314 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
VLBI imaging of OH absorption: The puzzle of the nuclear region of NGC 3079
Broad hydroxyl (OH) absorption-lines in the 1667 MHz and 1665 MHz transition
towards the central region of NGC 3079 have been observed at high resolution
with the European VLBI Network (EVN). Velocity fields of two OH absorption
components were resolved across the unresolved nuclear radio continuum of ~10
parsecs. The velocity field of the OH absorption close to the systemic velocity
shows rotation in nearly the same sense as the edge-on galactic-scale molecular
disk probed by CO(1-0) emission. The velocity field of the blue-shifted OH
absorption displays a gradient in almost the opposite direction. The
blue-shifted velocity field represents a non-rotational component, which may
trace an outflow from the nucleus, or material driven and shocked by the
kiloparsec-scale superbubble. This OH absorption component traces a structure
that does not support a counter-rotating disk suggested on the basis of the
neutral hydrogen absorption.Comment: 9 pages, 10 figures, submitted to MNRAS (03/12/2003
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
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
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
H2O MegaMasers: a RadioAstron success story
The RadioAstron space-VLBI mission has successfully detected extragalactic
H2O MegaMaser emission regions at very long Earth to space baselines ranging
between 1.4 and 26.7 Earth Diameters (ED). The preliminary results for two
galaxies, NGC3079 and NGC4258, at baselines longer than one ED indicate
masering environments and excitation conditions in these galaxies that are
distinctly different. Further observations of NGC4258 at longer baselines will
reveal more of the physics of individual emission regions.Comment: To be published in Astrophysical Masers: Unlocking the Mysteries of
the Universe, IAU Symposium 336, 201
- …