7,247 research outputs found
Statistical Properties of Radio Emission from the Palomar Seyfert Galaxies
We have carried out an analysis of the radio and optical properties of a
statistical sample of 45 Seyfert galaxies from the Palomar spectroscopic survey
of nearby galaxies. We find that the space density of bright galaxies (-22 mag
<= M_{B_T} <= -18 mag) showing Seyfert activity is (1.25 +/- 0.38) X 10^{-3}
Mpc^{-3}, considerably higher than found in other Seyfert samples. Host galaxy
types, radio spectra, and radio source sizes are uncorrelated with Seyfert
type, as predicted by the unified schemes for active galaxies. Approximately
half of the detected galaxies have flat or inverted radio spectra, more than
expected based on previous samples. Surprisingly, Seyfert 1 galaxies are found
to have somewhat stronger radio sources than Seyfert 2 galaxies at 6 and 20 cm,
particularly among the galaxies with the weakest nuclear activity. We suggest
that this difference can be accommodated in the unified schemes if a minimum
level of Seyfert activity is required for a radio source to emerge from the
vicinity of the active nucleus. Below this level, Seyfert radio sources might
be suppressed by free-free absorption associated with the nuclear torus or a
compact narrow-line region, thus accounting for both the weakness of the radio
emission and the preponderance of flat spectra. Alternatively, the flat spectra
and weak radio sources might indicate that the weak active nuclei are fed by
advection-dominated accretion disks.Comment: 18 pages using emulateapj5, 13 embedded figures, accepted by Ap
Response of thin-film SQUIDs to applied fields and vortex fields: Linear SQUIDs
In this paper we analyze the properties of a dc SQUID when the London
penetration depth \lambda is larger than the superconducting film thickness d.
We present equations that govern the static behavior for arbitrary values of
\Lambda = \lambda^2/d relative to the linear dimensions of the SQUID. The
SQUID's critical current I_c depends upon the effective flux \Phi, the magnetic
flux through a contour surrounding the central hole plus a term proportional to
the line integral of the current density around this contour. While it is well
known that the SQUID inductance depends upon \Lambda, we show here that the
focusing of magnetic flux from applied fields and vortex-generated fields into
the central hole of the SQUID also depends upon \Lambda. We apply this
formalism to the simplest case of a linear SQUID of width 2w, consisting of a
coplanar pair of long superconducting strips of separation 2a, connected by two
small Josephson junctions to a superconducting current-input lead at one end
and by a superconducting lead at the other end. The central region of this
SQUID shares many properties with a superconducting coplanar stripline. We
calculate magnetic-field and current-density profiles, the inductance
(including both geometric and kinetic inductances), magnetic moments, and the
effective area as a function of \Lambda/w and a/w.Comment: 18 pages, 20 figures, revised for Phys. Rev. B, the main revisions
being to denote the effective flux by \Phi rather than
Extremely Anisotropic Scintillations
A small number of quasars exhibit interstellar scintillation on time-scales
less than an hour; their scintillation patterns are all known to be
anisotropic. Here we consider a totally anisotropic model in which the
scintillation pattern is effectively one-dimensional. For the persistent rapid
scintillators J1819+3845 and PKS1257-326 we show that this model offers a good
description of the two-station time-delay measurements and the annual cycle in
the scintillation time-scale. Generalising the model to finite anisotropy
yields a better match to the data but the improvement is not significant and
the two additional parameters which are required to describe this model are not
justified by the existing data. The extreme anisotropy we infer for the
scintillation patterns must be attributed to the scattering medium rather than
a highly elongated source. For J1819+3845 the totally anisotropic model
predicts that the particular radio flux variations seen between mid July and
late August should repeat between late August and mid November, and then again
between mid November and late December as the Earth twice changes its direction
of motion across the scintillation pattern. If this effect can be observed then
the minor-axis velocity component of the screen and the orientation of that
axis can both be precisely determined. In reality the axis ratio is finite,
albeit large, and spatial decorrelation of the flux pattern along the major
axis may be observable via differences in the pairwise fluxes within this
overlap region; in this case we can also constrain both the major-axis velocity
component of the screen and the magnitude of the anisotropy.Comment: 5 pages, 4 figures, MNRAS submitte
The spin temperature of high-redshift damped Lyman- systems
We report results from a programme aimed at investigating the temperature of
neutral gas in high-redshift damped Lyman- absorbers (DLAs). This
involved (1) HI 21cm absorption studies of a large DLA sample, (2) VLBI studies
to measure the low-frequency quasar core fractions, and (3) optical/ultraviolet
spectroscopy to determine DLA metallicities and velocity widths.
Including literature data, our sample consists of 37 DLAs with estimates of
the spin temperature and the covering factor. We find a strong )
difference between the distributions in high-z (z>2.4) and low-z (z<2.4)
DLA samples. The high-z sample contains more systems with high values,
K. The distributions in DLAs and the Galaxy are also
clearly (~) different, with more high- sightlines in DLAs than in
the Milky Way. The high values in the high-z DLAs of our sample arise due
to low fractions of the cold neutral medium.
For 29 DLAs with metallicity [Z/H] estimates, we confirm the presence of an
anti-correlation between and [Z/H], at significance via a
non-parametric Kendall-tau test. This result was obtained with the assumption
that the DLA covering factor is equal to the core fraction. Monte Carlo
simulations show that the significance of the result is only marginally
decreased if the covering factor and the core fraction are uncorrelated, or if
there is a random error in the inferred covering factor.
We also find evidence for redshift evolution in DLA values even for the
z>1 sub-sample. Since z>1 DLAs have angular diameter distances comparable to or
larger than those of the background quasars, they have similar efficiency in
covering the quasars. Low covering factors in high-z DLAs thus cannot account
for the observed redshift evolution in spin temperatures. (Abstract abridged.)Comment: 37 pages, 22 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Nearest pattern interaction and global pattern formation
We studied the effect of nearest pattern interaction on a globally pattern
formation in a 2-dimensional space, where patterns are to grow initially from a
noise in the presence of periodic supply of energy. Although our approach is
general, we found that this study is relevant in particular to the pattern
formation on a periodically vibrated granular layer, as it gives a unified
perspective of the experimentally observed pattern dynamics such as oscillon
and stripe formations, skew-varicose and crossroll instabilities, and also a
kink formation and decoration
Radio Sources in Low-Luminosity Active Galactic Nuclei. I. VLA Detections of Compact, Flat-Spectrum Cores
We report a 0.2" resolution, 15 GHz survey of a sample of 48 low-luminosity
active galactic nuclei with the Very Large Array. Compact radio emission has
been detected in 57% (17 of 30) of LINERs and low-luminosity Seyferts, at least
15 of which have a flat to inverted radio spectrum (alpha > -0.3). The compact
radio cores are found in both type 1 (i.e. with broad Halpha) and type 2
(without broad Halpha) nuclei. The 2 cm radio power is significantly correlated
with the emission-line ([OI] lambda6300) luminosity. While the present
observations are consistent with the radio emission originating in star-forming
regions, higher resolution radio observations of 10 of the detected sources,
reported in a companion paper (Falcke et al. 2000), show that the cores are
very compact (= 10^8K) and
probably synchrotron self-absorbed, ruling out a starburst origin. Thus, our
results suggest that at least 50% of low-luminosity Seyferts and LINERs in the
sample are accretion powered, with the radio emission presumably coming from
jets or advection-dominated accretion flows. We have detected only 1 of 18
`transition' (i.e. LINER + HII) nuclei observed, indicating their radio cores
are significantly weaker than those of `pure' LINERs.Comment: To appear in the Astrophysical Journal, October 20, 200
CLASS B0827+525: `Dark lens' or binary radio-loud quasar?
We present radio, optical, near-infrared and spectroscopic observations of
the source B0827+525. We consider this source as the best candidate from the
Cosmic Lens All-Sky Survey (CLASS) for a `dark lens' system or binary
radio-loud quasar. The system consists of two radio components with somewhat
different spectral indices, separated by 2.815 arcsec. VLBA observations show
that each component has substructure on a scale of a few mas. A deep K-band
exposure with the W.M.Keck-II Telescope reveals emission near both radio
components. The K-band emission of the weaker radio component appears extended,
whereas the emission from the brighter radio component is consistent with a
point source. Hubble Space Telescope F160W-band observations with the NICMOS
instrument confirms this. A redshift of 2.064 is found for the brighter
component, using the LRIS instrument on the W.M.Keck-II Telescope. The
probability that B0827+525 consists of two unrelated compact flat-spectrum
radio sources is ~3%, although the presence of similar substructure in both
component might reduce this.
We discuss two scenarios to explain this system: (i) CLASS B0827+525 is a
`dark lens' system or (ii) B0827+525 is a binary radio-loud quasar. B0827+525
has met all criteria that thus far have in 100% of the cases confirmed a source
as an indisputable gravitational lens system. Despite this, no lens galaxy has
been detected with m_F160W<=23 mag. Hence, we might have found the first binary
radio-loud quasar. At this moment, however, we feel that the `dark lens'
hypothesis cannot yet be fully excluded.Comment: 9 pages, 6 figures; Accepted for publication in Astronomy &
Astrophysics; Full-res. images 1 and 3 can be obtained from L.V.E.
An Imaging and Spectroscopic Study of the z=3.38639 Damped Lyman Alpha System in Q0201+1120: Clues to Star Formation Rate at High Redshift
We present the results of a series of imaging and spectroscopic observations
aimed at identifying and studying the galaxy responsible for the z = 3.38639
damped lya system in the z = 3.61 QSO Q0201+1120. We find that the DLA is part
of a concentration of matter which includes at least four galaxies (probably
many more) over linear comoving dimensions, greater than 5h^-1Mpc. The absorber
may be a 0.7 L* galaxy at an impact parameter of 15 h^-1 kpc, but follow-up
spectroscopy is still required for positive identification. The gas is
turbulent, with many absorption components distributed over approximately 270
km/s and a large spin temperature, T_s greater than 4000K. The metallicity is
relatively high for this redshift, Z(DLA) approximately 1/20 Z(solar). From
consideration of the relative ratios of elements which have different
nucleosynthetic timescales, it would appear that the last major episode of star
formation in this DLA occurred at z greater than 4.3, more than approximately
500 Myr prior to the time when we observe it.Comment: Accepted for publication in Ap
Physical Conditions in the Narrow-Line Region of M51
We have investigated the physical conditions in the narrow-line region (NLR)
of M51 using long-slit spectra obtained with the Space Telescope Imaging
Spectrograph (STIS) aboard the Hubble Space Telescope (HST) and 3.6 cm radio
continuum observations obtained with the Very Large Array (VLA). Emission-line
diagnostics were employed for nine NLR clouds, which extend 2.5" (102 pc) from
the nucleus, to examine the electron density, temperature, and ionization state
of the NLR gas. The emission-line ratios are consistent with those typically
found in Seyfert nuclei and indicate that within the inner near-nuclear region
(r ~< 1") the ionization decreases with increasing radius. Upper-limits to the
[O III] electron temperature (T ~< 11,000 K) for the inner NLR clouds indicate
that photoionization is the dominant ionization mechanism close to the nucleus.
The emission-line fluxes for most of the NLR clouds can be reproduced
reasonably well by simple photoionization models using a central power-law
continuum source and supersolar nitrogen abundances. Shock+precursor models,
however, provide a better fit to the observed fluxes of an NLR cloud ~2.5"
south of the nucleus that is identified with the extra-nuclear cloud (XNC). The
large [O III] electron temperature of this cloud (T = 24,000 K) further
suggests the presence of shocks. This cloud is straddled by two radio knots and
lies near the location where a weak radio jet, ~2.5" (102pc) in extent,
connects the near-nuclear radio emission with a diffuse lobe structure spanning
\~4" (163 pc). It is plausible that this cloud represents the location where
the radio jet impinges on the disk ISM.Comment: 25 pages, 26 figures (9 color), 7 tables. Accepted for publication in
the Astrophysical Journa
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