64 research outputs found
Red-shifted H2O emission in NGC 3079: more evidence for a pc-scale circumnuclear torus?
Using the Effelsberg 100-m telescope, sensitive measurements of the H2O
megamaser in NGC 3079 are presented. During 2000 - 2001, `high velocity'
features are seen that are red-shifted up to 225 km/s with respect to the
systemic velocity of the galaxy (about 1120 km/s). Symmetrically bracketing the
systemic velocity, the H2O emission covers a velocity range of 450 km/s with
only one potential narrow gap (20 km/s) near the systemic velocity itself.
Velocity drifts of individual components are not convincingly detected. It is
shown that the presence of red-shifted emission and the absence of detectable
velocity drifts are not inconsistent with the existence of a rotating
circumnuclear maser disk at the very center of the galaxy. Significant
differences in the overall line profile compared to NGC 4258 and a complex
morphology of the radio continuum leave, however, space for scepticism.Comment: 4 pages, including 3 figures. Astronomy and Astrophysics Letters,
accepte
A Survey for H2O Megamasers III. Monitoring Water Vapor Masers in Active Galaxies
We present single-dish monitoring of the spectra of 13 extragalactic water
megamasers taken over a period of 9 years and a single epoch of sensitive
spectra for 7 others. Our data include the first K-band science observations
taken with the new 100 m Green Bank Telescope (GBT). In the context of a
circumnuclear, molecular disk model, our results suggest that either (a) the
maser lines seen are systemic features subject to a much smaller acceleration
than present in NGC 4258, presumably because the gas is farther from the
nuclear black hole, or (b) we are detecting ``satellite'' lines for which the
acceleration is in the plane of the sky.
We also report a search for water vapor masers towards the nuclei of 58
highly inclined, nearby galaxies.Comment: accepted by ApJ
High resolution imaging of the radio continuum and neutral gas in the inner kiloparsec of the radio galaxy 3C293
Using a combination of observations involving the VLA, MERLIN and global VLBI
networks we have made a detailed study of the radio continuum and the neutral
hydrogen (HI) kinematics and distribution within the central kiloparsec of the
radio galaxy 3C293. These observations trace the complex jet structure and
identify the position of the steeply inverted radio core at 1.3GHz.
Strong HI absorption is detected against the majority of the inner kiloparsec
of 3C293. This absorption is separated into two dynamically different and
spatially resolved systems. Against the eastern part of the inner radio jet
narrow HI absorption is detected and shown to have higher optical depths in
areas co-spatial with a central dust lane. Against the western jet emission and
core component, broad and complex HI absorption is detected. This broad and
complex absorption structure is discussed in terms of two possible
interpretations for the gas kinematics observed: that the gas is situated in
two gas layers or that it is in a sub-kiloparsec disk rotating about the core.
(Abridged)Comment: 14 pages, 8 figures, To be published in MNRAS. High resolution
version is available at http://www.jb.man.ac.uk/~rbeswick/3c293-paper.htm
Discovery of water vapor megamaser emission from Mrk1419 (NGC 2690): An analogue of NGC 4258?
Water vapor emission at 22 GHz is reported from the nucleus of the LINER
galaxy Mrk 1419 (NGC 2960). Single-dish spectra of the maser source show
properties that are similar to those seen in NGC 4258, namely (1) a cluster of
systemic H2O features, (2) two additional H2O clusters, one red- and one
blue-shifted by about 475 km/s, (3) a likely acceleration of the systemic
features, and (4) no detectable velocity drifts in the red- and blue-shifted
features. Interpreting the data in terms of the paradigm established for NGC
4258, i.e. assuming the presence of an edge-on Keplerian circumnuclear annulus
with the systemic emission arising from the near side of its inner edge, the
following parameters are derived: Rotational velocity: 330-600 km/s; radius:
0.13-0.43 pc; binding mass: about 10 million solar masses. With the galaxy
being approximately ten times farther away than NGC 4258, a comparison of
linear and angular scales (the latter via Very Long Baseline Interferometry)
may provide an accurate geometric distance to Mrk 1419 that could be used to
calibrate the cosmic distance scale.Comment: 4 pages, 5 Postscript figures, A&A Letter
The wideband backend at the MDSCC in Robledo. A new facility for radio astronomy at Q- and K- bands
The antennas of NASA's Madrid Deep Space Communications Complex (MDSCC) in
Robledo de Chavela are available as single-dish radio astronomical facilities
during a significant percentage of their operational time. Current
instrumentation includes two antennas of 70 and 34 m in diameter, equipped with
dual-polarization receivers in K (18 - 26 GHz) and Q (38 - 50 GHz) bands,
respectively. We have developed and built a new wideband backend for the
Robledo antennas, with the objectives (1) to optimize the available time and
enhance the efficiency of radio astronomy in MDSCC; and (2) to tackle new
scientific cases impossible to that were investigated with the old, narrow-band
autocorrelator. The backend consists of an IF processor, a FFT spectrometer
(FFTS), and the software that interfaces and manages the events among the
observing program, antenna control, the IF processor, the FFTS operation, and
data recording. The whole system was end-to-end assembled in August 2011, at
the start of commissioning activities, and the results are reported in this
paper. Frequency tunings and line intensities are stable over hours, even when
using different synthesizers and IF channels; no aliasing effects have been
measured, and the rejection of the image sideband was characterized. The first
setup provides 1.5 GHz of instantaneous bandwidth in a single polarization,
using 8192 channels and a frequency resolution of 212 kHz; upgrades under way
include a second FFTS card, and two high-resolution cores providing 100 MHz and
500 MHz of bandwidth, and 16384 channels. These upgrades will permit
simultaneous observations of the two polarizations with instantaneous
bandwidths from 100 MHz to 3 GHz, and spectral resolutions from 7 to 212 kHz.Comment: 9 pages, 8 figures. Accepted to Astronomy and Astrophysic
6.7 GHz methanol absorption toward the Seyfert 2 galaxy NGC 3079
The detection of the 6.7 GHz line of methanol (CH3OH) is reported for the
first time toward an object beyond the Magellanic Clouds. Using the Effelsberg
100 m telescope, two absorption features were identified toward the Seyfert 2
galaxy NGC 3079. Both components probably originated on lines-of-sight toward
the central region, presumably absorbing the radio continuum of the nuclear
sources A, B, and E of NGC 3079. One absorption feature, at the systemic
velocity, is narrow and may arise from gas not related to the nuclear
environment of the galaxy. The weaker blue-shifted component is wider and may
trace outflowing gas. Total A-type CH3OH column densities are estimated to be
between a few times 10^13 and a few times 10^15 cm^-2. Because of a highly
frequency-dependent continuum background, the overall similarity of HI, OH, and
CH3OH absorption profiles hints at molecular clouds that cover the entire area
occupied by the nuclear radio continuum sources ~ 4 pc.Comment: 4 pages, 1 figure, accepted for publication in A&A Letter
Jet-cloud collisions in the jet of the Seyfert galaxy NGC3079
We report the results from a six-year, multi-epoch very long baseline
interferomertry monitoring of the Seyfert galaxy NGC3079. We have observed
NGC3079 during eight epochs between 1999 and 2005 predominantly at 5GHz, but
covering the frequency range of 1.7GHz to 22GHz. Using our data and
observations going back to 1985, we find that the separation of two of the
three visible nuclear radio components underwent two decelerations. At the time
of these decelerations, the flux density of one of the components increased by
factors of five and two, respectively. We interpret these events as a radio jet
component undergoing compression, possibly as a result of a collision with ISM
material. This interpretation strongly supports the existence of jets
surrounded by a clumpy medium of dense clouds within the first few parsecs from
the central engine in NGC3079. Moreover, based on recently published
simulations of jet interactions with clumpy media, this scenario is able to
explain the nature of two additional regions of ageing synchrotron material
detected at the lower frequencies as by-products of such interactions, and also
the origin of the kpc-scale super bubble observed in NGC3079 as the result of
the spread of the momentum of the jets impeded from propagating freely. The
generalization of this scenario provides an explanation why jets in Seyfert
galaxies are not able to propagate to scales of kpc as do jets in radio-loud
AGN.Comment: 11 pages, 9 figures, needs aas_macros.sty and mn2e.cl
First images of 6.7-GHz methanol masers in DR21(OH) and DR21(OH)N
The first images of 6.7-GHz methanol masers in the massive star-forming
regions DR21(OH) and DR21(OH)N are presented. By measuring the shapes, radial
velocities and polarization properties of these masers it is possible to map
out the structure, kinematics and magnetic fields in the molecular gas that
surrounds newly-formed massive stars. The intrinsic angular resolution of the
observations was 43 mas (~100 AU at the distance of DR21), but structures far
smaller than this were revealed by employing a non-standard mapping technique.
This technique was used in an attempt to identify the physical structure (e.g.
disc, outflow, shock) associated with the methanol masers. Two distinct
star-forming centres were identified. In DR21(OH) the masers had a linear
morphology, and the individual maser spots each displayed an internal velocity
gradient in the same direction as the large-scale structure. They were detected
at the same position as the OH 1.7-GHz ground-state masers, close to the centre
of an outflow traced by CO and class I methanol masers. The shape and velocity
gradients of the masers suggests that they probably delineate a shock. In
DR21(OH)N the methanol masers trace an arc with a double-peaked profile and a
complex velocity gradient. This velocity gradient closely resembles that of a
Keplerian disc. The masers in the arc are 4.5% linearly polarized, with a
polarization angle that indicates that the magnetic field direction is roughly
perpendicular to the large-scale magnetic field in the region (indicated by
lower angular resolution measurements of the CO and dust polarization). The
suitability of channel-by-channel centroid mapping is discussed as an improved
and viable means to maximise the information gained from the data.Comment: Accepted by MNRA
Class I methanol masers in low-mass star formation regions
Four Class I maser sources were detected at 44, 84, and 95 GHz toward
chemically rich outflows in the regions of low-mass star formation NGC 1333I4A,
NGC 1333I2A, HH25, and L1157. One more maser was found at 36 GHz toward a
similar outflow, NGC 2023. Flux densities of the newly detected masers are no
more than 18 Jy, being much lower than those of strong masers in regions of
high-mass star formation. The brightness temperatures of the strongest peaks in
NGC 1333I4A, HH25, and L1157 at 44 GHz are higher than 2000 K, whereas that of
the peak in NGC 1333I2A is only 176 K. However, rotational diagram analysis
showed that the latter source is also a maser. The main properties of the newly
detected masers are similar to those of Class I methanol masers in regions of
massive star formation. The former masers are likely to be an extension of the
latter maser population toward low luminosities of both the masers and the
corresponding YSOs.Comment: 5 pages, 1 figure, Proc. IAU Symp. 287 "Cosmic Masers: from OH to
H0". LSR velocities of the HH25 masers, which are presented in Table 1, are
correcte
The structure of the magnetic field in the massive star-forming region W75N
A debated topic in star formation theory is the role of magnetic fields
during the protostellar phase of high-mass stars. It is still unclear how
magnetic fields influence the formation and dynamics of massive disks and
outflows. Most current information on magnetic fields close to high-mass
protostars comes from polarized maser emissions, which allows us to investigate
the magnetic field on small scales by using very long-baseline interferometry.
The massive star-forming region W75N contains three radio continuum sources
(VLA1, VLA2, and VLA3), at three different evolutionary stages, and associated
masers, while a large-scale molecular bipolar outflow is also present. Very
recently, polarization observations of the 6.7 GHz methanol masers at
milliarsecond resolution have been able to probe the strength and structure of
the magnetic field over more than 2000 AU around VLA1. The magnetic field is
parallel to the outflow, suggesting that VLA1 is its powering source. The
observations of water masers at 22 GHz can give more information about the gas
dynamics and the magnetic fields around VLA1 and VLA2. The NRAO Very Long
Baseline Array was used to measure the linear polarization and the
Zeeman-splitting of the 22 GHz water masers in the star-forming region W75N. We
detected 124 water masers, 36 around VLA1 and 88 around VLA2 of W75N, which
indicate two different physical environments around the two sources, where VLA1
is in a more evolved state. The linear polarization of the masers confirms the
tightly ordered magnetic field around VLA1, which is aligned with the
large-scale molecular outflow, and also reveals an ordered magnetic field
around VLA2, which is not parallel to the outflow. [abridged]Comment: 11 pages, 5 figures, 2 Tables, accepted by Astronomy & Astrophysic
- …