4,356 research outputs found
Pre-enriched, not primordial ellipticals
We follow the chemical evolution of a galaxy through star formation and its
feedback into the inter-stellar medium, starting from primordial gas and
allowing for gas to inflow into the region being modelled. We attempt to
reproduce observed spectral line-strengths for early-type galaxies to constrain
their star formation histories. The efficiencies and times of star formation
are varied as well as the amount and duration of inflow. We evaluate the
chemical enrichment and the mass of stars made with time. Single stellar
population (SSP) data are then used to predict line-strengths for composite
stellar populations. The results are compared with observed line-strengths in
ten ellipticals, including some features which help to break the problem of
age-metallicity degeneracy in old stellar populations. We find that the
elliptical galaxies modelled require high metallicity SSPs (>3 x solar) at
later times. In addition the strong lines observed cannot be produced by an
initial starburst in primordial gas, even if a large amount of inflow is
allowed for during the first few x 10E+8 years. This is because some
pre-enrichment is required for lines in the bulk of the stars to approach the
observed line-strengths in ellipticals.Comment: 18 pages, 8 figures, Latex, accepted for publication in MNRA
On the relation between viscoelastic and magnetohydrodynamic flows and their instabilities
We demonstrate a close analogy between a viscoelastic medium and an
electrically conducting fluid containing a magnetic field. Specifically, the
dynamics of the Oldroyd-B fluid in the limit of large Deborah number
corresponds to that of a magnetohydrodynamic (MHD) fluid in the limit of large
magnetic Reynolds number. As a definite example of this analogy, we compare the
stability properties of differentially rotating viscoelastic and MHD flows. We
show that there is an instability of the Oldroyd-B fluid that is physically
distinct from both the inertial and elastic instabilities described previously
in the literature, but is directly equivalent to the magnetorotational
instability in MHD. It occurs even when the specific angular momentum increases
outwards, provided that the angular velocity decreases outwards; it derives
from the kinetic energy of the shear flow and does not depend on the curvature
of the streamlines. However, we argue that the elastic instability of
viscoelastic Couette flow has no direct equivalent in MHD.Comment: 21 pages, 3 figures, to be published in J. Fluid Mec
A self-sustaining nonlinear dynamo process in Keplerian shear flows
A three-dimensional nonlinear dynamo process is identified in rotating plane
Couette flow in the Keplerian regime. It is analogous to the hydrodynamic
self-sustaining process in non-rotating shear flows and relies on the
magneto-rotational instability of a toroidal magnetic field. Steady nonlinear
solutions are computed numerically for a wide range of magnetic Reynolds
numbers but are restricted to low Reynolds numbers. This process may be
important to explain the sustenance of coherent fields and turbulent motions in
Keplerian accretion disks, where all its basic ingredients are present.Comment: 4 pages, 7 figures, accepted for publication in Physical Review
Letter
Constraining the Star Formation Histories of Spiral Bulges
Long-slit spectroscopic observations of line-strengths and kinematics made
along the minor axes of four spiral bulges are reported. Comparisons are made
between central line-strengths in spiral bulges and those in other
morphological types. The bulges are found to have central line-strengths
comparable with those of single stellar populations (SSPs) of approximately
solar abundance or above. Negative radial gradients are observed in
line-strengths, similar to those in elliptical galaxies. The bulge data are
consistent with correlations between Mg2, and central velocity dispersion
observed in elliptical galaxiess. In contrast to elliptical galaxies, central
line-strengths lie within the loci defining the range of and Mg2 achieved
by Worthey's (1994) solar abundance ratio, SSPs. The implication of solar
abundance ratios indicates differences in the star formation histories of
spiral bulges and elliptical galaxies. A ``single zone with in- fall'' model of
galactic chemical evolution, using Worthey's (1994) SSPs, is used to constrain
possible star formation histories in our sample. We show that , Mg2 and
Hbeta line-strengths observed in these bulges cannot be reproduced using
primordial collapse models of formation but can be reproduced by models with
extended in-fall of gas and star formation (2-17 Gyr) in the region modelled.
One galaxy (NGC 5689) shows a central population with luminosity weighted
average age of ~5 Gyr, supporting the idea of extended star formation.
Kinematic substructure, possibly associated with a central spike in
metallicity, is observed at the centre of the Sa galaxy NGC 3623.Comment: 14 pages. MNRAS latex file. Accepted for publication in MNRA
Effects of fluctuation on alpha-omega dynamo models
We analyse the role of a fluctuating alpha-effect in alpha-omega dynamo
models, and show that there is a mechanism for magnetic field generation, valid
at large scale separation, deriving from the interaction of mean shear and a
fluctuating alpha-effect. It is shown that this effect can act as a dynamo even
in the absence of a mean alpha-effect, and that the timescale for dynamo waves
is strongly affected by the presence of fluctuations.Comment: 4 pages, 3 figure
- …