48 research outputs found
Project Overview of the Beijing-Arizona Sky Survey
The Beijing-Arizona Sky Survey (BASS) is a wide-field two-band photometric
survey of the Northern Galactic Cap using the 90Prime imager on the 2.3 m Bok
telescope at Kitt Peak. It is a four-year collaboration between the National
Astronomical Observatory of China and Steward Observatory, the University of
Arizona, serving as one of the three imaging surveys to provide photometric
input catalogs for target selection of the Dark Energy Spectroscopic Instrument
(DESI) project. BASS will take up to 240 dark/grey nights to cover an area of
about 5400 deg in the and bands. The 5 limiting AB
magnitudes for point sources in the two bands, corrected for the Galactic
extinction, are 24.0 and 23.4 mag, respectively. BASS, together with other DESI
imaging surveys, will provide unique science opportunities that cover a wide
range of topics in both Galactic and extragalactic astronomy.Comment: 10 pages, submitted to PAS
SDSS-IV MaNGA: Stellar initial mass function variation inferred from Bayesian analysis of the integral field spectroscopy of early type galaxies
We analyze the stellar initial mass functions (IMF) of a large sample of
early type galaxies (ETGs) provided by MaNGA. The large number of IFU spectra
of individual galaxies provide high signal-to-noise composite spectra that are
essential for constraining IMF and to investigate possible radial gradients of
the IMF within individual galaxies. The large sample of ETGs also make it
possible to study how the IMF shape depends on various properties of galaxies.
We adopt a novel approach to study IMF variations in ETGs, use Bayesian
inferences based on full spectrum fitting. The Bayesian method provides a
statistically rigorous way to explore potential degeneracy in spectrum fitting,
and to distinguish different IMF models with Bayesian evidence. We find that
the IMF slope depends systematically on galaxy velocity dispersion, in that
galaxies of higher velocity dispersion prefer a more bottom-heavy IMF, but the
dependence is almost entirely due to the change of metallicity, , with
velocity dispersion. The IMF shape also depends on stellar age, , but the
dependence is completely degenerate with that on metallicity through a
combination . Using independent age and metallicity estimates we
find that the IMF variation is produced by metallicity instead of age. The IMF
near the centers of massive ETGs appears more bottom-heavy than that in the
outer parts, while a weak opposite trend is seen for low-mass ETGs.
Uncertainties produced by star formation history, dust extinction,
-element abundance enhancement and noise in the spectra are tested.Comment: 21 pages,20 figures, accepted for publication in MNRA
The First Data Release of the Beijing-Arizona Sky Survey
The Beijing-Arizona Sky Survey (BASS) is a new wide-field legacy imaging
survey in the northern Galactic cap using the 2.3m Bok telescope. The survey
will cover about 5400 deg in the and bands, and the expected
5 depths (corrected for the Galactic extinction) in the two bands are
24.0 and 23.4 mag, respectively. BASS started observations in January 2015, and
has completed about 41% of the whole area as of July 2016. The first data
release contains both calibrated images and photometric catalogs obtained in
2015 and 2016. The depths of single-epoch images in the two bands are 23.4 and
22.9 mag, and the full depths of three epochs are about 24.1 and 23.5 mag,
respectively.Comment: 16 pages, published by A
Chandra X-ray Analysis of Galaxy Cluster A168
We present Chandra X-ray observations of galaxy cluster A168 (z=0.045). Two
X-ray peaks with a projected distance of 676 kpc are found to be located close
to two dominant galaxies, respectively. Both peaks are significantly offset
from the peak of the number density distribution of galaxies. This suggests
that A168 consists of two subclusters, a northern subcluster (A168N) and a
southern subcluster (A168S). Further X-ray imaging analysis reveals that (1)
the X-ray isophotes surrounding the two X-ray peaks are heavily distorted, (2)
an elongated and ontinuous filament connects the two X-ray peaks. These suggest
that strong interactions have occurred between the two subclusters. Spectral
analysis shows that A168 has a mean temperature of 2.53 +/- 0.09 keV and a mean
metallicity of 0.31 +/- 0.04 Z_{solar}. The metallicity is roughly a constant
across the cluster but the temperature shows some systematic variations. Most
X-ray, optical and radio properties of A168 are consistent with it being an
off-axis merger several Gyrs after a core passage, although detailed numerical
simulations are required to see whether the observed properties, in particular
the significant offset between the optical and X-ray centers, can be reproduced
in such a scenario.Comment: 17 pages, 7 figures, to be published in ApJ. When this paper was
being refereed, Eric Hallman and Maxim Markevitch carried out a similar work
(astro-ph/0406322); our results are in broad agreemen
Chemical abundances and ages of the bulge stars in APOGEE high-velocity peaks
A cold high-velocity (HV, 200 km/s) peak was first reported in several
Galactic bulge fields based on the APOGEE commissioning observations. Both the
existence and the nature of the high-velocity peak are still under debate. Here
we revisit this feature with the latest APOGEE DR13 data. We find that most of
the low latitude bulge fields display a skewed Gaussian distribution with a HV
shoulder. However, only 3 out of 53 fields show distinct high-velocity peaks
around 200 km/s. The velocity distribution can be well described by
Gauss-Hermite polynomials, except the three fields showing clear HV peaks. We
find that the correlation between the skewness parameter () and the mean
velocity (), instead of a distinctive HV peak, is a strong indicator
of the bar. It was recently suggested that the HV peak is composed of
preferentially young stars. We choose three fields showing clear HV peaks to
test this hypothesis using the metallicity, [/M] and [C/N] as age
proxies. We find that both young and old stars show HV features. The similarity
between the chemical abundances of stars in the HV peaks and the main component
indicates that they are not systematically different in terms of chemical
abundance or age. In contrast, there are clear differences in chemical space
between stars in the Sagittarius dwarf and the bulge stars. The strong HV peaks
off-plane are still to be explained properly, and could be different in nature.Comment: 13 pages, 10 figures, published in ApJ. Updated to match the final
ApJ published version. Minor revisions to the text and Figure
Bayesian optimization - LSTM modeling and time frequency correlation mapping based probabilistic forecasting of ultra-short-term photovoltaic power outputs
Due to the fluctuation and randomness of photovoltaic power over time, accurate and reliable ultra-short-term photovoltaic power forecasting is significant for real-time dispatch and frequency regulation of power grids. In this paper, the improved BO-LSTM forecasting frame considering frequency correlation mapping is proposed. Firstly, the features of photovoltaic power are extracted and resolved according to power series frequency segments. Then, the established BO-LSTM forecasting model is adjusted based on the above extracted features in separate segment, and the results of deterministic forecasting are obtained. Furthermore, in order to obtain the reliable performance, the time-correlation algorithm is employed into the above deterministic forecasting model, which offers the base for probabilistic power forecasting. Finally, the above algorithms and forecasting framework are applied to the measurement data from a commercial photovoltaic power station in North China. Compared to the benchmark models, the Power Interval Normalized Average Width (PINAW) error of the proposed ultra-short-term forecasting algorithm has shown satisfied improvements. The PINAW has reduced by 8.4% (v.s. Adam-LSTM), 48.9% (v.s. Sgd-LSTM), 52.8% (v.s. Adagrad-LSTM), 9.1% (v.s. Rmsprop-LSTM), 97.2% (v.s. Adadelta-LSTM), 86.8% (v.s. Adam-mlp), 87.4% (v.s. Sgd-mlp), 90.9% (v.s. Adagrad-mlp), 86.5% (v.s. Rmsprop-mlp), and 99.7% (v.s. Adadelta-mlp)
Spatially resolved Spectro-photometry of M81: Age, Metallicity and Reddening Maps
In this paper, we present a multi-color photometric study of the nearby
spiral galaxy M81, using images obtained with the Beijing Astronomical
Observatory 60/90 cm Schmidt Telescope in 13 intermediate-band filters from
3800 to 10000{\AA}. The observations cover the whole area of M81 with a total
integration of 51 hours from February 1995 to February 1997. This provides a
multi-color map of M81 in pixels of 1\arcsec.7 \times 1\arcsec.7. Using
theoretical stellar population synthesis models, we demonstrate that some BATC
colors and color indices can be used to disentangle the age and metallicity
effect. We compare in detail the observed properties of M81 with the
predictions from population synthesis models and quantify the relative chemical
abundance, age and reddening distributions for different components of M81. We
find that the metallicity of M81 is about with no significant
difference over the whole galaxy. In contrast, an age gradient is found between
stellar populations of the central regions and of the bulge and disk regions of
M81: the stellar population in its central regions is older than 8 Gyr while
the disk stars are considerably younger, Gyr. We also give the
reddening distribution in M81. Some dust lanes are found in the galaxy bulge
region and the reddening in the outer disk is higher than that in the central
regions.Comment: Accepted for publication in AJ (May 2000 issue). 27 pages including 6
figures. Uses AASTeX aasms4 styl
Three low-mass companions around aged stars discovered by TESS
We report the discovery of three transiting low-mass companions to aged
stars: a brown dwarf (TOI-2336b) and two objects near the hydrogen burning mass
limit (TOI-1608b and TOI-2521b). These three systems were first identified
using data from the Transiting Exoplanet Survey Satellite (TESS). TOI-2336b has
a radius of , a mass of and an orbital
period of 7.71 days. TOI-1608b has a radius of , a mass of
and an orbital period of 2.47 days. TOI-2521b has a radius
of , a mass of and an orbital period of
5.56 days. We found all these low-mass companions are inflated. We fitted a
relation between radius, mass and incident flux using the sample of known
transiting brown dwarfs and low-mass M dwarfs. We found a positive correlation
between the flux and the radius for brown dwarfs and for low-mass stars that is
weaker than the correlation observed for giant planets.Comment: 20 pages, 13 figures; submitted to MNRA