4,356 research outputs found
Where are the missing cosmic metals ?
The majority of the heavy elements produced by stars 2 billion years after
the Big Bang (redshift z~3) are presently undetected at those epochs. We
propose a solution to this cosmic `missing metals' problem in which such
elements are stored in gaseous halos produced by supernova explosions around
star-forming galaxies. By using data from the ESO/VLT Large Program, we find
that:(i) only 5%-9% of the produced metals reside in the cold phase, the rest
being found in the hot (log T=5.8-6.4) phase; (ii) 1%-6% (3%-30%) of the
observed CIV (OVI) is in the hot phase. We conclude that at z~3 more than 90%
of the metals produced during the star forming history can be placed in a hot
phase of the IGM, without violating any observational constraint. The observed
galaxy mass-metallicity relation, and the intergalactic medium and intracluster
medium metallicity evolution are also naturally explained by this hypothesis.Comment: 9 pages, 2 figures, ApJ Letters, in pres
Spectroscopic analysis of DA white dwarfs from the McCook & Sion catalog
For some years now, we have been gathering optical spectra of DA white dwarfs
in an effort to study and define the empirical ZZ Ceti instability strip.
However, we have recently expanded this survey to include all the DA white
dwarfs in the McCook & Sion catalog down to a limiting visual magnitude of
V=17.5. We present here a spectroscopic analysis of over 1000 DA white dwarfs
from this ongoing survey. We have several specific areas of interest most
notably the hot DAO white dwarfs, the ZZ Ceti instability strip, and the DA+dM
binary systems. Furthermore, we present a comparison of the ensemble properties
of our sample with those of other large surveys of DA white dwarfs, paying
particular attention to the distribution of mass as a function of effective
temperature.Comment: 8 pages, 7 figures, to appear in Journal of Physics Conference
Proceedings for the 16th European White Dwarf Worksho
Damped Lyman Alpha Systems at z<1.65: The Expanded SDSS HST Sample
We present results of our HST Cycle 11 Survey for low-redshift (z<1.65) DLAs
in the UV spectra of quasars selected from the SDSS Early Data Release. These
quasars have strong intervening MgII-FeII systems which are known signatures of
high column density neutral gas. In total, UV observations of Ly-alpha
absorption in 197 MgII systems with z<1.65 and rest equivalent width (REW)
W2796 \ge 0.3A have now been obtained. The main results are: (1) 36(+/- 6)% of
systems with W2796 \ge 0.5 A and FeII W2600 \ge 0.5 A are DLAs. This increases
to 42(+/- 7)% for systems with W2796/W2600 0.1 A. (2) The
mean N(HI) of MgII systems with 0.3 A \le W2796 < 0.6 A is a factor of ~36
lower than that of systems with W2796 \ge 0.6 A. (3) The DLA incidence per unit
redshift is consistent with no evolution for z <~ 2 (Omega_L=0.7, Omega_M =
0.3), but exhibits significant evolution for z >~ 2. (4) Omega_{DLA} is
constant for 0.5<z<5.0 to within the uncertainties. This is larger than
Omega_{gas}(z=0) by a factor of ~2. (5) The slope of the N(HI) distribution
does not change significantly with redshift. However, the low redshift
distribution is marginally flatter due to the higher fraction of high N(HI)
systems in our sample. (6) Finally, using the precision of MgII survey
statistics, we find that there may be evidence of a decreasing Omega_{DLA} from
z=0.5 to z=0. We reiterate the conclusion of Hopkins, Rao, & Turnshek that very
high columns of neutral gas might be missed by DLA surveys because of their
very small cross sections, and therefore, that Omega_{DLA} might not include
the bulk of the neutral gas mass in the Universe. (Abridged)Comment: Accepted for publication in ApJ. 22 pages, 22 figure
Combinatorics of Labelled Parallelogram polyominoes
We obtain explicit formulas for the enumeration of labelled parallelogram
polyominoes. These are the polyominoes that are bounded, above and below, by
north-east lattice paths going from the origin to a point (k,n). The numbers
from 1 and n (the labels) are bijectively attached to the north steps of
the above-bounding path, with the condition that they appear in increasing
values along consecutive north steps. We calculate the Frobenius characteristic
of the action of the symmetric group S_n on these labels. All these enumeration
results are refined to take into account the area of these polyominoes. We make
a connection between our enumeration results and the theory of operators for
which the intergral Macdonald polynomials are joint eigenfunctions. We also
explain how these same polyominoes can be used to explicitly construct a linear
basis of a ring of SL_2-invariants.Comment: 25 pages, 9 figure
Parameter-free Stark Broadening of Hydrogen Lines in DA White Dwarfs
We present new calculations for the Stark broadening of the hydrogen line
profiles in the dense atmospheres of white dwarf stars. Our improved model is
based on the unified theory of Stark broadening from Vidal, Cooper & Smith, but
it also includes non-ideal gas effects from the Hummer & Mihalas occupation
probability formalism directly inside the line profile calculations. This
approach improves upon previous calculations that relied on the use of an
ad-hoc free parameter to describe the dissolution of the line wing opacity in
the presence of high electric microfields in the plasma. We present here the
first grid of model spectra for hot Teff >~ 12,000 K DA white dwarfs that has
no free parameters. The atmospheric parameters obtained from optical and UV
spectroscopic observations using these improved models are shown to differ
substantially from those published in previous studies.Comment: 8 pages, 8 figures, to appear in Journal of Physics Conference
Proceedings for the 16th European White Dwarf Worksho
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
Optical properties and spatial distribution of MgII absorbers from SDSS image stacking
We present a statistical analysis of the photometric properties and spatial
distribution of more than 2,800 MgII absorbers with 0.37<z<1 and rest
equivalent width W_0(\lambda2796)>0.8\AA detected in SDSS quasar spectra. Using
an improved image stacking technique, we measure the cross-correlation between
MgII gas and light (in the g, r, i and z-bands) from 10 to 200 kpc and infer
the light-weighted impact parameter distribution of MgII absorbers. Such a
quantity is well described by a power-law with an index that strongly depends
on W_0, ranging from ~-1 for W_0~ 1.5\AA. At redshift
0.37<z<0.55, we find the average luminosity enclosed within 100 kpc around MgII
absorbers to be M_g=-20.65+-0.11 mag, which is ~0.5 L_g*. The global
luminosity-weighted colors are typical of present-day intermediate type
galaxies. However, while the light of weaker absorbers originates mostly from
red passive galaxies, stronger systems display the colors of blue star-forming
galaxies. Based on these observations, we argue that the origin of strong MgII
absorber systems might be better explained by models of metal-enriched gas
outflows from star-forming/bursting galaxies. Our analysis does not show any
redshift dependence for both impact parameter and rest-frame colors up to z=1.
However, we do observe a brightening of the absorbers related light at high
redshift (~50% from z~0.4 to 1). We argue that MgII absorbers are a phenomenon
typical of a given evolutionary phase that more massive galaxies experience
earlier than less massive ones, in a downsizing fashion. (abridged)Comment: ApJ in press, 28 pages, 16 figures, using emulateapj. Only typo
corrections wrt the original submission (v1
Atomic resolution STM imaging of a twisted single-wall carbon nanotube
We present atomically-resolved STM images of single-wall carbon nanotubes
(SWNTs) embedded in a crystalline nanotube rope. Although they may be
interpreted as of a chiral nanotube, the images are more consistently explained
a an achiral armchair tube with a quenched twist distortion. The existence of
quenched twists in SWNTs in ropes might explain the fact that both as-grown
bulk nanotube material and individual ropes have insulator-like conductivity at
low temperature.Comment: preprint, 4 pages, and 4 gif figure
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
Discovery of Damped Lyman-Alpha Systems at Redshifts Less Than 1.65 and Results on their Incidence and Cosmological Mass Density
We report results on the incidence and cosmological mass density of damped
Lyman-alpha (DLA) systems at redshifts less that 1.65. We used HST and an
efficient non-traditional (but unbiased) survey technique to discover DLA
systems at redshifts z<1.65, where we observe the Lyman-alpha line in known
MgII absorption-line systems. We uncovered 14 DLA lines including 2
serendipitously. We find that (1) The DLA absorbers are drawn almost
exclusively from the population of MgII absorbers which have rest equivalent
widths W(2796)>0.6A. (2) The incidence of DLA systems per unit redshift,
n(DLA), is observed to decrease with decreasing redshift. (3) On the other
hand, the cosmological mass density of neutral gas in low-redshift DLA
absorbers, Omega(DLA), is observed to be comparable to that observed at high
redshift. (4) The low-redshift DLA absorbers exhibit a significantly larger
fraction of very high column density systems in comparison to determinations at
both high redshift and locally.Comment: 47 pages in LaTeX - emulateapj style with included tables and
encapsulated postscript figures. Accepted for Publication in Astrophysical
Journal Supplements. Results unchanged, text revise
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