498 research outputs found
Spectral Mapping Reconstruction of Extended Sources
Three dimensional spectroscopy of extended sources is typically performed
with dedicated integral field spectrographs. We describe a method of
reconstructing full spectral cubes, with two spatial and one spectral
dimension, from rastered spectral mapping observations employing a single slit
in a traditional slit spectrograph. When the background and image
characteristics are stable, as is often achieved in space, the use of
traditional long slits for integral field spectroscopy can substantially reduce
instrument complexity over dedicated integral field designs, without loss of
mapping efficiency -- particularly compelling when a long slit mode for single
unresolved source followup is separately required. We detail a custom
flux-conserving cube reconstruction algorithm, discuss issues of extended
source flux calibration, and describe CUBISM, a tool which implements these
methods for spectral maps obtained with ther Spitzer Space Telescope's Infrared
Spectrograph.Comment: 11 pages, 8 figures, accepted by PAS
Development of integrated mode reformatting components for diffraction-limited spectroscopy
We present the results of our work on developing fully integrated devices (photonic dicers) for reformatting multimode light to a diffraction limited pseudo-slit. These devices can be used to couple a seeing limited telescope point spread function to a spectrograph operating at the diffraction limit, thus potentially enabling compact, high-resolution spectrographs that are free of modal noise
The B3-VLA CSS sample. VIII: New optical identifications from the Sloan Digital Sky Survey. The ultraviolet-optical spectral energy distribution of the young radio sources
Compact steep-spectrum radio sources and giga-hertz peaked spectrum radio
sources (CSS/GPS) are generally considered to be mostly young radio sources. In
recent years we studied at many wavelengths a sample of these objects selected
from the B3-VLA catalog: the B3-VLA CSS sample. Only ~ 60 % of the sources were
optically identified. We aim to increase the number of optical identifications
and study the properties of the host galaxies of young radio sources. We
cross-correlated the CSS B3-VLA sample with the Sloan Digital Sky Survey
(SDSS), DR7, and complemented the SDSS photometry with available GALEX (DR 4/5
and 6) and near-IR data from UKIRT and 2MASS. We obtained new identifications
and photometric redshifts for eight faint galaxies and for one quasar and two
quasar candidates. Overall we have 27 galaxies with SDSS photometry in five
bands, for which we derived the ultraviolet-optical spectral energy
distribution (UV-O-SED). We extended our investigation to additional CSS/GPS
selected from the literature. Most of the galaxies show an excess of
ultra-violet (UV) radiation compared with the UV-O-SED of local radio-quiet
ellipticals. We found a strong dependence of the UV excess on redshift and
analyzed it assuming that it is generated either from the nucleus (hidden
quasar) or from a young stellar population (YSP). We also compare the UV-O-SEDs
of our CSS/GPS sources with those of a selection of large size (LSO) powerful
radio sources from the literature. If the major process of the UV excess is
caused by a YSP, our conclusion is that it is the result of the merger process
that also triggered the onset of the radio source with some time delay. We do
not see evidence for a major contribution from a YSP triggered by the radio
sources itself.Comment: 38 pages, 21 figures, 6 tables. Accepted for publication on A&
Moderating Readers and Reading Online
Despite the proliferation of online forums for the discussion of literary texts, very little has been written to date on the management of these spaces and how this helps frame the kinds of discussion and interpretative work that take place. This article draws on a series of interviews with moderators of online book-related sites, alongside close analysis of online interactions between moderators and users to consider issues of authority, hierarchy, power and control, asking how these act to structure or facilitate acts of interpretation taking place online. We begin by outlining the moderator's role before conducting a brief review of existing scholarship on offline reading groups and online communities, to identify how social infrastructures are established and negotiated. The main body of the article draws upon interviews with moderators of two online literary forums – The Republic of Pemberley and The Guardian’s online Reading Group – to explore the ways in which each of the respective moderators frames his or her role. This is accompanied by an in-depth exploration of how the forms of interpretation we find on the two sites are shaped and directed by the moderators. The article concludes by reflecting upon some of the issues raised by this study and its methodology, particularly with regards to digital dualism and the blurring of the boundaries between the public and the private in online spaces
PKS 0347+05: a radio-loud/radio-quiet double active galactic nucleus system triggered in a major galaxy merger
We present optical, infrared and radio observations of the powerful FRII
radio source PKS0347+05 (z=0.3390), and demonstrate that it is a rare example
of a radio-loud/radio-quiet double AGN system, comprising a weak line radio
galaxy (WLRG) separated by 25 kpc (in projection) from a Seyfert 1 nucleus at
the same redshift. Our deep Gemini optical images show a highly disturbed
morphology, with a warped dust lane crossing through the halo and nuclear
regions of the radio galaxy host, tidal tails, and a bridge connecting the
radio galaxy to the Seyfert 1 nucleus. Spectral synthesis modelling of our
Gemini optical spectrum of the radio galaxy shows evidence for a reddened young
stellar population of age <100 Myr. Further evidence for recent star formation
activity in this source is provided by the detection of strong PAH features in
mid-IR Spitzer/IRS spectra. Together, these observations support a model in
which both AGN have been triggered simultaneously in a major galaxy merger.
However, despite the presence of a powerful FRII radio source, and the
apparently plentiful supply of fuel provided by the merger, the nucleus of the
radio galaxy shows only weak, low ionization emission line activity. We
speculate that the fuel supply to nuclear regions of the radio galaxy has
recently switched off (within the last ~10^6 yr), but the information about the
resulting decrease in nuclear AGN activity has yet to reach the extended lobes
and hotspots of the FRII radio source. Based on this scenario, we estimate that
powerful, intermediate redshift FRII radio sources have lifetimes of t\sim
5x10^6 yr. Overall, our observations emphasise that the fuelling of AGN
activity in major galaxy mergers is likely to be highly intermittent.Comment: 12 pages, 10 figures, accepted for publication in MNRA
The History of Galaxy Formation in Groups: An Observational Perspective
We present a pedagogical review on the formation and evolution of galaxies in
groups, utilizing observational information from the Local Group to galaxies at
z~6. The majority of galaxies in the nearby universe are found in groups, and
galaxies at all redshifts up to z~6 tend to cluster on the scale of nearby
groups (~1 Mpc). This suggests that the group environment may play a role in
the formation of most galaxies. The Local Group, and other nearby groups,
display a diversity in star formation and morphological properties that puts
limits on how, and when, galaxies in groups formed. Effects that depend on an
intragroup medium, such as ram-pressure and strangulation, are likely not major
mechanisms driving group galaxy evolution. Simple dynamical friction arguments
however show that galaxy mergers should be common, and a dominant process for
driving evolution. While mergers between L_* galaxies are observed to be rare
at z < 1, they are much more common at earlier times. This is due to the
increased density of the universe, and to the fact that high mass galaxies are
highly clustered on the scale of groups. We furthermore discus why the local
number density environment of galaxies strongly correlates with galaxy
properties, and why the group environment may be the preferred method for
establishing the relationship between properties of galaxies and their local
density.Comment: Invited review, 16 pages, to be published in ESO Astrophysics
Symposia: "Groups of Galaxies in the Nearby Universe", eds. I. Saviane, V.
Ivanov, J. Borissov
Infrared Spectral Energy Distributions of Seyfert Galaxies: Spitzer Space Telescope Observations of the 12 micron Sample of Active Galaxies
The mid-far-infrared spectral energy distributions (SEDs) of 83 active
galaxies, mostly Seyfert galaxies, selected from the extended 12 micron sample
are presented. The data were collected using all three instruments, IRAC, IRS,
and MIPS, aboard the Spitzer Space Telescope. The IRS data were obtained in
spectral mapping mode, and the photometric data from IRAC and IRS were
extracted from matched, 20 arcsec diameter circular apertures. The MIPS data
were obtained in SED mode, providing very low resolution spectroscopy (R ~ 20)
between ~ 55 and 90 microns in a larger, 20 by 30 arcsec synthetic aperture. We
further present the data from a spectral decomposition of the SEDs, including
equivalent widths and fluxes of key emission lines; silicate 10 and 18 micron
emission and absorption strengths; IRAC magnitudes; and mid-far infrared
spectral indices. Finally, we examine the SEDs averaged within optical
classifications of activity. We find that the infrared SEDs of Seyfert 1s and
Seyfert 2s with hidden broad line regions (HBLR, as revealed by
spectropolarimetry or other technique) are qualitatively similar, except that
Seyfert 1s show silicate emission and HBLR Seyfert 2s show silicate absorption.
The infrared SEDs of other classes with the 12 micron sample, including Seyfert
1.8-1.9, non-HBLR Seyfert 2 (not yet shown to hide a type 1 nucleus), LINER and
HII galaxies, appear to be dominated by star-formation, as evidenced by blue
IRAC colors, strong PAH emission, and strong far-infrared continuum emission,
measured relative to mid-infrared continuum emission.Comment: 78 pages, 13 figure
Astronomical Spectroscopy
Spectroscopy is one of the most important tools that an astronomer has for
studying the universe. This chapter begins by discussing the basics, including
the different types of optical spectrographs, with extension to the ultraviolet
and the near-infrared. Emphasis is given to the fundamentals of how
spectrographs are used, and the trade-offs involved in designing an
observational experiment. It then covers observing and reduction techniques,
noting that some of the standard practices of flat-fielding often actually
degrade the quality of the data rather than improve it. Although the focus is
on point sources, spatially resolved spectroscopy of extended sources is also
briefly discussed. Discussion of differential extinction, the impact of
crowding, multi-object techniques, optimal extractions, flat-fielding
considerations, and determining radial velocities and velocity dispersions
provide the spectroscopist with the fundamentals needed to obtain the best
data. Finally the chapter combines the previous material by providing some
examples of real-life observing experiences with several typical instruments.Comment: An abridged version of a chapter to appear in Planets, Stars and
Stellar Systems, to be published in 2011 by Springer. Slightly revise
Remittances and land change: A systematic review
Remittances—funds sent by migrants to family and friends back home—are an important source of global monetary flows, and they have implications for the maintenance and transformation of land systems. A number of published reviews have synthesized work on a variety of aspects of remittances (e.g., rural livelihoods, disasters, and economic development). To our knowledge, there are no reviews of work investigating the linkages between remittances and land change, broadly understood. This knowledge gap is important to address because researchers have recognized that remittances flows are a mechanism that helps to explain how migration can affect land change. Thus, understanding the specific roles remittances play in land system changes should help to clarify the multiple processes associated with migration and their independent and interactive effects. To address the state of knowledge about the connection between remittances and land systems, this paper conducts a systematic review. Our review of 51 journal articles finds that the linkages uncovered were commonly subtle and/or indirect. Very few studies looked at the direct connections between receipt of remittances and quantitative changes in land. Most commonly, the relationship between remittances and land change was found to occur through pathways from labor migration to household income to agricultural development and productivity. We find four non-exclusive pathways through which households spend remittances with consequent changes to land systems: (1) agricultural crops and livestock, (2) agricultural labor and technologies, (3) land purchases, and (4) non-agricultural purchases and consumables. In the papers reviewed, these expenditures are linked to various land system change outcomes, including land use change, soil degradation, pasture degradation, afforestation/deforestation/degradation, agricultural intensification/extensification/diversification, and no impact. These findings suggest four avenues for future research. One avenue is the use of the theoretical lens of telecoupling to understand how remittances may produce wider-scale changes in land systems. A second avenue is further examination of the impacts of shocks and disturbances to remittance flows on land change both in migrant sending and in remittance receiving areas. A third avenue is scholarship that examines the extent that household uses of remittances have a “ripple effect” on land uses in nearby interlinked systems. A fourth avenue for future work is the use of spatially explicit modeling that leverages land cover and land use data based on imagery and other geospatial information
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