637 research outputs found
Lyman Alpha and MgII as Probes of Galaxies and their Environments
Ly{\alpha} emission, Ly{\alpha} absorption and MgII absorption are powerful
tracers of neutral hydrogen. Hydrogen is the most abundant element in the
universe and plays a central role in galaxy formation via gas accretion and
outflows, as well as being the precursor to molecular clouds, the sites of star
formation. Since 21cm emission from neutral hydrogen can only be directly
observed in the local universe, we rely on Ly{\alpha} emission, and Ly{\alpha}
and MgII absorption to probe the physics that drives galaxy evolution at higher
redshifts. Furthermore, these tracers are sensitive to a range of hydrogen
densities that cover the interstellar medium, the circumgalactic medium and the
intergalactic medium, providing an invaluable means of studying gas physics in
regimes where it is poorly understood. At high redshift, Ly{\alpha} emission
line searches have discovered thousands of star-forming galaxies out to z = 7.
The large Ly{\alpha} scattering cross-section makes observations of this line
sensitive to even very diffuse gas outside of galaxies. Several thousand more
high-redshift galaxies are known from damped Ly{\alpha} absorption lines and
absorption by the MgII doublet in quasar and GRB spectra. MgII, in particular,
probes metal-enriched neutral gas inside galaxy haloes in a wide range of
environments and redshifts (0.1 < z < 6.3), including the so-called redshift
desert. Here we review what observations and theoretical models of Ly{\alpha}
emission, Ly{\alpha} and MgII absorption have told us about the interstellar,
circumgalactic and intergalactic medium in the context of galaxy formation and
evolution.Comment: 59 Pages, 19 Figures, 1 Table. Accepted for publication in
Publications of the Astronomical Society of the Pacifi
Comparing BDD and SAT based techniques for model checking Chaum's Dining Cryptographers Protocol
We analyse different versions of the Dining Cryptographers protocol by means of automatic verification via model checking. Specifically we model the protocol in terms of a network of communicating automata and verify that the protocol meets the anonymity requirements specified. Two different model checking techniques (ordered binary decision diagrams and SAT-based bounded model checking) are evaluated and compared to verify the protocols
Discovery of Multi-Phase Cold Accretion in a Massive Galaxy at z=0.7
We present detailed photo+collisional ionization models and kinematic models
of the multi-phase absorbing gas, detected within the HST/COS, HST/STIS, and
Keck/HIRES spectra of the background quasar TON 153, at 104 kpc along the
projected minor axis of a star-forming spiral galaxy (z=0.6610). Complementary
g'r'i'Ks photometry and stellar population models indicate that the host galaxy
is dominated by a 4 Gyr stellar population with slightly greater than solar
metallicity and has an estimated log(M*)=11 and a log(Mvir)=13. Photoionization
models of the low ionization absorption, (MgI, SiII, MgII and CIII) which trace
the bulk of the hydrogen, constrain the multi-component gas to be cold
(logT=3.8-5.2) and metal poor (-1.68<[X/H]<-1.64). A lagging halo model
reproduces the low ionization absorption kinematics, suggesting gas coupled to
the disk angular momentum, consistent with cold accretion mode material in
simulations. The CIV and OVI absorption is best modeled in a separate
collisionally ionized metal-poor (-2.50<[X/H]<-1.93) warm phase with logT=5.3.
Although their kinematics are consistent with a wind model, given the 2-2.5dex
difference between the galaxy stellar metallicity and the absorption
metallicity indicates the gas cannot arise from galactic winds. We discuss and
conclude that although the quasar sight-line passes along the galaxy minor axis
at projected distance of 0.3 virial radii, well inside its virial shock radius,
the combination of the relative kinematics, temperatures, and relative
metallicities indicated that the multi-phase absorbing gas arises from cold
accretion around this massive galaxy. Our results appear to contradict recent
interpretations that absorption probing the projected minor axis of a galaxy is
sampling winds.Comment: 16 pages, 11 figures, accepted for publication in MNRA
Investigating MgII Absorption in Paired Quasar Sight-Lines
We test whether the Tinker & Chen model of MgII absorption due to the gaseous
halo around a galaxy can reproduce absorption in quasar pairs (both lensed and
physical) and lensed triples and quads from the literature. These quasars
exhibit absorption from a total of 38 MgII systems spanning z=0.043 - 2.066
with mean redshift =1.099 and weighted mean rest-frame equivalent width of
0.87 Ang. Using the Tinker & Chen model to generate simulated sight-lines, we
marginalize the unknown parameters of the absorbing galaxies: dark matter halo
mass, impact parameter, and azimuthal angle on the sky. We determine the
ability of the model to statistically reproduce the observed variation in MgII
absorption strength between paired sight-lines for different values of the gas
covering fraction f_c and the characteristic length scale ell_c, within which
the variation in absorption equivalent widths between sight-lines exponentially
decreases. We find a best-fit f_c=0.60 \pm 0.15 and ell_c<8/h_70 kpc (1\sigma
confidence limits), with smaller f_c allowed at larger ell_c. At 99.7%
confidence, we are able to rule out f_c>0.87 for all values of ell_c and the
region where ell_c<1.0/h_70 kpc and f_c<0.3.Comment: 24 pages, 11 figures, 2 tables, accepted for publication in MNRAS, 1
Dec 201
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SemTab 2019: Resources to Benchmark Tabular Data to Knowledge Graph Matching Systems
Tabular data to Knowledge Graph matching is the process of assigning semantic tags from knowledge graphs (e.g., Wikidata or DBpedia) to the elements of a table. This task is a challenging problem for various reasons, including the lack of metadata (e.g., table and column names), the noisiness, heterogeneity, incompleteness and ambiguity in the data. The results of this task provide significant insights about potentially highly valuable tabular data, as recent works have shown, enabling a new family of data analytics and data science applications. Despite significant amount of work on various flavors of this problem, there is a lack of a common framework to conduct a systematic evaluation of state-of-the-art systems. The creation of the Semantic Web Challenge on Tabular Data to Knowledge Graph Matching (SemTab) aims at filling this gap. In this paper, we report about the datasets, infrastructure and lessons learned from the first edition of the SemTab challenge
An Empirical Characterization of Extended Cool Gas Around Galaxies Using MgII Absorption Features
We report results from a survey of MgII absorbers in the spectra of
background QSOs that are within close angular distances to a foreground galaxy
at z<0.5, using the Magellan Echellette Spectrograph. We have established a
spectroscopic sample of 94 galaxies at a median redshift of = 0.24 in
fields around 70 distant background QSOs (z_QSO>0.6), 71 of which are in an
'isolated' environment with no known companions and located at rho <~ 120 h^-1
kpc from the line of sight of a background QSO. The rest-frame absolute B-band
magnitudes span a range from M_B-5log h=-16.4 to M_B-5log h=-21.4 and
rest-frame B_AB-R_AB colors range from B_AB-R_AB~0 to B_AB-R_AB~1.5. Of these
'isolated' galaxies, we find that 47 have corresponding MgII absorbers in the
spectra of background QSOs and rest-frame absorption equivalent width
W_r(2796)=0.1-2.34 A, and 24 do not give rise to MgII absorption to sensitive
upper limits. Our analysis shows that (1) Wr(2796) declines with increasing
distance from 'isolated' galaxies but shows no clear trend in 'group'
environments; (2) more luminous galaxies possess more extended MgII absorbing
halos with the gaseous radius scaled by B-band luminosity according to
R_gas=75x(L_B/L_B*)^(0.35+/-0.03) h^{-1} kpc; (3) there is little dependence
between the observed absorber strength and galaxy intrinsic colors; and (4)
within R_gas, we find a mean covering fraction of ~70% for absorbers
of Wr(2796)>=0.3 A and ~80% for absorbers of Wr(2796)>=0.1 A. The
lack of correlation between Wr(2796) and galaxy colors suggests a lack of
physical connection between the origin of extended MgII halos and recent star
formation history of the galaxies. Finally, we discuss the total gas mass in
galactic halos as traced by MgII absorbers. We also compare our results with
previous studies.Comment: 20 pages, 13 figures; to appear in the Astrophysical Journal 2010 May
10 issue; a version with higher resolution figures can be found at
http://lambda.uchicago.edu/public/tmp/mage_apj.pd
The Pittsburgh Sloan Digital Sky Survey MgII Quasar Absorption-Line Survey Catalog
We present a catalog of intervening MgII quasar absorption-line systems in
the redshift interval 0.36 <= z <= 2.28. The catalog was built from Sloan
Digital Sky Survey Data Release Four (SDSS DR4) quasar spectra. Currently, the
catalog contains > 17,000 measured MgII doublets. We also present data on the
~44,600 quasar spectra which were searched to construct the catalog, including
redshift and magnitude information, continuum-normalized spectra, and
corresponding arrays of redshift-dependent minimum rest equivalent widths
detectable at our confidence threshold. The catalog is available on the web. A
careful second search of 500 random spectra indicated that, for every 100
spectra searched, approximately one significant MgII system was accidentally
rejected. Current plans to expand the catalog beyond DR4 quasars are discussed.
Many MgII absorbers are known to be associated with galaxies. Therefore, the
combination of large size and well understood statistics makes this catalog
ideal for precision studies of the low-ionization and neutral gas regions
associated with galaxies at low to moderate redshift. An analysis of the
statistics of MgII absorbers using this catalog will be presented in a
subsequent paper.Comment: AJ, in pres
The UVES Spectral Quasar Absorption Database (SQUAD) Data Release 1: The first 10 million seconds
We present the first data release (DR1) of the UVES Spectral Quasar
Absorption Database (SQUAD), comprising 467 fully reduced, continuum-fitted
high-resolution quasar spectra from the Ultraviolet and Visual Echelle
Spectrograph (UVES) on the European Southern Observatory's Very Large
Telescope. The quasars have redshifts -5, and a total exposure time of 10
million seconds provides continuum-to-noise ratios of 4-342 (median 20) per
2.5-km/s pixel at 5500 \AA. The SQUAD spectra are fully reproducible from the
raw, archival UVES exposures with open-source software, including our
UVES_popler tool for combining multiple extracted echelle exposures which we
document here. All processing steps are completely transparent and can be
improved upon or modified for specific applications. A primary goal of SQUAD is
to enable statistical studies of large quasar and absorber samples, and we
provide tools and basic information to assist three broad scientific uses:
studies of damped Lyman- systems (DLAs), absorption-line surveys and
time-variable absorption lines. For example, we provide a catalogue of 155 DLAs
whose Lyman- lines are covered by the DR1 spectra, 18 of which are
reported for the first time. The HI column densities of these new DLAs are
measured from the DR1 spectra. DR1 is publicly available and includes all
reduced data and information to reproduce the final spectra.Comment: 21 pages, 18 figures. Accepted by MNRAS. All final quasar spectra,
reduced contributing exposures, and supplementary material available via
https://github.com/MTMurphy77/UVES_SQUAD_DR
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