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  • 2015-2019  (8)
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  • 1
    Publication Date: 2015-12-18
    Description: We use cosmological adaptive mesh refinement code enzo zoom-in simulations to study the long-term evolution of the collapsing gas within dark matter haloes at z . This direct collapse process is a leading candidate for rapid formation of supermassive black hole (SMBH) seeds. To circumvent the Courant condition at small radii, we apply the sink particle method, focusing on evolution on scales ~0.01–10 pc. The collapse proceeds in two stages, with the secondary runaway happening within the central 10 pc. The sink particles form when the collapsing gas requires additional refinement of the grid size at the highest refinement level. Their growth is negligible with the sole exception of the central seed which grows dramatically to M seed  ~ 2  x  10 6 M in ~2 Myr, confirming the feasibility of this path to the SMBH. The variability of angular momentum in the accreted gas results in the formation of two misaligned discs. Both discs lie within the Roche limit of the central seed. While the inner disc is geometrically thin and weakly asymmetric, the outer disc flares due to turbulent motions as a result of the massive inflow along a pair of penetrating filaments. The filamentary inflow determines the dominant Fourier modes in this disc – these modes have a non-self-gravitational origin. We do not confirm that m  = 1 is a dominant mode that drives the inflow in the presence of a central massive object. The overall configuration appears to be generic, and is expected to form when the central seed becomes sufficiently massive.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 2
    Publication Date: 2015-04-05
    Description: We examine how the small-scale (〈kpc) variation of metallicity within a galaxy, which is found in nearby galaxies, affect the observational estimates of metallicity in the explosion sites of transient events such as core-collapse supernovae (CC SNe) and gamma-ray bursts (GRBs). Assuming the same luminosity, metallicity, and spatial distributions of H ii regions (hereafter HIIRs) as observed in M31, we compute the apparent metallicities that we would obtain when the spectrum of a target region is blended with those of surrounding HIIRs within the length-scale of typical spatial resolution. When the spatial resolution of spectroscopy is 0.5 kpc, which is typical for the existing studies of CC SN sites, we find that the apparent metallicities reflect the metallicities of target regions, but with significant systematic uncertainties in some cases. When the spatial resolution is 1.0 kpc, regardless of the target regions (which has a wide range of metallicity that spans ~0.6 dex for the M31 HIIRs), we always obtain the apparent metallicities similar to the average metallicity of the M31 HIIRs. Given that the apparent metallicities measured with kpc scale resolution do not necessarily reflect the immediate environment of the stellar explosions, the current observational estimates of high metallicities for some of the long GRB host galaxies do not rule out the hypothesis that the long GRBs are exclusively born in a low-metallicity environment.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 3
    Publication Date: 2017-01-04
    Description: We perform smoothed particle hydrodynamics (SPH) simulations of an isolated galaxy with a new treatment for dust formation and destruction. To this aim, we treat dust and metal production self-consistently with star formation and supernova (SN) feedback. For dust, we consider a simplified model of grain size distribution by representing the entire range of grain sizes with large and small grains. We include dust production in stellar ejecta, dust destruction by SN shocks, grain growth by accretion and coagulation and grain disruption by shattering. We find that the assumption of fixed dust-to-metal mass ratio becomes no longer valid when the galaxy is older than 0.2 Gyr, at which point the grain growth by accretion starts to contribute to the non-linear rise of dust-to-gas ratio. As expected in our previous one-zone model, shattering triggers grain growth by accretion since it increases the total surface area of grains. Coagulation becomes significant when the galaxy age is greater than ~ 1 Gyr; at this epoch, the abundance of small grains becomes high enough to raise the coagulation rate of small grains. We further compare the radial profiles of dust-to-gas ratio $(\mathcal {D})$ and dust-to-metal ratio $(\mathcal {D}/Z$ , i.e. depletion) at various ages with observational data. We find that our simulations broadly reproduce the radial gradients of dust-to-gas ratio and depletion. In the early epoch ( 0.3 Gyr), the radial gradient of $\mathcal {D}$ follows the metallicity gradient with $\mathcal {D}/Z$ determined by the dust condensation efficiency in stellar ejecta, while the $\mathcal {D}$ gradient is steeper than the Z gradient at the later epochs because of grain growth by accretion. The framework developed in this paper is applicable to any SPH-based galaxy evolution simulations including cosmological ones.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 4
    Publication Date: 2016-12-04
    Description: We perform 3D smoothed particle hydrodynamics (SPH) simulations of gas accretion on to the seeds of binary stars to investigate their short-term evolution. Taking into account the dynamically evolving envelope with non-uniform distribution of gas density and angular momentum of accreting flow, our initial condition includes a seed binary and a surrounding gas envelope, modelling the phase of core collapse of gas cloud when the fragmentation has already occurred. We run multiple simulations with different values of initial mass ratio q 0 (the ratio of secondary over primary mass) and gas temperature. For our simulation setup, we find a critical value of q c = 0.25 which distinguishes the later evolution of mass ratio q as a function of time. If q 0 q c , the secondary seed grows faster and q increases monotonically towards unity. If q 0 q c , on the other hand, the primary seed grows faster and q is lower than q 0 at the end of the simulation. Based on our numerical results, we analytically calculate the long-term evolution of the seed binary including the growth of binary by gas accretion. We find that the seed binary with q 0 q c evolves towards an equal-mass binary star and that with q 0 q c evolves to a binary with an extreme value of q . Binary separation is a monotonically increasing function of time for any q 0 , suggesting that the binary growth by accretion does not lead to the formation of close binaries.
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  • 5
    Publication Date: 2015-07-31
    Description: The Bullet Cluster has provided some of the best evidence for the cold dark matter (CDM) model via direct empirical proof of the existence of collisionless dark matter, while posing a serious challenge owing to the unusually high inferred pairwise velocities of its progenitor clusters. Here, we investigate the probability of finding such a high-velocity pair in large-volume N -body simulations, particularly focusing on differences between halo-finding algorithms. We find that algorithms that do not account for the kinematics of infalling groups yield vastly different statistics and probabilities. When employing the rockstar halo finder that considers particle velocities, we find numerous Bullet-like pair candidates that closely match not only the high pairwise velocity, but also the mass, mass ratio, separation distance, and collision angle of the initial conditions that have been shown to produce the Bullet Cluster in non-cosmological hydrodynamic simulations. The probability of finding a high pairwise velocity pair among haloes with M halo  ≥ 10 14 M is 4.6  x  10 –4 using rockstar , while it is 34  x  lower using a friends-of-friends (FoF)-based approach as in previous studies. This is because the typical spatial extent of Bullet progenitors is such that FoF tends to group them into a single halo despite clearly distinct kinematics. Further requiring an appropriately high average mass among the two progenitors, we find the comoving number density of potential Bullet-like candidates to be of the order of 10 –10 Mpc –3 . Our findings suggest that CDM straightforwardly produces massive, high relative velocity halo pairs analogous to Bullet Cluster progenitors, and hence the Bullet Cluster does not present a challenge to the CDM model.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
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  • 6
    Publication Date: 2015-05-30
    Description: We use high-resolution zoom-in cosmological simulations of galaxies of Romano-Díaz et al., post-processing them with a panchromatic three-dimensional radiation transfer code to obtain the galaxy UV luminosity function (LF) at z ~= 6–12. The galaxies are followed in a rare, heavily overdense region within a ~5 density peak, which can host high- z quasars, and in an average density region, down to the stellar mass of M star  ~ 4 10 7 M . We find that the overdense regions evolve at a substantially accelerated pace – the most massive galaxy has grown to M star  ~ 8.4 10 10 M by z  = 6.3, contains dust of M dust  ~ 4.1 10 8 M , and is associated with a very high star formation rate, SFR ~ 745 M yr – 1 . The attained SFR– M star correlation results in the specific SFR slowly increasing with M star . Most of the UV radiation in massive galaxies is absorbed by the dust, its escape fraction f esc is low, increasing slowly with time. Galaxies in the average region have less dust, and agree with the observed UV LF. The LF of the overdense region is substantially higher, and contains much brighter galaxies. The massive galaxies are bright in the infrared (IR) due to the dust thermal emission, with L IR  ~ 3.7 10 12 L at z  = 6.3, while L IR  〈 10 11 L for the low-mass galaxies. Therefore, ALMA can probe massive galaxies in the overdense region up to z  ~ 10 with a reasonable integration time. The UV spectral properties of discy galaxies depend significantly upon the viewing angle. The stellar and dust masses of the most massive galaxy in the overdense region are comparable to those of the sub-millimetre galaxy found by Riechers et al. at z  = 6.3, while the modelled SFR and the sub-millimetre flux fall slightly below the observed one. Statistical significance of these similarities and differences will only become clear with the upcoming ALMA observations.
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    Electronic ISSN: 1365-2966
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  • 7
    Publication Date: 2016-05-12
    Description: We provide detailed comparison between the adaptive mesh refinement (AMR) code enzo -2.4 and the smoothed particle hydrodynamics (SPH)/ N -body code gadget -3 in the context of isolated or cosmological direct baryonic collapse within dark matter (DM) haloes to form supermassive black holes. Gas flow is examined by following evolution of basic parameters of accretion flows. Both codes show an overall agreement in the general features of the collapse; however, many subtle differences exist. For isolated models, the codes increase their spatial and mass resolutions at different pace, which leads to substantially earlier collapse in SPH than in AMR cases due to higher gravitational resolution in gadget -3. In cosmological runs, the AMR develops a slightly higher baryonic resolution than SPH during halo growth via cold accretion permeated by mergers. Still, both codes agree in the build-up of DM and baryonic structures. However, with the onset of collapse, this difference in mass and spatial resolution is amplified, so evolution of SPH models begins to lag behind. Such a delay can have effect on formation/destruction rate of H 2 due to UV background, and on basic properties of host haloes. Finally, isolated non-cosmological models in spinning haloes, with spin parameter ~ 0.01–0.07, show delayed collapse for greater , but pace of this increase is faster for AMR. Within our simulation set-up, gadget -3 requires significantly larger computational resources than enzo -2.4 during collapse, and needs similar resources, during the pre-collapse, cosmological structure formation phase. Yet it benefits from substantially higher gravitational force and hydrodynamic resolutions, except at the end of collapse.
    Print ISSN: 0035-8711
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  • 8
    Publication Date: 2015-06-12
    Description: We use high-resolution zoom-in cosmological simulations of galaxies of Romano-Díaz et al., post-processing them with a panchromatic three-dimensional radiation transfer code to obtain the galaxy UV luminosity function (LF) at z ~= 6–12. The galaxies are followed in a rare, heavily overdense region within a ~5 density peak, which can host high- z quasars, and in an average density region, down to the stellar mass of M star  ~ 4 10 7 M . We find that the overdense regions evolve at a substantially accelerated pace – the most massive galaxy has grown to M star  ~ 8.4 10 10 M by z  = 6.3, contains dust of M dust  ~ 4.1 10 8 M , and is associated with a very high star formation rate, SFR ~ 745 M yr – 1 . The attained SFR– M star correlation results in the specific SFR slowly increasing with M star . Most of the UV radiation in massive galaxies is absorbed by the dust, its escape fraction f esc is low, increasing slowly with time. Galaxies in the average region have less dust, and agree with the observed UV LF. The LF of the overdense region is substantially higher, and contains much brighter galaxies. The massive galaxies are bright in the infrared (IR) due to the dust thermal emission, with L IR  ~ 3.7 10 12 L at z  = 6.3, while L IR  〈 10 11 L for the low-mass galaxies. Therefore, ALMA can probe massive galaxies in the overdense region up to z  ~ 10 with a reasonable integration time. The UV spectral properties of discy galaxies depend significantly upon the viewing angle. The stellar and dust masses of the most massive galaxy in the overdense region are comparable to those of the sub-millimetre galaxy found by Riechers et al. at z  = 6.3, while the modelled SFR and the sub-millimetre flux fall slightly below the observed one. Statistical significance of these similarities and differences will only become clear with the upcoming ALMA observations.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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