Post-starburst galaxies can be identified via the presence of prominent Hydrogen Balmer absorption lines in their spectra. We present a comprehensive study of the origin of strong Balmer lines in a volume-limited sample of 189 galaxies with 0.01 < z < 0.05, log(M /M ) > 9.5 and projected axis ratio b/a > 0.32. We explore their structural properties, environments, emission lines and star formation histories, and compare them to control samples of star-forming and quiescent galaxies, and simulated galaxy mergers. Excluding contaminants, in which the strong Balmer lines are most likely caused by dust-star geometry, we find evidence for three different pathways through the post-starburst phase, with most events occurring in intermediate-density environments: (1) a significant disruptive event, such as a gas-rich major merger, causing a starburst and growth of a spheroidal component, followed by quenching of the star formation (70% of post-starburst galaxies at 9.5 < log(M /M ) < 10.5 and 60% at log(M /M ) > 10.5); (2) at 9.5 < log(M /M ) < 10.5, stochastic star formation in blue-sequence galaxies, causing a weak burst and subsequent return to the blue sequence (30%); (3) at log(M /M ) > 10.5, cyclic evolution of quiescent galaxies which gradually move towards the high-mass end of the red sequence through weak starbursts, possibly as a result of a merger with a smaller gas-rich companion (40%). Our analysis suggests that AGN are 'on' for 50% of the duration of the post-starburst phase, meaning that traditional samples of post-starburst galaxies with strict emission line cuts will be at least 50% incomplete due to the exclusion of narrow-line AGN.

We describe a population of young star clusters (SCs) formed in a hydrodynamical simulation of a gasrich dwarf galaxy merger resolved with individual massive stars at sub-parsec spatial resolution. The simulation is part of the griffin (Galaxy Realizations Including Feedback From INdividual massive stars) project. The star formation environment during the simulation spans seven orders of magnitude in gas surface density and thermal pressure, and the global star formation rate surface density (Σ SFR ) varies by more than three orders of magnitude during the simulation. Young SCs more massive than M * ,cl ∼ 10 2.5 M form along a mass function with a power-law index α ∼ −1.7 (α ∼ −2 for M * ,cl 10 3 M ) at all merger phases, while the normalization and the highest SC masses (up to ∼ 10 6 M ) correlate with Σ SFR . The cluster formation efficiency varies from Γ ∼ 20% in early merger phases to Γ ∼ 80% at the peak of the starburst and is compared to observations and model predictions. The massive SCs ( 10 4 M ) have sizes and mean surface densities similar to observed young massive SCs. Simulated lower mass clusters appear slightly more concentrated than observed. All SCs form on timescales of a few Myr and lose their gas rapidly resulting in typical stellar age spreads between σ ∼ 0.1 − 2 Myr (1σ), consistent with observations. The age spreads increase with cluster mass, with the most massive cluster (∼ 10 6 M ) reaching a spread of 5 Myr once its hierarchical formation finishes. Our study shows that it is now feasible to investigate the SC population of entire galaxies with novel high-resolution numerical simulations.

We present KETJU, a new extension of the widely-used smoothed particle hydrodynamics simulation code GADGET-3. The key feature of the code is the inclusion of algorithmically regularized regions around every supermassive black hole (SMBH). This allows for simultaneously following global galactic-scale dynamical and astrophysical processes, while solving the dynamics of SMBHs, SMBH binaries and surrounding stellar systems at sub-parsec scales. The KETJU code includes PostNewtonian terms in the equations of motions of the SMBHs which enables a new SMBH merger criterion based on the gravitational wave coalescence timescale pushing the merger separation of SMBHs down to ∼ 0.005 pc. We test the performance of our code by comparison to NBODY7 and rVINE. We set up dynamically stable multi-component merger progenitor galaxies to study the SMBH binary evolution during galaxy mergers. In our simulation sample the SMBH binaries do not suffer from the final-parsec problem, which we attribute to the non-spherical shape of the merger remnants. For bulge-only models, the hardening rate decreases with increasing resolution, whereas for models which in addition include massive dark matter halos the SMBH binary hardening rate becomes practically independent of the mass resolution of the stellar bulge. The SMBHs coalesce on average 200 Myr after the formation of the SMBH binary. However, small differences in the initial SMBH binary eccentricities can result in large differences in the SMBH coalescence times. Finally, we discuss the future prospects of KETJU, which allows for a straightforward inclusion of gas physics in the simulations.