772 research outputs found
Central activity in 60 micron peakers
The authors present charge coupled device (CCD) imaging results of their sample of Infrared Astronomy Satellite (IRAS) galaxies with spectral energy distributions peaking at 60 microns (Vader et al 1988). The results support the author's suggestion that the activity in 60 micron peaking galaxies is centrally concentrated, and represents an early stage of dust-embedded nuclear activity. This activity is probably triggered by a recent interaction/merger event as indicated by their peculiar optical morphologies. The authors propose that 60 micron peakers are the precursors of SO's in the case of amorphous systems, and ellipticals in the case of interacting galaxies
A Search for Low Surface Brightness Structure Around Compact Narrow Emission Line Galaxies
As the most extreme members of the rapidly evolving faint blue galaxy
population at intermediate redshift, the compact narrow emission line galaxies
(CNELGs) are intrinsically luminous (-22 < M_B < -18) with narrow emission
linewidths (30 < \sigma < 125 km/s). Their nature is heavily debated: they may
be low-mass starbursting galaxies that will fade to present-day dwarf galaxies
or bursts of star formation temporarily dominating the flux of more massive
galaxies, possibly related to in situ bulge formation or the formation of cores
of galaxies. We present deep, high-quality (~0.6 - 0.8 arcsec) images with CFHT
of 27 CNELGs. One galaxy shows clear evidence for a tidal tail; the others are
not unambiguously embedded in galactic disks. Approximately 55% of the CNELGS
have sizes consistent with local dwarfs of small-to-intermediate sizes, while
45% have sizes consistent with large dwarfs or disks galaxies. At least 4
CNELGs cannot harbor substantial underlying disk material; they are
low-luminosity galaxies at the present epoch (M_B > -18). Conversely, 15 are
not blue enough to fade to low-luminosity dwarfs (M_B > -15.2). The majority of
the CNELGs are consistent with progenitors of intermediate-luminosity dwarfs
and low-luminosity spiral galaxies with small disks. CNELGs are a heterogeneous
progenitor population with significant fractions (up to 44%) capable of fading
into today's faint dwarfs (M_B > -15.2), while 15 to 85% may only experience an
apparently extremely compact CNELG phase at intermediate redshift but remain
more luminous galaxies at the present epoch.Comment: 16 pages, 14 figures, emulateapj, published in Ap
Radiation exposure by radio-diagnostics in Switzerland: a pilot patient-oriented survey
A patient-oriented survey was conducted in Switzerland with the aim to establishing the distribution of medical X-ray examinations among the population. A sample of 1235 people aged between 15 and 74 y (589 males and 637 females) was contacted. The participants were asked to give the number of medical X-ray examinations performed during the previous 12 months. Three-fourths of the sample did not undergo any medical X-ray examination during a 12 month-period, and 10% of the population underwent more than 1 medical X-ray examination in a year. These results were independent of the geographic region. The age distributions of the participants who did not undergo any medical X-ray examination and those who underwent one medical X-ray examination were similar. On the other hand, the age distribution of the participants who underwent more than one medical X-ray examination was characterised by a higher contribution from the elderl
Starbursts versus Truncated Star Formation in Nearby Clusters of Galaxies
We present long-slit spectroscopy, B and R bandpass imaging, and 21 cm
observations of a sample of early-type galaxies in nearby clusters which are
known to be either in a star-forming phase or to have had star formation which
recently terminated. From the long-slit spectra, obtained with the Blanco 4-m
telescope, we find that emission lines in the star-forming cluster galaxies are
significantly more centrally concentrated than in a sample of field galaxies.
The broadband imaging reveals that two currently star-forming early-type
galaxies in the Pegasus I cluster have blue nuclei, again indicating that
recent star formation has been concentrated. In contrast, the two galaxies for
which star formation has already ended show no central color gradient. The
Pegasus I galaxy with the most evident signs of ongoing star formation
(NGC7648), exhibits signatures of a tidal encounter. Neutral hydrogen
observations of that galaxy with the Arecibo radiotelescope reveal the presence
of ~4 x 10^8 solar masses of HI. Arecibo observations of other current or
recent star-forming early-type galaxies in Pegasus I indicate smaller amounts
of gas in one of them, and only upper limits in others.Comment: to be published in Astronomical Journa
Radial Color Gradients in K+A Galaxies in Distant Clusters of Galaxies
Galaxies in rich clusters with z 0.3 are observed to have a higher
fraction of photometrically blue galaxies than their nearby counterparts. This
raises the important question of what environmental effects can cause the
termination of star formation between z 0.3 and the present. The star
formation may be truncated due to ram-pressure stripping, or the gas in the
disk may be depleted by an episode of star formation caused by some external
perturbation. To help resolve this issue, surface photometry was carried out
for a total of 70 early-type galaxies in the cluster Cl1358+62, at z
0.33, using two-color images from the Hubble Archive. The galaxies were divided
into two categories based on spectroscopic criteria: 24 are type K+A (e.g.,
strong Balmer lines, with no visible emission lines), while the remaining 46
are in the control sample with normal spectra. Radial color profiles were
produced to see if the K+A galaxies show bluer nuclei in relation to their
surrounding disks. Specifically, a linear gradient was fit to the radial color
profile of each galaxy. We find that the K+A galaxies on average tend to have
slightly bluer gradients towards the center than the normals. A
Kolmogorov-Smirnov two-sample test has been applied to the two sets of color
gradients. The result of the test indicates that there is only a 2%
probability that the K+A and normal samples are drawn from the same parent
distribution. There is a possible complication from a trend in the apparent
magnitude vs. color gradient relation, but overall our results favor the
centralized star formation scenario as an important process in the evolution of
galaxies in dense clusters.Comment: 16 pages, 12 figures, accepted for publication in A
Scenario-ontwikkeling voor het natuurbeleid
De Natuurverkenning 2010-2040 is als scenariostudie uitgevoerd om te verkennen welke richtingen het natuurbeleid op termijn zou kunnen inslaan. Via de scenario-onderdelen worden beleidsmakers stap voor stap in het langetermijndenken meegenomen. Dit wordt versterkt doordat de onderdelen het karakter van kwalitatieve verhalen hebben gekregen, die bovendien toegankelijk zijn verbeeld en op onderdelen kwantitatief zijn uitgewerkt. Doordat beleidsmakers zelf in de verkenning hebben geparticipeerd is hun ontvankelijkheid ervoor vergroot. Over: natuurverkenning, scenariomethode, normatieve scenario's en stakeholderparticipati
Use of and irradiation from plain lumbar spine radiography in Switzerland.
Plain lumbar spine radiographic examination (LSRE) is frequently used in medical practice and delivers a high dose of ionising radiation. The objectives of the study were to determine the annual frequency of LSRE in Switzerland and its distribution according to practitioners' and patients' characteristics, as well as the related population dose of ionising radiation.
Data were extrapolated from a nationwide questionnaire survey on radiation exposure resulting from medical imaging in 1998, involving physicians and other healthcare providers performing radiological examinations in Switzerland.
An estimated number of 273,000 LSRE are performed annually in Switzerland (39 LSRE per 1000 inhabitants per year). The collective dose to the population due to LSRE was 1130 Sv (0.16 mSv per person per year). 50-60% of these procedures were performed to confirm or rule out a diagnosis, the majority (85%) in the context of an illness.
LSRE is the third most frequent radiographic procedure performed and delivers the highest population dose of ionising radiation of any radiodiagnostic procedure. Efforts to reduce the frequency and the radiation dose of this procedure must be kept up, technically by optimising the equipment and radioprotection measures, and clinically by implementing evidence-based approaches to appropriate indications for this imaging technique
Line-Strength Indices in Bright Spheroidals: Evidence for a Stellar Population Dichotomy between Spheroidal and Elliptical Galaxies
We present new measurements of central line-strength indices (namely Mg2,
, and Hbeta and gradients for a sample of 6 bright spheroidal galaxies
(Sph's) in the Virgo cluster. Comparison with similar measurements for
elliptical galaxies (E's), galactic globular clusters (GGC's), and stellar
population models yield the following results: (1) In contrast with bright E's,
bright Sph's are consistent with solar abundance [Mg/Fe] ratios; (2) Bright
Sph's exhibit metallicities ranging from values typical for metal-rich GGC's to
those for E's; (3) Although absolute mean ages are quite model dependent, we
find evidence that the stellar populations of some (if not all) Sph's look
significantly younger than GGC's; and (4) Mg2 gradients of bright Sph's are
significantly shallower than those of E galaxies. We conclude that the
dichotomy found in the structural properties of Sph and E galaxies is also
observed in their stellar populations. A tentative interpretation in terms of
differences in star formation histories is suggested.Comment: 14 pages, LaTeX file + 2 PostScript figures, aasms4.sty require
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