MAGPI Magic I: Jostling galaxies

By Ryan Bagge
[email protected]

How galaxies evolve over cosmic time is a fundamental question in extragalactic astronomy. In my research, I try to disentangle what physical processes impact galaxy evolution the most by observing how the gas and stars move within galaxies.  Typically, stars and gas within galaxies move in elliptical orbits, but these orbits can become disturbed through mergers, interactions and other processes within galaxies. Using spatially resolved spectra from Integral Field Spectrographs (IFS), we can create kinematic maps of galaxies. Signatures of disturbed orbits on kinematic maps may indicate that these galaxies have experienced a recent merger or interaction.

We investigate the sources of kinematic disturbances in a recently published paper, where we measure the disturbances in the ionised gas from a sample of galaxies from the Middle Ages Galaxy Properties in IFS (MAGPI) Survey.  MAGPI is collecting data from galaxies in the Universe’s ‘middle ages’, 4 billion years in the past, using the European Southern Observatory’s Multi-Object Spectroscopic Explorer (MUSE) on the Very Large Telescope in Chile.

Group of MAGPI galaxies.
Fig. 1: An image of a. group of MAGPI galaxies (MAGP1207197197; lower centre, MAGPI1207128248; centre left, MAGPI1207181305; upper centre). The galaxies which are within the white circles all have disturbed kinematics. We suspect this is due to their proximity. The white arrow is pointing to the tidal stream between MAGPI1207197197 and MAGPI1207128248.

We found that galaxies close to a more massive neighbouring galaxy tend to display larger kinematic disturbances than galaxies without a close neighbour. We found one group consisting entirely of kinematically disturbed galaxies (Fig. 1). We suspect that this is because they are close to each other – two of the galaxies (MAGPI1207197197 & MAGPI1207128248) are interacting and are connected by a tidal tail.

Interactions between galaxies are not the only sources of kinematic disturbances. We found galaxies with large velocity disturbance at specific locations only, like in Fig. 2. Given how localised these disturbances are, it is unlikely that interactions are responsible. When stars are born, they release energy back into their surroundings and disturb the nearby gas. This is largely a local process, so star formation was the obvious culprit for this localised disturbance.

Fig. 2: A flux map (left) and line-of-sight velocity (right) map for MAGPI1202197197, a spiral galaxy in our sample that is rotating with a slight kinematic disturbance that is marked with the white square. The rest of the galaxy appears to be rotating normally.


Michael Murphy is the Australian representative on the ESO Science Technical Committee. Contact: [email protected]

Sarah Sweet is the Australian representative on the ESO Users Committee. Contact: [email protected]

Stuart Ryder is a Program Manager with AAL. Contact: [email protected]

Guest posts are also welcome – please submit these to [email protected]