309 research outputs found
High-Mg tholeiitic rocks and their significance in the Karroo Central Province
Averages for the composition of dolerites from the Southern and Eastern Cape, and the correlative basaltic lavas of the Stormberg, are presented for major elements and 8 of the more significant trace elements. The remarkable correspondence between these averages is indicative of the uniformity in composition of the magma emplaced over a very large area
The Karoo igneous province: an introduction
The Karoo rocks are amongst the earliest manifestations of igneous activity accompanying the break-up of Gondwanaland during the early Mesozoic. Remnants of the once extensive lava sequence and the associated dolerite sills are found throughout southern Africa south of latitude 150S, the present outcrop being ca. 140,000 km². The main peak of volcanic activity is dated at about 190 m.y. and is probably coincident with the earliest stages of the opening of the Indian Ocean by the separation of Antarctica from southern Africa
The Herschel Exploitation of Local Galaxy Andromeda (HELGA). VI. The distribution and properties of molecular cloud associations in M31
In this paper we present a catalog of Giant Molecular Clouds (GMCs) in the An- dromeda (M31) galaxy extracted from the Herschel Exploitation of Local Galaxy An- dromeda (HELGA) dataset. GMCs are identified from the Herschel maps using a hierarchical source extraction algorithm. We present the results of this new catalog and characterise the spatial distribution and spectral energy properties of its clouds based on the radial dust/gas properties found by Smith et al (2012). 326 GMCs in the mass range 104 − 107 M⊙ are identified, their cumulative mass distribution is found to be proportional to M −2.34 in agreement with earlier studies. The GMCs appear to follow the same cloud mass to LCO correlation observed in the Milky Way. However, comparison between this catalog and interferometry studies also shows that the GMCs are substructured below the Herschel resolution limit suggesting that we are observing associations of GMCs. Following Gordon et al. (2006), we study the spatial structure of M31 by splitting the observed structure into a set of spiral arms and offset rings. We fit radii of 10.3 and 15.5 kpc to the two most prominent rings. We then fit a logarithmic spiral with a pitch angle of 8fdg9 to the GMCs not associated with either ring. Last, we comment on the effects of deprojection on our results and investigate the effect different models for M31's inclination will have on the projection of an unperturbed spiral arm system
Some geochemical constraints upon models for the crystallization of the upper critical zone-main zone interval, northwestern Bushveld complex
Ratios between elements Mg, Fe, Co, Cr, Ni, V, and Sc are consistently different in mafic rocks of the upper critical zone, and those above the Bastard unit. Within the 300 m section above the Merensky Reef, 87Sr/86Sr ratios increase from c.0.7063 to c.0.7087, irrespective of rock type. Decoupling of Mg/(Mg + Fe2+) ratios and the Ca contents of plagioclase, and wide variations in the proportions of anorthosite within the Bastard, Merensky, and Merensky Footwall units, are inconsistent with anorthosite formation by simple fractional crystallization of magma batches of limited volume
Deep Westerbork 1.4 GHz Imaging of the Bootes Field
We present the results from our deep (16x12 hour) Westerbork Synthesis Radio
Telescope (WSRT) observations of the approximately 7 square degree Bootes Deep
Field, centered at 14h32m05.75s, 34d16'47.5" (J2000). Our survey consists of 42
discrete pointings, with enough overlap to ensure a uniform sensitivity across
the entire field, with a limiting sensitivity of 28 microJy (1 sigma rms). The
catalog contains 3172 distinct sources, of which 316 are resolved by the
13"x27" beam. The Bootes field is part of the optical/near-infrared imaging and
spectroscopy survey effort conducted at various institutions. The combination
of these data sets, and the deep nature of the radio observations will allow
unique studies of a large range of topics including the redshift evolution of
the luminosity function of radio sources, the K-z relation and the clustering
environment of radio galaxies, the radio / far-infrared correlation for distant
starbursts, and the nature of obscured radio loud AGN.Comment: Accepted for publication in AJ, 23 pages, 17 figures. Additional
information can be obtained through anonymous ftp to
ftp://ftp.nfra.nl/pub/Boote
PACS photometry of the Herschel Reference Survey - Far-infrared/sub-millimeter colours as tracers of dust properties in nearby galaxies
We present Herschel/PACS 100 and 160 micron integrated photometry for the 323
galaxies in the Herschel Reference Survey (HRS), a K-band-, volume-limited
sample of galaxies in the local Universe. Once combined with the Herschel/SPIRE
observations already available, these data make the HRS the largest
representative sample of nearby galaxies with homogeneous coverage across the
100-500 micron wavelength range. In this paper, we take advantage of this
unique dataset to investigate the properties and shape of the
far-infrared/sub-millimeter spectral energy distribution in nearby galaxies. We
show that, in the stellar mass range covered by the HRS (8<log(M*/Msun)<12),
the far-infrared/sub-millimeter colours are inconsistent with a single modified
black-body having the same dust emissivity index beta for all galaxies. In
particular, either beta decreases, or multiple temperature components are
needed, when moving from metal-rich/gas-poor to metal-poor/gas-rich galaxies.
We thus investigate how the dust temperature and mass obtained from a single
modified black-body depend on the assumptions made on beta. We show that, while
the correlations between dust temperature, galaxy structure and star formation
rate are strongly model dependent, the dust mass scaling relations are much
more reliable, and variations of beta only change the strength of the observed
trends.Comment: 26 pages, 8 figures, 2 tables. Accepted for publication in MNRA
The Herschel Exploitation of Local Galaxy Andromeda (HELGA). I: Global far-infrared and sub-mm morphology
We have obtained Herschel images at five wavelengths from 100 to 500 micron
of a ~5.5x2.5 degree area centred on the local galaxy M31 (Andromeda), our
nearest neighbour spiral galaxy, as part of the Herschel guaranteed time
project "HELGA". The main goals of HELGA are to study the characteristics of
the extended dust emission, focusing on larger scales than studied in previous
observations of Andromeda at an increased spatial resolution, and the obscured
star formation. In this paper we present data reduction and Herschel maps, and
provide a description of the far-infrared morphology, comparing it with
features seen at other wavelengths. We use high--resolution maps of the atomic
hydrogen, fully covering our fields, to identify dust emission features that
can be associated to M31 with confidence, distinguishing them from emission
coming from the foreground Galactic cirrus. Thanks to the very large extension
of our maps we detect, for the first time at far-infrared wavelengths, three
arc-like structures extending out to ~21, ~26 and ~31 kpc respectively, in the
south-western part of M31. The presence of these features, hosting ~2.2e6 Msol
of dust, is safely confirmed by their detection in HI maps. Overall, we
estimate a total dust mass of ~5.8e7 Msol, about 78% of which is contained in
the two main ring-like structures at 10 and 15 kpc, at an average temperature
of 16.5 K. We find that the gas-to-dust ratio declines exponentially as a
function of the galacto-centric distance, in agreement with the known
metallicity gradient, with values ranging from 66 in the nucleus to ~275 in the
outermost region. [Abridged]Comment: 15 Pages, 9 Figures. Accepted for publication in Astronomy and
Astrophysics. A high resolution version of the paper can be found at
http://wazn.ugent.be/jfritz/HelgaI_final.pd
Mid-infrared spectral evidence for a luminous dust enshrouded source in Arp220
We have re-analyzed the 6-12 micron ISO spectrum of the ultra-luminous
infrared galaxy Arp220 with the conclusion that it is not consistent with that
of a scaled up version of a typical starburst. Instead, both template fitting
with spectra of the galaxies NGC4418 and M83 and with dust models suggest that
it is best represented by combinations of a typical starburst component,
exhibiting PAH emission features, and a heavily absorbed dust continuum which
contributes ~40% of the 6-12 micron flux and likely dominates the luminosity.
Of particular significance relative to previous studies of Arp220 is the fact
that the emission feature at 7.7 micron comprises both PAH emission and a
broader component resulting from ice and silicate absorption against a heavily
absorbed continuum. Extinction to the PAH emitting source, however, appears to
be relatively low. We tentatively associate the PAH emitting and heavily
dust/ice absorbed components with the diffuse emission region and the two
compact nuclei respectively identified by Soifer et al. (2002) in their higher
spatial resolution 10 micron study. Both the similarity of the absorbed
continuum with that of the embedded Galactic protostars and results of the dust
models imply that the embedded source(s) in Arp220 could be powered by, albeit
extremely dense, starburst activity. Due to the high extinction, it is not
possible with the available data to exclude that AGN(s) also contribute some or
all of the observed luminosity. In this case, however, the upper limit measured
for its hard X-ray emission would require Arp220 to be the most highly obscured
AGN known.Comment: 11 pages, 9 figures. Accepted for publication in A&A. Also available
at http://www.astro.rug.nl/~spoon/publications.htm
Detection of an ultra-bright submillimeter galaxy in the Subaru/XMM-Newton Deep Field using AzTEC/ASTE
We report the detection of an extremely bright (37 mJy at 1100 m
and 91 mJy at 880 m) submillimeter galaxy (SMG),
AzTEC-ASTE-SXDF1100.001 (hereafter referred to as SXDF1100.001 or Orochi),
discovered in 1100 m observations of the Subaru/XMM-Newton Deep Field
using AzTEC on ASTE. Subsequent CARMA 1300 m and SMA 880 m
observations successfully pinpoint the location of Orochi and suggest that it
has two components, one extended (FWHM of 4) and one
compact (unresolved). Z-Spec on CSO has also been used to obtain a wide band
spectrum from 190 to 308 GHz, although no significant emission/absorption lines
are found. The derived upper limit to the line-to-continuum flux ratio is
0.1--0.3 (2 ) across the Z-Spec band.
Based on the analysis of the derived spectral energy distribution from
optical to radio wavelengths of possible counterparts near the SMA/CARMA peak
position, we suggest that Orochi is a lensed, optically dark SMG lying at behind a foreground, optically visible (but red) galaxy at . The deduced apparent (i.e., no correction for magnification) infrared
luminosity () and star formation rate (SFR) are
and 11000 yr, respectively, assuming that the
is dominated by star formation. These values suggest that Orochi
will consume its gas reservoir within a short time scale (
yr), which is indeed comparable to those in extreme starbursts like the centres
of local ULIRGs.Comment: 18 pages, 13 figure
Multiwavelength characterization of faint ultra steep spectrum radio sources: a search for high-redshift radio galaxies
Context. Ultra steep spectrum (USS) radio sources are one of the efficient tracers of powerful high-z radio galaxies (HzRGs). In contrast to searches for powerful HzRGs from radio surveys of moderate depths, fainter USS samples derived from deeper radio surveys can be useful in finding HzRGs at even higher redshifts and in unveiling a population of obscured weaker radio-loud AGN at moderate redshifts.
Aims. Using our 325 MHz GMRT observations (5σ ∼ 800 μJy) and 1.4 GHz VLA observations (5σ ∼ 80−100 μJy) available in two subfields (VLA-VIMOS VLT Deep Survey (VLA-VVDS) and Subaru X-ray Deep Field (SXDF)) of the XMM-LSS field, we derive a large sample of 160 faint USS radio sources and characterize their nature.
Methods. The optical and IR counterparts of our USS sample sources are searched using existing deep surveys, at respective wavelengths. We attempt to unveil the nature of our faint USS sources using diagnostic techniques based on mid-IR colors, flux ratios of radio to mid-IR, and radio luminosities. Results. Redshift estimates are available for 86/116 (∼74%) USS sources in the VLA-VVDS field and for 39/44 (∼87%) USS sources in the SXDF fields with median values (zmedian) ∼1.18 and ∼1.57, respectively, which are higher than estimates for non-USS radio sources (zmedian non−USS ∼ 0.99 and ∼0.96), in the two subfields. The MIR color–color diagnostic and radio luminosities are consistent with most of our USS sample sources at higher redshifts (z > 0.5) being AGN. The flux ratio of radio to mid-IR (S 1.4 GHz/S 3.6 μm) versus redshift diagnostic plot suggests that more than half of our USS sample sources distributed over z ∼ 0.5 to 3.8 are likely to be hosted in obscured environments. A significant fraction (∼26% in the VLA-VVDS and ∼13% in the SXDF) of our USS sources without redshift estimates mostly remain unidentified in the existing optical, IR surveys, and exhibit high radio to mid-IR flux ratio limits similar to HzRGs, and so, can be considered as potential HzRG candidates.
Conclusions. Our study shows that the criterion of ultra steep spectral index remains a reasonably efficient method to select high-z sources even at sub-mJy flux densities. In addition to powerful HzRG candidates, our faint USS sample also contains populations of weaker radio-loud AGNs potentially hosted in obscured environments
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