Thorsten Naab

Max Planck Institute for Astrophysics, Arching, Bavaria, Germany

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Publications (228)880.3 Total impact

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    ABSTRACT: The halo of the Milky Way contains a hot plasma with a surface brightness in soft X-rays of the order $10^{-12}$erg cm$^{-2}$ s$^{-1}$ deg$^{-2}$. The origin of this gas is unclear, but so far numerical models of galactic star formation have failed to reproduce such a large surface brightness by several orders of magnitude. In this paper, we analyze simulations of the turbulent, magnetized, multi-phase interstellar medium including thermal feedback by supernova explosions as well as cosmic-ray feedback. We include a time-dependent chemical network, self-shielding by gas and dust, and self-gravity. Pure thermal feedback alone is sufficient to produce the observed surface brightness, although it is very sensitive to the supernova rate. Cosmic rays suppress this sensitivity and reduce the surface brightness because they drive cooler outflows. Self-gravity has by far the largest effect because it accumulates the diffuse gas in the disk in dense clumps and filaments, so that supernovae exploding in voids can eject a large amount of hot gas into the halo. This can boost the surface brightness by several orders of magnitude. Although our simulations do not reach a steady state, all simulations produce surface brightness values of the same order of magnitude as the observations, with the exact value depending sensitively on the simulation parameters. We conclude that star formation feedback alone is sufficient to explain the origin of the hot halo gas, but measurements of the surface brightness alone do not provide useful diagnostics for the study of galactic star formation.
    Preview · Article · Oct 2015
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    ABSTRACT: We study the connection of star formation to atomic (HI) and molecular hydrogen (H$_2$) in isolated, low metallicity dwarf galaxies with high-resolution ($m_{\rm gas}$ = 4 M$_\odot$, $N_{\rm ngb}$ = 100) SPH simulations. The model includes self-gravity, non-equilibrium cooling, shielding from an interstellar radiation field, the chemistry of H$_2$ formation, H$_2$-independent star formation, supernova feedback and metal enrichment. We find that the H$_2$ mass fraction is sensitive to the adopted dust-to-gas ratio and the strength of the interstellar radiation field, while the star formation rate is not. Star formation is regulated by stellar feedback, keeping the gas out of thermal equilibrium for densities $n <$ 1 cm$^{-3}$. Because of the long chemical timescales, the H$_2$ mass remains out of chemical equilibrium throughout the simulation. Star formation is well-correlated with cold ( T $\leqslant$ 100 K ) gas, but this dense and cold gas - the reservoir for star formation - is dominated by HI, not H$_2$. In addition, a significant fraction of H$_2$ resides in a diffuse, warm phase, which is not star-forming. The ISM is dominated by warm gas (100 K $<$ T $\leqslant 3\times 10^4$ K) both in mass and in volume. The scale height of the gaseous disc increases with radius while the cold gas is always confined to a thin layer in the mid-plane. The cold gas fraction is regulated by feedback at small radii and by the assumed radiation field at large radii. The decreasing cold gas fractions result in a rapid increase in depletion time (up to 100 Gyrs) for total gas surface densities $\Sigma_{\rm HI+H_2} \lesssim$ 10 M$_\odot$pc$^{-2}$, in agreement with observations of dwarf galaxies in the Kennicutt-Schmidt plane.
    Preview · Article · Oct 2015
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    ABSTRACT: We analyze the angular momenta of massive star forming galaxies (SFGs) at the peak of the cosmic star formation epoch (z~0.8-2.6). Our sample of ~360 log(M*/Msun) ~ 9.3-11.8 SFGs is mainly based on the KMOS^3D and SINS/zC-SINF surveys of H\alpha\ kinematics, and collectively provides a representative subset of the massive star forming population. The inferred halo scale, angular momentum distribution is broadly consistent with that theoretically predicted for their dark matter halos, in terms of mean spin parameter <\lambda> ~ 0.037 and its dispersion ($\sigma_{log(\lambda)}$~0.2). Spin parameters correlate with the disk radial scale, and with their stellar surface density, but do not depend significantly on halo mass, stellar mass, or redshift. Our data thus support the long-standing assumption that on average the specific angular momentum of early disks reflects that of their dark matter halos (jd = jDM), despite the fact that gas enters the virial radius with substantially higher angular momentum, requiring subsequent angular momentum redistribution. The lack of correlation between \lambda x (jd/jDM) and the nuclear stellar density $\Sigma_{*}$(1kpc) favors that disk-internal angular momentum redistribution leads to "compaction" inside massive high-z disks. The average disk to dark halo mass ratio is ~5%, consistent with recent abundance matching results and implying that our high-z disks are strongly baryon dominated.
    Full-text · Article · Oct 2015
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    ABSTRACT: We present a hydrodynamical simulation of the turbulent, magnetized, supernova-driven interstellar medium (ISM) in a stratified box that dynamically couples the injection and evolution of cosmic rays (CRs) and a self-consistent evolution of the chemical composition. CRs are treated as a relativistic fluid in the advection-diffusion approximation. The thermodynamic evolution of the gas is computed using a chemical network that follows the abundances of H+, H, H2, CO, C+, and free electrons and includes (self-)shielding of the gas and dust. We find that CRs perceptibly thicken the disk with the heights of 90% (70%) enclosed mass reaching ~1.5 kpc (~0.2 kpc). The simulations indicate that CRs alone can launch and sustain strong outflows of atomic and ionized gas with mass loading factors of order unity, even in solar neighbourhood conditions and with a CR energy injection per supernova (SN) of 10^50 erg, 10% of the fiducial thermal energy of a SN. The CR-driven outflows have moderate launching velocities close to the midplane (~100 km/s) and are denser (\rho~1e-24 - 1e-26 g/cm^3), smoother and colder than the (thermal) SN-driven winds. The simulations support the importance of CRs for setting the vertical structure of the disk as well as the driving of winds.
    No preview · Article · Sep 2015
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    ABSTRACT: The SILCC project (SImulating the Life-Cycle of molecular Clouds) aims at a more self-consistent understanding of the interstellar medium (ISM) on small scales and its link to galaxy evolution. We present three-dimensional (magneto)hydrodynamic simulations of the ISM in a vertically stratified box including self-gravity, an external potential due to the stellar component of the galactic disc, and stellar feedback in the form of an interstellar radiation field and supernovae (SNe). The cooling of the gas is based on a chemical network that follows the abundances of H+, H, H2, C+, and CO and takes shielding into account consistently. We vary the SN feedback by comparing different SN rates, clustering and different positioning, in particular SNe in density peaks and at random positions, which has a major impact on the dynamics. Only for random SN positions the energy is injected in sufficiently low-density environments to reduce energy losses and enhance the effective kinetic coupling of the SNe with the gas. This leads to more realistic velocity dispersions ($\sigma _\mathrm{H\,{\small {I}}}\approx 0.8\sigma _{300\rm{-}8000\,\mathrm{K}}\sim 10\hbox{-}20\,\mathrm{km}\,\mathrm{s}^{-1}$, $\sigma _\mathrm{H\,\alpha }\approx 0.6\sigma _{8000-3\times 10^5\,\mathrm{K}}\sim 20\hbox{-}30\,\mathrm{km}\,\mathrm{s}^{-1}$), and strong outflows with mass loading factors (ratio of outflow to star formation rate) of up to 10 even for solar neighbourhood conditions. Clustered SNe abet the onset of outflows compared to individual SNe but do not influence the net outflow rate. The outflows do not contain any molecular gas and are mainly composed of atomic hydrogen. The bulk of the outflowing mass is dense (ρ ∼ 10−25–10−24 g cm−3) and slow (v ∼ 20–40 km s−1) but there is a high-velocity tail of up to v ∼ 500 km s−1 with ρ ∼ 10−28–10−27 g cm−3.
    No preview · Article · Aug 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: In around ≈25% of early-type galaxies (ETGs) UV emission from young stellar populations is present. Molecular gas reservoirs have been detected in these systems (e.g. Young et al. (2011), providing the fuel for this residual star-formation. The environment in which this molecular gas is found is quite different than that in spiral galaxies however, with harsher radiation fields, deeper potentials and high metallicity and alpha-element abundances. Here we report on one element of our multi-faceted programme to understand the similarities and differences between the gas reservoirs in spirals and ETGs. We use spatially resolved observations from the CARMA mm-wave interferometer to investigate the size of the molecular reservoirs in the the CO-rich ATLAS3D ETGs (survey described in Alatalo et al. 2012, submitted). We find that the molecular gas extent is smaller in absolute terms in ETGs than in late-type galaxies, but that the size distributions are similar once scaled by the galaxies optical/stellar characteristic scale-lengths (Fig 1, left). Amongst ETGs, we find that the extent of the molecular gas is independent of the kinematic misalignment, despite the many reasons why misaligned gas might have a smaller extent. The extent of the molecular gas does depend on environment, with Virgo cluster ETGs having less extended molecular gas reservoirs (Fig 1, right). Whatever the cause, this further emphases that cluster ETGs follow different evolutionary pathways from those in the field. Full details of this work will be presented in Davis et al. (2012), submitted.
    No preview · Conference Paper · Aug 2015
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    Stefanie Walch · Thorsten Naab
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    ABSTRACT: We investigate the early impact of single and binary supernova (SN) explosions on dense gas clouds with three-dimensional, high-resolution, hydrodynamic simulations. The effect of cloud structure, radiative cooling and ionizing radiation from the progenitor stars on the net input of kinetic energy, fkin = Ekin/ESN, thermal energy, ftherm = Etherm/ESN, and gas momentum, fP = P/PSN, to the interstellar medium (ISM) is tested. For clouds with $\bar{n} = 100\;{\rm cm}^{-3}$, the momentum generating Sedov and pressure-driven snowplough phases are terminated early (∝0.01 Myr) and radiative cooling limits the coupling to ftherm ∼ 0.01, fkin ∼ 0.05, and fP ∼ 9, significantly lower than for the case without cooling. For pre-ionized clouds, these numbers are only increased by ∼50 per cent, independent of the cloud structure. This only suffices to accelerate ∼5 per cent of the cloud to radial velocities ≳30 km s−1. A second SN might enhance the coupling efficiencies if delayed past the Sedov phase of the first explosion. Such very low coupling efficiencies cast doubts on many subresolution models for SN feedback, which are, in general, validated a posteriori. Ionizing radiation appears not to significantly enhance the coupling of SNe to the surrounding gas as it drives the ISM into inert dense shells and cold clumps, a process which is unresolved in galaxy-scale simulations. Our results indicate that the momentum input of SNe in ionized, structured clouds is larger (more than a factor of 10) than the corresponding momentum yield of the progenitor's stellar winds.
    Preview · Article · Jul 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present a hybrid code combining the OpenMP-parallel tree code vine with an algorithmic chain regularization scheme. The new code, called ‘rvine’, aims to significantly improve the accuracy of close encounters of massive bodies with supermassive black holes (SMBHs) in galaxy-scale numerical simulations. We demonstrate the capabilities of the code by studying two test problems, the sinking of a single massive black hole to the centre of a gas-free galaxy due to dynamical friction and the hardening of an SMBH binary due to close stellar encounters. We show that results obtained with rvine compare well with nbody7 for problems with particle numbers that can be simulated with nbody7. In particular, in both nbody7 and rvine we find a clear N-dependence of the binary hardening rate, a low binary eccentricity and moderate eccentricity evolution, as well as the conversion of the galaxy's inner density profile from a cusp to a core via the ejection of stars at high velocity. The much larger number of particles that can be handled by rvine will open up exciting opportunities to model stellar dynamics close to SMBHs much more accurately in a realistic galactic context. This will help to remedy the inherent limitations of commonly used tree solvers to follow the correct dynamical evolution of black holes in galaxy-scale simulations.
    Preview · Article · Jul 2015 · Monthly Notices of the Royal Astronomical Society
  • Michaela Hirschmann · Thorsten Naab
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    ABSTRACT: We investigate the origin of stellar metallicity gradients in massive galaxies at large radii (r > 2 Reff) using ten cosmological zoom simulations of halos with 6 × 1012M⊙ < Mhalo < 2 × 1013M⊙. The simulations follow metal cooling and enrichment from SNII, SNIa and AGB winds. We explore the differential impact of an empirical model for galactic winds that reproduces the evolution of the mass-metallicity relation. At larger radii, the galaxies become more dominated by stars accreted from satellite galaxies in major and minor mergers. In the wind model, fewer stars are accreted, but they are significantly more metal poor resulting in steep global metallicity (〈 ▽ Zstars 〉= -0.35 dex/dex) gradients in agreement with observations. Metallicity gradients of models without winds are inconsistent with observations. For the wind model, stellar accretion is steepening existing in-situ metallicity gradients by about 0.2 dex by the present day and is required to match observed gradients. Metallicity gradients are significantly steeper for systems, which have accreted stars in minor mergers. In contrast, galaxies with major mergers have relatively flat gradients, confirming previous results. We highlight the importance of stellar accretion for stellar population properties of massive galaxies at large radii, which provide important constraints for formation models.
    No preview · Article · Jul 2015 · Proceedings of the International Astronomical Union
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    ABSTRACT: Supernovae are the most energetic among stellar feedback processes, and are crucial for regulating the interstellar medium (ISM) and launching galactic winds. We explore how supernova remnants (SNRs) create a multiphase medium by performing high resolution, 3D hydrodynamical simulations at various SN rates, $S$, and ISM average densities, $n$. We find that the evolution of a SNR in a self-consistently generated three-phase ISM is qualitatively different from that in a uniform or a two-phase warm/cold medium. By traveling faster and further in the cooling-inefficient hot phase, the spatial-temporal domain of a SNR is enlarged by $>10^{2.5}$ in a hot-dominated multiphase medium (HDMM) compared to the uniform case. We then examine the resultant ISM as we vary $n$ and $S$, finding that a steady state can only be achieved when the hot gas volume fraction \fvh $\lesssim 0.6\pm 0.1$. Above that, overlapping SNRs render connecting topology of the hot gas, and such a HDMM is subjected to thermal runaway with growing pressure and \fvh. Photoelectric heating (PEH) has a surprisingly strong impact on \fvh. For $n \gtrsim 3 cm^{-3}$, a reasonable PEH rate is able to suppress the ISM from undergoing thermal runaway. Overall, we determine that the critical SN rate for the onset of thermal runaway is roughly $S_{crit} = 200 (n/1cm^{-3})^k (E_{SN}/10^{51} erg)^{-1} kpc^{-3} Myr^{-1}$, where k=(1.2,2.7) for $n$ < 1 and >1 cm$^{-3}$, respectively. We present a fitting formula of the ISM pressure $P(n, S)$, which can be used as an effective equation of state in cosmological simulations. The observed velocities of OB stars imply that the core collapse SN are almost randomly located on scales $\lesssim$ 150 pc. Despite the 5 orders of magnitude span of $(n,S)$, the average Mach number shows very small variations: $M \approx 0.5\pm 0.2, 1.2\pm 0.3, 2.3\pm 0.9$ for the hot, warm and cold phases, respectively.
    Preview · Article · Jun 2015 · The Astrophysical Journal
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    ABSTRACT: We compare the performance of mass estimators for elliptical galaxies that rely on the directly observable surface brightness and velocity dispersion profiles, without invoking computationally expensive detailed modelling. These methods recover the mass at a specific radius where the mass estimate is expected to be least sensitive to the anisotropy of stellar orbits. One uses the total luminosity-weighted velocity dispersion and evaluates the mass at a 3D half-light radius r1/2, i.e. it depends on the global galaxy properties. Another approach estimates the mass from the velocity dispersion at a radius R2 where the surface brightness declines as R−2, i.e. it depends on the local properties. We evaluate the accuracy of the two methods for analytical models, simulated galaxies and real elliptical galaxies that have already been modelled by the Schwarzschild's orbit-superposition technique. Both estimators recover an almost unbiased circular speed estimate with a modest rms scatter (≲10 per cent). Tests on analytical models and simulated galaxies indicate that the local estimator has a smaller rms scatter than the global one. We show by examination of simulated galaxies that the projected velocity dispersion at R2 could serve as a good proxy for the virial galaxy mass. For simulated galaxies the total halo mass scales with σp(R2) as $ M_{\rm vir} [\mathrm{M}_{{\odot }}\,h^{-1}] \approx 6\times 10^{12}({{\sigma _{\rm p}(R_2)}\over{200\, \rm km\, s^{-1}}})^{4}$ with rms scatter ≈40 per cent.
    Preview · Article · Apr 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: Accurate direct N-body simulations help to obtain detailed information about the dynamical evolution of star clusters. They also enable comparisons with analytical models and Fokker-Planck or Monte Carlo methods. nbody6 is a well-known direct N-body code for star clusters, and nbody6++ is the extended version designed for large particle number simulations by supercomputers. We present nbody6++gpu, an optimized version of nbody6++ with hybrid parallelization methods (MPI, GPU, OpenMP, and AVX/SSE) to accelerate large direct N-body simulations, and in particular to solve the million-body problem. We discuss the new features of the nbody6++gpu code, benchmarks, as well as the first results from a simulation of a realistic globular cluster initially containing a million particles. For million-body simulations, nbody6++gpu is 400–2000 times faster than nbody6 with 320 CPU cores and 32 NVIDIA K20X GPUs. With this computing cluster specification, the simulations of million-body globular clusters including 5 per cent primordial binaries require about an hour per half-mass crossing time.
    Full-text · Article · Apr 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present the stellar population content of early-type galaxies from the Atlas3D survey. Using spectra integrated within apertures covering up to one effective radius, we apply two methods: one based on measuring line-strength indices and applying single stellar population (SSP) models to derive SSP-equivalent values of stellar age, metallicity, and alpha enhancement; and one based on spectral fitting to derive non-parametric star-formation histories, mass-weighted average values of age, metallicity, and half-mass formation timescales. Using homogeneously derived effective radii and dynamically-determined galaxy masses, we present the distribution of stellar population parameters on the Mass Plane (M_JAM, Sigma_e, R_maj), showing that at fixed mass, compact early-type galaxies are on average older, more metal-rich, and more alpha-enhanced than their larger counterparts. From non-parametric star-formation histories, we find that the duration of star formation is systematically more extended in lower mass objects. Assuming that our sample represents most of the stellar content of today's local Universe, approximately 50% of all stars formed within the first 2 Gyr following the big bang. Most of these stars reside today in the most massive galaxies (>10^10.5 M_sun), which themselves formed 90% of their stars by z~2. The lower-mass objects, in contrast, have formed barely half their stars in this time interval. Stellar population properties are independent of environment over two orders of magnitude in local density, varying only with galaxy mass. In the highest-density regions of our volume (dominated by the Virgo cluster), galaxies are older, alpha-enhanced and have shorter star-formation histories with respect to lower density regions.
    Full-text · Article · Jan 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: The SILCC (SImulating the Life-Cycle of molecular Clouds) project aims to self-consistently understand the small-scale structure of the interstellar medium (ISM) and its link to galaxy evolution. We simulate the evolution of the multiphase ISM in a (500 pc)2 × ±5 kpc region of a galactic disc, with a gas surface density of $\Sigma _{_{\rm GAS}} = 10 \;{\rm M}_{\odot }\,{\rm pc}^{-2}$. The flash 4 simulations include an external potential, self-gravity, magnetic fields, heating and radiative cooling, time-dependent chemistry of H2 and CO considering (self-) shielding, and supernova (SN) feedback but omit shear due to galactic rotation. We explore SN explosions at different rates in high-density regions (peak), in random locations with a Gaussian distribution in the vertical direction (random), in a combination of both (mixed), or clustered in space and time (clus/clus2). Only models with self-gravity and a significant fraction of SNe that explode in low-density gas are in agreement with observations. Without self-gravity and in models with peak driving the formation of H2 is strongly suppressed. For decreasing SN rates, the H2 mass fraction increases significantly from <10 per cent for high SN rates, i.e. 0.5 dex above Kennicutt–Schmidt, to 70–85 per cent for low SN rates, i.e. 0.5 dex below KS. For an intermediate SN rate, clustered driving results in slightly more H2 than random driving due to the more coherent compression of the gas in larger bubbles. Magnetic fields have little impact on the final disc structure but affect the dense gas (n ≳ 10 cm−3) and delay H2 formation. Most of the volume is filled with hot gas (∼80 per cent within ±150 pc). For all but peak driving a vertically expanding warm component of atomic hydrogen indicates a fountain flow. We highlight that individual chemical species populate different ISM phases and cannot be accurately modelled with temperature-/density-based phase cut-offs.
    Preview · Article · Dec 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We use hydrodynamical simulations in a (256 pc)3 periodic box to model the impact of supernova (SN) explosions on the multiphase interstellar medium (ISM) for initial densities n = 0.5–30 cm−3 and SN rates 1–720 Myr−1. We include radiative cooling, diffuse heating, and the formation of molecular gas using a chemical network. The SNe explode either at random positions, at density peaks, or both. We further present a model combining thermal energy for resolved and momentum input for unresolved SNe. Random driving at high SN rates results in hot gas (T ≳ 106 K) filling >90 per cent of the volume. This gas reaches high pressures (104 < P/kB < 107 K cm−3) due to the combination of SN explosions in the hot, low-density medium and confinement in the periodic box. These pressures move the gas from a two-phase equilibrium to the single-phase, cold branch of the cooling curve. The molecular hydrogen dominates the mass (>50 per cent), residing in small, dense clumps. Such a model might resemble the dense ISM in high-redshift galaxies. Peak driving results in huge radiative losses, producing a filamentary ISM with virtually no hot gas, and a small molecular hydrogen mass fraction (≪1 per cent). Varying the ratio of peak to random SNe yields ISM properties in between the two extremes, with a sharp transition for equal contributions. The velocity dispersion in H i remains ≲10 km s−1 in all cases. For peak driving, the velocity dispersion in Hα can be as high as 70 km s−1 due to the contribution from young, embedded SN remnants.
    Full-text · Article · Oct 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: In this paper, we follow up on our previous detection of nuclear ionized outflows in the most massive (log(M */M ☉) ≥ 10.9) z ~ 1-3 star-forming galaxies by increasing the sample size by a factor of six (to 44 galaxies above log(M */M ☉) ≥ 10.9) from a combination of the SINS/zC-SINF, LUCI, GNIRS, and KMOS3Dspectroscopic surveys. We find a fairly sharp onset of the incidence of broad nuclear emission (FWHM in the Hα, [N II], and [S II] lines ~450-5300 km s–1), with large [N II]/Hα ratios, above log(M */M ☉) ~ 10.9, with about two-thirds of the galaxies in this mass range exhibiting this component. Broad nuclear components near and above the Schechter mass are similarly prevalent above and below the main sequence of star-forming galaxies, and at z ~ 1 and ~2. The line ratios of the nuclear component are fit by excitation from active galactic nuclei (AGNs), or by a combination of shocks and photoionization. The incidence of the most massive galaxies with broad nuclear components is at least as large as that of AGNs identified by X-ray, optical, infrared, or radio indicators. The mass loading of the nuclear outflows is near unity. Our findings provide compelling evidence for powerful, high-duty cycle, AGN-driven outflows near the Schechter mass, and acting across the peak of cosmic galaxy formation.
    Preview · Article · Oct 2014 · The Astrophysical Journal
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    ABSTRACT: We investigate the evolution of stellar population gradients from z = 2 to 0 in massive galaxies at large radii (r > 2Reff) using 10 cosmological zoom simulations of haloes with 6 × 1012 M⊙ < Mhalo < 2 × 1013 M⊙. The simulations follow metal cooling and enrichment from SNII, SNIa and asymptotic giant branch winds. We explore the differential impact of an empirical model for galactic winds that reproduces the mass–metallicity relation and its evolution with redshift. At larger radii the galaxies, for both models, become more dominated by stars accreted from satellite galaxies in major and minor mergers. In the wind model, fewer stars are accreted, but they are significantly more metal-poor resulting in steep global metallicity (〈∇Zstars〉 = −0.35 dex dex−1) and colour (e.g. 〈∇g − r〉 = −0.13 dex dex−1) gradients in agreement with observations. In contrast, colour and metallicity gradients of the models without winds are inconsistent with observations. Age gradients are in general mildly positive at z = 0 (〈∇Agestars〉 = 0.04 dex dex−1) with significant differences between the models at higher redshift. We demonstrate that for the wind model, stellar accretion is steepening existing in situ metallicity gradients by about 0.2 dex by the present day and helps to match observed gradients of massive early-type galaxies at large radii. Colour and metallicity gradients are significantly steeper for systems which have accreted stars in minor mergers, while galaxies with major mergers have relatively flat gradients, confirming previous results. The effect of stellar migration of in situ formed stars to large radii is discussed. This study highlights the importance of stellar accretion for stellar population properties of massive galaxies at large radii, which can provide important constraints for formation models.
    Full-text · Article · Oct 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: Galactic archeology based on star counts is instrumental to reconstruct the past mass assembly of Local Group galaxies. The development of new observing techniques and data-reduction, coupled with the use of sensitive large field of view cameras, now allows us to pursue this technique in more distant galaxies exploiting their diffuse low surface brightness (LSB) light. As part of the Atlas3D project, we have obtained with the MegaCam camera at the Canada-France Hawaii Telescope extremely deep, multi--band, images of nearby early-type galaxies. We present here a catalog of 92 galaxies from the Atlas3D sample, that are located in low to medium density environments. The observing strategy and data reduction pipeline, that achieve a gain of several magnitudes in the limiting surface brightness with respect to classical imaging surveys, are presented. The size and depth of the survey is compared to other recent deep imaging projects. The paper highlights the capability of LSB--optimized surveys at detecting new prominent structures that change the apparent morphology of galaxies. The intrinsic limitations of deep imaging observations are also discussed, among those, the contamination of the stellar halos of galaxies by extended ghost reflections, and the cirrus emission from Galactic dust. The detection and systematic census of fine structures that trace the present and past mass assembly of ETGs is one of the prime goals of the project. We provide specific examples of each type of observed structures -- tidal tails, stellar streams and shells --, and explain how they were identified and classified. We give an overview of the initial results. The detailed statistical analysis will be presented in future papers.
    Full-text · Article · Oct 2014 · Monthly Notices of the Royal Astronomical Society
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    S. K. Walch · T. Naab
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    ABSTRACT: We investigate the early impact of single and binary supernova (SN) explosions on dense gas clouds with three-dimensional, high-resolution, hydrodynamic simulations. The effect of cloud structure, radiative cooling, and ionising radiation from the progenitor stars on the net input of kinetic energy, f_kin=E_kin/E_SN, thermal energy, f_therm=E_therm/E_SN, and gas momentum f_P=P/P_SN to the interstellar medium (ISM) is tested. For clouds with n=100 cm^{-3}, the momentum generating Sedov and pressure-driven snowplough phases are terminated early (~ 0.01 Myr) and radiative cooling limits the coupling to f_therm ~ 0.01, f_kin ~ 0.05, and f_P ~ 9, significantly lower than for the case without cooling. For pre-ionised clouds these numbers are only increased by ~ 50%, independent of the cloud structure. This only suffices to accelerate ~ 5% of the cloud to radial velocities >30km/s. A second SN might further enhance the coupling efficiencies if delayed past the Sedov phase of the first explosion. Such very low coupling efficiencies cast doubts on many galaxy-scale sub-resolution models for supernova feedback, most of which are validated a posteriori by qualitative agreement of galaxy properties with observations. Ionising radiation appears not to significantly enhance the immediate coupling of SNe to the surrounding gas as it drives the ISM into inert dense shells and cold clumps, a process which is unresolved in galaxy scale simulations. Our results support previous conclusions that supernovae might only drive a wind if a significant fraction explodes in low-density environments or if they are supported by processes other than ionising radiation.
    Preview · Article · Sep 2014
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    ABSTRACT: We report on empirical trends between the dynamically determined stellar initial mass function (IMF) and stellar population properties for a complete, volume-limited sample of 260 early-type galaxies from the ATLAS(3D) project. We study trends between our dynamically derived IMF normalization alpha(dyn) equivalent to (M/L)(stars)/(M/L)(Salp) and absorption line strengths, and interpret these via single stellar population-equivalent ages, abundance ratios (measured as [alpha/Fe]), and total metallicity, [Z/H]. We find that old and alpha-enhanced galaxies tend to have on average heavier (Salpeter-like) mass normalization of the IMF, but stellar population does not appear to be a good predictor of the IMF, with a large range of alpha(dyn) at a given population parameter. As a result, we find weak alpha(dyn)-[alpha/Fe] and alpha(dyn)-Age correlations and no significant alpha(dyn)-[Z/H] correlation. The observed trends appear significantly weaker than those reported in studies that measure the IMF normalization via the low-mass star demographics inferred through stellar spectral analysis.
    No preview · Article · Sep 2014

Publication Stats

9k Citations
880.30 Total Impact Points

Institutions

  • 2009-2014
    • Max Planck Institute for Astrophysics
      Arching, Bavaria, Germany
  • 2001-2013
    • University of Cambridge
      • Institute of Astronomy
      Cambridge, England, United Kingdom
  • 2012
    • University of Turku
      • Finnish Centre for Astronomy with ESO
      Turku, Southwest Finland, Finland
    • University of Bonn
      • Argelander-Institute of Astronomy
      Bonn, North Rhine-Westphalia, Germany
    • University of Washington Seattle
      • Department of Astronomy
      Seattle, Washington, United States
  • 2011
    • University of Oxford
      • Department of Physics
      Oxford, England, United Kingdom
  • 2009-2010
    • Technische Universität München
      München, Bavaria, Germany
  • 2007-2010
    • Ludwig-Maximilians-University of Munich
      München, Bavaria, Germany
  • 2008
    • Tel Aviv University
      • Department of Physics and Astronomy
      Tel Aviv, Tel Aviv, Israel
  • 2003-2005
    • Cancer Research UK Cambridge Institute
      Cambridge, England, United Kingdom
  • 1999-2002
    • Max Planck Institute for Astronomy
      Heidelburg, Baden-Württemberg, Germany