83 research outputs found
N(HI) and jet power/emission in AGNs
Neutral hydrogen (HI) 21 cm absorption has been detected against more and
more powerful radio jets. In this work, based on the Guppta et al. 2006a
sample, we present our preliminary study of the correlations between the HI
column density N(HI) and the jet power, N(HI) versus the low frequency
luminosity at 408MHz, and N(HI) versus the radio luminosity at 1400MHz.Comment: 3 pages, 1 figures, Multiwavelength Variability of Blazars Guangzhou,
China, Sept. 22-24, 2010; will be published by JA
Observations of HI Absorbing Gas in Compact Radio Sources at Cosmological Redshifts
We present an overview of the occurrence and properties of atomic gas
associated with compact radio sources at redshifts up to z=0.85. Searches for
HI 21cm absorption were made with the Westerbork Synthesis Radio Telescope at
UHF-high frequencies (725-1200 MHz). Detections were obtained for 19 of the 57
sources with usable spectra (33%). We have found a large range in line depths,
from tau=0.16 to tau<=0.001. There is a substantial variety of line profiles,
including Gaussians of less than 10km/s, to more typically 150km/s, as well as
irregular and multi-peaked absorption profiles, sometimes spanning several
hundred km/s. Assuming uniform coverage of the entire radio source, we obtain
column depths of atomic gas between 1e19 and 3.3e21(Tsp/100K)(1/f)cm^(-2).
There is evidence for significant gas motions, but in contrast to earlier
results at low redshift, there are many sources in which the HI velocity is
substantially negative (up to v=-1420km/s) with respect to the optical
redshift, suggesting that in these sources the atomic gas, rather than falling
into the centre, may be be flowing out, interacting with the jets, or rotating
around the nucleus.Comment: 10 pages, accepted for publication in A&
HI Observations of the Supermassive Binary Black Hole System in 0402+379
We have recently discovered a supermassive binary black hole system with a
projected separation between the two black holes of 7.3 parsecs in the radio
galaxy 0402+379. This is the most compact supermassive binary black hole pair
yet imaged by more than two orders of magnitude. We present Global VLBI
observations at 1.3464 GHz of this radio galaxy, taken to improve the quality
of the HI data. Two absorption lines are found toward the southern jet of the
source, one redshifted by 370 +/- 10 km/s and the other blueshifted by 700 +/-
10 km/s with respect to the systemic velocity of the source, which, along with
the results obtained for the opacity distribution over the source, suggests the
presence of two mass clumps rotating around the central region of the source.
We propose a model consisting of a geometrically thick disk, of which we only
see a couple of clumps, that reproduces the velocities measured from the HI
absorption profiles. These clumps rotate in circular Keplerian orbits around an
axis that crosses one of the supermassive black holes of the binary system in
0402+379. We find an upper limit for the inclination angle of the twin jets of
the source to the line of sight of 66 degrees, which, according to the proposed
model, implies a lower limit on the central mass of ~7 x 10^8 Msun and a lower
limit for the scale height of the thick disk of ~12 pc .Comment: 20 pages, 7 figures. Accepted on the Astrophysical Journa
A survey for redshifted molecular and atomic absorption lines I
We are currently undertaking a large survey for redshifted atomic and
molecular absorption ... only one clear and one tentative detection were
obtained: HI absorption at z = 0.097 in PKS 1555-140 and OH absorption at z
=0.126 in PKS 2300-189, respectively... In order to determine why no clear
molecular absorption was detected in any of the 13 sources searched, we
investigate the properties of the five redshifted systems currently known to
exhibit OH absorption. In four of these, molecules were first detected via
millimetre-wave transitions and the flat radio spectra indicate compact
background continuum sources, which may suggest a high degree of coverage of
the background source by the molecular clouds in the absorber. Furthermore, for
these systems we find a relationship between the molecular line strength and
red optical--near infrared (V-K) colours, thus supporting the notion that the
reddening of these sources is due to dust, which provides an environment
conducive to the formation of molecules. Upon comparison with the V-K colours
of our sample, this relationship suggests that, presuming the reddening occurs
at the host galaxy redshift at least in some of the targets, many of our
observations still fall short of the sensitivityrequired to detect OH
absorption, although a confirmation of the ``detection'' of OH in 2300-189
could contravene this.Comment: 13 pages, loads of figures, accepted by MNRA
The unfriendly ISM in the radio galaxy 4C12.50 (PKS 1345+12)
The radio source 4C12.50 has often been suggested to be a prime candidate for
the link between ultraluminous infrared galaxies and young radio galaxies. A
VLBI study of the neutral hydrogen in the nuclear regions of this object shows
that most of the gas detected close to the systemic velocity is associated with
an off-nuclear cloud (~50 to 100 pc from the radio core) with a column density
of ~10^22 T_spin/100 K) cm^(-2) and an HI mass of a few times 10^5 to 10^6
M_sun. We consider a number of possibilities to explain the results. In
particular, we discus the possibility that this cloud indicates the presence of
a rich and clumpy interstellar medium in the centre, likely left over from the
merger that triggered the activity and that this medium influences the growth
of the radio source. The location of the cloud -- at the edge of the northern
radio jet/lobe -- suggests that the radio jet might be interacting with a gas
cloud. This interaction could be responsible for bending the young radio jet.
The velocity profile of the gas is relatively broad (~150$ km/s) and we
interpret this as kinematical evidence for interaction of the radio plasma with
the cloud. We also consider the model where the cloud is part of a broader
circumnuclear structure. Only a limited region of this structure would have
sufficient background radio brightness and large enough column depth in neutral
gas to obtain detectable HI absorption against the counterjet. The VLBI study
of the neutral hydrogen in 4C12.50 suggests that HI detected near the systemic
velocity (as it is often the case in radio galaxies) may not necessarily be
connected with a circumnuclear disk or torus (as is very often assumed) but
instead could be a tracer of the large-scale medium that surrounds the active
nucleus and that may influence the growth of the young radio source.Comment: 7 pages, 2 figures. Accepted for publication in A&
Detection of 4765 MHz OH Emission in a Pre-Planetary Nebula -- CRL 618
Jets and outflows are ubiquitous phenomena in astrophysics, found in our
Galaxy in diverse environments, from the formation of stars to late-type
stellar objects. We present observations conducted with the 305m Arecibo
Telescope of the pre-planetary nebula CRL 618 (Westbrook Nebula) - a well
studied late-type star that has developed bipolar jets. The observations
resulted in the first detection of 4765 MHz OH in a late-type stellar object.
The line was narrow (FWHM ~ 0.6 km/s) and ~40 km/s blueshifted with respect to
the systemic velocity, which suggests association with the expanding
jets/bullets in CRL 618. We also report non-detection at Arecibo of any other
OH transition between 1 and 9 GHz. The non-detections were obtained during the
observations in 2008, when the 4765 MHz OH line was first discovered, and also
in 2015 when the 4765 MHz OH line was not detected. Our data indicate that the
4765 MHz OH line was a variable maser. Modeling of the 4765 MHz OH detection
and non-detection of the other transitions is consistent with the physical
conditions expected in CRL 618. The 4765 MHz OH maser could originate from
dissociation of H2O by shocks after sublimation of icy objects in this dying
carbon-rich stellar system, although other alternatives such as OH in an
oxygen-rich circumstellar region associated with a binary companion are also
possible.Comment: 16 pages, 2 figure
Probing radio source environments via HI and OH absorption
We present the results of HI and OH absorption measurements towards a sample
of radio sources using the Arecibo 305-m telescope and the GMRT. In total, 27
radio sources were searched for associated 21-cm HI absorption. One totally new
HI absorption system was detected against the radio galaxy 3C258, while five
previously known HI absorption systems, and one galaxy detected in emission,
were studied with improved frequency resolution and/or sensitivity. Our sample
included 17 GPS and CSS objects, 4 of which exhibit HI absorption. This
detection rate of ~25% compares with a value of ~40% by Vermeulen et al. for
similar sources. We detected neither OH emission nor absorption towards any of
the sources that were observed at Arecibo, and estimate a limit on the
abundance ratio of N(HI)/N(OH)>4x10^6 for 3C258. We have combined our results
with those from other available HI searches to compile a heterogeneous sample
of 96 radio sources consisting of 27 GPS, 35 CSS, 13 flat spectrum and 21 large
sources. The HI absorption detection rate is highest (~45%) for the GPS sources
and least for the large sources. We find HI column density to be anticorrelated
with source size, as reported earlier by Pihlstr\"om et al. The HI column
density shows no significant dependence on either redshift or luminosity, which
are themselves strongly correlated. These results suggest that the environments
of radio sources on GPS/CSS scales are similar at different redshifts. Further,
in accordance with the unification scheme, the GPS/CSS galaxies have an HI
detection rate of ~40% which is significantly higher than the detection rate
(~20%) towards the GPS/CSS quasars. Also, the principal (strongest) absorption
component detected towards GPS sources appears blue-shifted in ~65% of the
cases, in agreement with the growing evidence for jet-cloud interactions.Comment: Abridged abstract, 22 pages, 21 figures, moderately revised, accepted
for publication in MNRA
The PARADIGM project I: a multiscale radio morphological analysis of local U/LIRGS
Disentangling the radio flux contribution from star formation (SF) and active-galactic-nuclei (AGNs) activity is a long-standing problem in extragalactic astronomy, since at frequencies of âČ 10 GHz, both processes emit synchrotron radiation. We present in this work the general objectives of the PARADIGM (PAnchromatic high-Resolution Analysis of DIstant Galaxy Mergers) project, a multi-instrument concept to explore SF and mass assembly of galaxies. We introduce two novel general approaches for a detailed multiscale study of the radio emission in local (ultra) luminous infrared galaxies (U/LIRGs). In this work, we use archival interferometric data from the Very Large Array (VLA) centred at ⌠6 GHz (C band) and present new observations from the e-Multi-Element Radio-Linked Interferometer Network (e-MERLIN) for UGCâ5101, VVâ705, VVâ250, and UGCâ8696. Using our image decomposition methods, we robustly disentangle the radio emission into distinct components by combining information from the two interferometric arrays. We use e-MERLIN as a probe of the core-compact radio emission (AGN or starburst) at ⌠20 pc scales, and as a probe of nuclear diffuse emission, at scales âŒ100â200 pc. With VLA, we characterize the source morphology and the flux density on scales from âŒ200 pc up to and above 1 kpc. As a result, we find deconvolved and convolved sizes for nuclear regions from âŒ10 to âŒ200 pc. At larger scales, we find sizes of 1.5â2 kpc for diffuse structures (with effective sizes of ⌠300â400 pc). We demonstrate that the radio emission from nuclear extended structures (⌠100 pc) can dominate over core-compact components, providing a significant fraction of the total multiscale SF output. We establish a multiscale radio tracer for SF by combining information from different instruments. Consequently, this work sets a starting point to potentially correct for overestimations of AGN fractions and underestimates of SF activity
Spin temperatures and covering factors for HI 21-cm absorption in damped Lyman-alpha systems
We investigate the practice of assigning high spin temperatures to damped
Lyman-alpha absorption systems (DLAs) not detected in HI 21-cm absorption. In
particular, Kanekar & Chengaulr (2003) have attributed the mix of 21-cm
detections and non-detections in low redshift (z<2.04) DLAs to a mix of spin
temperatures, while the non-detections at high redshift were attributed to high
spin temperatures. Below z=0.9, where some of the DLA host galaxy morphologies
are known, we find that 21-cm absorption is normally detected towards large
radio sources when the absorber is known to be associated with a large
intermediate (spiral) galaxy. Furthermore, at these redshifts, only one of the
six 21-cm non-detections has an optical identification and these DLAs tend to
lie along the sight-lines to the largest background radio continuum sources.
For these and many of the high redshift DLAs occulting large radio continua, we
therefore expect covering factors of less than the assumed/estimated value of
unity. This would have the effect of introducing a range of spin temperatures
considerably narrower than the current range of >9000 K, while still supporting
the hypothesis that the high redshift DLA sample comprises a larger proportion
of compact galaxies than the low redshift sample.Comment: Submitted to MNRAS, 11 pages, 6 figure
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