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Universe’s ultraviolet background could provide clues about missing galaxies

Last Updated on Monday, 20 March 2017 15:13
Published on Wednesday, 22 March 2017 00:00

Astronomers have developed a way to detect the ultraviolet (UV) background of the Universe, which could help explain why there are so few small galaxies in the cosmos. UV radiation is invisible but shows up as visible red light when it interacts with gas. An international team of researchers led by Durham University, UK, has now found a way to measure it using instruments on Earth. The findings are published in a paper in Monthly Notices of the Royal Astronomical Society.

 

thumb UGC 7321 starlightColour image of the starlight emitted by a nearby spiral galaxy called UGC 7321. Credit: M. Fumagalli/T. Theuns/S. Berry. Click for a larger image

The researchers said their method can be used to measure the evolution of the UV background through cosmic time, mapping how and when it suppresses the formation of small galaxies. The study could also help produce more accurate computer simulations of the evolution of the Universe.

 

UV radiation – a type of radiation also given out by our Sun – is found throughout the Universe and strips smaller galaxies of the gas that forms stars, effectively stunting their growth. It is believed to be the reason why some larger galaxies like our Milky Way don’t have many smaller companion galaxies.

 

Simulations show that there should be more small galaxies in the Universe, but UV radiation essentially stopped them from developing by depriving them of the gas they need to form stars. Larger galaxies like the Milky Way were able to withstand this cosmic blast because of the thick gas clouds surrounding them.

 

Lead author Dr Michele Fumagalli, in the Institute for Computational Cosmology and Centre for Extragalactic Astronomy, at Durham University, said: “Massive stars and supermassive black holes produce huge amounts of ultraviolet radiation, and their combined radiation builds up this ultraviolet background.”

thumb UGC 7321 fluorescenceGalaxy UGC 7321 is surrounded by hydrogen gas, and as this gas is irradiated with UV radiation, it emits a diffuse red glow through a process known as fluorescence. Credit: M. Fumagalli/T. Theuns/S. Berry. Click for a larger image

 

“This UV radiation excites the gas in the Universe, causing it to emit red light in a similar way that the gas inside a fluorescent bulb is excited to produce visible light.”

“Our research means we now have the ability to measure and map this UV radiation which will help us to further refine models of galaxy formation.”

 

Co-author Professor Simon Morris, in the Centre for Extragalactic Astronomy, Durham University, added: “Ultimately this could help us learn more about the evolution of the Universe and why there are so few small galaxies.”

 

Researchers pointed the Multi Unit Spectroscopic Explorer (MUSE), an instrument of the European Southern Observatory’s Very-Large Telescope, in Chile, at the galaxy UGC 7321, which lies at a distance of 30 million light years from Earth. MUSE provides a spectrum, or band of colours, for each pixel in the image allowing the researchers to map the red light produced by the UV radiation illuminating the gas in that galaxy.

 

The research, funded in the UK by the Science and Technology Facilities Council, could also help scientists predict the temperature of the cosmic gas with more accuracy.

 

Co-author Professor Tom Theuns, in Durham University’s Institute for Computational Cosmology, said: “Ultraviolet radiation heats the cosmic gas to temperatures higher than that of the surface of the Sun.”

“Such hot gas will not cool to make stars in small galaxies. This explains why there are so few small galaxies in the Universe, and also why our Milky Way has so few small satellite galaxies.”

 


Media contact

 

Durham University Marketing & Communications Office
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Science contacts

 

Dr Michele Fumagalli
+44 (0)191 33 43789
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Professor Tom Theuns
+44 (0) 191 33 43795
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Images and animations

 

A selection of pictures and simulation videos are available from Durham University Marketing & Communications Office on +44 (0)191 334 6075; This email address is being protected from spambots. You need JavaScript enabled to view it.

 

Details are as follows:


Pictures (NB – annotated pictures, showing the direction of the ultraviolet radiation and fluorescence, are also available)


UGC 7321 starlight: Colour image of the starlight emitted by a nearby spiral galaxy called UGC 7321. Stars in this galaxy lie in a disc, similar to that of our Galaxy, the Milky Way. We see this disc nearly perfectly edge-on. Other sources in the image are foreground or background objects (galaxies and stars), unrelated to galaxy UGC 7321. Credit: M. Fumagalli/T. Theuns/S. Berry


UGC 7321 fluorescence: Galaxy UGC 7321 is surrounded by hydrogen gas, and as this gas is irradiated with UV radiation, it emits a diffuse red glow through a process known as fluorescence. This image shows the light emitted by stars inside the galaxy, surrounded by a red ring that represents the fluorescent emission induced by the UV radiation. Credit: M. Fumagalli/T. Theuns/S. Berry


Simulation videos

 

Galaxy UGC7321.mp4:

Movie showing the effect of ultraviolet (UV) radiation (indicated by the purple arrows) on the hydrogen gas surrounding the nearby spiral galaxy UGC 7321. As the gas is irradiated by the UV it begins to glow or fluoresce, allowing researchers to map the intensity of the UV radiation. The movie shows the field of view of the MUSE instrument at the European Very Large Telescope, which scientists used to detect the fluorescence.

Credit: M. Fumagalli/T. Theuns/S. Berry

 

Movie file Apostle movie.mp4:

This movie follows the formation of galaxies with cosmic time, illustrating how ultraviolet (UV) radiation from other galaxies and from quasars suppresses the formation of stars inside small galaxies near to large galaxies similar to the Milky Way and Andromeda.

The left panel shows a simulation that includes such diffuse UV radiation as in the real Universe, where fewer smaller galaxies form.

For comparison, the right panel shows what would happen in the absence of such radiation, with more small galaxies forming.

Credit: S. McAlpine/S. Berry

 


Additional information re simulation videos: Calculations were performed on the DiRAC Data Centric system hosted by Durham University and operated by the Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk), and used resources provided by WestGrid (www.westgrid.ca) and ComputeCanada/Calcul Canada (www.computecanada.ca).

 


Further information

 

The new work appears in “A measurement of the z = 0 UV background from H-alpha fluorescence” Fumagalli, M, et al, Monthly Notices of the Royal Astronomical Society, Oxford University Press, DOI: 10.1093/mnras/stx398 (http://doi.org/10.1093/mnras/stx398)

 

A copy of this paper is available on request from Durham University Marketing & Communications Office on +44 (0)191 334 6075; This email address is being protected from spambots. You need JavaScript enabled to view it.


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Useful weblinks

Institute for Computational Cosmology: http://icc.dur.ac.uk/
Centre for Extragalactic Astronomy: http://astro.dur.ac.uk/cosmology/
Department of Physics, Durham University: https://www.dur.ac.uk/physics/
Science and Technology Facilities Council: http://www.stfc.ac.uk/
Monthly Notices of the Royal Astronomical Society: https://academic.oup.com/mnras

 


Notes for editors

 

About Durham University

A world top 100 university with a global reputation and performance in research and education (QS and THE World University Rankings 2016/17; https://www.dur.ac.uk/about/rankings)
In the 2017 Times and Sunday Times Good University Guide, Durham was ranked fourth in the UK
A member of the Russell Group of leading research-intensive UK universities
Research at Durham shapes local, national and international agendas, and directly informs the teaching of our students
Ranked 39 globally for the employability of its students by blue-chip companies world-wide (QS World University Rankings 2016/17)

 

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