We investigate the evolution of galaxy masses and star formation rates in the Evolution and Assembly of Galaxies and their Environment (eagle) simulations. These comprise a suite of hydrodynamical ...simulations in a Λ cold dark matter cosmogony with subgrid models for radiative cooling, star formation, stellar mass-loss and feedback from stars and accreting black holes. The subgrid feedback was calibrated to reproduce the observed present-day galaxy stellar mass function and galaxy sizes. Here, we demonstrate that the simulations reproduce the observed growth of the stellar mass density to within 20 per cent. The simulations also track the observed evolution of the galaxy stellar mass function out to redshift z = 7, with differences comparable to the plausible uncertainties in the interpretation of the data. Just as with observed galaxies, the specific star formation rates of simulated galaxies are bimodal, with distinct star forming and passive sequences. The specific star formation rates of star-forming galaxies are typically 0.2 to 0.5 dex lower than observed, but the evolution of the rates track the observations closely. The unprecedented level of agreement between simulation and data across cosmic time makes eagle a powerful resource to understand the physical processes that govern galaxy formation.
Feedback from energy liberated by gas accretion on to black holes (BHs) is an attractive mechanism to explain the exponential cut-off at the massive end of the galaxy stellar mass function. Most ...previous implementations of BH accretion in hydrodynamical simulations of galaxy formation have assumed that BHs grow at an accretion rate that is proportion to the Bondi rate. A major concern is that the Bondi accretion rate is inappropriate when the accreting material has significant angular momentum. We present an improved accretion model that takes into account the circularization and subsequent viscous transport of infalling material, and implemented as a ‘subgrid’ model in hydrodynamic simulations. The resulting accretion rates are generally low in low mass (≲ 1011.5 M⊙) haloes, but show outbursts of Eddington-limited accretion during galaxy mergers. During outbursts these objects strongly resemble quasars. In higher mass haloes, gas accretion peaks at ∼10 per cent of the Eddington rate, which is thought to be conducive to the formation of radio jets. The resulting accretion rate depends strongly on the effective pressure of the gas surrounding the BH, which in turn depends strongly on halo mass. This induces a sharp transition in the importance of BH feedback. In small haloes, the growth of galaxies is regulated by star formation and supernova feedback, but above a halo mass of 1011.5 M⊙, rapid BH growth leads to the suppression of star formation and reduced growth of stellar mass with increasing halo mass.
We present the first results from the KMOS (K-band Multi-Object Spectrograph) AGN (active galactic nuclei) Survey at High redshift (KASHz), a VLT/KMOS integral-field spectroscopic (IFS) survey of z ≳ ...0.6 AGN. We present galaxy-integrated spectra of 89 X-ray AGN (L
2–10 keV = 1042–1045 erg s−1), for which we observed O iii (z ≈ 1.1–1.7) or Hα emission (z ≈ 0.6–1.1). The targets have X-ray luminosities representative of the parent AGN population and we explore the emission-line luminosities as a function of X-ray luminosity. For the O iii targets, ≈50 per cent have ionized gas velocities indicative of gas that is dominated by outflows and/or highly turbulent material (i.e. overall line widths ≳600 km s−1). The most luminous half (i.e. L
X > 6 × 1043 erg s−1) have a ≳2 times higher incidence of such velocities. On the basis of our results, we find no evidence that X-ray obscured AGN are more likely to host extreme kinematics than unobscured AGN. Our KASHz sample has a distribution of gas velocities that is consistent with a luminosity-matched sample of z < 0.4 AGN. This implies little evolution in the prevalence of ionized outflows, for a fixed AGN luminosity, despite an order-of-magnitude decrease in average star formation rates over this redshift range. Furthermore, we compare our Hα targets to a redshift-matched sample of star-forming galaxies and despite a similar distribution of Hα luminosities and likely star formation rates, we find extreme ionized gas velocities are up to ≈10 times more prevalent in the AGN-host galaxies. Our results reveal a high prevalence of extreme ionized gas velocities in high-luminosity X-ray AGN and imply that the most powerful ionized outflows in high-redshift galaxies are driven by AGN activity.
We examine the radial entropy distribution and its scaling using 31 nearby galaxy clusters from the representative XMM-Newton cluster structure survey (REXCESS), a sample in the temperature range ...2-9 keV selected in X-ray luminosity only, with no bias toward any particular morphological type. The entropy profiles are robustly measured at least out to R1000 in all systems and out to R500 in thirteen systems. Compared to theoretical expectations from non-radiative cosmological simulations, the observed distributions show a radial and mass-dependent excess entropy, such that the excess is greater and extends to larger radii in lower mass systems. At R500, the mass dependence and entropy excess are both negligible within the large observational and theoretical uncertainties. Mirroring this behaviour, the scaling of gas entropy is shallower than self-similar in the inner regions, but steepens with radius, becoming consistent with self-similar at R500. There is a large dispersion in scaled entropy in the inner regions, apparently linked to the presence of cool cores and dynamical activity; at larger radii the dispersion decreases by approximately a factor of two to 30 per cent, and the dichotomy between subsamples disappears. There are two peaks in the distribution of both inner slope and, after parameterising the profiles with a power law plus constant model, in central entropy K0. However, we are unable to distinguish between a bimodal or a left-skewed distribution of K0 with the present data. The distribution of outer slopes is unimodal with a median value of 0.98, and there is a clear correlation of outer slope with temperature. Renormalising the dimensionless entropy profiles by the gas mass fraction profile fgas (<R), leads to a remarkable reduction in the scatter, implying that gas mass fraction variations with radius and mass are the cause of the observed entropy structural and scaling properties. The results are consistent with the picture of a cluster population in which entropy modification is centrally concentrated and extends to larger radii at lower mass, leading to both a radial and a mass-dependence in the gas mass fraction, but which is increasingly self-similar at large radius. The observed normalisation, however, would suggest entropy modification at least up to R1000, and even beyond, in all but the most massive systems. We discuss a tentative scenario to explain the observed behaviour of the entropy and gas mass fraction in the REXCESS sample, in which a combination of extra heating and merger mixing maintains an elevated central entropy level in the majority of the population, and a smaller fraction of systems is able to develop a cool core.
We present the evolution of galaxy sizes, from redshift 2 to 0, for actively star forming and passive galaxies in the cosmological hydrodynamical 100 super( 3) cMpc3 simulation of the EAGLE project. ...We find that the sizes increase with stellar mass, but that the relation weakens with increasing redshift. Separating galaxies by their star formation activity, we find that passive galaxies are typically smaller than active galaxies at a fixed stellar mass. These trends are consistent with those found in observations and the level of agreement between the predicted and observed size-mass relations is of the order of 0.1 dex for z < 1 and 0.2-0.3 dex from redshift 1 to 2. We use the simulation to compare the evolution of individual galaxies with that of the population as a whole. While the evolution of the size-stellar mass relation for active galaxies provides a good proxy for the evolution of individual galaxies, the evolution of individual passive galaxies is not well represented by the observed size-mass relation due to the evolving number density of passive galaxies. Observations of z ~ 2 galaxies have revealed an abundance of massive red compact galaxies, which depletes below z ~ 1. We find that a similar population forms naturally in the simulation. Comparing these galaxies with their z = 0 descendants, we find that all compact galaxies grow in size due to the high-redshift stars migrating outwards. Approximately 60 per cent of the compact galaxies increase in size further due to renewed star formation and/or mergers.
We use a coupled model of the formation and evolution of galaxies and black holes (BHs) to study the evolution of active galactic nuclei (AGNs) in a cold dark matter universe. The model is embedded ...in the galaxy formation code galform and predicts the masses, spins and mass accretion histories of BHs in tandem with the formation of their host galaxies. BHs grow by accretion during starbursts, triggered by discs becoming dynamically unstable or by galaxy mergers, and accretion from quasi-hydrostatic hot gas haloes. Using an empirical law for AGN obscuration, our model matches the observed luminosity functions (LFs) of AGNs over a wide range of redshifts. Due to the suppression of cooling in massive haloes by AGN feedback, at low redshift (z≲ 2), the brightest quasars (L
bol≳ 1046 erg s−1) are predicted preferentially to inhabit haloes with masses
. The model predicts a hierarchical buildup of BH mass, with the typical mass of actively growing BHs increasing with decreasing redshift. Nevertheless, the model displays clear 'downsizing' as reflected in the differential evolution of the space density of faint and bright AGNs. This arises naturally from the interplay between the starburst and hot gas halo accretion modes. The faint end of the LF is dominated by massive BHs accreting at low rates via a thick disc, primarily during the hot-halo mode. The bright end is populated by BHs accreting close to or above the Eddington limit during the starburst mode. Obscuration plays a central role in determining the observed abundance of AGNs and, hence, in their implied cosmic evolution.
The observed stellar mass function (SMF) is very different to the halo mass function predicted by Λ cold dark matter (ΛCDM), and it is widely accepted that this is due to energy feedback from ...supernovae and black holes. However, the strength and form of this feedback is not understood. In this paper, we use the phenomenological model galform to explore how galaxy formation depends on the strength and halo mass dependence of feedback. We focus on 'expulsion' models in which the wind mass loading, β, is proportional to
, with n= 0, 1, 2 and contrast these models with the successful Bower et al. model (B8W7), for which
. A crucial development is that our code explicitly accounts for the recapture of expelled gas as the system's halo mass (and thus gravitational potential) increases. While models with high wind speed and mass loading result in a poor match to the observed SMF, a model with slower wind speed matches the flat portion of the SMF at M
★∼ 109-1011 h
−1 M⊙. When combined with active galactic nucleus feedback, the model provides a good description of the observed SMF above 109 h
−1 M⊙. In order to explore the impact of different feedback schemes further, we examine how the expulsion models compare with a further range of observational data, contrasting the results with the B8W7 model. In the expulsion models, the brightest galaxies are assembled more recently, and the specific star formation rates of galaxies decrease strongly with decreasing stellar mass. The expulsion models tend to have a cosmic star formation density that is dominated by lower mass galaxies at z= 1-3, and dominated by high-mass galaxies at low redshift. These trends are in conflict with observational data, but the comparison highlights some deficiencies of the B8W7 model also. The experiments in this paper not only give us important physical insight into the impact of the feedback process on the formation histories of galaxies, but the strong mass dependence of feedback adopted in B8W7 still appears to provide the most promising description of the observed Universe.
Abstract
We present dynamical measurements for 586 Hα-detected star-forming galaxies from the KMOS (K-band Multi-Object Spectrograph) Redshift One Spectroscopic Survey (KROSS). The sample represents ...typical star-forming galaxies at this redshift (z = 0.6–1.0), with a median star formation rate of ≈7 M⊙ yr−1 and a stellar mass range of log (M⋆M⊙) ≈ 9–11. We find that the rotation velocity–stellar mass relationship (the inverse of the Tully–Fisher relationship) for our rotationally dominated sources (vC/σ0 > 1) has a consistent slope and normalization as that observed for z = 0 discs. In contrast, the specific angular momentum (j⋆; angular momentum divided by stellar mass) is ≈0.2–0.3 dex lower on average compared to z = 0 discs. The specific angular momentum scales as $j_{\rm s}\propto M_{\star }^{0.6\pm 0.2}$, consistent with that expected for dark matter (i.e. $j_{\rm DM}\propto M_{\rm DM}^{2/3}$). We find that z ≈ 0.9 star-forming galaxies have decreasing specific angular momentum with increasing Sérsic index. Visually, the sources with the highest specific angular momentum, for a given mass, have the most disc-dominated morphologies. This implies that an angular momentum–mass–morphology relationship, similar to that observed in local massive galaxies, is already in place by z ≈ 1.
We present adaptive optics-assisted integral field spectroscopy around the Hα or Hβ lines of 12 gravitationally lensed galaxies obtained with VLT/SINFONI, Keck/OSIRIS and Gemini/NIFS. We combine ...these data with previous observations and investigate the dynamics and star formation properties of 17 lensed galaxies at 1 < z < 4. Thanks to gravitational magnification of 1.4–90 times by foreground clusters, effective spatial resolutions of 40–700 pc are achieved. The magnification also allows us to probe lower star formation rates (SFRs) and stellar masses than unlensed samples; our target galaxies feature dust-corrected SFRs derived from Hα or Hβ emission of ∼0.8–40 M⊙ yr−1, and stellar masses M
* ∼ 4 × 108–6 × 1010 M⊙. All of the galaxies show velocity gradients, with 59 per cent consistent with being rotating discs and a likely merger fraction of 29 per cent, with the remaining 12 per cent classed as ‘undetermined’. We extract 50 star-forming clumps with sizes in the range 60 pc–1 kpc from the Hα (or Hβ) maps, and find that their surface brightnesses, Σclump and their characteristic luminosities, L
0, evolve to higher luminosities with redshift. We show that this evolution can be described by fragmentation on larger scales in gas-rich discs, and is likely to be driven by evolving gas fractions.
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