55187356
Technology developments for ACIGA high power test facility for advanced interferometry
The High Optical Power Test Facility for Advanced Interferometry has been built by the Australian Consortium for Interferometric Gravitational Astronomy north of Perth in Western Australia. An 80 m suspended cavity has been prepared in collaboration with LIGO, where a set of experiments to test suspension control and thermal compensation will soon take place. Future experiments will investigate radiation pressure instabilities and optical spring effects in a high power optical cavity with ~200 kW circulating power. The facility combines research and development undertaken by all consortium members, whose latest results are presented.
144912
Multicolor cavity metrology.
Long-baseline laser interferometers used for gravitational-wave detection have proven to be very complicated to control. In order to have sufficient sensitivity to astrophysical gravitational waves, a set of multiple coupled optical cavities comprising the interferometer must be brought into resonance with the laser field. A set of multi-input, multi-output servos then lock these cavities into place via feedback control. This procedure, known as lock acquisition, has proven to be a vexing problem and has reduced greatly the reliability and duty factor of the past generation of laser interferometers. In this article, we describe a technique for bringing the interferometer from an uncontrolled state into resonance by using harmonically related external fields to provide a deterministic hierarchical control. This technique reduces the effect of the external seismic disturbances by 4 orders of magnitude and promises to greatly enhance the stability and reliability of the current generation of gravitational-wave detectors. The possibility for using multicolor techniques to overcome current quantum and thermal noise limits is also discussed.
14265914
REPRODUCING THE STELLAR MASS/HALO MASS RELATION IN SIMULATED ΛCDM GALAXIES: THEORY VERSUS OBSERVATIONAL ESTIMATES
We examine the present-day total stellar-to-halo mass (SHM) ratio as a function of halo mass for a new sample of simulated field galaxies using fully cosmological, ?CDM, high-resolution SPH + N-body simulations. These simulations include an explicit treatment of metal line cooling, dust and self-shielding, H2-based star formation (SF), and supernova-driven gas outflows. The 18 simulated halos have masses ranging from a few times 108 to nearly 1012 M ?. At z = 0, our simulated galaxies have a baryon content and morphology typical of field galaxies. Over a stellar mass range of 2.2 ? 103-4.5 ? 1010 M ? we find extremely good agreement between the SHM ratio in simulations and the present-day predictions from the statistical abundance matching technique presented in Moster et?al. This improvement over past simulations is due to a number systematic factors, each decreasing the SHM ratios: (1) gas outflows that reduce the overall SF efficiency but allow for the formation of a cold gas component; (2)?estimating the stellar masses of simulated galaxies using artificial observations and photometric techniques similar to those used in observations; and (3) accounting for a systematic, up to 30% overestimate in total halo masses in DM-only simulations, due to the neglect of baryon loss over cosmic times. Our analysis suggests that stellar mass estimates based on photometric magnitudes can underestimate the contribution of old stellar populations to the total stellar mass, leading to stellar mass errors of up to 50% for individual galaxies. These results highlight that implementing a realistic high density threshold for SF considerably reduces the overall SF efficiency due to more effective feedback. However, we show that in order to reduce the perceived tension between the SF efficiency in galaxy formation models and in real galaxies, it is very important to use proper techniques to compare simulations with observations.
4805930
Testing rotational mixing predictions with new boron abundances in main-sequence B-type stars
New boron abundances for seven main-sequence B-type stars are determined from HST STIS spectroscopy around the B III 2066 A line. Boron abundances provide a unique and critical test of stellar evolution models that include rotational mixing, since boron is destroyed in the surface layers of stars through shallow mixing long before other elements are mixed from the stellar interior through deep mixing. The stars in this study are all on or near the main sequence and are members of young Galactic clusters. They show no evidence of mixing with gas from H-burning layers from their CNO abundances. Boron abundances range from 12 + log(B/H) ≤ 1.0 to 2.2. The boron abundances are compared to the published values of the stellar nitrogen abundances [all have 12 + log(N/H) ≤ 7.8] and to their host cluster ages (4-16 Myr) to investigate the predictions from models of massive star evolution with rotational mixing effects. We find that the variations in boron and nitrogen are generally within the range of the predictions from the stellar evolution models with rotation (where predictions for models with rotation rates from 0 to 450 km s-1 and μ-barriers are examined), especially given their age and mass ranges. Three stars (of 34 B-type stars with detailed boron abundance determinations) deviate from the model predictions, showing either much larger boron depletions than can be explained by the rotating model predictions or a spectroscopic mass that is lower than expected, given the rotating model predictions for its age and abundances. The results from these three stars suggest that rotational mixing could be more efficient than that currently modeled at the highest rotation rates.
39151409
The cold gas content of bulgeless dwarf galaxies
We present an analysis of the neutral hydrogen (H i) properties of a fully cosmological hydrodynamical dwarf galaxy, run with varying simulation parameters. As reported by Governato et al., the high-resolution, high star formation density threshold version of this galaxy is the first simulation to result in the successful reproduction of a (dwarf) spiral galaxy without any associated stellar bulge. We have set out to compare in detail the H i distribution and kinematics of this simulated bulgeless disc with what is observed in a sample of nearby dwarfs. To do so, we extracted the radial gas density profiles, velocity dispersion (e.g. velocity ellipsoid and turbulence) and the power spectrum of structure within the cold interstellar medium (ISM) from the simulations. The highest resolution dwarf, when using a high-density star formation threshold comparable to densities of giant molecular clouds, possesses bulk characteristics consistent with those observed in nature, though the cold gas is not as radially extended as that observed in nearby dwarfs, resulting in somewhat excessive surface densities. The lines-of-sight velocity dispersion radial profiles have values that are in good agreement with the observed dwarf galaxies, but due to the fact that only the streaming velocities of particles are tracked, a correction to include the thermal velocities can lead to profiles that are quite flat. The ISM power spectra of the simulations appear to possess more power on smaller spatial scales than that of the Small Magellanic Cloud. We conclude that unavoidable limitations remain due to the unresolved physics of star formation and feedback within parsec-scale molecular clouds.
17146882
The Central Slope of Dark Matter Cores in Dwarf Galaxies: Simulations versus THINGS
We make a direct comparison of the derived dark matter (DM) distributions between hydrodynamical simulations of dwarf galaxies assuming a ΛCDM cosmology and the observed dwarf galaxies sample from the THINGS survey in terms of (1) the rotation curve shape and (2) the logarithmic inner density slope α of mass density profiles. The simulations, which include the effect of baryonic feedback processes, such as gas cooling, star formation, cosmic UV background heating, and most importantly, physically motivated gas outflows driven by supernovae, form bulgeless galaxies with DM cores. We show that the stellar and baryonic mass is similar to that inferred from photometric and kinematic methods for galaxies of similar circular velocity. Analyzing the simulations in exactly the same way as the observational sample allows us to address directly the so-called cusp/core problem in the ΛCDM model. We show that the rotation curves of the simulated dwarf galaxies rise less steeply than cold dark matter rotation curves and are consistent with those of the THINGS dwarf galaxies. The mean value of the logarithmic inner density slopes α of the simulated galaxies' DM density profiles is ~–0.4 ± 0.1, which shows good agreement with α = –0.29 ± 0.07 of the THINGS dwarf galaxies. The effect of non-circular motions is not significant enough to affect the results. This confirms that the baryonic feedback processes included in the simulations are efficiently able to make the initial cusps with α ~–1.0 to –1.5 predicted by DM-only simulations shallower and induce DM halos with a central mass distribution similar to that observed in nearby dwarf galaxies.
34733315
The effect of models of the interstellar media on the central mass distribution of galaxies
We compare the central mass distribution of galaxies simulated with three different models of the interstellar medium (ISM) with increasing complexity: primordial (H+He) cooling down to 10 4 K, additional cooling via metal lines and to lower temperatures, and molecular hydrogen (H2) with shielding of atomic and molecular hydrogen, in addition to metal line cooling. The latter model includes a non-equilibrium calculation of the H2 abundance, self-shielding of H2, dust shielding of both HI and H2, and H2-based star formation efficiency. In order to analyze the effect ofthese models while holding all other parameters constant, we follow the evolution of four field galaxies with Vpeak < 120 km/s to a redshift of zero using high-resolution Smoothed Particle Hydrodynamic simulations in a �