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Register a new userGalaxies in a Simulated $Lambda$CDM Universe II: Observable Properties and Constraints on Feedback
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Duv{s}an Kerev{s}
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We compare the properties of galaxies that form in a cosmological simulation without strong feedback to observations at z=0. We confirm previous findings that models without strong feedback overproduce the observed galaxy baryonic mass function, especially at the low and high mass extremes. Through post-processing we investigate what kinds of feedback would be required to reproduce observed galaxy masses and star formation rates. To mimic an extreme form of preventive feedback (e.g., AGN radio mode) we remove all baryonic mass that was originally accreted via hot mode from shock-heated gas. This does not bring the high mass end of the galaxy mass function into agreement with observations because much of the stellar mass in these systems formed at high redshift from baryons that originally accreted via cold mode onto lower mass progenitors. An efficient ejective feedback mechanism, such as supernova driven winds, must reduce the masses of these progenitors. Feedback must also reduce the masses of lower mass z=0 galaxies, which assemble at lower redshifts and have much lower star formation rates. If we monotonically re-map galaxy masses to reproduce the observed mass function, but retain the simulations predicted star formation rates, we obtain fairly good agreement with the observed sequence of star-forming galaxies but fail to recover the observed population of passive, low star formation rate galaxies. Suppressing all hot mode accretion improves agreement for high mass galaxies but worsens the agreement at intermediate masses. Reproducing these z=0 observations requires a feedback mechanism that dramatically suppresses star formation in a fraction of galaxies, increasing with mass, while leaving star formation rates of other galaxies essentially unchanged.
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We study the formation of galaxies in a (50 Mpc/h)^3 cosmological simulation (2x288^3 particles), evolved using the entropy conserving SPH code Gadget-2. Most of the baryonic mass in galaxies of all masses is originally acquired through filamentary cold mode accretion of gas that was never shock heated to its halo virial temperature, confirming the key feature of our earlier results obtained with a different SPH code (Keres et al. 2005). Atmospheres of hot, virialized gas develop in halos above ~2.5e11 Msun, a transition mass that is nearly constant from z=3 to z=0. Cold accretion persists in halos above the transition mass, especially at z>=2. It dominates the growth of galaxies in low mass halos at all times, and it is the main driver of the cosmic star formation history. Satellite galaxies have accretion rates similar to central galaxies of the same baryonic mass at high redshifts, but they have less accretion than comparable central galaxies at low redshift. Relative to our earlier results, the Gadget-2 simulations predict much lower rates of hot mode accretion from the virialized gas component of massive halos. At z<=1, typical hot accretion rates in halos above 5e12 Msun are below 1 Msun/yr, even though our simulation does not include AGN heating or other forms of preventive feedback. The inner density profiles of hot gas in these halos are shallow, with long associated cooling times. The cooling recipes typically used in semi-analytic models can overestimate the accretion rates in these halos by orders of magnitude, so such models may overemphasize the role of preventive feedback in producing observed galaxy masses and colors. A fraction of the massive halos develop cuspy profiles and significant cooling rates between z=1 and z=0, a redshift trend similar to the observed trend in the frequency of cooling flow clusters.
Understanding the formation and evolution of early-type, spheroid-dominated galaxies is an open question within the context of the hierarchical clustering scenario, particularly, in low-density environments. Our goal is to study the main structural, dynamical, and stellar population properties and assembly histories of field spheroid-dominated galaxies formed in a LCDM scenario to assess to what extend they are consistent with observations. We selected spheroid-dominated systems from a LCDM simulation that includes star formation, chemical evolution and Supernova feedback. A sample of 18 field systems with Mstar <= 6x10^10 Msun that are dominated by the spheroid component. For this sample we estimate the fundamental relations of ellipticals and then compared with current observations. The simulated spheroid galaxies have sizes in good agreement with observations. The bulges follow a Sersic law with Sersic indexes that correlate with the bulge-to-total mass ratios. The structural-dynamical properties of the simulated galaxies are consistent with observed Faber-Jackson, Fundamental Plane, and Tully-Fisher relations. However, the simulated galaxies are bluer and with higher star formation rates than observed isolated early-type galaxies. The archaeological mass growth histories show a slightly delayed formation and more prominent inside-out growth mode than observational inferences based on the fossil record method. The main structural and dynamical properties of the simulated spheroid-dominated galaxies are consistent with observations. This is remarkable since none of them has been tuned to be reproduced. However, the simulated galaxies are blue and star-forming, and with later stellar mass growth histories as compared to observational inferences. This is mainly due to the persistence of extended discs in the simulations. Abridged
Associations of dwarf galaxies are loose systems composed exclusively of dwarf galaxies. These systems were identified in the Local Volume for the first time more than thirty years ago. We study these systems in the cosmological framework of the $Lambda$ Cold Dark Matter ($Lambda$CDM) model. We consider the Small MultiDark Planck simulation and populate its dark matter haloes by applying the semi-analytic model of galaxy formation SAG. We identify galaxy systems using a friends of friends algorithm with a linking length equal to $b=0.4 ,{rm Mpc},h^{-1}$, to reproduce the size of dwarf galaxy associations detected in the Local Volume. Our samples of dwarf systems are built up removing those systems that have one (or more) galaxies with stellar mass larger than a maximum threshold $M_{rm max}$. We analyse three different samples defined by ${rm log}_{10}(M_{rm max}[{rm M}_{odot},h^{-1}]) = 8.5, 9.0$ and $9.5$. On average, our systems have typical sizes of $sim 0.2,{rm Mpc},h^{-1}$, velocity dispersion of $sim 30 {rm km,s^{-1}} $ and estimated total mass of $sim 10^{11} {rm M}_{odot},h^{-1}$. Such large typical sizes suggest that individual members of a given dwarf association reside in different dark matter haloes and are generally not substructures of any other halo. Indeed, in more than 90 per cent of our dwarf systems their individual members inhabit different dark matter haloes, while only in the remaining 10 per cent members do reside in the same halo. Our results indicate that the $Lambda$CDM model can naturally reproduce the existence and properties of dwarf galaxies associations without much difficulty.
We use the Evolution and Assembly of GaLaxies and their Environments ( EAGLE ) suite of hydrodynamical cosmological simulations to measure offsets between the centres of stellar and dark matter components of galaxies. We find that the vast majority (>95%) of the simulated galaxies display an offset smaller than the gravitational softening length of the simulations (Plummer-equivalent $epsilon = 700$ pc), both for field galaxies and satellites in clusters and groups. We also find no systematic trailing or leading of the dark matter along a galaxys direction of motion. The offsets are consistent with being randomly drawn from a Maxwellian distribution with $sigma leq 196$ pc. Since astrophysical effects produce no feasible analogues for the $1.62^{+0.47}_{-0.49}$ kpc offset recently observed in Abell 3827, the observational result is in tension with the collisionless cold dark matter model assumed in our simulations.
Baryon acoustic oscillations (BAO) provide a robust standard ruler, and can be used to constrain the expansion history of the Universe at low redshift. Standard BAO analyses return a model-independent measurement of the expansion rate and the comoving angular diameter distance as function of redshift, normalized by the sound horizon at radiation drag. However, this methodology relies on anisotropic distance distortions of a fixed, pre-computed template (obtained in a given fiducial cosmology) in order to fit the observations. Therefore, it may be possible that extensions to the consensus $Lambda$CDM add contributions to the BAO feature that cannot be captured by the template fitting. We perform mock BAO fits to power spectra computed assuming cosmological models which modify the growth of perturbations prior to recombination in order to test the robustness of the standard BAO analysis. We find no significant bias in the BAO analysis for the models under study ($Lambda$CDM with a free effective number of relativistic species, early dark energy, and a model with interactions between neutrinos and a fraction of the dark matter), even for cases which do not provide a good fit to textit{Planck} measurements of the cosmic microwave background power spectra. This result supports the use of the standard BAO analysis and its measurements to perform cosmological parameter inference and to constrain exotic models. In addition, we provide a methodology to reproduce our study for different models and surveys, as well as discuss different options to handle eventual biases in the BAO measurements.
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