745 research outputs found
Measuring the Virial Masses of Disk Galaxies
I present detailed models for the formation of disk galaxies, and investigate
which observables are best suited as virial mass estimators. Contrary to naive
expectations, the luminosities and circular velocities of disk galaxies are
extremely poor indicators of total virial mass. Instead, I show that the
product of disk scale length and rotation velocity squared yields a much more
robust estimate. Finally, I show how this estimator may be used to put limits
on the efficiencies of cooling and feedback during the process of galaxy
formation.Comment: 6 pages, 2 fig. To appear in proceedings of "The Mass of Galaxies at
Low and High Redshift", eds. R. Bender and A. Renzini (ESO Astrophysics
Symposia, Springer-Verlag
A model of growth and development in copepods
We present a model for the growth and development of copepods based on the simple assumption that a constant fraction of assimilated carbon is invested in the formation of a new carapace. The individual molts into the next stage when the weight of the accumulated carapace building bricks is a fixed fraction of the body mass at the beginning of the stage. This simple stage-transition coordinating system is built into an individual growth model.
The model predicts decreasing stage duration and increasing body weight with increasing food concentrations. These trends closely agree with published data. Anisochronality, as found in many copepod species, agrees with model predictions. Acartia species usually show isochronal development. The model predicts isochronal development for one particular value of a parameter of the feeding process. Literature data show that this parameter value is realistic for Acartia spp. ''Equiproportionality'' is predicted by the model. The model thus shows close agreement with observed growth and development pattern
Nuclear stellar discs in early-type galaxies --- II. Photometric properties
Hubble Space Telescope images of two early-type galaxies harboring both
nuclear and outer stellar discs are studied in detail. By means of a
photometric decomposition, the images of NGC 4342 and NGC 4570 are analyzed and
the photometric properties of the nuclear discs investigated. We find a
continuity of properties in the parameter space defined by the central surface
brightness and the scalelength of discs in spirals, S0s and embedded discs in
ellipticals, in the sense that the nuclear discs extend the observed disc
properties even further towards smaller scalelengths and brighter central
surface brightnesses. When including the nuclear discs, disc properties span
more than four orders of magnitude in both scalelength and central surface
brightness. The nuclear discs studied here are the smallest and brightest
stellar discs known, and as such, they are as extreme in their photometric
properties as Malin I, when compared to typical galactic discs that obey
Freeman's law. We discuss a possible formation scenario in which the
double-disc structure observed in these galaxies has been shaped by now
dissolved bars. Based on the fact that the black holes known to exist in some
of these galaxies have masses comparable to those of the nuclear discs, we
explore a possible link between the black holes and the nuclear discs.Comment: 9 pages (TeX) with 7 figures (postscript). Accepted for publication
in MNRA
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