5,265 research outputs found
Spiral Structure Based Limits on the Disk Mass of the Low Surface Brightness Galaxies UGC 6614 and F568-6
Upper limits for the disk mass-to-light ratios for the low surface brightness
galaxies UGC 6614 and F568-6 (Malin 2) are estimated by considering the minimum
velocity perturbations in the HI velocity field that should result from the
spiral structure observed in the R band images. The weak observed response in
the velocity component limits the mass-to-light ratios of the disk
within a scale length to M/L <~ 3 and 6 for UGC 6614 for F568-6 respectively
(in solar units) based upon azimuthal variations observed in the R band images.
These limits are sufficiently strong to require a significant dark matter
component even in the central regions of these galaxies. Our limits furthermore
imply that this dark matter component cannot be in the form of a cold disk
since a cold disk would necessarily be involved in the spiral structure.
However, a more massive disk could be consistent with the observations because
of a non-linear gas response or if the gas is driven by bar-like distortions
instead of spiral structure. To produce the large observed arm/interarm HI
density variations it is likely that the spiral arm potential perturbation is
sufficiently strong to produce shocks in the gas. For a forcing that is greater
than 2% of the axisymmetric force, M/L >~ 1 is required in both galaxies in the
outer regions. These lower limits imply that the stellar surface density is at
least of the same order as the gas surface density. This is consistent with the
large scale morphology of the spiral structure, and the stability of the gas
disk, both which suggest that a moderate stellar component is required to
produce the observed spiral structure. (Shortened abstract)Comment: AAS Latex, Postscript and jpeg Figures, Accepted for publication in
A
Fe I Oscillator Strengths for the Gaia-ESO Survey
The Gaia-ESO Public Spectroscopic Survey (GES) is conducting a large-scale
study of multi-element chemical abundances of some 100 000 stars in the Milky
Way with the ultimate aim of quantifying the formation history and evolution of
young, mature and ancient Galactic populations. However, in preparing for the
analysis of GES spectra, it has been noted that atomic oscillator strengths of
important Fe I lines required to correctly model stellar line intensities are
missing from the atomic database. Here, we present new experimental oscillator
strengths derived from branching fractions and level lifetimes, for 142
transitions of Fe I between 3526 {\AA} and 10864 {\AA}, of which at least 38
are urgently needed by GES. We also assess the impact of these new data on
solar spectral synthesis and demonstrate that for 36 lines that appear
unblended in the Sun, Fe abundance measurements yield a small line-by-line
scatter (0.08 dex) with a mean abundance of 7.44 dex in good agreement with
recent publications.Comment: Accepted for publication in Mon. Not. R. Astron. So
- …