Galaxies fall into two clearly distinct types: 'blue-sequence' galaxies which are rapidly forming young stars, and 'red-sequence' galaxies in which star formation has almost completely ceased. Most ...galaxies more massive than ... follow the red sequence, while less massive central galaxies lie on the blue sequence. We show that these sequences are created by a competition between star formation-driven outflows and gas accretion on to the supermassive black hole at the galaxy's centre. We develop a simple analytic model for this interaction. In galaxies less massive than ..., young stars and supernovae drive a high-entropy outflow which is more buoyant than any tenuous corona. The outflow balances the rate of gas inflow, preventing high gas densities building up in the central regions. More massive galaxies, however, are surrounded by an increasingly hot corona. Above a halo mass of ..., the outflow ceases to be buoyant and star formation is unable to prevent the build-up of gas in the central regions. This triggers a strongly non-linear response from the black hole. Its accretion rate rises rapidly, heating the galaxy's corona, disrupting the incoming supply of cool gas and starving the galaxy of the fuel for star formation. The host galaxy makes a transition to the red sequence, and further growth predominantly occurs through galaxy mergers. We show that the analytic model provides a good description of galaxy evolution in the EAGLE hydrodynamic simulations. So long as star formation-driven outflows are present, the transition mass scale is almost independent of subgrid parameter choice. (ProQuest: ... denotes formulae/symbols omitted.)
We use the hydrodynamical EAGLE simulation to study the magnitude and origin of the scatter in the stellar mass-halo mass relation for central galaxies. We separate cause and effect by correlating ...stellar masses in the baryonic simulation with halo properties in a matched dark matter only (DMO) simulation. The scatter in stellar mass increases with redshift and decreases with halo mass. At z = 0.1, it declines from 0.25 dex at M..., DMO ... 10... M... to 0.12 dex at M200, DMO ... 10... M..., but the trend is weak above 1012 M... For M..., DMO < 10... M... up to 0.04 dex of the scatter is due to scatter in the halo concentration. At fixed halo mass, a larger stellar mass corresponds to a more concentrated halo. This is likely because higher concentrations imply earlier formation times and hence more time for accretion and star formation, and/or because feedback is less efficient in haloes with higher binding energies. The maximum circular velocity, Vmax, DMO, and binding energy are therefore more fundamental properties than halo mass, meaning that they are more accurate predictors of stellar mass, and we provide fitting formulae for their relations with stellar mass. However, concentration alone cannot explain the total scatter in the M...--M... relation, and it does not explain the scatter in Mstar-Vmax, DMO. Halo spin, sphericity, triaxiality, substructure and environment are also not responsible for the remaining scatter, which thus could be due to more complex halo properties or non-linear/stochastic baryonic effects. (ProQuest: ... denotes formulae/symbols omitted.)
Abstract
We investigate the connection between the star formation rate (SFR) of galaxies and their central black hole accretion rate (BHAR) using the eagle cosmological hydrodynamical simulation. We ...find, in striking concurrence with recent observational studies, that the 〈SFR〉–BHAR relation for an active galactic nucleus (AGN)-selected sample produces a relatively flat trend, whilst the 〈BHAR〉–SFR relation for an SFR-selected sample yields an approximately linear trend. These trends remain consistent with their instantaneous equivalents even when both SFR and BHAR are time averaged over a period of 100 Myr. There is no universal relationship between the two growth rates. Instead, SFR and BHAR evolve through distinct paths that depend strongly on the mass of the host dark matter halo. The galaxies hosted by haloes of mass M
200 ≲ 1011.5 M⊙ grow steadily, yet black holes (BHs) in these systems hardly grow, yielding a lack of correlation between SFR and BHAR. As haloes grow through the mass range 1011.5 ≲ M
200 ≲ 1012.5 M⊙ BHs undergo a rapid phase of non-linear growth. These systems yield a highly non-linear correlation between the SFR and BHAR, which are non-causally connected via the mass of the host halo. In massive haloes (M
200 ≳ 1012.5 M⊙), both SFR and BHAR decline on average with a roughly constant scaling of SFR/BHAR ∼ 103. Given the complexity of the full SFR–BHAR plane built from multiple behaviours, and from the large dynamic range of BHARs, we find the primary driver of the different observed trends in the 〈SFR〉–BHAR and 〈BHAR〉–SFR relationships are due to sampling considerably different regions of this plane.
ABSTRACT
We introduce the Assembly of high-ResoluTion Eagle-simulations of MIlky Way-type galaxieS (artemis) simulations, a new set of 42 zoomed-in, high-resolution (baryon particle mass of $\approx ...2\times 10^4 \, {\rm M}_{\odot }\, h^{-1}$), hydrodynamical simulations of galaxies residing in haloes of Milky Way mass, simulated with the eagle galaxy formation code with re-calibrated stellar feedback. In this study, we analyse the structure of stellar haloes, specifically the mass density, surface brightness, metallicity, colour, and age radial profiles, finding generally very good agreement with recent observations of local galaxies. The stellar density profiles are well fitted by broken power laws, with inner slopes of ≈−3, outer slopes of ≈−4, and break radii that are typically ≈20–40 kpc. The break radii generally mark the transition between in situ formation and accretion-driven formation of the halo. The metallicity, colour, and age profiles show mild large-scale gradients, particularly when spherically averaged or viewed along the major axes. Along the minor axes, however, the profiles are nearly flat, in agreement with observations. Overall, the structural properties can be understood by two factors: that in situ stars dominate the inner regions and that they reside in a spatially flattened distribution that is aligned with the disc. Observations targeting both the major and minor axes of galaxies are thus required to obtain a complete picture of stellar haloes.
The brighter galaxies reionized the Universe Sharma, Mahavir; Theuns, Tom; Frenk, Carlos ...
Monthly Notices of the Royal Astronomical Society Letters,
05/2016, Letnik:
458, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Hydrogen in the Universe was (re)ionized between redshifts z ≈ 10 and z ≈ 6. The nature of the sources of the ionizing radiation is hotly debated, with faint galaxies below current detection limits ...regarded as prime candidates. Here, we consider a scenario in which ionizing photons escape through channels punctured in the interstellar medium by outflows powered by starbursts. We take account of the observation that strong outflows occur only when the star formation density is sufficiently high, and estimate the galaxy-averaged escape fraction as a function of redshift and luminosity from the resolved star formation surface densities in the eagle cosmological hydrodynamical simulation. We find that the fraction of ionizing photons that escape from galaxies increases rapidly with redshift, reaching values of 5–20 per cent at z > 6, with the brighter galaxies having higher escape fractions. Combining the dependence of escape fraction on luminosity and redshift with the observed luminosity function, we demonstrate that galaxies emit enough ionizing photons to match the existing constraints on reionization while also matching the observed ultraviolet-background post-reionization. Our findings suggest that galaxies above the current Hubble Space Telescope detection limit emit half of the ionizing radiation required to reionize the Universe.
We present results from a subset of simulations from the ‘Evolution and Assembly of GaLaxies and their Environments’ (eagle) suite in which the formulation of the hydrodynamics scheme is varied. We ...compare simulations that use the same subgrid models without recalibration of the parameters but employing the standard gadget flavour of smoothed particle hydrodynamics (SPH) instead of the more recent state-of-the-art anarchy formulation of SPH that was used in the fiducial eagle runs. We find that the properties of most galaxies, including their masses and sizes, are not significantly affected by the details of the hydrodynamics solver. However, the star formation rates of the most massive objects are affected by the lack of phase mixing due to spurious surface tension in the simulation using standard SPH. This affects the efficiency with which AGN activity can quench star formation in these galaxies and it also leads to differences in the intragroup medium that affect the X-ray emission from these objects. The differences that can be attributed to the hydrodynamics solver are, however, likely to be less important at lower resolution. We also find that the use of a time-step limiter is important for achieving the feedback efficiency required to match observations of the low-mass end of the galaxy stellar mass function.
We introduce a series of 20 cosmological hydrodynamical simulations of L* (M
200 = 1011.7–1012.3 M⊙) and group-sized (M
200 = 1012.7–1013.3 M⊙) haloes run with the model used for the eagle project, ...which additionally includes a non-equilibrium ionization and cooling module that follows 136 ions. The simulations reproduce the observed correlation, revealed by COS-Halos at z ∼ 0.2, between
${{\rm O} \small{VI}}$
column density at impact parameters b < 150 kpc and the specific star formation rate (sSFR ≡ SFR/M
*) of the central galaxy at z ∼ 0.2. We find that the column density of circumgalactic
${O \small {VI}}$
is maximal in the haloes associated with L* galaxies, because their virial temperatures are close to the temperature at which the ionization fraction of
${O \small {VI}}$
peaks (T ∼ 105.5 K). The higher virial temperature of group haloes (>106 K) promotes oxygen to higher ionization states, suppressing the
${O \small {VI}}$
column density. The observed
$N_{\rm O\,\small {VI}}$
–sSFR correlation therefore does not imply a causal link, but reflects the changing characteristic ionization state of oxygen as halo mass is increased. In spite of the mass dependence of the oxygen ionization state, the most abundant circumgalactic oxygen ion in both L* and group haloes is
${O\,{\small VII}}$
;
${O \small {VI}}$
accounts for only 0.1 per cent of the oxygen in group haloes and 0.9–1.3 per cent with L* haloes. Nonetheless, the metals traced by
${O \small {VI}}$
absorbers represent a fossil record of the feedback history of galaxies over a Hubble time; their characteristic epoch of ejection corresponds to z > 1 and much of the ejected metal mass resides beyond the virial radius of galaxies. For both L* and group galaxies, more of the oxygen produced and released by stars in the circumgalactic medium (within twice the virial radius) than in the stars and interstellar medium of the galaxy.
We present results from 13 cosmological simulations that explore the parameter space of the ‘Evolution and Assembly of GaLaxies and their Environments’ (EAGLE) simulation project. Four of the ...simulations follow the evolution of a periodic cube L = 50 cMpc on a side, and each employs a different subgrid model of the energetic feedback associated with star formation. The relevant parameters were adjusted so that the simulations each reproduce the observed galaxy
stellar mass function at z = 0.1. Three of the simulations fail to form disc galaxies as extended as observed, and we show analytically that this is a consequence of numerical radiative losses that reduce the efficiency of stellar feedback in high-density gas. Such losses are greatly reduced in the fourth simulation – the EAGLE reference model – by injecting more energy in higher density gas. This model produces galaxies with the observed size distribution, and also reproduces many galaxy scaling relations. In the remaining nine simulations, a single parameter or process of the reference model was varied at a time. We find that the properties of galaxies with stellar mass ≲ M
⋆ (the ‘knee’ of the galaxy stellar mass function) are largely governed by feedback associated with star formation, while those of more massive galaxies are also controlled by feedback from accretion on to their central black holes. Both processes must be efficient in order to reproduce the observed galaxy population. In general, simulations that have been calibrated to reproduce the low-redshift galaxy stellar mass function will still not form realistic galaxies, but the additional requirement that galaxy sizes be acceptable leads to agreement with a large range of observables.
The observed high covering fractions of neutral hydrogen (
${\rm H\,\small {\rm I}}$
) with column densities above ∼1017 cm−2 around Lyman-Break Galaxies (LBGs) and bright quasars at redshifts ...z ∼ 2–3 has been identified as a challenge for simulations of galaxy formation. We use the Evolution and Assembly of Galaxies and their Environment (EAGLE) cosmological, hydrodynamical simulation, which has been shown to reproduce a wide range of galaxy properties and for which the subgrid feedback was calibrated without considering gas properties, to study the distribution of
${\rm H\,\small {\rm I}}$
around high-redshift galaxies. We predict the covering fractions of strong
${\rm H\,\small {\rm I}}$
absorbers (
${N_{\rm H\,\small {I}}}\gtrsim 10^{17} \,{\rm cm^{-2}}$
) inside haloes to increase rapidly with redshift but to depend only weakly on halo mass. For massive (M
200 ≳ 1012M⊙) haloes, the covering fraction profiles are nearly scale-invariant and we provide fitting functions that reproduce the simulation results. While efficient feedback is required to increase the
${\rm H\,\small {\rm I}}$
covering fractions to the high observed values, the distribution of strong absorbers in and around haloes of a fixed mass is insensitive to factor of 2 variations in the strength of the stellar feedback. In contrast, at fixed stellar mass the predicted
${\rm H\,\small {\rm I}}$
distribution is highly sensitive to the feedback efficiency. The fiducial EAGLE simulation reproduces both the observed global column density distribution function of
${\rm H\,\small {\rm I}}$
and the observed radial covering fraction profiles of strong
${\rm H\,\small {\rm I}}$
absorbers around LBGs and bright quasars.
The Local Group galaxies offer some of the most discriminating tests of models of cosmic structure formation. For example, observations of the Milky Way (MW) and Andromeda satellite populations ...appear to be in disagreement with N-body simulations of the ‘lambda cold dark matter’ (ΛCDM) model: there are far fewer satellite galaxies than substructures in CDM haloes (the ‘missing satellites’ problem); dwarf galaxies seem to avoid the most massive substructures (the ‘too-big-to-fail’ problem); and the brightest satellites appear to orbit their host galaxies on a thin plane (the ‘planes of satellites’ problem). Here we present results from apostle (A Project Of Simulating The Local Environment), a suite of cosmological hydrodynamic simulations of 12 volumes selected to match the kinematics of the Local Group (LG) members. Applying the eagle code to the LG environment, we find that our simulations match the observed abundance of LG galaxies, including the satellite galaxies of the MW and Andromeda. Due to changes to the structure of haloes and the evolution in the LG environment, the simulations reproduce the observed relation between stellar mass and velocity dispersion of individual dwarf spheroidal galaxies without necessitating the formation of cores in their dark matter profiles. Satellite systems form with a range of spatial anisotropies, including one similar to the MWs, confirming that such a configuration is not unexpected in ΛCDM. Finally, based on the observed velocity dispersion, size, and stellar mass, we provide estimates of the maximum circular velocity for the haloes of nine MW dwarf spheroidals.
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