760 research outputs found
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 jet power, radio loudness and black hole mass in radio loud AGNs
The jet formation is thought to be closely connected with the mass of central
supermassive black hole in Active Galactic Nuclei. The radio luminosity
commonly used in investigating this issue is merely an indirect measure of the
energy transported through the jets from the central engine, and severely
Doppler boosted in core-dominated radio quasars. In this work, we investigate
the relationship between the jet power and black hole mass, by estimating the
jet power using extrapolated extended 151 MHz flux density from the VLA 5 GHz
extended radio emission, for a sample of 146 radio loud quasars complied from
literature. After removing the effect of relativistic beaming in the radio and
optical emission, we find a significant intrinsic correlation between the jet
power and black hole mass. It strongly implies that the jet power, so as jet
formation, is closely connected with the black hole mass.To eliminate the
beaming effect in the conventional radio loudness, we define a new radio
loudness as the ratio of the radio extended luminosity to the optical
luminosity estimated from the broad line luminosity.In a tentatively combined
sample of radio quiet with our radio loud quasars, the apparent gap around the
conventional radio loudness R=10 is not prominent for the new-defined radio
loudness. In this combined sample, we find a significant correlation between
the black hole mass and new-defined radio loudness.Comment: 35 pages, 10 figures. accepted by Ap
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
Abundant dust found in intergalactic space
Galactic dust constitutes approximately half of the elements more massive
than helium produced in stellar nucleosynthesis. Notwithstanding the formation
of dust grains in the dense, cool atmospheres of late-type stars, there still
remain huge uncertainties concerning the origin and fate of galactic stardust.
In this paper, we identify the intergalactic medium (i.e. the region between
gravitationally-bound galaxies) as a major sink for galactic dust. We discover
a systematic shift in the colour of background galaxies viewed through the
intergalactic medium of the nearby M81 group. This reddening coincides with
atomic, neutral gas previously detected between the group members. The
dust-to-HI mass ratio is high (1/20) compared to that of the solar neighborhood
(1/120) suggesting that the dust originates from the centre of one or more of
the galaxies in the group. Indeed, M82, which is known to be ejecting dust and
gas in a starburst-driven superwind, is cited as the probable main source.Comment: 5 pages, 3 figures, 1 table. ApJ Letters in pres
Discovery of an Optical Jet in the BL Lac Object 3C 371
We have detected an optical jet in the BL Lac object 3C 371 that coincides
with the radio jet in this object in the central few kpc. The most notable
feature is a bright optical knot 3 arcsec (4 kpc) from the nucleus that occurs
at the location where the jet apparently changes its direction by ~30 degrees.
The radio, near-infrared and optical observations of this knot are consistent
with a single power-law spectrum with a radio-optical spectral index alpha =
-0.81. One possible scenario for the observed turn is that the jet is
interacting with the material in the bridge connecting 3C 371 to nearby
galaxies and the pressure gradient is deflecting the jet significantly.Comment: 11 pages, LaTeX, 4 figures (1 eps, 3 gifs), accepted for publication
in ApJ Letter
J16021+3326: New Multi-Frequency Observations of a Complex Source
We present multifrequency Very Long Baseline Array (VLBA) observations of
J16021+3326. These observations, along with variability data obtained from the
Owens Valley Radio Observatory (OVRO) candidate gamma-ray blazar monitoring
program, clearly indicate this source is a blazar. The peculiar characteristic
of this blazar, which daunted previous classification attempts, is that we
appear to be observing down a precessing jet, the mean orientation of which is
aligned with us almost exactly.Comment: 16 pages, 7 Figures, 2 Tables, accepted to Ap
High redshift AGNs from the 1Jy catalogue and the magnification bias
We have found a statistically significant (99.1 \%) excess of red ()
galaxies with photographic magnitudes , taken from the APM Sky
Catalogue around radiosources from the 1Jy catalogue. The amplitude,
scale and dependence on galaxy colours of the observed overdensity are
consistent with its being a result of the magnification bias caused by the weak
gravitational lensing of large scale structures at redshift
and are hardly explained by other causes, as obscuration by dust.Comment: uuencoded file containing 3 ps files: the main text, a table and a
figure. To appear in ApJ Letter
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
HST Observations of the Host Galaxies of BL Lacertae Objects
Six BL Lac objects from the complete 1 Jy radio-selected sample of 34 objects
were observed in Cycle 5 with the HST WFPC2 camera to an equivalent limiting
flux of mu_I~26 mag/arcsec^2. Here we report results for the second half of
this sample, as well as new results for the first three objects, discussed
previously by Falomo et al. (1997). In addition, we have analyzed in the same
way HST images of three X-ray-selected BL Lacs observed by Jannuzi et al.
(1997). The ensemble of 9 BL Lac objects spans the redshift range from z=0.19
to ~1. Host galaxies are clearly detected in seven cases, while the other two,
at z~0.258 (redshift highly uncertain) and z=0.997, are not resolved. The HST
images constitute a homogeneous data set with unprecedented morphological
information between a few tenths of an arcsecond and several arcseconds from
the nucleus, allowing us in 6 of the 7 detected host galaxies to rule out
definitively a pure disk light profile. The host galaxies are luminous
ellipticals with an average absolute magnitude of M_I~-24.6 mag (with
dispersion 0.7 mag), more than a magnitude brighter than L* and comparable to
brightest cluster galaxies. The morphologies are generally smooth and have
small ellipticities (epsilon<0.2). Given such roundness, there is no obvious
alignment with the more linear radio structures. In the six cases for which we
have HST WFPC2 images in two filters, the derived color profiles show no strong
spatial gradients and are as expected for K-corrected passively evolving
elliptical galaxies. The host galaxies of the radio-selected and X-ray-selected
BL Lacs for this very limited sample are comparable in both morphology and
luminosity.Comment: 23 pages, including 6 postscript figures and 3 tables (embedded).
Latex requires aaspp4.sty and psfig.sty (not included). Accepted for
publication in the Astrophysical Journa
The Sedentary Multi-Frequency Survey. I. Statistical Identification and Cosmological Properties of HBL BL Lacs
We have assembled a multi-frequency database by cross-correlating the NVSS
catalog of radio sources with the RASSBSC list of soft X-ray sources, obtaining
optical magnitude estimates from the Palomar and UK Schmidt surveys as provided
by the APM and COSMOS on-line services. By exploiting the nearly unique
broad-band properties of High-Energy Peaked (HBL) BL Lacs we have statistically
identified a sample of 218 objects that is expected to include about 85% of BL
Lacs and that is therefore several times larger than all other published
samples of HBLs. Using a subset (155 objects) that is radio flux limited and
statistically well-defined we have derived the \vovm distribution and the
LogN-LogS of extreme HBLs (fx/fr >= 3E-10 erg/cm2/s/Jansky) down to 3.5 mJy. We
find that the LogN-LogS flattens around 20 mJy and that = 0.42 +/- 0.02.
This extends to the radio band earlier results, based on much smaller X-ray
selected samples, about the anomalous cosmological observational properties of
HBL BL Lacs. A comparison with the expected radio LogN-LogS of all BL Lacs
(based on a beaming model) shows that extreme HBLs make up roughly 2% of the BL
Lac population, independently of radio flux. This result, together with the
flatness of the radio logN-logS at low fluxes, is in contrast with the
predictions of a recent model which assumes an anti-correlation between peak
frequency and bolometric luminosity. The extreme fx/fr flux ratios and high
X-ray fluxes of these BL Lacs makes them good candidate TeV sources, some of
the brighter (and closer) ones possibly detectable with the current generation
of Cerenkov telescopes.Comment: 11 pages, LaTeX, 6 ps figures. Accepted for publication in MNRA
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