pretty pictures

The images above show the neutral hydrogen gas around the galaxy NGC 1427A as seen by MeerKAT. This galaxy is located near the centre of the Fornax cluster, and is a prime example of the interaction between galaxies and the environment around them.

The neutral hydrogen gas is shown with four different colour schemes, all representing the gas density. The gas is overlaid on a sensitive, grey-scale optical image, which shows stellar light and was downloaded from the Legacy Survey database. The densest neutral hydrogen gas is found inside the stellar body of the galaxy, towards the bottom right, but we find plenty of gas outside the galaxy, too!

Indeed, the most notable feature of these images is the long and broad gas tail, extending towards the top left as far as 200,000 light years from the galaxy. Our analysis shows that the formation of the tail occurred in two steps. First, NGC 1427A was involved in a close encounter with another galaxy. The resulting gravitational interaction stirred NGC 1427A, weakening its grip on some of the gas originally located inside its stellar body. Second, the now loosely-bound gas was swept away by the pressure exerted on NGC 1427A by the surrounding hot gas, which fills the entire Fornax cluster. This pressure is caused by the fact that NGC 1427A moves towards the bottom right of the image within the hot gas.

We think that this two-step process is a common way of removing neutral hydrogen gas from galaxies in small clusters like Fornax. 

Image adapted from Serra et al. (2024), A&A, 690, A4.

The top image shows 3 galaxies in the centre of the Fornax A group observed in optical colours by the Fornax Deep Survey:  NGC 1316 (bottom), NGC 1317 (top left) and NGC 1310 (top-right).

The bottom image shows the same 3 galaxies, as well as the cool neutral hydrogen gas (blue) observed by MeerKAT and the hot ionised hydrogen gas (pink) observed by the VST telescope. The galaxies contain different amounts of gas due to galaxy interactions that can remove and transform it. 

The most striking feature is the cool (blue) and hot (pink) gas outside the galaxies. We detect over 1 billion solar masses of neutral hydrogen and 2.5 million solar masses of ionised hydrogen outside the galaxies, which is a result of gas being stripped and ejected from galaxies cannibalised by NGC 1316. For the first time in the Fornax A group, we observe huge filaments of hot ionised gas that are 325,000 light years in length and stretch between the galaxies. There are some regions where the ionised (hot) and neutral (cold) gas overlap, which is very rare and unusual as it is difficult for hot and cold gas to survive in the same location. This work speculates that the magnetic field from the super massive black hole of NGC 1316 is assisting the survival of the hot and cold gas. 

Image adapted from Kleiner et al. (2021), A&A, 648A, 32.

The image above shows radio plasma (in red) ejected through multiple episodes of nuclear activity in Fornax A (a.k.a. NGC 1316) as seen with MeerKAT. The radio emission is overlaid on a deep optical image from the Fornax Deep Survey.

Fornax A is one of the most fascinating radio sources in the local Universe because of its filamentary extended radio lobes, reaching ~ 500,000 light years from the galaxy centre. To the south of the galaxy, a 'bridge' of radio synchrotron emission connects the two lobes. In the centre, two small jets are embedded in the stellar body of the galaxy. For the first time we have been able to measure the timescale of the formation of the radio lobes and jets, finding that the nuclear activity of Fornax A is rapidly flickering. Multiple episodes of activity, the last of which ended 12 million years ago, must have formed the giant radio lobes. The jets in the centre are the remnant of a more recent activity episode, ended less than 3 million years ago. Currently, the galaxy core may be in a new active phase.

Image adapted from Maccagni et al. (2020), A&A, 26A, 634.