123 research outputs found
The electron temperatures of SDSS high-metallicity giant extragalactic HII regions
Spectra of high-metallicity (12+log(O/H) > 8.2) HII regions where oxygen
auroral lines are measurable in both the O+ and O++ zones, have been extracted
from the Data Release 6 of the Sloan Digital Sky Survey (SDSS). Our final
sample consists of 181 SDSS spectra of HII regions in galaxies in the redshift
range from ~0.025 to ~0.17. The t_2,O-t_3,O diagram is examined. In the SDSS
HII regions, the electron temperature t_2,O is found to have a large scatter at
a given value of the electron temperature t_3,O. The majority of the SDSS HII
regions lie below the t_2,O-t_3,O relation derived for HII regions in nearby
galaxies, i.e. the positions of the SDSS HII regions show a systematic shift
towards lower t_2,O temperatures or/and towards higher t_3,O temperatures. The
scatter and shift of the SDSS HII regions in the t_2,O-t_3,O diagram can be
understood if they are composite nebulae excited by two or more ionizing
sources of different temperatures.Comment: 14 pages, 11 figures accepted for publication in the MNRA
Convergence of expansions in Schr\"odinger and Dirac eigenfunctions, with an application to the R-matrix theory
Expansion of a wave function in a basis of eigenfunctions of a differential
eigenvalue problem lies at the heart of the R-matrix methods for both the
Schr\"odinger and Dirac particles. A central issue that should be carefully
analyzed when functional series are applied is their convergence. In the
present paper, we study the properties of the eigenfunction expansions
appearing in nonrelativistic and relativistic -matrix theories. In
particular, we confirm the findings of Rosenthal [J. Phys. G: Nucl. Phys. 13,
491 (1987)] and Szmytkowski and Hinze [J. Phys. B: At. Mol. Opt. Phys. 29, 761
(1996); J. Phys. A: Math. Gen. 29, 6125 (1996)] that in the most popular
formulation of the R-matrix theory for Dirac particles, the functional series
fails to converge to a claimed limit.Comment: Revised version, accepted for publication in Journal of Mathematical
Physics, 21 pages, 1 figur
Three-dimensional chemically homogeneous and bi-abundance photoionization models of the "super-metal-rich" planetary nebula NGC 6153
Deep spectroscopy of the planetary nebula (PN) NGC\,6153 shows that its heavy
element abundances derived from optical recombination lines (ORLs) are ten
times higher than those derived from collisionally excited lines (CELs), and
points to the existence of H-deficient inclusions embedded in the diffuse
nebula. In this study, we have constructed chemically homogeneous and
bi-abundance three-dimensional photoionization models, using the Monte Carlo
photoionization code {\sc mocassin}. We attempt to reproduce the multi-waveband
spectroscopic and imaging observations of NGC\,6153, and investigate the nature
and origin of the postulated H-deficient inclusions, as well as their impacts
on the empirical nebular analyses assuming a uniform chemical composition. Our
results show that chemically homogeneous models yield small electron
temperature fluctuations and fail to reproduce the strengths of ORLs from C, N,
O and Ne ions. In contrast, bi-abundance models incorporating a small amount of
metal-rich inclusions ( per cent of the total nebular mass) are able
to match all the observations within the measurement uncertainties. The
metal-rich clumps, cooled down to a very low temperature (~K) by
ionic infrared fine-structure lines, dominate the emission of heavy element
ORLs, but contribute almost nil to the emission of most CELs. We find that the
abundances of C, N, O and Ne derived empirically from CELs, assuming a uniform
chemical composition, are about 30 per cent lower than the corresponding
average values of the whole nebula, including the contribution from the
H-deficient inclusions. Ironically, in the presence of H-deficient inclusions,
the traditional standard analysis of the optical helium recombination lines,
assuming a chemically homogeneous nebula, overestimates the helium abundance by
40 per cent.Comment: 19 pages, 18 figures, accepted for publication in MNRA
Can X-rays provide a solution to the abundance discrepancy problem in photoionised nebulae?
We re-examine the well-known discrepancy between ionic abundances determined
via the analysis of recombination lines (RLs) and collisionally excited lines
(CELs). We show that abundance variations can be mimicked in a {\it chemically
homogeneous} medium by the presence of dense X-ray irradiated regions which
present different ionisation and temperature structures from those of the more
diffuse medium they are embedded in, which is predominantly ionised by
extreme-ultraviolet radiation. The presence of X-ray ionised dense clumps or
filaments also naturally explains the lower temperatures often measured from O
{\sc ii} recombination lines and from the Balmer jump when compared to
temperatures determined by CELs. We discuss the implications for abundances
determined via the analysis of CELs and RLs and provide a simple analytical
procedure to obtain upwards corrections for CEL-determined abundance. While we
show that the abundance discrepancy factor (ADF) and the Balmer Jump
temperature determined from observations of the Orion Nebula can simultaneously
be reproduced by this model (implying upward corrections for CELs by a factor
of 1.15), we find that the required X-ray fluxes exceed the known Orion's
stellar and diffuse X-ray budget, if we assume that the clumps are located at
the edge of the blister. We propose, however, that spatially resolved
observations may be used to empirically test the model, and we outline how the
framework developed in this letter may be applied in the future to objects with
better constrained geometries (e.g. planetary nebulae).Comment: 5 pages, accepted for publication by MNRAS Letter
Predicting spectral features in galaxy spectra from broad-band photometry
We explore the prospects of predicting emission line features present in
galaxy spectra given broad-band photometry alone. There is a general consent
that colours, and spectral features, most notably the 4000 A break, can predict
many properties of galaxies, including star formation rates and hence they
could infer some of the line properties. We argue that these techniques have
great prospects in helping us understand line emission in extragalactic objects
and might speed up future galaxy redshift surveys if they are to target
emission line objects only. We use two independent methods, Artifical Neural
Neworks (based on the ANNz code) and Locally Weighted Regression (LWR), to
retrieve correlations present in the colour N-dimensional space and to predict
the equivalent widths present in the corresponding spectra. We also investigate
how well it is possible to separate galaxies with and without lines from broad
band photometry only. We find, unsurprisingly, that recombination lines can be
well predicted by galaxy colours. However, among collisional lines some can and
some cannot be predicted well from galaxy colours alone, without any further
redshift information. We also use our techniques to estimate how much
information contained in spectral diagnostic diagrams can be recovered from
broad-band photometry alone. We find that it is possible to classify AGN and
star formation objects relatively well using colours only. We suggest that this
technique could be used to considerably improve redshift surveys such as the
upcoming FMOS survey and the planned WFMOS survey.Comment: 10 pages 7 figures summitted to MNRA
The Massive Stellar Content in the Starburst NGC3049: A Test for Hot-Star Mode
We have obtained high-spatial resolution ultraviolet and optical STIS
spectroscopy and imaging of the metal-rich nuclear starburst in NGC3049. The
stellar continuum and the absorption line spectrum in the ultraviolet are used
to constrain the massive stellar population. The strong, blueshifted stellar
lines of CIV and SiIV detected in the UV spectra indicate a metal-rich,
compact, massive (1E6 Msol) cluster of age 3--4 Myr emitting the UV-optical
continuum. We find strong evidence against a depletion of massive stars in this
metal-rich cluster. The derived age and the upper mass-limit cut-off of the
initial mass function are also consistent with the detection of Wolf-Rayet (WR)
features at optical wavelengths. As a second independentconstraint on the
massive stellar content, the nebular emission-line spectrum is modeled with
photoionization codes using stellar spectra from evolutionary synthesis models.
However, the nebular lines are badly reproduced by 3--4 Myr instantaneous
bursts, as required by the UV line spectrum, when unblanketed WR and/or Kurucz
stellar atmospheres are used. The corresponding number of photons above 24 and
54 eV in the synthetic models is too high in comparison with values suggested
by the observed line ratios. Since the ionizing spectrum in this regime is
dominated by emission from WR stars, this discrepancy between observations and
models is most likely the result of incorrect assumptions about the WR stars.
Thus we conclude that the nebular spectrum of high-metallicity starbursts is
poorly reproduced by models for WR dominated populations. However, the new
model set of Smith et al. (2002) with blanketed WR and O atmospheres and
adjusted WR temperatures predicts a softer far-UV radiation field, providing a
better match to the data.Comment: To be published in ApJ, Dec. issue 17 figures, 3 in gif forma
Stellar population gradients in the cores of nearby field E+A galaxies
We have selected a sample of local E+A galaxies from the Sloan Digital Sky
Survey (SDSS) Data Release 7 for follow up integral field spectroscopy with the
Wide Field Spectrograph (WiFeS) on the ANU 2.3-m telescope. The sample was
selected using the Halpha line in place of the [OII]3727 line as the indicator
of on-going star formation (or lack thereof). This allowed us to select a lower
redshift sample of galaxies than available in the literature since the
[OII]3727 falls off the blue end of the wavelength coverage in the SDSS for the
very lowest redshift objects. This low redshift selection means that the
galaxies have a large angular to physical scale which allows us to resolve the
central ~1kpc region of the galaxies; the region where stellar population
gradients are expected. Such observations have been difficult to make using
other higher redshift samples because even at redshifts z~0.1 the angular to
physical scale is similar to the resolution provided by ground based seeing.
Our integral field spectroscopy has enabled us to make the first robust
detections of Balmer line gradients in the centres of E+A galaxies. Six out of
our sample of seven, and all the galaxies with regular morphologies, are
observed to have compact and centrally-concentrated Balmer line absorption.
This is evidence for compact young cores and stellar population gradients which
are predicted from models of mergers and tidal interactions which funnel gas
into the galaxy core. Given the generally isolated nature of our sample this
argues for the galaxies being seen in the late stage of a merger where the
progenitors have already coalesced.Comment: accepted to MNRA
Beyond pure state entanglement for atomic ensembles
We analyze multipartite entanglement between atomic ensembles within quantum
matter-light interfaces. In our proposal, a polarized light beam crosses
sequentially several polarized atomic ensembles impinging on each of them at a
given angle \alpha_i. These angles are crucial parameters for shaping the
entanglement since they are directly connected to the appropriate combinations
of the collective atomic spins that are squeezed. We exploit such scheme to go
beyond the pure state paradigm proposing realistic experimental settings to
address multipartite mixed state entanglement in continuous variables.Comment: 23 pages, 5 figure
The primordial abundance of 4He: a self-consistent empirical analysis of systematic effects in a large sample of low-metallicity HII regions
We determine the primordial helium mass fraction Yp using 93 spectra of 86
low-metallicity extragalactic HII regions. This sample constitutes the largest
and most homogeneous high-quality data sets in existence for the determination
of Yp. For comparison and to improve the statistics in our investigation of
systematic effects affecting the Yp determination, we have also considered a
sample of 271 low-metallicity HII regions selected from the DR5 of the SDSS.
Although this larger sample shows more scatter, it gives results that are
consistent at the 2sigma level with our original sample. We have considered
known systematic effects which may affect the 4He abundance determination. They
include different sets of HeI line emissivities and reddening laws, collisional
and fluorescent enhancements of HeI recombination lines, underlying HeI stellar
absorption lines, collisional excitation of hydrogen lines, temperature and
ionization structure of the HII region, and deviation of HeI and H emission
line intensities from case B. However, the most likely value of Yp depends on
the adopted set of HeI line emissivities. Using Monte Carlo methods to solve
simultaneously the above systematic effects we find a primordial helium mass
fraction Yp = 0.2472+/-0.0012 when using the HeI emissivities from Benjamin et
al. (1999, 2002) and 0.2516+/-0.0011 when using those from Porter et al.
(2005). The first value agrees well with the value given by SBBN theory, while
the value obtained with likely more accurate emissivities of Porter et al.
(2005) is higher at the 2sigma level. This latter value, if confirmed, would
imply slight deviations from SBBN.Comment: 56 pages, 15 figures, accepted for publication in the Astrophysical
Journa
Black Hole Mass, Host galaxy classification and AGN activity
We investigate the role of host galaxy classification and black hole mass in
a heterogeneous sample of 276 mostly nearby (z<0.1) X-ray and IR selected AGN.
Around 90% of Seyfert 1 AGN in bulge-dominated host galaxies (without disk
contamination) span a very narrow range in the observed 12um to 2-10keV
luminosity ratio (1<R_{IR/X}<7). This narrow dispersion incorporates all
possible variations among AGN central engines, including accretion mechanism
and efficiency, disk opening angle, orientation to sightline, covering fraction
of absorbing material, patchiness of X-ray corona and measured variability. As
a result, all models of X-ray and IR production in AGN are very strongly
constrained. Among Seyfert 1 AGN, median X-ray and IR luminosities increase
with black hole mass at >99% confidence. Using ring morphology of the host
galaxy as a proxy for lack of tidal interaction, we find that AGN luminosity in
host galaxies within 70Mpc is independent of host galaxy interaction for
Gyrs, suggesting that the timescale of AGN activity due to secular evolution is
much shorter than that due to tidal interactions. We find that LINER hosts have
lower 12um luminosity than the median 12um luminosity of normal disk- and
bulge-dominated galaxies which may represent observational evidence for past
epochs of feedback that supressed star formation in LINER host galaxies. We
propose that nuclear ULXs may account for the X-ray emission from LINER 2s
without flat-spectrum, compact radio cores. We confirmed the robustness of our
results in X-rays by comparing them with the 14-195keV 22-month BAT survey of
AGN, which is all-sky and unbiased by photoelectric absorption.Comment: MNRAS accepted. 14 pages, 11 figures, complete Table 1 in online
journa
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