24 research outputs found
Dissecting Galaxies with Quantitative Spectroscopy of the Brightest Stars in the Universe
Measuring distances to galaxies, determining their chemical composition,
investigating the nature of their stellar populations and the absorbing
properties of their interstellar medium are fundamental activities in modern
extragalactic astronomy helping to understand the evolution of galaxies and the
expanding universe. The optically brightest stars in the universe, blue
supergiants of spectral A and B, are unique tools for these purposes. With
absolute visual magnitudes up to M_V = -9.5 they are the ideal to obtain
accurate quantitative information about galaxies through the powerful modern
methods of quantitative stellar spectroscopy. The spectral analyis of
individual blue supergiant targets provides invaluable information about
chemical abundances and abundance gradients, which is more comprehensive than
the one obtained from HII regions, as it includes additional atomic species,
and which is also more accurate, since it avoids the systematic uncertainties
inherent in the strong line studies usually applied to the HII regions of
spiral galaxies beyond the Local Group. Simultaneously, the spectral analysis
yields stellar parameters and interstellar extinction for each individual
supergiant target, which provides an alternative very accurate way to determine
extragalactic distances through a newly developed method, called the
Flux-weighted Gravity - Luminosity Relationship (FGLR). With the present
generation of 10m-class telescopes these spectroscopic studies can reach out to
distances of 10 Mpc. The new generation of 30m-class will allow to extend this
work out to 30 Mpc, a substantial volume of the local universe.Comment: Karl Schwarzschild Lecture 2009. To appear in Astronomische
Nachrichte
PopStar Evolutionary Synthesis Models II: Optical emission-line spectra from Giant H{\sc ii} regions
This is the second paper of a series reporting the results from the PopStar
evolutionary synthesis models. Here we present synthetic emission line spectra
of H{\sc ii} regions photoionized by young star clusters, for seven values of
cluster masses and for ages between 0.1 and 5.2 Myr. The ionizing Spectral
Energy Distributions (SEDs) are those obtained by the PopStar code
\citep*{mgb09} for six different metallicities, with a very low metallicity
set, Z=0.0001, not included in previous similar works. We assume that the
radius of the H{\sc ii} region is the distance at which the ionized gas is
deposited by the action of the mechanical energy of the winds and supernovae
from the central ionizing young cluster. In this way the ionization parameter
is eliminated as free argument, since now its value is obtained from the
cluster physical properties (mass, age and metallicity) and from the gaseous
medium characteristics (density and abundances). We discuss our results and
compare them with those from previous models and also with a large and data set
of giant H{\sc ii} regions for which abundances have been derived in a
homogeneous manner. The values of the [OIII] lines (at 4363,
4959, 5007\AA) in the lowest metallicity nebulae are found to be very weak and
similar to those coming from very high metallicity regions (solar or
over-solar). Thus, the sole use of the oxygen lines is not enough to
distinguish between very low and very high metallicity regions. In these cases
we emphasize the need of the additional support of alternative metallicity
tracers, like the [SIII] lines in the near-\textit{IR}.Comment: 20 pages, 26 figures, accepted for publication in MNRAS Main Journa
Ram pressure stripping of disc galaxies orbiting in clusters. II. Galactic wakes
We present 3D hydrodynamical simulations of ram pressure stripping of a disc
galaxy orbiting in a galaxy cluster. In this paper, we focus on the properties
of the galaxies' tails of stripped gas. The galactic wakes show a flaring
width, where the flaring angle depends on the gas disc's cross-section with
respect to the galaxy's direction of motion. The velocity in the wakes shows a
significant turbulent component of a few 100 km/s. The stripped gas is
deposited in the cluster rather locally, i.e. within ~150 kpc from where it was
stripped. We demonstrate that the most important quantity governing the tail
density, length and gas mass distribution along the orbit is the galaxy's mass
loss per orbital length. This in turn depends on the ram pressure as well as
the galaxy's orbital velocity. For a sensitivity limit of ~10^19 cm^-2 in
projected gas density, we find typical tail lengths of 40 kpc. Such long tails
are seen even at large distances (0.5 to 1 Mpc) from the cluster centre. At
this sensitivity limit, the tails show little flaring, but a width similar to
the gas disc's size. Morphologically, we find good agreement with the HI tails
observed in the Virgo cluster by Chung et al. (2007). However, the observed
tails show a much smaller velocity width than predicted from the simulation.
The few known X-ray and H tails are generally much narrower and much
straighter than the tails in our simulations. Thus, additional physics like a
viscous ICM, the influence of cooling and tidal effects may be needed to
explain the details of the observations. We discuss the hydrodynamical drag as
a heat source for the ICM but conclude that it is not likely to play an
important role, especially not in stopping cooling flows.Comment: 23 pages, 23 figures, accepted by MNRAS. Additions to method, result
and discussion section, references added. Results and conclusions essentially
unchanged. high resolution pdf available at
http://www.faculty.iu-bremen.de/eroediger/PAPERS/eroediger_wakes.pd
Evaluation of antibiotics as a methodological procedure to inhibit free-living and biofilm bacteria in marine zooplankton culture
Optimization of Simple Reaction-Diffusion PDE Simulations on a 64-Opteron Linux Cluster
We use a simple message-passing (MPI) algorithm on a parallel cluster to reduce the simulation times in our study of spiral and scroll waves in 2D and 3D excitable media. However, the efficiency of such parallel simulations can vary significantly based on a multitude of factors. We undertook a thorough investigation to find optimal ways of using our on-site 64-processor Linux cluster. This investigation includes a detailed 2D and 3D speed-up analysis, and a comparison of alternative MPI implementations, MPI configuration files, and a variety of other approaches and options. Due to the generic nature of our parallelization algorithm, the results of this study can be interesting to a broad body of scientific cluster users