Post-Newtonian Dynamical Modeling of Supermassive Black Holes in Galactic-scale 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 subparsec scales. The KETJU code includes post-Newtonian 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 similar to 0.005 pc. We test the performance of our code by comparison to NBODY7 and rVINE. We set up dynamically stable multicomponent 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 nonspherical shape of the merger remnants. For bulge-only models, the hardening rate decreases with increasing resolution, whereas for models that 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.Peer reviewe

The Origin of the [C II] Deficit in a Simulated Dwarf Galaxy Merger-driven Starburst

We present [C II] synthetic observations of smoothed particle hydrodynamics (SPH) simulations of a dwarf galaxy merger. The merging process varies the star formation rate (SFR) by more than three orders of magnitude. Several star clusters are formed, the feedback of which disperses and unbinds the dense gas through expanding H II regions and supernova (SN) explosions. For galaxies with properties similar to the modeled ones, we find that the [C II] emission remains optically thin throughout the merging process. We identify the warm neutral medium (3 2 chi(H2)) to be the primary source of [C II] emission (similar to 58% contribution), although at stages when the H II regions are young and dense (during star cluster formation or SNe in the form of ionized bubbles), they can contribute greater than or similar to 50% to the total [C II] emission. We find that the [C II]/far-IR (FIR) ratio decreases owing to thermal saturation of the [C II] emission caused by strong far-UV radiation fields emitted by the massive star clusters, leading to a [C II] deficit medium. We investigate the [C II]-SFR relation and find an approximately linear correlation that agrees well with observations, particularly those from the Dwarf Galaxy Survey. Our simulation reproduces the observed trends of [C II]/FIR versus Sigma(S)(FR) and Sigma(FIR), and it agrees well with the Kennicutt relation of SFR-FIR luminosity. We propose that local peaks of [C II] resolved observations may provide evidence for ongoing massive cluster formation.Peer reviewe

The Formation of Low-metallicity Globular Clusters in Dwarf Galaxy Mergers

We present a hydrodynamical simulation at sub-parsec and few-solar-mass resolution of a merger between two gas-rich dwarf galaxies. Our simulation includes a detailed model for the multi-phase interstellar medium and is able to follow the entire formation history of spatially resolved star clusters, including feedback from individual massive stars. Shortly after the merger we find a population of similar to 900 stellar clusters with masses above 10(2.5) M-circle dot and a cluster mass function (CMF), which is well fitted with a power law with a slope of alpha = -1.70 +/- 0.08. We describe here in detail the formation of the three most massive clusters (M-* greater than or similar to 10(5) M-circle dot), which populate the high-mass end of the CMF. The simulated clusters form rapidly on a timescale of 6-8 Myr in converging flows of dense gas. The embedded merger phase has extremely high star formation rate surface densities of Sigma(SFR) > 10 M-circle dot yr(-1) kpc(-2) and thermal gas pressures in excess of Pth similar to 10(7) K-B cm(-3))(-1). The formation process is terminated by rapid gas expulsion driven by the first generation of supernovae, after which the cluster centers relax and both their structure and kinematics become indistinguishable from observed local globular clusters (GCs). The simulation presented here provides a general model for the formation of metal-poor GCs in chemically unevolved starbursting environments of low-mass dwarf galaxies, which are common at high redshifts.Peer reviewe

Structure and Rotation of Young Massive Star Clusters in a Simulated Dwarf Starburst

We analyze the three-dimensional shapes and kinematics of the young star cluster population forming in a high-resolution griffin project simulation of a metal-poor dwarf galaxy starburst. The star clusters, which follow a power-law mass distribution, form from the cold phase interstellar medium with an initial mass function sampled with individual stars down to four solar masses at sub-parsec spatial resolution. Massive stars and their important feedback mechanisms are modeled in detail. The simulated clusters follow a surprisingly tight relation between the specific angular momentum and mass with indications of two sub-populations. Massive clusters (M-cl greater than or similar to 3 x 10(4) M) have the highest specific angular momenta at low ellipticities (epsilon similar to 0.2) and show alignment between their shapes and rotation. Lower mass clusters have lower specific angular momenta with larger scatter, show a broader range of elongations, and are typically misaligned indicating that they are not shaped by rotation. The most massive clusters (M greater than or similar to 10(5) M) accrete gas and protoclusters from a less than or similar to 100 pc scale local galactic environment on a t less than or similar to 10 Myr timescale, inheriting the ambient angular momentum properties. Their two-dimensional kinematic maps show ordered rotation at formation, up to v similar to 8.5 km s(-1), consistent with observed young massive clusters and old globular clusters, which they might evolve into. The massive clusters have angular momentum parameters lambda(R) less than or similar to 0.5 and show Gauss-Hermite coefficients h(3) that are anti-correlated with the velocity, indicating asymmetric line-of-sight velocity distributions as a signature of a dissipative formation process.Peer reviewe

Comparison of stellar populations in simulated and real post-starburst galaxies in MaNGA

Recent integral field spectroscopic (IFS) surveys have revealed radial gradients in the optical spectral indices of post-starburst (PSB) galaxies, which can be used to constrain their formation histories. We study the spectral indices of post-processed mock IFS datacubes of binary merger simulations, carefully matched to the properties of the MaNGA IFS survey, with a variety of black hole (BH) feedback models, progenitor galaxies, orbits, and mass ratios. Based on our simulation sample, we find that only major mergers on prograde-prograde or retrograde-prograde orbits in combination with a mechanical BH feedback model can form galaxies with weak enough ongoing star formation, and therefore absent H alpha emission, to be selected by traditional PSB selection methods. We find strong fluctuations in nebular emission line strengths, even within the PSB phase, suggesting that H alpha selected PSBs are only a subsample of the underlying population. The global PSB population can be more robustly identified using stellar continuum-based approaches. The difficulty in reproducing the very young PSBs in simulations potentially indicates that new sub-resolution star formation recipes are required to properly model the process of star formation quenching. In our simulations, we find that the starburst peaks at the same time at all radii, but is stronger and more prolonged in the inner regions. This results in a strong time evolution in the radial gradients of the spectral indices that can be used to estimate the age of the starburst without reliance on detailed star formation histories from spectral synthesis models.Peer reviewe

The GRIFFIN Project-Formation of Star Clusters with Individual Massive Stars in a Simulated Dwarf Galaxy Starburst

A correction to this article has published 2022 July 27: https://doi.org/10.3847/1538-4357/ac7ebbWe describe a population of young star clusters (SCs) formed in a hydrodynamical simulation of a gas-rich dwarf galaxy merger resolved with individual massive stars at subparsec 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 (Sigma(SFR)) varies by more than three orders of magnitude during the simulation. Young SCs more massive than M-*,M-cl similar to 10(2.5)M(circle dot) form along a mass function with a power-law index alpha similar to -1.7 (alpha similar to -2 for M-*,M-cl greater than or similar to 10(3) M-circle dot) at all merger phases, while the normalization and the highest SC masses (up to similar to 10(6) M-circle dot) correlate with SSFR. The cluster formation efficiency varies from Gamma similar to 20% in early merger phases to Gamma similar to 80% at the peak of the starburst and is compared to observations and model predictions. The massive SCs (greater than or similar to 10(4)M(circle dot)) 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 sigma similar to 0.1-2 Myr (1 sigma), consistent with observations. The age spreads increase with cluster mass, with the most massive cluster (similar to 10(6)M(circle dot)) 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.Peer reviewe