pretty pictures
A MeerKAT view of pre-processing in the Fornax A group
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 flickering nuclear activity of Fornax A
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.
Neutral hydrogen gas within and around NGC 1316
The image on the left shows neutral hydrogen gas (in green) discovered within and around the galaxy NGC 1316 (a.k.a. Fornax A). The neutral hydrogen was observed using MeerKAT and here it is overlaid on a deep optical image coming from the Fornax Deep Survey.
The important features in this image are the two long gaseous tails highlighted by the thin curved arcs, containing a total of 500 million solar masses of neutral hydrogen and partly corresponding with known stellar tails. Their discovery solves a long-standing puzzle on the origin of NGC 1316. It strengthens the theory that the galaxy formed through a merger between two galaxies a few billion years ago: one of these galaxies was similar to the Milky Way and hosted the gas now found in the two tails; while the other was 10 times more massive but devoid of neutral hydrogen.
Neutral hydrogen is also visible in a few clouds within and around the galaxy, as well as in three satellite galaxies, revealing the richness and complexity of NGC 1316's environment.
Image adapted from Serra et al. (2019), A&A, 628A, 122.