Pinning down the assembly of most massive galaxies with the M3G Survey
Final Report Abstract
Within the project "Pinning down the assembly of most massive galaxies with the M3G survey", we studied the stellar kinematics and stellar populations of some of the most massive galaxies in the Universe. These galaxies are rare, but they keep the records of past interaction and provide a unique view into the formation of galaxies. We used the data of 25 galaxies obtained within the M3G Survey, using a large Rield-of-view integral-Rield spectrograph, MUSE, mounted on the Very Large Telescope of the European Southern Observatory. Galaxies were selected to have stellar masses larger than 10^12 Msun, live in dense environments of rich galaxy clusters, either as the brightest cluster galaxies (BCGs) or their somewhat less bright, but still massive, satellites (non-BCGs). Such galaxies remain relatively unexplored family of systems even though they represent the end point in the evolution of galaxy assembly. The data provided an unprecedented maps of stellar kinematics revealing the complex internal shapes and the richness of orbits occupied by stars in massive galaxies. The main goal of the project was to construct dynamical models of such systems and link them with chemical properties of their stars. As the Rirst step, we selected a unique BCG, designated as PGC046382, which has the most complex velocity map, showing consecutive spin reversals, none of which is aligned with the galaxy stellar body. This implies that the galaxy is built of multiple orbital families, which can co-exist only in a triaxial gravitational potential. Our dynamical model was able to reproduce all major kinematic features and show that the shape of the galaxy actually changes with radius from a strongly triaxial (almost prolate) in the centre to the almost axisymmetric (oblate) in the outer parts. A comparison of the complex kinematic and morphological structure of PGC046832 and predictions from numerical simulations suggested that its central supermassive black hole (SMBH) could be exceptionally massive (>10^10 Msun). We attempted to determine the mass of the SMBH using the same dynamical models which successfully described the kinematics and the shape of the galaxy, but the results were inconclusive, providing only an upper limit of ~10^9 Msun for the SMBH mass. This is a challenging results, as PGC046832 has a large surface brightness core (central 400pc have essentially constant brightness), and the current hypothesis for the core formation is that they are created by interaction of massive SMBHs. The core in PGC046832 could have been created using a single coalescence of SMBHs with masses of ~5x10^9 Msun, or a larger number of similar mass mergers (~5) having "small" black holes (~10^8 Msun). This is an exciting discovery, which suggest an unexpectedly active mass assembly of a BCG, or a need to look for alternative scenarios of core formation. Within the project we investigated also the stellar populations of massive galaxies. A prerequisite for this was to Rind and remove all emission-lines from the spectra, essentially mapping the distribution and kinematics of the warm ionised gas. Massive galaxies are usually considered as being devoid of gas, but several galaxies in our sample, both BGCs and non-BCGs, had signiRicant gas components. Their origin seem mostly to be external as the gas was found to be misaligned and rotate differently from the host galaxies. Investigating the structure of the sample galaxies, we were able to establish that the fraction of accreted stellar mass in some of them is above 80% (20% of stars being made in-situ), and that this is predominately a consequence of gas-free major mergers. On the other hand, stellar populations of our sample galaxies are mostly old and metal rich, without strong changes within the observed spatial scales. We were not able to Rind a relation between different chemical properties and stellar orbits, but we developed a method of linking the stellar population parameters with the orbital structure provided by our dynamical models.
Publications
- "Assembly history of massive galaxies. A pilot project with VEGAS deep imaging and M3G integral field spectroscopy", 2021, A&A, 649,161
Spavone, Marilena; Krajnovic, Davor; Emsellem, Eric; lodice, Enrichetta; den Brok, Mark
(See online at https://doi.org/10.1051/0004-6361/202040186) - "Dynamical modelling of the twisted galaxy PGC046832", MNRAS, 508, 4786
den Brok, Mark; Krajnovic, Davor; Emsellem, Eric; Brinchmann, Jarle; Maseda, Michael
(See online at https://doi.org/10.1093/mnras/stab2852) - "Optical emission lines in the most massive galaxies: Morphology, kinematic and ionisation properties", 2021, A&A, 649, 63
Pagotto, Ilaria; Krajnovic, Davor; den Brok, Mark; Emsellem, Eric; Brinchmann, Jarle; Weilbacher, Peter M.; Kollatschny, Wolfram; Steinmetz Matthias
(See online at https://doi.org/10.1051/0004-6361/202039443)
