1,253 research outputs found
A New Einstein Cross: A Highly Magnified, Intrinsically Faint Lyman-Alpha Emitter at z=2.7
We report the discovery of a new Einstein cross at redshift z_S = 2.701 based
on Lyman-alpha emission in a cruciform configuration around an SDSS luminous
red galaxy (z_L = 0.331). The system was targeted as a possible lens based on
an anomalous emission line in the SDSS spectrum. Imaging and spectroscopy from
the W. M. Keck Observatory confirm the lensing nature of this system. This is
one of the widest-separation galaxy-scale lenses known, with an Einstein radius
of ~1.84 arcsec. We present simple gravitational lens models for the system and
compute the intrinsic properties of the lensed galaxy. The total mass of the
lensing galaxy within the 8.8 +/- 0.1 kpc enclosed by the lensed images is (5.2
+/- 0.1) x 10^11 M_sun. The lensed galaxy is a low mass galaxy (0.2 L*) with a
high equivalent-width Lyman-alpha line (EW_Lya_rest = 46 +/- 5 Angstroms).
Follow-up studies of this lens system can probe the mass structure of the
lensing galaxy, and can provide a unique view of an intrinsically faint,
high-redshift, star-forming galaxy at high signal-to-noise ratio.Comment: ApJ Letters, in pres
Tears Of Love / music by S.R. Henry; words by Frank K. Warren
Cover: a photo of a woman; Publisher: Jos. W. Stern and Co. (New York)https://egrove.olemiss.edu/sharris_c/1152/thumbnail.jp
An improved model of the Edgeworth-Kuiper debris disk
(Abridged) We access the expected EKB dust disk properties by modeling. We
treat the debiased population of the known transneptunian objects (TNOs) as
parent bodies and generate the dust with our collisional code. The resulting
dust distributions are modified to take into account the influence of
gravitational scattering and resonance trapping by planets on migrating dust
grains as well as the effect of sublimation. A difficulty is that the amount
and distribution of dust are largely determined by sub-kilometer-sized bodies.
These are directly unobservable, and their properties cannot be accessed by
collisional modeling, because objects larger than 10...60m in the present-day
EKB are not in a collisional equilibrium. To place additional constraints, we
use in-situ measurements of the New Horizons spacecraft within 20AU. We show
that the TNO population has to have a break in the size distribution at s<70km.
However, even this still leaves us with several models that all correctly
reproduce a nearly constant dust impact rates in the region of giant planet
orbits and do not violate the constraints from the non-detection of the EKB
dust thermal emission by the COBE spacecraft. The modeled EKB dust disks, which
conform to the observational constraints, can either be transport-dominated or
intermediate between the transport-dominated and collision-dominated regime.
The in-plane optical depth of such disks is tau(r>10AU)~10^-6 and their
fractional luminosity is f_d~10^-7. Planets and sublimation are found to have
little effect on dust impact fluxes and dust thermal emission. The spectral
energy distribution of an EKB analog, as would be seen from 10pc distance,
peaks at wavelengths of 40...50\mum at F~0.5mJy, which is less than 1% of the
photospheric flux at those wavelengths. Therefore, exact EKB analogs cannot be
detected with present-day instruments such as Herschel/PACS.Comment: 10 pages, 8 figures, accepted for publication in Astronomy and
Astophysic
Rest-frame optical continua of L ~ L*, z>3 quasars: probing the faint end of the high z quasar luminosity function
Near-IR photometry for 20 radio-loud z>3 quasars, 16 of which are radio-
selected, are presented. These data sample the rest-frame optical/UV continuum,
which is commonly interpreted as emission from an accretion disk. In a previous
study, we compared the rest-frame optical/UV continuum shapes of 15 optically
bright (V3 quasars with those of 27 low redshift (z~0.1) ones that
were matched to the high redshift sample in evolved luminosity (i.e. having
luminosities ranging from 1-7 times the characteristic luminosity, L*, where
L*~(1+z)^{~3}) to look for signs of evolution in the central engines. We found
the continuum shapes at z~0.1 and z>3 similar, consistent with no significant
change in the ratio mdot/M, where mdot is the accretion rate with respect to
the Eddington rate and M is the black hole mass. This study expands our earlier
high redshift sample to lower luminosity, away from extreme objects and towards
a luminosity overlap with lower redshift samples. The distribution of
rest-frame optical/UV continuum shapes for this fainter sample is broader,
extending further to the red than that of the brighter z>3 one. Three quasars
from this fainter sample, two radio-selected and one optically-selected, have
optical continuum slopes alpha<-1 (F_{nu}~nu^{alpha}). The optically-selected
one, LBQS0056+0125, appears to be reddened by dust along the line of sight or
in the host galaxy, whereas the radio-selected ones, PKS2215+02 and
TXS2358+189, could derive their red continua from the contribution of a
relatively strong synchrotron component to the rest-frame optical. These
objects may represent a bridge to a population of very red high redshift
quasars to which ongoing or future near-IR, optical and deep X-ray surveys will
be sensitive.Comment: 16 pages, 6 figures; accepted for publication in MNRA
Quasar candidates selection in the Virtual Observatory era
We present a method for the photometric selection of candidate quasars in
multiband surveys. The method makes use of a priori knowledge derived from a
subsample of spectroscopic confirmed QSOs to map the parameter space. The
disentanglement of QSOs candidates and stars is performed in the colour space
through the combined use of two algorithms, the Probabilistic Principal
Surfaces and the Negative Entropy clustering, which are for the first time used
in an astronomical context. Both methods have been implemented in the VONeural
package on the Astrogrid VO platform. Even though they belong to the class of
the unsupervised clustering tools, the performances of the method are optimized
by using the available sample of confirmed quasars and it is therefore possible
to learn from any improvement in the available "base of knowledge". The method
has been applied and tested on both optical and optical plus near infrared data
extracted from the visible SDSS and infrared UKIDSS-LAS public databases. In
all cases, the experiments lead to high values of both efficiency and
completeness, comparable if not better than the methods already known in the
literature. A catalogue of optical candidate QSOs extracted from the SDSS DR7
Legacy photometric dataset has been produced and is publicly available at the
URL voneural.na.infn.it/qso.html.Comment: 75 pages, 43 figure, 7 tables, accepted for publication in MNRA
The Edgeworth-Kuiper debris disk
(Abridged) The Edgeworth-Kuiper belt with its presumed dusty debris is a
natural reference for extrsolar debris disks. We employ a new algorithm to
eliminate the inclination and the distance selection effects in the known TNO
populations to derive expected parameters of the "true" EKB. Its estimated mass
is M_EKB=0.12 M_earth, which is by a factor of \sim 15 larger than the mass of
the EKB objects detected so far. About a half of the total EKB mass is in
classical and resonant objects and another half is in scattered ones. Treating
the debiased populations of EKB objects as dust parent bodies, we then
"generate" their dust disk with our collisional code. Apart from accurate
handling of collisions and direct radiation pressure, we include the
Poynting-Robertson (P-R) drag, which cannot be ignored for the EKB dust disk.
Outside the classical EKB, the radial profile of the optical depth
approximately follows tau \sim r^-2 which is roughly intermediate between the
slope predicted analytically for collision-dominated (r^-1.5) and
transport-dominated (r^-2.5) disks. The cross section-dominating grain size
still lies just above the blowout size (\sim 1...2 \microm), as it would
without the P-R transport. However, if the EKB were by one order of magnitude
less massive, the optical depth profile would fall off as tau \sim r^-3, and
the cross section-dominating grain size would shift from \sim 1...2\microm to
~100 \microm. These properties are seen if dust is assumed to be generated only
by known TNOs. If the solar system were observed from outside, the thermal
emission flux from the EKB dust would be about two orders of magnitude lower
than for solar-type stars with the brightest known infrared excesses observed
from the same distance. Herschel and other new-generation facilities should
reveal extrasolar debris disks nearly as tenuous as the EKB disk. The
Herschel/PACS instrument should be able to detect disks at a \sim 1...2M_EKB
level.Comment: 18 pages, 14 figures, accepted for publication in A&
The Optical and Near-Infrared Properties of 2837 Quasars in the UKIRT Infrared Deep Sky Survey (UKIDSS)
The UKIRT Infrared Deep Sky Survey (UKIDSS) is the first of a new generation
of hemispheric imaging projects to extend the work of the Two Micron All Sky
Survey (2MASS) by reaching three magnitudes deeper in YJHK imaging, to K=18.2
(5-sigma, Vega) over wide fields. Better complementing existing optical surveys
such as the Sloan Digital Sky Survey (SDSS), the resulting public imaging
catalogues provide new photometry of rare object samples too faint to be
reached previously. The first data release of UKIDSS has already surpassed
2MASS in terms of photons gathered, and using this new dataset we examine the
near-infrared properties of 2837 quasars found in the SDSS and newly catalogued
by the UKIDSS in ~189 square degrees. The matched quasars include the RA range
22hr to 4hr on the Southern Equatorial Stripe (SDSS Stripe 82), an area of
significant future followup possibilities with deeper surveys and pointed
observations. The sample covers the redshift and absolute magnitude ranges
0.08<z<5.03 and -29.5<M_i<-22.0, and 98 per cent of SDSS quasars have matching
UKIDSS data. We discuss the photometry, astrometry, and various colour
properties of the quasars. We also examine the effectiveness of quasar/star
separation using the near-infrared passbands. The combination of SDSS ugriz
photometry with the YJHK near-infrared photometry from UKIDSS over large areas
of sky has enormous potential for advancing our understanding of the quasar
population.Comment: Accepted for publication in MNRAS, 11 pages, 13 figures, full
resolution figures available for download (see links in text
The VMC Survey - VI. Quasars behind the Magellanic system
The number and spatial distribution of confirmed quasi-stellar objects (QSOs)
behind the Magellanic system is limited. This undermines their use as
astrometric reference objects for different types of studies. We have searched
for criteria to identify candidate QSOs using observations from the VISTA
survey of the Magellanic Clouds system (VMC) that provides photometry in the
YJKs bands and 12 epochs in the Ks band. The (Y-J) versus (J-Ks) diagram has
been used to distinguish QSO candidates from Milky Way stars and stars of the
Magellanic Clouds. Then, the slope of variation in the Ks band has been used to
identify a sample of high confidence candidates. These criteria were developed
based on the properties of 117 known QSOs presently observed by the VMC survey.
VMC YJKs magnitudes and Ks light-curves of known QSOs behind the Magellanic
system are presented. About 75% of them show a slope of variation in Ks>10^-4
mag/day and the shape of the light-curve is in general irregular and without
any clear periodicity. The number of QSO candidates found in tiles including
the South Ecliptic Pole and the 30 Doradus regions is 22 and 26, respectively,
with a ~20% contamination by young stellar objects, planetary nebulae, stars
and normal galaxies. By extrapolating the number of QSO candidates to the
entire VMC survey area we expect to find about 1200 QSOs behind the LMC, 400
behind the SMC, 200 behind the Bridge and 30 behind the Stream areas, but not
all will be suitable for astrometry. Further, the Ks band light-curves can help
support investigations of the mechanism responsible for the variations.Comment: 17 pages, 15 figures, replaced with accepted version by Astronomy &
Astrophysic
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