Metal-Ion Absorption in Conductively Evaporating Clouds

We present computations of the ionization structure and metal-absorption properties of thermally conductive interface layers that surround evaporating warm spherical clouds, embedded in a hot medium. We rely on the analytical formalism of Dalton & Balbus to calculate the temperature profile in the evaporating gas, and explicitly solve the time-dependent ionization equations for H, He, C, N, O, Si, and S in the interface. We include photoionization by an external field. We estimate how departures from equilibrium ionization affect the resonance-line cooling efficiencies in the evaporating gas, and determine the conditions for which radiative losses may be neglected in the solution for the evaporation dynamics and temperature profile. Our results indicate that non-equilibrium cooling significantly increases the value of the saturation parameter at which radiative losses begin to affect the flow dynamics. As applications we calculate the ion fractions and projected column densities arising in the evaporating layers surrounding dwarf-galaxy-scale objects that are also photoionized by metagalactic radiation. We compare our results to the UV metal-absorption column densities observed in local highly-ionized metal-absorbers, located in the Galactic corona or intergalactic medium. Conductive interfaces significantly enhance the formation of high-ions such as C^3+, N^4+, and O^5+ relative to purely photoionized clouds, especially for clouds embedded in a high-pressure corona. However, the enhanced columns are still too low to account for the O VI columns (~1e14 cm^-2) observed in the local high-velocity absorbers. We find that O VI columns larger than ~1e13 cm^-2 cannot be produced in evaporating clouds. Our results do support the conclusion of Savage & Lehner, that absorption due to evaporating O VI likely occurs in the local interstellar medium, with characteristic columns of ~1e13 cm^-2.Comment: Accepted for Publication in Ap

Implementation and Evaluation of Power Consumption of an Iris Pre-processing Algorithm on Modern FPGA

In this article, the efficiency and applicability of several power reduction techniques applied on a modern 65nm FPGA is described. For image erosion and dilation algorithms, two major solutions were tested and compared with respect to power and energy consumption. Firstly the algorithm was run on a general purpose processor (gpp) NIOS and then hardware architecture of an Intellectual Property (IP) was designed. Furthermore IPs design was improved by applying a number of power optimization techniques. They involved RTL level clock gating, power driven synthesis with fitting and appropriate coding style. Results show that hardware implementation is much more energy efficient than a general purpose processor and power optimization schemes can reduce the overall power consumption on an FPGA

Enhancement of Optical Coherence Tomography Images of the Retina by Normalization and Fusion

This paper describes an image processing method applied to Optical Coherence Tomography (OCT) images of the retina. The aim is to achieve improved OCT images from the fusion of sequential OCT scans obtained at identical retinal locations. The method is based on the normalization of the acquired images and their fusion. As a result, a noise reduction and an image enhancement are reached. Thanks to the resulting improvement in retinal imaging, clinical specialists are able to evaluate more efficiently eyes pathologies and anomalies. This paper presents the proposed method and gives some evaluation results

Co-evolution of Dust and Chemistry in Galaxy Simulations with a Resolved Interstellar Medium

Nearby dwarf irregular galaxies are ideal laboratories for studying the interstellar medium (ISM) at low metallicity, which is expected to be common for galaxies at very high redshift being observed by the James Webb Space Telescope. We present the first high-resolution ($\sim 0.2$~pc) hydrodynamical simulations of an isolated low-metallicity ($0.1~Z_\odot$) dwarf galaxy coupled with a time-dependent chemistry network and a dust evolution model where dust is locally produced and destroyed by various processes. To accurately model carbon monoxide (CO), we post-process the simulations with a detailed chemistry network including the time-dependent effect of molecular hydrogen (H$_2$). Our model successfully reproduces the observed star formation rate and CO(1-0) luminosity ($L_{\rm CO}$). We find that dust growth in dense gas is required to reproduce the observed $L_{\rm CO}$ as otherwise CO would be completely photodissociated. In contrast, the H$_2$ abundance is extremely small and is insensitive to dust growth, leading to a CO-to-H$_2$ conversion factor that is only slightly higher than the Milky Way value despite the low metallicity. An observationally inferred dust-to-gas ratio is thus underestimated if adopting the metallicity-dependent CO-to-H$_2$ conversion factor. The newly-produced dust in dense gas mixes with the ISM through supernova feedback without being completely destroyed by sputtering, which leads to galactic outflows 20% - 50% dustier than the ISM, providing a possible source for intergalactic dust.Comment: ApJ accepted. 19 pages, 10 figure

Metallicity dependence of the H/H2_2 and C+^+/C/CO distributions in a resolved self-regulating interstellar medium

We study the metallicity dependence of the H/H$_2$ and C$^+$/C/CO distributions in a self-regulated interstellar medium (ISM) across a broad range of metallicities ($0.1 < Z/Z_\odot < 3$). To this end, we conduct high-resolution (particle mass of $1\ {\rm M_\odot}$) hydrodynamical simulations coupled with a time-dependent H$_2$ chemistry network. The results are then post-processed with an accurate chemistry network to model the associated C$^+$/C/CO abundances, based on the time-dependent non-steady-state (``non-equilibrium'') H$_2$ abundances. We find that the time-averaged star formation rate and the ISM structure are insensitive to metallicity. The column densities relevant for molecular shielding appear correlated with the volume densities in gravitationally unstable gas. As metallicity decreases, H$_2$ progressively deviates from steady state (``equilibrium'') and shows shallow abundance profiles until they sharply truncate at the photodissociation fronts. In contrast, the CO profile is sharp and controlled by photodissociation as CO quickly reaches steady state. We construct effective one-dimensional cloud models that successfully capture the time-averaged chemical distributions in simulations. At low metallicities, the steady-state model significantly overestimates the abundance of H$_2$ in the diffuse medium. The overestimated H$_2$, however, has little impact on CO. Consequently, the mass fraction of CO-dark H$_2$ gas is significantly lower than what a fully steady-state model predicts. The mass ratios of H$_2$/C$^+$ and H$_2$/C both show a weaker dependence on $Z^{\prime}$ than H$_2$/CO, which potentially indicates that C$^+$ and C could be alternative tracers for H$_2$ at low $Z^{\prime}$ in terms of mass budget. Our chemistry code for post-processing is publicly available.Comment: ApJ accepted, 32 pages (main text 24 pages), chemistry code available at https://github.com/huchiayu/AstroChemistry.j

[CII] Emission in a Self-Regulated Interstellar Medium

The [CII] 157.74 $\mu$m fine structure transition is one of the brightest and most well-studied emission lines in the far-infrared (FIR), produced in the interstellar medium (ISM) of galaxies. We study its properties in sub-pc resolution hydrodynamical simulations for an ISM patch with gas surface density of $\Sigma_{\rm{g}}=10\;M_{\odot}\;\rm{pc}^{-2}$, coupled with time-dependent chemistry, far-ultraviolet (FUV) dust and gas shielding, star formation, photoionization and supernova (SN) feedback, and full line-radiative transfer. The [CII] 157.74 $\mu$m line intensity correlates with star formation rate (SFR), and scales linearly with metallicity, and is, therefore, a good SFR tracer even in metal-poor environments, where molecular lines might be undetectable. We find a [CII]-to-H$_2$ conversion factor that is well described by the power law $X_{\rm{[CII]}}=6.31\times 10^{19} \;Z^{\prime\;0.17}\; (\rm{cm}^{-2}\;(\rm{K}\;\rm{km}\;\rm{s}^{-1})^{-1})$. Our results are in agreement with galaxy surveys in all but the lowest metallicity run. We find that resolving the clumpy structure of the dense interstellar medium (ISM) is important as it dominates [CII] 157.74 $\mu$m emission. We compare our full radiative transfer computation with the optically-thin limit, and find that [CII] emission only begins approaching the optically thick limit at super-solar metallicity, for our assumed gas surface density

Description of a three-dimensional deconvolution reconstruction algorithm from cone beam projection

This paper presents the discretization and the application of a 3D reconstruction method front cone beam X-ray projections . The generalized back projection theorem, established in a previous work, is the theoretical basis for the method. It allows to reduce the reconstruction problem to a 3D deconvolution problem . The proposed algorithm essentially consists in two steps : (i) computation of the discrete corrected back projection of all the cone beam projections ; (ii) deconvolution of the result . After vectorization, this algorithm has been implemented on a CDC CYBER 205 computer. A simple and comprehensive test function is proposed to evaluate the algorithm relatively to various error criteria . The first simulations show that the reconstruction results are very satisfying when the X-ray sources are located in the whole space around the object, in accordance with the theory (4 it geometry). Furthermore, even in poor acquisition conditions the algorithm seems to give a first approximation of the object which can be sufficient to study its morphological aspect .Cet article présente la discrétisation et la mise en oeuvre d'une méthode de reconstruction 3D à partir de projections coniques . Le théorème de la rétroprojection, établi dans un travail précédent, est la base théorique de la méthode. Il permet de ramener le problème de reconstruction à un problème de déconvolution 3D. L'algorithme proposé comporte essentiellement deux étapes, consistant tout d'abord à calculer une rétroprojection corrigée des projections coniques, puis à déconvoluer le résultat obtenu. Cet algorithme a été implanté sur le CYBER 205 de Control Data après avoir été complètement vectorisé . Un exemple simple de simulation et différents critères d'écarts sont proposés pour l'évaluer . Les premières simulations montrent que les résultats obtenus sont très satisfaisants si les sources sont réparties dans l'espace tout autour de l'objet (géométie 4 7t) . De plus, même dans des conditions d'acquisition assez défavorables, l'algorithme semble donner une première approximation de l'objet qui peut être suffisante pour étudier son aspect morphologique

The Super Star Cluster NGC 1569-A Resolved on Sub-Parsec Scales with Hubble Space Telescope Spectroscopy

We present 3000--10000 Ang HST/STIS long-slit spectroscopy of the bright super star cluster A (SSC-A) in the dwarf starburst galaxy NGC 1569. The 0.05" HST angular resolution allows, for the first time, to probe for spatial variations in the stellar population of a ~ 10^6 M_sun SSC. Integrated ground-based spectra of SSC-A have previously revealed young Wolf-Rayet (WR) signatures that coexist with features from supposedly older, red supergiant (RSG), populations. We find that the WR emission complexes come solely from the subcluster A2, identified in previous HST imaging, and are absent from the main cluster A1, thus resolving the question of whether the WR and RSG features arise in a single or distinct clusters. The equivalent widths of the WR features in A2 --- including the CIV 5808 complex which we detect in this object for the first time --- are larger than previously observed in other WR galaxies. Models with sub-solar metallicity, as inferred from the nebular emission lines of this galaxy, predict much lower equivalent widths. On the ``clean'' side of A1, opposite to A2, we find no evidence for radial gradients in the observed stellar population at 0.05"<R<0.40" (~0.5 to 5 pc), neither in broad-band, low-resolution, spectra nor in medium-resolution spectra of the infrared CaII triplet.Comment: 5 pages, accepted for publication in ApJ Le