On the origin of bursts in blue compact dwarf galaxies: clues from kinematics and stellar populations

Blue compact dwarf galaxies (BCDs) form stars at, for their sizes, extraordinarily high rates. In this paper, we study what triggers this starburst and what is the fate of the galaxy once its gas fuel is exhausted. We select four BCDs with smooth outer regions, indicating them as possible progenitors of dwarf elliptical galaxies. We have obtained photometric and spectroscopic data with the FORS and ISAAC instruments on the VLT. We analyse their infrared spectra using a full spectrum fitting technique, which yields the kinematics of their stars and ionized gas together with their stellar population characteristics. We find that the stellar velocity to velocity dispersion ratio ((nu/sigma)(star)) of our BCDs is of the order of 1.5, similar to that of dwarf elliptical galaxies. Thus, those objects do not require significant (if any) loss of angular momentum to fade into early-type dwarfs. This finding is in discordance with previous studies, which however compared the stellar kinematics of dwarf elliptical galaxies with the gaseous kinematics of star-forming dwarfs. The stellar velocity fields of our objects are very disturbed and the star formation regions are often kinematically decoupled from the rest of the galaxy. These regions can be more or less metal rich with respect to the galactic body and sometimes they are long lived. These characteristics prevent us from pinpointing a unique trigger of the star formation, even within the same galaxy. Gas impacts, mergers, and in-spiraling gas clumps are all possible star formation igniters for our targets

The current and future risk of harmful algal blooms in the North Sea = Het huidig en toekomstig risico voor schadelijke algenbloei in de Noordzee

The growing need to feed the world’s population has led to significant advances in agricultural practices over the last 50 years. The Haber-Bosch process, in particular, enabled an intensification of the global fertilizer use, resulting in higher crop yields across the world. Yet, due to the inefficient incorporation of fertilizers into agricultural products, these nutrients led to significant environmental pollution and eutrophication. Fuelled by nutrient enriched runoff, rivers upset the biogeochemical balance of the marine environment, leading to a global increase in size, frequency and distribution of harmful algal blooms (HABs). During these events, a phytoplankton species is able to proliferate at the expense of others, causing severe harm to the environment through hypoxia, shading, physical disruption and the release of potent toxins. As a result, HABs are a severe threat to marine biodiversity as well as the safety and security of seafood. Now, with climate change looming over the horizon, scientists fear that HABs could become more prevalent in our future oceans. To date, however, there is a lack of experimental evidence that global change will affect HABs. Despite the implications for human health and ecosystem health, the link between eutrophication and HAB development is still not fully understood. Yet, while scientists are discussing the fundamental ecological importance of biological features such as toxicity, mixotrophy and allelopathy, we are slowly missing our opportunity to prevent a major escalation of HABs. For this reason, we urgently need to identify and employ model species in cross-validated, long-term multifactorial studies with co-occurring species to rapidly progress our understanding of HABs, and quantify the impact of HABs on the socioeconomic well-being of our species to persuade policy makers. During this PhD research, we tried to achieve both. Given our lack of knowledge on the occurrence of HABs in our own regional sea, the main objective of this thesis was to assess whether the ongoing changes in the environmental state of the Belgian Part of the North Sea (BPNS) have enhanced the risk of HAB development. In addition to regular sampling campaigns, we identified several knowledge gaps in the available literature that we first had to resolve in order to answer this main question. In particular, we needed to determine whether changes in the total or relative nutrient availability, both effects of eutrophication, affect the competitive traits of potentially harmful dinoflagellates. Next, we needed to verify that climate-change driven temperature increases would not affect these traits either.Lastly, we wondered if toxic dinoflagellates could have unknown mixed toxicity effects – by themselves or in together with marine pathogens - on keystone bivalve species like Mytilus edulis

A tale of two tails:Insights from simulations into the formation of the peculiar dwarf galaxy NGC 1427A

We present a scenario for the formation and the morphology of the arrow-shaped dwarf irregular galaxy NGC 1427A in the Fornax Cluster. This galaxy shows intriguing stellar and gaseous tails pointing in different directions for which alternative but not conclusive formation scenarios have been proposed in the literature. We performed N-body/SPH simulations of dwarf galaxies falling into a model of the Fornax cluster, exhibiting a jellyfish-like appearance while undergoing ram-pressure stripping. We noted that some of our models show interesting tail morphologies similar to that of NGC 1427A. In this way, the peculiar NGC 1427A structure can be studied using models whose stellar and neutral gas photometry and kinematics are in good agreement with the observed ones, without the need of invoking an interaction with a nearby galaxy. Thanks to the tails, we can identify the requirements for a galaxy to expose such a structure and assess the possible position and velocity of the galaxy in the cluster. This puts constraints on the orbit of the galaxy, its position in the cluster and the time since its pericentre passage. From the statistics of identified snapshots following our modelling, we found that the most likely position of the galaxy is around 200 kpc in front of the cluster centre, travelling towards the cluster with a velocity angle with respect to the line-of-sight direction of around 50 deg. This analysis can be useful in future observations of similar galaxies in clusters to characterize their position and velocity in the cluster and their formation

The counter-streaming instability in dwarf ellipticals with off-center nuclei

n many nucleated dwarf elliptical galaxies (dE,N's), the nucleus is offset by a significant fraction of the scale radius with respect to the center of the outer isophotes. Using a high-resolution N-body simulation, we demonstrate that the nucleus can be driven off-center by the m=1 counterstreaming instability, which is strong in flattened stellar systems with zero rotation. The model develops a nuclear offset on the order of 30% of the exponential scale length. We compare our numerical results with the photometry and kinematics of FCC 046, a Fornax Cluster dE,N with a nucleus offset by 1.2" we find good agreement between the model and FCC 046. We also discuss mechanisms that may cause counterrotation in dE,N's and conclude that the destruction of box orbits in an initially triaxial galaxy is the most promising.Comment: 5 pages, 4 figure

Stellar population and kinematics of NGC404

NGC404 is a nearly face-on nearby low-luminosity lenticular galaxy. Probing its characteristics provides a wealth of information on the details of possible evolution processes of dS0 galaxies which may not be possible in other, more distant objects. In order to study its kinematics and star formation history, we obtained long slit spectroscopy at the OHP 1m93 telescope along the major and minor axes of NGC404. The spectra have a resolution R = 3600 covering a wavelength range from 4600 to 5500 A. The data are fitted against the Pegase.HR stellar population models to derive simultaneously the internal stellar kinematics, ages and metallicities. Firstly, the global properties of the galaxy are analyzed by fitting a single model and to the data and looking at the kinematic variations and SSP equivalent age and metallicities as a function of radius. Afterwards, the stellar populations are decomposed into 4 components that are individually analyzed. NGC404 clearly shows two radial velocity inversions along its major axis. The kinematically decoupled core rotates in the same direction as the neutral hydrogen shell that surrounds the galaxy. We resolved the star formation history in the core of the galaxy ino 4 events: A very young (< 150 Myr, and [Fe/H] = 0.4) component with constant on-going star formation, a second young (430 Myr) component with [Fe/H] = 0.1, an intermediate population (1.7 Gyr) which has [Fe/H] = -0.05 and, finally, an old (12 Gyr) component with [Fe/H] = -1.26. The two young components fade very quickly with radius, leaving only the intermediate and old population at a radius of 25" (370 pc) from the centre. We conclude that NGC404 had a spiral morphology about 1 Gyr ago and that one or many merger events has triggered a morphological transition.Comment: 8 pages, 8 figures, accepted for publication in A&

Peanut-shaped bulges in face-on disk galaxies

We present high resolution absorption-line spectroscopy of 3 face-on galaxies, NGC 98, NGC 600, and NGC 1703 with the aim of searching for box/peanut (B/P)-shaped bulges. These observations test and confirm the prediction of Debattista et al. (2005) that face-on B/P-shaped bulges can be recognized by a double minimum in the profile of the fourth-order Gauss-Hermite moment h_4. In NGC 1703, which is an unbarred control galaxy, we found no evidence of a B/P bulge. In NGC 98, a clear double minimum in h_4 is present along the major axis of the bar and before the end of the bar, as predicted. In contrast, in NGC 600, which is also a barred galaxy but lacks a substantial bulge, we do not find a significant B/P shape.Comment: 4 pages, 1 figure. To appear in "Tumbling, twisting, and winding galaxies: Pattern speeds along the Hubble sequence", E. M. Corsini and V. P. Debattista (eds.), Memorie della Societa` Astronomica Italian

VLA Limits for Intermediate Mass Black Holes in Three Globular Clusters

The observational evidence for central black holes in globular clusters has been argued extensively, and their existence has important consequences for both the formation and evolution of the cluster. Most of the evidence comes from dynamical arguments, but the interpretation is difficult, given the short relaxation times and old ages of the clusters. One of the most robust signatures for the existence of a black hole is radio and/or X-ray emission. We observed three globular clusters, NGC6093 (M80), NGC6266 (M62), and NGC7078 (M15), with the VLA in the A and C configuration with a 3-sigma noise of 36, 36 and 25 microJy, respectively. We find no statistically-significant evidence for radio emission from the central region for any of the three clusters. NGC6266 shows a 2-sigma detection. It is difficult to infer a mass from these upper limits due to uncertainty about the central gas density, accretion rate, and accretion model.Comment: 5 pages, accepted for publication in the Astronomical Journa