Significant Small-scale Structure Research Articles

Hawking radiation from “phase horizons” in laser filaments?

Belgiorno et al. have reported on experiments aiming at the detection of (the analogue of) Hawking radiation using laser filaments [F. Belgiorno et al., Phys. Rev. Lett. 105, 203901 (2010)]. They sent intense focused Bessel pulses into a nonlinear dielectric medium in order to change its refractive index via the Kerr effect and saw creation of photons orthogonal to the direction of travel of the pulses. Since the refractive index change in the pulse generated a ``phase horizon'' (where the phase velocity of these photons equals the pulse speed), they concluded that they observed the analogue of Hawking radiation. We study this scenario in a model with a phase horizon and a phase velocity very similar to that of their experiment and find that the effective metric does not quite correspond to a black hole. The photons created in this model are not due to the analogue of black hole evaporation but have more similarities to cosmological particle creation. Nevertheless, even this effect cannot explain the observations---unless the pulse has significant small scale structure in both the longitudinal and transverse dimensions.

THE GALACTIC PLANE INFRARED POLARIZATION SURVEY (GPIPS): DATA RELEASE 1

The Galactic Plane Infrared Polarization Survey (GPIPS) covers 76 deg{sup 2} of the first Galactic quadrant midplane, 18 Degree-Sign {<=} l {<=} 56 Degree-Sign and -1 Degree-Sign {<=} b {<=} +1 Degree-Sign , in H-band (1.6 {mu}m) linear polarimetry to reveal the plane-of-the-sky orientation of the magnetic field in diffuse and denser atomic and molecular clouds. The Survey consists of 3237 overlapping 10 Multiplication-Sign 10 arcmin fields observed using the Mimir instrument on the 1.8 m Perkins telescope. Here, the first community release of GPIPS data for 559 fields (17% of the Survey) is announced and basic characteristics are described. Data products consist of H-band stellar photometry and polarimetry as well as combined images. The formats and contents of the products are described and quality cuts are explored to provide insight into opportunities and limitations of the data. The Survey probes to distances as far as the Galactic bulge, revealing magnetic field properties that correlate with spiral arms and also show significant small-scale structure. The polarizations are classified into three 'usage' samples, based on stellar brightness and polarimetric uncertainty. The brightest, lowest uncertainty polarizations are suitable for individual use and direct magnetic field mapping. The next two fainter samples aremore » useful, once averaged, for probing magnetic fields to greater distances, albeit with lower resolution. Based on this release, the full GPIPS data set will number about 5.6 million stars, with more than 1 million in the high-quality sample. This increases, by many orders of magnitude, the number of polarimetric probes of the Milky Way's magnetic field.« less

Ultraviolet absorption observations of intermediate-velocity cloud gas detected towards the M 15 globular cluster

Aims. We present medium resolution ultraviolet interstellar absorption measurements recorded with the HST-COS and FUSE spectrographs towards two post-AGB stars (K 559 & K 648) located within the M 15 globular cluster (l ∼ 65 ◦ , b ∼ –27 ◦ ). By sampling interstellar gas over the 10.4 kpc sight-line towards M 15 we wish to reveal spectral features that are associated with absorption due to the foreground g1 intermediate velocity cloud (IVC), whose distance has previously been constrained to lie between 1.8–3.8 kpc. Methods. Inspection of the UV line profiles recorded towards both stars have revealed measurable IVC absorption at Vlsr ∼ +61.5 ± 5k m s −1 in the profiles of the CI, CII, CII*, CIV, NI, NII, OI, OVI, AlII, SiII, SiIII, SiIV, PII, SII, FeII, FeIII and NiII ions. Line-profile fitting to the IVC absorption features has resulted in column density determinations for the low and high ions. The best-fit column density values for the various ions have near identical values along both sight-lines, such that our UV data does not reveal the significant small-scale structure that has been reported previously for the g1 cloud. Results. The observed column density ratios of the CIV, SiIV, OVI and NV IV components are consistent with what may be expected from turbulent mixing layers, shock ionization or halo SNR models, although no one model can predict all of the observed ratios. We derive sub-solar metallicity values of [O/H] = −1.22 ± 0.44 and [N/H] = −1.21 ± 0.38 for the neutral IVC gas, and also the IV components of C, Si, Al, Ni and Fe mainly possess less than solar abundance ratios relative to that of N. A possible origin for the g1 IVC is that of a low metallicity cloud that has been accreted towards our galaxy, has passed through the disk and is now moving away from the galactic plane.

Identifying Lenses with Small‐Scale Structure. I. Cusp Lenses

The inability of standard models to explain the flux ratios in many 4-image gravitational lenses has been cited as evidence for significant small-scale structure in lens galaxies. That claim has generally relied on detailed lens modeling, so it is both model dependent and somewhat difficult to interpret. We present a more robust and generic method for identifying lenses with small-scale structure. For a close triplet of images associated with a source near a cusp caustic, the sum of the signed magnifications should approximately vanish. We derive realistic upper bounds on the sum, and argue that lenses with flux ratios that significiantly violate the bounds can be said to have structure in the lens potential on scales smaller than the image separation. Five observed lenses have such flux ratio ``anomalies'': B2045+265, 1RXS J1131-1231, and SDSS J0924+0219 have strong anomalies; B0712+472 has a strong anomaly at optical/near-IR wavelengths and a marginal anomaly at radio wavelengths; and RX J0911+0551 appears to have an anomaly, but this conclusion is subject to uncertainties about octopole modes in early-type galaxies. Analysis of the cusp relation does not identify the known anomaly in B1422+231, so methods that are more sophisticated (and less generic) than the cusp relation may be necessary to uncover flux ratio anomalies in some systems. Although these flux ratio anomalies might represent milli- or micro-lensing, we cannot identify the cause; we can only conclude that the lenses have significant structure in the potential on scales smaller than the separation between the images. Additional arguments must be invoked to specify the nature of this small-scale structure. [Abridged]

Hubble Space TelescopePaα and 1.9 Micron Imaging of Sagittarius A West

We present HST/NICMOS images at 0.2" resolution of the HI Paschen Alpha (PaA) emission line in a 70" x 90" region of the Galactic center centered on the non-thermal radio source Sgr A*. The majority of the emission arises from ionized gas in the mini-spiral in the central parsec. PaA emission is also seen from 26 stellar sources, presumably early-type stars with mass-loss winds. The new data reveal significant small-scale structure (<1"~0.04pc) in the ionized gas of the mini-spiral; low surface brightness emission features are also seen for the first time. Extinction, estimated from the ratio of observed PaA emission to 6-cm continuum emission, varies from 20 to 50 mag with a median Av=31.1 mag, in excellent agreement with earlier estimates for the stellar sources and indepedent measurements derived using H92alpha recombination line data. Large increases in extinction are seen along the periphery of the ionized gas, suggesting that the ionized gas is partially extincted by dust in the molecular clouds at the outside of the ionized regions. The small-scale, filamentary structures in the ionized gas have a free thermal expansion time of only ~ 3000 yrs; either magnetic fields or mass-loss winds from the hot emission line stars may contain the ionized filaments. For both the ionized gas and the stellar continuum, the centroids of the emission remain within ~+/- 1" from a radius of 2" out to 40", providing further evidence that Sgr A* is indeed at or extremely close to the dynamical center of the Galactic nucleus stellar distribution. The 1.9 micron surface brightness increases inwards to 0.9" and then decreases or levels off closer to Sgr A*, possibly indicating the core radius of the central stellar distribution or depletion of the late-type stars by stellar collisions near the central black hole.

Matter accretion by cosmic string loops and wakes.

We develop quantative methods for the study of structure formation with cosmic strings in an expanding universe. The gravitational effects of arbitrary string configurations are calculated using linearized gravity. The growth of density fluctuations is then studied using these gravitational forces as a source term in the Zeldovich approximation. We use either the adhesion modification or an N-body tree code to project this fluctuation growth into the nonlinear regime. These methods are applied to specific loop and long string solutions beginning at equal matter and radiation ${\mathit{t}}_{\mathrm{eq}}$ on scales corresponding to about 10 Mpc today (h=0.5). We reproduce analytic results for spherical and planar collapse. We show that these methods are applicable to accretion about closed oscillating loops and in the wakes of moving long strings which possess significant small-scale structure, quantitatively confirming the wiggly string approximation with a renormalized string energy density \ensuremath{\mu}\ifmmode \tilde{}\else \~{}\fi{}. We demonstrate the efficiency of the fragmentation of wakes created by wiggly strings by the present day. These methods are sufficiently computationally efficient to employ in the study of an evolving string network. For the cosmic string scenario, we conclude that reliable quantitative predictions must take into account non-Gaussianity, vorticity generation, and nonlinear fragmentation effects.

String-induced density perturbations in hot dark matter

view Abstract Citations (9) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS String-induced Density Perturbations in Hot Dark Matter Vollick, Dan N. Abstract The density and velocity perturbations produced by cosmic strings in hot dark matter are examined using the Gilbert equation. We use the results from recent numerical simulations which show that the strings have significant small-scale structure. It is shown that the string network can produce structure whose size is comparable to the size of the observed large-scale structure. It is also shown that the large-scale peculiar velocities produced by the string network are coherent over distances which are too small to account for the observed large-scale streaming motions. The role of the small-scale structure in fragmenting the wakes produced by the strings is examined. It is found that small- scale structure below a critical size cannot fragment the wakes, whereas small-scale structure larger than this critical size may be able to fragment the wakes. Publication: The Astrophysical Journal Pub Date: September 1992 DOI: 10.1086/171760 Bibcode: 1992ApJ...397...14V Keywords: Cosmology; Dark Matter; Galactic Clusters; Galactic Evolution; String Theory; Equations Of Motion; Fourier Transformation; Hubble Constant; Neutrinos; Perturbation Theory; Astrophysics; COSMIC STRINGS; COSMOLOGY: DARK MATTER; COSMOLOGY: THEORY; GALAXIES: CLUSTERING; GALAXIES: FORMATION full text sources ADS |

Vorticity amplification and annihilation in a potential vortex

An analytical solution is given that describes the transient growth and decay of an axial disturbance in a line vortex flow. The solution serves two functions. First, it illustrates the essential mechanisms whereby vorticity is amplified by swirl and annihilated by viscous diffusion. Second, it provides a nontrivial test case, involving significant small-scale structure, for numerical solutions of the general incompressible Navier–Stokes equations at high Reynolds numbers.

Small-scale structure on cosmic strings and galaxy formation.

The density perturbations produced by cosmic strings in cold dark matter are examined using the Zel'dovich approximation. We use the results from recent numerical simulations which show that the strings have significant small-scale structure. It is shown that the string network produces wakelike overdensities which may be able to account for the observed large-scale structure of the Universe. In the process of producing wakes the strings also produce large-scale peculiar velocity fields. It is shown that these velocities are coherent over distances which are too small to account for the observed large-scale streaming motions. It is also shown that the small-scale structure on the strings can fragment the wakes into pieces which have the mass of a galaxy.

100-MICRON ARRAY POLARIMETRY FROM THE KUIPER AIRBORNE OBSERVATORY: INSTRUMENTATION, TECHNIQUES, AND FIRST RESULTS

ABSTRACT The University of Chicago far-infrared array polarimeter, 'STOKES', is the first multiple-beam polarimeter for far-infrared astronomy. Observations are made from the NASA Kuiper Airborne Observatory. Two orthogonal components of linear polarization are detected simultaneously by corresponding pairs of bolometers in two 32-detector arrays. Novel observing and data-analysis techniques are used to overcome the inherent difficulties of array polarimetry. Results from the first observing flights with the new instrument are reported for the molecular clouds W3 and W51. The measurements show that the magnetic-field structure in both clouds is nonuniform on the scale of 0.5-1.5 pc. This is consistent with molecular line and Zeeman observations that indicate the presence of turbulent velocities and significant small-scale structure. Preliminary results from the second flight series have yielded approximately 40 new measurements in the Sgr A complex. These results indicate that modifications made since the first flights have significantly improved the performance of STOKES.

Cosmic-string evolution: A numerical simulation.

A system of intercommuting cosmic strings is numerically evolved in a homogeneous radiation-dominated expanding universe. Further evidence for the existence of a scaling'' solution is presented, and results for the long-string energy density are given. Long strings exhibit significant small-scale structure---kinks and short-wavelength propagating modes. Net loop production is observed to be strongly peaked at scales considerably smaller than the horizon, and reasons for this severe fragmentation are briefly discussed. The new numerical techniques employed are also described.

Large and small scale changes in the disturbed upper atmosphere

ESRO 4 gas analyzer data are used to investigate changes in the upper atmosphere induced by a magnetic disturbance in late November 1973. The prominent feature observed is a significant enhancement in nitrogen densities which shows a complex and variable latitudinal structure. At the same time atomic oxygen exhibits a somewhat delayed decrease at lower altitudes (say, below 300 km) and a moderate increase at higher altitudes. Superimposed on these large scale changes is a significant small scale structure, part of which is thought to be caused by atmospheric gravity waves.

Study of galactic gas and dust using observations of elliptical galaxies

view Abstract Citations (34) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Study of galactic gas and dust using observations of elliptical galaxies. Knapp, G. R. Abstract The gas-to-dust ratio has been measured in the directions of 55 elliptical galaxies by comparing 21-cm H I column densities with color excesses and absorptions for the galaxies. The value of the gas-to-dust ratio found in this study is similar to previous measurements, but the data suggest significant small-scale structure of the interstellar medium. Publication: The Astronomical Journal Pub Date: February 1975 DOI: 10.1086/111719 Bibcode: 1975AJ.....80..111K Keywords: Cosmic Dust; Elliptical Galaxies; Galactic Clusters; H Lines; Interstellar Gas; Brightness Temperature; Interstellar Extinction; Mass Ratios; Microwave Emission; Radio Astronomy; Tables (Data); Astrophysics full text sources ADS |

High-Resolution Observations of Small-Diameter Radio Sources at 18-CENTIMETER Wavelength

view Abstract Citations (54) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS High-Resolution Observations of Small-Diameter Radio Sources at 18-CENTIMETER Wavelength Clark, B. G. ; Kellermann, K. I. ; Bare, C. C. ; Cohen, M. H. ; Jauncey, D. L. Abstract A long base-line interferometer has been operated at 18 cm over an 845-km (4.7 X 10~ X) base line between the NRAO 140-foot radio telescope at Green Bank, West Virginia, and the 120-foot telescope at the Haystack Microwave Facility near Boston, Massachusetts. Independent rubidium-controlled atomic clocks were used as time and frequency standards at each telescope. The intermediate-frequency signals from each antenna were recorded on magnetic tape and later correlated in a digital computer Reliable data were obtained for twenty-eight sources. Eleven appear unresolved and are therefore less than O'~O1-O'~O2 in diameter. Four others have components that contain a significant fraction of the total flux in a component less than this size; and two others are completely resolved and thus do not contain significant small-scale structure. The ten partially resolved sources have angular dimensions of the order of a few hundredths of a second of arc. Comparison of the radio-frequency spectra with interferometer measurements made at several wavelengths indicates that many sources consist of a number of well-defined, small-diameter compo- nents, each of which has a diameter independent of wavelength. The magnetic field deduced from the low-frequency cutoff in the radio spectra is of the order of 1O-~ gauss in the partially resolved compo- nents Publication: The Astrophysical Journal Pub Date: September 1968 DOI: 10.1086/149699 Bibcode: 1968ApJ...153..705C full text sources ADS | data products NED (28)