344 research outputs found
Observations of QSO J2233-606 in the Southern Hubble Deep Field
The Hubble Deep Field South (HDF-S) HST observations are expected to begin in
October 1998. We present a composite spectrum of the QSO in the HDF-S field
covering UV/optical/near IR wavelengths, obtained by combining data from the
ANU 2.3m Telescope with STIS on the HST. This intermediate resolution spectrum
covers the range 1600-10000A and allows us to derive some basic information on
the intervening absorption systems which will be important in planning future
higher resolution studies of this QSO.Comment: 9 pages and 2 figures, submitted to ApJ
A dominant mutation in a neuronal acetylcholine receptor subunit leads to motor neuron degeneration in Caenorhabditis elegans
Inappropriate or excessive activation of ionotropic receptors can have dramatic consequences for neuronal function and, in many instances, leads to cell death. In Caenorhabditis elegans, nicotinic acetylcholine receptor (nAChR) subunits are highly expressed in a neural circuit that controls movement. Here, we show that heteromeric nAChRs containing the acr-2 subunit are diffusely localized in the processes of excitatory motor neurons and act to modulate motor neuron activity. Excessive signaling through these receptors leads to cell-autonomous degeneration of cholinergic motor neurons and paralysis. C. elegans double mutants lacking calreticulin and calnexin-two genes previously implicated in the cellular events leading to necrotic-like cell death (Xu et al. 2001)-are resistant to nAChR-mediated toxicity and possess normal numbers of motor neuron cell bodies. Nonetheless, excess nAChR activation leads to progressive destabilization of the motor neuron processes and, ultimately, paralysis in these animals. Our results provide new evidence that chronic activation of ionotropic receptors can have devastating degenerative effects in neurons and reveal that ion channel-mediated toxicity may have distinct consequences in neuronal cell bodies and processes
The evolution of faint AGN between z~1 and z~5 from the COMBO-17 survey
We present a determination of the optical/UV AGN luminosity function and its
evolution, based on a large sample of faint (R<24) QSOs identified in the
COMBO-17 survey. Using multi-band photometry in 17 filters within 350nm <
lambda_obs < 930nm, we could simultaneously determine photometric redshifts
with an accuracy of sigma_z<0.03 and obtain spectral energy distributions. The
redshift range covered by the sample is 1.2<z<4.8, which implies that even at
z~3, the sample reaches below luminosities corresponding to M_B = -23,
conventionally employed to distinguish between Seyfert galaxies and quasars. We
clearly detect a broad plateau-like maximum of quasar activity around z~2 and
map out the smooth turnover between z~1 and z~4. The shape of the LF is
characterised by some mild curvature, but no sharp `break' is present within
the range of luminosities covered. Using only the COMBO-17 data, the evolving
LF can be adequately described by either a pure density evolution (PDE) or a
pure luminosity evolution (PLE) model. However, the absence of a strong L*-like
feature in the shape of the LF inhibits a robust distinction between these
modes. We present a robust estimate for the integrated UV luminosity generation
by AGN as a function of redshift. We find that the LF continues to rise even at
the lowest luminosities probed by our survey, but that the slope is
sufficiently shallow that the contribution of low-luminosity AGN to the UV
luminosity density is negligible. Although our sample reaches much fainter flux
levels than previous data sets, our results on space densities and LF slopes
are completely consistent with extrapolations from recent major surveys such as
SDSS and 2QZ.Comment: 17 pages, 14 figures, Astronomy & Astrophysics, in print, revised
versio
Requirement of JNK1 for endothelial cell injury in atherogenesis
AbstractObjectiveThe c-Jun N-terminal kinase (JNK) family regulates fundamental physiological processes including apoptosis and metabolism. Although JNK2 is known to promote foam cell formation during atherosclerosis, the potential role of JNK1 is uncertain. We examined the potential influence of JNK1 and its negative regulator, MAP kinase phosphatase-1 (MKP-1), on endothelial cell (EC) injury and early lesion formation using hypercholesterolemic LDLRâ/â mice.Methods and resultsTo assess the function of JNK1 in early atherogenesis, we measured EC apoptosis and lesion formation in LDLRâ/â or LDLRâ/â/JNK1â/â mice exposed to a high fat diet for 6 weeks. En face staining using antibodies that recognise active, cleaved caspase-3 (apoptosis) or using Sudan IV (lipid deposition) revealed that genetic deletion of JNK1 reduced EC apoptosis and lesion formation in hypercholesterolemic mice. By contrast, although EC apoptosis was enhanced in LDLRâ/â/MKP-1â/â mice compared to LDLRâ/â mice, lesion formation was unaltered.ConclusionWe conclude that JNK1 is required for EC apoptosis and lipid deposition during early atherogenesis. Thus pharmacological inhibitors of JNK may reduce atherosclerosis by preventing EC injury as well as by influencing foam cell formation
A Uniform Analysis of the Ly-alpha forest at z = 0 - 5: II. Measuring the mean intensity of the extragalactic ionizing background using the proximity effect
A homogeneous sample of 99 moderate resolution QSO spectra at z > 1.7 were
presented in Paper I, including 39 previously unpublished spectra from the
Multiple Mirror Telescope. The statistics of the Lyman alpha forest were
discussed. In this analysis, we demonstrate that a proximity effect is present
in the data, ie. there exists a significant (5.5) deficit of lines at
. Within 1.5 Mpc of the QSO emission redshift,
the significance does depend on QSO luminosity, in accordance with the theory
that this effect is caused by enhanced ionization of hydrogen in the vicinity
of the QSO from UV photons from the QSO itself. The photoionization model of
Bajtlik, Duncan, and Ostriker (1988) permits an estimate of the mean intensity
of the extragalactic background radiation at the Lyman limit. We compare the
results of this standard analysis with those obtained using a maximum
likelihood technique. The best fit value for is
7.0 x 10 ergs/s/cm/Hz/sr, over the redshift range
1.7 < z < 3.8, using QSO redshifts based on narrow emission lines. The best fit
value for the HI ionization rate is 1.9 x 10 s,
in good agreement with models of the background which incorporate QSOs only.
This large absorption line sample and these techniques for measuring the
background and understanding the systematics involved allow us to place what we
believe are are the firmest limits on the background at these redshifts.Comment: revised figures 13 and 14, and other minor corrections, 42 Latex
pages, 23 encapsulated Postscript figures, uses emulateapj.sty, To appear in
the Sept. 2000 ApJ
The BeppoSAX view of the hard X-ray background
First results on a medium-deep X-ray survey in the "new" 5-10 keV band
carried out with the MECS detectors onboard BeppoSAX are presented. The High
Energy Llarge Area Survey (HELLAS) is aimed to directly explore a band where
the energy density of the X-ray background is more than twice than that in the
soft (0.5-2.0 keV) band. The optical identification follow-up of the first ten
HELLAS hard X-ray sources indicate that Active Galactic Nuclei are the dominant
population at 5-10 keV fluxes of the order of 10e-13 cgs. We discuss the
implications of these findings for the AGN synthesis models for the XRB.Comment: 6 pages, 4 figures, uses psfig.sty. Accepted for publication in
Advances in Space Research, Proceedings of the 32nd Scientific Assembly of
COSPA
Constraints on the faint end of the quasar luminosity function at z~5 in the COSMOS field
We present the result of our low-luminosity quasar survey in the redshift
range of 4.5 < z < 5.5 in the COSMOS field. Using the COSMOS photometric
catalog, we selected 15 quasar candidates with 22 < i' < 24 at z~5, that are ~
3 mag fainter than the SDSS quasars in the same redshift range. We obtained
optical spectra for 14 of the 15 candidates using FOCAS on the Subaru Telescope
and did not identify any low-luminosity type-1 quasars at z~5 while a
low-luminosity type-2 quasar at z~5.07 was discovered. In order to constrain
the faint end of the quasar luminosity function at z~5, we calculated the
1sigma confidence upper limits of the space density of type-1 quasars. As a
result, the 1sigma confidence upper limits on the quasar space density are Phi<
1.33*10^{-7} Mpc^{-3} mag^{-1} for -24.52 < M_{1450} < -23.52 and Phi<
2.88*10^{-7} Mpc^{-3} mag^{-1} for -23.52 < M_{1450} < -22.52. The inferred
1sigma confidence upper limits of the space density are then used to provide
constrains on the faint-end slope and the break absolute magnitude of the
quasar luminosity function at z~5. We find that the quasar space density
decreases gradually as a function of redshift at low luminosity (M_{1450} ~
-23), being similar to the trend found for quasars with high luminosity
(M_{1450}<-26). This result is consistent with the so-called downsizing
evolution of quasars seen at lower redshifts.Comment: 8 pages, 9 figures, 1 table, accepted for publication in Ap
The quasar M_bh - M_host relation through Cosmic Time I - Dataset and black hole masses
We study the M_bh - M_host relation as a function of Cosmic Time in a sample
of 96 quasars from z=3 to the present epoch. In this paper we describe the
sample, the data sources and the new spectroscopic observations. We then
illustrate how we derive M_bh from single-epoch spectra, pointing out the
uncertainties in the procedure. In a companion paper, we address the dependence
of the ratio between the black hole mass and the host galaxy luminosity and
mass on Cosmic Time.Comment: 16 pages, 6 figures, 5 tables. Accepted for publication in MNRA
The Spectral Energy Distributions of Red 2MASS AGN
We present infrared (IR) to X-ray spectral energy distributions (SEDs) for 44
red AGN selected from the 2MASS survey on the basis of their red J-K color
(>2 mag) and later observed by Chandra. In comparison with optically-, radio-,
and X-ray selected AGN, their median SEDs are red in the optical and near-IR
with little/no blue bump. It thus seems that near-IR color selection isolates
the reddest subset of AGN that can be classified optically. The shape of the
SEDs is generally consistent with modest absorption by gas (in the X-ray) and
dust (in the optical-IR). The levels of obscuration, estimated from X-rays,
far-IR and our detailed optical/near-IR color modeling are all consistent
implying N_H < few*10^{22} cm^{-2}. We present SED models that show how the AGN
optical/near-IR colors change due to differing amounts of reddening, AGN to
host galaxy ratio, redshift and scattered light emission and apply them to the
sources in the sample. We find that the 2MASS AGN optical color, B-R, and to a
lesser extent the near-IR color, J-K, are strongly affected by reddening,
host galaxy emission, redshift, and in few, highly polarized objects, also by
scattered AGN light. The obscuration/inclination of the AGN allows us to see
weaker emission components which are generally swamped by the AGN.Comment: 52 pages, 17 figures, accepted for publication in Ap
Colors of 2625 Quasars at 0<z<5 Measured in the Sloan Digital Sky Survey Photometric System
We present an empirical investigation of the colors of quasars in the Sloan
Digital Sky Survey (SDSS) photometric system. The sample studied includes 2625
quasars with SDSS photometry. The quasars are distributed in a 2.5 degree wide
stripe centered on the Celestial Equator covering square degrees.
Positions and SDSS magnitudes are given for the 898 quasars known prior to SDSS
spectroscopic commissioning. New SDSS quasars represent an increase of over
200% in the number of known quasars in this area of the sky. The ensemble
average of the observed colors of quasars in the SDSS passbands are well
represented by a power-law continuum with (). However, the contributions of the bump
and other strong emission lines have a significant effect upon the colors. The
color-redshift relation exhibits considerable structure, which may be of use in
determining photometric redshifts for quasars. The range of colors can be
accounted for by a range in the optical spectral index with a distribution
(95% confidence), but there is a red tail in the
distribution. This tail may be a sign of internal reddening. Finally, we show
that there is a continuum of properties between quasars and Seyfert galaxies
and we test the validity of the traditional division between the two classes of
AGN.Comment: 66 pages, 15 figures (3 color), accepted by A
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