Surveys of Galaxy Clusters with the Sunyaev Zel'dovich Effect

We have created mock Sunyaev-Zel'dovich effect (SZE) surveys of galaxy clusters using high resolution N-body simulations. To the pure surveys we add `noise' contributions appropriate to instrument and primary CMB anisotropies. Applying various cluster finding strategies to these mock surveys we generate catalogues which can be compared to the known positions and masses of the clusters in the simulations. We thus show that the completeness and efficiency that can be achieved depend strongly on the frequency coverage, noise and beam characteristics of the instruments, as well as on the candidate threshold. We study the effects of matched filtering techniques on completeness, and bias. We suggest a gentler filtering method than matched filtering in single frequency analyses. We summarize the complications that arise when analyzing the SZE signal at a single frequency, and assess the limitations of such an analysis. Our results suggest that some sophistication is required when searching for `clusters' within an SZE map.Comment: 8 pages, 7 figure

Strategies to Mitigate the Effects of Soil Physical Disturbances Caused by Forest Machinery: a Comprehensive Review

Purpose of Review: Ground-based mechanized forest operations can cause severe soil disturbances that are often long lasting and detrimental to the health of forested ecosystems. To reduce these soil disturbances, focus is being increasingly directed at identifying and using appropriate mitigation techniques. This systematic review considered 104 scientific articles and reported the main findings according to four core themes: terrain-related factors, operational planning, machine modifications, and types of amendments used to mitigate machine-induced soil impacts. Recent Findings: For terrain-related factors, most severe disturbances occur on machine operating trails exceeding 20% slope and that soil bulk density and rut depth show greater increases in fine-textured soils. When considering operational planning, trafficability maps proved to be helpful in reducing the frequency and magnitude of soil damages as well as the length of trails needed within harvest sites, especially if they are regularly updated with weather information. Machine modifications, through high flotation tires, use of extra bogie axle, lower inflation pressure, and use of steel flexibles tracks, are highly researched topics because of the considerable upside in terms of machine ground pressure distribution and increased traction. Two main types of amendments emerged to mitigate soil disturbances: brush mats and mulch cover. Brush mats created from harvesting debris can spread the load of a machine to a greater area thereby lowering peak loads transferred to the soil. Brush mats of 15–20 kg m−2 are being recommended for adequate soil protection from harvesting operations. Summary: To conclude, we outline recommendations and strategies on the use of soil mitigation techniques within cut-to-length forest operations. New research opportunities are also identified and discussed. Considering single factors causing machine-induced soil disturbances remains important but there is a pressing need for having a multi-disciplinary approach to tackle the complex problems associated with machine/soil/plant interactions

The Radio Quiescence of Active Galaxies with High Accretion Rates

We present 6 cm Very Large Array observations of the Greene & Ho (2004) sample of 19 low-mass active galaxies with high accretion rates. This is one of the only studies of a uniform sample of narrow-line Seyfert 1 (NLS1) galaxies with such high sensitivity and resolution. Although we detect only one source, the entire sample is very radio-quiet down to strong limits. GH10 was found to have a radio power of 8.5 x 10^21 W/Hz, and a ratio R = f(6 cm)/f(4400 A) of 2.8. The 3 sigma upper limits for the remaining nondetections correspond to radio powers from 3 x 10^20 to 8 x 10^21 W/Hz and 0.47 < R <9.9. Stacking all nondetections yields an even stronger upper limit of R < 0.27. An assessment of existing observations in the literature confirms our finding that NLS1s are consistently radio-quiet, with a radio-loud fraction of 0%-6%, which is significantly lower than the 10%-20% observed in the general quasar population. By analogy with stellar-mass black holes, we argue that AGNs undergo a state transition at L_bol/L_Edd~0.01. Below this value a radiatively inefficient accretion flow effectively drives an outflow, which disappears when the flow turns into an optically thick, geometrically thin disk, or a radiation pressure-dominated slim disk at still higher L_bol/L_Edd.Comment: To appear in ApJ; 8 pages, 3 figures; uses emulateapj5.st

Molecular Gas, Dust and Star Formation in the Barred Spiral NGC 5383

We present multi-wavelength (interferometer and single-dish CO J=1-0, Halpha, broadband optical and near-infrared) observations of the classic barred spiral NGC 5383. We compare the observed central gas and dust morphology to the predictions of recent hydrodynamic simulations. In the nuclear region, our observations reveal three peaks lying along a S-shaped gas and dust distribution. In contrast, the model predicts a circumnuclear ring, not the observed S-shaped distribution; moreover, the predicted surface density contrast between the central gas accumulation and the bar dust lanes is an order of magnitude larger than observed. The discrepancies are not due to unexplored model parameter space or a nuclear bar but are probably due to the vigorous (7 solar masses per year) star formation activity in the center. As is common in similar bars, the star formation rate in the bar between the bar ends and the central region is low (~0.5 solar masses per yr), despite the high gas column density in the bar dust lanes; this is generally attributed to shear and shocks. We note a tendency for the HII regions to be associated with the spurs feeding the main bar dust lanes, but these are located on the leading side of the bar. We propose that stars form in the spurs, which provide a high column density but low shear environment. HII regions can therefore be found even on the leading side of the bar because the ionizing stars pass ballistically through the dust laneComment: Accepted for publication in The Astrophysical Journal, 33 pages (includes 10 figures

Optimal Image Reconstruction in Radio Interferometry

We introduce a method for analyzing radio interferometry data which produces maps which are optimal in the Bayesian sense of maximum posterior probability density, given certain prior assumptions. It is similar to maximum entropy techniques, but with an exact accounting of the multiplicity instead of the usual approximation involving Stirling's formula. It also incorporates an Occam factor, automatically limiting the effective amount of detail in the map to that justified by the data. We use Gibbs sampling to determine, to any desired degree of accuracy, the multi-dimensional posterior density distribution. From this we can construct a mean posterior map and other measures of the posterior density, including confidence limits on any well-defined function of the posterior map.Comment: 41 pages, 11 figures. High resolution figures 8 and 9 available at http://www.astro.uiuc.edu/~bwandelt/SuttonWandelt200

Environment, Ram Pressure, and Shell Formation in HoII

Neutral hydrogen VLA D-array observations of the dwarf irregular galaxy HoII, a prototype galaxy for studies of shell formation, are presented. HI is detected to radii over 16' or 4 R_25, and M_HI=6.44x10^8 M_sun. The total HI map has a comet-like appearance suggesting that HoII is affected by ram pressure from an intragroup medium (IGM). A rotation curve corrected for asymmetric drift was derived and an analysis of the mass distribution yields a total mass 6.3x10^9 M_sun, of which about 80% is dark. HoII lies northeast of the M81 group's core, along with Kar52 (M81dwA) and UGC4483. No signs of interaction are observed and it is argued that HoII is part of the NGC2403 subgroup, infalling towards M81. A case is made for ram pressure stripping and an IGM in the M81 group. Stripping of the disk outer parts would require an IGM density n_IGM>=4.0x10^-6 atoms/cm^3 at the location of HoII. This corresponds to 1% of the virial mass of the group uniformly distributed over a volume just enclosing HoII and is consistent with the X-ray properties of small groups. It is argued that existing observations of HoII do not support self-propagating star formation scenarios, whereby the HI holes and shells are created by supernova explosions and stellar winds. Many HI holes are located in low surface density regions of the disk, where no star formation is expected or observed. Ram pressure has the capacity to enlarge preexisting holes and lower their creation energies, helping to bridge the gap between the observed star formation rate and that required to create the holes. (abridged)Comment: 43 pages, including 7 figures. 4 figures available as JPEG only. Complete manuscript including full resolution figures available at http://www.strw.leidenuniv.nl/~bureau/pub_list.html . Accepted for publication in The Astronomical Journa

Diffraction-limited near-IR imaging at Keck reveals asymmetric, time-variable nebula around carbon star CIT 6

We present multi-epoch, diffraction-limited images of the nebula around the carbon star CIT 6 at 2.2 microns and 3.1 microns from aperture masking on the Keck-I telescope. The near-IR nebula is resolved into two main components, an elongated, bright feature showing time-variable asymmetry and a fainter component about 60 milliarcseconds away with a cooler color temperature. These images were precisely registered (~35 milliarcseconds) with respect to recent visible images from the Hubble Space Telescope (Trammell et al. 2000), which showed a bipolar structure in scattered light. The dominant near-IR feature is associated with the northern lobe of this scattering nebula, and the multi-wavelength dataset can be understood in terms of a bipolar dust shell around CIT 6. Variability of the near-IR morphology is qualitatively consistent with previously observed changes in red polarization, caused by varying illumination geometry due to non-uniform dust production. The blue emission morphology and polarization properties can not be explained by the above model alone, but require the presence of a wide binary companion in the vicinity of the southern polar lobe. The physical mechanisms responsible for the breaking of spherical symmetry around extreme carbon stars, such as CIT 6 and IRC+10216, remain uncertain.Comment: 18 pages, 5 figures (one in color), to appear in the Astrophysical Journa

An Automated Method for the Detection and Extraction of HI Self-Absorption in High-Resolution 21cm Line Surveys

We describe algorithms that detect 21cm line HI self-absorption (HISA) in large data sets and extract it for analysis. Our search method identifies HISA as spatially and spectrally confined dark HI features that appear as negative residuals after removing larger-scale emission components with a modified CLEAN algorithm. Adjacent HISA volume-pixels (voxels) are grouped into features in (l,b,v) space, and the HI brightness of voxels outside the 3-D feature boundaries is smoothly interpolated to estimate the absorption amplitude and the unabsorbed HI emission brightness. The reliability and completeness of our HISA detection scheme have been tested extensively with model data. We detect most features over a wide range of sizes, linewidths, amplitudes, and background levels, with poor detection only where the absorption brightness temperature amplitude is weak, the absorption scale approaches that of the correlated noise, or the background level is too faint for HISA to be distinguished reliably from emission gaps. False detection rates are very low in all parts of the parameter space except at sizes and amplitudes approaching those of noise fluctuations. Absorption measurement biases introduced by the method are generally small and appear to arise from cases of incomplete HISA detection. This paper is the third in a series examining HISA at high angular resolution. A companion paper (Paper II) uses our HISA search and extraction method to investigate the cold atomic gas distribution in the Canadian Galactic Plane Survey.Comment: 39 pages, including 14 figure pages; to appear in June 10 ApJ, volume 626; figure quality significantly reduced for astro-ph; for full resolution, please see http://www.ras.ucalgary.ca/~gibson/hisa/cgps1_survey

Spatially Resolved Observations of the Galactic Center Source, IRS 21

We present diffraction-limited 2-25 micron images obtained with the W. M. Keck 10-m telescopes that spatially resolve the cool source, IRS 21, one of a small group of enigmatic objects in the central parsec of our Galaxy that have eluded classification. Modeled as a Gaussian, the azimuthally-averaged intensity profile of IRS 21 has a half-width half-maximum (HWHM) size of 650+/-80 AU at 2.2 microns and an average HWHM size of 1600+/-200 AU at mid-infrared wavelengths. These large apparent sizes imply an extended distribution of dust. The mid-infrared color map indicates that IRS 21 is a self-luminous source rather than an externally heated dust clump as originally suggested. The spectral energy distribution has distinct near- and mid-infrared components. A simple radiative transfer code, which simultaneously fits the near- and mid- infrared photometry and intensity profiles, supports a model in which the near-infrared radiation is scattered and extincted light from an embedded central source, while the mid-infrared emission is from thermally re-radiating silicate dust. We argue that IRS 21 (and by analogy the other luminous sources along the Northern Arm) is a massive star experiencing rapid mass loss and plowing through the Northern Arm, thereby generating a bow shock, which is spatially resolved in our observations.Comment: 29 pages, 8 figures, Latex. Astrophysical Journal, accepte

The Extraordinary X-ray Light Curve of the Classical Nova V1494 Aquilae (1999 #2) in Outburst: The Discovery of Pulsations and a "Burst"

V1494 Aql (Nova Aql 1999 No. 2) was discovered on 2 December 1999. We obtained Chandra ACIS-I spectra on 15 April and 7 June 2000 which appear to show only emission lines. Our third observation, on 6 August, showed that its spectrum had evolved to that characteristic of a Super Soft X-ray Source. We then obtained Chandra LETG+HRC-S spectra on 28 September (8 ksec) and 1 October (17 ksec). We analyzed the X-ray light curve of our grating observations and found both a short time scale ``burst'' and oscillations. Neither of these phenomena have previously been seen in the light curve of a nova in outburst. The ``burst'' was a factor of 10 rise in X-ray counts near the middle of the second observation, and which lasted about 1000 sec; it exhibited at least two peaks, in addition to other structure. Our time series analysis of the combined 25 ksec observation shows a peak at 2500 s which is present in independent analyses of both the zeroth order image and the dispersed spectrum and is not present in similar analyses of grating data for HZ 43 and Sirius B. Further analyses of the V1494 Aql data find other periods present which implies that we are observing non-radial g+ modes from the pulsating, rekindled white dwarf.Comment: ApJ accepte