Simulations of Dwarf Galaxy Formation

Dwarf galaxies are related to important cosmological questions, and central to our understanding of the physics of galaxy formation. In this thesis, I present the results of cosmological, hydrodynamical simulations of the formation and evolution of dwarf galaxies. I compare the simulation results with observations, and interpret them in the context of a Lambda-CDM cosmology. In high resolution simulations of isolated dwarf galaxies, I show that a combination of supernova feedback and the cosmic UV background results in the formation of galaxies with properties similar to the Local Group dwarf spheroidals, and that both effects are strongly moderated by the depth of the gravitational potential. The simulations naturally reproduce the observed scaling relations between luminosity and mass-to-light ratio, and between total stellar mass and metallicities. The final objects have halo masses between 2.3 x 10^8 and 1.1 x 10^9 solar masses, mean velocity dispersions between 6.5 and 9.7 kms^-1, stellar masses ranging from 5 x 10^5 to 1.2 x 10^7 solar masses, median metallicities between [Fe/H]=-1.8 and -1.1, and half-light radii of the order of 200 to 300 pc, all comparable with Local Group dwarf spheroidals. The simulations also indicate that the dwarf spheroidal galaxies observed today lie near a mass threshold around 10^9 solar masses, in agreement with stellar kinematic data, where supernova feedback not only suffices to completely expel the interstellar medium and leave the residual gas-free, but where the combination of feedback, UV radiation and self-shielding establishes a dichotomy of age distributions similar to that observed in the Milky Way and M31 satellites. A second line of work has been the analysis of the dwarf galaxy population resulting from the Aquila simulation. By simultaneously including the formation of a Milky Way type galaxy along with ~500 dwarf-sized haloes in the mass range of ~10^8 - 10^10 solar masses, this simulation allows a study of the effect of the environment on dwarf galaxy evolution. I study the relative importance, and interplay, of the different mechanisms for gas loss, and compare the properties of the satellites with those of isolated dwarf galaxies. A third set of simulations focuses on the formation of dwarf galaxies in a representative sample of haloes extracted from the Millennium-II simulation. The six haloes in these simulations all have a z=0 mass of ~10^10 solar masses and show different mass assembly histories, which are reflected in different star formation histories. The galaxies reach final stellar masses in the range of 5 x 10^7 - 10^8 solar masses, consistent with other published simulations of galaxy formation in similar mass haloes. The resulting objects have structures and stellar populations consistent with dwarf elliptical and dwarf irregular galaxies. However, in a Lambda-CDM universe, 10^10 solar mass haloes must typically contain galaxies with much lower stellar mass than these simulations predict, if they are to match observed galaxy abundances. The dwarf galaxies formed in my own and all other current hydrodynamical simulations are more than an order of magnitude more luminous than expected for haloes of this mass. I discuss the significance and possible implications of this result for cosmological models, and for the assumptions about the physics of galaxy formation. Finally, I present preliminary results of a direct comparison between hydrodynamical simulations and semi-analytical models for the formation of dwarf galaxies. Current semi-analytical models, which are tuned to match the statistical properties of galaxies, do not agree with the predictions of hydrodynamical simulations for individual objects. Conversely, when tuned to accurately reproduce the simulations, semi-analytical models can give a more qualitative interpretation of the simulation results, in terms of equations of galaxy formation. The combination of the two methods allows an extrapolation from individual cases to cosmological volumes, not currently attainable with direct simulations alone

Structure formation with two periods of inflation : beyond PLaln Lambda CDM

We discuss structure formation in models with a spectator field in small-field inflation which accommodate a secondary period of inflation. In such models, subgalactic scale primordial fluctuations can be much suppressed in comparison to the usual power-law Lambda CDM model while the large scale fluctuations remain consistent with current observations. We discuss how a secondary inflationary epoch may give rise to observable features in the small scale power spectrum and hence be tested by the structures in the Local Universe.Peer reviewe

The Local Group's mass: probably no more than the sum of its parts

The total mass of the Local Group (LG) and the masses of its primary constituents, the Milky Way and M31, are important anchors for several cosmological questions. In recent years, independent measurements have consistently yielded halo masses close to $10^{12} \mathrm{M_\odot}$ for the MW, and $1-2 \times 10^{12} \mathrm{M_\odot}$ for M31, while estimates derived from the pair's kinematics via the `timing argument' have yielded a combined mass of around $5 \times 10^{12} \mathrm{M_\odot}$. Here, we analyse the extremely large Uchuu simulation to constrain the mass of the Local Group and its two most massive members. First, we demonstrate the importance of selecting LG analogues whose kinematics are dominated by mutual interactions to a similar extent as the LG. Adopting the observed separation and radial velocity, we obtain a weighted posterior of $75_{-40}^{+65}$ kms$^{-1}$ for the uncertain transverse velocity. Via Gaussian process regression, we infer a total mass of $3.2^{+1.2}_{-0.9} \times 10^{12} \mathrm{M_\odot}$, significantly below the timing argument prediction. Importantly, we show that the remaining uncertainty is not rooted in the analysis or observational errors, but in the irreducible scatter in the kinematics-mass relation. We further find a mass for the less massive halo of $0.9_{-0.3}^{+0.6} \times 10^{12} \mathrm{M_\odot}$ and for the more massive halo of $2.3_{-0.9}^{+1.0} \times 10^{12} \mathrm{M_\odot}$, consistent with independent measurements of the masses of MW and M31, respectively. Incorporating the mass of the MW as an additional prior allows us to further constrain all measurements and determine that the MW is very likely to be the lower mass object of the two.Comment: 15 pages, submitted to MNRAS. Full source code provided, comments are welcom

The Timeless Timing Argument and the Mass of the Local Group

The Timing Argument connects the motion of a two-body system to its mass in an expanding Universe with a finite age, under the assumption that it has evolved on a self-gravitating orbit. It is commonly applied to the present-day Milky Way-M31 system in order to infer its unknown mass from the measured kinematics. We use a set of Local Group analogues from the Uchuu simulation to investigate the Timing Argument over cosmic time. We find that the median inferred mass remains almost constant over the past 12 Gyr, even while the haloes themselves grew in mass by more than an order of magnitude. By contrast, we find a closer, and nearly time-invariant agreement between the Timing Argument value and the mass within a sphere of radius equal to the MW-M31 separation, and we identify this as the total mass of the system. We conclude that the comparatively close present-day agreement between the Timing Argument and the sum of the halo masses reflects no underlying relation, but merely echoes the fact that the MW and M31 now contain most (but not all) of the mass of the Local Group system.Comment: 6 pages, 4 figures, this version accepted to MNRAS Letter

The timeless timing argument and the total mass of the Local Group

The timing argument connects the motion of a two-body system to its mass in an expanding Universe with a finite age, under the assumption that it has evolved on a self-gravitating orbit. It is commonly applied to the present-day Milky Way (MW)–M31 system in order to infer its unknown mass from the measured kinematics. We use a set of Local Group analogues from the UCHUU simulation to investigate the timing argument over cosmic time. We find that the median inferred mass remains almost constant over the past 12 Gyr, even while the haloes themselves grew in mass by more than an order of magnitude. By contrast, we find a closer, and nearly time-invariant agreement between the timing argument value and the mass within a sphere of radius equal to the MW–M31 separation, and we identify this as the total mass of the system. We conclude that the comparatively close present-day agreement between the timing argument and the sum of the halo masses reflects no underlying relation, but merely echoes the fact that the MW and M31 now contain most (but not all) of the mass of the Local Group system

Formation of Isolated Dwarf Galaxies with Feedback

We present results of high resolution hydrodynamical simulations of the formation and evolution of dwarf galaxies. Our simulations start from cosmological initial conditions at high redshift. They include metal-dependent cooling, star formation, feedback from type II and type Ia supernovae and UV background radiation, with physical recipes identical to those applied in a previous study of Milky Way type galaxies. We find that a combination of feedback and the cosmic UV background results in the formation of galaxies with properties similar to the Local Group dwarf spheroidals, and that their effect is strongly moderated by the depth of the gravitational potential. Taking this into account, our models naturally reproduce the observed luminosities and metallicities. The final objects have halo masses between 2.3x10^8 and 1.1x10^9 solar masses, mean velocity dispersions between 6.5 and 9.7 kms-1, stellar masses ranging from 5x10^5 to 1.2x10^7 solar masses, median metallicities between [Fe/H] = -1.8 and -1.1, and half-light radii of the order of 200 to 300 pc, all comparable with Local Group dwarf spheroidals. Our simulations also indicate that the dwarf spheroidal galaxies observed today lie near a halo mass threshold around 10^9 solar masses, in agreement with stellar kinematic data, where supernova feedback not only suffices to completely expel the interstellar medium and leave the residual gas-free, but where the combination of feedback, UV radiation and self-shielding establishes a dichotomy of age distributions similar to that observed in the Milky Way and M31 satellites.Comment: 17 pages, 14 figures. MNRAS accepte

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 low abundance and insignificance of dark discs in simulated Milky Way galaxies

We investigate the presence and importance of dark matter discs in a sample of 24 simulated Milky Way galaxies in the apostle project, part of the eagle programme of hydrodynamic simulations in ΛCDM cosmology. It has been suggested that a dark disc in the Milky Way may boost the dark matter density and modify the velocity modulus relative to a smooth halo at the position of the Sun, with ramifications for direct detection experiments. From a kinematic decomposition of the dark matter and a real space analysis of all 24 haloes, we find that only one of the simulated Milky Way analogues has a detectable dark disc component. This unique event was caused by a merger at late time with an LMC-mass satellite at very low grazing angle. Considering that even this rare scenario only enhances the dark matter density at the solar radius by 35 per cent and affects the high-energy tail of the dark matter velocity distribution by less than 1 per cent, we conclude that the presence of a dark disc in the Milky Way is unlikely, and is very unlikely to have a significant effect on direct detection experiments