Hubble and HAWK-I explore a cluster with the mass of two quadrillion Suns: This image shows something spectacular: a massive galaxy cluster that it is warping the space around it! The cluster, whose heart is at the centre of the frame, is named RCS2 J2327, and is one of the most massive clusters known at its distance or beyond. Massive objects such as RCS2 J2327 have such a strong influence on their surroundings that they visibly warp the space around them. This effect is known as gravitational lensing. In this way, they cause the light from more distant objects to be bent, distorted, and magnified, allowing us to see galaxies that would otherwise be far too distant to detect. Gravitational lensing is one of the predictions of Einstein's theory of general relativity. Strong lensing produces stunning images of distorted galaxies and sweeping arcs; both of which can be seen in this image. Weak gravitational lensing, on the other hand, is more subtle, hardly seen directly in an image, and is mostly studied statistically — but it provides a way to measure the masses of cosmic objects, as in the case of this cluster. This image is a composite of observations from the HAWK-I instrument on ESO’s Very Large Telescope and the NASA/ESA Hubble Space Telescope’s Advanced Camera for Surveys. It demonstrates an impressively detailed collaborative approach to studying weak lensing in the cosmos. The study found RCS2 J2327 to contain the mass of two quadrillion Suns! The diffuse blue and white image covering the picture shows a mass map. It is connected to the amount of mass thought to be contained within each region.

Name: RCS2 J2327
Type: Early Universe : Galaxy : Grouping : Cluster
Category: Galaxies
Constellation: Pisces
Release date: 25 December 2017, 06:00
Credit: ESO, ESA/Hubble & NASA

Galaxy cluster RCS2 J2327: This image shows the galaxy cluster RCS2 J2327. The mass of the cluster causes both strong and weak gravitational lensing, which can be used to calculate the mass of the cluster. This image is a composite of observations from the HAWK-I instrument on ESO’s Very Large Telescope and the NASA/ESA Hubble Space Telescope’s Advanced Camera for Surveys. It demonstrates an impressively detailed collaborative approach to studying weak lensing in the cosmos.

Name: RCS2 J2327
Type: Early Universe : Galaxy : Grouping : Cluster
Category: Galaxies
Constellation: Pisces
Release date: 25 December 2017, 06:00
Credit: ESO & ESA/Hubble & NASA

Lasers and supermassive black holes: This image, captured by the NASA/ESA Hubble Space Telescope’s Wide Field Camera 3 (WFC3), shows a galaxy named UGC 6093. As can be easily seen, UGC 6093 is something known as a barred spiral galaxy — it has beautiful arms that swirl outwards from a bar slicing through the galaxy’s centre. It is classified as an active galaxy, which means that it hosts an active galactic nucleus, or AGN: a compact region at a galaxy’s centre within which material is dragged towards a supermassive black hole. As this black hole devours the surrounding matter it emits intense radiation, causing it to shine brightly. But UGC 6093 is more exotic still. The galaxy essentially acts as a giant astronomical laser that spews out light at microwave, not visible, wavelengths — this type of object is dubbed a megamaser (maser being the term for a microwave laser). Megamasers such as UGC 6093 can be some 100 million times brighter than masers found in galaxies like the Milky Way. Hubble’s WFC3 observes light spanning a range wavelengths — from the near-infrared, through the visible range, to the near-ultraviolet. It has two channels that detect and process different light, allowing astronomers to study a remarkable range of astrophysical phenomena; for example, the UV-visible channel can study galaxies undergoing massive star formation, while the near-infrared channel can study redshifted light from galaxies in the distant Universe. Such multi-band imaging makes Hubble invaluable in studying megamaser galaxies, as it is able to untangle their intriguing complexity.

Name: UGC 6093
Type: Early Universe : Galaxy : Activity : AGN
Category: Galaxies
Constellation: Leo
Distance: z=0.036 (redshift)
Release date: 1 January 2018, 06:00
Credit: ESA/Hubble & NASA

A gargantuan collision: In 2014, astronomers using the NASA/ESA Hubble Space Telescope found that this enormous galaxy cluster contains the mass of a staggering three million billion Suns — so it’s little wonder that it has earned the nickname of “El Gordo” (“the Fat One” in Spanish)! Known officially as ACT-CLJ0102-4915, it is the largest, hottest, and X-ray brightest galaxy cluster ever discovered in the distant Universe. Galaxy clusters are the largest objects in the Universe that are bound together by gravity. They form over billions of years as smaller groups of galaxies slowly come together. In 2012, observations from ESO’s Very Large Telescope, NASA’s Chandra X-ray Observatory and the Atacama Cosmology Telescope showed that El Gordo is actually composed of two galaxy clusters colliding at millions of kilometres per hour. The formation of galaxy clusters depends heavily on dark matter and dark energy; studying such clusters can therefore help shed light on these elusive phenomena. In 2014, Hubble found that most of El Gordo’s mass is concealed in the form of dark matter. Evidence suggests that El Gordo’s “normal” matter — largely composed of hot gas that is bright in the X-ray wavelength domain — is being torn from the dark matter in the collision. The hot gas is slowing down, while the dark matter is not. This image was taken by Hubble’s Advanced Camera for Surveys and Wide-Field Camera 3 as part of an observing programme called RELICS (Reionization Lensing Cluster Survey). RELICS imaged 41 massive galaxy clusters with the aim of finding the brightest distant galaxies for the forthcoming NASA/ESA/CSA James Webb Space Telescope (JWST) to study.

Name: ACT-CL J0102-4915
Type: Early Universe : Galaxy : Grouping : Cluster
Category: Galaxies
Constellation: Phoenix
Distance: z=0.87 (redshift)
Release date: 8 January 2018, 06:00
Credit: ESA/Hubble & NASA, RELICS