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