730 research outputs found
High Levels of Circularly Polarized Emission from the Radio Jet in NGC 1275 (3C 84)
We present multi-frequency, high resolution VLBA circular polarization images
of the radio source 3C 84 in the center of NGC 1275. Our images reveal a
complex distribution of circular polarization in the inner parsec of the radio
jet, with local levels exceeding 3% polarization, the highest yet detected with
VLBI techniques. The circular polarization changes sign along the jet, making
3C 84 also the first radio jet to show both signs of circular polarization
simultaneously. The spectrum and changing sign of the circular polarization
indicate that it is unlikely to be purely intrinsic to the emitted synchrotron
radiation. The Faraday conversion process makes a significant and perhaps
dominant contribution to the circular polarization, and the observed spectrum
suggests the conversion process is near saturation. The sign change in the
circular polarization along the jet may result from this saturation or may be
due to a change in magnetic field order after an apparent bend in the jet. From
the small spatial scales probed here, ~ 0.15 pc, and the comparably high levels
of circular polarization inferred for the intra-day variable source PKS
1519-273, we suggest a connection between small spatial scales and efficient
production of circular polarization.Comment: 4 pages, accepted in ApJ Letter
Peptic and Tryptic Digestion Products as Inexpensive Culture Mediums for Routine Bacteriologic Work
n/
An Investigation of Gravitational Lensing in the Southern BL Lac PKS 0537-441
The BL-Lac family of active galaxies possess almost featureless spectra and
exhibit rapid variability over their entire spectral range. A number of models
have been developed to explain these extreme properties, several of which have
invoked the action of microlensing by sub-stellar mass objects in a foreground
galaxy; this not only introduces variability, but also amplifies an otherwise
normal quasar source. Here we present recent spectroscopy and photometry of the
southern BL Lac PKS 0537-441; with an inferred redshift of z~0.9 it represents
one of the most distant and most luminous members of the BL Lac family. The
goal of the observations was not only to confirm the redshift of PKS~0537-441,
but also to determine the redshift of a putative galaxy along the line of sight
to the BL-Lac; it has been proposed that this galaxy is the host of
microlensing stars that account for PKS 0537-441's extreme properties. While
several observations have failed to detect any extended emission in PKS
0537-441, the HST imaging data presented here indicate the presence of a
galactic component, although we fail to identify any absorption features that
reveal the redshift of the emission. It is also noted that PKS 0537-441 is
accompanied by several small, but extended companions, located a few arcseconds
from the point-like BL-Lac source. Two possibilities present themselves; either
they represent true companions of PKS 0537-441, or are themselves
gravitationally lensed images of more distant sources.Comment: 13 Pages with 4 Figures; Accepted for Publication by the
Astrophysical Journa
The Properties of the Radio-Selected 1Jy Sample of BL Lacertae Objects
We present new optical and near-IR spectroscopy as well as new high dynamic
range, arcsecond-resolution VLA radio maps of BL Lacs from the complete
radio-selected "1 Jansky" (1Jy) sample (RBLs) for which such data were not
previously available. Unlike BL Lacs from the complete X-ray-selected Einstein
Medium Sensitivity Survey (EMSS) sample (XBLs), most RBLs possess weak but
moderately luminous emission lines. And whereas nearly all XBLs have extended
power levels consistent with FR-1s, more than half of the RBLs have extended
radio power levels too luminous to be beamed FR-1 radio galaxies. In fact, we
find evidence for and examples of three distinct mechanisms for creating the BL
Lac phenomenon in the 1Jy sample: beamed FR-1s, beamed FR-2s and possibly a few
gravitationally-lensed quasars. The v/v_max determined for the 1Jy sample is
0.614+/-0.047, which is markedly different from the negative evolution seen in
the EMSS and other XBL samples. A correlation between logarithmic X-ray to
radio flux ratio and v/v_max is observed across the EMSS and 1Jy samples, from
negative evolution in the more extreme XBLs to positive evolution in the more
extreme RBLs. There is evidence that the selection criteria chosen by Stickel
et al. eliminates some BL Lac objects from the 1Jy sample, although how many is
unknown. And several objects currently in the sample have exhibited strong
emission lines in one or more epochs, suggesting they should be reclassified as
FSRQs. However these selection effects cannot account for the observed
discrepancy in XBL and RBL properties. From these observational properties we
conclude that RBLs and XBLs cannot be related by viewing angle alone, and that
RBLs are more closely related to FSRQs.Comment: 29 pages, 47 figures, accepted A
Evidence for the evolutionary sequence of blazars: different types of accretion flows in BL Lac objects
The limits on the mass of the black hole in 23 BL Lac objects are obtained
from their luminosities of the broad emission line H\beta on the assumption
that broad emission lines are emitted from clouds ionized by the radiation of
the accretion disk surrounding a black hole. The distribution of line
luminosity L_{H\beta} of all these BL Lac objects suggests a bimodal nature,
although this cannot be statistically proven on the basis of the present,
rather small sample. We found that standard thin disks are probably in the
sources with L_{H\beta}>10^{41} erg s^{-1}. The central black holes in these
sources have masses of 10^{8-10} M_\odot, if the matter is accreting at the
rate of 0.025 {\dot M}_{\rm Edd}. For the sources with L_{H\beta}<10^{41} erg
s^{-1}, the accretion flows have transited from standard thin disk type to the
ADAF type. The lower limits on the mass of the black hole in these sources are
in the range of 1.66-24.5\times 10^{8} M_\odot. The results support the
evolutionary sequence of blazars: FSRQ\to LBL\to HBL.Comment: 5 pages, accepted for publication in ApJ Letter
The HST Survey of BL Lacertae Objects. I. Surface Brightness Profiles, Magnitudes, and Radii of Host Galaxies
We report on a large HST imaging survey of BL Lac objects, at spatial
resolution ~10 times better than previous ground-based surveys. We focus on
data reduction and analysis, describing the procedures used to model the host
galaxy surface brightness radial profiles. A total of 69 host galaxies were
resolved out of 110 objects observed, including almost all sources at z < 0.5.
We classify them morphologically by fitting with either an exponential disk or
a de~Vaucouleurs profile; when one fit is preferred over the other, in 58 of 69
cases, it is invariably the elliptical morphology. This is a very strong result
given the large number of BL Lac objects, the unprecedented spatial resolution,
and the homogeneity of the data set. With the present reclassification of the
host galaxy of 1418+546 as an elliptical, there remain no undisputed examples
of a disk galaxy hosting a BL Lac nucleus. This implies that, at 99%
confidence, fewer than 7% of BL Lacs can be in disk galaxies. The apparent
magnitude of the host galaxies varies with distance as expected if the absolute
magnitudes are approximately the same, with a spread of +-1 mag, out to
redshift z < 0.5. At larger redshifts, only 6 of 23 BL Lacs are resolved so the
present data do not constrain possible luminosity evolution of the host
galaxies. The collective Hubble diagram for BL Lac host galaxies and radio
galaxies strongly supports their unification.Comment: Accepted for publication on ApJS. 43 pages. 10 figures. Figure 1 can
also be downloaded from http://icarus.stsci.edu/~scarpa/tmp/hst_figure1.ta
Neutral Hydrogen 21cm Absorption at Redshift 0.673 towards 1504+377
We detect the 21 cm line of neutral hydrogen in absorption at a redshift of
0.673 towards the 1 Jy radio source 1504+377. The 1504+377 radio source is
located toward the center of what appears to be an inclined disk galaxy at z =
0.674. The 21 cm absorption line shows multiple velocity components over a
velocity range of about 100 km sec, with a total HI column density:
N(HI) = cm. The
velocity-integrated optical depth of this system is the largest yet seen for
redshifted HI 21 cm absorption line systems (Carilli 1995). The 21 cm
absorption line is coincident in redshift with a previously detected broad
molecular absorption line system (Wiklind and Combes 1996). We do not detect HI
21 cm absorption associated with the narrow molecular absorption line system at
z = 0.67150, nor do we detect absorption at these redshifts by the 18 cm lines
of OH, nor by the 2 cm transition of HCO. There is no evidence for a bright
optical AGN in 1504+377, suggesting significant obscuration through the disk --
a hypothesis supported by the strong absorption observed. The 1504+377 system
resembles the ``red quasar'' PKS 1413+135, which has been modeled as a
optically obscured AGN with a very young radio jet in the center of a gas rich
disk galaxy (Perlman et al. 1996). The presence of very bright radio jets at
the centers of these two disk galaxies presents a challenge to unification
schemes for extragalactic radio sources and to models for the formation of
radio loud AGN.Comment: 17 pages, postscrip
Encoding monomorphic and polymorphic types
Most automatic theorem provers are restricted to untyped logics, and existing translations from typed logics are bulky or unsound. Recent research proposes monotonicity as a means to remove some clutter. Here we pursue this approach systematically, analysing formally a variety of encodings that further improve on efficiency while retaining soundness and completeness. We extend the approach to rank-1 polymorphism and present alternative schemes that lighten
the translation of polymorphic symbols based on the novel notion of “cover”. The new encodings are implemented, and partly proved correct, in Isabelle/HOL. Our evaluation finds them vastly superior to previous schemes
A Compendium of Far-Infrared Line and Continuum Emission for 227 Galaxies Observed by the Infrared Space Observatory
Far-infrared line and continuum fluxes are presented for a sample of 227
galaxies observed with the Long Wavelength Spectrometer on the Infrared Space
Observatory. The galaxy sample includes normal star-forming systems,
starbursts, and active galactic nuclei covering a wide range of colors and
morphologies. The dataset spans some 1300 line fluxes, 600 line upper limits,
and 800 continuum fluxes. Several fine structure emission lines are detected
that arise in either photodissociation or HII regions: [OIII]52um, [NIII]57um,
[OI]63um, [OIII]88um, [NII]122um, [OI]145um, and [CII]158um. Molecular lines
such as OH at 53um, 79um, 84um, 119um, and 163um, and H2O at 58um, 66um, 75um,
101um, and 108um are also detected in some galaxies. In addition to those lines
emitted by the target galaxies, serendipitous detections of Milky Way
[CII]158um and an unidentified line near 74um in NGC1068 are also reported.
Finally, continuum fluxes at 52um, 57um, 63um, 88um, 122um, 145um, 158um, and
170um are derived for a subset of galaxies in which the far-infrared emission
is contained within the ~75" ISO LWS beam. The statistics of this large
database of continuum and line fluxes, including trends in line ratios with the
far-infrared color and infrared-to-optical ratio, are explored.Comment: Accepted for publication in the Astrophysical Journal Supplement
Serie
Mid-Infrared Emission from E+A Galaxies in the Coma Cluster
We have used ISO to observe at 12m seven E+A galaxies plus an additional
emission line galaxy, all in the Coma cluster. E+A galaxies lacking narrow
emission lines have 2.2m to 12m flux density ratios or limits similar
to old stellar populations (typical of early-type galaxies). Only galaxies with
emission lines have enhanced 12m flux density. Excess 12m emission is
therefore correlated with the presence of on-going star formation or an active
galactic nucleus (AGN).
By comparing the current star formation rates with previous rates estimated
from the Balmer absorption features, we divide the galaxies into two groups:
those for which star formation has declined significantly following a dramatic
peak 1 Gyr ago; and those with a significant level of ongoing star
formation or/and an AGN. There is no strong difference in the spatial
distribution on the sky between these two groups. However, the first group has
systemic velocities above the mean cluster value and the second group below
that value. This suggests that the two groups differ kinematically.
Based on surveys of the Coma cluster in the radio, the IRAS sources, and
galaxies detected in H emission, we sum the far infrared luminosity
function of galaxies in the cluster. We find that star formation in late type
galaxies is probably the dominant component of the Coma cluster far infrared
luminosity. The presence of significant emission from intracluster dust is not
yet firmly established. The member galaxies also account for most of the far
infrared output from nearby rich clusters in general.Comment: AAS Latex, accepted for publication in Ap
- …