2,669 research outputs found
Towards a fully consistent Milky Way disc model - II. The local disc model and SDSS data of the NGP region
We have used the self-consistent vertical disc models of the solar
neighbourhood presented in Just & Jahreiss (2010), which are based on different
star formation histories (SFR) and fit the local kinematics of main sequence
stars equally well, to predict star counts towards the North Galactic Pole
(NGP). We combined these four different models with the local main sequence in
the filter system of the SDSS and predicted the star counts in the NGP field
with b>80deg. All models fit the Hess diagrams in the F-K dwarf regime better
than 20 percent and the star number densities in the solar neighbourhood are
consistent with the observed values. The chi^2 analysis shows that model A is
clearly preferred with systematic deviations of a few percent only. The SFR of
model A is characterised by a maximum at an age of 10Gyr and a decline by a
factor of four to the present day value of 1.4Msun/pc^2/Gyr. The thick disc can
be modelled very well by an old isothermal simple stellar population. The
density profile can be approximated by a sech^(alpha_t) function. We found a
power law index alpha_t=1.16 and a scale height of 800pc corresponding to a
vertical velocity dispersion of 45.3km/s. About 6 percent of the stars in the
solar neighbourhood are thick disc stars.Comment: 11 pages, 12 figures, accepted by MNRAS. The qualtity of figures 10
and 12 are much better in the .ps version than the .pdf versio
A Survey of z~6 Quasars in the SDSS Deep Stripe. II. Discovery of Six Quasars at z_{AB}>21
We present the discovery of six new quasars at z~6 selected from the Sloan
Digital Sky Survey (SDSS) southern survey, a deep imaging survey obtained by
repeatedly scanning a stripe along the celestial equator. The six quasars are
about two magnitudes fainter than the luminous z~6 quasars found in the SDSS
main survey and one magnitude fainter than the quasars reported in Paper I
(Jiang et al. 2008). Four of them comprise a complete flux-limited sample at
21<z_AB<21.8 over an effective area of 195 deg^2. The other two quasars are
fainter than z_AB=22 and are not part of the complete sample. The quasar
luminosity function at z~6 is well described as a single power law
\Phi(L_{1450}) \propto L_{1450}^{\beta} over the luminosity range
-28<M_{1450}<-25. The best-fitting slope \beta varies from -2.6 to -3.1,
depending on the quasar samples used, with a statistical error of 0.3-0.4.
About 40% of the quasars discovered in the SDSS southern survey have very
narrow Lya emission lines, which may indicate small black hole masses and high
Eddington luminosity ratios, and therefore short black hole growth time scales
for these faint quasars at early epochs.Comment: Accepted for publication in A
Reliability of ultrasonic diaphragm thickness measurement in mechanically ventilated infants and children: A pilot study
Background. Diaphragmatic atrophy in mechanically ventilated infants and children may be due to ventilator-induced diaphragmatic dysfunction, which could lead to extubation failure. Ultrasound may be used as a means by which diaphragmatic atrophy can be reliably identified. There are currently no data reporting on the use of ultrasound to monitor diaphragm atrophy in the paediatric population. Objectives. To assess the inter- and intra-rater reliability of using ultrasound to measure diaphragm thickness in mechanically ventilated infants and children. Method. Diaphragm thickness measurements were compared between two individual researchers for inter-rater reliability and between multiple measurements from a single researcher for intra-rater reliability. Measurements were compared using Intraclass correlation coefficients and Bland- Altman plots. Results. Results indicated excellent reliability between measurements for both inter-and intra-rater reliability, with slightly better reliability for intra-rater compared with inter-rater reliability. Intraclass correlation coefficients for inter-rater reliability were between 0.77 and 0.98, and 0.94 for intra-rater reliability. Conclusion. Ultrasound measurements of diaphragm thickness can be used to reliably measure diaphragm thickness in mechanically ventilated infants and children. This modality could therefore be used as a reliable outcome measure for future clinical research studies to establish the relationship between ventilator-induced diaphragmatic atrophy and children who are at risk for extubation failure
Improved background subtraction for the Sloan Digital Sky Survey images
We describe a procedure for background subtracting Sloan Digital Sky Survey
(SDSS) imaging that improves the resulting detection and photometry of large
galaxies on the sky. Within each SDSS drift scan run, we mask out detected
sources and then fit a smooth function to the variation of the sky background.
This procedure has been applied to all SDSS-III Data Release 8 images, and the
results are available as part of that data set. We have tested the effect of
our background subtraction on the photometry of large galaxies by inserting
fake galaxies into the raw pixels, reanalyzing the data, and measuring them
after background subtraction. Our technique results in no size-dependent bias
in galaxy fluxes up to half-light radii of 100 arcsec; in contrast, for
galaxies of that size the standard SDSS photometric catalog underestimates
fluxes by about 1.5 mag. Our results represent a substantial improvement over
the standard SDSS catalog results and should form the basis of any analysis of
nearby galaxies using the SDSS imaging data.Comment: accepted by the Astronomical Journa
Discovery of Four Gravitationally Lensed Quasars from the Sloan Digital Sky Survey
We present the discovery of four gravitationally lensed quasars selected from
the spectroscopic quasar catalog of the Sloan Digital Sky Survey. We describe
imaging and spectroscopic follow-up observations that support the lensing
interpretation of the following four quasars: SDSS J0832+0404 (image separation
\theta=1.98", source redshift z_s=1.115, lens redshift z_l=0.659); SDSS
J1216+3529 (\theta=1.49", z_s=2.012); SDSS J1322+1052 (\theta=2.00",
z_s=1.716); and SDSS J1524+4409 (\theta=1.67", z_s=1.210, z_l=0.320). Each
system has two lensed images. We find that the fainter image component of SDSS
J0832+0404 is significantly redder than the brighter component, perhaps because
of differential reddening by the lensing galaxy. The lens potential of SDSS
J1216+3529 might be complicated by the presence of a secondary galaxy near the
main lensing galaxy.Comment: 25 pages, 10 figures, 6 tables, accepted for publication in A
SDSS J094604.90+183541.8: A Gravitationally Lensed Quasar at z=4.8
We report the discovery of a gravitationally lensed quasar identified
serendipitously in the Sloan Digital Sky Survey (SDSS). The object, SDSS
J094604.90+183541.8, was initially targeted for spectroscopy as a luminous red
galaxy, but the SDSS spectrum has the features of both a z=0.388 galaxy and a
z=4.8 quasar. We have obtained additional imaging that resolves the system into
two quasar images separated by 3.06 arcsec and a bright galaxy that is strongly
blended with one of the quasar images. We confirm spectroscopically that the
two quasar images represent a single lensed source at z=4.8 with a total
magnification of 3.2, and we derive a model for the lensing galaxy. This is the
highest redshift lensed quasar currently known. We examine the issues
surrounding the selection of such an unusual object from existing data and
briefly discuss implications for lensed quasar surveys.Comment: AJ accepted, 9 pages, 6 figures, referee suggestions include
Photometric Redshift Probability Distributions for Galaxies in the SDSS DR8
We present redshift probability distributions for galaxies in the SDSS DR8
imaging data. We used the nearest-neighbor weighting algorithm presented in
Lima et al. 2008 and Cunha et al. 2009 to derive the ensemble redshift
distribution N(z), and individual redshift probability distributions P(z) for
galaxies with r < 21.8. As part of this technique, we calculated weights for a
set of training galaxies with known redshifts such that their density
distribution in five dimensional color-magnitude space was proportional to that
of the photometry-only sample, producing a nearly fair sample in that space. We
then estimated the ensemble N(z) of the photometric sample by constructing a
weighted histogram of the training set redshifts. We derived P(z) s for
individual objects using the same technique, but limiting to training set
objects from the local color-magnitude space around each photometric object.
Using the P(z) for each galaxy, rather than an ensemble N(z), can reduce the
statistical error in measurements that depend on the redshifts of individual
galaxies. The spectroscopic training sample is substantially larger than that
used for the DR7 release, and the newly added PRIMUS catalog is now the most
important training set used in this analysis by a wide margin. We expect the
primary source of error in the N(z) reconstruction is sample variance: the
training sets are drawn from relatively small volumes of space. Using
simulations we estimated the uncertainty in N(z) at a given redshift is 10-15%.
The uncertainty on calculations incorporating N(z) or P(z) depends on how they
are used; we discuss the case of weak lensing measurements. The P(z) catalog is
publicly available from the SDSS website.Comment: 29 pages, 9 figures, single colum
The Size Distributions of Asteroid Families in the SDSS Moving Object Catalog 4
Asteroid families, traditionally defined as clusters of objects in orbital
parameter space, often have distinctive optical colors. We show that the
separation of family members from background interlopers can be improved with
the aid of SDSS colors as a qualifier for family membership. Based on an
~88,000 object subset of the Sloan Digital Sky Survey Moving Object Catalog 4
with available proper orbital elements, we define 37 statistically robust
asteroid families with at least 100 members using a simple Gaussian
distribution model in both orbital and color space. The interloper rejection
rate based on colors is typically ~10% for a given orbital family definition,
with four families that can be reliably isolated only with the aid of colors.
About 50% of all objects in this data set belong to families, and this fraction
varies from about 35% for objects brighter than an H magnitude of 13 and rises
to 60% for objects fainter than this. The fraction of C-type objects in
families decreases with increasing H magnitude for H > 13, while the fraction
of S-type objects above this limit remains effectively constant. This suggests
that S-type objects require a shorter timescale for equilibrating the
background and family size distributions via collisional processing. The size
distributions for 15 families display a well-defined change of slope and can be
modeled as a "broken" double power-law. Such "broken" size distributions are
twice as likely for S-type familes than for C-type families, and are dominated
by dynamically old families. The remaining families with size distributions
that can be modeled as a single power law are dominated by young families. When
size distribution requires a double power-law model, the two slopes are
correlated and are steeper for S-type families.Comment: 50 pages, 16 figures, accepted for publication in Icaru
New UltraCool and Halo White Dwarf Candidates in SDSS Stripe 82
A 2.5 x 100 degree region along the celestial equator (Stripe 82) has been
imaged repeatedly from 1998 to 2005 by the Sloan Digital Sky Survey. A new
catalogue of ~4 million light-motion curves, together with over 200 derived
statistical quantities, for objects in Stripe 82 brighter than r~21.5 has been
constructed by combining these data by Bramich et al. (2007). This catalogue is
at present the deepest catalogue of its kind. Extracting the ~130000 objects
with highest signal-to-noise ratio proper motions, we build a reduced proper
motion diagram to illustrate the scientific promise of the catalogue. In this
diagram disk and halo subdwarfs are well-separated from the cool white dwarf
sequence. Our sample of 1049 cool white dwarf candidates includes at least 8
and possibly 21 new ultracool H-rich white dwarfs (T_eff < 4000K) and one new
ultracool He-rich white dwarf candidate identified from their SDSS optical and
UKIDSS infrared photometry. At least 10 new halo white dwarfs are also
identified from their kinematics.Comment: 10 pages, 5 figures, published in MNRAS, minor text changes, final
versio
The Luminosity and Mass Functions of Low-Mass Stars in the Galactic Disk: I. The Calibration Region
We present measurements of the luminosity and mass functions of low-mass
stars constructed from a catalog of matched Sloan Digital Sky Survey (SDSS) and
2 Micron All Sky Survey (2MASS) detections. This photometric catalog contains
more than 25,000 matched SDSS and 2MASS point sources spanning ~30 square
degrees on the sky. We have obtained follow-up spectroscopy, complete to J=16,
of more than 500 low mass dwarf candidates within a 1 square degree sub-sample,
and thousands of additional dwarf candidates in the remaining 29 square
degrees. This spectroscopic sample verifies that the photometric sample is
complete, uncontaminated, and unbiased at the 99% level globally, and at the
95% level in each color range. We use this sample to derive the luminosity and
mass functions of low-mass stars over nearly a decade in mass (0.7 M_sun > M_*
> 0.1 M_sun). We find that the logarithmically binned mass function is best fit
with an M_c=0.29 log-normal distribution, with a 90% confidence interval of
M_c=0.20--0.50. These 90% confidence intervals correspond to linearly binned
mass functions peaking between 0.27 M_sun and 0.12 M_sun, where the best fit MF
turns over at 0.17 M_sun. A power law fit to the entire mass range sampled
here, however, returns a best fit of alpha=1.1 (where the Salpeter slope is
alpha = 2.35). These results agree well with most previous investigations,
though differences in the analytic formalisms adopted to describe those mass
functions can give the false impression of disagreement. Given the richness of
modern-day astronomical datasets, we are entering the regime whereby stronger
conclusions can be drawn by comparing the actual datapoints measured in
different mass functions, rather than the results of analytic analyses that
impose structure on the data a priori. (abridged)Comment: Accepted for publication in the Astronomical Journal. 21 pages,
emulateapj format, 12 figures. Figures 1, 4, 11 and 12 degraded for astroph;
full resolution version available for download at
http://www.cfa.harvard.edu/~kcovey
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