7,207 research outputs found
Star Formation in Cluster Galaxies at 0.2<z<0.55
The rest frame equivalent width of the [OII]3727 emission line, W(OII), has
been measured for cluster and field galaxies in the CNOC redshift survey of
rich clusters at 0.2<z<0.55. Emission lines of any strength in cluster galaxies
at all distances from the cluster centre, out to 2R_{200}, are less common than
in field galaxies. The mean W(OII) in cluster galaxies more luminous than
M_r^k<-18.5 + 5\log h (q_o=0.1) is 3.8 \pm 0.3 A (where the uncertainty is the
1 sigma error in the mean), significantly less than the field galaxy mean of
11.2 \pm 0.3 A. For the innermost cluster members (R<0.3R_{200}), the mean
W(OII) is only 0.3 \pm 0.4 A. Thus, it appears that neither the infall process
nor internal tides in the cluster induce detectable excess star formation in
cluster galaxies relative to the field. The colour-radius relation of the
sample is unable to fully account for the lack of cluster galaxies with
W(OII)>10 A, as expected in a model of cluster formation in which star
formation is truncated upon infall. Evidence of supressed star formation
relative to the field is present in the whole cluster sample, out to 2 R_{200},
so the mechanism responsible for the differential evolution must be acting at a
large distance from the cluster centre, and not just in the core. The mean star
formation rate in the cluster galaxies with the strongest emission corresponds
to an increase in the total stellar mass of less than about 4% if the star
formation is due to a secondary burst lasting 0.1 Gyr.Comment: aasms4 latex, 3 postscript figures, accepted for publication in ApJ
Letters. Also available at http://astrowww.phys.uvic.ca/~balogh
Transformation of stimulus correlations by the retina
Redundancies and correlations in the responses of sensory neurons seem to
waste neural resources but can carry cues about structured stimuli and may help
the brain to correct for response errors. To assess how the retina negotiates
this tradeoff, we measured simultaneous responses from populations of ganglion
cells presented with natural and artificial stimuli that varied greatly in
correlation structure. We found that pairwise correlations in the retinal
output remained similar across stimuli with widely different spatio-temporal
correlations including white noise and natural movies. Meanwhile, purely
spatial correlations tended to increase correlations in the retinal response.
Responding to more correlated stimuli, ganglion cells had faster temporal
kernels and tended to have stronger surrounds. These properties of individual
cells, along with gain changes that opposed changes in effective contrast at
the ganglion cell input, largely explained the similarity of pairwise
correlations across stimuli where receptive field measurements were possible.Comment: author list corrected in metadat
Optical studies of carrier and phonon dynamics in Ga_{1-x}Mn_{x}As
We present a time-resolved optical study of the dynamics of carriers and
phonons in Ga_{1-x}Mn_{x}As layers for a series of Mn and hole concentrations.
While band filling is the dominant effect in transient optical absorption in
low-temperature-grown (LT) GaAs, band gap renormalization effects become
important with increasing Mn concentration in Ga_{1-x}Mn_{x}As, as inferred
from the sign of the absorption change. We also report direct observation on
lattice vibrations in Ga1-xMnxAs layers via reflective electro-optic sampling
technique. The data show increasingly fast dephasing of LO phonon oscillations
for samples with increasing Mn and hole concentration, which can be understood
in term of phonon scattering by the holes.Comment: 13 pages, 3 figures replaced Fig.1 after finding a mistake in
previous versio
Chirality-Selective Excitation of Coherent Phonons in Carbon Nanotubes
Using pre-designed trains of femtosecond optical pulses, we have selectively
excited coherent phonons of the radial breathing mode of specific-chirality
single-walled carbon nanotubes within an ensemble sample. By analyzing the
initial phase of the phonon oscillations, we prove that the tube diameter
initially increases in response to ultrafast photoexcitation. Furthermore, from
excitation profiles, we demonstrate that an excitonic absorption peak of carbon
nanotubes periodically oscillates as a function of time when the tube diameter
undergoes radial breathing mode oscillations.Comment: 4 pages, 4 figure
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