231 research outputs found
The resolved structure of the extragalactic supernova remnant SNR 4449-1
We present very long baseline interferometry (VLBI) observations of the
milliarcsecond-scale radio structure of the supernova remnant SNR 44491 in
the galaxy NGC 4449. This young and superluminous remnant was observed at 1.6
GHz (\,cm) with the European VLBI Network. The observations
confirm earlier identifications of this object with a supernova remnant (SNR)
while revealing a somewhat different morphology compared with the structure
reported by Bietenholz et al. from VLBI observations at 1.4 GHz. This
difference is discussed here in the context of structural sensitivity of both
observations. The 1.6 GHz image yields accurate estimates of the size (0.0422
arcsec 0.0285 arcsec and 0.8 0.5 pc) and age (55 yr) of
SNR 44491. With a total flux of 6.1 0.6 mJy measured in the VLBI
image, the historical lightcurve of the source can be well represented by a
power-law decay with a power index of 1.19 0.07. The SNR exhibits a
decline rate of the radio emission of 2.2 0.1 yr and a radio
luminosity of 1.74 10 erg s.Comment: 7 pages, 6 figures, MNRAS preprint, arXiv:1309.401
The central parsecs of active galactic nuclei: challenges to the torus
Type 2 AGN are by definition nuclei in which the broad-line region and
continuum light are hidden at optical/UV wavelengths by dust. Via accurate
registration of infrared (IR) Very Large Telescope adaptive optics images with
optical \textit{Hubble Space Telescope} images we unambiguously identify the
precise location of the nucleus of a sample of nearby, type 2 AGN. Dust
extinction maps of the central few kpc of these galaxies are constructed from
optical-IR colour images, which allow tracing the dust morphology at scales of
few pc. In almost all cases, the IR nucleus is shifted by several tens of pc
from the optical peak and its location is behind a dust filament, prompting to
this being a major, if not the only, cause of the nucleus obscuration. These
nuclear dust lanes have extinctions mag, sufficient to at least
hide the low-luminosity AGN class, and in some cases are observed to connect
with kpc-scale dust structures, suggesting that these are the nuclear fueling
channels. A precise location of the ionised gas H and
[\textsc{Si\,vii}] 2.48 m coronal emission lines relative to those of the
IR nucleus and dust is determined. The H peak emission is often shifted
from the nucleus location and its sometimes conical morphology appears not to
be caused by a nuclear --torus-- collimation but to be strictly defined by the
morphology of the nuclear dust lanes. Conversely, [\textsc{Si\,vii}] 2.48
m emission, less subjected to dust extinction, reflects the truly, rather
isotropic, distribution of the ionised gas. All together, the precise location
of the dust, ionised gas and nucleus is found compelling enough to cast doubts
on the universality of the pc-scale torus and supports its vanishing in
low-luminosity AGN. Finally, we provide the most accurate position of the NGC
1068 nucleus, located at the South vertex of cloud B.Comment: 23 pages, 10 figures, accepted for publication in MNRA
Starbursts and black hole masses in X-shaped radio galaxies: Signatures of a merger event?
We present new spectroscopic identifications of 12 X-shaped radio galaxies
and use the spectral data to derive starburst histories and masses of the
nuclear supermassive black holes in these galaxies. The observations were done
with the 2.1-m telescope of the Observatorio Astron\'omico Nacional at San
Pedro M\'artir, M\'exico. The new spectroscopic results extend the sample of
X-shaped radio galaxies studied with optical spectroscopy. We show that the
combined sample of the X-shaped radio galaxies has statistically higher
black-hole masses and older episodes of star formation than a control sample of
canonical double-lobed radio sources with similar redshifts and luminosities.
The data reveal enhanced star-formation activity in the X-shaped sample on the
timescales expected in galactic mergers. We discuss the results obtained in the
framework of the merger scenario.Comment: 9 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
The central molecular gas structure in LINERs with low luminosity AGN: evidence for gradual disappearance of the torus
We present observations of the molecular gas in the nuclear environment of
three prototypical low luminosity AGN (LLAGN), based on VLT/SINFONI AO-assisted
integral-field spectroscopy of H2 1-0 S(1) emission at angular resolutions of
~0.17". On scales of 50-150 pc the spatial distribution and kinematics of the
molecular gas are consistent with a rotating thin disk, where the ratio of
rotation (V) to dispersion (sigma) exceeds unity. However, in the central 50
pc, the observations reveal a geometrically and optically thick structure of
molecular gas (V/sigma10^{23} cm^{-2}) that is likely to be
associated with the outer extent of any smaller scale obscuring structure. In
contrast to Seyfert galaxies, the molecular gas in LLAGN has a V/sigma<1 over
an area that is ~9 times smaller and column densities that are in average ~3
times smaller. We interpret these results as evidence for a gradual
disappearance of the nuclear obscuring structure. While a disk wind may not be
able to maintain a thick rotating structure at these luminosities, inflow of
material into the nuclear region could provide sufficient energy to sustain it.
In this context, LLAGN may represent the final phase of accretion in current
theories of torus evolution. While the inflow rate is considerable during the
Seyfert phase, it is slowly decreasing, and the collisional disk is gradually
transitioning to become geometrically thin. Furthermore, the nuclear region of
these LLAGN is dominated by intermediate-age/old stellar populations (with
little or no on-going star formation), consistent with a late stage of
evolution.Comment: 15 pages, including 4 figures and 1 table, Accepted for publication
in ApJ Letter
Radio Emission From a 10.3 Black Hole in UHZ1
The recent discovery of a 4 10 M black hole (BH) in
UHZ1 at 10.3, just 450 Myr after the big bang, suggests that the seeds of
the first quasars may have been direct-collapse black holes (DCBHs) from the
collapse of supermassive primordial stars at 20. This object was
identified in James Webb Space Telescope (JWST) NIRcam and Chandra X-ray data,
but recent studies suggest that radio emission from such a BH should also be
visible to the Square Kilometer Array (SKA) and the next-generation Very Large
Array (ngVLA). Here, we present estimates of radio flux for UHZ1 from 0.1 - 10
GHz, and find that SKA and ngVLA could detect it with integration times of 10 -
100 hr and just 1 - 10 hr, respectively. It may be possible to see this object
with VLA now with longer integration times. The detection of radio emission
from UHZ1 would be a first test of exciting new synergies between near infrared
(NIR) and radio observatories that could open the era of 5 - 15 quasar
astronomy in the coming decade.Comment: 5 pages, 2 figures, submitted to Ap
Radio Emission from the First Quasars at 6-15
Nearly 300 quasars have now been found at 6, including nine at 7.
They are thought to form from the collapse of supermassive primordial stars to
10 - 10 M black holes at 20 - 25, which then rapidly
grow in the low-shear environments of rare, massive halos fed by strong
accretion flows. Sensitive new radio telescopes such as the Next-Generation
Very Large Array (ngVLA) and the Square Kilometer Array (SKA) could probe the
evolution of these objects at much earlier times. Here, we estimate radio flux
from the first quasars at 6 - 15 at 0.5 - 12.5 GHz. We find that SKA
and ngVLA could detect a quasar like ULAS J1120+0641, a 1.35 10
M black hole at 7.1, at much earlier stages of evolution, 14 - 15, with 100 hr integration times in targeted searches. The advent
of these new observatories, together with the James Webb Space Telescope
(JWST), Euclid, and the Roman Space Telescope (RST), will inaugurate the era of
15 quasar astronomy in the coming decade.Comment: 5 pages, 3 figures, accepted by MNRAS Letter
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