A New Z=0 Metagalactic Ultraviolet Background Limit

We present new integral-field spectroscopy in the outskirts of two nearby, edge-on, late-type galaxies to search for the H alpha emission that is expected from the exposure of their hydrogen gas to the metagalactic ultraviolet background (UVB). Despite the sensitivity of the VIRUS-P spectrograph on the McDonald 2.7 m telescope to low surface brightness emission and the large field of view, we do not detect H alpha to 5 sigma upper limits of 6.4 x 10(-19) erg s(-1) cm(-2) arcsec(-2) in UGC 7321 and of 25 x 10(-19) erg s(-1) cm(-2) arcsec(-2) in UGC 1281 in each of the hundreds of independent spatial elements (fibers). We fit gas distribution models from overlapping 21 cm data of HI, extrapolate one scale length beyond the HI data, and estimate predicted H alpha surface brightness maps. We analyze three types of limits from the data with stacks formed from increasingly large spatial regions and compare to the model predictions: (1) single fibers, (2) convolution of the fiber grid with a Gaussian, circular kernel (10('') full width at half-maximum), and (3) the co-added spectra from a few hundred fibers over the brightest model regions. None of these methods produce a significant detection (>5 sigma) with the most stringent constraints on the Hi photoionization rate of Gamma(z = 0) < 1.7 x 10(-14) s(-1) in UGC 7321 and Gamma(z = 0) < 14 x 10(-14) s(-1) in UGC 1281. The UGC 7321 limit is below previous measurement limits and also below current theoretical models. Restricting the analysis to the fibers bound by the HI data leads to a comparable limit; the limit is Gamma(z = 0) < 2.3 x 10(-14) s(-1) in UGC 7321. We discuss how a low Lyman limit escape fraction in z similar to 0 redshift star-forming galaxies might explain this lower than predicted UVB strength and the prospects of deeper data to make a direct detection.U.S. Government NAG W-2166National Science FoundationUT David BrutonTexas Norman Hackerman Advanced Research Program 003658-0295-2007Cynthia and George Mitchell FoundationMcDonald Observator

Single phase microreactor for the continuous, high-temperature synthesis of <4¿nm superparamagnetic iron oxide nanoparticles

The reproducibility of key nanomaterial features is essential in nanomedicine applications where small changes of physical characteristics often lead to a very different behavior. In this regard, continuous microreactors are often advocated as a means to achieve highly precise synthesis of nanomaterials. However, when the synthesis must take place at high temperatures the use of these devices becomes restricted in terms of materials and practical problems (e.g. plugging of microchannels). Here we present the continuous synthesis of ultrasmall superparamagnetic iron oxide nanoparticles (SPIONs) through a polyol-based process at high temperatures (>200 °C). The microfluidic reactor designed allows SPION production at residence times under 1 min, was able to work continuously for 8 h without channel blockage and reached high production yields by coupling microreactors using stacked plates. The effect of operating conditions was optimized to produce homogeneous particles with a narrow particle size distribution. In summary, the microreactor developed in this work enables easy-to scale up, reproducible continuous production of SPIONs

Signatures of Galaxy-Cluster Interactions: Tully-Fisher Observations at z~0.1

We have obtained new optical imaging and spectroscopic observations of 78 galaxies in the fields of the rich clusters Abell 1413 (z = 0.14), Abell 2218 (z = 0.18) and Abell 2670 (z = 0.08). We have detected line emission from 25 cluster galaxies plus an additional six galaxies in the foreground and background, a much lower success rate than what was found (65%) for a sample of 52 lower-richness Abell clusters in the range 0.02 < z < 0.08. We have combined these data with our previous observations of Abell 2029 and Abell 2295 (both at z = 0.08), which yields a sample of 156 galaxies. We evaluate several parameters as a function of cluster environment: Tully-Fisher residuals, H-alpha equivalent width, and rotation curve asymmetry, shape and extent. Although H-alpha is more easily detectable in galaxies that are located further from the cluster cores, we fail to detect a correlation between H-alpha extent and galaxy location in those where it is detected, again in contrast with what is found in the clusters of lesser richness. We fail to detect any statistically significant trends for the other parameters in this study. The zero-point in the z~0.1 Tully-Fisher relation is marginally fainter (by 1.5 sigma) than that found in nearby clusters, but the scatter is essentially unchanged.Comment: 27 pages including 5 figures; accepted for publication in the Astronomical Journa

Pixelization and Dynamic Range in Radio Interferometry

This study investigates some of the consequences of representing the sky by a rectangular grid of pixels on the dynamic range of images derived from radio interferometric measurements. In particular, the effects of image pixelization coupled to the CLEAN deconvolution representation of the sky as a set of discrete delta functions can limit the dynamic range obtained when representing bright emission not confined to pixels on the grid. Sky curvature effects on non-coplanar arrays will limit the dynamic range even if strong sources are centered on a pixel in a "fly's eye" representation when such pixel is not located at the corresponding facet's tangent point. Uncertainties in the response function of the individual antennas as well as in the calibration of actual data due to ionospheric, atmospheric or other effects will limit the dynamic range even when using grid-less subtraction (i.e. in the visibility domain) of strong sources located within the field of view of the observation. A technique to reduce these effects is described and examples from an implementation in the Obit package are given. Application of this technique leads to significantly superior results without a significant increase in the computing time.Comment: 8 pages, 4 figures (one double, one triple), to appear in Astronomy and Astrophysics (accepted: September 5, 2008

Beyond gold: rediscovering tetrakis-(hydroxymethyl)-phosphonium chloride (THPC) as an effective agent for the synthesis of ultra-small noble metal nanoparticles and Pt-containing nanoalloys

The use of tetrakis-(hydroxymethyl)-phosphonium chloride (THPC) as simultaneous reducing agent and stabilizing ligand has been extended to the single-step synthesis at room temperature of a wide variety of monometallic nanoparticles and bi-/tri- metallic nanoalloys containing noble metals with potential application in catalysis. The colloidal suspensions exhibit mean diameters below 4 nm with narrow size distributions and high stability in aqueous solution for long periods of time