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, 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
$\bar{n} = 100\;{\rm cm}^{-3}$
, the momentum generating Sedov and pressure-driven snowplough phases are terminated early (∝0.01 Myr) and radiative cooling limits the coupling to f
therm ∼ 0.01, f
kin ∼ 0.05, and f
P ∼ 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.
Theoretical Challenges in Galaxy Formation Naab, Thorsten; Ostriker, Jeremiah P
Annual review of astronomy and astrophysics,
08/2017, Letnik:
55, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Numerical simulations have become a major tool for understanding galaxy formation and evolution. Over the decades the field has made significant progress. It is now possible to simulate the formation ...of individual galaxies and galaxy populations from well-defined initial conditions with realistic abundances and global properties. An essential component of the calculation is to correctly estimate the inflow to and outflow from forming galaxies because observations indicating low formation efficiency and strong circumgalactic presence of gas are persuasive. Energetic "feedback" from massive stars and accreting supermassive black holes-generally unresolved in cosmological simulations-plays a major role in driving galactic outflows, which have been shown to regulate many aspects of galaxy evolution. A surprisingly large variety of plausible subresolution models succeeds in this exercise. They capture the essential characteristics of the problem, i.e., outflows regulating galactic gas flows, but their predictive power is limited. In this review, we focus on one major challenge for galaxy formation theory: to understand the underlying physical processes that regulate the structure of the interstellar medium, star formation, and the driving of galactic outflows. This requires accurate physical models and numerical simulations, which can precisely describe the multiphase structure of the interstellar medium on the currently unresolved few hundred parsec scales of large-scale cosmological simulations. Such models ultimately require the full accounting for the dominant cooling and heating processes, the radiation and winds from massive stars and accreting black holes, and an accurate treatment of supernova explosions as well as the nonthermal components of the interstellar medium like magnetic fields and cosmic rays.
There is observational evidence for inside-out growth of giant elliptical galaxies since z 2-3, which is - in contrast to disc galaxies - not driven by in situ star formation. Many of the ∼1011 M ...systems at high redshift have small sizes ∼1 kpc and surface brightness profiles with low-Sérsic indices n. The most likely descendants at z = 0 have, on average, grown by a factor of 2 in mass and a factor of 4 in size, indicating r ∝ M
α with α 2. They also have surface brightness profiles with n 5. This evolution can be qualitatively explained on the basis of two assumptions: compact ellipticals predominantly grow by collisionless minor (mass-ratio 1:10) or intermediate (mass-ratio 1:5) 'dry' mergers, and they are embedded in massive dark matter haloes which support the stripping of merging satellite stars at large radii. We draw these conclusions from idealized collisionless mergers spheroidal galaxies - with and without dark matter - with mass ratios of 1:1, 1:5 and 1:10. The sizes evolve as r ∝ M
α with α < 2 for mass-ratios of 1:1 (and 1:5 without dark matter haloes) and, while doubling the stellar mass, the Sérsic index increases from n ∼ 4 to n ∼ 5. For minor mergers of galaxies embedded in dark matter haloes, the sizes grow significantly faster and the profile shapes change more rapidly. Surprisingly, already mergers with moderate mass-ratios of 1:5, well motivated by recent cosmological simulations, give α ∼ 2.3 and after only two merger generations (∼40 per cent added stellar mass) the Sérsic index has increased to n > 8 (n ∼ 5.5 without dark matter), reaching a final value of n = 9.5 after doubling the stellar mass. This is accompanied by a significant increase of the dark matter fraction (from ∼40 to 70 per cent) within the stellar half-mass radius, driven by the strong size increase probing larger, dark matter-dominated regions. For equal-mass mergers the effect is much weaker. We conclude that only a few intermediate mass-ratio mergers (∼ 3-5 with initial mass-ratios of 1:5) of galaxies embedded in massive dark matter haloes can result in the observed concurrent inside-out growth and the rapid evolution in profile shapes. This process might explain the existence of present-day giant ellipticals with sizes, r > 4 kpc, high Sérsic indices, n > 5, and a significant amount of dark matter within the half-light radius. Apart from negative stellar metallicity gradients such a 'minor' merger scenario also predicts significantly lower dark matter fractions for z ∼ 2 compact quiescent galaxies and their rare present-day analogues.
ABSTRACT
Molecular outflows contributing to the matter cycle of star-forming galaxies are now observed in small and large systems at low and high redshift. Their physical origin is still unclear. In ...most theoretical studies, only warm ionized/neutral and hot gas outflowing from the interstellar medium is generated by star formation. We investigate an in situ H2 formation scenario in the outflow using high-resolution simulations, including non-equilibrium chemistry and self-gravity, of turbulent, warm, and atomic clouds with densities 0.1, 0.5, and $1\, \mathrm{cm}^{-3}$ exposed to a magnetized hot wind. For cloud densities $\gtrsim 0.5\, \mathrm{cm}^{-3}$, a magnetized wind triggers H2 formation before cloud dispersal. Up to 3 per cent of the initial cloud mass can become molecular on $\sim \! 10\, \mathrm{Myr}$ time-scales. The effect is stronger for winds with perpendicular B-fields and intermediate density clouds ($n_\mathrm{c}\sim 0.5\, \mathrm{cm}^{-3}$). Here, H2 formation can be boosted by up to one order of magnitude compared to isolated cooling clouds independent of self-gravity. Self-gravity preserves the densest clouds well past their $\sim \! 15\, \mathrm{Myr}$ cloud crushing time-scales. This model could provide a plausible in situ origin for the observed molecular gas. All simulations form warm ionized gas, which represents an important observable phase. The amount of warm ionized gas is almost independent of the cloud density but solely depends on the magnetic field configuration in the wind. For low-density clouds ($0.1\, \mathrm{cm}^{-3}$), up to 60 per cent of the initially atomic cloud mass can become warm and ionized.
We present a new statistical method to determine the relationship between the stellar masses of galaxies and the masses of their host dark matter haloes over the entire cosmic history from z ∼ 4 to ...the present. This multi-epoch abundance matching (MEAM) model self-consistently takes into account that satellite galaxies first become satellites at times earlier than they are observed. We employ a redshift-dependent parametrization of the stellar-to-halo-mass relation to populate haloes and subhaloes in the Millennium simulations with galaxies, requiring that the observed stellar mass functions at several redshifts are reproduced simultaneously. We show that physically meaningful growth of massive galaxies is consistent with these data only if observational mass errors are taken into account. Using merger trees extracted from the dark matter simulations in combination with MEAM, we predict the average assembly histories of galaxies, separating into star formation within the galaxies (in situ) and accretion of stars (ex situ). Our main results are the peak star formation efficiency decreases with redshift from 23 per cent at z = 0 to 9 per cent at z =4 while the corresponding halo mass increases from 1011.8 to 1012.5 M. The star formation rate of central galaxies peaks at a redshift which depends on halo mass; for massive haloes this peak is at early cosmic times while for low-mass galaxies the peak has not been reached yet. In haloes similar to that of the Milky Way about half of the central stellar mass is assembled after z = 0.7. In low-mass haloes, the accretion of satellites contributes little to the assembly of their central galaxies, while in massive haloes more than half of the central stellar mass is formed ex situ with significant accretion of satellites at z < 2. We find that our method implies a cosmic star formation history and an evolution of specific star formation rates which are consistent with those inferred directly. We present convenient fitting functions for stellar masses, star formation rates and accretion rates as functions of halo mass and redshift.
We present high-resolution (∼0.1 pc), hydrodynamical and magnetohydrodynamical simulations to investigate whether the observed level of molecular cloud (MC) turbulence can be generated and maintained ...by external supernova (SN) explosions. The MCs are formed self-consistently within their large-scale galactic environment following the non-equilibrium formation of H2 and CO, including (self-) shielding and important heating and cooling processes. The MCs inherit their initial level of turbulence from the diffuse ISM, where turbulence is injected by SN explosions. However, by systematically exploring the effect of individual SNe going off outside the clouds, we show that at later stages the importance of SN-driven turbulence is decreased significantly. This holds for different MC masses as well as for MCs with and without magnetic fields. The SN impact also decreases rapidly with larger distances. Nearby SNe (d ∼ 25 pc) boost the turbulent velocity dispersions of the MC by up to 70% (up to a few km s−1). For d > 50 pc, however, their impact decreases fast with increasing d and is almost negligible. For all probed distances the gain in velocity dispersion decays rapidly within a few 100 kyr. This is significantly shorter than the average timescale for an MC to be hit by a nearby SN under solar neighborhood conditions (∼2 Myr). Hence, at these conditions SNe are not able to sustain the observed level of MC turbulence. However, in environments with high gas surface densities and SN rates, like the Central Molecular Zone, observed elevated MC dispersions could be triggered by external SNe.
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.
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