168 research outputs found
The Stellar Populations of the Cetus Dwarf Spheroidal Galaxy
We present Hubble Space Telescope Wide Field Planetary Camera 2 photometry in
the V and I passbands of the recently discovered Local Group dwarf spheroidal
galaxy in Cetus. Our color-magnitude diagram extends from above the first
ascent red giant branch (RGB) tip to approximately half a magnitude below the
horizontal branch (HB). Adopting a reddening of E(B-V) = 0.03, the magnitude of
the RGB tip yields a distance modulus of (m-M)o = 24.46 +/- 0.14. After
applying the reddening and distance modulus, we have utilized the color
distribution of RGB stars to determine a mean metal abundance of [Fe/H] = -1.7
on the Zinn & West scale with an intrinsic internal abundance dispersion of
+/-0.2 dex. An indirect calculation of the HB morphology of Cetus based on the
mean dereddened HB color yields (B-R)/(B+V+R) = -0.91 +/- 0.09, which
represents an HB that is redder than what can be attributed solely to Cetus'
metal abundance. As such, Cetus suffers from the `second parameter effect' in
which another parameter besides metallicity is controlling the HB morphology.
If we adopt the conventional `age hypothesis' explanation for the second
parameter effect, then this implies that Cetus is 2-3 Gyr younger than Galactic
globular clusters at its metallicity.Comment: 13 pages, 8 figures, Accepted for publication in the March 10, 2002
Ap
Relative fitness contribution of BoLA alleles in T.parva immune cattle: Interface with parasite genetic diversity
Tidal dwarfs in the M81 group: the second generation?
We derive quantitative star formation histories of the four suspected tidal
dwarf galaxies in the M81 group, HolmbergIX, BK3N, Arp-loop (A0952+69), and
Garland, using Hubble Space Telescope/Wide Field Planetary Camera2 images in
F606W and F814W obtained as part of a Snapshot survey of dwarf galaxies in the
Local Universe. We consider the spatial distribution and ages of resolved
stellar populations in these dwarf irregular galaxies. We use synthetic
color-magnitude diagrams to derive the ages of the major star formation
episodes, star formation rates, and approximate metallicity ranges. All the
galaxies show evidence of continuous star formation between about 20 and 200
Myr ago with star formation rates in the range 7.5*10^(-3)- 7.6*10^(-4)
M(sun)/yr. The metallicity of the detected stars spans a wide range, and have
lower than solar abundance. A possible scenario is that all four dwarf galaxies
were formed from material in the metal-poor outer part of the giant spiral
galaxy M81 after the tidal interaction between M81, M82, and NGC3077 about 200
Myr ago. While we do not directly detect pronounced old stellar populations,
the photometric limits of our data are such that the presence of such a
population is not entirely ruled out
Addendum: "The Dynamics of M15: Observations of the Velocity Dispersion Profile and Fokker-Planck Models" (ApJ, 481, 267 [1997])
It has recently come to our attention that there are axis scale errors in
three of the figures of Dull et al. (1997, hereafter D97). D97 presented
Fokker-Planck models for the collapsed-core globular cluster M15 that include a
dense, centrally concentrated population of neutron stars and massive white
dwarfs, but do not include a central black hole. In this Addendum, we present
corrected versions of Figures 9, 10, and 12, and an expanded version of Figure
6. This latter figure, which shows the full run of the velocity dispersion
profile, indicates that the D97 model predictions are in good agreement with
the moderately rising HST-STIS velocity dispersion profile for M15 reported by
Gerssen et al. (2002, astro-ph/0209315). Thus, a central black hole is not
required to fit the new STIS velocity measurements, provided that there is a
sufficient population of neutron stars and massive white dwarfs. This
conclusion is consistent with the findings of Gerssen et al. (2002,
astro-ph/0210158), based on a reapplication of their Jeans equation analysis
using the corrected mass-to-light profile (Figure 12) for the D97 models.Comment: 4 pages, 4 figures, submitted to Ap
Phylogenetically Driven Sequencing of Extremely Halophilic Archaea Reveals Strategies for Static and Dynamic Osmo-response
© 2014. Organisms across the tree of life use a variety of mechanisms to respond to stress-inducing fluctuations in osmotic conditions. Cellular response mechanisms and phenotypes associated with osmoadaptation also play important roles in bacterial virulence, human health, agricultural production and many other biological systems. To improve understanding of osmoadaptive strategies, we have generated 59 high-quality draft genomes for the haloarchaea (a euryarchaeal clade whose members thrive in hypersaline environments and routinely experience drastic changes in environmental salinity) and analyzed these new genomes in combination with those from 21 previously sequenced haloarchaeal isolates. We propose a generalized model for haloarchaeal management of cytoplasmic osmolarity in response to osmotic shifts, where potassium accumulation and sodium expulsion during osmotic upshock are accomplished via secondary transport using the proton gradient as an energy source, and potassium loss during downshock is via a combination of secondary transport and non-specific ion loss through mechanosensitive channels. We also propose new mechanisms for magnesium and chloride accumulation. We describe the expansion and differentiation of haloarchaeal general transcription factor families, including two novel expansions of the TATA-binding protein family, and discuss their potential for enabling rapid adaptation to environmental fluxes. We challenge a recent high-profile proposal regarding the evolutionary origins of the haloarchaea by showing that inclusion of additional genomes significantly reduces support for a proposed large-scale horizontal gene transfer into the ancestral haloarchaeon from the bacterial domain. The combination of broad (17 genera) and deep (≥5 species in four genera) sampling of a phenotypically unified clade has enabled us to uncover both highly conserved and specialized features of osmoadaptation. Finally, we demonstrate the broad utility of such datasets, for metagenomics, improvements to automated gene annotation and investigations of evolutionary processes
Gene Gangs of the Chloroviruses: Conserved Clusters of Collinear Monocistronic Genes
Chloroviruses (family Phycodnaviridae) are dsDNA viruses found throughout the world’s inland waters. The open reading frames in the genomes of 41 sequenced chloroviruses (330 + 40 kbp each) representing three virus types were analyzed for evidence of evolutionarily conserved local genomic “contexts”, the organization of biological information into units of a scale larger than a gene. Despite a general loss of synteny between virus types, we informatically detected a highly conserved genomic context defined by groups of three or more genes that we have termed “gene gangs”. Unlike previously described local genomic contexts, the definition of gene gangs requires only that member genes be consistently co-localized and are not constrained by strand, regulatory sites, or intervening sequences (and therefore represent a new type of conserved structural genomic element). An analysis of functional annotations and transcriptomic data suggests that some of the gene gangs may organize genes involved in specific biochemical processes, but that this organization does not involve their coordinated expression
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