104 research outputs found
Knowledge tree: Putting discourse into computerâbased learning
Most CBL materials currently in use model only the declarative aspects of the learning process. If such courseware is used without careful planning, this can be dangerous because one of the most fundamental aspects of education is the dialogue that occurs between teachers and the students. Traditionally, this has taken place in informal discussions as well as in formal smallâgroup learning sessions such as the conventional tutorial. However, as the studentâstaff ratio increases, so does the opportunity for this type of personal dialogue decrease. Modern networking technology offers a huge potential to add discourse to CBL, but there are many pedagogical problems involved with the intrinsically ephemeral and anarchic nature both of the Internet and of most conferencing or bulletinâboard systems. In this paper we describe a software system called Knowledge Tree (KT) which we have developed to address some of these issues. KT combines a hierarchical conceptâoriented database functionality with that of a Usenetâstyle bulletin board Using this, a knowledge garden may be developed for any subject area. These each contain a hypermedia database of frequently asked questions, together with answers provided by subject experts. There is provision for interâstudent discussions of problems and issues. When students ask new questions these are automatically emailed to a relevant subject expert (determined by a subjectâspecific concept thesaurus). The answer is then placed in the database which eventually grows to become a valuable teaching resource. KT is disciplineâindependent as the concept thesaurus can be changed to encapsulate any domain of knowledge. We have used it in support of conventional lecture courses, as an important component of a multimedia course, and for general IT support. These examples illustrate the role that this system can play both in basic information provision, and in facilitating the discussion of deep issues
Comparing Galaxy Morphology at Ultraviolet and Optical Wavelengths
We have undertaken an imaging survey of 34 nearby galaxies in far-ultraviolet
(FUV, ~1500A) and optical (UBVRI) passbands to characterize galaxy morphology
as a function of wavelength. This sample, which includes a range of classical
Hubble types from elliptical to irregular with emphasis on spirals at low
inclination angle, provides a valuable database for comparison with images of
high-z galaxies whose FUV light is redshifted into the optical and near-
infrared bands. Ultraviolet data are from the UIT Astro-2 mission. We present
images and surface brightness profiles for each galaxy, and we discuss the
wavelength-dependence of morphology for different Hubble types in the context
of understanding high-z objects. In general, the dominance of young stars in
the FUV produces the patchy appearance of a morphological type later than that
inferred from optical images. Prominent rings and circumnuclear star formation
regions are clearly evident in FUV images of spirals, while bulges, bars, and
old, red stellar disks are faint to invisible at these short wavelengths.
However, the magnitude of the change in apparent morphology ranges from
dramatic in early--type spirals with prominent optical bulges to slight in
late-type spirals and irregulars, in which young stars dominate both the UV and
optical emission. Starburst galaxies with centrally concentrated, symmetric
bursts display an apparent ``E/S0'' structure in the FUV, while starbursts
associated with rings or mergers produce a peculiar morphology. We briefly
discuss the inadequacy of the optically-defined Hubble sequence to describe FUV
galaxy images and estimate morphological k-corrections, and we suggest some
directions for future research with this dataset.Comment: Accepted for publication in the ApJS. 15 pages, 17 JPEG figures, 10
GIF figures. Paper and full resolution figures available at
http://nedwww.ipac.caltech.edu/level5/Kuchinski/frames.htm
The Opacity of Spiral Galaxy Disks VI: Extinction, stellar light and color
In this paper we explore the relation between dust extinction and stellar
light distribution in disks of spiral galaxies. Extinction influences our
dynamical and photometric perception of disks, since it can distort our
measurement of the contribution of the stellar component. To characterize the
total extinction by a foreground disk, Gonzalez et al. (1998) proposed the
``Synthetic Field Method'' (SFM), which uses the calibrated number of distant
galaxies seen through the foreground disk as a direct indication of extinction.
The method is described in Gonzalez et al. (1998) and Holwerda et al. (2005a).
To obtain good statistics, the method was applied to a set of HST/WFPC2 fields
Holwerda et al. (2005b) and radial extinction profiles were derived, based on
these counts. In the present paper, we explore the relation of opacity with
surface brightness or color from 2MASS images, as well as the relation between
the scalelengths for extinction and light in the I band. We find that there is
indeed a relation between the opacity (A_I) and the surface brightness,
particularly at the higher surface brightnesses. No strong relation between
near infrared (H-J, H-K) color and opacity is found. The scalelengths of the
extinction are uncertain for individual galaxies but seem to indicate that the
dust distribution is much more extended than the stellar light. The results
from the distant galaxy counts are also compared to the reddening derived from
the Cepheids light-curves Freedman et al. (2001). The extinction values are
consistent, provided the selection effect against Cepheids with higher values
of A_I is taken into account. The implications from these relations for disk
photometry, M/L conversion and galaxy dynamical modeling are briefly discussed.Comment: 9 pages, 2 tables, 10 figures, accepted by A&
Trapped in the darkness of the night: thermal and energetic constraints of daylight flight in bats
Bats are one of the most successful mammalian groups, even though their foraging activities are restricted to the hours of twilight and night-time. Some studies suggested that bats became nocturnal because of overheating when flying in daylight. This is becauseâin contrast to feathered wings of birdsâdark and naked wing membranes of bats efficiently absorb short-wave solar radiation. We hypothesized that bats face elevated flight costs during daylight flights, since we expected them to alter wing-beat kinematics to reduce heat load by solar radiation. To test this assumption, we measured metabolic rate and body temperature during short flights in the tropical short-tailed fruit bat Carollia perspicillata at night and during the day. Core body temperature of flying bats differed by no more than 2°C between night and daytime flights, whereas mass-specific CO2 production rates were higher by 15 per cent during daytime. We conclude that increased flight costs only render diurnal bat flights profitable when the relative energy gain during daytime is high and risk of predation is low. Ancestral bats possibly have evolved dark-skinned wing membranes to reduce nocturnal predation, but a low degree of reflectance of wing membranes made them also prone to overheating and elevated energy costs during daylight flights. In consequence, bats may have become trapped in the darkness of the night once dark-skinned wing membranes had evolved
Quantitative Morphology of Galaxies Observed in the Ultraviolet
We present a quantitative study of the far-ultraviolet (FUV) and optical
morphology in 32 nearby galaxies and estimate the ``morphological
k-correction'' expected if these objects were observed unevolved at high
redshift. Using the common indices of central concentration (C) and rotational
asymmetry (A) to quantify morphology, we consider independently two phenomena
that give rise to this k-correction. Bandshifting, the decrease in rest-frame
wavelength of light observed through optical filters, is explored by measuring
these indices in several passbands for each galaxy, and it is found to be the
primary driver of changes in C and A. In general, the optical trend found for
decreasing C and increasing A when going to shorter wavelengths extends to the
FUV. However, the patchy nature of recent star-formation in late-type galaxies,
which is accentuated in the FUV, results in poor quantitative correspondence
between morphologies determined in the optical and FUV. We then artificially
redshift our FUV images into the Hubble Deep Field (HDF) filters to simulate
various cosmological distance effects such as surface brightness dimming and
loss of spatial resolution. Hubble types of many galaxies in our sample are not
readily identifiable at redshifts beyond z ~ 1, and the galaxies themselves are
difficult to detect beyond z ~ 3. Only features of the highest surface
brightness remain visible at cosmological distances. Our simulations suggest
that k-corrections alone are indeed capable of producing the peculiar
morphologies observed at high redshift.Comment: accepted to the Astrophysical Journa
Nuclear starburst-driven evolution of the central region in NGC 6764
We study the CO and the radiocontinuum emission in an active galaxy to
analyze the interplay between the central activity and the molecular gas. We
present new high-resolution observations of the CO(1-0) and CO(2-1) emission
lines, and 3.5 cm and 20 cm radio continuum emission in the central region of
the LINER/starburst galaxy NGC 6764. The galaxy has an outflow morphology in
radio continuum, spatially coincident with the CO and H emission, and
centered slightly off the radio continuum peak at the LINER nucleus. The total
molecular gas mass in the center is about 7x10^8 \msun, using a CO luminosity
to total molecular gas conversion factor that is three times lower than the
standard one. CO(1-0) emission is found near the boundaries of the radio
continuum emission cone. The outflow has a projected expansion velocity of 25
km/s relative to the systemic velocity of NGC6764. About 4x 10^6 \msun of
molecular gas is detected in the outflow. The approximate location (~1 kpc) of
the dynamical inner Lindblad resonance has been derived from the rotation
curve. The peak of the CO emission is slightly (< 200 pc) offset from the peak
of the radio continuum.
The molecular gas has most likely been ejected by the stellar winds from the
recent starburst, but the CO line ratios show indication of an interaction with
the AGN. The energy released by the nuclear starburst is sufficient to explain
the observed outflow, even if the data cannot exclude the AGN from being the
major energy source. Comparison of the outflow with hydrodynamical simulations
suggests that the nuclear starburst is 3--7 Myr old and the bubble-like outflow
is still confined and not freely expanding.Comment: Accepted for publication in A&
Obscured Activity: AGN, Quasars, Starbursts and ULIGs observed by the Infrared Space Observatory
Some of the most active galaxies in the Universe are obscured by large
quantities of dust and emit a substantial fraction of their bolometric
luminosity in the infrared. Observations of these infrared luminous galaxies
with the Infrared Space Observatory (ISO) have provided a relatively unabsorbed
view to the sources fuelling this active emission. The improved sensitivity,
spatial resolution and spectroscopic capability of ISO over its predecessor
Infrared Astronomical Satellite (IRAS), has enabled significant advances in the
understanding of the infrared properties of active galaxies. ISO surveyed a
wide range of active galaxies which, in the context of this review, includes
those powered by intense bursts of star-formation as well as those containing a
dominant active galactic nucleus (AGN). Mid infrared imaging resolved for the
first time the dust enshrouded nuclei in many nearby galaxies, while a new era
in infrared spectroscopy was opened by probing a wealth of atomic, ionic and
molecular lines as well as broad band features in the mid and far infrared.
This was particularly useful since it resulted in the understanding of the
power production, excitation and fuelling mechanisms in the nuclei of active
galaxies including the intriguing but so far elusive ultraluminous infrared
galaxies. Detailed studies of various classes of AGN and quasars greatly
improved our understanding of the unification scenario. Far-infrared imaging
and photometry also revealed the presence of a new very cold dust component in
galaxies and furthered our knowledge of the far-infrared properties of faint
starbursts, ULIGs and quasars. We summarise almost nine years of key results
based upon ISO data spanning the full range of luminosity and type of active
galaxies.Comment: Accepted for publication in 'ISO science legacy - a compact review of
ISO major achievements', Space Science Reviews - dedicated ISO issue. To be
published by Springer in 2005. 62 pages (low resolution figures version).
Higher resolution PDFs available from
http://users.physics.uoc.gr/~vassilis/papers/VermaA.pdf or
http://www.iso.vilspa.esa.es/science/SSR/Verma.pd
The Opacity of Spiral Galaxy Disks V: dust opacity, HI distributions and sub-mm emission
The opacity of spiral galaxy disks, from counts of distant galaxies, is
compared to HI column densities. The opacity measurements are calibrated using
the ``Synthetic Field Method'' from Gonzalez et al (1998) and Holwerda et al.
(2005a).
When compared for individual disks, the HI column density and dust opacity do
not seem to be correlated as HI and opacity follow different radial profiles.
To improve statistics, an average radial opacity profile is compared to an
average HI profile. Compared to dust-to-HI estimates from the literature, more
extinction is found in this profile. This difference may be accounted for by an
underestimate of the dust in earlier measurements due to their dependence on
dust temperature. Since the SFM is insensitive to the dust temperature, the
ratio between the SFM opacity and HI could very well be indicative of the true
ratio.
Earlier claims for a radially extended cold dust disk were based on sub-mm
observations. A comparison between sub-mm observations and counts of distant
galaxies is therefore desirable. We present the best current example of such a
comparison, M51, for which the measurements seem to agree. However, this
remains an area where improved counts of distant galaxies, sub-mm observations
and our understanding of dust emissivity are needed.Comment: 8 pages, 5 figures, 1 table, accepted by A&
The Dust Opacity of Star-Forming Galaxies
Presence of dust in galaxies removes half or more of the stellar energy from
the UV-optical budget of the Universe and has profound impact on our
understanding of how galaxies evolve. Measures of opacity in local galaxies are
reviewed together with widely used theoretical and empirical methods for
quantifying its effects. Existing evidence shows that the dust content of
nearby galaxies depends not only on their morphology, but also on their
luminosity and activity level. A digression is devoted to starbursts in view of
their potential relevance for measures of opacity in distant galaxies. Scarcity
of coherent multiwavelength datasets hampers our ability to derive reliable
obscuration estimates in intermediate and high redshift galaxies. This, in
turn, limits the reliability of inferred physical quantities, such as star
formation rates, stellar population ages, galaxy luminosity functions, and
others.Comment: Invited Review for PASP, to appear in December 2001. 84 pages,
including 6 tables and 11 embedded figures; uses AAS Latex macr
The Herschel Virgo Cluster Survey - VIII. The Bright Galaxy Sample
We describe the Herschel Virgo Cluster Survey (HeViCS) and the first data
that cover the complete survey area (four 4 x 4 deg2 regions). We use these
data to measure and compare the global far infrared properties of 78 optically
bright galaxies that are selected at 500 \mum and detected in all five
far-infrared bands. We show that our measurements and calibration are broadly
consistent with previous data obtained by IRAS, ISO, Spitzer and Planck. We use
SPIRE and PACS photometry data to produce 100, 160, 250, 350 and 500 \mum
cluster luminosity distributions. These luminosity distributions are not power
laws, but peaked, with small numbers of both faint and bright galaxies. We
measure a cluster 100-500 micron far-infrared luminosity density of 1.6(7.0)
\pm 0.2 x 10^9 Lsun/Mpc3. This compares to a cluster 0.4-2.5 \mum optical
luminosity density of 5.0(20.0) x 10^9 Lsun/Mpc3, some 3.2(2.9) times larger
than the far-infrared. A typical photon originates from an optical depth of
0.4\pm0.1. Most of our sample galaxies are well fitted by a single modified
blackbody (beta=2), leading to a mean dust mass of log Mdust = 7.31 Msun and
temperature of 20.0 K. We also derive both stellar and atomic hydrogen masses
from which we calculate mean values for the stars:gas(atomic) and gas(atomic):
dust mass ratios of 15.1 and 58.2 respectively. Using our derived dust, atomic
gas and stellar masses we estimate cluster mass densities of 8.6(27.8) x 10^6,
4.6(13.9) x 10^8, 7.8(29.7) x 10^9 Msun/Mpc3, respectively for dust, atomic gas
and stars. These values are higher than those derived for field galaxies by
factors of 39(126), 6(18) and 34(129) respectively. In the above
luminosity/mass densities are given using the whole sample with values in
brackets using just those galaxies that lie between 17 and 23 Mpc. We provide a
data table of flux densities in all the Herschel bands for all 78 bright Virgo
cluster galaxies.Comment: 17 pages, 9 figures, 4 tables, accepted for publication in MNRA
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