118456248
Observation of three-mode parametric interactions in long optical cavities
C. Zhao, L. Ju, Y. Fan, S. Gras, B. J. J. Slagmolen, H. Miao, P. Barriga, and D. G. Blair, D. J. Hosken, A. F. Brooks, P. J. Veitch, D. Mudge, and J. Munch
121643265
The Science benefits and Preliminary Design of the Southern hemisphere Gravitational Wave Detector AIGO.
The proposed southern hemisphere gravitational wave detector AIGO increases the projected average baseline of the global array of ground based gravitational wave detectors by a factor ~4. This allows the world array to be substantially improved. The orientation of AIGO allows much better resolution of both wave polarisations. This enables better distance estimates for inspiral events, allowing unambiguous optical identification of host galaxies for about 25% of neutron star binary inspiral events. This can allow Hubble Law estimation without optical identification of an outburst, and can also allow deep exposure imaging with electromagnetic telescopes to search for weak afterglows. This allows independent estimates of cosmological acceleration and dark energy as well as improved understanding of the physics of neutron star and black hole coalescences. This paper reviews and summarises the science benefits of AIGO and presents a preliminary conceptual design.
15931132
Compensation of strong thermal lensing in high-optical-power cavities.
In an experiment to simulate the conditions in high optical power advanced gravitational wave detectors, we show for the first time that the time evolution of strong thermal lenses follows the predicted infinite sum of exponentials (approximated by a double exponential), and that such lenses can be compensated using an intracavity compensation plate heated on its cylindrical surface. We show that high finesse approximately 1400 can be achieved in cavities with internal compensation plates, and that mode matching can be maintained. The experiment achieves a wave front distortion similar to that expected for the input test mass substrate in the Advanced Laser Interferometer Gravitational Wave Observatory, and shows that thermal compensation schemes are viable. It is also shown that the measurements allow a direct measurement of substrate optical absorption in the test mass and the compensation plate.
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.
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.
11935446
The Milky Way Tomography with SDSS. I. Stellar Number Density Distribution
Using the photometric parallax method we estimate the distances to ~48 million stars detected by the Sloan Digital Sky Survey (SDSS) and map their three-dimensional number density distribution in the Galaxy. The currently available data sample the distance range from 100 pc to 20 kpc and cover 6500 deg2 of sky, mostly at high Galactic latitudes (|b| > 25). These stellar number density maps allow an investigation of the Galactic structure with no a priori assumptions about the functional form of its components. The data show strong evidence for a Galaxy consisting of an oblate halo, a disk component, and a number of localized overdensities. The number density distribution of stars as traced by M dwarfs in the solar neighborhood (D < 2 kpc) is well fit by two exponential disks (the thin and thick disk) with scale heights and lengths, bias corrected for an assumed 35% binary fraction, of H1 = 300 pc and L1 = 2600 pc, and H2 = 900 pc and L2 = 3600 pc, and local thick-to-thin disk density normalization ρthick(R☉)/ρthin(R☉) = 12% . We use the stars near main-sequence turnoff to measure the shape of the Galactic halo. We find a strong preference for oblate halo models, with best-fit axis ratio c/a = 0.64, ρH ∝ r−2.8 power-law profile, and the local halo-to-thin disk normalization of 0.5%. Based on a series of Monte Carlo simulations, we estimate the errors of derived model parameters not to be larger than ~20% for the disk scales and ~10% for the density normalization, with largest contributions to error coming from the uncertainty in calibration of the photometric parallax relation and poorly constrained binary fraction. While generally consistent with the above model, the measured density distribution shows a number of statistically significant localized deviations. In addition to known features, such as the Monoceros stream, we detect two overdensities in the thick disk region at cylindrical galactocentric radii and heights (R,Z) ~ (6.5,1.5) kpc and (R,Z) ~ (9.5,0.8) kpc and a remarkable density enhancement in the halo covering over 1000 deg2 of sky toward the constellation of Virgo, at distances of ~6-20 kpc. Compared to counts in a region symmetric with respect to the l = 0° line and with the same Galactic latitude, the Virgo overdensity is responsible for a factor of 2 number density excess and may be a nearby tidal stream or a low-surface brightness dwarf galaxy merging with the Milky Way. The u − g color distribution of stars associated with it implies metallicity lower than that of thick disk stars and consistent with the halo metallicity distribution. After removal of the resolved overdensities, the remaining data are consistent with a smooth density distribution; we detect no evidence of further unresolved clumpy substructure at scales ranging from ~50 pc in the disk to ~1-2 kpc in the halo.
8132532
Forming a large disc galaxy from a z < 1 major merger
Using high-resolution SPH simulations in a fully cosmological Λ cold dark matter context, we study the formation of a bright disc-dominated galaxy that originates from a 'wet' major merger at z = 0.8 . The progenitors of the disc galaxy are themselves disc galaxies that formed from early major mergers between galaxies with blue colours. A substantial thin stellar disc grows rapidly following the last major merger and the present-day properties of the final remnant are typical of early-type spiral galaxies, with an i-band bulge-to-disc ratio ~0.65, a disc scalelength of 7.2 kpc, g−r = 0.5 mag , an HI linewidth (W_(20)/2) of 238 km s^(−1) and total magnitude i = −22.4. The key ingredients for the formation of a dominant stellar disc component after a major merger are (i) substantial and rapid accretion of gas through cold flows followed at late times by cooling of gas from the hot phase, (ii) supernova feedback that is able to partially suppress star formation during mergers and (iii) relative fading of the spheroidal component. The gas fraction of the progenitors' discs does not exceed 25 per cent at z < 3 , emphasizing that the continuous supply of gas from the local environment plays a major role in the regrowth of discs and in keeping the galaxies blue. The results of this simulation alleviate the problem posed for the existence of disc galaxies by the high likelihood of interactions and mergers for galaxy-sized haloes at relatively low z.
35904988
Misaligned angular momentum in hydrodynamic cosmological simulations: warps, outer discs and thick discs
We present a detailed analysis of a disc galaxy forming in a high-resolution fully cosmological simulation to investigate the nature of the outer regions of discs and their relevance for the disc formation process. Specifically, we focus on the phenomenon of misaligned disc components and find that the outer disc warp is a consequence of the misalignment between the inner disc and the surrounding hot gaseous halo. As the infalling cold gas sinks towards the centre of the galaxy, it is strongly torqued by the hot gas halo. By the time the fresh gas reaches the central disc-forming region, its angular momentum is completely aligned with the spin of the hot gas halo. If the spin of the hot gas halo, in turn, is not aligned with that of the inner disc, a misaligned outer disc forms, comprised of newly accreted material. The inner and outer components are misaligned with each other because they respond differently to infalling substructure and accretion. The warped disc feeds the main gas disc due to viscous angular momentum losses, but small amounts of star formation in the warp itself form a low-metallicity thick disc. We show that observations of resolved stellar populations in warped galaxies in the local Universe could provide evidence for the presence of these processes and therefore indirectly reveal ongoing gas accretion and the existence of hot gas haloes.
14106085
The nature of HI absorbers in GRB afterglows: clues from hydrodynamic simulations
In recent work, we have shown that it is possible to link quantitatively many aspects of damped Lyman alpha (DLA) absorbers in the spectra of quasars to high resolution simulations of galaxy formation. Using runs from the same series of hydrodynamic numerical studies, we consider the expected properties of intrinsic Lyman alpha absorbers seen in the spectra of high redshift (z > 2) gamma ray burst afterglows (GRB-DLAs). If GRBs are associated with the death of massive stars, their afterglows provide insights into otherwise unprobed regions of protogalactic objects, but detailed physical interpretations are currently embryonic. We find that median impact parameters (measured from the potential minimum) are approximately 1 kpc for GRBs compared with 4 kpc for QSO-DLAs. However, an equally important difference is that GRB-DLAs are predominantly associated with halos of mass 10 < Mvir/M < 10, an order of magnitude larger than the hosts of QSO-DLAs. Accordingly, there are differences in the stellar properties of hosts. For instance mean star formation rates are higher: 〈Ṁ?〉 ' 10 M yr−1 for GRB-DLAs compared with 〈Ṁ?〉 ' 1 M yr−1 for QSO-DLAs. Our simulations accurately predict the form of the GRB-DLA H I column density distribution, producing quantitative agreement for NHI > 10 cm−2, but they somewhat underpredict the incidence of low column densities NHI < 10 cm−2. This is reflected in our estimate of the ionizing photon escape fraction, fesc ' 1%, which is lower than the observational GRB-derived escape fraction (2%). Line-of-sight neutral gas metallicities predicted by our simulations (10−2 < Z/Z < 1) are consistent with the modest observational constraints. Because of large internal dispersions in gas metallicities, this agreement is not significantly compromised by imposing a cut-off on the metallicity of stars able to launch GRBs (Z? < Z /3), confounding claims that the observed metallicity of GRB-DLAs poses a severe challenge to current GRB models.