436 research outputs found
A relation between circumnuclear HI, dust, and optical cores in low-power radio galaxies
From new observations and literature data we investigate the presence of HI,
dust, and optical cores in the central kiloparsec of low-power radio galaxies.
The goal of this pilot study is to identify physical relations between these
components, which can help us to study kinematics and feeding mechanisms in
future samples of active galaxies. Our results are consistent with neutral gas
being associated with dust on sub-kiloparsec scales. Objects that have HI
absorption always have significant amounts of dust in their host galaxy. If
there is no visible dust in the host galaxy, there is also no HI absorption.
The presence of an unresolved optical core correlates with the HI column
density, with the core being absent in high column density sources. This work
opens a path for studying the kinematics of cold material in the central
regions of active galaxies by combining information of HI absorption and
molecular lines. Consistent with previous work, we find no evidence for a
compact, parsec-scale obscuring torus in low-power radio galaxies.Comment: Accepted for publication in A&
Large-scale HI in nearby radio galaxies: segregation in neutral gas content with radio source size
We present results of a study of neutral hydrogen (HI) in a complete sample
of nearby non-cluster radio galaxies. We find that radio galaxies with large
amounts of extended HI (M_HI >= 10^9 M_solar) all have a compact radio source.
The host galaxies of the more extended radio sources, all of Fanaroff & Riley
type-I, do not contain these amounts of HI. We discuss several possible
explanations for this segregation. The large-scale HI is mainly distributed in
disk- and ring-like structures with sizes up to 190 kpc and masses up to 2 x
10^10 M_solar. The formation of these structures could be related to past
merger events, although in some cases it may also be consistent with a
cold-accretion scenario.Comment: 4 pages, 2 figures. Accepted for publication in A&A Letters. A
version with full resolution figures can be found at
http://www.astro.rug.nl/~emonts/emonts_HIletter_jan07.pd
Coral diversity matches marine park zonation but not economic value of coral reef sites at St. Eustatius, eastern Caribbean
Stony corals play a key role in the marine biodiversity of many tropical coastal areas as suppliers of substrate, food and shelter for other reef organisms. Therefore, it is remarkable that coral diversity usually does not play a role in the planning of protected areas in coral reef areas. In the present study we examine how stony coral diversity patterns relate to marine park zonation and the economic value of reefs around St. Eustatius, a small island in the eastern Caribbean, with fisheries and tourism as important sources of income. The marine park contains two no-take reserves. A biodiversity survey was performed at 39 sites, 24 inside the reserves and 15 outside; 22 had a maximum depth >18 m and 17 were shallower. Data on economic value per site were obtained from the literature. Corals were photographed for the verification of identifications made in the field. Coral species richness (n = 49) was highest in the no-take reserves and species composition was mainly affected by maximum depth. No distinct relation is observed between coral diversity and fishery value or total economic value. Based on the outcome of this study we suggest that in future designs of marine park zonation in reef areas, coral diversity should be taken into consideration. This is best served by including reef areas with a continuous depth gradient from shallow flats to deep slopes
How Dry Are Red Mergers?
The focus of current research in galaxy evolution has increasingly turned to
understanding the effect that mergers have on the evolution of systems on the
red sequence. For those interactions purported to occur dissipationlessly (so
called "dry mergers"), it would appear that the role of gas is minimal.
However, if these mergers are not completely dry, then even low levels of gas
may be detectable. The purpose of our study is to test whether early type
galaxies with HI in or around them, or "wet" ellipticals, would have been
selected as dry mergers by the criteria in van Dokkum (2005, AJ, 130, 2647). To
that end, we examine a sample of 20 early types from the HI Rogues Gallery with
neutral hydrogen in their immediate environs. Of these, the 15 brightest and
reddest galaxies match the optical dry merger criteria, but in each case, the
presence of HI means that they are not truly dry.Comment: 8 pages plus 1 table and 5 figures; accepted for publication in A
The Mass-to-Light Ratio of Binary Galaxies
We report on the mass-to-light ratio determination based on a newly selected
binary galaxy sample, which includes a large number of pairs whose separations
exceed a few hundred kpc. The probability distributions of the projected
separation and the velocity difference have been calculated considering the
contamination of optical pairs, and the mass-to-light ratio has been determined
based on the maximum likelihood method. The best estimate of in the B
band for 57 pairs is found to be 28 36 depending on the orbital
parameters and the distribution of optical pairs (solar unit, km
s Mpc). The best estimate of for 30 pure spiral pairs is
found to be 12 16. These results are relatively smaller than those
obtained in previous studies, but consistent with each other within the errors.
Although the number of pairs with large separation is significantly increased
compared to previous samples, does not show any tendency of increase, but
found to be almost independent of the separation of pairs beyond 100 kpc. The
constancy of beyond 100 kpc may indicate that the typical halo size of
spiral galaxies is less than kpc.Comment: 18 pages + 8 figures, to appear in ApJ Vol. 516 (May 10
Senior Design Final Report: Destination Distillation
The goal of this project is to retrofit the distillation column owned by the Trinity Engineering Department in order to make it safe and easy to use in the learning environment. A final design has been agreed upon, parts have been ordered, installed and tested, and the final design is complete and determined to be working as it should
Neutral Hydrogen in the Interacting Magellanic Spirals NGC 4618/4625
Asymmetry is a common trait in spiral galaxies and is particularly frequent
among Magellanic spirals. To explore how morphological and kinematic asymmetry
are affected by companion galaxies, we analyze neutral hydrogen observations of
the interacting Magellanic spirals NGC 4618 and 4625. The analysis of the H I
distribution revealed that about 10% of the total H I mass of NGC 4618 resides
in a looping tidal structure that appears to wrap all the way around the
galaxy. Through calculations based on derived H I profiles, we show that NGC
4618 and 4625 are no more asymmetric than non-interacting Magellanic spirals
analyzed by Wilcots & Prescott (2004). We also derive rotation curves for the
approaching and receding sides of each galaxy. By fitting the mean curves with
an isothermal halo model, we calculate dynamical masses of 4.7 times 10^{9}
M_sun and 9.8 times 10^{9} M_sun out to 6.7 kpc, for NGC 4618 and 4625
respectively. While the rotation curves had systematically higher velocities on
the receding side of each galaxy, the effect was no more pronounced than in
studies of non-interacting spirals (Swaters et. al. 1999). The degree of
interaction-driven asymmetry in both galaxies is indistinguishable from the
intrinsic degree of asymmetry of lopsided galaxies.Comment: 20 pages, 11 figures, accepted for publication in the Astronomical
Journa
Performance Modelling and Evaluation of Enterprise Information Security Technologies
The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.By providing effective access control mechanisms, enterprise information security technologies have been proven successful in protecting the confidentiality of sensitive information in business organizations. However, such security mechanisms typically reduce the work productivity of the staff, by making them spend time working on non-project related tasks. Therefore, organizations have to invest a signification amount of capital in the information security technologies, and then to continue incurring additional costs. In this study, we investigate the performance of administrators in an information help desk, and the non-productive time (NPT) in an organization, resulting from the implementation of information security technologies. An approximate analytical solution is discussed first, and the loss of staff member productivity is quantified using non-productive time. Stochastic Petri nets are then used to provide simulation results. The presented study can help information security managers to make investment decisions, and to take actions toward reducing the cost of information security technologies, so that a balance is kept between information security expense, resource drain and effectiveness of security technologies
The Westerbork HI Survey of spiral and irregular galaxies III: HI observations of early-type disk galaxies
We present HI observations of 68 early-type disk galaxies from the WHISP
survey. They have morphological types between S0 and Sab and absolute B-band
magnitudes between -14 and -22. These galaxies form the massive, high
surface-brightness extreme of the disk galaxy population, few of which have
been imaged in HI before.
The HI properties of the galaxies in our sample span a large range; the
average values of M_HI/L_B and D_HI/D_25 are comparable to the ones found in
later-type spirals, but the dispersions around the mean are larger. No
significant differences are found between the S0/S0a and the Sa/Sab galaxies.
Our early-type disk galaxies follow the same HI mass-diameter relation as
later-type spiral galaxies, but their effective HI surface densities are
slightly lower than those found in later-type systems.
In some galaxies, distinct rings of HI emission coincide with regions of
enhanced star formation, even though the average gas densities are far below
the threshold of star formation derived by Kennicutt (1989). Apparently,
additional mechanisms, as yet unknown, regulate star formation at low surface
densities.
Many of the galaxies in our sample have lopsided gas morphologies; in most
cases this can be linked to recent or ongoing interactions or merger events.
Asymmetries are rare in quiescent galaxies. Kinematic lopsidedness is rare,
both in interacting and isolated systems.
In the appendix, we present an atlas of the HI observations: for all galaxies
we show HI surface density maps, global profiles, velocity fields and radial
surface density profiles.Comment: 24 pages, 11 figures. Accepted for publication in A&A. A version with
the full atlas can be downloaded from
http://www.astro.rug.nl/~edo/WHISPIII.ps.gz (gzipped postscript, 9.3Mb
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