On the use of fractional Brownian motion simulations to determine the 3D statistical properties of interstellar gas

Based on fractional Brownian motion (fBm) simulations of 3D gas density and velocity fields, we present a study of the statistical properties of spectro-imagery observations (channel maps, integrated emission, and line centroid velocity) in the case of an optically thin medium at various temperatures. The power spectral index gamma_W of the integrated emission is identified with that of the 3D density field (gamma_n) provided the medium's depth is at least of the order of the largest transverse scale in the image, and the power spectrum of the centroid velocity map is found to have the same index gamma_C as that of the velocity field (gamma_v). Further tests with non-fBm density and velocity fields show that this last result holds, and is not modified either by the effects of density-velocity correlations. A comparison is made with the theoretical predictions of Lazarian & Pogosyan (2000).Comment: 28 pages, 14 figures, accepted for publication in ApJ. For preprint with higher-resolution figures, see http://www.cita.utoronto.ca/~mamd/miville_fbm2003.pd

Detection of a Corrugated Velocity Pattern in the Spiral Galaxy NGC 5427

Here we report the detection, in Halpha emission, of a radial corrugation in the velocity field of the spiral galaxy NGC 5427. The central velocity of the Halpha line displays coherent, wavy-like variations in the vicinity of the spiral arms. The spectra along three different arm segments show that the maximum amplitude of the sinusoidal line variations are displaced some 500 pc from the central part of the spiral arms. The peak blueshifted velocities appear some 500 pc upstream the arm, whereas the peak redshifted velocities are located some 500 pc downstream the arm. This kinematical behavior is similar to the one expected in a galactic bore generated by the interaction of a spiral density wave with a thick gaseous disk, as recently modeled by Martos & Cox (1998).Comment: Accepted for publication in Ap

The Parker Instability in 3-D: Corrugations and Superclouds Along the Carina-Sagittarius Arm

Here we present three-dimensional MHD models for the Parker instability in a thick magnetized disk, including the presence of a spiral arm. The $B$-field is assumed parallel to the arm, and the model results are applied to the optical segment of the Carina-Sagittarius arm. The characteristic features of the undular and interchange modes are clearly apparent in the simulations. The undular mode creates large gas concentrations distributed along the arm. This results in a clear arm/inter-arm difference: the instability triggers the formation of large interstellar clouds inside the arms, but generates only small structures with slight density enhancements in the inter-arm regions. The resulting clouds are distributed in an antisymmetric way with respect to the midplane, creating an azimuthal corrugation along the arm. For conditions similar to those of the optical segment of the Carina-Sagittarius arm, it has a wavelength of about 2.4 kpc. This structuring can explain the origin of both HI superclouds and the azimuthal corrugations in spiral arms. The wavelength matches the corrugation length derived with the young stellar groups located in the optical segment of the Carina-Sagittarius arm. Keywords: Galaxy: kinematics and dynamics -- Galaxy: structure -- Instabilities -- ISM: clouds -- ISM: magnetic fields -- ISM: structure -- MHDComment: 29 pages, 12 figures, Latex, Accepted by the Astrophysical Journa

Perceived mismatches between needs and services in the health care of elderly people.

A postal enquiry to professionals involved in service delivery points to a number of problems in the process of referring elderly people appropriately. These problems related to lack of choice and constrained options, rather than mis-referral through inadequate assessment

Matching services to needs in the health care of elderly people.

This paper is based on the results of a postal enquiry to health care professionals working in the care of elderly people in a Scottish Health Board. Responses fall into three main categories. The first refers to issues of process, where lack of information or delays in response create problems. Second, there are problems of scarcity, with both lack of provision and pressures on existing provision. Third, the responses relate to the impact of constrained choices, which are the consequences of that scarcity: either people are sent to existing services because of a lack of alternatives, or people cannot be referred on appropriately because of the lack of alternatives. The process of referral is, then, affected directly by the options that are available. In the process, compromises have to be made, and this produces mismatches between needs and services

The Parker instability under a linear gravity

A linear stability analysis has been done to a magnetized disk under a linear gravity. We have reduced the linearized perturbation equations to a second-order differential equation that resembles the Schrodinger equation with the potential of a harmonic oscillator. Depending on the signs of energy and potential terms, eigensolutions can be classified into ''continuum'' and ''discrete'' families. When the magnetic field is ignored, the continuum family is identified as the convective mode, while the discrete family is identified as acoustic-gravity waves. If the effective adiabatic index gamma is less than unity, the former develops into the convective instability. When a magnetic field is included, the continuum and discrete families further branch into several solutions with different characters. The continuum family is divided into two modes: one is the original Parker mode, which is a slow MHD mode modulated by the gravity, and the other is a stable Alfven mode. The Parker modes can be either stable or unstable depending on gamma. When gamma is smaller than a critical value gamma(cr), the Parker mode becomes unstable. The discrete family is divided into three modes: a stable fast MHD mode modulated by the gravity, a stable slow MHD mode modulated by the gravity, and an unstable mode that is also attributed to a slow MHD mode. The unstable discrete mode does not always exist. Even though the unstable discrete mode exists, the Parker mode dominates it if the Parker mode is unstable. However, if gamma greater than or equal to gamma(cr), then the discrete mode could be the only unstable one. When gamma is equal gamma(cr), the minimum growth time of the unstable discrete mode is 1.3 x 10(8) yr, with a corresponding length scale of 2.4 kpc. It is suggestive that the corrugated features seen in the Galaxy and external galaxies are related to the unstable discrete modeopen131

Modelling the spectral energy distribution of galaxies. V. The dust and PAH emission SEDs of disk galaxies

We present a self-consistent model of the spectral energy distributions (SEDs) of spiral galaxies from the ultraviolet (UV) to the mid-infrared (MIR)/far-infrared (FIR)/submillimeter (submm) based on a full radiative transfer calculation of the propagation of starlight in galaxy disks. This model predicts not only the total integrated energy absorbed in the UV/optical and re-emitted in the infrared/submm, but also the colours of the dust emission based on an explicit calculation of the strength and colour of the UV/optical radiation fields heating the dust, and incorporating a full calculation of the stochastic heating of small dust grains and PAH molecules. The geometry of the translucent components of the model is empirically constrained using the results from the radiation transfer analysis of Xilouris et al. on spirals in the middle range of the Hubble sequence, while the geometry of the optically thick components is constrained from physical considerations with a posteriori checks of the model predictions with observational data. These geometrical constraints enable the dust emission to be predicted in terms of a minimum set of free parameters: the central face-on dust opacity in the B-band tau^f_B, a clumpiness factor F for the star-forming regions, the star-formation rate SFR, the normalised luminosity of the old stellar population old and the bulge-to-disk ratio B/D. We show that these parameters are almost orthogonal in their predicted effect on the colours of the dust/PAH emission. The results of the calculations are made available in the form of a large library of simulated dust emission SEDs spanning the whole parameter space of our model, together with the corresponding library of dust attenuation calculated using the same model. (see full abstract in the paper)Comment: 39 pages; accepted for publication in Astronomy & Astrophysics; For a higher resolution version of Fig.1 and Fig.20 see http://www.star.uclan.ac.uk/~ccp/index.shtm

The Vertical Structure of the Outer Milky Way HI Disk

We examine the outer Galactic HI disk for deviations from the b=0 plane by constructing maps of disk surface density, mean height, and thickness. We find that the Galactic warp is well described by a vertical offset plus two Fourier modes of frequency 1 and 2, all of which grow with Galactocentric radius. Adding the m=2 mode accounts for the large asymmetry between the northern and southern warps. We use a Morlet wavelet transform to investigate the spatial and frequency localization of higher frequency modes; these modes are often referred to as "scalloping." We find that the m=10 and 15 scalloping modes are well above the noise, but localized; this suggests that the scalloping does not pervade the whole disk, but only local regions.Comment: Accepted by ApJ. 17 pages, 18 figures. Color maps are available at http://astron.berkeley.edu/~elevin

Increasing response to a postal survey of sedentary patients – a randomised controlled trial [ISRCTN45665423]

BACKGROUND: A systematic review identified a range of methods, which can influence response rates. However, analysis specific to a healthcare setting, and in particular, involving people expected to be poor responders, was missing, We examined the effect of pre-warning letters on response rates to a postal survey of sedentary patients whom we expected a low rate of response. METHODS: Participants were randomised to receive a pre-warning letter or no pre-warning letter, seven days before sending the main questionnaire. The main questionnaire included a covering letter and pre-paid return envelope. After seven days, non-responders were sent a reminder letter and seven days later, another reminder letter with a further copy of the questionnaire and return envelope. RESULTS: 627 adults, with a mean age of 48 years (SD 13, range 18 to 78) of whom 69.2% (434/627) were women, were randomised. 49.0% (307/627) of patients were allocated to receive a pre-warning letter and 51.0% (320/627) no pre-warning letter, seven days in advance of posting the main questionnaire. The final response rate to the main questionnaire was 30.0% (92/307) amongst those sent a pre-warning letter and 20.9% (67/320) not sent a pre-warning letter, with an adjusted odds ratio of 1.60 (95% CI 1.1, 2.30). CONCLUSIONS: The relatively low cost method of sending a pre-warning letter had a modest impact on increasing response rates to a postal questionnaire sent to a group of patients for whom a low response rate was anticipated. Investigators should consider incorporating this simple intervention when conducting postal surveys, to reduce the potential for nonresponse bias and to increase the study power. Methods other than postal surveys may be needed however when a low response rate to postal surveys is likely