707 research outputs found
VVV High Proper Motion Survey
Here we present survey of proper motion stars towards the Galactic Bulge and an adjacent plane region base on VISTA-VVV data. The searching method based on cross-matching photometric Ks-band CASU catalogs. The most interesting discoveries are shown.Peer reviewe
Stromgren Photometry from z=0 to z~1. The Method
We use rest-frame Stromgren photometry to observe clusters of galaxies in a
self-consistent manner from z=0 to z=0.8. Stromgren photometry of galaxies is
an efficient compromise between standard broad-band photometry and
spectroscopy, in the sense that it is more sensitive to subtle variations in
spectral energy distributions than the former, yet much less time-consuming
than the latter. Principal Component Analysis (PCA) is used to extract maximum
information from the Stromgren data. By calibrating the Principal Components
using well-studied galaxies (and stellar population models), we develop a
purely empirical method to detect, and subsequently classify, cluster galaxies
at all redshifts smaller than 0.8. Interlopers are discarded with unprecedented
efficiency (up to 100%). The first Principal Component essentially reproduces
the Hubble Sequence, and can thus be used to determine the global star
formation history of cluster members. The (PC2, PC3) plane allows us to
identify Seyfert galaxies (and distinguish them from starbursts) based on
photometric colors alone. In the case of E/S0 galaxies with known redshift, we
are able to resolve the age-dust- metallicity degeneracy, albeit at the
accuracy limit of our present observations. This technique will allow us to
probe galaxy clusters well beyond their cores and to fainter magnitudes than
spectroscopy can achieve. We are able to directly compare these data over the
entire redshift range without a priori assumptions because our observations do
not require k-corrections. The compilation of such data for different cluster
types over a wide redshift range is likely to set important constraints on the
evolution of galaxies and on the clustering process.Comment: 35 pages, 18 figures, accepted by ApJ
Identifying Ultra-Cool Dwarfs at Low Galactic Latitudes: A Southern Candidate Catalogue
We present an Ultra-Cool Dwarf (UCD) catalogue compiled from low southern
Galactic latitudes and mid-plane, from a cross-correlation of the 2MASS and
SuperCOSMOS surveys. The catalogue contains 246 members identified from 5042
sq. deg. within 220 deg. <= l <= 360 deg. and 0 deg. < l <= 30 deg., for |b| <=
15 deg. Sixteen candidates are spectroscopically confirmed in the near-IR as
UCDs with spectral types from M7.5V to L9. Our catalogue selection method is
presented enabling UCDs from ~M8V to the L-T transition to be selected down to
a 2MASS limiting magnitude of Ks ~= 14.5 mag. This method does not require
candidates to have optical detections for catalogue inclusion. An optimal set
of optical/near-IR and reduced proper-motion selection criteria have been
defined that includes: an Rf and Ivn photometric surface gravity test, a dual
Rf-band variability check, and an additional photometric classification scheme
to selectively limit contaminants. We identify four candidates as possible
companions to nearby Hipparcos stars -- observations are needed to identify
these as potential benchmark UCD companions. We also identify twelve UCDs
within a possible distance 20 pc, three are previously unknown of which two are
estimated within 10 pc, complimenting the nearby volume-limited census of UCDs.
An analysis of the catalogue spatial completeness provides estimates for
distance completeness over three UCD MJ ranges, while Monte-Carlo simulations
provide an estimate of catalogue areal completeness at the 75 per cent level.
We estimate a UCD space density of Rho (total) = (6.41+-3.01)x10^3/pc^3 over
the range of 10.5 <= MJ ~< 14.9, similar to values measured at higher Galactic
latitudes (|b| ~> 10 deg.) in the field population and obtained from more
robust spectroscopically confirmed UCD samples.Comment: MNRAS accepted April 2012. Contains 30 figures and 11 tables. Tables
2 and 6 to be published in full and on-line only. The on-line tables can also
be obtained by contacting the author
Structural and Photometric Classification of Galaxies - I. Calibration Based on a Nearby Galaxy Sample
In this paper we define an observationally robust, multi-parameter space for
the classification of nearby and distant galaxies. The parameters include
luminosity, color, and the image-structure parameters: size, image
concentration, asymmetry, and surface brightness. Based on an initial
calibration of this parameter space using the ``normal'' Hubble-types surveyed
by Frei et al. (1996), we find that only a subset of the parameters provide
useful classification boundaries for this sample. Interestingly, this subset
does not include distance-dependent scale parameters, such as size or
luminosity. The essential ingredient is the combination of a spectral index
(e.g., color) with parameters of image structure and scale: concentration,
asymmetry, and surface-brightness. We refer to the image structure parameters
(concentration and asymmetry) as indices of ``form.'' We define a preliminary
classification based on spectral index, form, and surface-brightness (a scale)
that successfully separates normal galaxies into three classes. We
intentionally identify these classes with the familiar labels of Early,
Intermediate, and Late. This classification, or others based on the above four
parameters can be used reliably to define comparable samples over a broad range
in redshift. The size and luminosity distribution of such samples will not be
biased by this selection process except through astrophysical correlations
between spectral index, form, and surface-brightness.Comment: to appear in AJ (June, 2000); 34 pages including 4 tables and 12
figure
Low voltage electrical properties of polypropylene filled with stainless steel fibres and a model of sample conductivity based on fibre geometry
Magneto-infrared modes in InAs-AlSb-GaSb coupled quantum wells
We have studied a series of InAs/GaSb coupled quantum wells using
magneto-infrared spectroscopy for high magnetic fields up to 33T within
temperatures ranging from 4K to 45K in both Faraday and tilted field
geometries. This type of coupled quantum wells consists of an electron layer in
the InAs quantum well and a hole layer in the GaSb quantum well, forming the
so-called two dimensional electron-hole bilayer system. Unlike the samples
studied in the past, the hybridization of the electron and hole subbands in our
samples is largely reduced by having narrower wells and an AlSb barrier layer
interposed between the InAs and the GaSb quantum wells, rendering them weakly
hybridized. Previous studies have revealed multiple absorption modes near the
electron cyclotron resonance of the InAs layer in moderately and strongly
hybridized samples, while only a single absorption mode was observed in the
weakly hybridized samples. We have observed a pair of absorption modes
occurring only at magnetic fields higher than 14T, which exhibited several
interesting phenomena. Among which we found two unique types of behavior that
distinguishes this work from the ones reported in the literature. This pair of
modes is very robust against rising thermal excitations and increasing magnetic
fields alligned parallel to the heterostructures. While the previous results
were aptly explained by the antilevel crossing gap due to the hybridization of
the electron and hole wavefunctions, i.e. conduction-valence Landau level
mixing, the unique features reported in this paper cannot be explained within
the same concept. The unusual properties found in this study and their
connection to the known models for InAs/GaSb heterostructures will be
disccused; in addition, several alternative ideas will be proposed in this
paper and it appears that a spontaneous phase separation can account for most
of the observed features
2MASS J03105986+1648155AB - A new binary at the L/T transition
The transition from the L to the T spectral type of brown dwarfs is marked by
a very rapid transition phase, remarkable brightening in the J-band and a
higher binary frequency. Despite being an active area of inquiry, this
transition regime still remains one of the most poorly understood phases of
brown dwarf evolution. We resolved the L dwarf 2MASS J03105986+1648155 for the
first time into two almost equally bright components straddling the L/T
transition. Since such a co-eval system with common age and composition
provides crucial information of this special transition phase, we monitored the
system over 3 years to derive first orbital parameters and dynamical mass
estimates, as well as a spectral type determination. We obtained resolved high
angular resolution, near-IR images with HST and the adaptive optics instrument
NACO at the VLT including the laser guide star system PARSEC. Based on two
epochs of astrometric data we derive a minimum semi-major axis of 5.2 +- 0.8
AU. The assumption of a face-on circular orbit yields an orbital period of 72
+- 4 years and a total system mass of 30-60 Mjup. This places the masses of the
individual components of the system at the lower end of the mass regime of
brown dwarfs. The achieved photometry allowed a first spectral type
determination of L9 +- 1 for each component. In addition, this seems to be only
the fifth resolved L/T transition binary with a flux reversal. While ultimate
explanations for this effect are still owing, the 2MASS J03105986+1648155
system adds an important benchmark object for improving our understanding of
this remarkable evolutionary phase of brown dwarfs. Additionally, the
observational results of 2MASS J03105986+1648155 AB derived with the new PARSEC
AO system at the VLT show the importance of this technical capability. The
updated AO system allows us to significantly extend the sample of brown dwarfs
observable with high-resolution from the ground and hence to reveal more of
their physical properties.Comment: 6 pages, 2 figures, 3 tables, accepted for publication by A&
The sub-stellar birth rate from UKIDSS
We present a new sample of mid-L to mid-T dwarfs with effective temperatures of 11001700 K selected from the UKIDSS Large Area Survey (LAS) and confirmed with infrared spectra from X-shooter/Very Large Telescope. This effective temperature range is especially sensitive to the formation history of Galactic brown dwarfs and allows us to constrain the form of the sub-stellar birth rate, with sensitivity to differentiate between a flat (stellar like) birth rate and an exponentially declining form. We present the discovery of 63 new L and T dwarfs from the UKIDSS LAS DR7, including the identification of 12 likely unresolved binaries, which form the first complete sub-set from our programme, covering 495 square degrees of sky, complete to J = 18.1. We compare our results for this sub-sample with simulations of differing birth rates for objects of masses 0.10-0.03 M-circle dot and ages 1-10 Gyr. We find that the more extreme birth rates (e. g. a halo type form) can likely be excluded as the true form of the birth rate. In addition, we find that although there is substantial scatter we find a preference for a mass function, with a power-law index a in the range -1 <alpha <0 that is consistent (within the errors) with the studies of late T dwarfs.Peer reviewe
Decorrelating the Power Spectrum of Galaxies
It is shown how to decorrelate the (prewhitened) power spectrum measured from
a galaxy survey into a set of high resolution uncorrelated band-powers. The
treatment includes nonlinearity, but not redshift distortions. Amongst the
infinitely many possible decorrelation matrices, the square root of the Fisher
matrix, or a scaled version thereof, offers a particularly good choice, in the
sense that the band-power windows are narrow, approximately symmetric, and
well-behaved in the presence of noise. We use this method to compute band-power
windows for, and the information content of, the Sloan Digital Sky Survey, the
Las Campanas Redshift Survey, and the IRAS 1.2 Jy Survey.Comment: 11 pages, including 8 embedded PostScript figures. Minor changes to
agree with published versio
Dynamical segregation of galaxies in groups and clusters
We have performed a systematic analysis of the dynamics of different galaxy
populations in galaxy groups from the 2dFGRS. For this purpose we have combined
all the groups into a single system, where velocities v and radius r are
expressed adimensionally. We have used several methods to compare the
distributions of relative velocities of galaxies with respect to the group
centre for samples selected according to their spectral type (as defined by
Madgwick et al., 2002), bj band luminosity and B-R colour index. We have found
strong segregation effects: spectral type I objects show a statistically
narrower velocity distribution than that of galaxies with a substantial star
formation activity (type II-IV). Similarly, the same behavior is observed for
galaxies with colour index B-R>1 compared to galaxies with B-R<1. Bright
(Mb-19) galaxies show the same segregation. It is not
important once the sample is restricted to a given spectral type. These effects
are particularly important in the central region (Rp<Rvirial/2) and do not have
a strong dependence on the mass of the parent group. These trends show a strong
correlation between the dynamics of galaxies in groups and star formation rate
reflected both by spectral type and by colour index.Comment: 7 pages, 8 figures. Accepted for publication in MNRA
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