Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userA numerical twist on the observational spin parameter, $lambda_R$
58
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
A primary goal of integral field spectroscopic (IFS) surveys is to provide a statistical census of galaxies classified by their internal kinematics. As a result, the observational spin parameter, $lambda_R$, has become one of the most popular methods of quantifying the relative importance of velocity dispersion and rotation in supporting a galaxys inner structure. The goal of this paper is to examine the relationship between the observationally deduced $lambda_R$ and one of the most commonly used theoretical spin parameters in the literature, the Bullock et al. (2001) $lambda$. Using a set of $N$-body realisations of galaxies from which we construct mock IFS observations, we measure $lambda_R$ as an observer would, incorporating the effects of beam smearing and seeing conditions. Assuming parameters typical of current IFS surveys, we confirm that there are strong positive correlations between $lambda_R$ and measurement radius, and strong negative correlations between $lambda_R$ and size of the PSF, for late-type galaxies; these biases can be reduced using a recently proposed empirical correction. Once observational biases are corrected for, we find that $lambda_R$ provides a good approximation to $sim sqrt{3}/2 ; lambda(rm R_{rm eff})$, where $lambda$ is evaluated for the galactic stellar component within 1 R$_{rm eff}$.
rate research
Read More
We use two-dimensional kinematic maps of simulated binary disc mergers to investigate the $lambda_R$-parameter, which is a luminosity weighted measure of projected angular momentum per unit mass. This parameter was introduced to subdivide the SAURON sample of early type galaxies in so called fast $lambda_R > 0.1$ and slow rotators $lambda_R < 0.1$. Tests on merger remnants reveal that $lambda_R$ is a robust indicator of the true angular momentum content in elliptical galaxies. We find the same range of $lambda_R$ values in our merger remnants as in the SAURON galaxies. The merger mass ratio is decisive in creating a slow or a fast rotator in a single binary merger, the former being created mostly in an equal mass merger. Slow rotators have a $lambda_R$ which does not vary with projection. The confusion rate with face-on fast rotators is very small. Merger with low gas fractions form slow rotators with smaller ellipticities and are in much better agreement with the SAURON slow rotators. Remergers of merger remnants are slow rotators but tend to have too high ellipticities. Fast rotators maintain the angular momentum content from the progenitor disc galaxy if merger mass ratio is high. Some SAURON galaxies have values of $lambda_R$ as high as our progenitor disc galaxies.
Photoionization fronts play a dominant role in many astrophysical situations, but remain difficult to achieve in a laboratory experiment. We present the results from a computational parameter study evaluating the feasibility of the photoionization experiment presented in the design paper by Drake, R. P., Hazak, G., Keiter, P. A., Davis, J. S., Patterson, C. R., Frank, A., Blackman, E. G., & Busquet, M. 2016, ApJ, 833, 249 in which a photoionization front is generated in a nitrogen medium . The nitrogen gas density and the Planckian radiation temperature of the x-ray source define each simulation. Simulations modeled experiments in which the x-ray flux is generated by a laser-heated gold foil, suitable for experiments using many kJ of laser energy, and experiments in which the flux is generated by a z-pinch device, which implodes a cylindrical shell of conducting wires. The models are run using CRASH, our block-adaptive-mesh code for multi-material radiation hydrodynamics. The radiative transfer model uses multi-group, flux-limited diffusion with thirty radiation groups. In addition, electron heat conduction is modeled using a single-group, flux-limited diffusion. In the theory, a photoionization front can exist only when the ratios of the electron recombination rate to the photoionization rate and the electron impact ionization rate to the recombination rate lie in certain ranges. These ratios are computed for several ionization states of nitrogen. Photoionization fronts are found to exist for laser driven models with moderate nitrogen densities ($sim$10$^{21}$ cm$^{-3}$) and radiation temperatures above 90 eV. For z-pinch driven models, lower nitrogen densities are preferred ($<$10$^{21}$ cm$^{-3}$). We conclude that the proposed experiments are likely to generate photoionization fronts.
In this work, we use observations of the Hubble parameter from the differential ages of passively evolving galaxies and the recent detection of the Baryon Acoustic Oscillations (BAO) at $z_1=0.35$ to constrain the Dvali-Gabadadze-Porrati (DGP) universe. For the case with a curvature term, we set a prior $h=0.73pm0.03$ and the best-fit values suggest a spatially closed Universe. For a flat Universe, we set $h$ free and we get consistent results with other recent analyses.
Observers experience a series of limitations when measuring galaxy kinematics, such as variable seeing conditions and aperture size. These effects can be reduced using empirical corrections, but these equations are usually applicable within a restrictive set of boundary conditions (e.g. Sersic indices within a given range) which can lead to biases when trying to compare measurements made across a full kinematic survey. In this work, we present new corrections for two widely used kinematic parameters, $lambda_R$ and $V/sigma$, that are applicable across a broad range of galaxy shapes, measurement radii and ellipticities. We take a series of mock observations of N-body galaxy models and use these to quantify the relationship between the observed kinematic parameters, structural properties and different seeing conditions. Derived corrections are then tested using the full catalogue of galaxies, including hydro-dynamic models from the EAGLE simulation. Our correction is most effective for regularly-rotating systems, yet the kinematic parameters of all galaxies -- fast, slow and irregularly rotating systems -- are recovered successfully. We find that $lambda_R$ is more easily corrected than $V/sigma$, with relative deviations of 0.02 and 0.06 dex respectively. The relationship between $lambda_R$ and $V/sigma$, as described by the parameter $kappa$, also has a minor dependence on seeing conditions. These corrections will be particularly useful for stellar kinematic measurements in current and future integral field spectroscopic (IFS) surveys of galaxies.
Most cosmological structures in the universe spin. Although structures in the universe form on a wide variety of scales from small dwarf galaxies to large super clusters, the generation of angular momentum across these scales is poorly understood. We have investigated the possibility that filaments of galaxies - cylindrical tendrils of matter hundreds of millions of light-years across, are themselves spinning. By stacking thousands of filaments together and examining the velocity of galaxies perpendicular to the filaments axis (via their red and blue shift), we have found that these objects too display motion consistent with rotation making them the largest objects known to have angular momentum. The strength of the rotation signal is directly dependent on the viewing angle and the dynamical state of the filament. Just as it is easiest to measure rotation in a spinning disk galaxy viewed edge on, so too is filament rotation clearly detected under similar geometric alignment. Furthermore, the mass of the haloes that sit at either end of the filaments also increases the spin speed. The more massive the haloes, the more rotation is detected. These results signify that angular momentum can be generated on unprecedented scales.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
