arXiv subjects

Philip F. Hopkins

Publications and source records attributed to Philip F. Hopkins.

At least 361 records · Page 20Linked to original sources

arXiv2011

Dynamical Delays Between Starburst and AGN Activity in Galaxy Nuclei

Observations of AGN have suggested a possible delay between the peak of star formation (on some scale) and AGN activity. Inefficient fueling (and/or feedback) from fast stellar winds has been invoked to explain this, but we argue this is unlikely in bright systems accreting primarily cold dense gas. We show that such a delay can arise even in bright quasars for purely dynamical reasons. If some large-scale process produces rapid inflow, smaller scales will quickly become gas-dominated. As the gas density peaks, so does the SFR. However, gravitational torques which govern further inflow are relatively inefficient in gas-dominated systems; as more gas is turned into stars, the stars provide an efficient angular momentum sink allowing more rapid inflow. Moreover, the gas provided to the central regions in mergers or strong disk instabilities will typically be ~100 times larger than that needed to fuel the BH; the system is effectively in the 'infinite gas supply' limit. BH growth can therefore continue for some time while the gas supply exhausts, until it has significantly depleted to the point where the BH is finally 'starved.' Both of these effects act together with comparable magnitude, and mean that the peak of BH growth can lag the peak in the SFR measured at a given scale by a timescale corresponding to the gas exhaustion time on that scale (~ 10-100 local dynamical times). This predicts that the inferred delay will vary in a specific manner with the radius over which the star formation rate is measured. We discuss possible implications for the role of AGN feedback in suppressing star formation activity.

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arXiv2011

CANDELS: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey - The Hubble Space Telescope Observations, Imaging Data Products and Mosaics

This paper describes the Hubble Space Telescope imaging data products and data reduction procedures for the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS). This survey is designed to document the evolution of galaxies and black holes at $z\sim1.5-8$, and to study Type Ia SNe beyond $z>1.5$. Five premier multi-wavelength sky regions are selected, each with extensive multiwavelength observations. The primary CANDELS data consist of imaging obtained in the Wide Field Camera 3 / infrared channel (WFC3/IR) and UVIS channel, along with the Advanced Camera for Surveys (ACS). The CANDELS/Deep survey covers \sim125 square arcminutes within GOODS-N and GOODS-S, while the remainder consists of the CANDELS/Wide survey, achieving a total of \sim800 square arcminutes across GOODS and three additional fields (EGS, COSMOS, and UDS). We summarize the observational aspects of the survey as motivated by the scientific goals and present a detailed description of the data reduction procedures and products from the survey. Our data reduction methods utilize the most up to date calibration files and image combination procedures. We have paid special attention to correcting a range of instrumental effects, including CTE degradation for ACS, removal of electronic bias-striping present in ACS data after SM4, and persistence effects and other artifacts in WFC3/IR. For each field, we release mosaics for individual epochs and eventual mosaics containing data from all epochs combined, to facilitate photometric variability studies and the deepest possible photometry. A more detailed overview of the science goals and observational design of the survey are presented in a companion paper.

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arXiv2011

CANDELS: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey

The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) is designed to document the first third of galactic evolution, over the approximate redshift (z) range 8--1.5. It will image >250,000 distant galaxies using three separate cameras on the Hubble Space Telescope, from the mid-ultraviolet to the near-infrared, and will find and measure Type Ia supernovae at z>1.5 to test their accuracy as standardizable candles for cosmology. Five premier multi-wavelength sky regions are selected, each with extensive ancillary data. The use of five widely separated fields mitigates cosmic variance and yields statistically robust and complete samples of galaxies down to a stellar mass of 10^9 M_\odot to z \approx 2, reaching the knee of the ultraviolet luminosity function (UVLF) of galaxies to z \approx 8. The survey covers approximately 800 arcmin^2 and is divided into two parts. The CANDELS/Deep survey (5σ point-source limit H=27.7 mag) covers \sim 125 arcmin^2 within GOODS-N and GOODS-S. The CANDELS/Wide survey includes GOODS and three additional fields (EGS, COSMOS, and UDS) and covers the full area to a 5σ point-source limit of H \gtrsim 27.0 mag. Together with the Hubble Ultra Deep Fields, the strategy creates a three-tiered "wedding cake" approach that has proven efficient for extragalactic surveys. Data from the survey are nonproprietary and are useful for a wide variety of science investigations. In this paper, we describe the basic motivations for the survey, the CANDELS team science goals and the resulting observational requirements, the field selection and geometry, and the observing design. The Hubble data processing and products are described in a companion paper.

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arXiv2011

Self-Regulated Star Formation in Galaxies via Momentum Input from Massive Stars

Feedback from massive stars is believed to play a critical role in shaping the galaxy mass function, the structure of the interstellar medium (ISM), and the low efficiency of star formation, but the exact form of the feedback is uncertain. In this paper, the first in a series, we present and test a novel numerical implementation of stellar feedback resulting from momentum imparted to the ISM by radiation, supernovae, and stellar winds. We employ a realistic cooling function, and find that a large fraction of the gas cools to <100K, so that the ISM becomes highly inhomogeneous. Despite this, our simulated galaxies reach an approximate steady state, in which gas gravitationally collapses to form giant molecular clouds (GMCs), dense clumps, and stars; subsequently, stellar feedback disperses the GMCs, repopulating the diffuse ISM. This collapse and dispersal cycle is seen in models of SMC-like dwarfs, the Milky-Way, and z~2 clumpy disk analogues. The simulated global star formation efficiencies are consistent with the observed Kennicutt-Schmidt relation. Moreover, the star formation rates are nearly independent of the numerically imposed high-density star formation efficiency, density threshold, and density scaling. This is a consequence of the fact that, in our simulations, star formation is regulated by stellar feedback limiting the amount of very dense gas available for forming stars. In contrast, in simulations without stellar feedback, i.e. under the action of only gravity and gravitationally-induced turbulence, the ISM experiences runaway collapse to very high densities. In these simulations without feedback, the global star formation rates exceed observed galactic star formation rates by 1-2 orders of magnitude, demonstrating that stellar feedback is crucial to the regulation of star formation in galaxies.

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arXiv2011

A Multiwavelength Study of Binary Quasars and Their Environments

We present Chandra X-ray imaging and spectroscopy for 14 quasars in spatially resolved pairs, part of a complete sample of binary quasars with small transverse separations drawn from Sloan Digital Sky Survey (DR6) photometry. We find no significant difference in X-ray properties when compared with large control samples of isolated quasars. We present infrared photometry from our observations with SWIRC at the MMT, and from the WISE Preliminary Data Release, and fit simple spectral energy distributions to all 14 QSOs. We find preliminary evidence that substantial contributions from star formation are required, but possibly no more so than for isolated X-ray-detected QSOs. Sensitive searches of the X-ray images for extended emission, and the optical images for optical galaxy excess show that these binary QSOs are not preferentially found in rich cluster environments. While larger binary QSO samples with richer far-IR and sub-millimeter multiwavelength data might better reveal signatures of merging and triggering, optical color-selection of QSO pairs may be biased against such signatures. X-ray and/or variability selection of QSO pairs, while challenging, should be attempted. We present in our Appendix a primer on X-ray flux and luminosity calculations.

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arXiv2011

Relation Between Globular Clusters and Supermassive Black Holes in Ellipticals as a Manifestation of the Black Hole Fundamental Plane

We analyze the relation between the mass of the central supermassive black hole (Mbh) and the number of globular clusters (Ngc) in elliptical galaxies and bulges as a ramification of the black hole fundamental plane, the theoretically predicted and observed multi-variable correlation between Mbh and bulge binding energy. Although the tightness of the Mbh-Ngc correlation suggests an unlikely causal link between supermassive black holes and globular clusters, such a correspondence can exhibit small scatter even if the physical relationship is indirect. We show that the relatively small scatter of the Mbh-Ngc relation owes to the mutual residual correlation of Mbh and Ngc with stellar mass when the velocity dispersion is held fixed. Thus, present observations lend evidence for feedback-regulated models in which the bulge binding energy is most important; they do not necessarily imply any `special' connection between globular clusters and Mbh. This raises the question of why Ngc traces the formation of ellipticals and bulges sufficiently well to be correlated with binding energy.

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arXiv2010

Mergers in Lambda-CDM: Uncertainties in Theoretical Predictions and Interpretations of the Merger Rate

Different methodologies lead to order-of-magnitude variations in predicted galaxy merger rates. We examine and quantify the dominant uncertainties. Different halo merger rates and subhalo 'destruction' rates agree to within a factor ~2 given proper care in definitions. If however (sub)halo masses are not appropriately defined or are under-resolved, the major merger rate can be dramatically suppressed. The dominant differences in galaxy merger rates owe to baryonic physics. Hydrodynamic simulations without feedback and older models that do not agree with the observed galaxy mass function propagate factor ~5 bias in the resulting merger rates. However, if the model matches the galaxy mass function, properties of central galaxies are sufficiently converged to give small differences in merger rates. But variations in baryonic physics of satellites also have dramatic effects. The known problem of satellite 'over-quenching' in most semi-analytic models (SAMs), whereby SAM satellites are too efficiently stripped of gas, could lead to order-of-magnitude under-estimates of merger rates for low-mass, gas-rich galaxies. Fixing the satellite properties to observations tends to predict higher merger rates, but with factor ~2 empirical uncertainties. Choice of mass ratio definition matters: at low masses, most true major mergers (in baryonic/dynamical galaxy mass) will appear to be minor mergers in their stellar or luminosity mass ratio. Observations and models using these criteria may underestimate major merger rates by factors ~5. Orbital parameters and gas fractions also introduce factor ~3 differences in amount of bulge formed by mergers, even for fixed mass ratio encounters.

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arXiv2010

Origins, Structure, and Inflows of m=1 Modes in Quasi-Keplerian Disks

Simulations show eccentric disks (m=1 modes) forming around quasi-Keplerian potentials, a topic of interest for fueling quasars, forming super-massive BHs, planet formation and migration, explaining the origin and properties of nuclear eccentric stellar disks like that in M31, and driving the formation of the obscuring AGN torus. We consider the global, linear normal m=1 modes in collisionless disks, without the restriction that the disk mass be negligible relative to the central (Keplerian) mass. We derive their structure and key resonance features, and show how they arise, propagate inwards, and drive both inflow/outflow and eccentricities in the disk. We compare with hydrodynamic simulations of such disks around a super-massive BH, with star formation, gas cooling, and feedback. We derive the dependence of the normal mode structure on disk structure, mass profiles, and thickness, and mode pattern speeds and growth rates. We show that, if the disk at some radii has mass of >~10% the central point mass, the modes are linearly unstable and are self-generating. They arise as 'fast modes' with pattern speed of order the local angular velocity at these radii. The characteristic global normal modes have pattern speeds comparable to the linear growth rate, of order (G*M_0*R_0^{-3})^{1/2}, where M_0 is the central mass and R_{0} is the radius where the enclosed disk mass ~M_{0}. They propagate inwards by exciting eccentricities towards smaller and smaller radii, until at small radii these are 'slow modes.' With moderate amplitude, the global normal modes can lead to shocks and significant gas inflows at near-Eddington rates at all radii inside several ~R_0.

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arXiv2010

Do sub-millimeter galaxy number counts provide evidence for a top-heavy IMF?

Theoretical models have had difficulty matching the observed number density of sub-millimeter galaxies (SMGs), causing some authors (e.g., Baugh et al. 2005) to suggest that SMGs provide evidence for a top-heavy initial mass function (IMF). To test this claim, we have, for the first time, combined high-resolution 3-D hydrodynamic simulations of isolated and merging massive, gas-rich galaxies, radiative transfer, and a semi-empirical merger rate model to predict the number density of SMGs. Our model can reproduce the observed SMG number density even when using a standard (Kroupa) IMF. The agreement is due to a combination of relatively long sub-mm duty cycles for mergers (a few times 10^8 years for our most massive models), which owe to our combination of high-resolution 3-D hydrodynamic simulations and dust radiative transfer; sufficient number densities of massive, gas-rich mergers; and the decrease in sub-mm counts observed by recent deep/wide surveys (e.g., Austermann et al. 2010) relative to previous surveys. Our results suggest that the observed SMG number counts do not provide evidence for a top-heavy IMF at high redshift.

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arXiv2010

On Sizes, Kinematics, M/L Gradients, and Light Profiles of Massive Compact Galaxies at z~2

We present a detailed analysis of the structure and resolved stellar populations of simulated merger remnants, and compare them to observations of compact quiescent galaxies at z ~ 2. We find that major merging is a viable mechanism to produce systems of ~ 10^11 Msun and ~ 1 kpc size, provided the gas fraction at the time of final coalescence is high (~ 40%), and provided that the progenitors are compact star-forming galaxies, as expected at high redshift. Their integrated spectral energy distributions and velocity dispersions are in good agreement with the observations, and their position in the (v_{maj}/sigma, ellipticity) diagram traces the upper envelope of the distribution of lower redshift early-type galaxies. The simulated merger remnants show time- and sightline-dependent M/L ratio gradients that result from a superposition of radially dependent stellar age, stellar metallicity, and extinction. The median ratio of effective radius in rest-frame V-band light to that in mass surface density is ~ 2 during the quiescent remnant phase. This is typically expressed by a negative color gradient (i.e., red core), which we expect to correlate with the integrated color of the system. Finally, the simulations differ from the observations in their surface brightness profile shape. The simulated remnants are typically best fit by high (n >> 4) Sersic indices, whereas observed quiescent galaxies at z ~ 2 tend to be less cuspy (median n ~ 2.3). Limiting early star formation in the progenitors may be required to prevent the simulated merger remnants from having extended wings.

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arXiv2010

A Non-Parametric Estimate of Mass 'Scoured' in Galaxy Cores

We present a simple estimate of the mass 'deficits' in cored spheroids, as a function of galaxy mass and radius within the galaxy. Previous attempts to measure such deficits depended on fitting some functional form to the profile at large radii and extrapolating inwards; this is sensitive to the assumed functional form and does not allow for variation in nuclear profile shapes. We take advantage of larger data sets to directly construct stellar mass profiles of observed systems and measure the stellar mass enclosed in a series of physical radii (M(<R)), for samples of cusp and core spheroids at the same stellar mass. There is a significant bimodality in this distribution at small radii, and we non-parametrically measure the median offset between core and cusp populations (the deficit Delta_M(<R)). We construct the scoured mass profile as a function of radius, without reference to any assumed functional form. The mass deficit rises in power-law fashion (Delta_M(<R) R^{1.3-1.8}) from a significant but small mass at R<10pc, to asymptote to a maximum ~0.5-2 M_BH at ~100pc. At larger radii there is no statistically significant separation between populations; the upper limit to the cumulative scoured mass at ~kpc is ~2-4 M_BH. This does not depend strongly on stellar mass. The dispersion in M(<R) appears larger in the core population, possibly reflecting the fact that scouring increases the scatter in profile shapes. These results are in good agreement with models of scouring from BH binary systems.

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arXiv2010

How Do Massive Black Holes Get Their Gas?

We use multi-scale SPH simulations to follow the inflow of gas from galactic scales to <0.1pc, where the gas begins to resemble a traditional Keplerian accretion disk. The key ingredients are gas, stars, black holes (BHs), self-gravity, star formation, and stellar feedback. We use ~100 simulations to survey a large parameter space of galaxy properties and subgrid models for the ISM physics. We generate initial conditions for our simulations of galactic nuclei (<~300pc) using galaxy scale simulations, including both major mergers and isolated bar-(un)stable disk galaxies. For sufficiently gas-rich, disk-dominated systems, a series of gravitational instabilities generates large accretion rates of up to 1-10 M_sun/yr onto the BH (at <<0.1pc); sufficient to fuel the most luminous quasars. The BH accretion rate is highly time variable, given fixed conditions at ~kpc. At >~10pc, our simulations resemble the 'bars within bars' model, but the gas exhibits diverse morphologies, including spirals, rings, clumps, and bars; their duty cycle is modest, complicating attempts to correlate BH accretion with nuclear morphology. At ~1-10pc, the gravitational potential becomes dominated by the BH and bar-like modes are no longer present. However, the gas becomes unstable to a standing, eccentric disk or a single-armed spiral mode (m=1), driving the gas to sub-pc scales. Proper treatment of this mode requires including star formation and the self-gravity of both the stars and gas. We predict correlations between BHAR and SFR at different galactic nuclei: nuclear SF is more tightly coupled to AGN activity, but correlations exist at all scales.

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arXiv2010

The Nuclear Stellar Disk in Andromeda: A Fossil from the Era of Black Hole Growth

The physics of angular momentum transport from galactic scales (~10-100 pc) to much smaller radii is one of the oustanding problems in our understanding of the formation and evolution of super-massive black holes (BHs). Seemingly unrelated observations have discovered that there is a lopsided stellar disk of unknown origin orbiting the BH in M31, and possibly many other systems. We show that these nominally independent puzzles are in fact closely related. Multi-scale simulations of gas inflow from galactic to BH scales show that when sufficient gas is driven towards a BH, gravitational instabilities form a lopsided, eccentric disk that propagates inwards from larger radii. The lopsided stellar disk exerts a strong torque on the remaining gas, driving inflows that fuel the growth of the BH and produce quasar-level luminosities. The same disk can produce significant obscuration along many sightlines and thus may be the putative 'torus' invoked to explain obscured active galactic nuclei and the cosmic X-ray background. The stellar relic of this disk is long lived and retains the eccentric pattern. Simulations that yield quasar-level accretion rates produce relic stellar disks with kinematics, eccentric patterns, precession rates, and surface density profiles in reasonable agreement with observations of M31. The observed properties of nuclear stellar disks can thus be used to constrain the formation history of super-massive BHs.

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arXiv2010

A physical model for the origin of the diffuse cosmic infrared background

We present a physical model for origin of the cosmic diffuse infrared background (CDIRB). By utilizing the observed stellar mass function and its evolution as input to a semi-empirical model of galaxy formation, we isolate the physics driving diffuse IR emission. The model includes contributions from three primary sources of IR emission: steady-state star formation owing to isolated disk galaxies, interaction-driven bursts of star formation owing to close encounters and mergers, and obscured active galactic nuclei (AGN). We find that most of the CDIRB is produced by equal contributions from objects at z=0.5-1 and z>1, as suggested by recent observations. Of those sources, the vast majority of the emission originates in systems with low to moderate IR luminosities (L_{IR}<10^{12} $L_sun); the most luminous objects contribute significant flux only at high-redshifts (z>2). All star formation in ongoing mergers accounts for <10% of the total at all wavelengths and redshifts, while emission directly attributable to the interaction-driven burst itself accounts for <5%. We furthermore find that obscured AGN contribute <1-2% of the CDIRB at all wavelengths and redshifts, with a strong upper limit of less than 4% of the total emission. Finally, since electron-positron pair production interactions with the CDIRB represent the primary source of opacity to very high energy (VHE: E_γ> 1 TeV) γ-rays, the model provides predictions for the optical depth of the Universe to the most energetic photons. We find that these predictions agree with observations of high-energy cutoffs at TeV energies in nearby blazars, and suggest that while the Universe is extremely optically thick at >10 TeV, the next generation of VHE γ-ray telescopes can reasonably expect detections from out to 50-150 Mpc.

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arXiv2010

Mergers and Bulge Formation in Lambda-CDM: Which Mergers Matter?

We use a suite of semi-empirical models to predict galaxy merger rates and contributions to bulge growth as functions of merger mass, redshift, and mass ratio. The models use empirical halo occupation constraints to identify mergers, together with high-resolution simulations to quantify how mergers with different properties contribute to the bulge population. We find good agreement with a variety of observational constraints, and provide fitting functions for merger rates and contributions to bulge growth. We identify several robust conclusions. (1) Major mergers dominate formation and assembly of L* bulges and the spheroid mass density, minor mergers contribute ~30%. (2) This is mass-dependent: bulge formation is dominated by more minor mergers in lower-mass systems. At higher masses, bulges form in major mergers near L*, but subsequently assemble in minor mergers. (3) The minor/major contribution is also morphology-dependent: higher B/T systems form in more major mergers, lower B/T systems form in situ from minor mergers. (4) Low-mass galaxies, being gas-rich, require more major mergers to reach the same B/T as high-mass systems. (5) Absolute merger rates increase with galaxy mass. (6) Predicted rates agree well with observations, but suggest that some morphology-selected samples include contamination from minor mergers. (7) Predicted rates agree with integrated growth in bulge mass with cosmic time, but with factor ~2 uncertainty - half the bulge mass density could come from non-mergers. We consider ~1000 model variations and quantify resulting uncertainties. Conclusions regarding the major/minor contribution to bulge growth are very robust, absolute merger rates have systematic factor ~2 uncertainties.

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arXiv2009

Mergers, AGN, and 'Normal' Galaxies: Contributions to the Distribution of Star Formation Rates and Infrared Luminosity Functions

We use a novel method to predict the contribution of normal star-forming galaxies, merger-induced bursts, and obscured AGN, to IR luminosity functions (LFs) and global SFR densities. We use empirical halo occupation constraints to populate halos with galaxies and determine the distribution of normal and merging galaxies. Each system can then be associated with high-resolution hydrodynamic simulations. We predict the distribution of observed luminosities and SFRs, from different galaxy classes, as a function of redshift from z=0-6. We provide fitting functions for the predicted LFs, quantify the uncertainties, and compare with observations. At all redshifts, 'normal' galaxies dominate the LF at moderate luminosities ~L* (the 'knee'). Merger-induced bursts increasingly dominate at L>>L*; at the most extreme luminosities, AGN are important. However, all populations increase in luminosity at higher redshifts, owing to increasing gas fractions. Thus the 'transition' between normal and merger-dominated sources increases from the LIRG-ULIRG threshold at z~0 to bright Hyper-LIRG thresholds at z~2. The transition to dominance by obscured AGN evolves similarly, at factor of several higher L_IR. At all redshifts, non-merging systems dominate the total luminosity/SFR density, with merger-induced bursts constituting ~5-10% and AGN ~1-5%. Bursts contribute little to scatter in the SFR-stellar mass relation. In fact, many systems identified as 'ongoing' mergers will be forming stars in their 'normal' (non-burst) mode. Counting this as 'merger-induced' star formation leads to a stronger apparent redshift evolution in the contribution of mergers to the SFR density.

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arXiv2009

A New Empirical Method to Infer the Starburst History of the Universe from Local Galaxy Properties

The centers of bulges are formed dissipationally via gas inflows over short timescales: the 'starburst' mode of star formation (SF). Recent work has shown that detailed observations can be used to separate the stellar mass profile of these 'burst relic' components in local systems. Together with the assumption that some Kennicutt-Schmidt law holds, and that the burst was indeed a dissipational gas-rich event, we show that the observed profiles can be inverted to obtain the time and space-dependent SF history of each burst. Performing this with a large sample of well-studied spheroids, we show that the implied bursts scale in magnitude, mass, peak SFR, and spatial extent with galaxy mass in simple manner, and provide fits to these correlations. Burst masses are ~10% the total spheroid mass; timescales a mass-independent ~10^8 yr; peak SFR ~M_burst/t_burst; and they decay in power-law fashion ~t^-2.4. Sizes are ~0.1 R_e(spheroid), but grow with time. Combined with measurements of the nuclear stellar population ages of these systems (i.e. burst times), it is possible to re-construct the burst contribution to the distribution of SFRs and IR luminosity functions at all redshifts. The burst LF agrees well with observed IR LFs at the brightest luminosities, at z=0-2. At low-L, bursts are always unimportant; the transition to their importance increases from ULIRG luminosities at z~0 to HyLIRG luminosities at z~2. At all redshifts, bursts are a small fraction (~5-10%) of the total SFR density. We discuss possible implications of tension between maximum relic stellar mass densities in massive systems, and estimated number counts of brightest sub-millimeter galaxies.

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arXiv2009

A Maximum Stellar Surface Density in Dense Stellar Systems

We compile observations of the surface mass density profiles of dense stellar systems, including globular clusters in the Milky Way and nearby galaxies, massive star clusters in nearby starbursts, nuclear star clusters in dwarf spheroidals and late-type disks, ultra-compact dwarfs, and galaxy spheroids spanning the range from low-mass cusp bulges and ellipticals to massive core ellipticals. We show that in all cases the maximum stellar surface density attained in the central regions of these systems is similar, Sigma_max ~ 10^11 M_sun/kpc^2 (~20 g/cm^2), despite the fact that the systems span 7 orders of magnitude in total stellar mass M_star, 5 in effective radius R_e, and have a wide range in effective surface density M_star/R_e^2. The surface density limit is reached on a wide variety of physical scales in different systems and is thus not a limit on three-dimensional stellar density. Given the very different formation mechanisms involved in these different classes of objects, we argue that a single piece of physics likely determines Sigma_max. The radiation fields and winds produced by massive stars can have a significant influence on the formation of both star clusters and galaxies, while neither supernovae nor black hole accretion are important in star cluster formation. We thus conclude that feedback from massive stars likely accounts for the observed Sigma_max, plausibly because star formation reaches an Eddington-like flux that regulates the growth of these diverse systems. This suggests that current models of galaxy formation, which focus on feedback from supernovae and active galactic nuclei, are missing a crucial ingredient.

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