ALBERT

Description: We use cosmological hydrodynamical zoom-in simulations with the smoothed particle hydrodynamics code gasoline of four haloes of mass M 200  ~ 10 13 M to study the response of the dark matter to elliptical galaxy formation. Our simulations include metallicity-dependent gas cooling, star formation and feedback from massive stars and supernovae, but not active galactic nuclei (AGN). At z  = 2 the progenitor galaxies have stellar-to-halo mass ratios consistent with halo abundance matching, assuming a Salpeter initial mass function. However, by z  = 0 the standard runs suffer from the well-known overcooling problem, overpredicting the stellar masses by a factor of  4. To mimic a suppressive halo quenching scenario, in our forced quenching (FQ) simulations, cooling and star formation are switched off at z  = 2. The resulting z  = 0 galaxies have stellar masses, sizes and circular velocities close to what is observed. Relative to the control simulations, the dark matter haloes in the FQ simulations have contracted, with central dark matter density slopes d log /d log r  ~ –1.5, showing that dry merging alone is unable to fully reverse the contraction that occurs at z  〉 2. Simulations in the literature with AGN feedback, however, have found expansion or no net change in the dark matter halo. Thus, the response of the dark matter halo to galaxy formation may provide a new test to distinguish between ejective and suppressive quenching mechanisms.

Description: We use the NIHAO (Numerical Investigation of Hundred Astrophysical Objects) cosmological simulations to study the effects of galaxy formation on key properties of dark matter (DM) haloes. NIHAO consists of 90 high-resolution smoothed particle hydrodynamics simulations that include (metal-line) cooling, star formation, and feedback from massive stars and supernovae, and cover a wide stellar and halo mass range: 10 6 M * /M 10 11 (10 9.5 M halo /M 10 12.5 ). When compared to DM-only simulations, the NIHAO haloes have similar shapes at the virial radius, R vir , but are substantially rounder inside 0.1 R vir . In NIHAO simulations, c / a increases with halo mass and integrated star formation efficiency, reaching ~0.8 at the Milky Way mass (compared to 0.5 in DM-only), providing a plausible solution to the long-standing conflict between observations and DM-only simulations. The radial profile of the phase-space Q parameter (/ 3 ) is best fit with a single power law in DM-only simulations, but shows a flattening within 0.1 R vir for NIHAO for total masses M 〉 10 11 M . Finally, the global velocity distribution of DM is similar in both DM-only and NIHAO simulations, but in the solar neighbourhood, NIHAO galaxies deviate substantially from Maxwellian. The distribution is more symmetric, roughly Gaussian, with a peak that shifts to higher velocities for Milky Way mass haloes. We provide the distribution parameters which can be used for predictions for direct DM detection experiments. Our results underline the ability of the galaxy formation processes to modify the properties of DM haloes.

Description: The radial density profiles of stellar galaxy discs can be well approximated as an exponential. Compared to this canonical form, however, the profiles in the majority of disc galaxies show downward or upward breaks at large radii. Currently, there is no coherent explanation in a galaxy formation context of the radial profile per se, along with the two types of profile breaks. Using a set of controlled hydrodynamic simulations of disc galaxy formation, we find a correlation between the host halo's initial angular momentum and the resulting radial profile of the stellar disc: galaxies that live in haloes with a low spin parameter 0.03 show an up-bending break in their disc density profiles, while galaxies in haloes of higher angular momentum show a down-bending break. We find that the case of pure exponential profiles (  0.035) coincides with the peak of the spin parameter distribution from cosmological simulations. Our simulations not only imply an explanation of the observed behaviours, but also suggest that the physical origin of this effect is related to the amount of radial redistribution of stellar mass, which is anticorrelated with .

Description: We introduce project Nihao (Numerical Investigation of a Hundred Astrophysical Objects), a set of 100 cosmological zoom-in hydrodynamical simulations performed using the gasoline code, with an improved implementation of the SPH algorithm. The haloes in our study range from dwarf ( M 200 ~ 5 x 10 9 M ) to Milky Way ( M 200 ~ 2 x 10 12 M ) masses, and represent an unbiased sampling of merger histories, concentrations and spin parameters. The particle masses and force softenings are chosen to resolve the mass profile to below 1 per cent of the virial radius at all masses, ensuring that galaxy half-light radii are well resolved. Using the same treatment of star formation and stellar feedback for every object, the simulated galaxies reproduce the observed inefficiency of galaxy formation across cosmic time as expressed through the stellar mass versus halo mass relation, and the star formation rate versus stellar mass relation. We thus conclude that stellar feedback is the chief piece of physics required to limit the efficiency of star formation in galaxies less massive than the Milky Way.

Description: We show that the cool gas masses of galactic discs reach a steady state that lasts many Gyr after their last major merger in cosmological hydrodynamic simulations. The mass of disc gas, M gas , depends mostly upon a galaxy virial mass and halo's spin, and less upon stellar feedback. Haloes with low spin have high star formation efficiency and lower disc gas mass. Similarly, lower stellar feedback leads to more star formation so the gas mass ends up being nearly the same regardless of stellar feedback strength. Rather than regulating cool gas mass, stellar feedback regulates the mass of stars that forms. Even considering spin, the M gas relation with halo mass, M 200 only shows a factor of 3 scatter. The simulated M gas – M 200 relation shows a break at M 200 = 2 x 10 11 M that corresponds to an observed break in the M gas – M * relation. The galaxies that maintain constant disc masses share a common halo gas density profile shape in all the simulated galaxies. In their outer regions, the profiles are isothermal. Where the profile rises above n = 10 –3  cm –3 , the gas readily cools and the profile steepens. Inside the disc, rotation supports gas with a flatter density profile. Energy injection from stellar feedback provides pressure support to the halo gas to prevent runaway cooling flows. The constant gas mass makes simpler models for galaxy formation possible, either using a ‘bathtub’ model for star formation rates or when modelling chemical evolution.

Description: We compare the half-light circular velocities, V 1/2 , of dwarf galaxies in the Local Group to the predicted circular velocity curves of galaxies in the Numerical Investigations of Hundred Astrophysical Objects (NIHAO) suite of cold dark matter (CDM) simulations. We use a subset of 34 simulations in which the central galaxy has a stellar luminosity in the range 0.5  x  10 5 〈 L V / L 〈 2  x  10 8 . The NIHAO galaxy simulations reproduce the relation between stellar mass and halo mass from abundance matching, as well as the observed half-light size versus luminosity relation. The corresponding dissipationless simulations overpredict the V 1/2 , recovering the problem known as too big to fail (TBTF). By contrast, the NIHAO simulations have expanded dark matter haloes, and provide an excellent match to the distribution of V 1/2 for galaxies with L V 2  x  10 6 L . For lower luminosities, our simulations predict very little halo response, and tend to overpredict the observed circular velocities. In the context of CDM, this could signal the increased stochasticity of star formation in haloes below M halo ~ 10 10 M , or the role of environmental effects. Thus, haloes that are ‘TBTF’, do not fail CDM, but haloes that are ‘too small to pass’ (the galaxy formation threshold) provide a future test of CDM.

Description: We study the effect of warm dark matter (WDM) on hydrodynamic simulations of galaxy formation as part of the Making Galaxies in a Cosmological Context (MaGICC) project. We simulate three different galaxies using three WDM candidates of 1, 2 and 5 keV and compare results with pure cold dark matter simulations. WDM slightly reduces star formation and produces less centrally concentrated stellar profiles. These effects are most evident for the 1 keV candidate but almost disappear for $m_ {\rm \small {wdm}} 〉 2$  keV. All simulations form similar stellar discs independent of WDM particle mass. In particular, the disc scalelength does not change when WDM is considered. The reduced amount of star formation in the case of 1 keV particles is due to the effects of WDM on merging satellites which are on average less concentrated and less gas rich. The altered satellites cause a reduced starburst during mergers because they trigger weaker disc instabilities in the main galaxy. Nevertheless we show that disc galaxy evolution is much more sensitive to stellar feedback than it is to WDM candidate mass. Overall, we find that WDM, especially when restricted to current observational constraints ( $m_ {\rm \small {wdm}} 〉 2$  keV), has a minor impact on disc galaxy formation.

Description: We use a suite of 31 simulated galaxies drawn from the MaGICC project to investigate the effects of baryonic feedback on the density profiles of dark matter haloes. The sample covers a wide mass range: 9.4 10 9  〈  M halo / M  〈 7.8 10 11 , hosting galaxies with stellar masses in the range 5.0 10 5  〈  M * / M  〈 8.3 10 10 , i.e. from dwarf to L*. The galaxies are simulated with blastwave supernova feedback and, for some of them, an additional source of energy from massive stars is included. Within this feedback scheme we vary several parameters, such as the initial mass function, the density threshold for star formation, and energy from supernovae and massive stars. The main result is a clear dependence of the inner slope of the dark matter density profile, α in    r α , on the stellar-to-halo mass ratio, M * / M halo . This relation is independent of the particular choice of parameters within our stellar feedback scheme, allowing a prediction for cusp versus core formation. When M * / M halo is low, 0.01 per cent, energy from stellar feedback is insufficient to significantly alter the inner dark matter density, and the galaxy retains a cuspy profile. At higher stellar-to-halo mass ratios, feedback drives the expansion of the dark matter and generates cored profiles. The flattest profiles form where M * / M halo  ~ 0.5 per cent. Above this ratio, stars formed in the central regions deepen the gravitational potential enough to oppose the supernova-driven expansion process, resulting in cuspier profiles. Combining the dependence of α on M * / M halo with the empirical abundance matching relation between M * and M halo provides a prediction for how α varies as a function of stellar mass. Further, using the Tully–Fisher relation allows a prediction for the dependence of the dark matter inner slope on the observed rotation velocity of galaxies. The most cored galaxies are expected to have V rot  ~ 50 km s –1 , with α decreasing for more massive disc galaxies: spirals with V rot  ~ 150 km s –1 have central slopes α ≤ –0.8, approaching again the Navarro–Frenk–White profile. This novel prediction for the dependence of α on disc galaxy mass can be tested using observational data sets and can be applied to theoretical modelling of mass profiles and populations of disc galaxies.

Description: We present the Dark MaGICC (Making Galaxies in a Cosmological Context) project, which aims to investigate the effect of dark energy (DE) modelling on disc galaxy formation via hydrodynamical cosmological simulations. Dark MaGICC includes four dynamical DE scenarios with time varying equations of state, one with a self-interacting Ratra–Peebles model. In each scenario, we simulate three disc galaxies with high resolution using smoothed particle hydrodynamics. The baryonic physics model is the same used in the MaGICC project, and we varied only the background cosmology. We find that the DE parametrization has a surprisingly important impact on galaxy evolution and on structural properties of galaxies at z  = 0, in striking contrast with predictions from pure N -body simulations. The different background evolutions can (depending on the behaviour of the DE equation of state) either enhance or quench star formation with respect to a cold dark matter model, at a level similar to the variation of the stellar feedback parametrization, with strong effects on the final galaxy rotation curves. While overall stellar feedback is still the driving force in shaping galaxies, we show that the effect of the DE parametrization plays a larger role than previously thought, especially at lower redshifts. For this reason, the influence of DE parametrization on galaxy formation must be taken into account, especially in the era of precision cosmology.

Description: We present an analysis of the role of feedback in shaping the neutral hydrogen (H  i ) content of simulated disc galaxies. For our analysis, we have used two realizations of two separate Milky Way-like (~ L *) discs – one employing a conservative feedback scheme (McMaster Unbiased Galaxy Survey), the other significantly more energetic [Making Galaxies In a Cosmological Context (MaGICC)]. To quantify the impact of these schemes, we generate zeroth moment (surface density) maps of the inferred H  i distribution; construct power spectra associated with the underlying structure of the simulated cold interstellar medium, in addition to their radial surface density and velocity dispersion profiles. Our results are compared with a parallel, self-consistent, analysis of empirical data from The H  i Nearby Galaxy Survey (THINGS). Single power-law fits ( P    k ) to the power spectra of the stronger feedback (MaGICC) runs (over spatial scales corresponding to ~0.5 to ~20 kpc) result in slopes consistent with those seen in the THINGS sample ( ~ –2.5). The weaker feedback (MUGS) runs exhibit shallower power-law slopes ( ~ –1.2). The power spectra of the MaGICC simulations are more consistent though with a two-component fit, with a flatter distribution of power on larger scales (i.e.  ~ –1.4 for scales in excess of ~2 kpc) and a steeper slope on scales below ~1 kpc ( ~ –5), qualitatively consistent with empirical claims, as well as our earlier work on dwarf discs. The radial H  i surface density profiles of the MaGICC discs show a clear exponential behaviour, while those of the MUGS suite are essentially flat; both behaviours are encountered in nature, although the THINGS sample is more consistent with our stronger (MaGICC) feedback runs.