Research

The research of Theoretical Extragalactic group concentrates on trying to understand how galaxies have formed and evolved into the galaxy population that we observe in the local Universe. In order to tackle this problem we typically employ high resolution numerical simulations of structure formation using both smooth particle hydrodynamics codes (Gadget-3) and adaptive mesh refinement codes (Enzo). Below a brief overview of recent research projects is given followed by information on the Helsinki Astronomy seminar and Astrophysics Journal club.

3) The cosmological formation of Early-type galaxies

Description: 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 on this topic:

  1. 1) R.S Remus, K. Dolag, T. Naab, A. Burkert, M. Hirschmann, T.L. Hoffmann, P.H. Johansson, 2017, MNRAS, 464, 3742

  2. 2) T. Naab et al. (including P.H. Johansson), 2014, MNRAS, 444, 3357

  3. 3) P.H. Johansson, T. Naab,  J.P. Ostriker, 2012, ApJ, 754, 115

  4. 4) L. Oser,  T. Naab, J.P. Ostriker, P.H. Johansson, 2012, ApJ, 744, 63

  5. 5) L. Oser, J.P. Ostriker, T. Naab, P.H. Johansson, A. Burkert, 2010, ApJ, 725, 2312

  6. 6) P.H. Johansson, T. Naab, J.P. Ostriker, 2009, ApJL, 697, 38

  7. 7) T. Naab, P.H. Johansson, J.P. Ostriker, 2009, ApJL, 699, 178

  8. 8) T. Naab, P.H. Johansson, G. Efstathiou. J.P. Ostriker, 2007, ApJ, 658, 710

5) The formation of galaxies through galaxy mergers

Description: 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 on this topic:

  1. 1. E. Choi, T. Naab, J.P. Ostriker, P.H. Johansson, B.P. Moster, 2014, MNRAS, 442, 440

  2. 2. A.T.P. Schauer, R.-S. Remus, A. Burkert, P.H. Johansson, 2014, ApJL, 783, 32

  3. 3. R.-S. Remus, A. Burkert, K. Dolag, P.H. Johansson, T. Naab, L. Oser, J. Thomas, 2013, ApJ, 766, 71

  4. 4. E. Choi, J.P. Ostriker, T. Naab, P.H. Johansson, B.P. Moster, 2012, MNRAS, 442, 440

  5. 5. P.H. Johansson, T. Naab, A. Burkert, 2009, ApJ, 690, 802

  6. 6. P.H. Johansson, A. Burkert, T. Naab, 2009, ApJL, 707, 184

  7. 7. P.H. Johansson, T. Naab, A. Burkert, 2008, AN, 329, 956

6) The reproduction of observed galaxy mergers with simulations

Description: 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 merger 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.

Selected research papers on this topic:

  1. 1. N. Lahén, P.H. Johansson, A. Rantala, T. Naab, M. Frigo, 2018,
    MNRAS, 475,3934

  2. 2. P. Väisänen, J. Reunanen, J. Kotilainen, S. Mattila, P.H. Johansson et al., 2017, MNRAS, 471, 2059

  3. 3.M.M. Pawlik, V. Vild, C.J. Walcher, P.H. Johansson, C. Villforth, K. Rowlands, J. Mendez-Abreu, T. Hewlett, 2016, MNRAS, 456, 3032

  4. 4. V. Wild, et al. (including P.H. Johansson), 2014, A&A, 567, 132

  5. 5. S.J. Karl, T. Lunttila, T. Naab, P.H. Johansson, U. Klaas, M. Juvela, 2013, MNRAS, 434, 696

  6. 6. H. Engel, et al. (including P.H. Johansson), 2010, A&A, 524, 56

  7. 7. P. Väisänen, et al. (including P.H. Johansson), 2008, MNRAS, 384, 886

4) The formation of the first supermassive black holes in the Universe

Description: 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. Recently we have been studying this formation mechanism using adaptive mesh refinement simulations that include live radiative transfer. In the most recent simulations, we have explored the synchronized formation scenario, in which the near simultaneous collapse of two haloes triggers the formation of a direct collapse blackhole in the secondary halo.

Selected research papers on this topic:

  1. 1. J.A. Regan, E. Visbal, J.H. Wise, Z. Haiman, P.H. Johansson, G.L. Bryan, 2017, Nature Astronomy, 1, 0075

  2. 2. J.A. Regan, P.H. Johansson, J.H. Wise, 2016, MNRAS, 461, 111

  3. 3. J.A. Regan, P.H. Johansson, J.H. Wise, 2016, MNRAS, 459, 3377

  4. 4. J.A. Regan, P.H. Johansson, J.H. Wise, 2015, MNRAS, 449, 3766

  5. 5. J.A. Regan, P.H. Johansson, J.H. Wise, 2014, ApJ, 795, 137

  6. 6. J.A. Regan, P.H. Johansson, M.G., Haehnelt, 2014, MNRAS, 439, 1160

Helsinki Astrophysics seminar

Description: The Helsinki Astrophysics seminar is the prime source for disseminating new astronomy knowledge at the University of Helsinki. The seminar program is very broad containing all the subfields of astronomy with currently active Helsinki astronomers, including planetary astronomy, stellar astronomy, interstellar medium studies, extragalactic astronomy and cosmology. The seminar is arranged regularly on Fridays at 10.15am in Room A315 during term time.

The current seminar program can be found here

Helsinki Astrophysics Journal club

Description: In the Helsinki Astrophysics Journal club we get together on a weekly basis and discuss recent interesting papers that have appeared on Astro-ph or in Journals. The meetings are open to everyone and the aim is to stimulate discussion about recent results and provide a possibility to learn about research that is not necessarily connected to one’s own field of expertise. The journal club is arranged regularly on Thursdays at 10.15am in Room C310 during term time.
The current journal club program can be found here

1) Dynamical modelling of supermassive black hole binaries

Description: We have developed a new simulation code KETJU (‘chain’ in Finnish), which is built on the GADGET-3 code and includes regions around every supermassive black hole (SMBH) in which the dynamics of SMBHs and stellar particles is modelled using a non-softened Post-Newtonian algorithmic chain regularisation technique. Using KETJU we can study at unprecedented accuracy the dynamics of SMBHs at sub-parsec separations down to ~10 Schwarz-schild radii, the formation of cores in massive galaxies, the formation of nuclear stellar clusters and finally provide a realistic prediction for the amplitude and frequency distribution of the cosmological gravitational wave background.

2) The formation of the Local Group of galaxies

Description: We study the Local Group (LG) of galaxies using numerical simulations, as it provides us with a unique window into the Universe. In the Local Group we can observe the faintest galaxies, which enable us to study the nature of dark matter. In particular, the abundance, anisotropy and inner structure of LG galaxies have all been considered challenges to the prevailing Lambda cold dark matter paradigm. While recent advanced in cosmological hydrodynamical simulations indicate that baryonic effects may resolve some of these issues, other questions remain, such as the peculiar orbits and alignments of dwarf galaxies within the Local Group.