216 research outputs found
Identification and functional characterization of a highly divergent N-acetylglucosaminyltransferase I (TbGnTI) in <em>Trypanosoma brucei</em>
Trypanosoma brucei expresses a diverse repertoire of N-glycans, ranging from oligomannose and paucimannose structures to exceptionally large complex N-glycans. Despite the presence of the latter, no obvious homologues of known β1–4-galactosyltransferase or β1–2- or β1–6-N-acetylglucosaminyltransferase genes have been found in the parasite genome. However, we previously reported a family of putative UDP-sugar-dependent glycosyltransferases with similarity to the mammalian β1–3-glycosyltransferase family. Here we characterize one of these genes, TbGT11, and show that it encodes a Golgi apparatus resident UDP-GlcNAc:α3-d-mannoside β1–2-N-acetylglucosaminyltransferase I activity (TbGnTI). The bloodstream-form TbGT11 null mutant exhibited significantly modified protein N-glycans but normal growth in vitro and infectivity to rodents. In contrast to multicellular organisms, where the GnTI reaction is essential for biosynthesis of both complex and hybrid N-glycans, T. brucei TbGT11 null mutants expressed atypical “pseudohybrid” glycans, indicating that TbGnTII activity is not dependent on prior TbGnTI action. Using a functional in vitro assay, we showed that TbGnTI transfers UDP-GlcNAc to biantennary Man(3)GlcNAc(2), but not to triantennary Man(5)GlcNAc(2), which is the preferred substrate for metazoan GnTIs. Sequence alignment reveals that the T. brucei enzyme is far removed from the metazoan GnTI family and suggests that the parasite has adapted the β3-glycosyltransferase family to catalyze β1–2 linkages
The Globular Cluster Systems in the Coma Ellipticals. III: The Unique Case of IC 4051
Using archival \hst WFPC2 data, we derive the metallicity distribution,
luminosity function, and spatial structure of the globular cluster system
around IC 4051, a giant E galaxy on the outskirts of the Coma cluster core. The
metallicity distribution derived from the (V-I) colors has a mean [Fe/H] =
-0.3, a near-complete lack of metal-poor clusters, and only a small metallicity
gradient with radius; it may, however, have two roughly equal metallicity
subcomponents, centered at [Fe/H] ~ 0.0 and -1.0. The luminosity distribution
(GCLF) has the Gaussian-like form observed in all other giant E galaxies, with
a peak (turnover) at V = 27.8, consistent with a Coma distance of 100 Mpc. The
radial profiles of both the GCS and the halo light show an unusually steep
falloff which may indicate that the halo of this galaxy has been tidally
truncated. Lastly, the specific frequency of the GCS is remarkably large: we
find S_N = 11 +- 2, resembling the central cD-type galaxies even though IC 4051
is not a cD or brightest cluster elliptical. A formation model consistent with
most of the observations would be that this galaxy was subjected to removal of
a large fraction of its protogalactic gas shortly after its main phase of
globular cluster formation, probably by its first passage through the Coma
core. Since then, no significant additions due to accretions or mergers have
taken place.Comment: 24 pp. plus 13 Figures. Postscript file for the complete paper can
also be downloaded from http://www.physun.mcmaster.ca/~harris/WEHarris.html.
Astron.J., in pres
NGC 770: A Counter-Rotating Core in a Low-Luminosity Elliptical Galaxy
We present evidence for a counter-rotating core in the low-luminosity (M_B =
-18.2) elliptical galaxy NGC 770 based on internal stellar kinematic data. This
counter-rotating core is unusual as NGC 770 is not the primary galaxy in the
region and it lies in an environment with evidence of on-going tidal
interactions. We discovered the counter-rotating core via single-slit Keck/ESI
echelle spectroscopy; subsequent integral field spectroscopy was obtained with
the Gemini/GMOS IFU. The counter-rotating region has a peak rotation velocity
of 21 km/s as compared to the main galaxy's rotation speed of greater than 45
km/s in the opposite direction. The counter-rotating region extends to a radius
of 4'' (0.6 kpc), slightly smaller than the half-light radius of the galaxy
which is 5.3'' (0.8 kpc) and is confined to a disk whose scale height is less
than 0.8'' (0.1 kpc). We compute an age and metallicity of the inner
counter-rotating region of 3 +/- 0.5 Gyr and [Fe/H] = 0.2 +/- 0.2 dex, based on
Lick absorption-line indices. The lack of other large galaxies in this region
limits possible scenarios for the formation of the counter-rotating core. We
discuss several scenarios and favor one in which NGC 770 accreted a small
gas-rich dwarf galaxy during a very minor merging event. If this scenario is
correct, it represents one of the few known examples of merging between two
dwarf-sized galaxies.Comment: 26 pages, 9 figures. Accepted to AJ. See this
http://www.ociw.edu/~mgeha/geha.ps.gz for version with high resolution
figure
The FORS Deep Field Spectroscopic Survey
We present a catalogue and atlas of low-resolution spectra of a well defined
sample of 341 objects in the FORS Deep Field. All spectra were obtained with
the FORS instruments at the ESO VLT with essentially the same spectroscopic
set-up. The observed extragalactic objects cover the redshift range 0.1 to 5.0.
98 objects are starburst galaxies and QSOs at z > 2. Using this data set we
investigated the evolution of the characteristic spectral properties of bright
starburst galaxies and their mutual relations as a function of the redshift.
Significant evolutionary effects were found for redshifts 2 < z < 4. Most
conspicuous are the increase of the average C IV absorption strength, of the
dust reddening, and of the intrinsic UV luminosity, and the decrease of the
average Ly alpha emission strength with decreasing redshift. In part the
observed evolutionary effects can be attributed to an increase of the
metallicity of the galaxies with cosmic age. Moreover, the increase of the
total star-formation rates and the stronger obscuration of the starburst cores
by dusty gas clouds suggest the occurrence of more massive starbursts at later
cosmic epochs.Comment: 24 pages, 25 figures (35 PS files), 4 tables, accepted for
publication in A&A. v2: minor typos corrected and references update
Nuclear stellar discs in low-luminosity elliptical galaxies: NGC 4458 and NGC 4478
We present the detection of nuclear stellar discs in the low-luminosity
elliptical galaxies NGC 4458 and NGC 4478, which are known to host a
kinematically-decoupled core. Using archival HST imaging, and available
absorption line-strength index data based on ground-based spectroscopy, we
investigate the photometric parameters and the properties of the stellar
populations of these central structures. Their scale length, h, and face-on
central surface brightness, mu_0^c, fit on mu_0^c-h relation for galaxy discs.
For NGC 4458 these parameters are typical for nuclear discs, while the same
quantities for NGC 4478 lie between those of nuclear discs and the discs of
discy ellipticals. We present Lick/IDS absorption line-strength measurements of
Hbeta, Mgb, along the major and minor axes of the galaxies. We model these
data with simple stellar populations that account for the alpha/Fe
overabundance. The counter-rotating central disc of NGC 4458 is found to have
similar properties to the decoupled cores of bright ellipticals. This galaxy
has been found to be uniformly old despite being counter-rotating. In contrast,
the cold central disc of NGC 4478 is younger, richer in metals and less
overabundant than the main body of the galaxy. This points to a prolonged star
formation history, typical of an undisturbed disc-like, gas-rich (possibly
pre-enriched) structure.Comment: 11 pages, 8 figures, accepted for pubblication on MNRA
The Bulge-Disk Orthogonal Decoupling in Galaxies: NGC 4698
The R-band isophotal map of the Sa galaxy NGC 4698 shows that the inner
region of the bulge structure is elongated perpendicularly to the major axis of
the disk, this is also true for the outer parts of the bulge if a parametric
photometric decomposition is adopted. At the same time the stellar component is
characterized by an inner velocity gradient and a central zero-velocity plateau
along the minor and major axis of the disk respectively. This remarkable
geometric and kinematic decoupling suggests that a second event occurred in the
formation history of this galaxy.Comment: 12 pages, LaTex, with 4 PostScript figures. Accepted for publication
in The Astrophysical Journal Letter
The Tully-Fisher relation at intermediate redshift
Using the Very Large Telescope in Multi Object Spectroscopy mode, we have
observed a sample of 113 field spiral galaxies in the FORS Deep Field (FDF)
with redshifts in the range 0.1<z<1.0. The galaxies were selected upon apparent
brightness (R<23) and encompass all late spectrophotometric types from Sa to
Sdm/Im. Spatially resolved rotation curves have been extracted for 77 galaxies
and fitted with synthetic velocity fields taking into account all observational
effects from inclination and slit misalignment to seeing and slit width. We
also compared different shapes for the intrinsic rotation curve. To gain robust
values of V_max, our analysis is focussed on galaxies with rotation curves
which extend well into the region of constant rotation velocity at large radii.
If the slope of the local Tully-Fisher relation (TFR) is held fixed, we find
evidence for a mass-dependent luminosity evolution which is as large as up to 2
mag for the lowest-mass galaxies, but is small or even negligible for the
highest-mass systems in our sample. In effect, the TFR slope is shallower at
z~0.5 in comparison to the local sample. We argue for a mass-dependent
evolution of the mass-to-light ratio. An additional population of blue,
low-mass spirals does not seem a very appealing explanation. The flatter tilt
we find for the distant TFR is in contradiction to the predictions of recent
semi-analytic simulations.Comment: 18 pages, 14 figures, A&A, in press. Section on sample completeness
added. Please note that the entire analysis is based on undisturbed, high
quality rotation curves! Potential effects of tidal interactions are also
discusse
The Tully-Fisher relation for S0 galaxies
We present a study of the local B and K-band Tully-Fisher Relation (TFR)
between absolute magnitude and maximum circular speed in S0 galaxies. To make
this study, we have combined kinematic data, including a new high-quality
spectral data set from the Fornax Cluster, with homogeneous photometry from the
RC3 and 2MASS catalogues, to construct the largest sample of S0 galaxies ever
used in a study of the TFR. Independent of environment, S0 galaxies are found
to lie systematically below the TFR for nearby spirals in both optical and
infrared bands. This offset can be crudely interpreted as arising from the
luminosity evolution of spiral galaxies that have faded since ceasing star
formation. However, we also find a large scatter in the TFR. We show that most
of this scatter is intrinsic, not due to the observational uncertainties. The
presence of such a large scatter means that the population of S0 galaxies
cannot have formed exclusively by the above simple fading mechanism after all
transforming at a single epoch. To better understand the complexity of the
transformation mechanism, we have searched for correlations between the offset
from the TFR and other properties of the galaxies such as their structural
properties, central velocity dispersions and ages (as estimated from line
indices). For the Fornax Cluster data, the offset from the TFR relates with the
estimated age of the stars in the individual galaxies, in the sense and of the
magnitude expected if S0 galaxies had passively faded since being converted
from spirals. This correlation implies that a significant part of the scatter
in the TFR arises from the different times at which galaxies began their
transformation.Comment: 17 pages, 11 figures, 3 tables, accepted for publication in MNRA
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