1,002 research outputs found
The Mgb-sigma Relation of Elliptical Galaxies at z=0.37
We derive absorption indices of ellipticals in clusters at z=0.37 from
medium-resolution spectroscopy together with kinematical parameters. These
galaxies exhibit a relationship between the Mgb linestrength and their internal
velocity dispersion similar to local dynamically hot galaxies. But for any
given sigma, Mgb of the distant ellipticals is significantly lower than the
mean value of the nearby sample. The difference of Mgb between the two samples
is small and can be fully attributed to the younger age of the distant stellar
populations in accordance with the passive evolution model. The low reduction
of Mgb at a look-back time of about 5 Gyrs requires that the bulk of the stars
in cluster ellipticals have formed at very high redshifts of z_f>2. For the
most massive galaxies, where the reduction is even lower, z_f probably exceeds
4.
Unlike most methods to measure the evolution of ellipticals using
luminosities, surface brightnesses or colours, the Mgb linestrength does not
depend on corrections for extinction and cosmic expansion (K-correction) and
only very little on the slope of the initial mass function. The combination of
a kinematical parameter with a stellar population indicator allows us to study
the evolution of very similar objects. In addition, the good mass estimate
provided by sigma means that the selection criteria for the galaxy sample as a
whole are well controlled.
(abriged)Comment: 25 pages, Latex, uses mn.sty and mncite.sty, accepted by MNRAS, also
available at http://www.usm.uni-muenchen.de:8001/people/ziegler/pubs.htm
Approximation Algorithms for Multi-Criteria Traveling Salesman Problems
In multi-criteria optimization problems, several objective functions have to
be optimized. Since the different objective functions are usually in conflict
with each other, one cannot consider only one particular solution as the
optimal solution. Instead, the aim is to compute a so-called Pareto curve of
solutions. Since Pareto curves cannot be computed efficiently in general, we
have to be content with approximations to them.
We design a deterministic polynomial-time algorithm for multi-criteria
g-metric STSP that computes (min{1 +g, 2g^2/(2g^2 -2g +1)} + eps)-approximate
Pareto curves for all 1/2<=g<=1. In particular, we obtain a
(2+eps)-approximation for multi-criteria metric STSP. We also present two
randomized approximation algorithms for multi-criteria g-metric STSP that
achieve approximation ratios of (2g^3 +2g^2)/(3g^2 -2g +1) + eps and (1 +g)/(1
+3g -4g^2) + eps, respectively.
Moreover, we present randomized approximation algorithms for multi-criteria
g-metric ATSP (ratio 1/2 + g^3/(1 -3g^2) + eps) for g < 1/sqrt(3)), STSP with
weights 1 and 2 (ratio 4/3) and ATSP with weights 1 and 2 (ratio 3/2). To do
this, we design randomized approximation schemes for multi-criteria cycle cover
and graph factor problems.Comment: To appear in Algorithmica. A preliminary version has been presented
at the 4th Workshop on Approximation and Online Algorithms (WAOA 2006
Disk galaxy scaling relations at intermediate redshifts - I. The Tully-Fisher and velocity-size relations
We took spatially resolved slit spectra of 261 field disk galaxies at
redshifts up to z~1 using the FORS instruments of the ESO Very Large Telescope.
Our spectroscopy was complemented with HST/ACS imaging in the F814W filter. We
analyzed the ionized gas kinematics by extracting rotation curves from the 2-D
spectra. Taking into account all geometrical, observational and instrumental
effects, these rotation curves were used to derive the intrinsic Vmax.
Neglecting galaxies with disturbed kinematics or insufficient spatial rotation
curve extent, Vmax could be robustly determined for 124 galaxies covering
redshifts 0.05<z<0.97. This is one of the largest kinematic samples of distant
disk galaxies to date. We compared this data set to the local B-band
Tully-Fisher relation and the local velocity-size relation. The scatter in both
scaling relations is a factor of ~2 larger at z~0.5 than at z~0. The deviations
of individual distant galaxies from the local Tully-Fisher relation are
systematic in the sense that the galaxies are increasingly overluminous towards
higher redshifts, corresponding to an over-luminosity of -(1.2+-0.5) mag at
z=1. This luminosity evolution at given Vmax is probably driven by younger
stellar populations of distant galaxies with respect to their local
counterparts, potentially combined with modest changes in dark matter mass
fractions. The analysis of the velocity-size relation reveals that disk
galaxies of a given Vmax have grown in size by a factor of ~1.5 over the past
~8 Gyr, likely via accretion of cold gas and/or small satellites. Scrutinizing
the combined evolution in luminosity and size, we find that the galaxies which
show the strongest evolution towards smaller sizes at z~1 are not those which
feature the strongest evolution in luminosity, and vice versa. [abstract
abbreviated]Comment: A&A, accepted. 12 pages, 6 figures, 1 tabl
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