Norman Murray

University of Toronto, Toronto, Ontario, Canada

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Publications (68)363.7 Total impact

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    ABSTRACT: Observations of quasar pairs reveal that quasar host halos at z ~ 2 have large covering fractions of cool dense gas (≳ 60% for Lyman limit systems within a projected virial radius). Most simulations have so far failed to explain these large observed covering fractions. We analyze a new set of 15 simulated massive halos with explicit stellar feedback from the FIRE project, covering the halo mass range M_h ≈ 2 x 10^(12) - 10^(13) M_☉ at z = 2. This extends our previous analysis of the circum-galactic medium of high-redshift galaxies to more massive halos. Feedback from active galactic nuclei (AGN) is not included in these simulations. We find covering fractions consistent with those observed around z ~ 2 quasars. The large HI covering fractions arise from star formation-driven galactic winds, including winds from low-mass satellite galaxies that interact with the cosmological infalling filaments in which they are typically embedded. The simulated covering fractions increase with both halo mass and redshift over the ranges covered, as well as with resolution. Our simulations predict that galaxies occupying dark matter halos of mass similar to quasars but without a luminous AGN should have Lyman limit system covering fractions comparable to quasars. This prediction can be tested by measuring covering fractions transverse to sub-millimeter galaxies or to more quiescent galaxies selected based on their high stellar mass.
    Full-text · Article · Jan 2016
  • Daniel W. Murray · Philip Chang · Norman W. Murray · John Pittman
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    ABSTRACT: We perform simulations of star formation in self-gravitating turbulently driven gas. We find that star formation is not a self-similar process; two length scales enter, the radius of the rotationally supported disk $r_d$, and the radius $r_*$ of the sphere of influence of the nascent star, where the enclosed gas mass exceeds the stellar mass. The character of the flow changes at these two scales. We do not see any examples of inside-out collapse. Rather, the accretion of mass starts at large scales where we see large infall velocities $|u_r(r)| \approx (1/3) v_{ff} \sim (1/3)\sqrt{GM(r)/r}\gtrsim c_s$ out to $r \sim 1 \, \rm{pc}$ hundreds of thousands of years before a star forms. The density evolves to a fixed attractor, $\rho(r,t ) \rightarrow \rho(r)$, for $r_d<r<r_*$; mass flows through this structure onto a sporadically gravitationally unstable disk, and from thence onto the star. In the bulk of the molecular cloud, we find that the turbulent velocity $v_T \sim r^p$ with $p \sim 0.5$, in agreement with Larson's size-linewidth relation. But in the vicinity of star forming regions we find $ p \sim 0.2-0.3$, as seen in observations of massive star forming regions. For $r<r_*$, $v_T$ increases inward, with $p=-1/2$, i.e., it increases with increasing density, as seen in observations of massive star forming regions. The acceleration due to the turbulent pressure gradient is comparable to that due to gravity at all $r>r_d$ and rotational support becomes important for $r<r_d$. As a result, the infall velocity is substantially smaller than the free fall velocity; for $r_d<r<r_*$, we find $|u_r| \approx (1/3) v_{ff}$. Finally, we find the forming stars acquire mass from much larger radii than a typical hydrostatic core and the star forming efficiency is nonlinear with time, i.e., $M_*(t)\sim t^2$.
    No preview · Article · Sep 2015
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    Dong Zhang · Todd A. Thompson · Eliot Quataert · Norman Murray
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    ABSTRACT: Efficient thermalization of overlapping supernovae within star-forming galaxies may produce a supernova-heated fluid that drives galactic winds. For fiducial assumptions about the timescale for Kelvin-Helmholz (KH) instabilities from high-resolution simulations (which neglect magnetic fields) we show that cool clouds with temperature from T_c ~ 10^2-10^4 K seen in emission and absorption in galactic winds cannot be accelerated to observed velocities by the ram pressure of a hot wind. Taking into account both the radial structure of the hot flow and gravity, we show that this conclusion holds over a wide range of galaxy, cloud, and hot wind properties. This finding calls into question the prevailing picture whereby the cool atomic gas seen in galactic winds is entrained and accelerated by the hot flow. Given these difficulties with ram pressure acceleration, we discuss alternative models for the origin of high velocity cool gas outflows. Another possibility is that magnetic fields in cool clouds are sufficiently important that they prolong the cloud's life. For T_c = 10^3 K and 10^4 K clouds, we show that if conductive evaporation can be neglected, the KH timescale must be ~ 10 and 3 times longer, respectively, than the values from hydrodynamical simulations in order for cool cloud velocities to reach those seen in observations.
    Preview · Article · Jul 2015
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    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.
    No preview · Article · Apr 2015 · Monthly Notices of the Royal Astronomical Society
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    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.
    No preview · Article · Apr 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present a series of high-resolution (20–2000 M⊙, 0.1–4 pc) cosmological zoom-in simulations at z ≳ 6 from the Feedback In Realistic Environment (FIRE) project. These simulations cover halo masses 109–1011 M⊙ 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 per cent), while the time-averaged fesc over long time-scales generally remains ≲ 5 per cent, 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 and 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 overpredict fesc.
    Preview · Article · Mar 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present multiple ultrahigh resolution cosmological hydrodynamic simulations of M⋆ ≃ 104–6.3 M⊙ dwarf galaxies that form within two Mvir = 109.5–10 M⊙ dark matter halo initial conditions. Our simulations rely on the Feedback in Realistic Environments (fire) implementation of star formation feedback and were run with high enough force and mass resolution to directly resolve structure on the ∼200 pc scales. The resultant galaxies sit on the M⋆ versus Mvir relation required to match the Local Group stellar mass function via abundance matching. 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. We demonstrate that it is possible to create a large (∼1 kpc) constant-density dark matter core in a cosmological simulation of an M⋆ ≃ 106.3 M⊙ dwarf galaxy within a typical Mvir = 1010 M⊙ 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 ≲ 2), after the early epoch of cusp building mergers has ended. Our M⋆ ≃ 104 M⊙ dwarf retains a cuspy dark matter halo density profile that matches that of a dark-matter-only run of the same system. Though ancient, most of the stars in our ultrafaint form after reionization; the ultraviolet field acts mainly to suppress fresh gas accretion, not to boil away gas that is already present in the protodwarf.
    Preview · Article · Feb 2015 · Monthly Notices of the Royal Astronomical Society
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    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]
    Preview · Article · Jan 2015 · Monthly Notices of the Royal Astronomical Society
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    Snezana Prodan · Norman Murray
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    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.
    Preview · Article · Nov 2014 · The Astrophysical Journal
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    ABSTRACT: We use high-resolution cosmological zoom-in simulations from the FIRE (Feedback in Realistic Environments) 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 haloes covering the mass range Mh ≈ 109–6 × 1012 M⊙ at z = 2, including 12 haloes in the mass range Mh ∼ 1011–1012 M⊙ 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 H i covering fractions in galaxy haloes by direct ejection of cool gas from galaxies and through interactions with gas inflowing from the intergalactic medium. Our simulations predict H i 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 H i absorbers arising in inflows and in outflows are on average ∼50 per cent but exhibit significant time variability, ranging from ∼10 to ∼90 per cent. For our most massive haloes, 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, which decrease with time, peak at Mh ∼ 1011–1012 M⊙, near the peak of the star formation efficiency in dark matter haloes. In our simulations, star formation and galactic outflows are highly time dependent; H i covering fractions are also time variable but less so because they represent averages over large areas.
    Preview · Article · Sep 2014 · Monthly Notices of the Royal Astronomical Society
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    Norman W. Murray · Philip Chang
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    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.
    Preview · Article · Jul 2014 · The Astrophysical Journal
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    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.
    Preview · Article · Jun 2014
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    Eve J. Lee · Philip Chang · Norman Murray
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    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.
    Preview · Article · Jun 2014 · The Astrophysical Journal
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    Dong Zhang · Todd A. Thompson · Norman Murray · Eliot Quataert
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    ABSTRACT: Galactic superwinds may be driven by very hot outflows generated by overlapping supernovae within the host galaxy. We use the Chevalier & Clegg (CC85) wind model and the observed correlation between X-ray luminosities of galaxies and their star formation rates (SFRs) to constrain the mass-loss rates () across a wide range of SFRs, from dwarf starbursts to ultraluminous infrared galaxies. We show that for fixed thermalization and mass-loading efficiencies, the X-ray luminosity of the hot wind scales as LX∝SFR2, significantly steeper than is observed for star-forming galaxies: LX∝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 ☉ yr–1 we find that , which is 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.
    Preview · Article · Oct 2013 · The Astrophysical Journal
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    Snezana Prodan · Norman Murray · Todd A. Thompson
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    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.
    Preview · Article · May 2013
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    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.
    Full-text · Article · Apr 2013 · The Astrophysical Journal
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    Norman Murray
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    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.
    Preview · Article · Mar 2013 · Proceedings of the International Astronomical Union
  • Philip F. Hopkins · Desika Narayanan · Norman Murray
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    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.
    No preview · Article · Mar 2013 · Monthly Notices of the Royal Astronomical Society
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    Philip F. Hopkins · Dusan Keres · Norman Murray
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    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.
    Preview · Article · Jan 2013 · Monthly Notices of the Royal Astronomical Society
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    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.
    Full-text · Article · Jan 2013 · Monthly Notices of the Royal Astronomical Society