We present two‐dimensional spectroscopy covering the rest‐frame wavelengths of strong optical emission lines in six luminous submillimetre galaxies (SMGs) at z= 1.3–2.5. Using this near‐infrared integral field spectroscopy together with Hubble Space Telescope ACS and NICMOS imaging, we map the dynamics and morphologies of these systems on scales from 4–11 kpc. Four of the systems show multiple components in their spatially resolved spectra with average velocity offsets of ∼180 km s−1 across 8 kpc in projection. From the ensemble properties of eight galaxies, from our survey and the literature, we estimate the typical dynamical masses of bright SMGs as 5 ± 3 × 1011 M⊙. This is similar to recent estimates of their stellar masses – suggesting that the dynamics of the central regions of these galaxies are baryon dominated, with a substantial fraction of those baryons in stars by the epoch of observation. Combining our dynamical mass estimates with stellar luminosities for this population, we investigate whether SMGs can evolve on to the Faber–Jackson (FJ) relation for local ellipticals. Adopting a typical lifetime of τburst∼ 300 Myr for the submillimetre‐luminous phase – using the latest estimates of gas masses, star formation rates and active galactic nucleus contribution to the bolometric luminosities – we find that the stellar populations of SMGs should fade to place them on the FJ relation, at MK∼−25.1. Furthermore, using the same starburst lifetime we correct the observed space density of SMGs for the duty cycle to derive a volume density of the progenitors of ∼1 × 10−4 Mpc−3. This is consistent with the space density of local luminous early‐type galaxies with MK∼−25.1, indicating that SMGs can evolve on to the scaling relations observed for local early‐type galaxies, and the observed population at z∼ 2 is then sufficient to account for the formation of the whole population of ≳3 L*K ellipticals seen at z∼ 0.
We present axisymmetric hydrodynamical simulations of the long-term accretion of a rotating gamma-ray burst (GRB) progenitor star, a "collapsar," onto the central compact object, which we take to be a black hole. The simulations were carried out with the adaptive-mesh-refinement code FLASH in two spatial dimensions and with an explicit shear viscosity. The evolution of the central accretion rate exhibits phases reminiscent of the long GRB γ -ray and X-ray light curve, which lends support to the proposal by Kumar et al. that the luminosity is modulated by the central accretion rate. In the first "prompt" phase, the black hole acquires most of its final mass through supersonic quasiradial accretion occurring at a steady rate of ∼0.2 M s −1 . After a few tens of seconds, an accretion shock sweeps outward through the star. The formation and outward expansion of the accretion shock is accompanied with a sudden and rapid power-law decline in the central accretion rateṀ ∝ t −2.8 , which resembles the L X ∝ t −3decline observed in the X-ray light curves. The collapsed, shock-heated stellar envelope settles into a thick, low-mass equatorial disk embedded within a massive, pressure-supported atmosphere. After a few hundred seconds, the inflow of low angular momentum material in the axial funnel reverses into an outflow from the thick disk. Meanwhile, the rapid decline of the accretion rate slows down, which is potentially suggestive of the "plateau" phase in the X-ray light curve. We complement our adiabatic simulations with an analytical model that takes into account the cooling by neutrino emission and estimate that the duration of the prompt phase can be ∼20 s. The model suggests that the steep decline in GRB X-ray light curves is triggered by the circularization of the infalling stellar envelope at radii where the virial temperature is below 10 10 K, such that neutrino cooling is inefficient and an outward expansion of the accretion shock becomes imminent; GRBs with longer prompt γ -ray emission should have more slowly rotating envelopes.
In recent work we presented the first results of global general relativistic magnetohydrodynamic (GRMHD) simulations of tilted (or misaligned) accretion disks around rotating black holes. The simulated tilted disks showed dramatic differences from comparable untilted disks, such as asymmetrical accretion onto the hole through opposing "plunging streams" and global precession of the disk powered by a torque provided by the black hole. However, those simulations used a traditional spherical-polar grid that was purposefully underresolved along the pole, which prevented us from assessing the behavior of any jets that may have been associated with the tilted disks. To address this shortcoming we have added a block-structured "cubed-sphere" grid option to the Cosmos++ GRMHD code, which will allow us to simultaneously resolve the disk and polar regions. Here we present our implementation of this grid and the results of a small suite of validation tests intended to demonstrate that the new grid performs as expected. The most important test in this work is a comparison of identical tilted disks, one evolved using our spherical-polar grid and the other with the cubed-sphere grid. We also demonstrate an interesting dependence of the early-time evolution of our disks on their orientation with respect to the grid alignment. This dependence arises from the differing treatment of current sheets within the disks, especially whether they are aligned with symmetry planes of the grid or not.
The association of long-duration gamma-ray bursts (LGRBs) with Type Ic supernovae presents a challenge to supernova explosion models. In the collapsar model for LGRBs, gamma rays are produced in an ultrarelativistic jet launching from the magnetosphere of the black hole that forms in the aftermath of the collapse of a rotating progenitor star. The jet is collimated along the star's rotation axis, but the concomitant luminous supernova should be relatively-though certainly not entirely-spherical, and should synthesize a substantial mass of 56 Ni. Our goal is to provide a qualitative assessment of the possibility that accretion of the progenitor envelope onto the black hole, which powers the LGRB, could also deposit sufficient energy and nickel mass in the envelope to produce a luminous supernova. For this, the energy dissipated near the black hole during accretion must be transported outward, where it can drive a supernova-like shockwave. Here we suggest that the energy is transported by convection and develop an analytical toy model, relying on global mass and energy conservation, for the dynamics of stellar collapse. The model suggests that a ∼ 10, 000 km s −1 shock can be driven into the envelope and that ∼ 10 51 erg explosions are possible. The efficiency with which the accretion energy is being transferred to the envelope is governed by the competition of advection and convection at distances ∼ 100 − 1, 000 km from the black hole and is sensitive to the values of the convective mixing length, the magnitude of the effective viscous stress, and the specific angular momentum of the infalling envelope. Substantial masses of 56 Ni may be synthesized in the convective accretion flow over the course of tens of seconds from the initial circularization of the infalling envelope around the black hole. The synthesized nickel is convectively mixed with a much larger mass of unburned ejecta.
Observational evidence suggests a link between long duration gamma ray bursts (LGRBs) and Type Ic supernovae. Here, we propose a potential mechanism for Type Ic supernovae in LGRB progenitors powered solely by accretion energy. We present spherically-symmetric hydrodynamic simulations of the long-term accretion of a rotating gamma-ray burst progenitor star, a "collapsar," onto the central compact object, which we take to be a black hole. The simulations were carried out with the adaptive mesh refinement code FLASH in one spatial dimension and with rotation, an explicit shear viscosity, and convection in the mixing length theory approximation. Once the accretion flow becomes rotationally supported outside of the black hole, an accretion shock forms and traverses the stellar envelope. Energy is carried from the central geometrically thick accretion disk to the stellar envelope by convection. Energy losses through neutrino emission and nuclear photodisintegration are calculated but do not seem important following the rapid early drop of the accretion rate following circularization. We find that the shock velocity, energy, and unbound mass are sensitive to convective efficiency, effective viscosity, and initial stellar angular momentum. Our simulations show that given the appropriate combinations of stellar and physical parameters, explosions with energies ∼ 5 × 10 50 ergs, velocities ∼ 3000 km s −1 , and unbound material masses 6 M are possible in a rapidly rotating 16 M main sequence progenitor star. Further work is needed to constrain the values of these parameters, to identify the likely outcomes in more plausible and massive LRGB progenitors, and to explore nucleosynthetic implications.
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