Faint companions in the close environment of star-forming dwarf galaxies: possible overlooked starburst triggers?
Using the NASA Extragalactic Database, we have searched the close environment of 98 star– forming dwarf galaxies (SFDGs) from field– and low density environments for companion galaxies. Most of the found companions are dwarf galaxies, previously disregarded in environmental studies of SFDGs. Using a subsample at low redshifts, cz < 2000 kms 1 , i.e. less biased against dwarf companions, we find that 30% of the SFDGs have close companions within a projected ]
Optical and Near Infrared studies of the photometric structure and starburst activity of Blue Compact Dwarf Galaxies
This thesis describes three studies of Blue Compact Dwarf Galaxies (BCDs), focussed on the structure of the stellar components, the star-forming activity, and the environment of such galaxies, as well as the physical background of their morphological variety.The analysis of deep Near Infrared (NIR) image data of a significant sample of BCDs allows to study the evolved stellar low surface brightness (LSB) components of BCDs more precisely than previous studies at visible wavelengths. Azimuthally averaged radial surface brightness profiles (SBPs) show an exponential intensity distribution of the stellar LSB components at large galactocentric radii. This result, along with the derived exponential scale lengths which are systematically smaller than those of dwarf Irregular and dwarf Elliptical galaxies, agrees with previous optical studies. Towards smaller radii, however, the NIR data reveal an inwards-flattening of the SBPs of the stellar LSB components with respect to their outer exponential slopes in more than half of the BCDs under study. Such inwards-flattening exponential SBPs are frequent in dwarf Irregulars and dwarf Ellipticals, but were hitherto largely undiscovered in the stellar hosts of BCDs. The physical origin of such SBPs in dwarf galaxies is to date not understood. Empirical approaches to their systematization and quantitative investigation are discussed, along with the various implications of the discovery of such SBPs in many BCDs for the understanding of such galaxies.Based on the derived structural information on the stellar LSB components and the starburst components, the hypothesis is raised that below a certain threshold density of the stellar LSB component, of the order of 0.4 solar masses per cubic parsec, burst-like star formation does not occur in gas-rich dwarf galaxies. On this hypothesis, the observed relations between the structure of the stellar LSB components of BCDs and their luminosity can be reproduced, as well as the systematic differences between the host structure - luminosity parameter spaces of LSB components of BCDs and other types of dwarf galaxies. These findings are discussed in view of different scenarios of the starburst-driven evolution of dwarf galaxies. The above mentioned stellar LSB density threshold for the occurrence of starburst activity, along with other results, suggests that the stellar mass density strongly influences the star-forming activity of a dwarf galaxy. The derived relations might be explained by a modified Jeans-criterion that includes the influence of a compact stellar background on the Jeans mass.Results of a detailed study of two prototypical examples of the "cometary" morphological subclass of BCDs are interpreted in connection with literature data for such systems. This comparison suggests that cometary BCDs may systematically differ from the more common elliptical BCDs with respect to several physical properties, such as the structure and age of their stellar LSB components. The hypothesis that the development of a cometary morphology may be connected to an early evolutionary stage of BCDs is discussed.A third study focuses on the environment of star-forming dwarf galaxies (SFDGs), which are known to typically reside in regions of low density of luminous galaxies. In the close environment of more than 30 % of the studied SFDGs, the present investigation reveals low-luminosity dwarf companion galaxies which may be frequently undetectable at larger distances due to observational selection effects. These companions, which frequently show hints of physical association with the sample SFDGs, address the question of interaction-induced star-forming activity in SFDGs, similar to earlier surveys for low-mass gaseous companions of SFDGs. A significant influence of the found companions on the star-forming activity of the studied SFDGs is not found.
5.63 and 5.80 MeV levels of Ne20; de-excitation branching
Abstract The ratio Г γ /Г for the 5.63 and 5.80 MeV levels of Ne 20 has been determined to be 0.077 (±0.008) and −4 , respectively, by using the C 12 (C 12 , α)Ne 20 reaction and determining the fraction of the events in which the alpha-particle and the Ne 20 recoil are present in coincidence at the kinematically correct angles. In both cases these results in themselves are shown to imply radiative E1 transition (5.63→1.63 and 5.80→0, respectively) probabilities ⪡5 × 10 −5 Weisskopf units. When taken together with the life-time for the 5.63 MeV level measured by Evans et al . values of Г( E1 ) ≲ 5 × 10 −6 and Г α ≈ 6% of the particle estimate are obtained for this state.