Dynamical condition of neutral hydrogen envelopes of dwarf galaxies and their possible morphological evolution

We investigate the star-formation history of gas-rich dwarf galaxies, taking account of the dynamical evolution of their neutral hydrogen (H{\sc i}) envelope. Gas-rich dwarfs are classified into blue compact dwarfs (BCDs) and dwarf irregulars (dIrrs). In this paper, their H{\sc i} envelope is clearly shown not to be blown away by their stellar feedback. This is concluded since the observed star-formation rate (SFR) of gas-rich dwarfs is generally smaller than a critical SFR, $\psi_{\rm crit}$, at which stellar feedback accelerates the H{\sc i} envelope to the escape velocity. From this standpoint and the chemical property of sample BCDs, we suggest two possibilities; (1) The H{\sc i} gas in the envelope of BCDs is consumed to fuel their star-formation; and (2) BCDs have a similar star-formation history. We also discuss morphological evolution among dwarf galaxies. As long as gas-rich dwarfs are isolated, it is difficult for them to evolve into dwarf ellipticals (dEs). When the H{\sc i} envelope in gas-rich dwarfs is consumed in subsequent star-formation, a morphological exchange between BCDs and dIrrs is still expected, consistent with previous studies. If the SFR of gas-rich dwarfs was much higher than $\psi_{\rm crit}$ in the past, interestingly, an evolutionary scenario from dEs to gas-rich dwarfs is possible.Comment: 15 pages, 2 figures, accepted by A&A

Anomalous Star-Formation Activity of Less-Luminous Galaxies in Cluster Environment

We discuss a correlation between star formation activity (SFA) and luminosity of star-forming galaxies at intermediate redshifts of $0.2\le z\le 0.6$ in both cluster and field environments. Equivalent width (EW) of [O{\sc ii}] is used for measurement of the SFA, and $R$-band absolute magnitude, $M_R$, for the luminosity. In less-luminous (M_R \gsim -20.7) galaxies, we find : (1) the mean EW([O{\sc ii}]) of cluster galaxies is smaller than that of field galaxies; but (2) some cluster galaxies have as large EW([O{\sc ii}]) as that of actively star-forming field galaxies. Based on both our results, we discuss a new possible mechanism for the Butcher-Oemler (BO) effect, assuming that the luminosity of a galaxy is proportional to its dynamical mass. Our proposal is that BO galaxies are less-massive cluster galaxies with smaller peculiar velocities. They are then stable against Kelvin-Helmholtz instability (KHI), and are not affected by tidal interaction between clusters and themselves. Their interstellar medium (ISM) would be hardly stripped, and their SFA would be little suppressed. Hence, as long as such galaxies keep up their SFA, the fraction of blue galaxies in a cluster does not decrease. As a cluster becomes virialized, however, such galaxies become more accelerated, the ISM available for SFA is stripped by KHI, and their color evolves redward, which produces the BO effect.Comment: accepted for publication in ApJ

Fatigue crack paths and properties in A356-T6 aluminum alloy microstructurally modified by friction stir processing under different conditions

A356-T6 cast aluminum alloy is a light weight structural material, but fatigue crack initiates and propagates from a casting defect leading to final fracture. Thus it is important to eliminate casting defects. In this study, friction stir processing (FSP) was applied to A356-T6, in which rotating tool with probe and shoulder was plunged into the material and travels along the longitudinal direction to induce severe plastic deformation,resulting in the modification of microstructure. Two different processing conditions with low and high toolrotational speeds were tried and subsequently fully reversed fatigue tests were performed to investigate theeffect of processing conditions on the crack initiation and propagation behavior. The fatigue strengths weresuccessfully improved by both conditions due to the elimination of casting defects. But the lower tool rotationalspeed could further improve fatigue strength than the higher speed. EBSD analyses revealed that the higher tool rotational speed resulted in the severer texture having detrimental effects on fatigue crack initiation andpropagation resistances.&nbsp

From nonassociativity to solutions of the KP hierarchy

A recently observed relation between 'weakly nonassociative' algebras A (for which the associator (A,A^2,A) vanishes) and the KP hierarchy (with dependent variable in the middle nucleus A' of A) is recalled. For any such algebra there is a nonassociative hierarchy of ODEs, the solutions of which determine solutions of the KP hierarchy. In a special case, and with A' a matrix algebra, this becomes a matrix Riccati hierarchy which is easily solved. The matrix solution then leads to solutions of the scalar KP hierarchy. We discuss some classes of solutions obtained in this way.Comment: 7 pages, 4 figures, International Colloquium 'Integrable Systems and Quantum Symmetries', Prague, 15-17 June 200