1,228 research outputs found
A Census of Object Types and Redshift Estimates in the SDSS Photometric Catalog from a Trained Decision-Tree Classifier
We have applied ClassX, an oblique decision tree classifier optimized for
astronomical analysis, to the homogeneous multicolor imaging data base of the
Sloan Digital Sky Survey (SDSS), training the software on subsets of SDSS
objects whose nature is precisely known via spectroscopy. We find that the
software, using photometric data only, correctly classifies a very large
fraction of the objects with existing SDSS spectra, both stellar and
extragalactic. ClassX also accurately predicts the redshifts of both normal and
active galaxies in SDSS. To illustrate ClassX applications in SDSS research, we
(a) derive the object content of the SDSS DR2 photometric catalog and (b)
provide a sample catalog of resolved SDSS objects that contains a large number
of candidate AGN galaxies, 27,000, along with 63,000 candidate normal galaxies
at magnitudes substantially fainter than typical magnitudes of SDSS
spectroscopic objects. The surface density of AGN selected by ClassX to i~19 is
in agreement with that quoted by SDSS. When ClassX is applied to the
photometric data fainter than the SDSS spectroscopic limit, the inferred
surface density of AGN rises sharply, as expected. The ability of the
classifier to accurately constrain the redshifts of huge numbers (ultimately ~
10^7) of active galaxies in the photometric data base promises new insights
into fundamental issues of AGN research, such as the evolution of the AGN
luminosity function with cosmic time, the starburst--AGN connection, and
AGN--galactic morphology relationships.Comment: Accepted for publication in The Astronomical Journal, Vol. 130, 2005;
33 pages, 10 figures, 5 tables, AASTeX v5.0. Table 5 will be electronic in
the published journal, but available now at
http://www-int.stsci.edu/~margon/table5.ascii and
http://www-int.stsci.edu/~margon/table5.ascii.g
Evidence for Prolonged Main Sequence Stellar Evolution of F Stars in close binaries
Binary F stars exhibit large brightness anomaly, which is defined here as the
difference between the absolute magnitude from the uvby photometry and the
actual absolute magnitude of the star. We have found that the anomaly inversely
correlates with the binary components separation. There is evidence that the
correlation reflects actual population differences between close and wide
binary pairs, in which case it indicates that the anomaly is somehow associated
with the interaction of binary's components. The anomaly has also been found to
correlate with both kinematics and metallicity. The sense of the correlations
implies that the anomaly increases as the star evolves, suggesting a peculiar
evolution of a primary F star in a tight binary pair. This conclusion has
further been supported by the study of the age-velocity relation (AVR) of F
stars that are cataloged in the HIPPARCOS as single. Among these stars, those
with brightness anomaly were previously shown to be most likely unidentified
close binaries. We have found that the AVR of these binary candidates is
different from that of the ``truly single'' F stars. The discrepancy between
the two AVRs indicates that the putative binaries are, on average, older than
similar normal single F stars at the same effective temperature and luminosity,
which is consistent with the inferred peculiar evolution in close binaries. It
appears that this peculiarity is caused by the impact of the components
interaction in a tight pair on stellar evolution, which results in the
prolonged main sequence lifetime of the primary F star.Comment: 8 pages, 7 figures, accepted by Astronomy and Astrophysic
The Discovery of Soft X-ray Loud Broad Absorption Line Quasars
It is been known for more than a decade that BALQSOs (broad absorption line
quasars) are highly attenuated in the X-ray regime compared to other quasars,
especially in the soft band ( 1 keV). Using X-ray selection techniques we
have found "soft X-ray loud" BALQSOs that, by definition, have soft X-ray (0.3
keV) to UV () flux density ratios that are higher than typical nonBAL
radio quiet quasars. Our sample of 3 sources includes one LoBALQSO (low
ionization BALQSO) which are generally considered to be the most highly
attenuated in the X-rays. The three QSOs are the only known BALQSOs that have
X-ray observations that are consistent with no intrinsic soft X-ray absorption.
The existence of a large X-ray luminosity and the hard ionizing continuum that
it presents to potential UV absorption gas is in conflict with the ionization
states that are conducive to line driving forces within BAL winds (especially
for the LoBALs).Comment: To appear in ApJ Letter
Candidate Pre-Mainsequence F Stars with Circumstellar Dust Identified Using Combined 2MASS and uvby Data
We propose a method that uses near-infrared plus uvby photometry to identify
potentially extensive circumstellar dusty environment about F and A stars. The
method has been applied to a sample of ~900 metal rich reddened F stars with
2MASS and uvby data, suggesting the presence of circumstellar dust emitting in
the near infrared for ~70 stars. The log T_e - M_V diagram suggests that most,
if not all, of them are likely pre-mainsequence (PMS). They seem to be
consistent with being a continuation of the class of Herbig Ae/Be PMS stars
into the spectral type F. Their number drops sharply downward of log T_e ~ 3.84
(spectral types later than ~F5), which may provide new clues to the PMS
evolution of stars with 1 to 2 solar mass. We present a list of 21 most
conspicuous candidate stars with circumstellar dust. About half of them are
associated with the extended star-forming region around rho Oph. The brightest
of these 21 stars, with V < 7.5, turn out to be IRAS sources, suggesting the
presence of heated dust emitting in the far infrared. Also in this list, HD
81270 is reported as a very unusual star moving away from the Galactic plane at
a projected speed of 70 km/sec.Comment: 8 pages, 4 figures, 1 table. To appear in ApJ, part 2, v. 570, 2002
May
The Asymmetric Wind in M82
We have obtained detailed imaging Fabry-Perot observations of the nearby
galaxy M82, in order to understand the physical association between the
high-velocity outflow and the starburst nucleus. The observed velocities of the
emitting gas in M82 reveal a bipolar outflow of material, originating from the
bright starburst regions in the galaxy's inner disk, but misaligned with
respect to the galaxy spin axis. The deprojected outflow velocity increases
with radius from 525 to 655 km/s. Spectral lines show double components in the
centers of the outflowing lobes, with the H-alpha line split by ~300 km/s over
a region almost a kiloparsec in size. The filaments are not simple surfaces of
revolution, nor is the emission distributed evenly over the surfaces. We model
these lobes as a composite of cylindrical and conical structures, collimated in
the inner ~500 pc but expanding at a larger opening angle of ~25 degrees beyond
that radius. We compare our kinematic model with simulations of
starburst-driven winds in which disk material surrounding the source is
entrained by the wind. The data also reveal a remarkably low [NII]/H-alpha
ratio in the region of the outflow, indicating that photoionization by the
nuclear starburst may play a significant role in the excitation of the optical
filament gas, particularly near the nucleus.Comment: 42 pages AASTeX with 16 figures; accepted for publication in ApJ;
figures reformatted for better printin
Nonlinear switching and solitons in PT-symmetric photonic systems
One of the challenges of the modern photonics is to develop all-optical
devices enabling increased speed and energy efficiency for transmitting and
processing information on an optical chip. It is believed that the recently
suggested Parity-Time (PT) symmetric photonic systems with alternating regions
of gain and loss can bring novel functionalities. In such systems, losses are
as important as gain and, depending on the structural parameters, gain
compensates losses. Generally, PT systems demonstrate nontrivial
non-conservative wave interactions and phase transitions, which can be employed
for signal filtering and switching, opening new prospects for active control of
light. In this review, we discuss a broad range of problems involving nonlinear
PT-symmetric photonic systems with an intensity-dependent refractive index.
Nonlinearity in such PT symmetric systems provides a basis for many effects
such as the formation of localized modes, nonlinearly-induced PT-symmetry
breaking, and all-optical switching. Nonlinear PT-symmetric systems can serve
as powerful building blocks for the development of novel photonic devices
targeting an active light control.Comment: 33 pages, 33 figure
Wave scattering on a domain wall in a chain of PT-symmetric couplers
We study wave propagation in linear arrays composed of pairs of conjugate
waveguides with balanced gain and loss, i.e. arrays of the PT-symmetric
couplers, where the linear spectrum is known to feature high-frequency and
low-frequency branches. We introduce a domain wall by switching the gain and
loss in a half of the array, and analyze the scattering of linear waves on this
defect. The analysis reveals two major effects: amplification of both reflected
and transmitted waves, and excitation of the reflected and transmitted
low-frequency and high-frequency waves by the incident high-frequency and
low-frequency waves, respectively.Comment: 7 pages, 9 figures, Physical Review A, in pres
On the Energy Required to Eject Processed Matter from Galaxies
We evaluate the minimum energy input rate that starbursts require for
expelling their newly processed matter from their host galaxies. Special
attention is given to the pressure caused by the environment in which a galaxy
is situated, as well as to the intrinsic rotation of the gaseous component. We
account for these factors and for a massive dark matter distribution, and
develop a self-consistent solution for the interstellar matter gas
distribution. Our results are in excellent agreement with the results of Mac
Low & Ferrara (1999) for galaxies with a flattened disk-like ISM density
distribution and a low intergalactic gas pressure ( 1
cm K). However, our solution also requires a much larger energy input
rate threshold when one takes into consideration both a larger intergalactic
pressure and the possible existence of a low-density, non-rotating, extended
gaseous halo component.Comment: 7 pages, 4 figures, 1 table, Accepted for publication in Ap
- …