105 research outputs found
Tracing the merger-driven evolution of active galaxies using the CJF sample
In the context of the evolution of large structures in the Universe, it is
unclear whether active galaxies are a phase which each galaxy undergoes, and
what is the importance of the evolution of black holes in their centers. Binary
black hole (BBH) systems could play a key role in our understanding of the
above question.
We investigate the Caltech-Jodrell Bank flat-spectrum (CJF) sample for
evidence in favor of the merger-driven evolution scheme of active galaxies and
search tracer-systems of AGN evolution and possible indications of BBH
candidates. We discuss the validity and ambiguity of such indications and
formulate a set of selection criteria for the detection of such systems. We
conduct an extensive literature search for all available multi-wavelength
information, concentrating on the optical and infrared regime, in addition to
morphological information of the CJF sources. We analyze the statistics of this
sample, in terms of these properties.
We find 1 ULIRG (Mrk 231) included in the CJF, prototype of a transitory
system. In total 28.6% of the CJF sources with z<0.4 are distorted or have a
companion. Given the unbiased sample used here, this provides strong evidence
for the ubiquity of the merger phenomenon in the context of active galaxies. We
find a correlation between the radio and the near-infrared luminosity for the
high-luminosity sources, interpreted in the context of the interplay between a
star-formation and AGN component. We find a connection between variability and
evolutionary transitory systems, as selected through their near-infrared
colors. We select 28 sources that trace the different evolution phases of an
AGN, as well as a number of the most promising BBH candidates. We find 4
sources with almost periodical variability in the optical and radio on similar
timescales.Comment: 18 pages, 6 figures, accepted for publication in A&A (updated to
match proofs
Distribution of Radio Spectral Slopes of Galaxies in Optical Diagnostic Diagrams
For about 500 intermediate-redshift sources (), whose radio flux
densities at are larger than 10 mJy, we performed additional
observations at 4.85 GHz and 10.45 GHz using 100-m Effelsberg telescope. Our
radio-optical galaxies are located preferentially in the composite and AGN
spectral classes in the narrow line optical diagnostic diagrams (ODD). In the
analysis, we focused on the distribution of radio spectral indices of radio
synchrotron power-law profiles, , in the ODDs.
Using different analysis techniques, both observationally motivated and
machine-learning based, we found three distinct groups--clusters in the radio
loudness, [OIII]/H ratio, and spectral index volume: (1) sources with
steep radio spectral index, large radio loudness and large [OIII]/H
ratio; (2) sources with flat radio spectral index, intermediate radio loudness
and lower [OIII]/H ratio; (3) sources with inverted radio spectral
index, low radio loudness and low [OIII]/H. The groups (1), (2), and (3)
are located along the Seyfert-LINER spectral classes towards lower ionization
ratios in the ODDs and hence can represent different activity cycles/accretion
modes of AGNs, which could be in some cases associated with different merger
stages.Comment: 7 pages, 3 figures, 2 tables; accepted for publication by the Polish
Astronomical Society Proceedings (volume 123
Intranight optical variability of TeV blazars with parsec-scale jets dominated by slow-moving radio knots
BL Lac objects detected at TeV energies preferentially belong to the subclass
called 'high-frequency-peaked' BL Lacs (HBLs). Parsec-scale radio jets in these
TeV-HBLs often show dominant, slow moving radio knots that are at most mildly
superluminal. We report the first systematic campaign to characterise the
Intra-Night Optical Variability (INOV) of TeV-HBLs using a representative
sample of 6 such sources, all showing a fairly high degree of optical
polarization. Our campaign consists of high-sensitivity monitoring of this
sample in 24 sessions of more than 3 hour duration each. For these TeV-HBLs, we
find a striking lack of INOV and based on this, we discuss the importance of
superluminal motion of the radio knots vis-a-vis the optical polarization, as
the key diagnostic for INOV detection.Comment: accepted for publication in MNRAS letter
Effect of Electromagnetic Interaction on Galactic Center Flare Components
Recently, near-infrared GRAVITY@ESO observations at have
announced the detection of three bright "flares" in the vicinity of the
Galactic center supermassive black hole (SMBH) that exhibited orbital motion at
a distance of about gravitational radii from an black hole. There are indications of the presence of a large-scale,
organized component of the magnetic field at the Galactic center.
Electromagnetic effects on the flare dynamics were previously not taken into
account despite the relativistic motion of a plasma in magnetic field leading
to the charge separation and nonnegligible net charge density in the plasma.
Applying various approaches, we find the net charge number density of the flare
components of the order of cm, while the particles'
total number density is of the order of cm. However,
even such a tiny excess of charged particles in the quasi-neutral plasma can
significantly affect the dynamics of flare components, which can then lead to
the degeneracy in the measurements of spin of the SMBH. Analyzing the dynamics
of recent flares in the case of the rapidly rotating black hole, we also
constrain the inclination angle between the magnetic field and spin axis to
, as for larger angles, the motion of the hot spot is
strongly chaotic.Comment: 28 pages, 7 figures; to appear in the Astrophysical Journal; accepted
versio
Precession-induced Variability in AGN Jets and OJ 287
The combined study of the flaring of Active Galactic Nuclei (AGN) at radio
wavelengths and pc-scale jet kinematics with Very Long Baseline Interferometry
(VLBI) has led to the view that i) the observed flares are associated with
ejections of synchrotron blobs from the core, and ii) most of the flaring would
follow a one-to-one correlation with the component ejection. Recent results
have provided mounting evidence that the quasi-regular component injections
into the relativistic jet may not be the only cause of the flux variability. We
propose that AGN flux variability and jet morphology changes can both be of
deterministic nature, i.e. having a geometric/kinetic origin linked to the
time-variable Doppler beaming of the jet emission as its direction changes due
to precession (and nutation). The physics of the underlying jet leads to
shocks, instabilities, or to ejections of plasmoids. The appearance
(morphology, flux, etc.) of the jet can, however, be strongly affected and
modulated by precession. We demonstrate this modulating power of precession for
OJ 287. For the first time, we show that the spectral state of the Spectral
Energy Distribution (SED) can be directly related to the jet's precession
phase. We model the SED evolution and reproduce the precession parameters.
Further, we apply our precession model to eleven prominent AGN. We show that
for OJ 287 precession seems to dominate the long-term variability () of the AGN flux, SED spectral state, and jet morphology, while
stochastic processes affect the variability on short timescales ().Comment: 48 pages, 26 figures, 14 tables; published in the Astrophysical
Journa
Constraining the radio jet proper motion of the high-redshift quasar J2134-0419 at z = 4.3
To date, PMN J2134-0419 (at a redshift z = 4.33) is the second most distant quasar known with a milliarcsecond-scale morphology permitting direct estimates of the jet proper motion. Based on two-epoch observations, we constrained its radio jet proper motion using the very long baseline interferometry (VLBI) technique. The observations were conducted with the European VLBI Network (EVN) at 5 GHz on 1999 November 26 and 2015 October 6. We imaged the central 10-pc scale radio jet emission and modeled its brightness distribution. By identifying a jet component at both epochs separated by 15.86 yr, a proper motion of μ = 0.035 ± 0.023 mas yr-1 is found. It corresponds to an apparent superluminal speed of βa = 4.1 ± 2.7 c. Relativistic beaming at both epochs suggests that the jet viewing angle with respect to the line of sight is smaller than 20°, with a minimum bulk Lorentz factor Γ = 4.3. The small value of the proper motion is in good agreement with the expectations from the cosmological interpretation of the redshift and the current cosmological model. Additionally we analyzed archival Very Large Array observations of J2143-0419 and found indication of a bent jet extending to ˜30 kpc
Molecular gas in the immediate vicinity of Sgr A* seen with ALMA
We report serendipitous detections of line emission with ALMA in band 3, 6,
and 7 in the central parsec of the Galactic center at an up to now highest
resolution (<0.7''). Among the highlights are the very first and highly
resolved images of sub-mm molecular emission of CS, H13CO+, HC3N, SiO, SO, C2H,
and CH3OH in the immediate vicinity (~1'' in projection) of Sgr A* and in the
circumnuclear disk (CND). The central association (CA) of molecular clouds
shows three times higher CS/X (X: any other observed molecule) luminosity
ratios than the CND suggesting a combination of higher excitation - by a
temperature gradient and/or IR-pumping - and abundance enhancement due to UV-
and/or X-ray emission. We conclude that the CA is closer to the center than the
CND is and could be an infalling clump consisting of denser cloud cores
embedded in diffuse gas. Moreover, we identified further regions in and outside
the CND that are ideally suited for future studies in the scope of hot/cold
core and extreme PDR/XDR chemistry and consequent star formation in the central
few parsecs
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