The star formation history of the Sculptor Dwarf Irregular Galaxy

[abridged] We study the resolved stellar populations and derive the SFH of the SDIG, a gas-rich dwarf galaxy member of the NGC7793 subgroup in the Sculptor group. We construct a CMD using archival HST observations and examine its stellar content. We derive its SFH using a maximum-likelihood fit to the CMD. The CMD shows that SDIG contains stars from 10Myr to several Gyr old, as revealed from the MS, BL, luminous AGB, and RGB stars. The young stars with ages less than ~250Myr show a spatial distribution confined to its central regions, and additionally the young MS stars exhibit an off-center density peak. The intermediate-age and older stars are more spatially extended. SDIG is dominated by intermediate-age stars with an average age of 6.4Gyr. The average metallicity inferred is [M/H]\approx -1.5dex. Its SFH is consistent with a constant SFR, except for ages younger than ~200Myr. The lifetime average SFR is 1.3x10^{-3} Mo/yr. More recently than 100Myr, there has been a burst of SF at a rate ~2-3 times higher than the average SFR. The inferred recent SFR from CMD modelling is higher than inferred from the Ha flux of the galaxy; we interpret this to mean that the upper end of the IMF is not being fully sampled due to the low SFR. Additionally, an observed lack of bright blue stars in the CMD could indicate a downturn in SFR on 10^7-yr timescales. A previous SF enhancement appears to have occurred between 600-1100Myr ago, with amplitude similar to the most recent 100Myr. Older bursts of similar peak SFR and duration would not be resolvable with these data. The observed enhancements in SF suggest that SDIG is able to sustain a complex SFH without the effect of interactions with its nearest massive galaxy. Integrating the SFR over the entire history of SDIG yields a total stellar mass 1.77x10^{7}Mo, and a current V-band stellar mass-to-light ratio 3.2Mo/Lo.Comment: A&A accepted; 10 pages, 9 figure

Stochasticity in Colonial Growth Dynamics of Individual Bacterial Cells

Conventional bacterial growth studies rely on large bacterial populations without considering the individual cells. Individual cells, however, can exhibit marked behavioral heterogeneity. Here, we present experimental observations on the colonial growth of 220 individual cells of Salmonella enterica serotype Typhimurium using time-lapse microscopy videos. We found a highly heterogeneous behavior. Some cells did not grow, showing filamentation or lysis before division. Cells that were able to grow and form microcolonies showed highly diverse growth dynamics. The quality of the videos allowed for counting the cells over time and estimating the kinetic parameters lag time ( ) and maximum specific growth rate ( max) for each microcolony originating from a single cell. To interpret the observations, the variability of the kinetic parameters was characterized using appropriate probability distributions and introduced to a stochastic model that allows for taking into account heterogeneity using Monte Carlo simulation. The model provides stochastic growth curves demonstrating that growth of single cells or small microbial populations is a pool of events each one of which has its own probability to occur. Simulations of the model illustrated how the apparent variability in population growth gradually decreases with increasing initial population size (N0). For bacterial populations with N 0 of >100 cells, the variability is almost eliminated and the system seems to behave deterministically, even though the underlying law is stochastic. We also used the model to demonstrate the effect of the presence and extent of a nongrowing population fraction on the stochastic growth of bacterial populations

Star formation laws in the Andromeda galaxy: Gas, stars, metals and the surface density of star formation

We use hierarchical Bayesian regression analysis to investigate star formation laws in the Andromeda galaxy (M31) in both local (30, 155 and 750 pc) and global cases. We study and compare the well-known Kennicutt-Schmidt law, the extended Schmidt law and the metallicity/star formation correlation. Using a combination of Hα and 24 μm emission, a combination of far-ultraviolet and 24 μm, and the total infrared emission, we estimate the total star formation rate (SFR) in M31 to be between 0.35 ± 0.04 and 0.4 ± 0.04 M⊙ yr-1. We produce a stellar mass surface density map using IRAC 3.6 μm emission and measured the total stellar mass to be 6.9 × 1010 M⊙. For the Kennicutt-Schmidt law in M31, we find the power-law index N to be between 0.49 and 1.18; for all the laws, the power-law index varies more with changing gas tracer than with SFR tracer. The power-law index also changes with distance from the centre of the galaxy. We also applied the commonly used ordinary least-squares fitting method and showed that using different fitting methods leads to different power-law indices. There is a correlation between the surface density of SFR and the stellar mass surface density, which confirms that the Kennicutt-Schmidt law needs to be extended to consider the other physical properties of galaxies. We found a weak correlation between metallicity, the SFR and the stellar mass surface density

Modelling biofilm formation of Salmonella enterica ser. Newport as a function of pH and water activity

The effect of pH and water activity (aw) on the formation of biofilm by Salmonella enterica ser. Newport, previously identified as a strong biofilm producer, was assessed. Biofilm formation was evaluated in tryptone soy broth at 37 C and at different combinations of pH (3.3e7.8) and aw (0.894e0.997). In total, 540 biofilm formation tests in 108 pH and aw combinations were carried out in polystyrene microtiter plates using crystal violet staining and optical density (OD; 580 nm) measurements. Since the individual effects of pH and aw on biofilm formation had a similar pattern to that observed for microbial growth rate, cardinal parameter models (CPMs) were used to describe these effects. CPMs described successfully the effects of these two environmental parameters, with the estimated cardinal values of pHmin, pHopt, pHmax, awmin and awopt being 3.58, 6.02, 9.71, 0.894 and 0.994, respectively. The CPMs assumption of the multiplicative inhibitory effect of environmental factors was validated in the case of biofilm formation using additional independent data (i.e. 430 OD data at 86 different combinations of pH and aw). The validation results showed a good agreement (r2 ¼ 0.938) between observed and predicted OD with no systematic error. In the second part of this study, a probabilistic model predicting the pathogen's biofilm formation boundaries was developed, and the degree of agreement between predicted probabilities and observations was as high as 99.8%. Hence, the effect of environmental parameters on biofilm formation can be quantitatively expressed using mathematical models, with the latter models, in turn, providing useful information for biofilm control in food industry environments

Strain effect on the heterogeneity of individual-cell growth kinetics of Salmonella Typhimurium

The present study showed that the variability of individual-cell growth kinetics may differ among strains of S. Typhimurium. The results provide useful quantitative information for incorporating strain variability and heterogeneity in individual-cell behavior in stochastic growth models and risk assessment studies

The dustier early-type galaxies deviate from late-type galaxies\u27 scaling relations

Several dedicated surveys focusing on early-type galaxies (ETGs) reveal that significant fractions of them are detectable in all interstellar medium phases studied to date. We select ETGs from the Herschel Reference Survey that have both far-infrared Herschel and either H I or CO detection (or both).We derive their star formation rates (SFRs), stellar masses and dust masses viamodelling their spectral energy distributions.We combine these with literature information on their atomic and molecular gas properties, in order to relate their star formation, total gas mass and dust mass on global scales. The ETGs deviate from the dust mass-SFR relation and the Schmidt-Kennicutt relation that SDSS star-forming galaxies define: compared to SDSS galaxies, ETGs have more dust at the same SFR, or less SFR at the same dust mass. When placing them in the M*-SFR plane, ETGs show a much lower specific SFR as compared to normal star-forming galaxies. ETGs show a large scatter compared to the Schmidt-Kennicutt relation found locally within our Galaxy, extending to lower SFRs and gas mass surface densities. Using an ETG\u27s SFR and the Schmidt-Kennicutt law to predict its gas mass leads to an underestimate. ETGs have similar observed-gas-to-modelled-dust mass ratios to star-forming galaxies of the same stellar mass, as well as they exhibit a similar scatter

Single-cell growth kinetic behavior of pathogenic bacteria in the presence of microbial supernatants containing autoinducer-2 signal compounds

The findings of this study constitute preliminary data on the role of QS compounds on the single-cell growth behavior of important pathogens, knowledge that maybe useful in understanding the mechanisms underlying their behavior as well as in developing strategies for their control in situ

The interacting M81 group of galaxies

Das Thema dieser Arbeit ist die Rolle der Umgebung und Wechselwirkungen auf die Entstehung und Entwicklung von Zwerggalaxien in nahegelegenen Gruppen. Die M81-Gruppe ist eine hochgradig wechselwirkende Gruppe, die eine einzigartige Gelegenheit bietet, den Einfluss der Umgebung auf die Eigenschaften ihrer Zwerggalaxienpopulation zu studieren. Wir verwenden die Tully-Fisher-Relation, um nach möglichen Gezeiten-Zwerggalaxien in der M81-Gruppe zu suchen. Keine potentiellen Gezeiten-Zwerggalaxien wurden identifiziert. Darüber hinaus leiten wir photometrische Metallizitäts-Verteilungs-funktionen her und untersuchen die Gegenwart von Metallizitätsgradienten bei neun Zwerggalaxien frühen Typs der M81-Gruppe. Der Vergleich ihrer mittleren Metallizitätseigenschaften mit denen der Zwerggalaxien in der lokalen Gruppe zeigt, dass diese sich ähneln. Nicht alle der Zwerggalaxien weisen einen Metallizitätsgradienten auf, wie es für die Zwerggalaxien der lokalen Gruppe der Fall ist. Der Anteil an leuchtkräftigen AGB-Sternen in jeder Zwerggalaxie ist gering, während ihr Bruchteil als Funktion ihres Abstandes von der Galaxie M81 keinerlei Trend zeigt. Die Resultate deuten darauf hin, dass die untersuchten Eigenschaften durch interne Prozesse beeinflusst werden. Schließlich wurde unter Verwendung der Galaktischen kugelförmigen Zwerggalaxien die Methode der Herleitung der photometrischen Metallizitäten für Zwerggalaxien ausgewertet, die komplexe Sternentstehungsgeschichten aufweisen. Die resultierenden mittleren photometrischen Metallizitätseigenschaften befinden sich in guter Übereinstimmung mit spektroskopischen Messungen, während die Unterschiede in den individuellen Sternen größer werden, je komplexer die Sternentstehung ist