Effects of Clumping on the Observed Properties of Dusty Galaxies

We present Monte Carlo radiative transfer simulations for spiral galaxies modelled as a stellar disk and a two-phase clumpy dust distribution. We divide the volume occupied by the dust into a three-dimensional grid and assign each cell a clump or smooth medium status. Cell dimension, clump dust mass and spatial distribution are derived from the observed properties of Giant Molecular Clouds and molecular gas in the Galaxy. We produce models for several values of the optical depth and fraction of the interstellar medium residing in clumps. As a general result, clumpy models are less opaque than the corresponding homogeneous models. For the adopted parameters, the increase in the fraction of energy that escapes the disk is moderate, resulting in surface brightness profiles that are less than one magnitude brighter than those of the homogeneous models. The effects of clumping are larger for edge-on views of the disk. This is in contrast with previous preliminary results for clumping in the literature. We show how differences arise from the different parametrisation and clump distribution adopted. We also consider models in which a fraction of the stellar radiation is emitted within the clumps. In this case, galaxies are less transparent than in the case when only dust is clumped. The opacity can be even higher than in the homogeneous case, depending on the fraction of embedded stellar emission. We point out the implications of the results for the determination of the opacity and dust mass of spiral galaxies.Comment: 11 pages, 8 figures, MNRAS accepted, also available at http://www.arcetri.astro.it/~sbianchi/clumping.htm

Glittering Logic in a Minor Key

Glorious Catastrophe: Jack Smith, Performance and Visual Culture by Dominic Johnson. Rethinking Art’s Histories. Manchester, England: Manchester University Press, 2012. Pp. 256; 40 black-and-white illustrations. $95.00 cloth,$32.95 paper

Transforming Leadership in the California Literature Project: Ethnographic Narrative as True Fiction

With rare exception leadership scholars and practitioners focus on individuals as leaders. However, to investigate leadership only through the activities of individual leaders is to miss its essential feature, namely, that leadership is a particular kind of relationship in which leaders and followers create changes. Relationships are difficult to describe because researchers cannot observe them directly; instead, they must infer them indirectly. This study investigated leadership relationships in the California Literature Project. An exploration of leadership relationships required that I implement research methods that would support the construction of detailed descriptions of transforming leadership processes. These methods developed in part out of three background assumptions about social science research: (1) social truths are socially constructed and represent only a partial reality; (2) the narrative paradigm and related concepts of true fiction and story shape how a researcher reads and interprets the experiences in the field and, later, authors the text; and (3) the researcher\u27s experiences in the field setting and the written reconstruction of those experiences constitutes two distinct components of the research process. To that end, following an emergent design, qualitative study format, I became a participant observer for a year at the California Literature Project, in which collaborative leadership is part of the organization\u27s stated methods of operation. I attended a project academy and regional summer institute and conducted informal, open-ended interviews with selected members. From my field notes, transcripts, and selected memory, I then reconstructed a translation of the field experiences into an ethnographic narrative of collaborative leadership relationships. The narrative depicts aspects of influence relationships among leaders and followers who intended real changes that reflected their common purposes. From the narrative I develop a leadership framework under the headings leadership, intending changes, common purposes, and influence and power. As a result this narrative contributes to a new metaphor--relationship--for reconceptualizing leadership as collaborative, nonhierarchical and episodic in nature

Resolving the brainstem contributions to attentional analgesia

Previous human imaging studies manipulating attention or expectancy have identified the periaqueductal gray (PAG) as a key brainstem structure implicated in endogenous analgesia. However, animal studies indicate that PAG analgesia is mediated largely via caudal brainstem structures, such as the rostral ventromedial medulla (RVM) and locus coeruleus (LC). To identify their involvement in endogenous analgesia, we used brainstem optimized, whole-brain imaging to record responses to concurrent thermal stimulation (left forearm) and visual attention tasks of titrated difficulty in 20 healthy subjects. The PAG, LC, and RVM were anatomically discriminated using a probabilistic atlas. Pain ratings disclosed the anticipated analgesic interaction between task difficulty and pain intensity (p < 0.001). Main effects of noxious thermal stimulation were observed across several brain regions, including operculoinsular, primary somatosensory, and cingulate cortices, whereas hard task difficulty was represented in anterior insular, parietal, and prefrontal cortices. Permutation testing within the brainstem nuclei revealed the following: main effects of task in dorsal PAG and right LC; and main effect of temperature in RVM and a task × temperature interaction in right LC. Intrasubject regression revealed a distributed network of supratentorial brain regions and the RVM whose activity was linearly related to pain intensity. Intersubject analgesia scores correlated to activity within a distinct region of the RVM alone. These results identify distinct roles for a brainstem triumvirate in attentional analgesia: with the PAG activated by attentional load; specific RVM regions showing pronociceptive and antinociceptive processes (in line with previous animal studies); and the LC showing lateralized activity during conflicting attentional demands. SIGNIFICANCE STATEMENT Attention modulates pain intensity, and human studies have identified roles for a network of forebrain structures plus the periaqueductal gray (PAG). Animal data indicate that the PAG acts via caudal brainstem structures to control nociception. We investigated this issue within an attentional analgesia paradigm with brainstem-optimized fMRI and analysis using a probabilistic brainstem atlas. We find pain intensity encoding in several forebrain structures, including the insula and attentional activation of the PAG. Discrete regions of the rostral ventromedial medulla bidirectionally influence pain perception, and locus coeruleus activity mirrors the interaction between attention and nociception. This approach has enabled the resolution of contributions from a hub of key brainstem structures to endogenous analgesia