UK-led robotic sky scanner reveals its first galactic fingerprint
12 Dec 2022



A major telescope upgrade has peered through to the distant Universe to reveal the spectra of a pair of galaxies 280 million light years away from Earth.


​​​Blue, green and red colours, according to velocities derived from the WEAVE spectra, are overlaid on a composite image of Stephan's Quintet, which features galaxy star light (CFH telescope), and X-ray emission of hot gas (blue vertical band, Chandra X-ray observatory). 


The spectra provide a first glimpse of the sky from the WHT Enhanced Area Velocity Explorer (WEAVE) – a unique upgrade to the William Herschel Telescope (WHT) in La Palma on the Canary Islands.

After its integration into the WHT last year, WEAVE has now begun its on-sky commissioning phase, ready to reveal more than 12 million spectra of stars and galaxies over the next five years.

The Science and Technology Facilities Council (STFC) is one of the key partners in the operation of the WHT.

Understanding the Universe through spectra

Spectroscopy is an essential element in an astronomer's toolbox. Analysing light detected with a telescope reveals useful scientific information e.g. the speed of the object observed, the atoms it is made of and its temperature.

If an image tells us what an astronomical object looks like, its spectrum tells us what it is.

First galactic spectra with WEAVE

A galactic spectrum is the combination of spectra from the millions of stars in an observed galaxy. Studying the features of a galaxy spectrum allows astronomers to understand what types of stars the galaxy contains, and the relative abundances of each type of star. This tells us about how the galaxy formed and changed over time.

First-light observations with WEAVE were carried out with the large integral-field unit (LIFU) fibre bundle, one of WEAVE's three fibre systems. The team observed the heart of the galaxy group Stephan's Quintet, a group of five interacting galaxies.

The instrument was aimed at NGC 7318a and NGC 7318b, a pair of galaxies at the centre of a major galaxy collision 280 million light-years from Earth in the constellation Pegasus.

Professor Gavin Dalton, WEAVE Principal Investigator, University of Oxford and STFC RAL Space, said:

“The wealth of complexity revealed in this way by a single detailed observation of this pair of nearby galaxies provides insights into the interpretation of the many millions of spectra that WEAVE will obtain from galaxies in the distant Universe and provides an excellent illustration of the power and flexibility of the WEAVE facility."

The WEAVE LIFU measures separate spectra for 547 different regions in and around the two galaxies, recording the colours of their light from the ultraviolet to the near-infrared.

These spectra reveal the motions of stars and gas, the chemical composition of the stars, the temperatures and densities of the gas clouds, and more. This data will help astronomers learn how galaxy collisions transform the galaxies in the group.


The JWST image with the WEAVE LIFU pointing at Stephan's Quintet for the first-light observation. The LIFU gathers light from 547 points on the sky for analysis by the WEAVE spectrograph (each circle indicates an optical fibre 2.6 arcseconds in diameter). The observation provides physical information from each separate region of each galaxy as well as the space in between. Credits: NASA, ESA, CSA, STScI (background image); Aladin (overlay with fibres). 

Eight surveys using WEAVE

In the coming five years, the ING will assign 70% of the time available on the WHT to eight major surveys with WEAVE, selected out of those proposed by the astronomical communities of the partner countries. All these surveys require spectra of up to millions of individual stars and galaxies, a goal now obtainable thanks to WEAVE's ability to observe almost 1000 objects at a time.

Over 500 astronomers from across Europe have organized these eight surveys, covering studies of stellar evolution, Milky Way science, galaxy evolution and cosmology. WEAVE will study galaxies near and far to learn the history of their growth, and will obtain millions of spectra of stars in the Milky Way.

Professor Mark Thomson, STFC Executive Chair, said:

“As one of the key partners operating the William Herschel Telescope, STFC is proud to have a major contribution in the overall design, systems engineering and software control for its newest upgrade, WEAVE.

This first light event is a milestone for both the international and UK astronomy communities: WEAVE will provide spectra of millions of stars and galaxies over the next five years. After ten years in development, WEAVE will now finally offer astronomers a new eye to the sky to help them answer questions such as what is dark matter and how did stars form in distant galaxies?"


The advantage of the LIFU comes from the sheer amount of information contained in each observation. WEAVE produces spectra for each of 31,500 points or regions in and around the galaxies in two hours. The intensity of light from the fibres builds the image of the galaxies shown in the centre. The individual spectra (intensity at each wavelength; seven examples shown) provide a wealth of information about the physical conditions at each location. At the two galaxy nuclei (top-right) the spectra indicate moderately-old stars (one billion years) and no on-going star formation.

The narrow, peaked spectra in the lower-right are typical of gas (hydrogen, oxygen, nitrogen, sulfur) heated to over 10,000 degrees by very young stars, whereas the broad, asymmetric peaks in the spectra shown on the left indicate turbulent shocks between gas clouds. WEAVE is particularly accurate at measuring wavelengths, or velocities. In the bottom-left panel (in red) obtained in the high spectral resolving power mode, velocity distributions as narrow as 12.8 km/s can be measured. 


The main components of WEAVE are:

  • Fibre-positioner, developed by the University of Oxford and RAL Space in the UK, with support from the IAC
  • Prime-focus corrector, designed by ING and SENER, provided by the Instituto de Astrofísica de Canarias (IAC) in Spain and manufactured by SENER. Support from Konkoly Observatory (HU)
  • Lenses were polished by KiwiStar in New Zealand, funded from STFC, NOVA, INAF and ING, and mounted at SENER Aeroespacial (Spain) by SENER and ING.
  • Spectrograph, built by NOVA in the Netherlands with optical design by RAL Space in the UK, optics manufactured at INAOE (MX) and support from INAF (IT) and the IAC (ES).
  • Field Rotator, provided by the Instituto de Astrofísica de Canarias (IAC) in Spain and manufactured by IDOM (Spain)
  • Optical fibres, provided by the Observatoire de Paris in France, manufactured in France, Canada and the USA.
  • LIFU, built by NOVA (NL)
  • CCD detectors system, provided by Liverpool John Moores University in the UK.
  • Data processing, analysis and archiving, led by the University of Cambridge (UK) with support from the IAC (ES) and INAF (IT).
  • Observation control system, built by the ING

Isaac Newton Group of Telescopes (ING)

The Isaac Newton Group of Telescopes is operated on behalf of:

  • STFC, United Kingdom
  • Nederlanse Organisatie voor Wetenschappelijk Onderzoek (NWO), The Netherlands
  • Instituto de Astrofísica de Canarias (IAC), Spain