A ring of fireworks around a nearby galactic collision

October 15, 2015

Using the 1.2m UKST, a team of astronomers has discovered the closest collisional ring around a galaxy. This very rare structure, that has been dubbed “Kathryn’s Wheel”, is quickly converting gas into stars in a massive fireworks display.

An international team of astronomers led by Prof. Quentin Parker (The University of Hong Kong / Australian Astronomical Observatory) has discovered that the nearby galaxy ESO 179-13 possesses a giant ring rich of nebulae and new-formed stars around it. This rare structure has been induced by the direct collision of two galaxies, that has triggered the fireworks in the system. This huge star-forming ring has been dubbed “Kathryn’s Wheel” in honour of the wife of one of its discoverers, Prof. Albert Zijlstra, (University of Manchester, UK). 

The system was discovered as part of the observations of the AAO/UK Schmidt Telescope (UKST) Survey for Galactic H-alpha emission. Completed in late 2003, this survey used the 1.2m UKST at Siding Spring Observatory (NSW, Australia) to get wide-field photographic data of the Southern Galactic Plane and the Magellanic Clouds using a H-alpha filter. This special filter is able to trace the gaseous hydrogen (and not the stellar emission) within galaxies, allowing astronomers to detect the ionized gas from nebulae. The survey films were scanned by the SuperCosmos measuring machine at the Royal Observatory, Edinburgh (UK), to provide the online digital atlas “SuperCOSMOS H-alpha Survey” (SHS).

Kathryn Wheel Fig 1

(Left) Colour image of the collision, made by combining data obtained at the Cerro-Tololo InterAmerican Observatory (CTIO) 4-metre telescope in Chile. The H-alpha image is shown in red and reveals the star-forming ring around the galaxy ESO 179-13, that has been dubbed “Kathryn’s Wheel”. Image credit: Ivan Bojicic / the research team. (Right) Image showing only the pure H-alpha emission of the system  highlighting just the areas of active star formation. For clarity any remaining stellar residuals have been removed. Image credit: Quentin Parker / the research team.

The galaxy ESO 179-13 is located in the Ara (the Altar) constellation, at a distance of 30 million light years. The reason why this magnificent collisional ring structure has been unknown by astronomers is that the galaxy is located behind our own Milky Way and very close to a bright foreground star.

Prof. Parker and his team used the SHS survey to search for dying sun-like stars in our Galaxy traced by the emission of the atmospheres released by the dying stars (planetary nebulae), that usually show ring morphologies. The astronomers were very surprised to find such ring structures not around a star, but around a nearby galaxy (ESO 179-13). Follow-up observations of the system confirmed that the ring is consequence of the collision of two galaxies of similar mass. The ring structure is created by a powerful shock wave that sweeps up gas and dust, triggering the formation of new stars as the shock wave moves outwards. The most famous collisional ring galaxy is the Cartwheel (ESO 350-40) galaxy, which is located at 500 million light-years away in the Southern constellation of the Sculptor (that is, 40 times further than Kathryn’s Wheel).

The discovery SHS images of the system reveal 3 main structures (A, B and C) plus tens of H-alpha emitting knots making the ring. Component A is the remnant of the main galaxy, the collisional ring is centred on it. Component A does not possess ionized gas (that is, it does not have star-formation at the moment). On the other hand, component B seems to be the irregular, dwarf galaxy (“the bullet”) that impacted with the main galaxy. Component B does possess a clumpy and intense H-alpha emission.

Kathryn Wheel Fig 2

Discovery images of the “Kathryn’s Wheel" using the data obtained at the 1.2m UKST by the “SuperCOSMOS H-alpha Survey” (SHS). The left panel (SR) shows the red image tracing mainly the stars. The three main components of the system are labelled. The central panel shows the image using the H-alpha filter (H?), which sees both the diffuse ionized gas and the stars. The right panel (H?-SR) shows the continuum-substracted image of the system, revealling for the very first time the intense collisional star-forming ring. Image credit: Quentin Parker / the research team.

Furthermore, Kathryn’s Wheel possesses a lot of diffuse, neutral hydrogen in its surroundings. This cold gas is the raw fuel that galaxies need to create new stars. Observations using the 64-m Parkes radiotelescope (“The Dish”, Parkes, NSW) as part of the “HI Parkes All-Sky Survey” (HIPASS) revealed that the amount of neutral gas around Kathryn’s Wheel is similar to the amount of mass found in stars in the system. Astronomers are unsure about from where this cold gas is coming from, although they suspect it mainly belonged to the main galaxy before the collision started. However, as the remnant of the galaxy (component A) does not have star-formation at the moment, it seems that the diffuse gas has been expelled from the centre of the system to the outskirts of the galaxy.

Collisional ring galaxies are extremely rare in the Universe, only 20 of these objects are known. Kathryn’s Wheel is an ideal target for detailed studies aiming to understand how these rare collisional ring galaxies are formed, the physics behind these structures, and their role in galaxy evolution. Interestingly, the collisional ring is not very massive: its mass is only a few thousand million Suns. This is less than ~1% of the Milky Way mass, indicating that ring galaxies can be formed around small galaxies, something that was not considered so far.

 

The results were published in MNRAS in August 2015.

MNRAS 452, 3759–3775 (2015) doi:10.1093/mnras/stv1432
Kathryn’s Wheel: a spectacular galaxy collision discovered in the Galactic neighbourhood
Authors: Quentin A. Parker (quentinp@hku.hk), Albert A. Zijlstra, Milorad Stupar, Michelle Cluver, David J. Frew, George Bendo and Ivan Bojicic

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