501 research outputs found
Spitzer IRAC Imaging of the Relativistic Jet from Superluminal Quasar PKS 0637-752
Emission from the relativistic jet located at hundreds of kpc from the core
of the superluminal quasar PKS 0637-752 was detected at 3.6 and 5.8 microns
with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. The
unprecedented sensitivity and arcsecond resolution of IRAC allows us to explore
the mid-infrared emission from kiloparsec-scale quasar jets for the first time.
The mid-infrared flux from the jet knots, when combined with radio and optical
fluxes, confirms a synchrotron origin of the radio-to-optical emission and
constrains very well the high energy end of the nonthermal electron
distribution. Assuming the X-rays are produced in the relativistically moving
knots via inverse Compton scattering of cosmic microwave background (CMB)
radiation, the infrared observation puts constraints on the matter content of
the quasar extended jet. Specifically, pure electron-positoron pair jet models
are unfavorable based on the lack of an infrared bump associated with ``bulk
Comptonization'' of CMB photons by an ultrarelativistic jet.Comment: 4 pages, 3 figures; accepted for publication in ApJ Letter
An Infrared Study of the Large-scale Jet in Quasar PKS 1136-135
We present Spitzer IRAC imaging of the large-scale jet in the quasar PKS
1136-135 at wavelengths of 3.6 and 5.8 um, combined with previous VLA, HST, and
Chandra observations. We clearly detect infrared emission from the jet,
resulting in the most detailed multifrequency data among the jets in
lobe-dominated quasars. The spectral energy distributions of the jet knots have
significant variations along the jet, like the archetypal jet in 3C 273. The
infrared measurements with IRAC are consistent with the previous idea that the
jet has two spectral components, namely (1) the low-energy synchrotron spectrum
extending from radio to infrared, and (2) the high-energy component responsible
for the X-ray flux. The optical fluxes may be a mixture of the two components.
We consider three radiation models for the high-energy component: inverse
Compton scattering off CMB photons by radio-emitting electrons in a highly
relativistic jet, synchrotron radiation by a second distinct electron
population, and synchrotron radiation by ultra high energy protons. Each
hypothesis leads to important insights into and constraints on particle
acceleration in the jet, as well as the basic physical properties of the jet
such as bulk velocity, transporting power, and particle contents.Comment: 9 pages, 5 figures (2 color figures), accepted for publication in
ApJ; one typo in Table 1 is correcte
Optical properties of metal nanoparticles with no center of inversion symmetry: observation of volume plasmons
We present theoretical and experimental studies of the optical response of
L-shaped silver nanoparticles. The scattering spectrum exhibits several plasmon
resonances that depend sensitively on the polarization of the incident
electromagnetic field. The physical origin of the resonances is traced to
different plasmon phenomena. In particular, a high energy band with unusual
properties is interpreted in terms of volume plasmon oscillations arising from
the asymmetry of a nanoparticle.Comment: 14 pages, 5 figures. Physical Review B, 2007, accepte
Shedding New Light on the 3C 273 Jet with the Spitzer Space Telescope
We have performed infrared imaging of the jet of the quasar 3C 273 at
wavelengths 3.6 and 5.8 microns with the Infrared Array Camera (IRAC) on the
Spitzer Space Telescope. When combined with the radio, optical and X-ray
measurements, the IRAC photometry clearly shows that the optical emission is
dominated by the high-energy component of the jet, not by the radio synchrotron
component, as had been assumed to date. The high-energy component may be due to
a second synchrotron component or to IC scattering of ambient photons. In the
former case, we argue that the acceleration of protons exceeding 10^16 eV or
possibly even to 10^19 eV would be taking place in the jet. In contrast, the IC
model, into which highly relativistic Doppler beaming has to be incorporated,
requires very low-energy electrons (~ 1 MeV). The present polarization data in
the radio and optical would favor the former interpretation in the case of the
3C 273 jet. Sensitive and detailed measurements of optical polarization are
important to establish the radiation mechanism responsible for the high-energy
emission. The present study offers new clues as to the controversial origin of
the X-ray emission seen in many quasar jets.Comment: 12 pages, 8 figures (2 color figures), accepted for publication in
ApJ, color images are also available at
http://www.astro.isas.jaxa.jp/~uchiyama/Site2/Spitzer_3C273.htm
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Radio Continuum Emission at 1.4 GHz from KISS Emission-Line Galaxies
We have searched the Faint Images of the Radio Sky at Twenty centimeters
(FIRST) and the NRAO VLA Sky Survey (NVSS) 1.4 GHz radio surveys for sources
that are coincident with emission-line galaxy (ELG) candidates from the KPNO
International Spectroscopic Survey (KISS). A total of 207 of the 2157 KISS ELGs
(~10%) in the first two H-alpha-selected survey lists were found to possess
radio detections in FIRST and/or NVSS. Follow-up spectra exist for all of the
radio detections, allowing us to determine the activity type (star-forming vs.
AGN) for the entire sample. We explore the properties of the radio-detected
KISS galaxies in order to gain a better insight into the nature of
radio-emitting galaxies in the local universe (z < 0.1). No dwarf galaxies were
detected, despite the large numbers of low-luminosity galaxies present in KISS,
suggesting that lower mass, lower luminosity objects do not possess strong
galaxian-scale magnetic fields. Due to the selection technique used for KISS,
our radio ELGs represent a quasi-volume-limited sample, which allows us to
develop a clearer picture of the radio galaxy population at low redshift.
Nearly 2/3rds of the KISS radio galaxies are starburst/star-forming galaxies,
which is in stark contrast to the results of flux-limited radio surveys that
are dominated by AGNs and elliptical galaxies (i.e., classic radio galaxies).
While there are many AGNs among the KISS radio galaxies, there are no objects
with large radio powers in our local volume. We derive a radio luminosity
function (RLF) for the KISS ELGs that agrees very well with previous RLFs that
adequately sample the lower-luminosity radio population.Comment: Accepted for publication in the Astronomical Journal (April 2004); 23
pages, 16 figure
Halo Star Streams in the Solar Neighborhood
We have assembled a sample of halo stars in the solar neighborhood to look
for halo substructure in velocity and angular momentum space. Our sample
includes red giants, RR Lyrae, and red horizontal branch stars within 2.5 kpc
of the Sun with [Fe/H] less than -1.0. It was chosen to include stars with
accurate distances, space velocities, and metallicities as well as
well-quantified errors. We confirm the existence of the streams found by Helmi
and coworkers, which we refer to as the H99 streams. These streams have a
double-peaked velocity distribution in the z direction. We use the results of
modeling of the H99 streams by Helmi and collaborators to test how one might
use v_z velocity information and radial velocity information to detect
kinematic substructure in the halo. We find that detecting the H99 streams with
radial velocities alone would require a large sample. We use the velocity
distribution of the H99 streams to estimate their age. From our model of the
progenitor of the H99 streams, we determine that it was accreted between 6 and
9 Gyr ago. The H99 streams have [alpha/Fe] abundances similar to other halo
stars in the solar neighborhood, suggesting that the gas that formed these
stars were enriched mostly by Type II SNe. We have also discovered in angular
momentum space two other possible substructures, which we refer to as the
retrograde and prograde outliers. The retrograde outliers are likely to be halo
substructure, but the prograde outliers are most likely part of the smooth
halo. The retrograde outliers have significant structure in the v_phi direction
and show a range of [alpha/Fe]. The methods presented in this paper can be used
to exploit the kinematic information present in future large databases like
RAVE, SDSSII/SEGUE, and Gaia.Comment: 46 pages, 13 figures, and 9 tables. Minor changes to text to match
proofed version of the paper. Low resolution figures. High resolution version
at http://www.astro.wisc.edu/~kepley/solar_streams.p
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