1,032 research outputs found
Search for IR Emission from Intracluster Dust in A2029
We have searched for IR emission from the intracluster dust (ICD) in the
galaxy cluster A2029. Weak signals of enhanced extended emission in the cluster
are detected at both 24 and 70 micron. However, the signals are
indistinguishable from the foreground fluctuations. The 24 versus 70 micron
color map does not discriminate the dust emission in the cluster from the
cirrus emission. After excluding the contamination from the point sources, we
obtain upper limits for the extended ICD emission in A2029, 5 x 10^3 Jy/sr at
24 micron and 5 x 10^4 Jy/sr at 70 micron. The upper limits are generally
consistent with the expectation from theoretical calculations and support a
dust deficiency in the cluster compared to the ISM in our galaxy. Our results
suggest that even with the much improved sensitivity of current IR telescopes,
a clear detection of the IR emission from ICD may be difficult due to cirrus
noise.Comment: 5 pages, 4 figures, accepted by ApJ
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
Performance of three-photon PET imaging: Monte Carlo simulations
We have recently introduced the idea of making use of three-photon positron
annihilations in positron emission tomography. In this paper the basic
characteristics of the three-gamma imaging in PET are studied by means of Monte
Carlo simulations and analytical computations. Two typical configurations of
human and small animal scanners are considered. Three-photon imaging requires
high energy resolution detectors. Parameters currently attainable by CdZnTe
semiconductor detectors, the technology of choice for the future development of
radiation imaging, are assumed. Spatial resolution is calculated as a function
of detector energy resolution and size, position in the field of view, scanner
size, and the energies of the three gamma annihilation photons. Possible ways
to improve the spatial resolution obtained for nominal parameters: 1.5 cm and
3.2 mm FWHM for human and small animal scanners, respectively, are indicated.
Counting rates of true and random three-photon events for typical human and
small animal scanning configurations are assessed. A simple formula for minimum
size of lesions detectable in the three-gamma based images is derived.
Depending on the contrast and total number of registered counts, lesions of a
few mm size for human and sub mm for small animal scanners can be detected
Extending the limits of globule detection -- ISOPHOT Serendipity Survey Observations of interstellar clouds
A faint MJysr bipolar globule was discovered with the
ISOPHOT 170 m Serendipity Survey (ISOSS). ISOSS J 20246+6541 is a cold
( K) FIR source without an IRAS pointsource counterpart.
In the Digitized Sky Survey B band it is seen as a 3\arcmin size bipolar
nebulosity with an average excess surface brightness of
mag/\arcsec . The CO column density distribution determined by
multi-isotopic, multi-level CO measurements with the IRAM-30m telescope agrees
well with the optical appearance. An average hydrogen column density of
cm was derived from both the FIR and CO data. Using a
kinematic distance estimate of 400 pc the NLTE modelling of the CO, HCO,
and CS measurements gives a peak density of cm. The
multiwavelength data characterise ISOSS 20246+6541 as a representative of a
class of globules which has not been discovered so far due to their small
angular size and low 100m brightness. A significant overabundance of
CO is found . This is likely due to
isotope selective chemical processes.Comment: 5 pages, 3 figure
An Excess of Mg II Absorbers in BL Lac Objects
Two new Mg II absorbers are presented (z=1.340 in S5 0454+844 and z=1.117 in
PKS 2029+121), bringing the total number of Mg II systems in the 1 Jy
radio-selected BL Lac sample to 10. Five of the ten absorption systems are at W
> 1A; this is a factor of four to five greater than the number expected based
upon quasar sightlines, and is 2.5 to 3 sigma greater than the expectation
value. Interpretations of this possible excess include either that some of the
Mg II absorbers might be intrinsic to the BL Lac or that there is a correlation
between the presence of absorbing gas in the foreground and the nearly
featureless spectra of these BL Lac Objects compared to quasars. Such a
correlation can be created by gravitational microlensing as suggested by
Ostriker & Vietri. The similarity between the optical spectra of BL Lacs with
Mg II absorption and the spectrum of the Gamma-ray burst source GRB 970508
suggests that models of Gamma-ray bursts as microlensed AGN should be
investigated.Comment: LaTeX, AASTeX, 3 figures: jy0138.eps, jy0454.eps, jy2029.eps ApJL,
Accepted September 2nd, 199
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
Intra-Day Variability and the Interstellar Medium Towards 0917+624
The intra-day variable source 0917+624 displays annual changes in its
timescale of variability. This is explained in terms of a scintillation model
in which changes in the variability timescale are due to changes in the
relative velocity of the scintillation pattern as the Earth orbits the sun.
(see also astro-ph/0102050)Comment: 4 pages, 1 figure. Accepted for A&A Letter
The Infrared Luminosity of Galaxy Clusters
The aim of this study is to quantify the infrared luminosity of clusters as a
function of redshift and compare this with the X-ray luminosity. This can
potentially constrain the origin of the infrared emission to be intracluster
dust and/or dust heated by star formation in the cluster galaxies. We perform a
statistical analysis of a large sample of galaxy clusters selected from
existing databases and catalogues.We coadd the infrared IRAS and X-ray RASS
images in the direction of the selected clusters within successive redshift
intervals up to z = 1. We find that the total infrared luminosity is very high
and on average 20 times higher than the X-ray luminosity. If all the infrared
luminosity is to be attributed to emission from diffuse intracluster dust, then
the IR to X-ray ratio implies a dust-to-gas mass abundance of 5e-4. However,
the infrared luminosity shows a strong enhancement for 0.1 < z < 1, which
cannot be attributed to cluster selection effects. We show that this
enhancement is compatible with a star formation rate in the member galaxies
that is typical of the central Mpc of the Coma cluster at z = 0 and evolves
with the redshift as (1+z)^5. It is likely that most of the infrared luminosity
that we measure is generated by the ongoing star formation in the member
galaxies. From theoretical predictions calibrated on extinction measurements
(dust mass abundance equal to 1e-5), we expect only a minor contribution, of a
few percent, from intracluster dust.Comment: 9 pages, 7 figures, accepted july 31st 2008 for publication in
Astronomy and Astrophysics, language improved for this versio
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