New Constraints on the Escape of Ionizing Photons From Starburst Galaxies Using Ionization-Parameter Mapping

The fate of ionizing radiation in starburst galaxies is key to understanding cosmic reionization. However, the galactic parameters on which the escape fraction of ionizing radiation depend are not well understood. Ionization-parameter mapping provides a simple, yet effective, way to study the radiative transfer in starburst galaxies. We obtain emission-line ratio maps of [SIII]/[SII] for six, nearby, dwarf starbursts: NGC 178, NGC 1482, NGC 1705, NGC 3125, NGC 7126, and He 2-10. The narrow-band images are obtained with the Maryland-Magellan Tunable Filter at Las Campanas Observatory. Using these data, we previously reported the discovery of an optically thin ionization cone in NGC 5253, and here we also discover a similar ionization cone in NGC 3125. This latter cone has an opening angle of 40+/-5 degrees (0.4 ster), indicating that the passageways through which ionizing radiation may travel correspond to a small solid angle. Additionally, there are three sample galaxies that have winds and/or superbubble activity, which should be conducive to escaping radiation, yet they are optically thick. These results support the scenario that an orientation bias limits our ability to directly detect escaping Lyman continuum in many starburst galaxies. A comparison of the star-formation properties and histories of the optically thin and thick galaxies is consistent with the model that high escape fractions are limited to galaxies that are old enough (> 3 Myr) for mechanical feedback to have cleared optically thin passageways in the ISM, but young enough (< 5 Myr) that the ionizing stars are still present.Comment: Accepted for publication in Ap

An Ionization Cone in the Dwarf Starburst Galaxy NGC 5253

There are few observational constraints on how the escape of ionizing photons from starburst galaxies depends on galactic parameters. Here, we report on the first major detection of an ionization cone in NGC 5253, a nearby starburst galaxy. This high-excitation feature is identified by mapping the emission-line ratios in the galaxy using [S III] lambda 9069, [S II] lambda 6716, and H_alpha narrow-band images from the Maryland-Magellan Tunable Filter at Las Campanas Observatory. The ionization cone appears optically thin, which is suggestive of the escape of ionizing photons. The cone morphology is narrow with an estimated solid angle covering just 3% of 4pi steradians, and the young, massive clusters of the nuclear starburst can easily generate the radiation required to ionize the cone. Although less likely, we cannot rule out the possibility of an obscured AGN source. An echelle spectrum along the minor axis shows complex kinematics that are consistent with outflow activity. The narrow morphology of the ionization cone supports the scenario that an orientation bias contributes to the difficulty in detecting Lyman continuum emission from starbursts and Lyman break galaxies.Comment: 5 pages, 4 figures, Accepted to ApJ Letter

Photophysics of closed- and open-ring isomers of a diarylethene with a carboxylic anchor group

We study the transient photophysical properties of a diarylethene with a carboxylic anchor group by a combination of steady-state and ultrafast emission and absorption spectroscopy. After excitation of the closed-ring form, fluorescence with a quantum yield of 10−5 is observed and separated into different spectro-temporal components. The S1 state of the closed-ring form shows a lifetime of 1.3 ps and decays mainly by internal conversion to the S0 state of this isomer. This vibrationally hot ground state cools on a time scale of 10 ps

Scaling of spontaneous rotation with temperature and plasma current in tokamaks

Using theoretical arguments, a simple scaling law for the size of the intrinsic rotation observed in tokamaks in the absence of momentum injection is found: the velocity generated in the core of a tokamak must be proportional to the ion temperature difference in the core divided by the plasma current, independent of the size of the device. The constant of proportionality is of the order of $10\,\mathrm{km \cdot s^{-1} \cdot MA \cdot keV^{-1}}$. When the intrinsic rotation profile is hollow, i.e. it is counter-current in the core of the tokamak and co-current in the edge, the scaling law presented in this Letter fits the data remarkably well for several tokamaks of vastly different size and heated by different mechanisms.Comment: 5 pages, 3 figure

Seeking critical nodes in digraphs

The Critical Node Detection Problem (CNDP) consists in finding the set of nodes, defined critical, whose removal maximally degrades the graph. In this work we focus on finding the set of critical nodes whose removal minimizes the pairwise connectivity of a direct graph (digraph). Such problem has been proved to be NP-hard, thus we need efficient heuristics to detect critical nodes in real-world applications. We aim at understanding which is the best heuristic we can apply to identify critical nodes in practice, i.e., taking into account time constrains and real-world networks. We present an in-depth analysis of several heuristics we ran on both real-world and on synthetic graphs. We define and evaluate two different strategies for each heuristic: standard and iterative. Our main findings show that an algorithm recently proposed to solve the CNDP and that can be used as heuristic for the general case provides the best results in real-world graphs, and it is also the fastest. However, there are few exceptions that are thoroughly analyzed and discussed. We show that among the heuristics we analyzed, few of them cannot be applied to very large graphs, when the iterative strategy is used, due to their time complexity. Finally, we suggest possible directions to further improve the heuristic providing the best results

Deuterium Balmer/Stark spectroscopy and impurity profiles: first results from mirror-link divertor spectroscopy system on the JET ITER-like wall

For the ITER-like wall, the JET mirror link divertor spectroscopy system was redesigned to fully cover the tungsten horizontal strike plate with faster time resolution and improved near-UV performance. Since the ITER-like wall project involves a change in JET from a carbon dominated machine to a beryllium and tungsten dominated machine with residual carbon, the aim of the system is to provide the recycling flux, equivalent, to the impinging deuterium ion flux, the impurity fluxes (C, Be, O) and tungsten sputtering fluxes and hence give information on the tungsten divertor source. In order to do this self-consistently, the system also needs to provide plasma characterization through the deuterium Balmer spectra measurements of electron density and temperature during high density. L-Mode results at the density limit from Stark broadening/line ratio analysis will be presented and compared to Langmuir probe profiles and 2D-tomography of low-n Balmer emission [1]. Comparison with other diagnostics will be vital for modelling attempts with the EDGE2D-EIRENE code[2] as the best possible data sets need to be provided to study detachment behaviour.Comment: 18 pages, 11 figure

The Optical Depth of H II Regions in the Magellanic Clouds

We exploit ionization-parameter mapping (IPM) as a powerful tool to measure the optical depth of star-forming H II regions. Our simulations using the photoionization code CLOUDY and our new, SURFBRIGHT surface-brightness simulator demonstrate that this technique can directly diagnose most density-bounded, optically thin nebulae using spatially resolved emission-line data. We apply this method to the Large and Small Magellanic Clouds (LMC and SMC), using the data from the Magellanic Clouds Emission Line Survey. We generate new H II region catalogs based on photoionization criteria set by the observed ionization structure in the [S II ]/[O III ] ratio and Hα surface brightness. The luminosity functions from these catalogs generally agree with those from Hα-only surveys. We then use IPM to crudely classify all the nebulae into optically thick versus optically thin categories, yielding fundamental new insights into Lyman-continuum (LyC) radiation transfer. We find that in both galaxies, the frequency of optically thin objects correlates with Hα luminosity, and that the numbers of these objects dominate above log L /(erg s –1 ) ≥ 37.0. The frequencies of optically thin objects are 40% and 33% in the LMC and SMC, respectively. Similarly, the frequency of optically thick regions correlates with H I column density, with optically thin objects dominating at the lowest N (H I ). The integrated escape luminosity of ionizing radiation is dominated by the largest regions and corresponds to luminosity-weighted, ionizing escape fractions from the H II region population of ≥0.42 and ≥0.40 in the LMC and SMC, respectively. These values correspond to global galactic escape fractions of 4% and 11%, respectively. This is sufficient to power the ionization rate of the observed diffuse ionized gas in both galaxies. Since our optical depth estimates tend to be underestimates, and also omit the contribution from field stars without nebulae, our results suggest the possibility of significant galactic escape fractions of LyC radiation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98568/1/0004-637X_755_1_40.pd

Occurrence of testicular microlithiasis in androgen insensitive hypogonadal mice

&lt;b&gt;Background&lt;/b&gt;: Testicular microliths are calcifications found within the seminiferous tubules. In humans, testicular microlithiasis (TM) has an unknown etiology but may be significantly associated with testicular germ cell tumors. Factors inducing microlith development may also, therefore, act as susceptibility factors for malignant testicular conditions. Studies to identify the mechanisms of microlith development have been hampered by the lack of suitable animal models for TM.&lt;BR/&gt; &lt;b&gt;Methods&lt;/b&gt;: This was an observational study of the testicular phenotype of different mouse models. The mouse models were: cryptorchid mice, mice lacking androgen receptors (ARs) on the Sertoli cells (SCARKO), mice with a ubiquitous loss of androgen ARs (ARKO), hypogonadal (hpg) mice which lack circulating gonadotrophins, and hpg mice crossed with SCARKO (hpg.SCARKO) and ARKO (hpg.ARKO) mice.&lt;BR/&gt; &lt;b&gt;Results&lt;/b&gt;: Microscopic TM was seen in 94% of hpg.ARKO mice (n=16) and the mean number of microliths per testis was 81 +/- 54. Occasional small microliths were seen in 36% (n=11) of hpg testes (mean 2 +/- 0.5 per testis) and 30% (n=10) of hpg.SCARKO testes (mean 8 +/- 6 per testis). No microliths were seen in cryptorchid, ARKO or SCARKO mice. There was no significant effect of FSH or androgen on TM in hpg.ARKO mice.&lt;BR/&gt; &lt;b&gt;Conclusions&lt;/b&gt;: We have identified a mouse model of TM and show that lack of endocrine stimulation is a cause of TM. Importantly, this model will provide a means with which to identify the mechanisms of TM development and the underlying changes in protein and gene expression