ABSTRACT
Simulations of active galactic nuclei (AGN) jets have thus far been performed almost exclusively using grid-based codes. We present the first results from hydrodynamical tests of AGN jets, ...and their interaction with the intracluster medium (ICM), using smoothed particle hydrodynamics as implemented in the swift code. We launch these jets into a constant-density ICM, as well as ones with a power-law density profile. We also vary the jet power, velocity, opening angle, and numerical resolution. In all cases we find broad agreement between our jets and theoretical predictions for the lengths of the jets and the lobes they inflate, as well as the radii of the lobes. The jets first evolve ballistically, and then transition to a self-similar phase, during which the lobes expand in a self-similar fashion (keeping a constant shape). In this phase the kinetic and thermal energies in the lobes and in the shocked ICM are constant fractions of the total injected energy. In our standard simulation, two thirds of the initially injected energy is transferred to the ICM by the time the jets are turned off, mainly through a bow shock. Of that, $70{{\%}}$ is in kinetic form, indicating that the bow shock does not fully and efficiently thermalize while the jet is active. At resolutions typical of large cosmological simulations (mgas ≈ 107 M⊙), the shape of the lobes is close to self-similar predictions to an accuracy of $15{{\%}}$. This indicates that the basic physics of jet-inflated lobes can be correctly simulated even at such resolutions (≈500 particles per jet).
ABSTRACT
We use SWIFT, a smoothed particle hydrodynamics code, to simulate the evolution of bubbles inflated by active galactic nuclei (AGNs) jets, as well as their interactions with the ambient ...intracluster medium (ICM). These jets inflate lobes that turn into bubbles after the jets are turned off (at t = 50 Myr). Almost all of the energy injected into the jets is transferred to the ICM very quickly after they are turned off, with roughly 70 per cent of it in thermal form and the rest in kinetic. At late times (t > 500 Myr) we find the following: (1) the bubbles draw out trailing filaments of low-entropy gas, similar to those recently observed, (2) the action of buoyancy and the uplift of the filaments dominates the energetics of both the bubbles and the ICM, and (3) almost all of the originally injected energy is in the form of gravitational potential energy, with the bubbles containing 15 per cent of it, and the rest contained in the ICM. These findings indicate that feedback proceeds mainly through the displacement of gas to larger radii. We find that the uplift of these filaments permanently changes the thermodynamic properties of the ICM by reducing the central density and increasing the central temperature (within 30 kpc). We propose that jet feedback proceeds not only through the heating of the ICM (which can delay cooling), but also through the uplift-related reduction of the central gas density. The latter also delays cooling, on top of reducing the amount of gas available to cool.
The estimated stellar masses of galaxies are widely used to characterize how the galaxy population evolves over cosmic time. If stellar masses can be estimated in a robust manner, free from any bias, ...global diagnostics such as the stellar mass function can be used to constrain the physics of galaxy formation. We explore how galaxy stellar masses, estimated by fitting broad-band spectral energy distributions (SEDs) with stellar population models, can be biased as a result of commonly adopted assumptions for the star formation and chemical enrichment histories, recycled fractions and dust attenuation curves of galaxies. We apply the observational technique of broad-band SED fitting to model galaxy SEDs calculated by the theoretical galaxy formation model GALFORM, isolating the effect of each of these assumptions. We find that, averaged over the entire galaxy population, the common assumption of exponentially declining star formation histories does not, by itself, adversely affect stellar mass estimation. However, we also show that this result does not hold when considering galaxies that have undergone a recent burst of star formation. We show that fixing the metallicity in SED fitting or using sparsely sampled metallicity grids can introduce mass-dependent systematics into stellar mass estimates. We find that the common assumption of a star-dust geometry corresponding to a uniform foreground dust screen can cause the stellar masses of dusty model galaxies to be significantly underestimated. Finally, we show that stellar mass functions recovered by applying SED fitting to model galaxies at high redshift can differ significantly in both shape and normalization from the intrinsic mass functions predicted by a given model. In particular, the effects of dust can reduce the normalization at the high-mass end by up to 0.6 dex in some cases. Given these differences, our methodology of using stellar masses estimated from model galaxy SEDs offers a new, self-consistent way to compare model predictions with observations. We conclude that great care should be taken when comparing theoretical galaxy formation models to observational results based on the estimated stellar masses of high-redshift galaxies.
ABSTRACT
We present a study of galaxy mergers up to z = 10 using the Planck Millennium cosmological dark matter simulation and the GALFORM semi-analytical model of galaxy formation. Utilizing the ...full 800 Mpc3 volume of the simulation, we studied the statistics of galaxy mergers in terms of merger rates and close pair fractions. We predict that merger rates begin to drop rapidly for high-mass galaxies (M* > 1011.3–1010.5 M⊙ for z = 0–4), as a result of the exponential decline in the galaxy stellar mass function. The predicted merger rates for massive galaxies (M* > 1010 M⊙) increase and then turn over with increasing redshift, by z = 3.5, in disagreement with hydrodynamical simulations and semi-empirical models. In agreement with most other models and observations, we find that close pair fractions flatten or turn over at some redshift (dependent on the mass selection). We conduct an extensive comparison of close pair fractions, and highlight inconsistencies among models, but also between different observations. We provide a fitting formula for the major merger time-scale for close galaxy pairs, in which the slope of the stellar mass dependence is redshift dependent. This is in disagreement with previous theoretical results that implied a constant slope. Instead, we find a weak redshift dependence only for massive galaxies (M* > 1010 M⊙): in this case the merger time-scale varies approximately as $M_*^{-0.55}$. We find that close pair fractions and merger time-scales depend on the maximum projected separation as $r_\mathrm{max}^{1.32}$, in agreement with observations of small-scale clustering of galaxies.
ABSTRACT
We use the GALFORM semi-analytical model of galaxy formation and the Planck-Millennium simulation to investigate the origins of stellar mass in galaxies and their spheroids. We compare the ...importance of mergers and disc instabilities, as well as the starbursts that they trigger. We find that the fraction of galaxy stellar mass formed ex situ (i.e. through mergers; fex) increases sharply from M* = 1011 M⊙ upwards, reaching 80 per cent at M* = 1011.3 M⊙. The massive end of the fex–M* relation does not evolve with redshift, in disagreement with other models. For low-mass galaxies we find larger ex situ contributions at z = 0 than in other models (7–12 per cent), with a decrease towards higher redshifts. Major mergers contribute roughly half of the ex situ mass, with minor mergers and smooth accretion of satellites both accounting for ≈25 per cent, almost independent of stellar mass and redshift. Mergers dominate in building up high-mass (M*, sph > 1011 M⊙) and low-mass (M*, sph < 108.5 M⊙) spheroids. Disc instabilities and their associated starbursts dominate for intermediate-mass spheroids (108.5 < M*, sph < 1011 M⊙) at z = 0. The mass regime where pseudo-bulges dominate is in agreement with observed pseudo-bulge fractions, but the peak value in the pseudo-bulge fraction predicted by GALFORM is likely too high. Starbursts induced by disc instabilities are the dominant channel for spheroid growth at all redshifts, while merger-induced starbursts are relatively negligible, except at very high redshifts (z > 5).
A unified multiwavelength model of galaxy formation Lacey, Cedric G.; Baugh, Carlton M.; Frenk, Carlos S. ...
Monthly Notices of the Royal Astronomical Society,
11/2016, Volume:
462, Issue:
4
Journal Article
Peer reviewed
Open access
We present a new version of the galform semi-analytical model of galaxy formation. This brings together several previous developments of galform into a single unified model, including a different ...initial mass function (IMF) in quiescent star formation and in starbursts, feedback from active galactic nuclei suppressing gas cooling in massive haloes, and a new empirical star formation law in galaxy discs based on their molecular gas content. In addition, we have updated the cosmology, introduced a more accurate treatment of dynamical friction acting on satellite galaxies, and updated the stellar population model. The new model is able to simultaneously explain both the observed evolution of the K-band luminosity function and stellar mass function, and the number counts and redshift distribution of sub-mm galaxies selected at 850 μm. This was not previously achieved by a single physical model within the Λcold dark matter framework, but requires having an IMF in starbursts that is somewhat top-heavy. The new model is tested against a wide variety of observational data covering wavelengths from the far-UV to sub-mm, and redshifts from z = 0 to 6, and is found to be generally successful. These observations include the optical and near-infrared (IR) luminosity functions, H i mass function, fraction of early type galaxies, Tully–Fisher, metallicity–luminosity and size–luminosity relations at z = 0, as well as far-IR number counts, and far-UV luminosity functions at z ∼ 3–6. Discrepancies are, however, found in galaxy sizes and metallicities at low luminosities, and in the abundance of low-mass galaxies at high-z, suggesting the need for a more sophisticated model of supernova feedback.
We compare global predictions from the eagle hydrodynamical simulation, and two semi-analytic (SA) models of galaxy formation, l-galaxies and galform. All three models include the key physical ...processes for the formation and evolution of galaxies and their parameters are calibrated against a small number of observables at z ≈ 0. The two SA models have been applied to merger trees constructed from the eagle dark matter only simulation. We find that at z ≤ 2, both the galaxy stellar mass functions for stellar masses M
* < 1010.5 M⊙ and the median specific star formation rates (sSFRs) in the three models agree to better than 0.4 dex. The evolution of the sSFR predicted by the three models closely follows the mass assembly history of dark matter haloes. In both eagle and l-galaxies there are more central passive galaxies with M
* < 109.5 M⊙ than in galform. This difference is related to galaxies that have entered and then left a larger halo and which are treated as satellites in galform. In the range 0 < z < 1, the slope of the evolution of the star formation rate density in eagle is a factor of ≈1.5 steeper than for the two SA models. The median sizes for galaxies with M
* > 109.5 M⊙ differ in some instances by an order of magnitude, while the stellar mass–size relation in eagle is a factor of ≈2 tighter than for the two SA models. Our results suggest the need for a revision of how SA models treat the effect of baryonic self-gravity on the underlying dark matter. The treatment of gas flows in the models needs to be revised based on detailed comparison with observations to understand in particular the evolution of the stellar mass–metallicity relation.
ABSTRACT
We implement a black hole spin evolution and jet feedback model into SWIFT, a smoothed particle hydrodynamics code. The jet power is determined self-consistently assuming that the black hole ...accretion rate is equal to the Bondi rate (i.e. the accretion efficiency is 100 per cent), and using a realistic, spin-dependent efficiency. The jets are launched along the spin axis of the black hole, resulting in natural reorientation and precession. We apply the model to idealized simulations of galaxy groups and clusters, finding that jet feedback successfully quenches gas cooling and star formation in all systems. Our group-size halo (M200 = 1013 M⊙) is quenched by a strong jet episode triggered by a cooling flow, and it is kept quenched by a low-power jet fed from hot halo accretion. In more massive systems (M200 ≳ 1014 M⊙), hot halo accretion is insufficient to quench the galaxies, or to keep them quenched after the first cooling episode. These galaxies experience multiple episodes of gas cooling, star formation, and jet feedback. In the most massive galaxy cluster that we simulate (M200 = 1015 M⊙), we find peak cold gas masses of 1010 M⊙ and peak star formation rates of a few times 100 $\mathrm{M}_\odot \,\, \mathrm{yr}^{-1}$. These values are achieved during strong cooling flows, which also trigger the strongest jets with peak powers of 1047$\mathrm{erg}\, \mathrm{s}^{-1}$. These jets subsequently shut off the cooling flows and any associated star formation. Jet-inflated bubbles draw out low-entropy gas that subsequently forms dense cooling filaments in their wakes, as seen in observations.
We study the evolution of the cold gas content of galaxies by splitting the interstellar medium into its atomic and molecular hydrogen components, using the galaxy formation model galform in the Λ ...cold dark matter framework. We calculate the molecular-to-atomic hydrogen mass ratio, H2/H i, in each galaxy using two different approaches, the pressure-based empirical relation of Blitz & Rosolowsky and the theoretical model of Krumholz, McKeee & Tumlinson, and apply them to consistently calculate the star formation rates of galaxies. We find that the model based on the Blitz & Rosolowsky law predicts an H i mass function, 12CO (1-0) luminosity function, correlations between H2/H i and stellar and cold gas mass, and infrared-12CO molecule luminosity relation in good agreement with local and high-redshift observations. The H i mass function evolves weakly with redshift, with the number density of high-mass galaxies decreasing with increasing redshift. In the case of the H2 mass function, the number density of massive galaxies increases strongly from z= 0 to 2, followed by weak evolution up to z= 4. We also find that H2/H i of galaxies is strongly dependent on stellar and cold gas mass, and also on redshift. The slopes of the correlations between H2/H i and stellar and cold gas mass hardly evolve, but the normalization increases by up to two orders of magnitude from z= 0 to 8. The strong evolution in the H2 mass function and H2/H i is primarily due to the evolution in the sizes of galaxies and, secondarily, in the gas fractions. The predicted cosmic density evolution of H i agrees with the observed evolution inferred from damped Lyα systems, and is always dominated by the H i content of low- and intermediate-mass haloes. We find that previous theoretical studies have largely overestimated the redshift evolution of the global H2/H i due to limited resolution. We predict a maximum of
at z≈ 3.5.
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