1,165 research outputs found
Neutral Gas Distributions and Kinematics of Five Blue Compact Dwarf Galaxies
We present the results of high spatial resolution HI observations of five
intrinsically compact dwarf galaxies which are currently experiencing a strong
burst of star formation. The HI maps indicate that these systems have a complex
and clumpy interstellar medium. Unlike typical dwarf irregular galaxies, these
Blue Compact Dwarf (BCD) galaxies exhibit strong central concentrations in
their neutral gas distributions which may provide a clue to the origin of their
strong star-burst activity. Furthermore, while all of the systems do appear to
be rotating, based on observed velocity gradients, the kinematics are complex.
All systems have non-ordered kinematic structure at some level; some of the
extended gas is not necessarily kinematically connected to the main system.
The observed gas distributions and kinematics place constraints on
evolutionary scenarios for BCDs. Evolutionary links between BCDs, dwarf
irregulars, and dwarf ellipticals have been postulated to explain their high
star formation rates and low luminosity, low metallicity nature. The BCDs
appear to have higher central mass concentrations in both gas and stellar
content than the dwarf irregulars, indicating that evolutionary scenarios
connecting these two classes will require mass redistribution. In addition, the
fact that BCDs are rotationally supported systems indicates that BCDs are
unlikely to evolve into dwarf ellipticals without substantial loss of angular
momentum. Thus, while such evolutionary scenarios may still be possible with
the aid of mergers or tidal interactions, the isolated nature of BCDs suggests
that the majority of BCDs will not fade to become objects similar to the
present day dwarf ellipticals.Comment: 19 pages, 15 figures. To appear in A
The structure and dynamics of the AC114 galaxy cluster revisited
We present a dynamical analysis of the galaxy cluster AC114 based on a
catalogue of 524 velocities. Of these, 169 (32%) are newly obtained at ESO
(Chile) with the VLT and the VIMOS spectrograph. Data on individual galaxies
are presented and the accuracy of the measured velocities is discussed.
Dynamical properties of the cluster are derived. We obtain an improved mean
redshift value z= 0.31665 +/- 0.0008 and velocity dispersion \sigma= 1893+73-82
\kms. A large velocity dispersion within the core radius and the shape of the
infall pattern suggests that this part of the cluster is in a radial phase of
relaxation with a very elongated radial filament spanning 12000 \kms. A radial
foreground structure is detected within the central 0.5/h Mpc radius,
recognizable as a redshift group at the same central redshift value. We analyze
the color distribution for this archetype Butcher-Oemler galaxy cluster and
identify the separate red and blue galaxy sequences. The latter subset contains
44% of confirmed members of the cluster, reaching magnitudes as faint as R_{f}=
21.1 (1.0 magnitude fainter than previous studies). We derive a mass M_{200}=
(4.3 \pm 0.7) x 10^15 Msun/h. In a subsequent paper we will utilize the
spectral data presented here to explore the mass-metallicity relation for this
intermediate redshift cluster.Comment: 17 pages, 11 figures, 4 tables, accepted for publication in MNRA
Auslesemethoden für magnetoelektrische Sensoren
The detection of weak magnetic fields has the potential to provide additional, non-redundant information in scientific fields such as medical diagnostics, geomagnetic investigations, data storage, amongst others.
Many substances feature a low permeability and magnetic fields can penetrate them nearly unhindered which yields the possibility to detect signals that originate from within a volume without contact.
Thin-film magnetoelectric sensors are mm-sized magnetometers that transform magnetic fields into a measurable polarisation via a mechanical coupling of a magnetostrictive and a piezoelectric layer.
They do not need to be cooled and their high dynamic range allows them to be operated in unshielded environments.
The output signal of the cantilever-shaped sensors is enhanced at their resonance frequency which can be exploited to increase the signal-to-noise ratio.
This dissertation treats the signal processing for thin-film magnetoelectric sensors from a system point of view.
Four main readout methods are investigated, modelled, and evaluated with the aim to lower the limit of detection:
The direct detection, magnetic frequency conversion, electric frequency conversion, and a completely novel method utilising the sensor as a microwave resonator.
With the focus on the signal-to-noise ratio, the noise sources of the measurement systems are discussed in depth and the dominant noise sources identified.
The ultimate noise limit is given by the thermal-mechanical noise of the sensors.
Acoustic environmental interference can be reduced with a tuning fork assembly that discriminates magnetic and mechanical excitation of two cantilevers clamped face-to-face.
The best limit of detection for thin-film magnetoelectric sensors at 10 Hz is 50 pT/Hz^1/2 achieved with the magnetic frequency conversion leading the way towards measurements of biomagnetic signals.Die Detektion von schwachen magnetischen Feldern hat das Potential in Forschungsfeldern wie der medizinischen Diagnostik, geomagnetischen Untersuchungen, Datenspeicherung, usw. zusätzliche, nicht redundante Informationen verfügbar zu machen.
Viele Stoffe haben eine geringe Permeabilität und sind daher für Magnetfelder nahezu uneingeschränkt durchlässig.
Dadurch entsteht die Möglichkeit tieferliegende Signale kontaktlos zu detektieren.
Dünnfilm magnetoelektrische Sensoren sind millimeter große Magnetometer, welche magnetische Felder über die mechanische Kopplung eines magnetostriktiven und eines piezoelektrischen Materials in eine messbare Polarisation transformieren.
Die Sensoren brauchen nicht gekühlt werden und weisen einen hohen Dynamikbereich auf, wodurch sie in ungeschirmten Umgebungen betrieben werden können.
In der mechanischen Resonanzfrequenz der balkenförmigen Sensoren wird deren Ausgangssignal erhöht, wodurch das Signal-zu-Rausch Verhältnis verbessert wird.
In dieser Dissertation wird die Signalverarbeitung von diesen Sensoren aus Systemsicht behandelt.
Mit dem Ziel das Detektionslimit zu senken werden vier Ausleseverfahren untersucht, modelliert und bewertet: Die direkte Detektion, die magnetische Frequenzumsetzung, die elektrische Frequenzumsetzung und ein komplett neues Verfahren, in dem der Biegebalken als Mikrowellen Resonator verwendet wird.
Mit dem Fokus auf dem Signal-zu-Rausch Verhältnis werden die einzelnen Rauschquellen der Messsysteme diskutiert und die jeweils dominanten Rauschquellen ermittelt.
Das grundlegende Rauschlimit ist das thermisch-mechanische Rauschen des Sensors.
Akustische Umweltstörungen können mit einer Stimmgabelanordnung bestehend aus zwei Einzelsensoren unterdrückt werden, welche magnetische und mechanische Einkopplungen bauartbedingt unterscheiden kann.
Als bestes Detektionslimit für dünnfilm magnetoelektrische Sensoren bei 10 Hz wird 50 pT/Hz^1/2 gemessen
HI Properties of Low Luminosity Star-Forming Galaxies in the KPNO International Spectroscopic Survey
New HI observations are presented for a complete sample of 109 low luminosity
star-forming galaxies taken from the KPNO International Spectroscopic Survey
(KISS), the first CCD-based wide-field objective-prism survey for emission-line
galaxies. This sample consists of all star-forming galaxies with M_B > -18.0
and cz < 11,000 km/s from the first Halpha-selected survey list. Overall, 97
out of 109 galaxies have been detected in HI. We confirm the weak trend of
increasing gas richness with decreasing luminosity found by previous authors.
Gas richness is also shown to be weakly anti-correlated with metallicity. The
dependence of star formation rates (SFRs) and HI gas depletion timescales on
metallicity is examined. The median solar metallicity based SFR and gas
depletion timescale are 0.1639 M_sun/yr and 5 Gyrs, respectively. Corrections
for variations in metallicity decreases SFRs by about 0.5 dex and increases gas
depletion timescales by an average of about 8 Gyrs. The majority of galaxies in
this sample still have large reservoirs of HI gas, and despite their large
current star formation rates, could have formed stars in a quasi-continuous
manner for a Hubble time. Finally, we present the first HI mass function for
low luminosity star-forming galaxies and show that this subpopulation
contributes 10-15% of the overall HI density in the local universe. We conclude
that if the HI mass function of the Universe does indeed have a steeply rising
low-mass slope as suggested by previous authors, it is not due to the
population of low luminosity star-forming galaxies.Comment: 27 pages, 13 figures, 4 tables. Accepted for publication in the
Astronomical Journa
Metal Abundances of KISS Galaxies III. Nebular Abundances for Fourteen Galaxies and the Luminosity-Metallicity Relationship for HII Galaxies
We report results from the third in a series of nebular abundance studies of
emission-line galaxies from the KPNO International Spectroscopic Survey (KISS).
Galaxies with coarse metallicity estimates of 12 + log(O/H) less than 8.2 dex
were selected for observation. Spectra of 14 galaxies, which cover the full
optical region from [OII]3727,3729 to beyond [SII]6717,6731, are presented, and
abundance ratios of N, O, Ne, S, and Ar are computed. The auroral [OIII]4363
line is detected in all 14 galaxies. Oxygen abundances determined through the
direct electron temperature T_e method confirm that the sample is metal-poor
with 7.61 <= 12 + log(O/H) <= 8.32. By using these abundances in conjunction
with other T_e-based measurements from the literature, we demonstrate that HII
galaxies and more quiescent dwarf irregular galaxies follow similar
metallicity-luminosity (L-Z) relationships. The primary difference is a
zero-point shift between the correlations such that HII galaxies are brighter
by an average of 0.8 B magnitudes at a given metallicity. This offset can be
used as evidence to argue that low-luminosity HII galaxies typically undergo
factor of two luminosity enhancements, and starbursts that elevate the
luminosities of their host galaxies by 2 to 3 magnitudes are not as common. We
also demonstrate that the inclusion of interacting galaxies can increase the
scatter in the L-Z relation and may force the observed correlation towards
lower metallicities and/or larger luminosities. This must be taken into account
when attempting to infer metal abundance evolution by comparing local L-Z
relations with ones based on higher redshift samples since the fraction of
interacting galaxies should increase with look-back time.Comment: 36 pages, 5 figures. ApJ, in pres
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