Showing papers by "J. M. van der Hulst published in 2010"

Large-scale HI in nearby radio galaxies (II): the nature of classical low-power radio sources

Abstract: An important aspect of solving the long-standing question as to what triggers various types of Active Galactic Nuclei involves a thorough understanding of the overall properties and formation history of their host galaxies. This is the second in a series of papers that systematically study the large-scale properties of cold neutral hydrogen (HI) gas in nearby radio galaxies. The main goal is to investigate the importance of gas-rich galaxy mergers and interactions among radio-loud AGN. In this paper we present results of a complete sample of classical low-power radio galaxies. We find that extended Fanaroff & Riley type-I radio sources are generally not associated with gas-rich galaxy mergers or ongoing violent interactions, but occur in early-type galaxies without large (> 10^8 M_sun) amounts of extended neutral hydrogen gas. In contrast, enormous discs/rings of HI gas (with sizes up to 190 kpc and masses up to 2 x 10^10 M_sun) are detected around the host galaxies of a significant fraction of the compact radio sources in our sample. This segregation in HI mass with radio source size likely indicates that these compact radio sources are either confined by large amounts of gas in the central region, or that their fuelling is inefficient and different from the fuelling process of classical FR-I radio sources. To first order, the overall HI properties of our complete sample (detection rate, mass and morphology) appear similar to those of radio-quiet early-type galaxies. If confirmed by better statistics, this would imply that low-power radio-AGN activity may be a short and recurrent phase that occurs at some point during the lifetime of many early-type galaxies.

F-TCP: A Delay-Based Protocol with Fair Co-existence

Abstract: Various studies have shown that regular TCP is inefficient in high-speed networks. This paper proposes F-TCP, a delay-based TCP variant, which is able to operate efficiently in high-speed networks. The slowstart phase of F-TCP continues until a threshold determined from probing the available bandwidth. When competing with loss-based flows F-TCP reduces it's window to a value derived from the available bandwidth. More specifically, an adaptive bandwidth share estimation with a delay-sensitive instability measure is employed to guide window backoff when congestion is detected by F-TCP. Using ns-2 simulations we show that F-TCP has good throughput efficiency, intra-protocol fairness and TCP friendliness properties. Our results also illustrate fair co-existence between a delay-based protocol (F-TCP) and a loss-based protocol (regular TCP) thus F-TCP maintains it's fair share of the link. In addition we show that F-TCP avoids self-induced packet losses by using delay as the congestion signal hence zero packet loss is experienced in all the simulations where all the flows are F-TCP. In simulations with regular TCP, packet losses are inevitable since it is loss-based. We therefore conclude that when treated carefully, delay (or RTT) can indeed be a useful congestion signal indicator. Bandwidth estimation is a good indication of available bandwidth for delay-based flows if made aware of the early congestion back off experienced by these flows.

Polycyclic Aromatic Hydrocarbons in Galaxies at z 0.1 : The Effect of Star Formation and AGN

Abstract: We present the analysis of the Polycyclic Aromatic Hydrocarbon (PAH) spectra of a sample of 92 typical star forming galaxies at 0.03

The Cosmically Depressed : Life, Sociology and Identity of Voids

Abstract: In this contribution we review and discuss several aspects of Cosmic Voids, as a background for our void galaxy project (accompanying paper by Stanonik et al.). Voids are a major component of the large scale distribution of matter and galaxies in the Universe. Following a sketch of the general characteristics of void formation and evolution, we describe the influence of the environment on their development and structure and the characteristic hierarchical buildup of the cosmic void population. In order to be able to study the resulting tenuous void substructure and the galaxies populating the interior of voids, we subsequently set out to describe our parameter free tessellation-based watershed void finding technique. It allows us to trace the outline, shape and size of voids in galaxy redshift surveys. The application of this technique enables us to find galaxies in the deepest troughs of the cosmic galaxy distribution, and has formed the basis of our void galaxy program.