We analyze 40 cosmological re-simulations of individual massive galaxies with present-day stellar masses of M * > 6.3 X 1010 M in order to investigate the physical origin of the observed strong ...increase in galaxy sizes and the decrease of the stellar velocity dispersions since redshift z 2. At present 25 out of 40 galaxies are quiescent with structural parameters (sizes and velocity dispersions) in agreement with local early-type galaxies. At z = 2 all simulated galaxies with M * 1011 M (11 out of 40) at z = 2 are compact with projected half-mass radii of 0.77 (?0.24) kpc and line-of-sight velocity dispersions within the projected half-mass radius of 262 (?28) km s--1 (3 out of 11 are already quiescent). Similar to observed compact early-type galaxies at high redshift, the simulated galaxies are clearly offset from the local mass-size and mass-velocity dispersion relations. Toward redshift zero the sizes increase by a factor of ~5-6, following R 1/2(1 + z) Delta *a with Delta *a = --1.44 for quiescent galaxies ( Delta *a = --1.12 for all galaxies). The velocity dispersions drop by about one-third since z 2, following Delta *s1/2(1 + z) Delta *b with Delta *b = 0.44 for the quiescent galaxies ( Delta *b = 0.37 for all galaxies). The simulated size and dispersion evolution is in good agreement with observations and results from the subsequent accretion and merging of stellar systems at z 2, which is a natural consequence of the hierarchical structure formation. A significant number of the simulated massive galaxies (7 out of 40) experience no merger more massive than 1:4 (usually considered as major mergers). On average, the dominant accretion mode is stellar minor mergers with a mass-weighted mass ratio of 1:5. We therefore conclude that the evolution of massive early-type galaxies since z 2 and their present-day properties are predominantly determined by frequent 'minor' mergers of moderate mass ratios and not by major mergers alone.
Given its velocity dispersion, the early-type galaxy NGC 1600 has an unusually massive (M = 1.7 × 1010 M ) central supermassive black hole (SMBH) surrounded by a large core (rb = 0.7 kpc) with a ...tangentially biased stellar distribution. We present high-resolution equal-mass merger simulations including SMBHs to study the formation of such systems. The structural parameters of the progenitor ellipticals were chosen to produce merger remnants resembling NGC 1600. We test initial stellar density slopes of ∝ r−1 and ∝ r−3/2 and vary the initial SMBH masses from 8.5 × 108 to 8.5 × 109 M . With increasing SMBH mass, the merger remnants show a systematic decrease in central surface brightness, an increasing core size, and an increasingly tangentially biased central velocity anisotropy. Two-dimensional kinematic maps reveal decoupled, rotating core regions for the most massive SMBHs. The stellar cores form rapidly as the SMBHs become bound, while the velocity anisotropy develops more slowly after the SMBH binaries become hard. The simulated merger remnants follow distinct relations between the core radius and the sphere of influence, and the SMBH mass, similar to observed systems. We find a systematic change in the relations as a function of the progenitor density slope and present a simple scouring model reproducing this behavior. Finally, we find the best agreement with NGC 1600 using SMBH masses totaling the observed value of M = 1.7 × 1010 M . In general, density slopes of ∝ r−3/2 for the progenitor galaxies are strongly favored for the equal-mass merger scenario.
Using a high-resolution hydrodynamical cosmological simulation of the formation of a massive spheroidal galaxy we show that elliptical galaxies can be very compact and massive at high redshift in ...agreement with recent observations. Accretion of stripped infalling stellar material increases the size of the system with time and the central concentration is reduced by dynamical friction of the surviving stellar cores. In a specific case of a spheroidal galaxy with a final stellar mass of 1.5 X 1011 M we find that the effective radius re increases from 0.7 - 0.2 kpc at z = 3 to re = 2.4 - 0.4 kpc at z = 0 with a concomitant decrease in the effective density of an order of magnitude and a decrease of the central velocity dispersion by approximately 20% over this time interval. A simple argument based on the virial theorem shows that during the accretion of weakly bound material (minor mergers) the radius can increase as the square of the mass in contrast to the usual linear rate of increase for major mergers. By undergoing minor mergers compact high-redshift spheroids can evolve into present-day systems with sizes and concentrations similar to observed local ellipticals. This indicates that minor mergers may be the main driver for the late evolution of sizes and densities of early-type galaxies.
The direct collapse model of supermassive black hole seed formation requires that the gas cools predominantly via atomic hydrogen. To this end we simulate the effect of an anisotropic radiation ...source on the collapse of a halo at high redshift. The radiation source is placed at a distance of 3 kpc (physical) from the collapsing object and is set to emit monochromatically in the center of the Lyman-Werner (LW) band. The LW radiation emitted from the high redshift source is followed self-consistently using ray tracing techniques. Due to self-shielding, a small amount of H sub(2) is able to form at the very center of the collapsing halo even under very strong LW radiation. Furthermore, we find that a radiation source, emitting >10 super(54) (~10 super(3) J sub(21) ) photons s super(-1), is required to cause the collapse of a clump of M ~ 10 super(5) M sub(middot in circle). The resulting accretion rate onto the collapsing object is ~0.25 M sub(middot in circle) yr super(-1). Our results display significant differences, compared to the isotropic radiation field case, in terms of the H sub(2) fraction at an equivalent radius. These differences will significantly affect the dynamics of the collapse. With the inclusion of a strong anisotropic radiation source, the final mass of the collapsing object is found to be M ~ 10 super(5) M sub(middot in circle). This is consistent with predictions for the formation of a supermassive star or quasi-star leading to a supermassive black hole.
We study the growth of black holes (BHs) in galaxies using three-dimensional smoothed particle hydrodynamic simulations with new implementations of the momentum mechanical feedback, and restriction ...of accreted elements to those that are gravitationally bound to the BH. We also include the feedback from the X-ray radiation emitted by the BH, which heats the surrounding gas in the host galaxies, and adds radial momentum to the fluid. We perform simulations of isolated galaxies and merging galaxies and test various feedback models with the new treatment of the Bondi radius criterion. We find that overall the BH growth is similar to what has been obtained by earlier works using the Springel, Di Matteo, & Hernquist algorithms. However, the outflowing wind velocities and mechanical energy emitted by winds are considerably higher (v sub(w) ~ 1000-3000 km s super(-1)) compared to the standard thermal feedback model (v sub(w) ~ 50-100 km s super(-1)). While the thermal feedback model emits only 0.1% of BH released energy in winds, the momentum feedback model emits more than 30% of the total energy released by the BH in winds. In the momentum feedback model, the degree of fluctuation in both radiant and wind output is considerably larger than in standard treatments. We check that the new model of BH mass accretion agrees with analytic results for the standard Bondi problem.
We present a hydrodynamical simulation at sub-parsec and few-solar-mass resolution of a merger between two gas-rich dwarf galaxies. Our simulation includes a detailed model for the multi-phase ...interstellar medium and is able to follow the entire formation history of spatially resolved star clusters, including feedback from individual massive stars. Shortly after the merger we find a population of ∼900 stellar clusters with masses above and a cluster mass function (CMF), which is well fitted with a power law with a slope of = −1.70 0.08. We describe here in detail the formation of the three most massive clusters (M* 105 M ), which populate the high-mass end of the CMF. The simulated clusters form rapidly on a timescale of 6-8 Myr in converging flows of dense gas. The embedded merger phase has extremely high star formation rate surface densities of M yr−1 kpc−2 and thermal gas pressures in excess of . The formation process is terminated by rapid gas expulsion driven by the first generation of supernovae, after which the cluster centers relax and both their structure and kinematics become indistinguishable from observed local globular clusters (GCs). The simulation presented here provides a general model for the formation of metal-poor GCs in chemically unevolved starbursting environments of low-mass dwarf galaxies, which are common at high redshifts.
ABSTRACT
The total mass of the Local Group and the masses of its primary constituents, the Milky Way (MW) and M31, are important anchors for several cosmological questions. Recent independent ...measurements have consistently yielded halo masses close to 1012M⊙ for the MW, and 1–2 × 1012M⊙ for M31, while estimates derived from the pair’s kinematics via the ‘timing argument’ have yielded a combined mass of around 5 × 1012M⊙. We analyse the extremely large Uchuu simulation to constrain the mass of the Local Group and its two most massive members. First, we demonstrate the importance of selecting pairs whose kinematics reflect their mutual interactions. Adopting the observed separation and radial velocity, we obtain a weighted posterior of $75_{-40}^{+65}$ km s−1 for the uncertain transverse velocity. Via Gaussian process regression, we infer a total mass of $3.2^{+1.2}_{-0.9} \times 10^{12} \mathrm{M}_\odot$, significantly below the timing argument value. Importantly, the remaining uncertainty is not rooted in the analysis or observational errors, but in the irreducible scatter in the kinematics–mass relation. We further find a mass for the less massive halo of $0.9_{-0.3}^{+0.6} \times 10^{12} \mathrm{M}_\odot$ and for the more massive halo of $2.3_{-0.9}^{+1.0} \times 10^{12} \mathrm{M}_\odot$, consistent with independent measurements of the masses of MW and M31, respectively. Incorporating the MW mass as an additional prior let us constrain all measurements further and determine that the MW is very likely less massive than M31.
The Two Phases of Galaxy Formation Oser, Ludwig; Ostriker, Jeremiah P; Naab, Thorsten ...
The Astrophysical journal,
12/2010, Letnik:
725, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Cosmological simulations of galaxy formation appear to show a 'two-phase' character with a rapid early phase at z 2 during which 'in situ' stars are formed within the galaxy from infalling cold gas ...followed by an extended phase since z 3 during which 'ex situ' stars are primarily accreted. In the latter phase, massive systems grow considerably in mass and radius by accretion of smaller satellite stellar systems formed at quite early times (z>3) outside of the virial radius of the forming central galaxy. These tentative conclusions are obtained from high-resolution re-simulations of 39 individual galaxies in a full cosmological context with present-day virial halo masses ranging from 7 X 1011 M h --1 M vir 2.7 X 1013 M h --1 (h = 0.72) and central galaxy masses between 4.5 X 1010 M h --1 M * 3.6 X 1011 M h --1. The simulations include the effects of a uniform UV background, radiative cooling, star formation, and energetic feedback from Type II supernova. The importance of stellar accretion increases with galaxy mass and toward lower redshift. In our simulations, lower mass galaxies (M * 9 X 1010 M h --1) accrete about 60% of their present-day stellar mass. High-mass galaxy (M * 1.7 X 1011 M h --1) assembly is dominated by accretion and merging with about 80% of the stars added by the present day. In general the simulated galaxies approximately double their mass since z = 1. For massive systems this mass growth is not accompanied by significant star formation. The majority of the in situ created stars are formed at z>2, primarily out of cold gas flows. We recover the observational result of 'archaeological downsizing,' where the most massive galaxies harbor the oldest stars. We find that this is not in contradiction with hierarchical structure formation. Most stars in the massive galaxies are formed early on in smaller structures; the galaxies themselves are assembled late.
We describe high-resolution smoothed particle hydrodynamics (SPH) simulations of three approximately M* field galaxies starting from ACDM initial conditions. The simulations are made intentionally ...simple, and include photoionization, cooling of the intergalactic medium, and star formation, but not feedback from AGNs or supernovae. All of the galaxies undergo an initial burst of star formation at z -5, accompanied by the formation of a bubble of heated gas. Two out of three galaxies show early-type properties at present, whereas only one of them experienced a major merger. Heating from shocks and PdV work dominates over cooling so that for most of the gas the temperature is an increasing function of time. By z -1 a significant fraction of the final stellar system is in place and the spectral energy distribution resembles those of observed massive red galaxies. The galaxies have grown from z = 1 10 on average by 25% in mass and in size by gas-poor (dry) stellar mergers. By the present day the simulated galaxies are old (-10 Gyr), kinematically hot stellar systems surrounded by hot gaseous haloes. Stars dominate the mass of the galaxies up to -4 effective radii (-10 kpc). Kinematic and most photometric properties are in good agreement with those of observed elliptical galaxies. The galaxy with a major merger develops a counter-rotating core. Our simulations show that realistic intermediate-mass giant elliptical galaxies with plausible formation histories can be formed from ACDM initial conditions even without requiring recent major mergers or feedback from supernovae or AGNs.
We present KETJU, a new extension of the widely used smoothed particle hydrodynamics simulation code GADGET-3. The key feature of the code is the inclusion of algorithmically regularized regions ...around every supermassive black hole (SMBH). This allows for simultaneously following global galactic-scale dynamical and astrophysical processes, while solving the dynamics of SMBHs, SMBH binaries, and surrounding stellar systems at subparsec scales. The KETJU code includes post-Newtonian terms in the equations of motions of the SMBHs, which enables a new SMBH merger criterion based on the gravitational wave coalescence timescale, pushing the merger separation of SMBHs down to ∼0.005 pc. We test the performance of our code by comparison to NBODY7 and rVINE. We set up dynamically stable multicomponent merger progenitor galaxies to study the SMBH binary evolution during galaxy mergers. In our simulation sample the SMBH binaries do not suffer from the final-parsec problem, which we attribute to the nonspherical shape of the merger remnants. For bulge-only models, the hardening rate decreases with increasing resolution, whereas for models that in addition include massive dark matter halos, the SMBH binary hardening rate becomes practically independent of the mass resolution of the stellar bulge. The SMBHs coalesce on average 200 Myr after the formation of the SMBH binary. However, small differences in the initial SMBH binary eccentricities can result in large differences in the SMBH coalescence times. Finally, we discuss the future prospects of KETJU, which allows for a straightforward inclusion of gas physics in the simulations.
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