Norman Murray

University of Toronto, Toronto, Ontario, Canada

Are you Norman Murray?

Claim your profile

Publications (76)459.93 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: It is typically assumed that radiation-pressure-driven winds are accelerated to an asymptotic velocity of v∞ ≃ vesc, where vesc is the escape velocity from the central source. We note that this is not the case for dusty shells and clouds. Instead, if the shell or cloud is initially optically thick to the UV emission from the source of luminosity L, then there is a significant boost in v∞ that reflects the integral of the momentum absorbed as it is accelerated. For shells reaching a generalized Eddington limit, we show that v∞ ≃ (4RUVL/Mshc)1/2, in both point-mass and isothermal-sphere potentials, where RUV is the radius where the shell becomes optically thin to UV photons, and Msh is the mass of the shell. The asymptotic velocity significantly exceeds vesc for typical parameters, and can explain the ̃1000-2000 km s-1 outflows observed from rapidly star-forming galaxies and active galactic nuclei (AGN) if the surrounding halo has low gas density. Similarly fast outflows from massive stars can be accelerated on ̃few-103 yr time-scales. These results carry over to clouds that subtend only a small fraction of the solid angle from the source of radiation and that expand as a consequence of their internal sound speed. We further consider the dynamics of shells that sweep up a dense circumstellar or circumgalactic medium. We calculate the `momentum ratio' dot{M} v/(L/c) in the shell limit and show that it can only significantly exceed ̃2 if the effective optical depth of the shell to re-radiated far-infrared photons is much larger than unity. We discuss simple prescriptions for the properties of galactic outflows for use in large-scale cosmological simulations. We also briefly discuss applications to the dusty ejection episodes of massive stars, the disruption of giant molecular clouds, and AGN.
    Monthly Notices of the Royal Astronomical Society 04/2015; 449(1). DOI:10.1093/mnras/stv246 · 5.23 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We use hydrodynamic simulations to study the interaction of realistic active galactic nucleus (AGN) feedback mechanisms (accretion-disk winds & Compton heating) with a multi-phase interstellar medium (ISM). Our ISM model includes radiative cooling and explicit stellar feedback from multiple processes. We simulate radii ~0.1-100 pc around an isolated (non-merging) black hole. These are the scales where the accretion rate onto the black hole is determined and where AGN-powered winds and radiation couple to the ISM. Our primary results include: (1) The black hole accretion rate on these scales is determined by exchange of angular momentum between gas and stars in gravitational instabilities. This produces accretion rates of ~0.03-1 Msun/yr, sufficient to power a luminous AGN. (2) The gas disk in the galactic nucleus undergoes an initial burst of star formation followed by several Myrs where stellar feedback suppresses the star formation rate per dynamical time. (3) AGN winds injected at small radii with momentum fluxes ~L/c couple efficiently to the ISM and have a dramatic effect on the ISM properties in the central ~100 pc. AGN winds suppress the nuclear star formation rate by a factor of ~10-30 and the black hole accretion rate by a factor of ~3-30. They increase the total outflow rate from the galactic nucleus by a factor of ~10. The latter is broadly consistent with observational evidence for galaxy-scale atomic and molecular outflows driven by AGN rather than star formation. (4) In simulations that include AGN feedback, the predicted column density distribution towards the black hole is reasonably consistent with observations, whereas absent AGN feedback, the black hole is isotropically obscured and there are not enough optically-thin sight lines to explain observed Type I AGN. A 'torus-like' geometry arises self-consistently because AGN feedback evacuates the gas in the polar regions.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a series of high-resolution (20-2000 Msun, 0.1-4 pc) cosmological zoom-in simulations at z~6 from the Feedback In Realistic Environment (FIRE) project. These simulations cover halo masses 10^9-10^11 Msun and rest-frame ultraviolet magnitude Muv = -9 to -19. These simulations include explicit models of the multi-phase ISM, star formation, and stellar feedback, which produce reasonable galaxy properties at z = 0-6. We post-process the snapshots with a radiative transfer code to evaluate the escape fraction (fesc) of hydrogen ionizing photons. We find that the instantaneous fesc has large time variability (0.01%-20%), while the time-averaged fesc over long time-scales generally remains ~5%, considerably lower than the estimate in many reionization models. We find no strong dependence of fesc on galaxy mass or redshift. In our simulations, the intrinsic ionizing photon budgets are dominated by stellar populations younger than 3 Myr, which tend to be buried in dense birth clouds. The escaping photons mostly come from populations between 3-10 Myr, whose birth clouds have been largely cleared by stellar feedback. However, these populations only contribute a small fraction of intrinsic ionizing photon budgets according to standard stellar population models. We show that fesc can be boosted to high values, if stellar populations older than 3 Myr produce more ionizing photons than standard stellar population models (as motivated by, e.g., models including binaries). By contrast, runaway stars with velocities suggested by observations can enhance fesc by only a small fraction. We show that "sub-grid" star formation models, which do not explicitly resolve star formation in dense clouds with n >> 1 cm^-3, will dramatically over-predict fesc.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present ultra-high resolution cosmological hydrodynamic simulations of $M_*\simeq10^{4-6}M_{\odot}$ dwarf galaxies that form within $M_{v}=10^{9.5-10}M_{\odot}$ dark matter halos. Our simulations rely on the FIRE implementation of star formation feedback and were run with high enough force and mass resolution to directly resolve stellar and dark matter structure on the ~200 pc scales of interest for classical and ultra-faint dwarfs in the Local Group. The resultant galaxies sit on the $M_*$ vs. $M_{v}$ relation required to match the Local Group stellar mass function. They have bursty star formation histories and also form with half-light radii and metallicities that broadly match those observed for local dwarfs at the same stellar mass. For the first time we demonstrate that it is possible to create a large (~1 kpc) dark matter core in a cosmological simulation of an $M_*\simeq10^6M_{\odot}$ dwarf galaxy that resides within an $M_{v}=10^{10}M_{\odot}$ halo -- precisely the scale of interest for resolving the Too Big to Fail problem. However, these large cores are not ubiquitous and appear to correlate closely with the star formation histories of the dwarfs: dark matter cores are largest in systems that form their stars late ($z\lesssim2$), after the early epoch of cusp building mergers has ended. Our $M_*\simeq10^4M_{\odot}$ dwarf retains a cuspy dark matter halo density profile that matches almost identically that of a dark-matter only run of the same system. Despite forming in a field environment, this very low mass dwarf has observable properties that match closely to those of ultra-faint satellite galaxies of the Milky Way, including a uniformly old stellar population (>10 Gyr). Though ancient, most of the stars in our ultra-faint form after reionization; the UV field acts mainly to suppress fresh gas accretion, not to boil away gas that is already present in the proto-dwarf.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present an analysis of the galaxy-scale gaseous outflows from the FIRE (Feedback in Realistic Environments) simulations. This suite of hydrodynamic cosmological zoom simulations provides a sample of halos where star-forming giant molecular clouds are resolved to z=0, and features an explicit stellar feedback model on small scales. In this work, we focus on quantifying the gas mass ejected out of galaxies in winds and how this material travels through the halo. We correlate these quantities to star formation in galaxies throughout cosmic history. Our simulations reveal that a significant portion of every galaxy's evolution, particularly at high redshift, is dominated by bursts of star formation, which are followed by powerful gusts of galactic outflow that sweep up a large fraction of gas in the interstellar medium and send it through the circumgalactic medium. The dynamical effect of these outflows can significantly limit the amount of star formation within the affected galaxy. At low redshift, however, sufficiently massive galaxies corresponding to L*-progenitors develop stable disks and switch into a continuous and quiescent mode of star formation that does not drive outflows into the halo. We find inflow to be more continuous than outflow, although filamentary accretion onto the galaxy can be temporarily disrupted by recently ejected outflows. Using a variety of techniques, we measure outflow rates and use them to derive mass-loading factors, and their dependence on circular velocity, halo mass, and stellar mass for a large sample of galaxies in the FIRE simulation suite, spanning four decades in halo mass, six decades in stellar mass, and a redshift range of 4.0 > z > 0. Mass-loading factors for L*-progenitors are eta ~= 10 at high redshift, but decrease to eta << 1 at low redshift. [continued in text]
  • Source
    Snezana Prodan, Norman Murray
    [Show abstract] [Hide abstract]
    ABSTRACT: In this work we extend our dynamical study of Ultra Compact X-ray Binaries (UCXB) 4U 1820-30 from Prodan and Murray 2012 to three more UCXBs in globular clusters: 4U 1850-087, 4U 0513-40 and M15 X-2. These three UCXBs have orbital periods < 20 mins. Two of them, 4U 1850-087 and 4U 0513-40, have suspected luminosity variations of order of ~ 1yr. There is insufficient observational data to make any statements regarding the long periodicity in the light curve of M15 X-2 at this point. The properties of these three systems are quite similar to 4U 1820-30, which prompt us to model their dynamics in the same manner. As in the case of 4U 1820-30, we interpret the suspected long periods as the period of small oscillations around a stable fixed point in the Kozai resonance. We provide a lower limit on the tidal dissipation factor Q which is in agreement with results obtained for the case of 4U 1820-30.
    The Astrophysical Journal 11/2014; 798(2). DOI:10.1088/0004-637X/798/2/117 · 6.28 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Gas inflows and outflows regulate star formation in galaxies. Probing these processes is one of the central motivations for spectroscopic measurements of the circum-galactic medium. We use high-resolution cosmological zoom-in simulations from the FIRE project to make predictions for the covering fractions of neutral hydrogen around galaxies at z=2-4. These simulations resolve the interstellar medium of galaxies and explicitly implement a comprehensive set of stellar feedback mechanisms. Our simulation sample consists of 16 main halos covering the mass range M_h~2x10^9-8x10^12 Msun at z=2, including 12 halos in the mass range M_h~10^11-10^12 Msun corresponding to Lyman break galaxies (LBGs). We process our simulations with a ray tracing method to compute the ionization state of the gas. Galactic winds increase the HI covering fractions in galaxy halos by direct ejection of cool gas from galaxies and through interactions with gas inflowing from the intergalactic medium. Our simulations predict HI covering fractions for Lyman limit systems (LLSs) consistent with measurements around z~2-2.5 LBGs; these covering fractions are a factor ~2 higher than our previous calculations without galactic winds. The fractions of HI absorbers arising in inflows and in outflows are on average ~50% but exhibit significant time variability. For our most massive halos, we find a factor ~3 deficit in the LLS covering fraction relative to what is measured around quasars at z~2, suggesting that the presence of a quasar may affect the properties of halo gas on ~100 kpc scales. The predicted covering fractions peak at M_h~10^11-10^12 Msun, near the peak of the star formation efficiency in dark matter halos. In our simulations, star formation and galactic outflows are highly time dependent; HI covering fractions are also time variable but less so because they represent averages over large areas.
  • Source
    Norman W. Murray, Philip Chang
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a model of star formation in self-gravitating turbulent gas. We treat the turbulent velocity $v_T$ as a dynamical variable, and assume that it is adiabatically heated by the collapse. The theory predicts the run of density, infall velocity, and turbulent velocity, and the rate of star formation in compact massive gas clouds. The turbulent pressure is dynamically important at all radii, a result of the adiabatic heating. The system evolves toward a coherent spatial structure with a fixed run of density, $\rho(r,t)\to\rho(r)$; mass flows through this structure onto the central star or star cluster. We define the sphere of influence of the accreted matter by $m_*=M_g(r_*)$, where $m_*$ is the stellar plus disk mass in the nascent star cluster and $M_g(r)$ is the gas mass inside radius $r$. The density is given by a broken power law with a slope $-1.5$ inside $r_*$ and $\sim -1.6$ to $-1.8$ outside $r_*$. Both $v_T$ and the infall velocity $|u_r|$ decrease with decreasing $r$ for $r>r_*$; $v_T(r)\sim r^p$, the size-linewidth relation, with $p\approx0.2-0.3$, explaining the observation that Larson's Law is altered in massive star forming regions. The infall velocity is generally smaller than the turbulent velocity at $r>r_*$. For $r<r_*$, the infall and turbulent velocities are again similar, and both increase with decreasing $r$ as $r^{-1/2}$, with a magnitude about half of the free-fall velocity. The accreted (stellar) mass grows super-linearly with time, $\dot M_*=\phi M_{\rm cl}(t/\tau_{ff})^2$, with $\phi$ a dimensionless number somewhat less than unity, $M_{\rm cl}$ the clump mass and $\tau_{ff}$ the free-fall time of the clump. We suggest that small values of p can be used as a tracer of convergent collapsing flows.
    The Astrophysical Journal 07/2014; 804(1). DOI:10.1088/0004-637X/804/1/44 · 6.28 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: It is typically assumed that radiation pressure driven winds are accelerated to an asymptotic velocity of v_inf ~ v_esc, where v_esc is the escape velocity from the central source. We note that this is not the case for dusty shells. Instead, if the shell is initially optically-thick to the UV emission from the source of luminosity L, then there is a significant boost in v_inf that reflects the integral of the momentum absorbed by the shell as it is accelerated. For shells reaching a generalized Eddington limit, we show that v_inf ~ (4 R_UV L / M_sh c)^{1/2}, in both point-mass and isothermal-sphere potentials, where R_UV is the radius where the shell becomes optically-thin to UV photons, and M_sh is the mass of the shell. The asymptotic velocity significantly exceeds v_esc for typical parameters, and can explain the ~1000-2000 km/s outflows observed from rapidly star-forming galaxies and active galactic nuclei if their geometry is shell-like and if the surrounding halo has low gas density. Similarly fast shells from massive stars can be accelerated on ~ few -1000 yr timescales. We further consider the dynamics of shells that sweep up a dense circumstellar or circumgalactic medium. We calculate the "momentum ratio" Mdot v / (L/c) in the shell limit and show that it can only significantly exceed ~2 if the effective optical depth of the shell to re-radiated FIR photons is much larger than unity. We discuss simple prescriptions for the properties of galactic outflows for use in large-scale cosmological simulations. We also briefly discuss applications to the dusty ejection episodes of massive stars, the disruption of giant molecular clouds, and AGN.
  • Source
    Eve J. Lee, Philip Chang, Norman Murray
    [Show abstract] [Hide abstract]
    ABSTRACT: We present numerical evidence of dynamic star formation in which the accreted stellar mass grows superlinearly with time, roughly as $t^2$. We perform simulations of star formation in self-gravitating hydrodynamic and magneto-hydrodynamic turbulence that is continuously driven. By turning the self-gravity of the gas in the simulations on or off, we demonstrate that self-gravity is the dominant physical effect setting the mass accretion rate at early times before feedback effects take over, contrary to theories of turbulence-regulated star formation. We find that gravitational collapse steepens the density profile around stars, generating the power-law tail on what is otherwise a lognormal density probability distribution function. Furthermore, we find turbulent velocity profiles to flatten inside collapsing regions, altering the size-linewidth relation. This local flattening reflects enhancements of turbulent velocity on small scales, as verified by changes to the velocity power spectra. Our results indicate that gas self-gravity dynamically alters both density and velocity structures in clouds, giving rise to a time-varying star formation rate. We find that a substantial fraction of the gas that forms stars arrives via low density flows, as opposed to accreting through high density filaments.
    The Astrophysical Journal 06/2014; 800(1). DOI:10.1088/0004-637X/800/1/49 · 6.28 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Galactic superwinds may be driven by very hot outflows generated by overlapping supernovae within the host galaxy, and the cold gas seen in absorption may be accelerated by the ram pressure of this hot wind. We use the Chevalier & Clegg (CC85) wind model and the observed correlation between X-ray luminosities of galaxies and their SFRs to constrain the mass loss rates (\dot{M}_hot) across a wide range of star formation rates (SFRs), from dwarf starbursts to ultra-luminous infrared galaxies. We show that for fixed thermalization efficiency and mass loading rate, the X-ray luminosity of the hot wind scales as L_X ~ SFR^2, significantly steeper than is observed for star-forming galaxies: L_X ~ SFR. Using this difference we constrain the mass-loading and thermalization efficiency of hot galactic winds. For reasonable values of the thermalization efficiency (<~ 1) and for SFR >~ 10 M_sun/yr we find that \dot{M}_hot/SFR <~ 1, significantly lower than required by integrated constraints on the efficiency of stellar feedback in galaxies, and potentially too low to explain observations of winds from rapidly star-forming galaxies. In addition, we highlight the fact that heavily mass-loaded winds cannot be described by the adiabatic CC85 model because they become strongly radiative.
    The Astrophysical Journal 10/2013; 784(2). DOI:10.1088/0004-637X/784/2/93 · 6.28 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: White dwarf-white dwarf (WD-WD) mergers may lead to type Ia supernovae events. Thompson (2011) suggested that many such binaries are produced in hierarchical triple systems. The tertiary induces eccentricity oscillations in the inner binary via the Kozai-Lidov mechanism, driving the binary to high eccentricities, and significantly reducing the gravitational wave merger timescale (T_GW) over a broad range of parameter space. Here, we investigate the role of tidal forces in these systems. We show that tidal effects are important in the regime of moderately high initial relative inclination between the inner binary and the outer tertiary. For 85 < i_0 < 90 degrees (prograde) and 97 < i_0 < 102 degrees (retrograde), tides combine with GW radiation to dramatically decrease T_GW. In the regime of high inclinations between 91 < i_0 < 96 degrees, the inner binary likely suffers a direct collision, as in the work of Katz & Dong (2012) and tidal effects do not play an important role.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Mapping Mg II resonance emission scattered by galactic winds offers a means to determine the spatial extent and density of the warm outflow. Using Keck/LRIS spectroscopy, we have resolved scattered Mg II emission to the east of 32016857, a star-forming galaxy at z =0.9392 with an outflow. The Mg II emission from this galaxy exhibits a P-Cygni profile, extends further than both the continuum and [O II] emission along the eastern side of the slit, and has a constant Doppler shift along the slit which does not follow the velocity gradient of the nebular [O II] emission. Using the Sobolev approximation, we derive the density of Mg+ ions at a radius of 12 to 18 kpc in the outflow. We model the ionization correction and find that much of the outflowing Mg is in Mg++. We estimate that the total mass flux could be as large as 330 - 500 solar masses per year, with the largest uncertainties coming from the depletion of Mg onto grains and the clumpiness of the warm outflow. We show that confining the warm clouds with a hot wind reduces the estimated mass flux of the warm outflow and indicates amass-loading factor near unity in the warm phase alone. Based on the high blue luminosities that distinguish 32016857 and TKRS 4389, described by Rubin et al. 2011, from other galaxies with P-Cygni emission, we suggest that, as sensitivity to diffuse emission improves, scattering halos may prove to be a generic property of star-forming galaxies at intermediate redshifts.
    The Astrophysical Journal 04/2013; 770(1). DOI:10.1088/0004-637X/770/1/41 · 6.28 Impact Factor
  • Norman Murray
    [Show abstract] [Hide abstract]
    ABSTRACT: The low stellar and gas mass fractions, low galaxy-wide star formation rates (relative to galactic dynamical times) and observations of rapid outflows from galaxies, all suggest that stars and active galactic nuclei violently alter the state of the interstellar and even inter-halo gas in galaxies. I argue that the low galaxy wide star formation rates are not the result of turbulent suppression of star formation on small scale, but rather the result of a balance between dynamical pressure and the force (or rate of momentum deposition) provided by stellar feedback, either in the form of radiation pressure or by supernovae. Galaxy scale winds can also be driven by feedback, either from stars or active galactic nuclei, although the exact mechanisms involved are still not well determined.
    Proceedings of the International Astronomical Union 03/2013; 8(S292):343-350. DOI:10.1017/S1743921313001671
  • [Show abstract] [Hide abstract]
    ABSTRACT: 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.
    Monthly Notices of the Royal Astronomical Society 03/2013; 432(4). DOI:10.1093/mnras/stt723 · 5.23 Impact Factor
  • Source
    Philip F. Hopkins, Dusan Keres, Norman Murray
    [Show abstract] [Hide abstract]
    ABSTRACT: Rapid accretion of cold gas plays a crucial role in getting gas into galaxies. It has been suggested that this accretion proceeds along narrow streams that might also directly drive the turbulence in galactic gas, dynamical disturbances, and bulge formation. In cosmological simulations, however, it is impossible to isolate and hence disentangle the effect of accretion from internal instabilities and mergers. Moreover, in most cosmological simulations, the phase structure and turbulence in the ISM arising from stellar feedback are treated in a sub-grid manner, so that feedback cannot generate ISM turbulence. In this paper we therefore test the effects of cold streams in extremely high-resolution simulations of otherwise isolated galaxy disks using detailed models for star formation and feedback; we then include or exclude mock cold flows falling onto the galaxies with accretion rates, velocities and geometry set to maximize their effect on the disk. We find: (1) Turbulent velocity dispersions in gas disks are identical with or without the cold flow; the energy injected by the flow is dissipated where it meets the disk. (2) In runs without stellar feedback, the presence of a cold flow has essentially no effect on runaway local collapse, resulting in star formation rates (SFRs) that are far too large. (3) Disks in runs with feedback and cold flows have higher SFRs, but only insofar as they have more gas. (4) Because flows are extended relative to the disk, they do not trigger strong resonant responses and so induce weak morphological perturbation (bulge formation via instabilities is not accelerated). (5) However, flows can thicken the disk by direct contribution of out-of-plane streams. We conclude that while inflows are critical over cosmological timescales to determine the supply and angular momentum of gas disks, they have weak instantaneous dynamical effects on galaxies.
    Monthly Notices of the Royal Astronomical Society 01/2013; 432(4). DOI:10.1093/mnras/stt472 · 5.23 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: 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.
    Monthly Notices of the Royal Astronomical Society 01/2013; 433(1). DOI:10.1093/mnras/stt690 · 5.23 Impact Factor
  • Mubdi Rahman, N. W. Murray
    [Show abstract] [Hide abstract]
    ABSTRACT: The mechanism controlling the efficiency of star formation remains contested. However, stellar feedback has been thought to contribute significantly to the quenching of star formation at the giant molecular cloud (GMC) level. We conduct radiative hydrodynamic simulations using the adaptive mesh refinement code ENZO to simulate a GMC through the process of star formation to disruption, including the feedback effects of radiation and supernovae. We find that turbulence has little effect on the rate of star formation at early times in the GMC and likely does not control the star formation efficiency at the GMC scale. The radiative feedback is sufficient to disrupt the cloud on timescales faster than the onset of the first embedded supernova. Through these simulations, we find that radiative feedback is the dominant mechanism that controls the efficiency of star formation within a GMC. This is in contrast to earlier suggestions that turbulence and supernovae feedback is required to control the efficiency of star formation.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: 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).
    Monthly Notices of the Royal Astronomical Society 09/2012; 433(1). DOI:10.1093/mnras/stt688 · 5.23 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We use simulations with realistic models for stellar feedback to study galaxy mergers. These high resolution (1 pc) simulations follow formation and destruction of individual GMCs and star clusters. The final starburst is dominated by in situ star formation, fueled by gas which flows inwards due to global torques. The resulting high gas density results in rapid star formation. The gas is self gravitating, and forms massive (~10^10 M_sun) GMCs and subsequent super-starclusters (masses up to 10^8 M_sun). However, in contrast to some recent simulations, the bulk of new stars which eventually form the central bulge are not born in superclusters which then sink to the center of the galaxy, because feedback efficiently disperses GMCs after they turn several percent of their mass into stars. Most of the mass that reaches the nucleus does so in the form of gas. The Kennicutt-Schmidt law emerges naturally as a consequence of feedback balancing gravitational collapse, independent of the small-scale star formation microphysics. The same mechanisms that drive this relation in isolated galaxies, in particular radiation pressure from IR photons, extend over seven decades in SFR to regulate star formation in the most extreme starbursts (densities >10^4 M_sun/pc^2). Feedback also drives super-winds with large mass loss rates; but a significant fraction of the wind material falls back onto the disks at later times, leading to higher post-starburst SFRs in the presence of stellar feedback. Strong AGN feedback is required to explain sharp cutoffs in star formation rate. We compare the predicted relic structure, mass profile, morphology, and efficiency of disk survival to simulations which do not explicitly resolve GMCs or feedback. Global galaxy properties are similar, but sub-galactic properties and star formation rates can differ significantly.
    Monthly Notices of the Royal Astronomical Society 05/2012; 430(3). DOI:10.1093/mnras/stt017 · 5.23 Impact Factor

Publication Stats

3k Citations
459.93 Total Impact Points

Institutions

  • 1998–2015
    • University of Toronto
      • Canadian Institute for Theoretical Astrophysics
      Toronto, Ontario, Canada
  • 2006
    • Weizmann Institute of Science
      • Faculty of Physics
      Tell Afif, Tel Aviv, Israel