389 research outputs found
Steadily Increasing Star Formation Rates in Galaxies Observed at 3 <~ z <~ 5 in the CANDELS/GOODS-S Field
We investigate the star formation histories (SFHs) of high redshift (3 <~ z
<~ 5) star-forming galaxies selected based on their rest-frame ultraviolet (UV)
colors in the CANDELS/GOODS-S field. By comparing the results from the
spectral-energy-distribution-fitting analysis with two different assumptions
about the SFHs --- i.e., exponentially declining SFHs as well as increasing
ones, we conclude that the SFHs of high-redshift star-forming galaxies increase
with time rather than exponentially decline. We also examine the correlations
between the star formation rates (SFRs) and the stellar masses. When the
galaxies are fit with rising SFRs, we find that the trend seen in the data
qualitatively matches the expectations from a semi-analytic model of galaxy
formation. The mean specific SFR is shown to increase with redshift, also in
agreement with the theoretical prediction. From the derived tight correlation
between stellar masses and SFRs, we derive the mean SFH of star-forming
galaxies in the redshift range of 3 <~ z <~ 5, which shows a steep power-law
(with power alpha = 5.85) increase with time. We also investigate the formation
timescales and the mean stellar population ages of these star-forming galaxies.
Our analysis reveals that UV-selected star-forming galaxies have a broad range
of the formation redshift. The derived stellar masses and the stellar
population ages show positive correlation in a sense that more massive galaxies
are on average older, but with significant scatter. This large scatter implies
that the galaxies' mass is not the only factor which affects the growth or star
formation of high-redshift galaxies.Comment: 31 pages, 8 figures, 2 table
Selective Algicidal Action of Peptides against Harmful Algal Bloom Species
Recently, harmful algal bloom (HAB), also termed “red tide”, has been recognized as a serious problem in marine environments according to climate changes worldwide. Many novel materials or methods to prevent HAB have not yet been employed except for clay dispersion, in which can the resulting sedimentation on the seafloor can also cause alteration in marine ecology or secondary environmental pollution. In the current study, we investigated that antimicrobial peptide have a potential in controlling HAB without cytotoxicity to harmless marine organisms. Here, antimicrobial peptides are proposed as new algicidal compounds in combating HAB cells. HPA3 and HPA3NT3 peptides which exert potent antimicrobial activity via pore forming action in plasma membrane showed that HPA3NT3 reduced the motility of algal cells, disrupted their plasma membrane, and induced the efflux of intracellular components. Against raphidoflagellate such as Heterosigma akashiwo, Chattonella sp., and C. marina, it displayed a rapid lysing action in cell membranes at 1∼4 µM within 2 min. Comparatively, its lysing effects occurred at 8 µM within 1 h in dinoflagellate such as Cochlodium polykrikoides, Prorocentrum micans, and P. minimum. Moreover, its lysing action induced the lysis of chloroplasts and loss of chlorophyll a. In the contrary, this peptide was not effective against Skeletonema costatum, harmless algal cell, even at 256 µM, moreover, it killed only H. akashiwo or C. marina in co-cultivation with S. costatum, indicating to its selective algicidal activity between harmful and harmless algal cells. The peptide was non-hemolytic against red blood cells of Sebastes schlegeli, the black rockfish, at 120 µM. HAB cells were quickly and selectively lysed following treatment of antimicrobial peptides without cytotoxicity to harmless marine organisms. Thus, the antibiotic peptides examined in our study appear to have much potential in effectively controlling HAB with minimal impact on marine ecology
Antimicrobial HPA3NT3 peptide analogs: Placement of aromatic rings and positive charges are key determinants for cell selectivity and mechanism of action
AbstractIn an earlier study, we determined that HP(2‐20) (residues 2‐20 of parental HP derived from the N-terminus of the Helicobacter pylori ribosomal protein L1) and its analog, HPA3NT3, had potent antimicrobial effects. However, HPA3NT3 also showed undesirable cytotoxicity against HaCaT cells. In the present study, we designed peptide analogs including HPA3NT3-F1A (‐F1A), HPA3NT3-F8A (‐F8A), HPA3NT3-F1AF8A (‐F1AF8A), HPA3NT3-A1 (‐A1) and HPA3NT3-A2 (‐A2) in an effort to investigate the effects of amino acid substitutions in reducing their hydrophobicity or increasing their cationicity, and any resulting effects on their selectivity in their interactions with human cells and pathogens, as well as their mechanism of antimicrobial action. With the exception of HPA3NT3-A1, all of these peptides showed potent antimicrobial activity. Moreover, substitution of Ala for Phe at positions 1 and/or 8 of the HPA3NT3 peptides (‐F1A, -F8A and -F1AF8A) dramatically reduced their cytotoxicity. Thus the cytotoxicity of HPA3NT3 appears to be related to its Phe residues (positions 1 and 8), which strongly interact with sphingomyelin in the mammalian cell membrane. HPA3NT3 exerted its bactericidal effects through membrane permeabilization mediated by pore formation. In contrast, fluorescent dye leakage and nucleic acid gel retardation assays showed that ‐A2 acted by penetrating into the cytoplasm, where it bound to nucleic acids and inhibited protein synthesis. Notably, Staphylococcus aureus did not develop resistance to -A2 as it did with rifampin. These results suggest that the -A2 peptide could potentially serve as an effective antibiotic agent against multidrug-resistant bacterial strains
Improving the Estimation of Star formation Rates and Stellar Population Ages of High-redshift Galaxies from Broadband Photometry
We explore methods to improve the estimates of star formation rates and mean
stellar population ages from broadband photometry of high redshift star-forming
galaxies. We use synthetic spectral templates with a variety of simple
parametric star formation histories to fit broadband spectral energy
distributions. These parametric models are used to infer ages, star formation
rates and stellar masses for a mock data set drawn from a hierarchical
semi-analytic model of galaxy evolution. Traditional parametric models
generally assume an exponentially declining rate of star-formation after an
initial instantaneous rise. Our results show that star formation histories with
a much more gradual rise in the star formation rate are likely to be better
templates, and are likely to give better overall estimates of the age
distribution and star formation rate distribution of Lyman break galaxies. For
B- and V-dropouts, we find the best simple parametric model to be one where the
star formation rate increases linearly with time. The exponentially-declining
model overpredicts the age by 100 % and 120 % for B- and V-dropouts, on
average, while for a linearly-increasing model, the age is overpredicted by 9 %
and 16 %, respectively. Similarly, the exponential model underpredicts
star-formation rates by 56 % and 60 %, while the linearly-increasing model
underpredicts by 15 % 22 %, respectively. For U-dropouts, the models where the
star-formation rate has a peak (near z ~ 3) provide the best match for age --
overprediction is reduced from 110 % to 26 % -- and star-formation rate --
underprediction is reduced from 58 % to 22 %. We classify different types of
star-formation histories in the semi-analytic models and show how the biases
behave for the different classes. We also provide two-band calibration formulae
for stellar mass and star formation rate estimations.Comment: 28 pages, 7 figures, minor changes; published in Ap
- …