Other Research Projects

The formation of globular clusters and IMBHs

Surface density of stars (left panels) and gas (middle panels), and thermal gas pressure (right panels) in a 200 pc cube centered at the formation region of the most massive cluster (white ellipses). The top and bottom rows show the region 5 Myr and 3 Myr before the final cluster assembly at t0, respectively. Tens of stellar clusters are forming within the filamentary gas structures (Lahén et al. 2019, ApJL, 879, L18).

Using a hydrodynamical simulation of a merger of two gas-rich dwarf galaxies, run at sub-parsec and a few solar-mass resolution we have studied the formation of globular clusters. We found that the formation process is terminated by rapid gas expulsion driven by the first generation of supernovae, after which the stellar clusters relax and both their structure and kinematics become indistinguishable from observed local globular clusters. Our simulations present a general model for the formation of metal-poor globular clusters in chemically unevolved starbursting environments of low-mass dwarf galaxies, which are common at high redshifts. The simulations are carried out as part of the GRIFFIN (Galaxy Realizations Including Feedback From INdividual massive stars) project.

Recently we have also studied the formation of intermediate mass black holes (IMBHs) in dense stellar clusters through tidal capture of stars and tidal disruption events (TDEs). We find that in sufficiently dense regions, such as nuclear stellar clusters, TDEs can lead a 300 M_☉ BH to reach 7000 M_☉ within a Gyr.

Image of simulated stellar cluster — Synthetic observation of a simulated stellar cluster, with the insets showing the central system of black holes and a comparable real stellar cluster. The simulation shown here is presented in Rizzuto et al. (2022), MNRAS, 512, 884.

Selected Research Papers

Radiative transfer in the vicinity of compact objects

Intensity maps from a self-lensing event in a simple model of a binary SMBH system, computed with Arcmancer. Adapted from Pihajoki et al. 2018, ApJ, 863, 8.

The Arcmancer code (Pihajoki et al., 2018) was developed to allow performing general relativistic ray-tracing and polarised radiative transfer in the curved spacetimes around compact objects such as black holes and neutron stars. The code also includes features for more general calculations involving geodesics and other curves on manifolds of arbitrary dimension with user-specified metrics, including parallel transport of tensors and custom objects, and finding intersections of curves with surfaces. An application leveraging these more advanced capabilities is the generalised barycentric interpolation algorithm described in Pihajoki et al. (2019).

The Arcmancer code is publicly available from here.

Selected Research Papers

The cosmological formation of Early-type galaxies

Evolution of gas surface density (left panel), the mass-weighted mean gas temperature, T (middle panel), and the mass-weighted mean gas entropy, (right panel) for a high-resolution simulation of Halo A2 at redshifts z = 0, z = 1, z = 3, z = 5 from, top to bottom. Adapted from Johansson et al. 2012, ApJ, 754, 115.

Using cosmological SPH simulations we have been studying the formation of early-type galaxies (ETGs). Our simulations advocate a two-phased formation mechanism, in which the initial growth of the ETGs is dominated by compact in situ star formation, whereas the late growth is dominated by accretion of stars formed in subunits outside the main galaxy. The accreted stars assemble predominantly at larger radii explaining both the size and mass growth of the simulated galaxies in broad agreement with recent observations.

Selected Research Papers

The formation of the first supermassive black holes in the Universe

Recent observations have shown that supermassive black holes with masses in excess of 1 billion Solar masses existed already in the very early Universe at redshifts of above z~7. One possible scenario for forming massive black holes in the early Universe is through the direct collapse of gas haloes in which cooling by molecular hydrogen and metals is not important resulting in a suppression of fragmentation. We have been studying this formation mechanism using adaptive mesh refinement simulations that include live radiative transfer. In a study published in Nature Astronomy, we explored the synchronized formation scenario, in which the near simultaneous collapse of two haloes triggers the formation of a direct collapse black hole in the secondary halo.

Selected Research Papers

The formation of galaxies through galaxy mergers

Numerical merger simulations in which two galaxies are set in isolation on a parabolic collision trajectory is a powerful tool in studying the formation and subsequent evolution of galaxies. Using this merger setup we have studied in particular the impact of supermassive black hole feedback on merging galaxies. In addition, we have studied the kinematic properties of the resulting merger remnants and compared their properties to observations, in order to assess if galaxy mergers are a viable method for forming early-type galaxies.

Selected Research Papers

The reproduction of observed galaxy mergers with simulations

In these research projects, which are carried out in close collaboration with observers, we use numerical merger simulations in an attempt to reproduce the properties of observed merging galaxies. The correct merger initial conditions are sought by matching the general geometry, kinematics and star formation histories of the merging galaxies. Using numerical simulations enables us to better understand how different galaxy mergers evolve as a function of their initial morphology and gas fraction. In addition, with the help of simulations we can better understand the past and future of observed galaxy mergers.