Observational Astrophysics

X-ray Astronomy

We use X-rays to identify and study the active galactic nuclei (AGN) that exist at the centres of many galaxies. Almost all galaxies have a supermassive black hole at their centres, with masses millions to billions of times that of the Sun. Some of these AGN are accreting material from the central regions of their host galaxies, and the energy released is emitted over a wide range of wavelengths from radio all the way to X-rays, sometimes outshining the rest of the galaxy. Often there is significant amounts of dense material surrounding the centres of these galaxies that acts to obscure this emission. The high energy of the X-ray photons mean they can penetrate this opaque material, so X-ray observations give the most complete picture of these highly energetic objects.

Links to Galaxy Evolution and Clusters and Active Galaxies and Black Holes

Optical and Infrared Astronomy

Optical and infrared astronomy is used to observe everything from stars, galaxies, and even planets. These two wavelength are often joined together as they use the same detector architecture and therefore their instrumentation often overlaps. While the Earth's atmosphere transmits optical light (which we use to see) the water vapor and other greenhouse gases in the atmosphere are effective blockers of IR radiation from distant astonmoical objects which requires the use of specific spectral windows when using telescopes on the ground or the use of space-based telescopes. Here at Bristol we work with a range of ground- and space-based optical and infrared telescopes to measure different astrophysical phenomena.

Euclid

Euclid is an M-Class European Space Agency (ESA) mission launched successfully on July 1st, 2023. It is designed to poin-point the cosmological parameters of the Universe, revealing the nature of dark matter and dark energy. The two main instruments Visual Imager (VIS) and the Near-Infrared Spectrometer and Photometer (NISP), will deliver images and spectra of spatial resolution comparable to the Hubble Space Telescope (HST). Over the course of its 6-year mission lifetime, Euclid will observer the majority of the extragalactic sky (14,500 square degrees) and provide measurements for billlions of galaxies. In addition to cosmology, Euclid will be very impactful for the study of clusters, galaxies, and AGN. We are involved in all aspects of the mission. Bristol researchers have been at the forefront of the first quick look data release (Q1) in March 2025, in particular leading the creation of the first Euclid-AGN catalogue and novel machine-learning methods for the detection of AGN.

Members of the Euclid Consortium benefit from one year proprietary access to the data, which are then released to the public through the ESA archives.

Links to High-performance scientific computing and AI, Active Galaxies and Black Holes and Galaxy Evolution and Clusters

European Southern Observatory

ESO is an intergovernmental organisation responsible for a world-class suite of telescopes and instrumentation located at the best observing sites in the Chilean Atacama Desert. The 8-m class Very Large Telescopes (VLT) provide images of galaxies over cosmic time, and the new 4MOST multi-object spectrograph will measure thousands of spectra each night, providing distance, star formation, and chemical composition information for distant galaxies and AGN. Bristol astronomers have used these telescopes for their own research, and are members of collaborations undertaking large surveys.

Links to Active Galaxies and Black Holes and Galaxy Evolution and Clusters

Hubble Space Telescope

The NASA/ESA Hubble Space Telescope (Hubble) was launched in 1990 as the first great Flagship Observatory in space. Hubble operates over the UV-optical-nearIR wavelengths using a wide range of instruments (ACS, COS, STIS, WFC3). Hubble was a servicable telescope and had four Servicing missions between 1993 - 2009. The final service mission installed new computers for the STIS UV-optical instrument, and the new version of the Wide Field Camera instrument (WFC3) which operates in the UV to near-IR. Here at Bristol we have active research using Hubble to measure the atmospheric properties of exoplanets orbiting distant stars in our galaxy. These measurements make use of the wide UV-optical wavelength coverage to better understand the absorption properties of the atmosphere of other planets through the transmission of their host starlight through the upper atmosphere during a transit event (when the planet passes in front of the star from our point of view). The gases that make up the atmosphere of the distant planet are imprinted on the stars spectrum and this can be extracted and analysed. We are also able to better measure the impact of aerosols (solid or liquid droplets suspended in the gas) as their scattering properties greatly change the way the light is filters.

Links to Planetary formation and atmospheres

JWST

JWST is the latest Flagship Observatory from NASA/ESA/CSA and was launched on 25th December 2021. JWST operates in the IR from the red optical to the mid-IR (30 microns). JWST has four instruments (NIRISS, NIRSpec, NIRCam, MIRI) with multiple observing images and spectrographs to cover its wide wavelength range. Both Hubble and JWST are used for all of astrophysics from the first stars and galaxies to exoplanets and our own solar system. Here at Bristol we specilise in exoplanet time series observations with both observatories which allow us to precicely measure the transit of distant planets as they orbit their stars. This requirs incredible stabilty of the telescope pointing systems, precise timing of observations to capture the astrophysical events, and high precision spectrographs to capture part per million signals.

Since the start of science operations of JWST in summer 2022 Bristol astronomers have been work on the new observations, inculding leading some of the very first observations the telescope took of transiting exoplanets as part of the Transiting Exoplanet Community Early Release Science Program (see video above). Bristol is also a leading member of collaborations working with Guranteed Time Observations awarded to those who helped get the telescope into operations, as well as a large number of General Observing programs.

Hubble and JWST operate on a General Observing call for proposals where scientific programs are anonomously proposed by people and competitively selected to be observed. Our researchers at Bristol have been highly sucessful at winning time on these state-of-the-art telescopes with MScR and PhD students able to serve as PIs. At Bristol we have been involved in over 20 individual JWST programs looking at nearly 50 exoplanets and their atmospheres.

Links to Planet formation and atmospheres

Vera Rubin Observatory - LSST Consortium

The VRO is situated in Chile and hosts the 8m Simonyi Survey Telescope. It has been constructed over the last decade and is starting to produce scientific data in 2025. The UK has access to it through agreement with the NSF and DoE in the USA. The telescope will be carrying out a decade-long multi-band imaging survey of most of the southern sky - the LSST - the Legacy Survey of Space and Time. The huge data set produced by the survey will have many uses, including looking for rare objects based on their variability, transient nature or specific colour, and generating an unprecedentedly complete census of galaxies in terms of their brightness on the sky and their colours, from the most nearby objects to those so distant that they were detected from light emitted when the universe was less than a billion years old (it is now 14 billion years old). Astronomers in Bristol are interested in using this catalogue in conjunction with others, for example that generated by the Euclid mission, to study the characteristics and evolution of galaxy properties as a function of both time and environment, feeding into many of the research interests the group has in galaxy and cluster evolution.

Links to Active Galaxies and Black Holes

Radio Astronomy and Sub-mm

Radio astronomy uses radio antennas, rather than mirrors and cameras, to detect radio emission from astronomical objects. Many things in the Universe emit radio waves, including stars, planets, supernova remnants, pulsars, fast radio bursts, and active galactic nuclei. The neutral hydrogen atom also emits very weak radio waves. A number of Bristol Astrophysics theme members use radio telescopes for their research. The larger the telescope, the fainter and more distant objects it can detect. The resolution, or the ability to see fine details, also improves with increasing telescope size. Therefore, we want to build the largest telescopes possible.

We use radio telescopes to investigate the formation and evolution of galaxies, giving us a view into processes not visible at other wavelengths. Observing the hydrogen reservoirs of galaxies not only tells us about their past star formation history, but also their future star formation potential. It is also very sensitive to disturbances, so gives clues about a galaxy's surroundings, and interactions with other galaxies. We are observing both the neutral hydrogen, as the raw material for star formation, and the radio continuum emission from star formation and active galactic nuclei, using state of the art radio interferometers, including the South African telescope MeerKAT, and in the future, the Square Kilometre Array.

The sub-millimetre wavelength regime is sensitive to emission from cool material (from about 100 Kelvin down to a few Kelvin) and from molecular and atomic/ionized gas. Astronomers call much of this cool material "dust", in reality particulate matter comparable in size to smoke particles. The dust radiates in the sub-mm due it it absorbing shorter wavelength light from stars and other objects which heats the dust to a few tens of Kelvin, resulting in the dust re-radiating the absorbed energy at sub-mm wavelengths. Consequently, the sub-mm regime is ideal for studying star formation in galaxies near and far, which is often hidden behind a shroud of dusty material.

The emission from gas tells us about the energy state of hot atomic and ionised gas along with cold molecular gas which, again, traces regions of star formation in galaxies. We trace these giant molecular clouds through emission from other molecular species such as carbon monoxide (CO), hydrogen cyanide (HCN), with different species tracing molecular gas at different densities.

Links to Galactic Evolution and Clusters and Active Galaxies and Black Holes

Towards the SKAO

The global astronomy community is collaborating on building the next large radio telescope interferometer, called the Square Kilometre Array Observatory (SKAO). With one telescope located across southern Africa, and another in Western Australia, these facilities represent the next generation of radio telescopes, enabling observations and discoveries not possible with existing instruments. The operational headquarters are located in Jodrell Bank, near Manchester, so the UK is a core partner in the SKAO project. By using existing telescopes for our research, such as MeerKAT, we will build expertise in advance of the start of SKAO operations in the coming decade, and be ready for the science the SKAO will enable.

Virtual Observatory

We are developing software and standards to connect astronomers to data providers around the world as part of the International Virtual Observatory Alliance. This work is being led by Dr Mark Taylor.