Booms and Busts: the Burstiness of Star Formation in Nearby Dwarf Galaxies

In this review I summarise recent advances in our understanding of the importance of starburst events to the evolutionary histories of nearby galaxies. Ongoing bursts are easily diagnosed in emission-line surveys, but assessing the timing and intensity of fossil bursts requires more effort, usually demanding color-magnitude diagrams or spectroscopy of individual stars. For ages older than ~1 Gyr, this type of observation is currently limited to the Local Group and its immediate surroundings. However, if the Local Volume is representative of the Universe as a whole, then studies of the age and metallicity distributions of star clusters and resolved stellar populations should give statistical clues as to the frequency and importance of bursts to the histories of galaxies in general. Based on starburst statistics in the literature and synthetic colour-magnitude diagram studies of Local Group galaxies, I attempt to distinguish between systemic starbursts that strongly impact galaxy evolution and stochastic bursts that can appear impressive but are ultimately of little significance on gigayear timescales. As a specific case, it appears as though IC 10, the only starburst galaxy in the Local Group, falls into the latter category and is not fundamentally different from other nearby dwarf irregular galaxies.Comment: Accepted by the Publications of the Astronomical Society of Australia (PASA). Summary of a review talk given at the Southern Cross Astrophysics Conference on "Galaxy Metabolism" held in Sydney, 22-26 June 2009. 9 pages, 2 figure

Reevaluating the Nika Riot & Placing it in Conversation with the Antioch Riot of 387

The reign of the Byzantine emperor Justinian, from 527 to 565, despite its many high points and achievements, was a struggle for him and his people. One of his most famous, or infamous, challenges was during the Nika riots. During this time Justinian sees the people of Constantinople attempt to crown a usurper as emperor instead of him. As Justinian convened with his advisors, hiding from the mob in his palace, one question must have rung true in his mind: how did this happen? This presentation answers this question while also giving an overview to the time and the people involved. First, by looking at the tradition and history of Roman and Byzantine chariot racing and circus factions, a picture of who these rioters were starts to take shape. Next, the presentation investigates the social and political background of the riots. Of the 13 riots that occurred during Justinian’s reign, this one posed the greatest threat to his power and rule over the empire. In the chaos, the rioters list their demands for Justinian. Previous research has taken these demands at face value, but the real sparks that started this conflict are much deeper. Instead, the unique political and economic situation in which the people of Constantinople find themselves helped to fan this flame. At last, the actual riot is looked at, particularly through the lens of these outside influences. To show how these situations helped to fuel the riots, the Nika riot will be compared to other riots and mob activity from around the same time, particularly the Antioch riot. Since both of these riots had a similar unfolding of events and consequences, the question of how did the demands and background to each riot effect these similar structure

Faint, moving objects in the Hubble Deep Field: components of the dark halo?

The deepest optical image of the sky, the Hubble Deep Field (HDF), obtained with the Hubble Space Telescope (HST) in December 1995, has been compared to a similar image taken in December 1997. Two very faint, blue, isolated and unresolved objects are found to display a substantial apparent proper motion, 23+/-5 mas/yr and 26+/-5 mas/yr; a further three objects at the detection limit of the second epoch observations may also be moving. Galactic structure models predict a general absence of stars in the color-magnitude range in which these objects are found. However, these observations are consistent with recently-developed models of old white dwarfs with hydrogen atmospheres, whose color, contrary to previous expectations, has been shown to be blue. If these apparently moving objects are indeed old white dwarfs with hydrogen atmospheres and masses near 0.5 M_Sun, they have ages of approximately 12 Gyr, and a local mass density that is sufficient, within the large uncertainties arising from the small size of the sample, to account for the entire missing Galactic dynamical mass.Comment: 6 pages, using emulateapj, including 2 colour figures, accepted for publication in ApJ Letter

Cell synchronization and dynamic G-banding of equine chromosomes by bromodeoxyurldine

Both dynamic G-banding and cell synchronization produced by bromodeoxyuridine (BrdU), were applied to equine chromosomes. BrdU incorporated during the first half of the S-phase is taken up into the R-bands that are early replicating. These bands, which have incorporated BrdU, cannot contract as usual and remain elongated; only the other regions of the chromosome, i.e., the G-bands, contract normally and are sharply defined. BrdU also can be used for cell synchronization. The addition of BrdU in a high concentration, 15 hours before harvest, and its removal 11 hours later, has two effects: initially the BrdU is incorporated during the first part of the S-phase and then it blocks the cells at mid-S-phase. Within the cell cycle, mid-S-phase appears to be the most vuinerable time to various blocking agents. To differentiate the regions of BrdU incorporation from those that have not been substituted, the fluorescence-photolysis-Glemsa (FPG) technique was applied as modified for horse chromosomes. This dynamic technique, which produces many prometaphase and prophase chromosomes showing very sharp G-bands, is certain to enhance the accuracy of cytogenetic analysis and aid in the standardization of equine chromosome

High resolution R-bands produced in equine chromosomes after incorporation of bromodeoxyuridine

Cell synchronization was used to obtain an adequate percentage of very long chromosomes in equine mitotic spreads. Reported here is our variation, adapted to horse chromosomes, of a method using excess thymidine followed by bromodeoxyuridine incorporation. This technique routinely yields excellent quality cells, predominantly in prometaphase and prophase. Among other differences with the standard technique, this addltion does not use Colcemid, which, in addltion to Inhibiting spindle fiber formation, also increases chromosome contraction resulting in thicker and thus fewer bands. Consequently, horse prometaphase chromosomes, which have incorporated BrdU in the late-S-phase, are very long and display a large number of R-bands after the fluorescence-photolysis Glemsa method. This technique should definitely be useful for the analysis of structural anomalies and the standardization of equine R-band

Local Search for the Maximum Parsimony Problem

Four local search algorithms are investigated for the phylogenetic tree reconstruction problem under the Maximum Parsimony criterion. A new subtree swapping neighborhood is introduced and studied in combination with an effective array-based tree representation. Computational results are shown on a set of randomly generated benchmark instances as well as on 8 real problems (sequences of phytopathogen γ-proteobacteria) and compared with two references from the literature

The kinematics of the most oxygen-poor planetary nebula PN G135.9+55.9

PN G135.9+55.9 is a compact, high excitation nebula that has been identified recently as the most oxygen-poor halo planetary nebula. Given its very peculiar characteristics and potential implications in the realms of stellar and Galactic evolution, additional data are needed to firmly establish its true nature and evolutionary history. Here we present the first long-slit, high spectral resolution observations of this object in the lines of H$\alpha$ and He II 4686. The position-velocity data are shown to be compatible with the interpretation of PN G135.9+55.9 being a halo planetary nebula. In both emission lines, we find the same two velocity components that characterize the kinematics as that of an expanding elliptical envelope. The kinematics is consistent with a prolate ellipsoidal model with axis ratio about 2:1, a radially decreasing emissivity distribution, a velocity distribution that is radial, and an expansion velocity of 30 km/s for the bulk of the material. To fit the observed line profiles, this model requires an asymmetric matter distribution, with the blue-shifted emission considerably stronger than the red-shifted emission. We find that the widths of the two velocity components are substantially wider than those expected due to thermal motions, but kinematic structure in the projected area covered by the slit appears to be sufficient to explain the line widths. The present data also rule out the possible presence of an accretion disk in the system that could have been responsible for a fraction of the H$\alpha$ flux, further supporting the planetary nebula nature of PN G135.9+55.9.Comment: accepted by Astronomy & Astrophysic

Progressive Tree Neighborhood applied to the Maximum Parsimony Problem

The Maximum Parsimony (MP) problem aims at reconstructing a phylogenetic tree from DNA sequences while minimizing the number of genetic transformations. To solve this NP-complete problem, heuristic methods have been developed, often based on local search. In this paper, we focus on the influence of the neighborhood relations. After analyzing the advantages and drawbacks of the well-known Nearest Neighbor Interchange (NNI), Subtree Pruning Regrafting (SPR), and Tree-Bisection-Reconnection (TBR) neighborhoods, we introduce the concept of Progressive Neighborhood (PN), which consists of constraining progressively the size of the neighborhood as the search advances. We empirically show that applied to the MP problem, this PN turns out to be more efficient and robust than the classic neighborhoods using a descent algorithm. Indeed, it allows us to find better solutions with a smaller number of iterations or trees evaluated