748 research outputs found
Plant Mol Biol
Proteomic studies have addressed the composition of plant chloroplast ribosomes and 70S ribosomes from the unicellular organism Chlamydomonas reinhardtii But comprehensive characterization of cytoplasmic 80S ribosomes from higher plants has been lacking. We have used two-dimensional gel electrophoresis (2-DE) and mass spectrometry (MS) to analyse the cytoplasmic 80S ribosomes from the model flowering plant Arabidopsis thaliana. Of the 80 ribosomal protein families predicted to comprise the cytoplasmic 80S ribosome, we have confirmed the presence of 61; specifically, 27 (84%) of the small 40S subunit and 34 (71%) of the large 60S subunit. Nearly half (45%) of the ribosomal proteins identified are represented by two or more distinct spots in the 2-DE gel indicating that these proteins are either post-translationally modified or present as different isoforms. Consistently, MS-based protein identification revealed that at least one-third (34%) of the identified ribosomal protein families showed expression of two or more family members. In addition, we have identified a number of non-ribosomal proteins that co-migrate with the plant 80S ribosomes during gradient centrifugation suggesting their possible association with the 80S ribosomes. Among them, RACK1 has recently been proposed to be a ribosome-associated protein that promotes efficient translation in yeast. The study, thus provides the basis for further investigation into the function of the other identified non-ribosomal proteins as well as the biological meaning of the various ribosomal protein isoforms
Lyman Break Galaxies at z>4 and the Evolution of the UV Luminosity Density at High Redshift
We present initial results of a survey for star-forming galaxies in the
redshift range 3.8 < z < 4.5. This sample consists of a photometric catalog of
244 galaxies culled from a total solid angle of 0.23 square degrees to an
apparent magnitude of I_{AB}=25.0. Spectroscopic redshifts in the range 3.61 <
z < 4.81 have been obtained for 48 of these galaxies; their median redshift is
=4.13. Selecting these galaxies in a manner entirely analogous to our large
survey for Lyman break galaxies at smaller redshift (2.7 < z < 3.4) allows a
relatively clean differential comparison between the populations and integrated
luminosity density at these two cosmic epochs. Over the same range of UV
luminosity, the spectroscopic properties of the galaxy samples at z~4 and z~3
are indistinguishable, as are the luminosity function shapes and the total
integrated UV luminosity densities (rho_{UV}(z=3)/rho_{UV}(z=4) = 1.1 +/-0.3).
We see no evidence at these bright magnitudes for the steep decline in the star
formation density inferred from fainter photometric Lyman-break galaxies in the
Hubble Deep Field (HDF). If the true luminosity density at z~4 is somewhat
higher than implied by the HDF, as our ground-based sample suggests, then the
emissivity of star formation as a function of redshift is essentially constant
for all z>1 once internally consistent corrections for dust are made. This
suggests that there is no obvious peak in star formation activity, and that the
onset of substantial star formation in galaxies occurs at z > 4.5. [abridged
abstract]Comment: To appear in the ApJ, minor revisions to match accepted versio
Deep U band and R imaging of GOODS-South: Observations,data reduction and first results
We present deep imaging in the {\em U} band covering an area of 630
arcmin centered on the southern field of the Great Observatories Origins
Deep Survey (GOODS). The data were obtained with the VIMOS instrument at the
ESO Very Large Telescope. The final images reach a magnitude limit (AB, 1, in a 1\arcsec radius aperture), and have good
image quality, with full width at half maximum \approx 0.8\arcsec. They are
significantly deeper than previous U--band images available for the GOODS
fields, and better match the sensitivity of other multi--wavelength GOODS
photometry. The deeper U--band data yield significantly improved photometric
redshifts, especially in key redshift ranges such as , and deeper
color--selected galaxy samples, e.g., Lyman--break galaxies at . We
also present the coaddition of archival ESO VIMOS R band data, with (AB, 1, 1\arcsec radius aperture), and image quality
\approx 0.75 \arcsec. We discuss the strategies for the observations and data
reduction, and present the first results from the analysis of the coadded
images.Comment: Accepted for publication ApJS, 54 pages, 27 figures. Released data
and full-quality paper version available at
http://archive.eso.org/cms/eso-data/data-packages/goods-vimos-imaging-data-release-version-1.
Spectroscopic Observations of Lyman-Break Galaxies at Redshift ~ 4, 5 and 6 in the GOODS-South Field
We report on observations of Lyman-break galaxies (LBGs) selected from the
Great Observatories Origins Deep Survey at mean redshift z~4, 5 and 6, obtained
with FORS2 spectrograph at the ESO-VLT.This program has yielded spectroscopic
identifications for 114 galaxies of which 51 are at z~4, 31 at z~5, and 32 at
z~6. The adopted selection criteria are effective, identifying galaxies at the
expected redshift with minimal foreground contamination. Once selection effects
are properly accounted for, the rest-frame UV spectra of the higher-redshift
LBGs appear to be similar to their counterparts at z~3. As at z~3, LBGs at z~4
and z~5 show Lya both in emission and in absorption; when in absorption, strong
interstellar lines and red UV spectra are observed, a fact also measured at z~2
and 3. At z~6, sensitivity issues bias our sample towards galaxies with Lya in
emission; nevertheless, these spectra appear to be similar to their
lower-redshift counterparts. As in other studies at similar redshifts, we find
clear evidence that brighter LBGs tend to have weaker Lya emission lines. At
fixed rest-frame UV luminosity, the equivalent width of the Lya emission line
is larger at higher redshifts. At all redshifts where the measurements can be
reliably made, the redshift of the Lya emission line turns out to be larger
than that of the interstellar absorption lines,with a median velocity
difference DeltaV~400 km/s at z~4 and 5, consistent with results at lower
redshifts. This shows that powerful, large-scale winds are common at high
redshift.In general,there is no strong correlation between the morphology of
the UV light and the spectroscopic properties. However, galaxies with deep
interstellar absorption lines and strong Lya absorption appear to be more
diffuse than galaxies with Lya in emission.(abridged)Comment: 58 pages, 19 figures, accepted for publication in The Astrophysical
Journa
The lack of intense Lyman~alpha in ultradeep spectra of z=7 candidates in GOODS-S: imprint of reionization?
We present ultradeep optical spectroscopy obtained with FORS2 on VLT of seven
Lyman-break galaxy (LBG) candidates at z>6.5 selected in the GOODS-S field from
Hawk-I/VLT and WFC3/HST imaging. For one galaxy we detect a low significance
emission line (S/N< 7), located at 9691.5 +/- 0.5A and with flux 3.4 x
10^(-18)erg/cm^2/s. If identified as Lyman alpha, it places the LBG at redshift
z=6.972+/- 0.002, with a rest-frame equivalent width EW}=13A. Using Monte Carlo
simulations and conservative EW distribution functions at 2<z<6, we estimate
that the probability of observing no galaxies in our data with S/N>10 is ~ 2%,
and that of observing only one galaxy out of seven with S/N=5 is ~4%, but these
can be as small as ~1E-3, depending on the details of the EW distribution. We
conclude that either a significant fraction of the candidates is not at high
redshift or that some physical mechanism quenches the Lyman alpha emission
emerging from the galaxies at z>6.5, abruptly reversing the trend of the
increasing fraction of strong emitters with increasing redshift observed up to
z~ 6.5. We discuss the possibility that an increasingly neutral intergalactic
medium is responsible for such quenching.Comment: Submitted to ApJL: re-submitted version after modification responding
to the referee's repor
The Great Observatories Origins Deep Survey. VLT/FORS2 Spectroscopy in the GOODS-South Field: Part III
Aims. We present the full data set of the spectroscopic campaign of the
ESO/GOODS program in the GOODS-South field, obtained with the
FORS2 spectrograph at the ESO/VLT. Method. Objects were selected as
candidates for VLT/FORS2 observations primarily based on the expectation that
the detection and measurement of their spectral features would benefit from the
high throughput and spectral resolution of FORS2. The reliability of the
redshift estimates is assessed using the redshift-magnitude and color-redshift
diagrams, and comparing the results with public data. Results. Including the
third part of the spectroscopic campaign (12 masks) to the previous work (26
masks, Vanzella et al. 2005, 2006), 1715 spectra of 1225 individual targets
have been analyzed. The actual spectroscopic catalog provides 887 redshift
determinations. The typical redshift uncertainty is estimated to be sigma(z) ~
0.001. Galaxies have been selected adopting different color criteria and using
photometric redshifts. The resulting redshift distribution typically spans two
domains: from z=0.5 to 2 and z=3.5 to 6.3. The reduced spectra and the derived
redshifts are released to the community through the ESO web page
http://www.eso.org/science/goods/Comment: 11 pages, 11 figures; accepted for publication in Astronomy &
Astrophysics. Data are available at http://www.eso.org/science/goods
- …