Red halos are faint, extended and extremely red structures that have been reported around various types of galaxies since the mid-1990s. The colours of these halos are too red to be reconciled with ...any hitherto known type of stellar population, and instead indicative of a very bottom-heavy stellar initial mass function (IMF). Due to the large mass-to-light ratios of such stellar halos, they could contribute substantially to the baryonic masses of galaxies while adding very little to their overall luminosities. The red halos of galaxies therefore constitute potential reservoirs for some of the baryons still missing from inventories in the low-redshift Universe. While most studies of red halos have focused on disk galaxies, a red excess has also been reported in the faint outskirts of blue compact galaxies (BCGs). A bottom-heavy IMF can explain the colours of these structures as well, but due to model degeneracies, stellar populations with standard IMFs and abnormally high metallicities have also been demonstrated to fit the data. Here, we show that due to recent developments in the field of spectral synthesis, the metallicities required in this alternative scenario may be less extreme than previously thought. This suggests that the red excess seen in the outskirts of BCGs may stem from a normal, intermediate-metallicity host galaxy rather than a red halo of the type seen around disk galaxies. The inferred host metallicity does, however, still require the host to be more metal-rich than the gas in the central starburst of BCGs, in contradiction with current simulations of how BCGs form.
We have used multi-band imaging to investigate the nature of the extreme starburst environment in Haro 11 galaxy. The central starburst region has been observed in 8 HST wavebands and at 2.16 micron ...at the ESO-VLT. We constructed integrated spectral energy distributions (SEDs) for about 200 star clusters and compared them with single stellar population models in order to derive ages, masses and extinctions of thestar clusters. The present starburst has lasted for 40 Myr, and shows a peak of cluster formation only 3.5 Myr old. With such an extremely young cluster population, Haro 11 represents a unique opportunity to investigate the youngest phase of the cluster formation process and evolution in starburst systems. Extinction tends to diminish as function of the cluster age, but the spread is large and for clusters in partial embedded phases (< 5 Myr). A fraction of low-mass (> 10^4 Msun), very young (1-3 Myr) clusters is missing, either because they are embedded inthe parental molecular cloud and heavily extinguished, or because of blending. Almost half of the cluster sample is affected by flux excesses at wavelengths 8000 \AA which cannot be explained by simple stellar evolutionary models. Fitting SED models over all wavebands leads to systematic overestimates of cluster ages and incorrect masses for the stellar population supplying the light in these clusters. We show that the red excess affects also the HST F814W filter, which is typically used to constrain cluster physical properties. The clusters which show the red excess are younger than 40 Myr; we propose possible physical explanations for the phenomenon. Finally, we estimate that Haro 11 hasproduced bound clusters at a rate almost a factor of 10 higher than the massive and regular spirals, like the Milky Way. (Abriged)
We present a HST multiband analysis of the large population of star clusters in the blue compact galaxy (BCG) Mrk 930. We have reconstructed the spectral energy distributions of the star clusters and ...estimated age, mass, and extinction for a representative sample. We observe a very young cluster population with 70% of the systems formed less than 10 Myr ago. The peak in the star cluster age distribution at 4 Myr is corroborated by the presence of Wolf-Rayet spectral features, and by the observed optical and IR lines ratios OIII/H_beta and NeIII/NeII. The recovered extinction in these very young clusters shows large variations, with a decrease at older ages. It is likely that our analysis is limited to the optically brightest objects (i.e. systems only partially embedded in their natal cocoons). We map the extinction across the galaxy using low-resolution spectra and the H_alpha/H_beta ratio, as obtained from ground-based narrow band imaging. We find that the mean optical extinction derived in the starburst regions is close to the averaged value observed in the clusters, but locally, do not trace the more extinguished clusters. Previous HST studies of BCGs have revealed a population of young and extremely red super star clusters. We detect a considerable fraction of clusters affected by a red excess also in Mrk 930. The nature of the red excess, which turns up at near-IR wavelengths remains unknown. We compare the cluster and the star formation history, the latter derived from the fit of spectral population synthesis models to the spectra. We find a general agreement between the two independently estimated quantities. Using the cluster properties we perform a study of the host environmental properties. We find that the cluster formation efficiency is significantly higher, suggesting a key role of the environment for the formation of these massive objects.Abridged
Astrophys.J.650:812-817,2006 Optical and near-IR observations of the halos of disk galaxies and blue
compact galaxies have revealed a very red spectral energy distribution, which
cannot easily be ...reconciled with a normal, metal-poor stellar population like
that in the stellar halo of the Milky Way. Here, spectral evolutionary models
are used to explore the consequences of these observations. We demonstrate that
a stellar population of low to intermediate metallicity, but with an extremely
bottom-heavy initial mass function, can explain the red halos around both types
of objects. Other previously suggested explanations, like nebular emission or
very metal-rich stars, are shown to fail in this respect. This indicates that,
if the reported halo colours are correct, halo populations dominated by
low-mass stars may be a phenomenon common to galaxies of very different Hubble
types. Potential tests of this hypothesis are discussed, along with its
implications for the baryonic dark matter content of galaxies.
Deep optical/near-IR surface photometry of galaxies outside the Local Group have revealed faint and very red halos around objects as diverse as disk galaxies and starbursting dwarf galaxies. The ...colours of these structures are too extreme to be reconciled with stellar populations similar to those seen in the stellar halos of the Milky Way or M31, and alternative explanations like dust reddening, high metallicities or nebular emission are also disfavoured. A stellar population obeying an extremely bottom-heavy initial mass function (IMF), is on the other hand consistent with all available data. Because of its high mass-to-light ratio, such a population would effectively behave as baryonic dark matter and could account for some of the baryons still missing in the low-redshift Universe. Here, we give an overview of current red halo detections, alternative explanations for the origin of the red colours and ongoing searches for red halos around types of galaxies for which this phenomenon has not yet been reported. A number of potential tests of the bottom-heavy IMF hypothesis are also discussed.
The effect of support nature on activity of low-temperature carbon monoxide oxidation catalysts was studied. It is shown on the base of X-ray Diffraction, that y-Al2O3 is the best support for Pd-Cu ...catalyst
Optical and near-IR observations of the halos of disk galaxies and blue compact galaxies have revealed a very red spectral energy distribution, which cannot easily be reconciled with a normal, ...metal-poor stellar population like that in the stellar halo of the Milky Way. Here, spectral evolutionary models are used to explore the consequences of these observations. We demonstrate that a stellar population of low to intermediate metallicity, but with an extremely bottom-heavy initial mass function, can explain the red halos around both types of objects. Other previously suggested explanations, like nebular emission or very metal-rich stars, are shown to fail in this respect. This indicates that, if the reported halo colours are correct, halo populations dominated by low-mass stars may be a phenomenon common to galaxies of very different Hubble types. Potential tests of this hypothesis are discussed, along with its implications for the baryonic dark matter content of galaxies.
Burkholderia pseudomallei is the causative agent of melioidosis, a disease with a mortality rate of up to 40% even with treatment. Despite the ability of certain antibiotics to control initial ...infection, relapse occurs in treated patients. The inability of antibiotics to clear this bacterial infection is in part due to persistence, an evasion mechanism against antibiotics and the effect of host defenses. Evaluation of antibiotic efficacy against B. pseudomallei revealed that up to 48% of in vitro grown populations can survive in a persister state. Toxin–antitoxin (TA) systems have been previously implicated in modulating bacterial persistence. We generated three isogenic TA mutants and found that loss of each toxin gene did not alter antibiotic persistence or macrophage survival. In response to macrophage‐induced persistence, all three toxin mutants demonstrated increased intracellular susceptibility to levofloxacin which in part was due to the inability of the mutants to induce persistence after nitric oxide or nutrient starvation. In an inhalational model of murine melioidosis, both ΔBPSS0395 and ΔBPSS1584 strains were attenuated, and treatment with levofloxacin led to significant reduction in lung colonisation and reduced splenic colonisation by ΔBPSS0395. Based on our findings, these toxins deserve additional evaluation as putative therapeutic targets.
During development, the formation of mature neural circuits requires the selective elimination of inappropriate synaptic connections. Here we show that C1q, the initiating protein in the classical ...complement cascade, is expressed by postnatal neurons in response to immature astrocytes and is localized to synapses throughout the postnatal CNS and retina. Mice deficient in complement protein C1q or the downstream complement protein C3 exhibit large sustained defects in CNS synapse elimination, as shown by the failure of anatomical refinement of retinogeniculate connections and the retention of excess retinal innervation by lateral geniculate neurons. Neuronal C1q is normally downregulated in the adult CNS; however, in a mouse model of glaucoma, C1q becomes upregulated and synaptically relocalized in the adult retina early in the disease. These findings support a model in which unwanted synapses are tagged by complement for elimination and suggest that complement-mediated synapse elimination may become aberrantly reactivated in neurodegenerative disease.
Bacterial persistence, known as noninherited antibacterial resistance, is a factor contributing to the establishment of long-lasting chronic bacterial infections. In this study, we examined the ...ability of nicotinamide (NA) to potentiate the activity of different classes of antibiotics against
persister cells. Here we demonstrate that addition of NA in
models of
infection resulted in a significant depletion of the persister population in response to various classes of antibiotics. We applied microfluidic bioreactors with a continuous medium flow to study the effect of supplementation with an NA gradient on the recovery of
persister populations. A coculture of human neutrophils preactivated with 50 µM NA and
resulted in the most efficient reduction in the persister population. Applying single-cell RNA fluorescence
hybridization analysis and quantitative PCR, we found that NA inhibited gene expression of the stringent response regulator
, implicated in the regulation of the persister metabolic state. We also demonstrate that a therapeutic dose of NA (250 mg/kg of body weight), previously applied as immunoprophylaxis against antibiotic-resistant bacterial species, produced adverse effects in an
murine model of infection with the highly pathogenic bacterium
, indicating that therapeutic dose and metabolite effects have to be carefully evaluated and tailored for every case of potential clinical application.
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