3,201 research outputs found
On the Connection Between Metal Absorbers and Quasar Nebulae
We establish a simple model for the distribution of cold gas around L*
galaxies using a large set of observational constraints on the properties of
strong MgII absorber systems. Our analysis suggests that the halos of L*
galaxies are filled with cool gaseous clouds having sizes of order 1kpc and
densities of ~10^{-2} cm^{-3}. We then investigate the physical effects of
cloud irradiation by a quasar and study the resulting spectral signatures. We
show that quasar activity gives rise to (i) extended narrow-line emission on
~100kpc scales and (ii) an anisotropy in the properties of the absorbing gas
arising from the geometry of the quasar radiation field. Provided that quasars
reside in halos several times more massive than those of L* galaxies, our model
predictions appear to be in agreement with observations of narrow emission-line
nebulae around quasars and the recent detections of ~100kpc cold gaseous
envelopes around those objects, suggesting a common origin for these phenomena.
We discuss the implications of our results for understanding absorption
systems, probing quasar environments at high redshifts, and testing the quasar
unification scheme.Comment: 15 pages, 13 figures (ApJ submitted
QSO Absorption Line Constraints on Intragroup High-Velocity Clouds
We show that the number statistics of moderate redshift MgII and Lyman limit
absorbers may rule out the hypothesis that high velocity clouds are infalling
intragroup material.Comment: 4 pages, no figures; submitted to Astrophysical Journal Letters;
revised version, more general and includes more about Braun and Burton CHVC
Binary Adaptive Semi-Global Matching Based on Image Edges
Image-based modeling and rendering is currently one of the most challenging topics in Computer Vision and Photogrammetry. The key issue here is building a set of dense correspondence points between two images, namely dense matching or stereo matching. Among all dense matching algorithms, Semi-Global Matching (SGM) is arguably one of the most promising algorithms for real-time stereo vision. Compared with global matching algorithms, SGM aggregates matching cost from several (eight or sixteen) directions rather than only the epipolar line using Dynamic Programming (DP). Thus, SGM eliminates the classical “streaking problem” and greatly improves its accuracy and efficiency. In this paper, we aim at further improvement of SGM accuracy without increasing the computational cost. We propose setting the penalty parameters adaptively according to image edges extracted by edge detectors. We have carried out experiments on the standard Middlebury stereo dataset and evaluated the performance of our modified method with the ground truth. The results have shown a noticeable accuracy improvement compared with the results using fixed penalty parameters while the runtime computational cost was not increased
Physical Conditions in the Inner Narrow-Line Region of the Seyfert 2 Galaxy NGC 1068
The physical conditions in the inner narrow line region (NLR) of the Seyfert
2 galaxy, NGC 1068, are examined using ultraviolet and optical spectra and
photoionization models. The spectra are Hubble Space Telescope (HST) Harchive
data obtained with the Faint Object Spectrograph (FOS). We selected spectra of
four regions, taken through the 0.3" FOS aperture, covering the full FOS 1200A
to 6800A waveband. Each region is approximately 20 pc in extent, and all are
within 100 pc of the apparent nucleus of NGC 1068. The spectra show similar
emission-line ratios from wide range of ionization states for the most abundant
elements. After extensive photoionization modeling, we interpret this result as
an indication that each region includes a range of gas densities, which we
included in the models as separate components. Supersolar abundances were
required for several elements to fit the observed emission line ratios. Dust
was included in the models but apparently dust to gas fraction varies within
these regions. The low ionization lines in these spectra can be best explained
as arising in gas that is partially shielded from the ionizing continuum.
Although the predicted line ratios from the photoionization models provide a
good fit to the observed ratios, it is apparent that the model predictions of
electron temperatures in the ionized gas are too low. We interpret this as an
indication of additional collisional heating due to shocks and/or energetic
particles associated with the radio jet that traverses the NLR of NGC 1068. The
density structure within each region may also be the result of compression by
the jet.Comment: 38 pages, Latex, includes 5 figures (postscript), to appear in Ap
XMM observations of the narrow-line QSO PHL 1092: Detection of a high and variable soft component
We present results based on an XMM-Newton observation of the high luminosity
narrow-line QSO PHL 1092 performed in 2003 January. The 0.3 - 10 keV spectrum
is well described by a model which includes a power-law (Gamma ~ 2.1) and two
blackbody components (kT ~ 130 eV and kT ~ 50 eV). The soft X-ray excess
emission is featureless and contributes ~ 80% to the total X-ray emission in
the 0.3 - 10 keV band. The most remarkable feature of the present observation
is the detection of X-ray variability at very short time scale: the X-ray
emission varied by 35% in about 5000 s. We find that this variability can be
explained by assuming that only the overall normalization varied during the
observation. There was no evidence for any short term spectral variability and
the spectral shape was similar even during the ASCA observation carried out in
1997. Considering the high intrinsic luminosity (~ 2x10^45 erg/s) and the large
inferred mass of the putative black hole (~ 1.6x10^8 M_sun), the observed time
scale of variability indicates emission at close to Eddington luminosity
arising from very close to the black hole. We suggest that PHL 1092 in
particular (and narrow line Seyfert galaxies in general) is a fast rotating
black hole emitting close to its Eddington luminosity and the X-ray emission
corresponds to the high-soft state seen in Galactic black hole sources.Comment: 7 figures, 8 pages, emulateapj style, ApJ in pres
Precise Atmospheric Parameters for the Shortest Period Binary White Dwarfs: Gravitational Waves, Metals, and Pulsations
We present a detailed spectroscopic analysis of 61 low mass white dwarfs and
provide precise atmospheric parameters, masses, and updated binary system
parameters based on our new model atmosphere grids and the most recent
evolutionary model calculations. For the first time, we measure systematic
abundances of He, Ca and Mg for metal-rich extremely low mass white dwarfs and
examine the distribution of these abundances as a function of effective
temperature and mass. Based on our preliminary results, we discuss the
possibility that shell flashes may be responsible for the presence of the
observed He and metals. We compare stellar radii derived from our spectroscopic
analysis to model-independent measurements and find good agreement except for
those white dwarfs with Teff < 10,000 K. We also calculate the expected
gravitational wave strain for each system and discuss their significance to the
eLISA space-borne gravitational wave observatory. Finally, we provide an update
on the instability strip of extremely low mass white dwarf pulsators.Comment: 18 pages, 13 figures, 3 tables, accepted for publication in Ap
Lyman-alpha absorption around nearby galaxies
We have used STIS aboard HST to search for Lyman-alpha (Lya) absorption lines
in the outer regions of eight nearby galaxies using background QSOs and AGN as
probes. Lya lines are detected within a few hundred km/s of the systemic
velocity of the galaxy in all cases. We conclude that a background
line-of-sight which passes within 26-200 h-1 kpc of a foreground galaxy is
likely to intercept low column density neutral hydrogen with log N(HI) >~ 13.0.
The ubiquity of detections implies a covering factor of ~ 100% for low N(HI)
gas around galaxies within 200 h-1 kpc. We discuss the difficulty in trying to
associate individual absorption components with the selected galaxies and their
neighbors, but show that by degrading our STIS data to lower resolutions, we
are able to reproduce the anti-correlation of Lya equivalent width and impact
parameter found at higher redshift. We also show that the equivalent width and
column density of Lya complexes (when individual components are summed over ~
1000 km/s) correlate well with a simple estimate of the volume density of
galaxies brighter than M(B) = -17.5 at the same redshift as a Lya complex. We
do not reject the hypothesis that the selected galaxies are directly
responsible for the observed Lya lines, but our analysis indicates that
absorption by clumpy intragroup gas is an equally likely explanation. (Abriged)Comment: Accepted for publication in Nov 20, 2002 issue of ApJ. Paper with all
figures can be found at http://www.astro.princeton.edu/~dvb/lyapaper.ps
(preferable). Minor typos fixe
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