect methods to estimate the fraction of ionizing photons that escape the galaxies. In  one popular method, the gas is assumed to have a “picket fence” distribution, where the space between the stars and the edges of galaxies is assumed to be composed of either regions of very little gas, which are transparent to ionizing light, or regions of dense gas, which are opaque. Researchers can determine the fraction of each of these regions by studying the light (spectra) emerging from the galaxies.

In this new study, astronomers directly measured the fraction of ionizing photons escaping from the Cosmic Horseshoe. The Horseshoe is a distant galaxy that is gravitationally lensed. Gravitational lensing is the  deformation and amplification of a background object by the curving of space and time due to the mass of a foreground galaxy”, said Kaveh Vasei, graduate student of astronomy at UC Riverside and lead author of the new study.  “The details of the galaxy in the background are therefore magnified, allowing us to study its light and physical properties more clearly.”


Based on the picket fence model, an escape fraction of 40% for ionizing photons from the Horseshoe was expected. Therefore, the Horseshoe represented an ideal opportunity to get a clear, resolved image of leaking ionizing photons for the first time, to help us understand the mechanisms by which they escape their host galaxies. 

The research team obtained a deep-image of the Horseshoe with the Hubble Space Telescope in an ultraviolet filter, enabling them to directly detect escaping ionizing photons. Surprisingly, the image did not detect ionizing photons coming from the Horseshoe. This team constrained the fraction of escaping photons to be less than 8%, five times smaller than what had been inferred by indirect methods widely used by astronomers.

“The study concludes that the previously determined fraction of escaping ionizing radiation of galaxies, as estimated by the most popular indirect method, is likely overestimated in many galaxies,” added Prof. Brian Siana, co-author of the research paper and a professor at UC Riverside. “The team is now focusing on direct determination the fraction of escaping ionizing photons that do not rely on indirect estimates.”

This paper has been published in the Astrophysical Journal and is authored by Kaveh Vasei (UC Riverside), Brian Siana (UC Riverside), Alice E. Shapley (UCLA), Anna M. Quider (University of Cambridge, UK), Anahita Alavi (UC Riverside), Marc Rafelski (Goddard Space Flight Center / NASA), Charles C. Steidel (Caltech), Max Pettini (University of Cambridge, UK), Geraint F. Lewis (University of Sydney)


The research paper can be found in the Astrophysical Journal.


Image: The Cosmic Horseshoe, as photographed by the Hubble Space Telescope. Credit: ESA/Hubble & NASA