ALBERT

Description: In this first paper we discuss the linear theory and the background evolution of a new class of models we dub SCDEW: Strongly Coupled DE, plus WDM. In these models, WDM dominates today's matter density; like baryons, WDM is uncoupled. Dark energy is a scalar field ; its coupling to ancillary cold dark matter (CDM), whose today's density is 〈〈1 per cent, is an essential model feature. Such coupling, in fact, allows the formation of cosmic structures, in spite of very low WDM particle masses (~100 eV). SCDEW models yield cosmic microwave background and linear large scale features substantially undistinguishable from CDM, but thanks to the very low WDM masses they strongly alleviate CDM issues on small scales, as confirmed via numerical simulations in the second associated paper. Moreover SCDEW cosmologies significantly ease the coincidence and fine tuning problems of CDM and, by using a field theory approach, we also outline possible links with inflationary models. We also discuss a possible fading of the coupling at low redshifts which prevents non-linearities on the CDM component to cause computational problems. The (possible) low- z coupling suppression, its mechanism, and its consequences are however still open questions – not necessarily problems – for SCDEW models. The coupling intensity and the WDM particle mass, although being extra parameters in respect to CDM, are found to be substantially constrained a priori so that, if SCDEW is the underlying cosmology, we expect most data to fit also CDM predictions.

Description: We study the effect of mergers on the morphology of galaxies by means of the simulated merger tree approach first proposed by Moster et al. This method combines N -body cosmological simulations and semi-analytic techniques to extract realistic initial conditions for galaxy mergers. These are then evolved using high-resolution hydrodynamical simulations, which include dark matter, stars, cold gas in the disc and hot gas in the halo. We show that the satellite mass accretion is not as effective as previously thought, as there is substantial stellar stripping before the final merger. The fraction of stellar disc mass transferred to the bulge is quite low, even in the case of a major merger, mainly due to the dispersion of part of the stellar disc mass into the halo. We confirm the findings of Hopkins et al., that a gas-rich disc is able to survive major mergers more efficiently. The enhanced star formation associated with the merger is not localized to the bulge of galaxy, but a substantial fraction takes place in the disc too. The inclusion of the hot gas reservoir in the galaxy model contributes to reducing the efficiency of bulge formation. Overall, our findings suggest that mergers are not as efficient as previously thought in transforming discs into bulges. This possibly alleviates some of the tensions between observations of bulgeless galaxies and the hierarchical scenario for structure formation.

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 ~100 cosmological galaxy formation ‘zoom-in’ simulations using the smoothed particle hydrodynamics code gasoline to study the effect of baryonic processes on the mass profiles of cold dark matter haloes. The haloes in our study range from dwarf ( M 200  ~ 10 10 M ) to Milky Way ( M 200  ~ 10 12 M ) masses. Our simulations exhibit a wide range of halo responses, primarily varying with mass, from expansion to contraction, with up to factor ~10 changes in the enclosed dark matter mass at 1 per cent of the virial radius. Confirming previous studies, the halo response is correlated with the integrated efficiency of star formation: SF ( M star / M 200 )/( b / m ). In addition, we report a new correlation with the compactness of the stellar system: R r 1/2 / R 200 . We provide an analytic formula depending on SF and R for the response of cold dark matter haloes to baryonic processes. An observationally testable prediction is that, at fixed mass, larger galaxies experience more halo expansion, while the smaller galaxies more halo contraction. This diversity of dark halo response is captured by a toy model consisting of cycles of adiabatic inflow (causing contraction) and impulsive gas outflow (causing expansion). For net outflow, or equal inflow and outflow fractions, f , the overall effect is expansion, with more expansion with larger f . For net inflow, contraction occurs for small f (large radii), while expansion occurs for large f (small radii), recovering the phenomenology seen in our simulations. These regularities in the galaxy formation process provide a step towards a fully predictive model for the structure of cold dark matter haloes.

Description: We present the first numerical simulations in coupled dark energy cosmologies with high enough resolution to investigate the effects of the coupling on galactic and subgalactic scales. We choose two constant couplings and a time-varying coupling function and we run simulations of three Milky Way-sized haloes (~10 12  M ), a lower mass halo (6 x 10 11  M ) and a dwarf galaxy halo (5 x 10 9  M ). We resolve each halo with several million dark matter particles. On all scales, the coupling causes lower halo concentrations and a reduced number of substructures with respect to cold dark matter (CDM). We show that the reduced concentrations are not due to different formation times. We ascribe them to the extra terms that appear in the equations describing the gravitational dynamics. On the scale of the Milky Way satellites, we show that the lower concentrations can help in reconciling observed and simulated rotation curves, but the coupling values necessary to have a significant difference from CDM are outside the current observational constraints. On the other hand, if other modifications to the standard model allowing a higher coupling (e.g. massive neutrinos) are considered, coupled dark energy can become an interesting scenario to alleviate the small-scale issues of the CDM model.

Description: We use the relations between aperture stellar velocity dispersion ( ap ), stellar mass ( M SPS ) and galaxy size ( R e ) for a sample of ~150 000 early-type galaxies from Sloan Digital Sky Survey/DR7 to place constraints on the stellar initial mass function (IMF) and dark halo response to galaxy formation. We build cold dark matter-based mass models that reproduce, by construction, the relations between galaxy size, light concentration and stellar mass, and use the spherical Jeans equations to predict ap . Given our model assumptions (including those in the stellar population synthesis models), we find that reproducing the median ap versus M SPS relation is not possible with both a universal IMF and a universal dark halo response. Significant departures from a universal IMF and/or dark halo response are required, but there is a degeneracy between these two solutions. We show that this degeneracy can be broken using the strength of the correlation between residuals of the velocity–mass (log ap ) and size–mass (log R e ) relations. The slope of this correlation, VR log ap /log R e , varies systematically with galaxy mass from VR ~= –0.45 at M SPS  ~ 10 10 M to VR ~= –0.15 at M SPS  ~ 10 11.6 M . The virial Fundamental Plane (FP) has VR = –1/2, and thus we find that the tilt of the observed FP is mass dependent. Reproducing this tilt requires both a non-universal IMF and a non-universal halo response. Our best model has mass-follows-light at low masses ( M SPS   10 11.2 M ) and unmodified Navarro, Frenk and White haloes at M SPS  ~ 10 11.5 M . The stellar masses imply a mass-dependent IMF which is ‘lighter’ than Salpeter at low masses and ‘heavier’ than Salpeter at high masses.

Description: The distribution of galaxy morphological types is a key test for models of galaxy formation and evolution, providing strong constraints on the relative contribution of different physical processes responsible for the growth of the spheroidal components. In this paper, we make use of a suite of semi-analytic models to study the efficiency of galaxy mergers in disrupting galaxy discs and building galaxy bulges. In particular, we compare standard prescriptions usually adopted in semi-analytic models, with new prescriptions proposed by Kannan et al., based on results from high-resolution hydrodynamical simulations, and we show that these new implementations reduce the efficiency of bulge formation through mergers. In addition, we compare our model results with a variety of observational measurements of the fraction of spheroid-dominated galaxies as a function of stellar and halo mass, showing that the present uncertainties in the data represent an important limitation to our understanding of spheroid formation. Our results indicate that the main tension between theoretical models and observations does not stem from the survival of purely disc structures (i.e. bulgeless galaxies), rather from the distribution of galaxies of different morphological types, as a function of their stellar mass.

Description: In this second paper, we present the first N -body cosmological simulations of strongly coupled Dark Energy (SCDEW) models, a class of models that alleviates theoretical issues related to the nature of dark energy (DE). SCDEW models assume a strong coupling between DE and an ancillary cold dark matter (CDM) component together with the presence of an uncoupled warm dark matter (WDM) component. The strong coupling between CDM and DE allows us to preserve small-scale fluctuations even if the warm particle is quite light (100 eV). Our large-scale simulations show that, for 10 11  〈  M /M  〈 10 14 , SCDEW haloes exhibit a number density and distribution similar to a standard lambda cold dark matter (CDM) model, even though they have lower concentration parameters. High-resolution simulation of a galactic halo ( M  ~ 10 12 M ) shows ~60 per cent less substructures than its CDM counterpart, but the same cuspy density profile. On the scale of galactic satellites ( M  ~ 10 9 M ), SCDEW haloes dramatically differ from CDM. Due to the high thermal velocities of the WDM component they are almost devoid of any substructures and present strongly cored dark matter density profiles. These density cores extend for several hundreds of parsecs, in very good agreement with Milky Way satellites observations. Strongly coupled models, thanks to their ability to match observations on both large and small scales, might represent a valid alternative to a simple CDM model.

Description: A large fraction of the dwarf satellites orbiting the Andromeda galaxy are surprisingly aligned in a thin, extended and apparently kinematically coherent planar structure. Such a structure is not easily found in simulations based on the cold dark matter model (CDM). Using 21 high-resolution cosmological simulations, we analyse the kinematics of planes of satellites similar to the one around Andromeda. We find good agreement when co-rotation is characterized by the line-of-sight velocity. At the same time, when co-rotation is inferred by the angular momenta of the satellites, the planes are in agreement with the plane around our Galaxy. We find such planes to be common in our high-concentration haloes. The number of co-rotating satellites obtained from the sign of the line-of-sight velocity shows large variations depending on the viewing angle and is consistent with that obtained from a sample with random velocities. We find that the clustering of angular momentum vectors of the satellites in the plane is a better measure of the kinematic coherence. Thus we conclude that the line-of-sight velocity is not well suited as a proxy for the kinematical coherence of the plane. Analysis of the kinematics of our planes shows a fraction of ~30 per cent chance-aligned satellites. Tracking the satellites in the plane back in time reveals that these planes are a transient feature and not kinematically coherent as would appear at first sight. Thus we expect some of the satellites in the plane around Andromeda to have high velocities perpendicular to the plane.

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.