Sloan Fellows

About: Sloan Fellows is a based out in . It is known for research contribution in the topics: Galaxy & Star formation. The organization has 55 authors who have published 253 publications receiving 35008 citations. The organization is also known as: Sloan Fellows.

Large Stellar Disks in Small Elliptical Galaxies

Abstract: We present stellar kinematics along the principal axes of seven elliptical galaxies less luminous than MB=-19.5, which extend beyond the half-light radii for all systems in this photometrically selected sample. At large radii, the kinematics not only confirm that rotation and "diskiness" are important in faint elliptical galaxies, as was previously known, but show that rotation dominates: the stars at large galactocentric distances have (V/σ)max~2, similar to the disks in bona fide S0 galaxies. A comparison with published simulations of dissipationless mergers is not straightforward. Yet, within Re, the observed galaxies seem to rotate somewhat faster than 3:1 merger remnants, arguing against major mergers as the dominant mechanism in the final shaping of low-luminosity elliptical galaxies and favoring instead the dissipative formation of a disk.

Structure and evolution of circumbinary disks around supermassive black hole binaries

Abstract: We explore properties of circumbinary disks around supermassive black hole (SMBH) binaries in centers of galaxies by reformulating standard viscous disk evolution in terms of the viscous angular momentum flux FJ . If the binary stops gas inflow and opens a cavity in the disk, then the inner disk evolves toward a constant-FJ (rather than a constant ) state. We compute disk properties in different physical regimes relevant for SMBH binaries, focusing on the gas-assisted evolution of systems starting at separations 10–4 – 10–2 pc, and find the following. (1) Mass pileup at the inner disk edge caused by the tidal barrier accelerates binary inspiral. (2) Binaries can be forced to merge even by a disk with a mass below that of the secondary. (3) Torque on the binary is set non-locally, at radii far larger than the binary semi-major axis; its magnitude does not reflect disk properties in the vicinity of the binary. (4) Binary inspiral exhibits hysteresis—it depends on the past evolution of the disk. (5) The Eddington limit can be important for circumbinary disks even if they accrete at sub-Eddington rates, but only at late stages of the inspiral. (6) Gas overflow across the orbit of the secondary can be important for low secondary mass, high- systems, but mainly during the inspiral phase dominated by the gravitational wave emission. (7) Circumbinary disks emit more power and have harder spectra than constant disks; their spectra are very sensitive to the amount of overflow across the secondary orbit.

Density Waves Excited by Low-Mass Planets in Protoplanetary Disks II: High-Resolution Simulations of the Nonlinear Regime

Abstract: We investigate numerically the propagation of density waves excited by a low-mass planet in a protoplanetary disk in the nonlinear regime, using 2D local shearing box simulations with the grid-based code Athena at high spatial resolution (256 grid points per scale height h). The nonlinear evolution results in the wave steepening into a shock, causing damping and angular momentum transfer to the disk. On long timescales this leads to spatial redistribution of the disk density, causing migration feedback and potentially resulting in gap opening. Previous numerical studies concentrated on exploring these secondary phenomena as probes of the nonlinear wave evolution. Here we focus on exploring the evolution of the basic wave properties, such as its density profile evolution, shock formation, post-shock wave behavior, and provide comparison with analytical theory. The generation of potential vorticity at the shock is computed analytically and is subsequently verified by simulations and used to pinpoint the shock location. We confirm the theoretical relation between the shocking length and the planet mass (including the effect of the equation of state), and the post-shock decay of the angular momentum flux carried by the wave. The post-shock evolution of the wave profile is explored, and we quantitatively confirm its convergence to the theoretically expected N-wave shape. The accuracy of various numerical algorithms used to compute the nonlinear wave evolution is also investigated: we find that higher order spatial reconstruction and high resolution are crucial for capturing the shock formation correctly.

The Physics of the Neutrino Mechanism of Core-Collapse Supernovae

Abstract: (Abridged) Neutrino heating may drive core-collapse supernova explosions. Although it is known that the stalled accretion shock turns into explosion when the neutrino luminosity from the collapsed core exceeds a critical value (L_crit) (the "neutrino mechanism"), the physics of L_crit, as well as its dependence on the properties of the proto-neutron star (PNS) and changes to the microphysics has never been systematically explored. We solve the one-dimensional steady-state accretion problem between the PNS surface and the accretion shock. We quantify the deep connection between the solution space of steady-state accretion flows with bounding shocks and the neutrino mechanism. We show that there is a maximum, critical sound speed above which it is impossible to maintain accretion with a standoff shock, because the shock jump conditions cannot be satisfied. The physics of this critical sound speed is general and does not depend on a specific heating mechanism. For the simple model of pressure-less free-fall onto a shock bounding an isothermal accretion flow with sound speed c_T, we show that if c_T^2/v_escape^2 > 3/16 explosion results. We generalize this result to the more complete supernova problem, showing explicitly that the same physics determines L_crit. We find that the critical condition for explosion can be written as c_S^2/v_escape^2 = 0.19, where c_S is the adiabatic sound speed. This "antesonic" condition describes L_crit over a broad range in accretion rate and microphysics. We show that the addition of the accretion luminosity (L_acc) reduces L_crit non-trivially. As in previous work, we find that L_crit is always significantly higher than the maximum possible value of L_acc. Finally, we provide evidence that the reduction in L_crit seen in recent multi-dimensional simulations results from a reduction in the efficiency of cooling, rather than an increase in the heating rate.

Quantitative Analysis of Galaxy-Galaxy Lensing

Abstract: Gravitational light deflection due to mass along the line of sight will distort the images of background sources. Although an individual galaxy is not massive enough to cause a detectable lensing distortion in the background population, this effect can be measured statistically for a population of galaxies, and a first detection was claimed recently by Brainerd, Blandford, & Smail (BBS). BBS modeled their observations by describing galaxy halos as isothermal spheres of velocity dispersion σ, truncated at a radius s, where σ and s scale with the luminosity of the galaxy. Through Monte Carlo simulations they predicted the mean image polarization as a function of radius and compared it to the observations. In this paper we follow up on this discovery by developing a maximum-likelihood analysis that can constrain the halo properties of distant galaxy populations through "galaxy-galaxy" lensing; with it we show that the mean masses and sizes of halos can be estimated accurately, without excessive data requirements. The proposed maximum-likelihood analysis contains several important new elements: (1) it takes full account of the actual image ellipticities, positions, and apparent magnitudes, and as a consequence, it provides more efficient parameter estimation; (2) it provides automatically the proper relative weight for images of different ellipticities; (3) it uses a redshift probability distribution for each galaxy image and does not require a foreground lens-background image dichotomy; (4) it provides a rigorous means to investigate the covariances among the parameters that describe the halo model. We apply this analysis technique to simulated observations, using for ease of comparison the same lens model as BBS, and determine the best-fitting values, σ* and s*, corresponding to an L* galaxy. We explore two different observing strategies: (1) taking deep images (e.g., with HST) on small fields, and (2) using shallower images on larger fields. From these simulations we find that σ* can be determined to 10% accuracy if a sample of about 5000 galaxies with measured ellipticities are available, down to R 23. The corresponding data can be obtained on a 4 m class telescope in a few nights of very good seeing. Alternatively, the same accuracy in the determination of σ* can be achieved from about 10 moderately deep WFPC2 fields, on which galaxy shapes can be measured to about R ~ 25 and for which ground-based images are available on which the WFPC2 fields are centered. Firm lower limits can be set on the radial extent of the halo, but the maximal halo extent is poorly constrained. We show that this likelihood approach can also be used to constrain other parameters of the galaxy population, such as the Tully-Fisher index or the mean redshift of the galaxies as a function of apparent magnitude. Finally, we show how multicolor information, constraining the redshift of individual galaxies, can dramatically improve the accuracy of the parameter determination.