arXiv subjects

Philip F. Hopkins

Publications and source records attributed to Philip F. Hopkins.

At least 343 records · Page 19Linked to original sources

arXiv2013

Dense Molecular Gas: A Sensitive Probe of Stellar Feedback Models

We show that the mass fraction of GMC gas (n>100 cm^-3) in dense (n>>10^4 cm^-3) star-forming clumps, observable in dense molecular tracers (L_HCN/L_CO(1-0)), is a sensitive probe of the strength and mechanism(s) of stellar feedback. 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. In models with weak/no feedback, much of the mass in GMCs collapses into dense sub-units, predicting L_HCN/L_CO(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 L_HCN-L_CO 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 L_HCN than on L_CO. We predict that the fraction of dense gas (L_HCN/L_CO(1-0)) increases with increasing GMC surface density; this drives a trend in L_HCN/L_CO(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 ULIRGs and galactic nuclei (including the galactic center).

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arXiv2013

Resolving The Generation of Starburst Winds in Galaxy Mergers

We study galaxy super-winds driven in major mergers, using pc-resolution simulations with detailed models for stellar feedback that can self-consistently follow the formation/destruction of GMCs and generation of winds. The models include molecular cooling, star formation at high densities in GMCs, and gas recycling and feedback from SNe (I&II), stellar winds, and radiation pressure. We study mergers of systems from SMC-like dwarfs and Milky Way analogues to z~2 starburst disks. Multi-phase super-winds are generated in all passages, with outflow rates up to ~1000 M_sun/yr. However, the wind mass-loading efficiency (outflow rate divided by SFR) is similar to that in isolated galaxy counterparts of each merger: it depends more on global galaxy properties (mass, size, escape velocity) than on the dynamical state of the merger. Winds tend to be bi- or uni-polar, but multiple 'events' build up complex morphologies with overlapping, differently-oriented bubbles/shells at a range of radii. The winds have complex velocity and phase structure, with material at a range of speeds up to ~1000 km/s, and a mix of molecular, ionized, and hot gas that depends on galaxy properties and different feedback mechanisms. These simulations resolve a problem in some 'sub-grid' models, where simple wind prescriptions can dramatically suppress merger-induced starbursts. But despite large mass-loading factors (>~10) in the winds, the peak SFRs are comparable to those in 'no wind' simulations. Wind acceleration does not act equally, so cold dense gas can still lose angular momentum and form stars, while blowing out gas that would not have participated in the starburst in the first place. Considerable wind material is not unbound, and falls back on the disk at later times post-merger, leading to higher post-starburst SFRs in the presence of stellar feedback. This may require AGN feedback to explain galaxy quenching.

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arXiv2013

Variations in the Stellar IMF: from Bottom to Top

We use a recently-developed analytic model for the ISM structure from scales of GMCs through star-forming cores to explore how the pre-stellar core mass function (CMF) and, by extrapolation, stellar initial mass function (IMF) should depend on both local and galactic properties. If the ISM is supersonically turbulent, the statistical properties of the density field follow from the turbulent velocity spectrum, and the excursion set formalism can be applied to analytically calculate the mass function of collapsing cores on the smallest scales on which they are self-gravitating (non-fragmenting). Two parameters determine the model: the disk-scale Mach number M_h (which sets the shape of the CMF), and the absolute velocity (to assign an absolute scale). For 'normal' variation in disk properties and core gas temperatures in the MW and local galaxies, there is almost no variation in the predicted high-mass behavior of the CMF/IMF. The slope is always close to Salpeter down to <1 M_sun. We predict modest variation in the sub-solar regime, mostly from variation in M_h, but within the observed scatter in sub-solar IMFs in local regions. For fixed galaxy properties, there is little variation in shape or 'upper mass limit' with parent GMC mass. However, in extreme starbursts (e.g. ULIRGs) we predict a bottom-heavy CMF. This agrees with the IMF inferred for the centers of Virgo ellipticals, believed to form in such a nuclear starburst. The CMF is bottom heavy despite the gas temperature being an order of magnitude larger, because M_h is also much larger. Larger M_h values make the 'parent' cloud mass (turbulent Jeans mass) larger, but promote fragmentation to smaller scales; this steepens the slope of the low-mass CMF and shifts the turnover mass. The model may predict a top-heavy CMF for the sub-pc disks around Sgr A*, but the relevant input parameters are uncertain.

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arXiv2013

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

We consider the effects of different star formation criteria on galactic scales, in high-resolution simulations with explicitly resolved GMCs and stellar feedback. 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 free-fall time), (2) a fixed density threshold, (3) a molecular-gas law, (4) a temperature threshold, (5) a Jeans-instability requirement, (6) a criteria that cooling times be shorter than dynamical times, and (7) a convergent-flow criterion. We consider these both MW-like and high-density (starburst) galaxies. With feedback present, all models produce identical integrated star formation rates (SFRs), in agreement with the Kennicutt relation. Without feedback all produce orders-of-magnitude excessive SFRs. This is totally dependent on feedback and independent of the SF law. However, the spatial and density distribution of SF depend strongly on the SF criteria. Because cooling rates are generally fast and gas is turbulent, criteria (4)-(7) are 'weak' and spread SF uniformly over the disk (above densities n~0.01-0.1 cm^-3). A molecular criterion (3) localizes to higher densities, but still a wide range; for Z Z_solar, it is similar to a density threshold at n~1 cm^-3 (well below mean densities in the MW center or starbursts). Fixed density thresholds (2) can always select the highest densities, but must be adjusted for simulation resolution and galaxy properties; the same threshold that works in a MW-like simulation will select nearly all gas in a starburst. Binding criteria (1) tend to adaptively select the largest over-densities, independent of galaxy model or resolution, and automatically predict clustered SF. We argue that this SF model is most physically-motivated and presents significant numerical advantages in large-dynamic range simulations.

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arXiv2013

Modeling Mid-Infrared Diagnostics of Obscured Quasars and Starbursts

We analyze the link between active galactic nuclei (AGN) and mid-infrared flux using dust radiative transfer calculations of starbursts realized in hydrodynamical simulations. Focusing on the effects of galaxy dust, we evaluate diagnostics commonly used to disentangle AGN and star formation in ultraluminous infrared galaxies (ULIRGs). We examine these quantities as a function of time, viewing angle, dust model, AGN spectrum, and AGN strength in merger simulations representing two possible extremes of the ULIRG population: one is a typical gas-rich merger at z ~ 0, and the other is characteristic of extremely obscured starbursts at z ~ 2 to 4. This highly obscured burst begins star-formation-dominated with significant PAH emission, and ends with a ~10^9 yr period of red near-IR colors. At coalescence, when the AGN is most luminous, dust obscures the near-infrared AGN signature, reduces the relative emission from polycyclic aromatic hydrocarbons (PAHs), and enhances the 9.7 micron absorption by silicate grains. Although generally consistent with previous interpretations, our results imply none of these indicators can unambiguously estimate the AGN luminosity fraction in all cases. Motivated by the simulations, we show that a combination of the extinction feature at 9.7 micron, the PAH strength, and a near-infrared slope can simultaneously constrain the AGN fraction and dust grain distribution for a wide range of obscuration. We find that this indicator, accessible to the James Webb Space Telescope, may estimate the AGN power as tightly as the hard X-ray flux alone, thereby providing a valuable future cross-check and constraint for large samples of distant ULIRGs.

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arXiv2012

Why Do Stars Form In Clusters? An Analytic Model for Stellar Correlation Functions

Recently, we have shown that if the ISM is governed by super-sonic turbulent flows, the excursion-set formalism can be used to calculate the statistics of self-gravitating objects over a wide range of scales. On the largest self-gravitating scales ('first crossing'), these correspond to GMCs, and on the smallest non-fragmenting self-gravitating scales ('last crossing'), to protostellar cores. Here, we extend this formalism to rigorously calculate the auto and cross-correlation functions of cores (and by extension, young stars) as a function of spatial separation and mass, in analogy to the cosmological calculation of halo clustering. We show that this generically predicts that star formation is very strongly clustered on small scales: stars form in clusters, themselves inside GMCs. Outside the binary-star regime, the projected correlation function declines as a weak power-law, until a characteristic scale which corresponds to the characteristic mass scale of GMCs. On much larger scales the clustering declines such that star formation is not strongly biased on galactic scales, relative to the actual dense gas distribution. The precise correlation function shape depends on properties of the turbulent spectrum, but its qualitative behavior is quite general. The predictions agree well with observations of young star and core autocorrelation functions over ~4 dex in radius. Clustered star formation is a generic consequence of supersonic turbulence if most of the power in the velocity field, hence the contribution to density fluctuations, comes from large scales. The distribution of self-gravitating masses near the sonic length is then imprinted by fluctuations on larger scales. We similarly show that the fraction of stars formed in 'isolated' modes should be small (\lesssim10%).

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arXiv2012

Stellar Feedback & Bulge Formation in Clumpy Disks

We use numerical simulations of isolated galaxies to study the effects of stellar feedback on the formation and evolution of giant star-forming gas 'clumps' in high-redshift, gas-rich galaxies. Such galactic disks are unstable to the formation of bound gas-rich clumps whose properties initially depend only on global disk properties, not the microphysics of feedback. In simulations without stellar feedback, clumps turn an order-unity fraction of their mass into stars and sink to the center, forming a large bulge and kicking most of the stars out into a much more extended stellar envelope. By contrast, strong radiative stellar feedback disrupts even the most massive clumps after they turn ~10-20% of their mass into stars, in a timescale of ~10-100 Myr, ejecting some material into a super-wind and recycling the rest of the gas into the diffuse ISM. This suppresses the bulge formation rate by direct 'clump coalescence' by a factor of several. However, the galactic disks do undergo significant internal evolution in the absence of mergers: clumps form and disrupt continuously and torque gas to the galactic center. The resulting evolution is qualitatively similar to bar/spiral evolution in simulations with a more homogeneous ISM.

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arXiv2012

Submillimetre galaxies in a hierarchical universe: number counts, redshift distribution, and implications for the IMF

High-redshift submillimetre galaxies (SMGs) are some of the most rapidly star-forming galaxies in the Universe. Historically, galaxy formation models have had difficulty explaining the observed number counts of SMGs. We combine a semi-empirical model with 3-D hydrodynamical simulations and 3-D dust radiative transfer to predict the number counts of unlensed SMGs. Because the stellar mass functions, gas and dust masses, and sizes of our galaxies are constrained to match observations, we can isolate uncertainties related to the dynamical evolution of galaxy mergers and the dust radiative transfer. The number counts and redshift distributions predicted by our model agree well with observations. Isolated disc galaxies dominate the faint (S_1.1 < ~1 mJy, or S_850 < ~2 mJy) population. The brighter sources are a mix of merger-induced starbursts and galaxy-pair SMGs; the latter subpopulation accounts for ~30 - 50 per cent of all SMGs at all S_1.1 > ~0.5 mJy (S_850 > ~1 mJy). The mean redshifts are ~3.0 - 3.5, depending on the flux cut, and the brightest sources tend to be at higher redshifts. Because the galaxy-pair SMGs will be resolved into multiple fainter sources by ALMA, the bright ALMA counts should be as much as 2 times less than those observed using single-dish telescopes. The agreement between our model, which uses a Kroupa IMF, and observations suggests that the IMF in high-redshifts starbursts need not be top-heavy; if the IMF were top-heavy, our model would over-predict the number counts. We conclude that the difficulty some models have reproducing the observed SMG counts is likely indicative of more general problems -- such as an under-prediction of the abundance of massive galaxies or a star formation rate--stellar mass relation normalisation lower than that observed -- rather than a problem specific to the SMG population.

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arXiv2012

3-D Radiative Transfer Calculations of Radiation Feedback from Massive Black Holes: Outflow of Mass from the Dusty "Torus"

Observational and theoretical arguments suggest that the momentum carried in mass outflows from AGN can reach several times L / c, corresponding to outflow rates of hundreds of solar masses per year. Radiation pressure on lines alone may not be sufficient to provide this momentum deposition, and the transfer of reprocessed IR radiation in dusty nuclear gas has been postulated to provide the extra enhancement. The efficacy of this mechanism, however, will be sensitive to multi-dimensional effects such as the tendency for the reprocessed radiation to preferentially escape along sight-lines of lower column density. We use Monte Carlo radiative transfer calculations to determine the radiation force on dusty gas residing within approximately 10 parsecs from an accreting super-massive black hole. We calculate the net rate of momentum deposition in the surrounding gas and estimate the mass-loss rate in the resulting outflow as a function of solid angle for different black hole luminosities, sightline-averaged column densities, clumping parameters, and opening angles of the dusty gas. We find that these dust-driven winds carry momentum fluxes of 1-5 times L / c and correspond to mass-loss rates of 10-100 solar masses per year for a 10^8 solar mass black hole radiating at or near its Eddington limit. These results help to explain the origin of high velocity molecular and atomic outflows in local ULIRGs, and can inform numerical simulations of galaxy evolution including AGN feedback.

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arXiv2012

An Excursion-Set Model for the Structure of GMCs and the ISM

The ISM is governed by supersonic turbulence on a range of scales. We use this to develop a rigorous excursion-set model for the formation and time evolution of dense gas structures (GMCs, massive clumps, and cores). Supersonic turbulence drives the density distribution to a lognormal with dispersion increasing with Mach number; we generalize this to include scales >h (the disk scale height), and use it to construct the statistical properties of the density field smoothed on a scale R. We then compare conditions for self-gravitating collapse including thermal, turbulent, and rotational support. We show this becomes a well-defined barrier crossing problem. As such, an exact 'bound object mass function' can be derived, from scales of the sonic length to above the disk Jeans mass. This agrees remarkably well with observed GMC mass functions in the MW and other galaxies; the only inputs are the mass and size of the galaxies (to normalize the model). This explains the mass function cutoff and its power-law slope (close to, but shallower than, -2). The model also predicts the linewidth-size and size-mass relations of clouds and the dependence of their residuals on surface density/pressure. We use this to predict the spatial correlation function/clustering of clouds and star clusters; these also agree well with observations. We predict the size/mass function of ISM 'bubbles' or 'holes', and show this can account for observed HI hole distributions without any local feedback. We generalize the model to construct time-dependent 'merger/fragmentation trees' which can be used to follow cloud evolution and construct semi-analytic models for the ISM. We provide explicit recipes to construct the trees. We use a simple example to show that, if clouds are not destroyed in ~1-5 crossing times, then all ISM mass would be trapped in collapsing objects even if the large-scale turbulence were maintained.

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arXiv2012

The Stellar IMF, Core Mass Function, & The Last-Crossing Distribution

Hennebelle & Chabrier 2008 (HC08) attempted to derive the stellar IMF as a consequence of turbulent density fluctuations, using an argument similar to Press & Schechter 1974 for Gaussian random fields. Like that example, however, this solution does not resolve the 'cloud in cloud' problem; it also does not extend to large scales that dominate the velocity/density fluctuations. In principle, these can change the results at the order-of-magnitude level. Here, we use the results from Hopkins 2011 (H11) to generalize the excursion set formalism and derive the exact solution in this regime. We argue that the stellar IMF and core mass function (CMF) should be associated with the last-crossing distribution, i.e. the mass spectrum of bound objects defined on the smallest scale on which they are self-gravitating. This differs from the first-crossing distribution (mass function on the largest self-gravitating scale) which is defined cosmologically and which H11 show corresponds to the GMC mass function in disks. We derive an analytic equation for the last-crossing distribution that can be used for an arbitrary collapse threshold in ISM and cosmological studies. With this, we show that the same model that predicts the GMC mass function and large-scale structure of galaxy disks also predicts the CMF (and by extrapolation IMF) in good agreement with observations. The only adjustable parameter in the model is the turbulent velocity power spectrum, which in the range p~5/3-2 gives similar results. We also use this to justify why the approximate solution in HC08 is reasonable (up to a normalization) over the CMF/IMF mass range; however there are significant corrections at intermediate and high masses. We discuss how the exact solutions here can be embedded into time-dependent models that follow density fluctuations, fragmentation, successive generations of star formation.

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arXiv2012

CANDELS: The progenitors of compact quiescent galaxies at z~2

We combine high-resolution HST/WFC3 images with multi-wavelength photometry to track the evolution of structure and activity of massive (log(M*) > 10) galaxies at redshifts z = 1.4 - 3 in two fields of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). We detect compact, star-forming galaxies (cSFGs) whose number densities, masses, sizes, and star formation rates qualify them as likely progenitors of compact, quiescent, massive galaxies (cQGs) at z = 1.5 - 3. At z > 2 most cSFGs have specific star-formation rates (sSFR = 10^-9 yr^-1) half that of typical, massive SFGs at the same epoch, and host X-ray luminous AGN 30 times (~30%) more frequently. These properties suggest that cSFGs are formed by gas-rich processes (mergers or disk-instabilities) that induce a compact starburst and feed an AGN, which, in turn, quench the star formation on dynamical timescales (few 10^8 yr). The cSFGs are continuously being formed at z = 2 - 3 and fade to cQGs by z = 1.5. After this epoch, cSFGs are rare, thereby truncating the formation of new cQGs. Meanwhile, down to z = 1, existing cQGs continue to enlarge to match local QGs in size, while less-gas-rich mergers and other secular mechanisms shepherd (larger) SFGs as later arrivals to the red sequence. In summary, we propose two evolutionary scenarios of QG formation: an early (z > 2), fast-formation path of rapidly-quenched cSFGs that evolve into cQGs that later enlarge within the quiescent phase, and a slow, late-arrival (z < 2) path for SFGs to form QGs without passing through a compact state.

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arXiv2012

The Structure of the Interstellar Medium of Star Forming Galaxies

We present numerical methods for including stellar feedback in galaxy-scale simulations. We include heating by SNe (I & II), gas recycling and shock-heating from O-star & AGB winds, HII photoionization, and radiation pressure from stellar photons. The energetics and time-dependence are taken directly from stellar evolution models. We implement these in simulations with pc-scale resolution, modeling galaxies from SMC-like dwarfs and MW analogues to massive z~2 starburst disks. Absent feedback, gas cools and collapses without limit. With feedback, the ISM reaches a multi-phase steady state in which GMCs continuously form, disperse, and re-form. Our primary results include: (1) Star forming galaxies generically self-regulate at Toomre Q~1. Most of the volume is in diffuse hot gas with most of the mass in dense GMC complexes. The phase structure and gas mass at high densities are much more sensitive probes of stellar feedback physics than integrated quantities (Toomre Q or gas velocity dispersion). (2) Different feedback mechanisms act on different scales: radiation & HII pressure are critical to prevent runaway collapse of dense gas in GMCs. SNe and stellar winds dominate the dynamics of volume-filling hot gas; however this primarily vents out of the disk. (3) The galaxy-averaged SFR is determined by feedback. For given feedback efficiency, restricting star formation to molecular gas or modifying the cooling function has little effect; but changing feedback mechanisms directly translates to shifts off the Kennicutt-Schmidt relation. (4) Self-gravity leads to marginally-bound GMCs with an ~M^-2 mass function with a cutoff at the Jeans mass; they live a few dynamical times before being disrupted by stellar feedback and turn ~1-10% of their mass into stars (increasing from dwarfs through starburst galaxies). Low-mass GMCs are preferentially unbound.

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arXiv2012

Stellar Feedback in Galaxies and the Origin of Galaxy-scale Winds

Feedback from massive stars is believed to play a critical role in driving galactic super-winds that enrich the IGM and shape the galaxy mass function and mass-metallicity relation. In previous papers, we introduced new numerical methods for implementing stellar feedback on sub-GMC through galactic scales in galaxy simulations. This includes radiation pressure (UV through IR), SNe (Type-I & II), stellar winds ('fast' O-star through 'slow' AGB winds), and HII photoionization. Here, we show that these feedback mechanisms drive galactic winds with outflow rates as high as ~10-20 times the galaxy SFR. The mass-loading efficiency (wind mass loss rate divided by SFR) scales inversely with circular velocity, consistent with momentum-conservation expectations. We study the contributions of each feedback mechanism to galactic winds in a range of galaxy models, from SMC-like dwarfs & MW-analogues to z~2 clumpy disks. In massive, gas-rich systems (local starbursts and high-z galaxies), radiation pressure dominates the wind generation. For MW-like spirals and dwarf galaxies the gas densities are much lower, and shock-heated gas from SNe and stellar winds dominates production of large-scale outflows. In all models, however, winds have a multi-phase structure that depends on interactions between multiple feedback mechanisms operating on different spatial & time scales: any single mechanism fails to reproduce the winds observed. We provide fitting functions for wind mass-loading and velocities as a function of galaxy properties, for use in cosmological simulations and semi-analytic models. These differ from typically-adopted formulae with explicit dependence on gas surface density that can be very important in both low-density dwarf galaxies and high-density gas-rich galaxies.

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arXiv2012

Why Are AGN and Host Galaxies Misaligned?

It is well-established observationally that the characteristic angular momentum axis on small scales around AGN, traced by radio jets and the putative torus, is not well-correlated with the large-scale angular momentum axis of the host galaxy. In this paper, we show that such misalignments arise naturally in high-resolution simulations in which we follow angular momentum transport and inflows from galaxy to sub-pc scales near AGN, triggered either during galaxy mergers or by instabilities in isolated disks. Sudden misalignments can sometimes be caused by single massive clumps falling into the center slightly off-axis, but more generally, they arise even when the gas inflows are smooth and trace only global gravitational instabilities. When several nested, self-gravitating modes are present, the inner ones can precess and tumble in the potential of the outer modes. Resonant angular momentum exchange can flip or re-align the spin of an inner mode on a short timescale, even without the presence of massive clumps. We therefore do not expect that AGN and their host galaxies will be preferentially aligned, nor should the relative alignment be an indicator of the AGN fueling mechanism. We discuss implications of this conclusion for AGN feedback and BH spin evolution. The misalignments may mean that even BHs accreting from smooth large-scale disks will not be spun up to maximal rotation, and so have more modest radiative efficiencies and inefficient jet formation. Even more random orientations are possible if there is further, un-resolved clumpiness in the gas, and more ordered accretion may occur if the inflow is slower and not self-gravitating.

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arXiv2012

An Analytic Model of Angular Momentum Transport by Gravitational Torques: From Galaxies to Massive Black Holes

We present analytic calculations of angular momentum transport and gas inflow in galaxies, from scales of ~kpc to deep in the potential of a central black hole (BH). We compare these analytic calculations to numerical simulations and use them to develop a sub-grid model of BH growth that can be incorporated into semi-analytic models or cosmological simulations. Both analytic calculations and simulations argue that the strongest torque on gas arises when non-axisymmetric perturbations to the stellar gravitational potential produces orbit crossings and shocks in the gas. This is true both at large radii ~0.01-1 kpc, where bar-like modes dominate the non-axisymmetric potential, and at smaller radii <10 pc, where a lopsided/eccentric disk dominates. The traditional orbit crossing criterion is not always adequate to predict the locations of, and inflow due to, shocks in gas+stellar disks with finite sound speeds. We derive a modified criterion that predicts the presence of shocks in stellar dominated systems even absent formal orbit crossing. We then derive analytic expressions for the loss of angular momentum and the resulting gas inflow rates in the presence of such shocks. We test our analytic predictions using hydrodynamic simulations at a range of galactic scales, and show that they successfully predict the mass inflow rates and quasi-steady gas surface densities with small scatter (0.3 dex). We use our analytic results to construct a new estimate of the BH accretion rate given galaxy properties at larger radii. This captures the key scalings in the numerical simulations. Alternate estimates such as the local viscous accretion rate or the spherical Bondi rate fail systematically to reproduce the simulations.

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arXiv2012

An Explanation for the Slopes of Stellar Cusps in Galaxy Spheroids

The stellar surface mass density profiles at the centers of typical ~L* and lower-mass spheroids exhibit power law 'cusps' with $Σ\propto R^(-n)$, where 0.5<n<1 for radii ~1-100 pc. Observations and theory support models in which these cusps are formed by dissipative gas inflows and nuclear starbursts in gas-rich mergers. At these comparatively large radii, stellar relaxation is unlikely to account for or strongly modify the cuspy stellar profiles. We argue that the power-law surface density profiles observed are a natural consequence of the gravitational instabilities that dominate angular momentum transport in the gravitational potential of a central massive black hole. The dominant mode at these radii is an m=1 lopsided/eccentric disk instability, in which stars torquing the gas can drive rapid inflow and accretion. Such a mode first generically appears at large radii and propagates inwards by exciting eccentricities at smaller and smaller radii, where M*(<R)<<M_BH. When the stellar surface density profile is comparatively shallow with n<1/2, the modes cannot efficiently propagate to R=0 and so gas piles up and star formation steepens the profile. But if the profile is steeper than n=1, the inwards propagation of eccentricity is strongly damped, suppressing inflow and bringing n down again. Together these results produce an equilibrium slope of 1/2 < n < 1 in the potential of the central black hole. These physical arguments are supported by nonlinear numerical simulations of gas inflow in galactic nuclei. Together, these results naturally explain the observed stellar density profiles of 'cusp' elliptical galaxies.

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arXiv2011

The Origins of AGN Obscuration: The 'Torus' as a Dynamical, Unstable Driver of Accretion

Multi-scale simulations have made it possible to follow gas inflows onto massive black holes (BHs) from galactic scales to the accretion disk. When sufficient gas is driven towards the BH, gravitational instabilities generically form lopsided, eccentric disks that propagate inwards. The lopsided stellar disk exerts a strong torque on the gas disk, driving inflows that fuel rapid BH growth. Here, we investigate whether the same gas disk is the 'torus' invoked to explain obscured AGN. The disk is generically thick and has characteristic ~1-10 pc sizes and masses resembling those required of the torus. The scale heights and obscured fractions of the predicted torii are substantial even in the absence of strong stellar feedback providing the vertical support. Rather, they can be maintained by strong bending modes and warps excited by the inflow-generating instabilities. Other properties commonly attributed to feedback processes may be explained by dynamical effects: misalignment between torus and host galaxy, correlations between local SFR and turbulent gas velocities, and dependence of obscured fractions on AGN luminosity or SFR. We compare the predicted torus properties with observations of gas surface density profiles, kinematics, scale heights, and SFR densities in AGN nuclei, and find that they are consistent. We argue that it is not possible to reproduce these observations and the observed column density (N_H) distribution without a clumpy gas distribution, but allowing for clumping on small scales the predicted N_H distribution is in good agreement with observations from 10^20-27 cm^-2. We examine how N_H scales with galaxy and AGN properties, and find that AGN feedback may be necessary to explain some trends with luminosity and/or redshift. The torus is not merely a bystander or passive fuel source for accretion, but is itself the mechanism driving accretion.

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