2,198 research outputs found
Near-infrared luminosity function and colours of dwarf galaxies in the Coma Cluster
We present K-band observations of the low-luminosity galaxies in the Coma
cluster, which are responsible for the steep upturn in the optical luminosity
function at M_R ~ -16, discovered recently. The main results of this study are
(i) The opticalnear-infrared colours of these galaxies imply that they are
dwarf spheroidals. The median M-K colour for galaxies with -19.3 < M_K < -16.3
is 3.6 mag. (ii) The K-band luminosity function in the Coma cluster at the
faint-end is not wee constrained, because of the uncertainties due to the
field-to-field variance of the background. However, within the estimate large
errors, it is consistent with the R-band luminosity function, shifted by
magnitudes. (iii) Many of the cluster dwarfs lie in a region of the B-K
vs. B-R colour-colour diagram where background galaxies are rare Local dwarf
spheroidal galaxies lie in this region too. This suggests that a better
measurement of the K-band cluster luminosity function can be made if the
field-to-field variance of the background can be measured as a function of
colour. (iv) If we assume that none of the galaxies in the region of the B-K
vs. B-R plane given in (iii) in our cluster fields are background, and that all
the cluster galaxies with lie in this region of the plane,
then we measure alpha = -1.41 +/- 0.35 for -19.3 < M_K < -16.3, where alpha is
the logarithmic slope of the luminosity function.Comment: 6 pages, 8 figs, 2 tabs, MNRAS in press; email: [email protected],
[email protected]
Stellar Population of Ellipticals in Different Environments: Near-infrared Spectroscopic Observations
Near-infrared spectra of 50 elliptical galaxies in the Pisces, A2199 and
A2634 clusters, and in the general field, have been obtained. The strength of
the CO (2.3 micron) absorption feature in these galaxies is used to explore the
presence of an intermediate-age population (e.g. Asymptotic Giant Branch stars)
in ellipticals in different environments. We find the strongest evidence for
such a population comes from ellipticals in groups of a few members, which we
interpret as the result of recent minor merging of these galaxies with later
type galaxies. Field galaxies from very isolated environments, on the other
hand, show no evidence for young or intermediate-age stars as revealed by H
beta and CO absorptions, and appear to form a very uniform, old population with
very little scatter in metallicity and star formation history.Comment: 13 pages, 6 figures, accepted for publication in the Monthly Notices
of the Royal Astronomical Societ
The stellar content of brightest cluster galaxies
We present near-infrared K-band spectroscopy of 21 elliptical or cD Brightest
Cluster Galaxies (BCGs), for which we have measured the strength of the 2.293
micron CO stellar absorption feature. We find that the strength of this feature
is remarkably uniform among these galaxies, with a smaller scatter in
equivalent width than for the normal elliptical population in the field or
clusters. The scatter for BCGs is 0.156 nm, compared with 0.240 nm for Coma
cluster ellipticals, 0.337 nm for ellipticals from a variety of other clusters,
and 0.422 nm for field ellipticals. We interpret this homogeneity as being due
to a greater age, or more uniform history, of star formation in BCGs than in
other ellipticals; only a small fraction of the scatter can be due to
metallicity variations, even in the BCGs. Notwithstanding the small scatter,
correlations are found between CO strength and various galaxy properties,
including R-band absolute magnitude, which could improve the precision of these
galaxies as distance indicators in measurements of cosmological parameters and
velocity flows.Comment: 7 pages, 8 figures, accepted for publication by MNRA
Spectroscopic Constraints on the Stellar Population of Elliptical Galaxies in the Coma Cluster
Near-IR spectra for a sample of 31 elliptical galaxies in the Coma cluster
are obtained. The galaxies are selected to be ellipticals (no lenticulars),
with a large spatial distribution, covering both the core and outskirt of the
cluster (ie. corresponding to regions with large density contrasts).
Spectroscopic CO (2.3 micron) absorption indices, measuring contribution from
intermediate-age red giant and supergiant stars to the near-IR light of the
ellipticals, are then estimated.
It is found that the strength of spectroscopic CO features in elliptical
galaxies increases from the core (r 0.2 deg)
of the Coma cluster. Using the Mg2 strengths, it is shown that the observed
effect is not due to metallicity and is mostly caused by the presence of a
younger population (giant and supergiant stars) in ellipticals in outskirts
(low density region) of the cluster.
Using the spectroscopic CO features, the origin of the scatter on the near-IR
Fundamental Plane of elliptical galaxies is studied. Correcting this relation
for contributions from the red giant and supergiant stars, the rms scatter
reduces from 0.077dex to 0.073dex. Although measurable, the contribution from
these intermediate-age stars to the scatter on the near-IR Fundamental Plane of
ellipticals is only marginal.
A relation is found between the CO and V-K colours of ellipticals with a
slope 0.036 +/- 0.016. This is studied using stellar synthesis models.Comment: 17 pages, 7 figures, Accepted for publication in MNRAS Repoprt-no
Communication Over MIMO Broadcast Channels Using Lattice-Basis Reduction
A simple scheme for communication over MIMO broadcast channels is introduced
which adopts the lattice reduction technique to improve the naive channel
inversion method. Lattice basis reduction helps us to reduce the average
transmitted energy by modifying the region which includes the constellation
points. Simulation results show that the proposed scheme performs well, and as
compared to the more complex methods (such as the perturbation method) has a
negligible loss. Moreover, the proposed method is extended to the case of
different rates for different users. The asymptotic behavior of the symbol
error rate of the proposed method and the perturbation technique, and also the
outage probability for the case of fixed-rate users is analyzed. It is shown
that the proposed method, based on LLL lattice reduction, achieves the optimum
asymptotic slope of symbol-error-rate (called the precoding diversity). Also,
the outage probability for the case of fixed sum-rate is analyzed.Comment: Submitted to IEEE Trans. on Info. Theory (Jan. 15, 2006), Revised
(Jun. 12, 2007
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