1,978 research outputs found
Atomic hydrogen, star formation and feedback in the lowest mass Blue Compact Dwarf galaxies
We present the results from a search for HI emission from a sample of newly
discovered dwarf galaxies in the M81 group. HI is detected in three galaxies,
all of which are classified as BCDs. The HI masses of these galaxies are ~ 10^6
M_sun, making these some of the lowest mass BCDs known. For these three
galaxies FUV images (from GALEX) and H-alpha images (from the Russian 6m BTA
telescope) are available.The H-alpha emission is very faint, and, in principle
could be produced by a single O star. Further, in all cases we find offsets
between the peak of the FUV emission and that of the H-alpha emission. Offsets
between the most recent sites of star formation (i.e. those traced by H-alpha)
and the older sites (i.e. those traced by FUV) would be natural if the star
formation is stochastic. In spite of the expectation that the effects of
mechanical feedback from star formation would be most directly seen in the
smallest galaxies with low gravitational potentials, we only see tentative
evidence of outflowing HI gas associated with the star forming region in one of
the galaxies.Comment: 9 pages, 5 figures, 5 tables. Accepted for publication in MNRAS. The
definitive version is available at www.blackwell-synergy.co
Constraints on the star formation histories of galaxies in the Local Cosmological Volume
IB is supported by an Alexander von Humboldt postdoctoral research fellowship. PK acknowledges support from the Grant Agency of the Czech Republic under grant number 20-21855S. This work benefited from the International Space Science Institute (ISSI/ISSI-BJ) in Bern and Beijing, thanks to the funding of the team âChemical abundances in the ISM: the litmus test of stellar IMF variations in galaxies across cosmic timeâ (Donatella Romano and Zhi-Yu Zhang).The majority of galaxies with current star formation rates (SFRs), SFRoâ„10â3Mâyrâ1â , in the Local Cosmological Volume, where observations should be reliable, have the property that their observed SFRo is larger than their average SFR. This is in tension with the evolution of galaxies described by delayed-Ï models, according to which the opposite would be expected. The tension is apparent in that local galaxies imply the star formation time-scale Ï â 6.7 Gyr, much longer than the 3.5â4.5 Gyr obtained using an empirically determined main sequence at several redshifts. Using models where the SFR is a power law in time of the form â(t â t1)η for t1 = 1.8 Gyr (with no stars forming prior to t1) implies that η = 0.18 ± 0.03. This suggested near-constancy of a galaxyâs SFR over time raises non-trivial problems for the evolution and formation time of galaxies, but is broadly consistent with the observed decreasing main sequence with increasing age of the Universe.Publisher PDFPeer reviewe
Small Bites: Star formation recipes in extreme dwarfs
We study the relationship between the gas column density (Sigma_HI) and the
star formation rate surface density (Sigma_SFR) for a sample of extremely small
(M_B ~ -13, Delta V_50 ~ 30 km/s) dwarf irregular galaxies. We find a clear
stochasticity in the relation between the gas column density and star
formation. All gas with Sigma_HI >~ 10 M_sun/pc^2 has some ongoing star
formation, but the fraction of gas with ongoing star formation decreases as the
gas column density decreases, and falls to about 50% at Sigma_HI ~ 3
M_sun/pc^2. Further, even for the most dense gas, the star formation efficiency
is at least a factor of ~ 2 smaller than typical of star forming regions in
spirals. We also find that the ratio of H-alpha emission to FUV emission
increases with increasing gas column density. This is unlikely to be due to
increasing dust extinction because the required dust to gas ratios are too
high. We suggest instead that this correlation arises because massive (i.e.
H-alpha producing) stars are formed preferentially in regions with high gas
density.Comment: 5 pages, 5 figures, 1 table. Accepted for publication in MNRAS
Letters. The definitive version is available at www.blackwell- synergy.co
A Laser System for the Spectroscopy of Highly-Charged Bismuth Ions
We present and characterize a laser system for the spectroscopy on
highly-charged ^209Bi^82+ ions at a wavelength of 243.87 nm. For absolute
frequency stabilization, the laser system is locked to a near-infra-red laser
stabilized to a rubidium transition line using a transfer cavity based locking
scheme. Tuning of the output frequency with high precision is achieved via a
tunable rf offset lock. A sample-and-hold technique gives an extended tuning
range of several THz in the UV. This scheme is universally applicable to the
stabilization of laser systems at wavelengths not directly accessible to atomic
or molecular resonances. We determine the frequency accuracy of the laser
system using Doppler-free absorption spectroscopy of Te_2 vapour at 488 nm.
Scaled to the target wavelength of 244 nm, we achieve a frequency uncertainty
of \sigma_{244nm} = 6.14 MHz (one standard deviation) over six days of
operation.Comment: Contribution to the special issue on "Trapped Ions" in "Applied
Physics B
Effects of the integrated galactic IMF on the chemical evolution of the solar neighbourhood
The initial mass function determines the fraction of stars of different
intial mass born per stellar generation. In this paper, we test the effects of
the integrated galactic initial mass function (IGIMF) on the chemical evolution
of the solar neighbourhood. The IGIMF (Weidner & Kroupa 2005) is computed from
the combination of the stellar intial mass function (IMF), i.e. the mass
function of single star clusters, and the embedded cluster mass function, i.e.
a power law with index beta. By taking into account also the fact that the
maximum achievable stellar mass is a function of the total mass of the cluster,
the IGIMF becomes a time-varying IMF which depends on the star formation rate.
We applied this formalism to a chemical evolution model for the solar
neighbourhood and compared the results obtained by assuming three possible
values for beta with the results obtained by means of a standard, well-tested,
constant IMF. In general, a lower absolute value of beta implies a flatter
IGIMF, hence a larger number of massive stars and larger metal ejection rates.
This translates into higher type Ia and II supernova rates, higher mass
ejection rates from massive stars and a larger amount of gas available for star
formation, coupled with lower present-day stellar mass densities. (abridged) We
also discuss the importance of the present day stellar mass function (PDMF) in
providing a way to disentangle among various assumptions for beta. Our results
indicate that the model adopting the IGIMF computed with beta ~2 should be
considered the best since it allows us to reproduce the observed PDMF and to
account for most of the chemical evolution constraints considered in this work.Comment: 22 pages, 19 figure
Constraints on mass loss and self-enrichment scenarios for the globular clusters of the Fornax dSph
Recently, high-dispersion spectroscopy has demonstrated conclusively that
four of the five globular clusters (GCs) in the Fornax dwarf spheroidal galaxy
are very metal-poor with [Fe/H]<-2. The remaining cluster, Fornax 4, has
[Fe/H]=-1.4. This is in stark contrast to the field star metallicity
distribution which shows a broad peak around [Fe/H]=-1 with only a few percent
of the stars having [Fe/H]<-2. If we only consider stars and clusters with
[Fe/H]<-2 we thus find an extremely high GC specific frequency, SN=400,
implying by far the highest ratio of GCs to field stars known anywhere. We
estimate that about 1/5-1/4 of all stars in the Fornax dSph with [Fe/H]<-2
belong to the four most metal-poor GCs. These GCs could, therefore, at most
have been a factor of 4-5 more massive initially. Yet, the Fornax GCs appear to
share the same anomalous chemical abundance patterns known from Milky Way GCs,
commonly attributed to the presence of multiple stellar generations within the
clusters. The extreme ratio of metal-poor GC- versus field stars in the Fornax
dSph is difficult to reconcile with scenarios for self-enrichment and early
evolution of GCs in which a large fraction (90%-95%) of the first-generation
stars have been lost. It also suggests that the GCs may not have formed as part
of a larger population of now disrupted clusters with an initial power-law mass
distribution. The Fornax dSph may be a rosetta stone for constraining theories
of the formation, self-enrichment and early dynamical evolution of star
clusters.Comment: 4 pages, 2 figures, accepted for A&A Letter
Nutzung von KI-Methoden fĂŒr die Kupplungsentwicklung in automobilen AntriebsstrĂ€ngen = Use of AI methods for clutch development in automotive drivetrains
Im Spannungsfeld steigender Erwartungen an Fahrkomfort und Energieeffizienz stoĂen aktuelle Entwicklungsmethoden der Modellbildung und Optimierung fĂŒr Fahrzeugkupplungen an ihre Grenzen. In diesem Beitrag wird der Einsatz von KI-Methoden fĂŒr die Kupplungsentwicklung untersucht und ein Ăberblick anhand verschiedener Anwendungsbeispiele in aktuellen Forschungsprojekten der Mercedes-Benz AG gegeben. Mittels ĂŒberwachten Lernens und tiefen neuronalen Netzen werden ein Reibungszahlmodell sowie ein Temperaturmodell einer Kupplung mit hoher AbbildungsgĂŒte entwickelt. VerstĂ€rkendes Lernen mit tiefen neuronalen Netzen wird zur Synthese von Regelungen fĂŒr verschiedene Gangwechsel eingesetzt. Fahrzeugmessdaten werden mit Cluster-Algorithmen analysiert, um Handlungsempfehlungen fĂŒr die Applikation des Motorwiederstarts eines hybriden Antriebsstrangs abzuleiten. Mit den gezeigten Methoden steigt das Automatisierungspotential in der Entwicklung und der Aufwand fĂŒr die Ăbernahme komplexer Entwicklungsprozesse auf neue Getriebevarianten sinkt
Survey of H-alpha emission from thirty nearby dwarf galaxies
Measurements of the H-alpha flux from 30 neighboring dwarf galaxies are
presented. After correction for absorption, these fluxes are used to estimate
the star formation rate (SFR). The SFR for 18 of the galaxies according to the
H-alpha emission are compared with estimates of the SFR from FUV magnitudes
obtained with the GALEX telescope. These are in good agreement over the range
log[SFR] = [-3,0]M sun/yr.Comment: 18 pages, 10 figures, 3 table
Recommended from our members
IN-SITU DEFECT DETECTION FOR LASER POWDER BED FUSION WITH ACTIVE LASER THERMOGRAPHY
Defects are still common in metal components built with Additive Manufacturing (AM). Process
monitoring methods for laser powder bed fusion (PBF-LB/M) are used in industry, but
relationships between monitoring data and defect formation are not fully understood yet.
Additionally, defects and deformations may develop with a time delay to the laser energy input.
Thus, currently, the component quality is only determinable after the finished process.
Here, active laser thermography, a nondestructive testing method, is adapted to PBF-LB/M,
using the defocused process laser as heat source. The testing can be performed layer by layer
throughout the manufacturing process. We study our proposed testing method along experiments
carried out on a custom research PBF-LB/M machine using infrared (IR) cameras.
Our work enables a shift from post-process testing of components towards in-situ testing during
the AM process. The actual component quality is evaluated in the process chamber and defects can
be detected between layers.Mechanical Engineerin
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