Dust-to-Gas Ratio and Metallicity in Dwarf Galaxies

We examine the dust-to-gas ratio as a function of metallicity for dwarf galaxies [dwarf irregular galaxies (dIrrs) and blue compact dwarf galaxies (BCDGs)]. Using a one-zone model and adopting the instantaneous recycling approximation, we prepare a set of basic equations which describes processes of dust formation and destruction in a galaxy. Four terms are included for the processes: dust formation from heavy elements ejected by stellar mass loss, dust destruction in supernova remnants, dust destruction in star-forming regions, and accretion of heavy elements onto preexisting dust grains. Solving the equations, we compare the result with observational data of nearby dIrrs and BCDGs. The solution is consistent with the data within the reasonable ranges of model parameters constrained by the previous examinations. This means that the model is successful in understanding the dust amount of nearby galaxies. We also show that the accretion rate of heavy element onto preexisting dust grains is less effective than the condensation of heavy elements in dwarf galaxies.Comment: 14 pages LaTeX, 4 figures, to appear in Ap

The Cool ISM in S0 Galaxies. I. A Survey of Molecular Gas

Lenticular galaxies remain remarkably mysterious as a class. Observations to date have not led to any broad consensus about their origins, properties and evolution, though they are often thought to have formed in one big burst of star formation early in the history of the Universe, and to have evolved relatively passively since then. In that picture, current theory predicts that stellar evolution returns substantial quantities of gas to the interstellar medium; most is ejected from the galaxy, but significant amounts of cool gas might be retained. Past searches for that material, though, have provided unclear results. We present results from a survey of molecular gas in a volume-limited sample of field S0 galaxies, selected from the Nearby Galaxies Catalog. CO emission is detected from 78 percent of the sample galaxies. We find that the molecular gas is almost always located inside the central few kiloparses of a lenticular galaxy, meaning that in general it is more centrally concentrated than in spirals. We combine our data with HI observations from the literature to determine the total masses of cool and cold gas. Curiously, we find that, across a wide range of luminosity, the most gas rich galaxies have about 10 percent of the total amount of gas ever returned by their stars. That result is difficult to understand within the context of either monolithic or hierarchical models of evolution of the interstellar medium.Comment: 26 pages of text, 15 pages of tables, 10 figures. Accepted for publication in the Astrophysical Journa

Spitzer/IRS Imaging and Spectroscopy of the luminous infrared galaxy NGC 6052 (Mrk 297)

We present photometric and spectroscopic data of the interacting starburst galaxy NGC 6052 obtained with the Spitzer Space Telescope. The mid-infrared (MIR) spectra of the three brightest spatially resolved regions in the galaxy are remarkably similar and are consistent with dust emission from young nearly coeval stellar populations. Analysis of the brightest infrared region of the system, which contributes ~18.5 % of the total 16\micron flux, indicates that unlike similar off-nuclear infrared-bright regions found in Arp 299 or NGC 4038/9, its MIR spectrum is inconsistent with an enshrouded hot dust (T > 300K) component. Instead, the three brightest MIR regions all display dust continua of temperatures less than ~ 200K. These low dust temperatures indicate the dust is likely in the form of a patchy screen of relatively cold material situated along the line of sight. We also find that emission from polycyclic aromatic hydrocarbons (PAHs) and the forbidden atomic lines is very similar for each region. We conclude that the ionization regions are self-similar and come from young (about 6 Myr) stellar populations. A fourth region, for which we have no MIR spectra, exhibits MIR emission similar to tidal tail features in other interacting galaxies.Comment: 20 pages in preprint form, estimated 7 pages in ApJ Aeptember 10, 2007, v666n 2 issue, six encapsulated postscript figure

Discovery of Recent Star Formation in the Extreme Outer Regions of Disk Galaxies

We present deep Halpha images of three nearby late-type spiral galaxies (NGC628, NGC1058 and NGC6946), which reveal the presence of HII regions out to, and beyond, two optical radii (defined by the 25th B-band isophote). The outermost HII regions appear small, faint and isolated, compared to their inner disk counterparts, and are distributed in organized spiral arm structures, likely associated with underlying HI arms and faint stellar arms. The relationship between the azimuthally--averaged Halpha surface brightness (proportional to star formation rate per unit area) and the total gas surface density is observed to steepen considerably at low gas surface densities. We find that this effect is largely driven by a sharp decrease in the covering factor of star formation at large radii, and not by changes in the rate at which stars form locally. An azimuthally--averaged analysis of the gravitational stability of the disk of NGC6946 reveals that while the existence of star formation in the extreme outer disk is consistent with the Toomre-Q instability model, the low rates observed are only compatible with the model when a constant gaseous velocity dispersion is assumed. We suggest that observed behaviour could also be explained by a model in which the star formation rate has an intrinsic dependence on the azimuthally-averaged gas volume density, which decreases rapidly in the outer disk due to the vertical flaring of the gas layer.Comment: 10 pages, 2 embedded postscript files, 3 jpeg images; accepted for publication in ApJ Letter

A non-autonomous stochastic discrete time system with uniform disturbances

The main objective of this article is to present Bayesian optimal control over a class of non-autonomous linear stochastic discrete time systems with disturbances belonging to a family of the one parameter uniform distributions. It is proved that the Bayes control for the Pareto priors is the solution of a linear system of algebraic equations. For the case that this linear system is singular, we apply optimization techniques to gain the Bayesian optimal control. These results are extended to generalized linear stochastic systems of difference equations and provide the Bayesian optimal control for the case where the coefficients of these type of systems are non-square matrices. The paper extends the results of the authors developed for system with disturbances belonging to the exponential family

PPARγ agonists negatively regulate αIIbβ3 integrin outside-in signalling and platelet function through upregulation of protein kinase A activity

BACKGROUND: Agonists for the peroxisome proliferator activated receptor PPARγ, have been shown to have inhibitory effects on platelet activity following stimulation by GPVI and GPCR agonists. OBJECTIVES: Profound effects on thrombus formation led us to suspect a role for PPARγ agonists in the regulation of integrin αIIbβ3 mediated signalling. Both GPVI and GPCR signalling pathways lead to αIIbβ3 activation, and signalling through αIIbβ3 plays a critical role in platelet function and normal haemostasis. METHODS: The effects of PPARγ agonists on the regulation of αIIbβ3 outside-in signalling was determined by monitoring the ability of platelets to adhere and spread on fibrinogen and undergo clot retraction. Effects on signalling components downstream of αIIbβ3 activation were also determined following adhesion to fibrinogen by western blotting. RESULTS: Treatment of platelets with PPARγ agonists inhibited platelet adhesion and spreading on fibrinogen and diminished clot retraction. A reduction in phosphorylation of several components of αIIbβ3 signalling, including the integrin β3 subunit, Syk, PLCγ2, FAK and Akt was also observed as a result of reduced interaction of the integrin β3 subunit with Gα13. Studies of VASP phosphorylation revealed that this was a due to an increase in PKA activity following treatment with PPARγ receptor agonists. CONCLUSIONS: This study provides further evidence for anti-platelet actions of PPARγ agonists, identifies a negative regulatory role for PPARγ agonists in the control of integrin αIIbβ3 outside-in signalling, and provides a molecular basis by which the PPARγ agonists negatively regulate platelet activation and thrombus formation

Bubble control, levitation and manipulation using dielectrophoresis

Bubbles attached to surfaces are ubiquitous in nature and in industry. However, bubbles are problematic in important technologies, including causing damage to the operation of microfluidic devices and being parasitic during heat transfer processes, so considerable efforts have been made to develop mechanical and electrical methods to remove bubbles from surfaces. In this work liquid dielectrophoresis is used to force a captive air bubble to detach away from an inverted solid surface and, crucially, the detached bubble is then held stationary in place below the surface at a distance controlled by the voltage. In this “levitated” state the bubble is separated from the surface by liquid layer with a voltage-selected thickness at which the dielectrophoresis force exactly counterbalances the gravitational buoyancy force. The techniques described here provide exceptional command over repeatable cycles of bubble detachment, levitation, and re-attachment. A theoretical analysis is presented that explains the observed detachment-reattachment hysteresis in which bubble levitation is maintained with voltages an order of magnitude lower than those used to create detachment. Our precision surface bubble removal and control concepts are relevant to situations such as nucleate boiling and micro-gravity environments, and offer an approach towards "wall-less" bubble microfluidic devices

Multiwavelength Observations of the Low Metallicity Blue Compact Dwarf Galaxy SBS 0335-052

New infrared and millimeter observations from Keck, Palomar, ISO, and OVRO and archival data from the NRAO VLA and IRAS are presented for the low metallicity blue compact dwarf galaxy SBS 0335-052. Mid-infrared imaging shows this young star-forming system is compact (0.31"; 80 pc) at 12.5 microns. The large Br-gamma equivalent width (235 Angstroms) measured from integral field spectroscopy is indicative of a ~5 Myr starburst. The central source appears to be optically thin in emission, containing both a warm (~80 K) and a hot (~210 K) dust component, and the overall interstellar radiation field is quite intense, about 10,000 times the intensity in the solar neighborhood. CO emission is not detected, though the galaxy shows an extremely high global H I gas-to-dust mass ratio, high even for blue compact dwarfs. Finally, the galaxy's mid-infrared-to-optical and mid-to-near-infrared luminosity ratios are quite high, whereas its far-infrared-to-radio and far-infrared-to-optical flux ratios are surprisingly similar to what is seen in normal star-forming galaxies. The relatively high bolometric infrared-to-radio ratio is more easily understood in the context of such a young system with negligible nonthermal radio continuum emission. These new lines of evidence may outline features common to primordial galaxies found at high redshift.Comment: 28 pages including 6 figures; accepted for publication in the Astronomical Journa