88 research outputs found
The Emission Structure of Formaldehyde MegaMasers
The formaldehyde MegaMaser emission has been mapped for the three host
galaxies IC\,860. IRAS\,151070724, and Arp\,220. Elongated emission
components are found at the nuclear centres of all galaxies with an extent
ranging between 30 to 100 pc. These components are superposed on the peaks of
the nuclear continuum. Additional isolated emission components are found
superposed in the outskirts of the radio continuum structure. The brightness
temperatures of the detected features ranges from 0.6 to 13.4
K, which confirms their masering nature. The masering scenario is interpreted
as amplification of the radio continuum by foreground molecular gas that is
pumped by far-infrared radiation fields in these starburst environments of the
host galaxies.Comment: Accepted MNRA
Opportunities for maser studies with the Square Kilometre Array
The Square Kilometre Array (SKA) is the radio telescope of the next
generation, providing an increase in sensitivity and angular resolution of two
orders of magnitude over existing telescopes. Currently, the SKA is expected to
span the frequency range 0.1-25 GHz with capabilities including a wide
field-of-view and measurement of polarised emission. Such a telescope has
enormous potential for testing fundamental physical laws and producing
transformational discoveries. Important science goals include using H2O
megamasers to make precise estimates of H0, which will anchor the extragalactic
distance scale, and to probe the central structures of accretion disks around
supermassive black holes in AGNs, to study OH megamasers associated with
extreme starburst activity in distant galaxies and to study with unprecedented
precision molecular gas and star formation in our Galaxy.Comment: 5 pages, to appear in: IAU Symposium 242 Astrophysical Masers and
their Environment
Off-nuclear HO maser and dense molecular gas in NGC1068
The results of high-resolution spectral-line observations of dense molecular
gas are presented towards the nuclear region of the type 2 Seyfert galaxy
NGC1068. MERLIN observations of the 22 GHz HO maser were made for imaging
the known off-nuclear maser emission at radio jet component located about 0.3"
north-east of the radio nucleus in the galaxy. High angular resolution ALMA
observations have spatially resolved the molecular gas emissions of HCN and
HCO in this region. The off-nuclear maser spots are found to nearly
overlap with a ring-like molecular gas structure and are tracing an evolving
shock-like structure, which appears to be energized by interaction between the
radio jet and circumnuclear medium. A dynamic jet-ISM interaction is further
supported by a systematic shift of the centroid velocities of the off-nuclear
maser features over a period of 35 years. The integrated flux ratios of the
HCO line emission features at component C suggest a kinetic temperature
T 300K and an H density of 10 cm,
which are conditions where water masers may be formed. The diagnostics of the
masering action in this jet-ISM interaction region is exemplary for galaxies
hosting off-nuclear HO maser emission.Comment: 11 pages, 9 figures, submitted to MNRAS on 30 March 202
A radio structure resolved at the deca-parsec scale in radio-quiet quasar PDS 456 with an extremely powerful X-ray outflow
Active galactic nuclei (AGN) accreting at rates close to the Eddington limit
can host radiatively driven mildly relativistic outflows. Some of these X-ray
absorbing but powerful outflows may produce strong shocks resulting in a
significant non-thermal emission. This outflow-driven radio emission may be
detectable in the radio-quiet quasar PDS 456 since it has a bolometric
luminosity reaching the Eddington limit and a relativistic wide-aperture X-ray
outflow with a kinetic power high enough to quench the star formation in its
host galaxy. To investigate this possibility, we performed very-long-baseline
interferometric (VLBI) observations of the quasar with the European VLBI
Network (EVN) at 5 GHz. The EVN image with the full resolution reveals two
faint and diffuse radio components with a projected separation of about 20 pc
and an average brightness temperature of around two million Kelvin. In relation
to the optical sub-mas-accuracy position measured by the Gaia mission, the two
components are very likely on opposite sides of an undetected radio core. The
VLBI structure at the deca-pc scale can thus be either a young jet or a
bidirectional radio-emitting outflow, launched in the vicinity of a strongly
accreting central engine. Two diffuse components at the hecto-pc scale, likely
the relic radio emission from the past AGN activity, are tentatively detected
on each side in the low-resolution EVN image.Comment: 6 pages, 2 figures, 1 table. Accepted for publication in MNRA
Detection of a methanol megamaser in a major-merger galaxy
We have detected emission from both the 4_{-1}-3_{0} E (36.2~GHz) class I and
7_{-2}-8_{-1} E (37.7~GHz) class II methanol transitions towards the centre of
the closest ultra-luminous infrared galaxy Arp 220. The emission in both the
methanol transitions show narrow spectral features and have luminosities
approximately 8 orders of magnitude stronger than that observed from typical
class I methanol masers observed in Galactic star formation regions. The
emission is also orders of magnitude stronger than the expected intensity of
thermal emission from these transitions and based on these findings we suggest
that the emission from the two transitions are masers. These observations
provides the first detection of a methanol megamaser in the 36.2 and 37.7 GHz
transitions and represents only the second detection of a methanol megamaser,
following the recent report of an 84 GHz methanol megamaser in NGC1068. We find
the methanol megamasers are significantly offset from the nuclear region and
arise towards regions where there is Ha emission, suggesting that it is
associated with starburst activity. The high degree of correlation between the
spatial distribution of the 36.2 GHz methanol and X-ray plume emission suggests
that the production of strong extragalactic class I methanol masers is related
to galactic outflow driven shocks and perhaps cosmic rays. In contrast to OH
and H2O megamasers which originate close to the nucleus, methanol megamasers
provide a new probe of feedback (e.g. outflows) processes on larger-scales and
of star formation beyond the circumnuclear starburst regions of active
galaxies.Comment: Accepted for publication in ApJ
Trigonometric distance and proper motions of H2O maser bowshocks in AFGL 5142
We present the results of multi-epoch VLBI observations of water masers in
the AGFL 5142 massive star forming region. We measure an annual parallax of
mas, corresponding to a source distance of
kpc. Proper motion and line of sight velocities
reveal the 3D kinematics of masers in this region, most of which associate with
millimeter sources from the literature. In particular we find remarkable
bipolar bowshocks expanding from the most massive member, AFGL 5142 MM1, which
are used to investigate the physical properties of its protostellar jet. We
attempt to link the known outflows in this region to possible progenitors by
considering a precessing jet scenario and we discuss the episodic nature of
ejections in AFGL 5142
VLBI Observations of NGC6240: resolving the double nuclei and radio supernovae
The European VLBI Network (EVN) has been used at two epochs in 2003 and 2009
to obtain multi-frequency high-resolution images of the merger galaxy NGC6240
in order to study the radio properties of all compact high-brightness
components in the galaxy. Our observations at milli-arcsecond resolution
detected the northern and southern nuclei and two radio components, which we
interpret as long-lived luminous supernovae associated with the circum-nuclear
starburst activity at the southern nucleus. The new VLBI data support the
presence of an active galactic nucleus (AGN) together with starburst activity
at the southern nucleus and provides some evidence for an AGN at the northern
nucleus. The two nuclei both display an inverted spectrum at lower GHz
frequencies. The spectrum of the southern nucleus indicates thermal free-free
absorption on parsec scales, consistent with the presence of an AGN.Comment: 19 pages, 6 figures, accepted for publication in the Astronomical
Journa
A two-sided but significantly beamed jet in the supercritical accretion quasar IRAS F11119+3257
Highly accreting quasars are quite luminous in the X-ray and optical regimes.
While, they tend to become radio quiet and have optically thin radio spectra.
Among the known quasars, IRAS F11119+3257 is a supercritical accretion source
because it has a bolometric luminosity above the Eddington limit and extremely
powerful X-ray outflows. To probe its radio structure, we investigated its
radio spectrum between 0.15 and 96.15 GHz and performed very-long-baseline
interferometric (VLBI) observations with the European VLBI Network (EVN) at
1.66 and 4.93 GHz. The deep EVN image at 1.66 GHz shows a two-sided jet with a
projected separation about two hundred parsec and a very high flux density
ratio of about 290. Together with the best-fit value of the integrated spectral
index of -1.31+/-0.02 in the optically thin part, we infer that the approaching
jet has an intrinsic speed at least 0.57 times of the light speed. This is a
new record among the known all kinds of super-Eddington accreting sources and
unlikely accelerated by the radiation pressure. We propose a scenario in which
IRAS F11119+3257 is an unusual compact symmetric object with a small jet
viewing angle and a radio spectrum peaking at 0.53+/-0.06 GHz mainly due to the
synchrotron self-absorption.Comment: Accepted for publication in MNRA
Dense circum-nuclear molecular gas in starburst galaxies
We present results from a study of the dense circum-nuclear molecular gas of
starburst galaxies. The study aims to investigate the interplay between
starbursts, active galactic nuclei and molecular gas. We characterise the dense
gas traced by HCN, HCO and HNC and examine its kinematics in the
circum-nuclear regions of nine starburst galaxies observed with the Australia
Telescope Compact Array. We detect HCN (10) and HCO (10) in seven
of the nine galaxies and HNC (10) in four. Approximately 7 arcsec resolution
maps of the circum-nuclear molecular gas are presented. The velocity integrated
intensity ratios, HCO (10)/HCN (10) and HNC (10)/HCN (10),
are calculated. Using these integrated intensity ratios and spatial intensity
ratio maps we identify photon dominated regions (PDRs) in NGC 1097, NGC 1365
and NGC 1808. We find no galaxy which shows the PDR signature in only one part
of the observed nuclear region. We also observe unusually strong HNC emission
in NGC 5236, but it is not strong enough to be consistent with X-ray dominated
region (XDR) chemistry. Rotation curves are derived for five of the galaxies
and dynamical mass estimates of the inner regions of three of the galaxies are
made.Comment: Accepted for publication in MNRAS, 22 December 2015. Main manuscript
is 13 pages, containing 3 figures. Also has 4 appendices of 13 pages total
containing numerous figures and details of calculation
VLBI observations of a sample of Palomar-Green quasars - I. Parsec-scale morphology
We observed 20 Palomar-Green quasars at low redshi ft (z < 0.5) with total flux density >1 mJy, including four radio-loud quasars (RLQs) and 16 radio-quiet quasars (RQQs), using the Very Long Baseline Array (VLBA) at 5 GHz. 10 RQQs are clearly detected in the VLBA images, and a compact radio core is identified in eight of them, indicating the prevalence of active galactic nucleus (AGN)-related radio emission in this flux-density-limited RQQ sample. The RQQs and RLQs in our sample have a division at similar to 30 mJy. The radio emission from RQQs appears to be the result of a combination of star formation and AGN-associated activities. All RQQs in our sample have a 5 GHz flux density ratio of Very Large Array (VLA) A-array to D-array f(c) = S-A(VLA)/S-D(VLA) above 0.2. The RQQs with f(a)(VLBA and VLA flux density ratio S-VLBA/S-A(VLA)) > 0.2 versus f(a) < 0.2 show significant differences in morphology, compactness, and total flux density. f(a) of RQQs is systematically lower than that of RLQs, probably due to the extended jets or relic jets of RQQs on tens to hundreds parsecs that are resolved out in VLBA images. Future larger samples, especially with the addition of milliarcsec resolution radio images of RQQs with total flux densities below 1 mJy, can test the conclusions of this paper and contribute to the understanding of the radio emission mechanism of RQQs, and the dichotomy and physical connection between RQQs and RLQs
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