The Meaning and Consequences of Star Formation Criteria in Galaxy Models with Resolved Stellar Feedback

We consider the effects of different criteria for determining where stars will form in gas on galactic scales, in simulations with high (1 pc) resolution, with explicitly resolved physics of Giant Molecular Cloud (GMC) formation and destruction, and stellar feedback from supernovae, radiation pressure, stellar winds and photoheating. We compare: (1) a self-gravity criterion (based on the local virial parameter and the assumption that self-gravitating gas collapses to high density in a single free-fall time), (2) a fixed density threshold, (3) a molecular-gas law, (4) a temperature threshold, (5) a requirement that the gas be Jeans unstable, (6) a criteria that cooling times be shorter than dynamical times and (7) a convergent-flow criterion. We consider all of these in both a Milky Way (MW)-like and high-density (starburst or high-redshift) galaxy. With feedback present, all models produce identical integrated star formation rates (SFRs), in good agreement with the Kennicutt relation; without feedback all produce orders-of-magnitude excessive SFRs. This is totally dependent on feedback and independent of the star formation (SF) law, even if the ‘local’ collapse efficiency is 100 per cent. However, the predicted spatial and density distribution depend strongly on the SF criteria. Because cooling rates are generally fast within galaxy discs, and gas is turbulent, criteria (4)–(7) are very ‘weak’ and spread the SF uniformly over most of the disc (down to densities n ∼ 0.01–0.1 cm−3). A molecular criterion (3) localizes to slightly higher densities, but still a wide range; for metallicity near solar, it is almost identical to a fixed density threshold at n ∼ 1 cm−3 (well below the mean density in the central MW or starburst systems). A fixed density threshold (2) can always select the highest resolved densities, but must be adjusted both for simulation resolution and individual galaxy properties – the same threshold that works well in a MW-like simulation will select nearly all gas in a starburst. Binding criteria (1) tend to adaptively select the largest local overdensities, independent of galaxy model or resolution, and automatically predict clustered SF. We argue that this SF model (possible with other secondary criteria) is most physically motivated and presents significant numerical advantages in simulations with a large dynamic range

Morphologies of z ∼ 0.7 AGN host galaxies in CANDELS: no trend of merger incidence with AGN luminosity

The processes that trigger active galactic nuclei (AGN) remain poorly understood. While lower luminosity AGN may be triggered by minor disturbances to the host galaxy, stronger disturbances are likely required to trigger luminous AGN. Major wet mergers of galaxies are ideal environments for AGN triggering since they provide large gas supplies and galaxy scale torques. There is however little observational evidence for a strong connection between AGN and major mergers. We analyse the morphological properties of AGN host galaxies as a function of AGN and host galaxy luminosity and compare them to a carefully matched sample of control galaxies. AGN are X-ray selected in the redshift range 0.5 < z < 0.8 and have luminosities 41 ≲ log (L_X [erg s^(−1)]) ≲ 44.5. ‘Fake AGN’ are simulated in the control galaxies by adding point sources with the magnitude of the matched AGN. We find that AGN host and control galaxies have comparable asymmetries, Sérsic indices and ellipticities at rest frame ∼950 nm. AGN host galaxies show neither higher average asymmetries nor higher fractions of very disturbed objects. There is no increase in the prevalence of merger signatures with AGN luminosity. At 95 per cent confidence we find that major mergers are responsible for <6 per cent of all AGN in our sample as well as <40 per cent of the highest luminosity AGN (log  (L_X [erg s^(−1)]) ∼ 43.5). Major mergers therefore either play only a very minor role in the triggering of AGN in the luminosity range studied or time delays are too long for merger features to remain visible

Dense Molecular Gas: A Sensitive Probe of Stellar Feedback Models

We show that the mass fraction of giant molecular cloud (GMC) gas (n ≳ 100 cm−3) in dense (n ≫ 104 cm−3) star-forming clumps, observable in dense molecular tracers (LHCN/LCO(1–0)), is a sensitive probe of the strength and mechanism(s) of stellar feedback, as well as the star formation efficiencies in the most dense gas. Using high-resolution galaxy-scale simulations with pc-scale resolution and explicit models for feedback from radiation pressure, photoionization heating, stellar winds and supernovae (SNe), we make predictions for the dense molecular gas tracers as a function of GMC and galaxy properties and the efficiency of stellar feedback/star formation. In models with weak/no feedback, much of the mass in GMCs collapses into dense subunits, predicting LHCN/LCO(1–0) ratios order-of-magnitude larger than observed. By contrast, models with feedback properties taken directly from stellar evolution calculations predict dense gas tracers in good agreement with observations. Changing the strength or timing of SNe tends to move systems along, rather than off, the LHCN–LCO relation (because SNe heat lower density material, not the high-density gas). Changing the strength of radiation pressure (which acts efficiently in the highest density gas), however, has a much stronger effect on LHCN than on LCO. We show that degeneracies between the strength of feedback, and efficiency of star formation on small scales, can be broken by the combination of dense gas, intermediate-density gas and total star formation rate (SFR) tracers, and favour models where the galaxy-integrated star formation efficiency in dense gas is low. We also predict that the fraction of dense gas (LHCN/LCO(1–0)) increases with increasing GMC surface density; this drives a trend in LHCN/LCO(1–0) with SFR and luminosity which has tentatively been observed. Our results make specific predictions for enhancements in the dense gas tracers in unusually dense environments such as ultraluminous infrared galaxies and galactic nuclei (including the galactic centre)

Numerical Problems in Coupling Photon Momentum (Radiation Pressure) to Gas

Radiation pressure (RP; or photon momentum absorbed by gas) is important in a tremendous range of astrophysical systems. But we show the usual method for assigning absorbed photon momentum to gas in numerical radiation-hydrodynamics simulations (integrating over cell volumes or evaluating at cell centers) can severely under-estimate the RP force in the immediate vicinity around un-resolved (point/discrete) sources (and subsequently under-estimate its effects on bulk gas properties), unless photon mean-free-paths are highly-resolved in the fluid grid. The existence of this error is independent of the numerical radiation transfer (RT) method (even in exact ray-tracing/Monte-Carlo methods), because it depends on how the RT solution is interpolated back onto fluid elements. Brute-force convergence (resolving mean-free paths) is impossible in many cases (especially where UV/ionizing photons are involved). Instead, we show a 'face-integrated' method -- integrating and applying the momentum fluxes at interfaces between fluid elements -- better approximates the correct solution at all resolution levels. The 'fix' is simple and we provide example implementations for ray-tracing, Monte-Carlo, and moments RT methods in both grid and mesh-free fluid schemes. We consider an example of star formation in a molecular cloud with UV/ionizing RP. At state-of-the-art resolution, cell-integrated methods under-estimate the net effects of RP by an order of magnitude, leading (incorrectly) to the conclusion that RP is unimportant, while face-integrated methods predict strong self-regulation of star formation and cloud destruction via RP.Comment: 9 pages, 4 figures. Updated to match accepted MNRAS versio

Galaxy disks do not need to survive in the L-CDM paradigm: the galaxy merger rate out to z~1.5 from morpho-kinematic data

About two-thirds of present-day, large galaxies are spirals such as the Milky Way or Andromeda, but the way their thin rotating disks formed remains uncertain. Observations have revealed that half of their progenitors, six billion years ago, had peculiar morphologies and/or kinematics, which exclude them from the Hubble sequence. Major mergers, i.e., fusions between galaxies of similar mass, are found to be the likeliest driver for such strong peculiarities. However, thin disks are fragile and easily destroyed by such violent collisions, which creates a critical tension between the observed fraction of thin disks and their survival within the L-CDM paradigm. Here we show that the observed high occurrence of mergers amongst their progenitors is only apparent and is resolved when using morpho-kinematic observations which are sensitive to all the phases of the merging process. This provides an original way of narrowing down observational estimates of the galaxy merger rate and leads to a perfect match with predictions by state-of-the-art L-CDM semi-empirical models with no particular fine-tuning needed. These results imply that half of local thin disks do not survive but are actually rebuilt after a gas-rich major merger occurring in the past nine billion years, i.e., two-thirds of the lifetime of the Universe. This emphasizes the need to study how thin disks can form in halos with a more active merger history than previously considered, and to investigate what is the origin of the gas reservoir from which local disks would reform.Comment: 19 pages, 7 figures, 2 tables. Accepted in ApJ. V2 to match proof corrections and added reference

How Much Mass do Supermassive Black Holes Eat in their Old Age?

We consider the distribution of local supermassive black hole Eddington ratios and accretion rates, accounting for the dependence of radiative efficiency and bolometric corrections on the accretion rate. We find that black hole mass growth, both of the integrated mass density and the masses of most individual objects, must be dominated by an earlier, radiatively efficient, high accretion rate stage, and not by the radiatively inefficient low accretion rate phase in which most local supermassive black holes are currently observed. This conclusion is particularly true of supermassive black holes in elliptical host galaxies, as expected if they have undergone merger activity in the past which would fuel quasar activity and rapid growth. We discuss models of the time evolution of accretion rates and show that they all predict significant mass growth in a prior radiatively efficient state. The only way to avoid this conclusion is through careful fine-tuning of the accretion/quasar timescale to a value that is inconsistent with observations. Our results agree with a wide range of observational inferences drawn from the quasar luminosity function and X-ray background synthesis models, but our approach has the virtue of being independent of the modeling of source populations. Models in which black holes spend the great majority of their time in low accretion rate phases are thus completely consistent both with observations implying mass gain in relatively short, high accretion rate phases and with the local distribution of accretion rates.Comment: 11 pages, 4 figures, matches version accepted to Ap

A Theoretical Interpretation of the Black Hole Fundamental Plane

We examine the origin and evolution of correlations between properties of supermassive black holes (BHs) and their host galaxies using simulations of major galaxy mergers, including the effects of gas dissipation, cooling, star formation, and BH accretion and feedback. We demonstrate that the simulations predict the existence of a BH 'fundamental plane' (BHFP), of the form M_BH sigma^(3.0+-0.3)*R_e^(0.43+-0.19) or M_BH M_bulge^(0.54+-0.17)*sigma^(2.2+-0.5), similar to relations found observationally. The simulations indicate that the BHFP can be understood roughly as a tilted intrinsic correlation between BH mass and spheroid binding energy, or the condition for feedback coupling to power a pressure-driven outflow. While changes in halo circular velocity, merger orbital parameters, progenitor disk redshifts and gas fractions, ISM gas pressurization, and other parameters can drive changes in e.g. sigma at fixed M_bulge, and therefore changes in the M_BH-sigma or M_BH-M_bulge relations, the BHFP is robust. Given the empirical trend of decreasing R_e for a given M_bulge at high redshift, the BHFP predicts that BHs will be more massive at fixed M_bulge, in good agreement with recent observations. This evolution in the structural properties of merger remnants, to smaller R_e and larger sigma (and therefore larger M_BH, conserving the BHFP) at a given M_bulge, is driven by the fact that bulge progenitors have characteristically larger gas fractions at high redshifts. Adopting the observed evolution of disk gas fractions with redshift, our simulations predict the observed trends in both R_e(M_bulge) and M_BH(M_bulge).Comment: 22 pages, 19 figures, replaced with version accepted to ApJ. Companion paper to arXiv:0707.400

Cluster Alignments and Ellipticities in LCDM Cosmology

The ellipticities and alignments of clusters of galaxies, and their evolution with redshift, are examined in the context of a Lambda-dominated cold dark matter cosmology. We use a large-scale, high-resolution N-body simulation to model the matter distribution in a light cone containing ~10^6 clusters out to redshifts of z=3. Cluster ellipticities are determined as a function of mass, radius, and redshift, both in 3D and in projection. We find strong cluster ellipticities: the mean ellipticity increases with redshift from 0.3 at z=0 to 0.5 at z=3, for both 3D and 2D ellipticities; the evolution is well-fit by e=0.33+0.05z. The ellipticities increase with cluster mass and with cluster radius; the main cluster body is more elliptical than the cluster cores, but the increase of ellipticities with redshift is preserved. Using the fitted cluster ellipsoids, we determine the alignment of clusters as a function of their separation. We find strong alignment of clusters for separations <100 Mpc/h; the alignment increases with decreasing separation and with increasing redshift. The evolution of clusters from highly aligned and elongated systems at early times to lower alignment and elongation at present reflects the hierarchical and filamentary nature of structure formation. These measures of cluster ellipticity and alignment will provide a new test of the current cosmological model when compared with upcoming cluster surveys.Comment: 29 pages including 13 figures, to appear in ApJ Jan. 2005 (corrected typos, added reference