We develop semi-empirical models of the supermassive black hole and active galactic nucleus (AGN) populations, which incorporate the black hole growth implied by the observed AGN luminosity function ...assuming a radiative efficiency and a distribution of Eddington ratios λ. By generalizing these continuity-equation models to allow a distribution P(λ | M
BH, z), we are able to draw on constraints from observationally estimated λ distributions and active galaxy fractions while accounting for the luminosity thresholds of observational samples. We consider models with a Gaussian distribution of log λ and Gaussians augmented with a power-law tail to low λ. Within our framework, reproducing the high observed AGN fractions at low redshift requires a characteristic Eddington ratio λc that declines at late times, and matching observed Eddington ratio distributions requires a P(λ) that broadens at low redshift. To reproduce the observed increase of AGN fraction with black hole or galaxy mass, we also require a λc that decreases with increasing black hole mass, reducing the AGN luminosity associated with the most massive black holes. Finally, achieving a good match to the high-mass end of the local black hole mass function requires an increased radiative efficiency at high black hole mass. We discuss the potential impact of black hole mergers or a λ-dependent bolometric correction, and we compute evolutionary predictions for black hole and galaxy specific accretion rates. Despite the flexibility of our framework, no one model provides a good fit to all the data we consider; it is particularly difficult to reconcile the relatively narrow λ distributions and low duty cycles estimated for luminous broad-line AGN with the broader λ distributions and higher duty cycles found in more widely selected AGN samples, which typically have lower luminosity thresholds.
ABSTRACT
We investigate the impact of bursts in star formation on the predictions of one-zone chemical evolution models, adopting oxygen (O), iron (Fe), and strontium (Sr), as representative α, ...iron-peak, and s-process elements, respectively. To this end, we develop and make use of the Versatile Integrator for Chemical Evolution (VICE), a python package designed to handle flexible user-specified evolutionary parameters. Starbursts driven by a temporary boost of gas accretion rate create loops in O/Fe–Fe/H evolutionary tracks and a peak in the stellar O/Fe distribution at intermediate values. Bursts driven by a temporary boost of star formation efficiency have similar effects, and they also produce a population of α-deficient stars during the depressed star formation phase following the burst. This α-deficient population is more prominent if the outflow rate is tied to a time-averaged star formation rate (SFR) instead of the instantaneous SFR. Theoretical models of Sr production predict a strong metallicity dependence of supernova and asymptotic giant branch star yields, though comparison to data suggests an additional, nearly metallicity-independent source. Evolution of Sr/Fe and Sr/O during a starburst is complex because of this metallicity dependence and the multiple time-scales at play. Moderate amplitude (10–20 per cent) sinusoidal oscillations in SFR produce loops in O/Fe–Fe/H tracks and multiple peaks in O/Fe distributions, a potential source of intrinsic scatter in observed sequences. We investigate the impact of a factor ∼2 enhancement of Galactic star formation ∼2 Gyr ago, as suggested by some recent observations. VICE is publicly available at <http://pypi.org/project/vice/>.
Equilibrium and Sudden Events in Chemical Evolution Weinberg, David H.; Andrews, Brett H.; Freudenburg, Jenna
Astrophysical journal/The Astrophysical journal,
03/2017, Letnik:
837, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We present new analytic solutions for one-zone (fully mixed) chemical evolution models that incorporate a realistic delay time distribution for Type Ia supernovae (SNe Ia) and can therefore track the ...separate evolution of -elements produced by core collapse supernovae (CCSNe) and iron peak elements synthesized in both CCSNe and SNe Ia. Our solutions allow constant, exponential, or linear-exponential ( ) star formation histories, or combinations thereof. In generic cases, and iron abundances evolve to an equilibrium at which element production is balanced by metal consumption and gas dilution, instead of continuing to increase over time. The equilibrium absolute abundances depend principally on supernova yields and the outflow mass loading parameter , while the equilibrium abundance ratio depends mainly on yields and secondarily on star formation history. A stellar population can be metal-poor either because it has not yet evolved to equilibrium or because high outflow efficiency makes the equilibrium abundance itself low. Systems with ongoing gas accretion develop metallicity distribution functions (MDFs) that are sharply peaked, while "gas starved" systems with rapidly declining star formation, such as the conventional "closed box" model, have broadly peaked MDFs. A burst of star formation that consumes a significant fraction of a system's available gas and retains its metals can temporarily boost by 0.1-0.3 dex, a possible origin for rare, -enhanced stars with intermediate age and/or high metallicity. Other sudden transitions in system properties can produce surprising behavior, including backward evolution of a stellar population from high to low metallicity.
Observational studies show that the global deuterium-to-hydrogen ratio in the local interstellar medium (ISM) is about 90% of the primordial ratio predicted by Big Bang nucleosynthesis. The high ...implies that only a small fraction of interstellar gas has been processed through stars, which destroy any deuterium they are born with. Using analytic arguments for one-zone chemical evolution models that include accretion and outflow, I show that the deuterium abundance is tightly coupled to the abundance of core collapse supernova (CCSN) elements, such as oxygen. These models predict that the ratio of the ISM deuterium abundance to the primordial abundance is , where r is the recycling fraction, is the ISM oxygen mass fraction, and is the population-averaged CCSN yield of oxygen. Using values r = 0.4 and appropriate to a Kroupa initial mass function and recent CCSN yield calculations, solar oxygen abundance corresponds to , consistent with the observations. This approximation is accurate for a wide range of parameter values, and physical arguments and numerical tests suggest that it should remain accurate for more complex chemical evolution models. The good agreement with the upper range of observed values supports the long-standing suggestion that sightline-to-sightline variations of deuterium are a consequence of dust depletion, rather than a low global enhanced by localized accretion of primordial composition gas. This agreement limits deviations from conventional yield and recycling values, including models in which most high-mass stars collapse to form black holes without expelling their oxygen in supernovae, and it implies that Galactic outflows eject ISM hydrogen as efficiently as they eject CCSN metals.
The physical origin of high-velocity cool gas seen in galactic winds remains unknown. Following work by B. Wang, we argue that radiative cooling in initially hot thermally-driven outflows can produce ...fast neutral atomic and photoionized cool gas. The inevitability of adiabatic cooling from the flow's initial 107–108 K temperature and the shape of the cooling function for T ≲ 107 K imply that outflows with hot gas mass-loss rate relative to star formation rate of
$\beta =\dot{M}_{\rm hot}/\dot{M}_\star \gtrsim 0.5$
cool radiatively on scales ranging from the size of the energy injection region to tens of kpc. We highlight the β and star formation rate surface density dependence of the column density, emission measure, radiative efficiency, and velocity. At r
cool, the gas produces X-ray and then UV/optical line emission with a total power bounded by ∼10−2 L
⋆ if the flow is powered by steady-state star formation with luminosity L
⋆. The wind is thermally unstable at r
cool, potentially leading to a multiphase medium. Cooled winds decelerate significantly in the extended gravitational potential of galaxies. The cool gas precipitated from hot outflows may explain its prevalence in galactic haloes. We forward a picture of winds whereby cool clouds are initially accelerated by the ram pressure of the hot flow, but are rapidly shredded by hydrodynamical instabilities, thereby increasing β, seeding radiative and thermal instability, and cool gas rebirth. If the cooled wind shocks as it sweeps up the circumgalactic medium, its cooling time is short, thus depositing cool gas far out into the halo. Finally, conduction can dominate energy transport in low-β hot winds, leading to flatter temperature profiles than otherwise expected, potentially consistent with X-ray observations of some starbursts.
Chemical evolution models are powerful tools for interpreting stellar abundance surveys and understanding galaxy evolution. However, their predictions depend heavily on the treatment of inflow, ...outflow, star formation efficiency (SFE), the stellar initial mass function, the SN Ia delay time distribution, stellar yields, and stellar population mixing. Using flexCE, a flexible one-zone chemical evolution code, we investigate the effects of and trade-offs between parameters. Two critical parameters are SFE and the outflow mass-loading parameter, which shift the knee in O/Fe-Fe/H and the equilibrium abundances that the simulations asymptotically approach, respectively. One-zone models with simple star formation histories follow narrow tracks in O/Fe-Fe/H unlike the observed bimodality (separate high- and low- sequences) in this plane. A mix of one-zone models with inflow timescale and outflow mass-loading parameter variations, motivated by the inside-out galaxy formation scenario with radial mixing, reproduces the two sequences better than a one-zone model with two infall epochs. We present X/Fe-Fe/H tracks for 20 elements assuming three different supernova yield models and find some significant discrepancies with solar neighborhood observations, especially for elements with strongly metallicity-dependent yields. We apply principal component abundance analysis to the simulations and existing data to reveal the main correlations among abundances and quantify their contributions to variation in abundance space. For the stellar population mixing scenario, the abundances of -elements and elements with metallicity-dependent yields dominate the first and second principal components, respectively, and collectively explain 99% of the variance in the model. flexCE is a python package available at https://github.com/bretthandrews/flexCE.
Cold dark matter: Controversies on small scales Weinberg, David H; James S. Bullock; Fabio Governato ...
Proceedings of the National Academy of Sciences - PNAS,
10/2015, Letnik:
112, Številka:
40
Journal Article
Recenzirano
Odprti dostop
The cold dark matter (CDM) cosmological model has been remarkably successful in explaining cosmic structure over an enormous span of redshift, but it has faced persistent challenges from observations ...that probe the innermost regions of dark matter halos and the properties of the Milky Way’s dwarf galaxy satellites. We review the current observational and theoretical status of these “small-scale controversies.” Cosmological simulations that incorporate only gravity and collisionless CDM predict halos with abundant substructure and central densities that are too high to match constraints from galaxy dynamics. The solution could lie in baryonic physics: Recent numerical simulations and analytical models suggest that gravitational potential fluctuations tied to efficient supernova feedback can flatten the central cusps of halos in massive galaxies, and a combination of feedback and low star formation efficiency could explain why most of the dark matter subhalos orbiting the Milky Way do not host visible galaxies. However, it is not clear that this solution can work in the lowest mass galaxies, where discrepancies are observed. Alternatively, the small-scale conflicts could be evidence of more complex physics in the dark sector itself. For example, elastic scattering from strong dark matter self-interactions can alter predicted halo mass profiles, leading to good agreement with observations across a wide range of galaxy mass. Gravitational lensing and dynamical perturbations of tidal streams in the stellar halo provide evidence for an abundant population of low-mass subhalos in accord with CDM predictions. These observational approaches will get more powerful over the next few years.
We analyze the present day structure and assembly history of a high-resolution hydrodynamic simulation of the formation of a Milky-Way-(MW)-like disk galaxy, from the "Eris" simulation suite, ...dissecting it into cohorts of stars formed at different epochs of cosmic history. The younger age cohorts populate disks of progressively longer radial scale lengths and shorter vertical scale heights. At a given radius, the vertical density profiles and velocity dispersions of stars vary smoothly as a function of age, and the superposition of old, vertically extended and young, vertically compact cohorts gives rise to a double-exponential profile like that observed in the MW. Turning to formation history, we find that the trends of spatial structure and kinematics with stellar age are largely imprinted at birth, or immediately thereafter. The predicted correlations of stellar age with spatial and kinematic structure are in good qualitative agreement with the correlations observed for mono-abundance stellar populations in the MW.
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