11 research outputs found
OH masers in the Milky Way and Local Group galaxies in the SKA era
The intense line emission of OH masers is a perfect tracer of regions where
new stars are born aswell as of evolved stars, shedding large amounts of
processed matter into the interstellar medium. From SKA deep surveys at 18 cm,
where the maser lines from the ground-state of the OH molecule arise, we
predict the discovery of more than 20000 sources of stellar and interstellar
origin throughout the Galaxy. The study of this maser emission has many
applications, including the determination of magnetic field strengths from
polarisation measurements, studies of stellar kinematics using the precisely
determined radial velocities, and distance determinations from VLBI astrometry.
A new opportunity to study shocked gas in different galactic environments is
expected to arise with the detection of lower luminosity masers. For the first
time, larger numbers of OH masers will be detected in Local Group galaxies. New
insights are expected in structure formation in galaxies by comparing maser
populations in galaxies of different metallicity, as both their properties as
well as their numbers depend on it. With the full capabilities of SKA, further
maser transitions such as from excited OH and from methanol will be accessible,
providing new tools to study the evolution of star-forming regions in
particular.Comment: Contribution to the conference on "Advancing Astrophysics with the
Square Kilometre Array" for the SKA science book, Giardini-Naxos, Sicily,
June 2014; in Proceedings of Science, 14 page
Searching for Radio Outflows from M31* with VLBI Observations
As one of the nearest and most dormant supermassive black holes (SMBHs), M31*
provides a rare but promising opportunity for studying the physics of black
hole accretion and feedback at the quiescent state. Previous Karl G. Jansky
Very Large Array (VLA) observations with an arcsec resolution have detected
M31* as a compact radio source over centimeter wavelengths, but the steep radio
spectrum suggests optically-thin synchrotron radiation from an outflow driven
by a hot accretion flow onto the SMBH. Aiming to probe the putative radio
outflow, we have conducted milli-arcsec-resolution very long baseline
interferometric (VLBI) observations of M31* in 2016, primarily at 5 GHz and
combining the Very Long Baseline Array, Tianma-65m and Shanghai-25m Radio
Telescopes. Despite the unprecedented simultaneous resolution and sensitivity
achieved, no significant () signal is detected at the putative
position of M31* given an RMS level of , thus ruling
out a point-like source with a peak flux density comparable to that
() measured by the VLA observations taken in 2012. We
disfavor the possibility that M31* has substantially faded since 2012, in view
that a 2017 VLA observation successfully detected M31* at a historically-high
peak flux density ( at 6 GHz). Instead, the
non-detection of the VLBI observations is best interpreted as the arcsec-scale
core being resolved out at the milli-arcsec-scale, suggesting an intrinsic size
of M31* at 5 GHz larger than times the Schwarzschild radius. Such
extended radio emission may originate from a hot wind driven by the weakly
accreting SMBH.Comment: 9 pages, 2 figures. Accepted for publication in the Astrophysical
Journa
Structure of W3(OH) from Very High Spectral Resolution Observations of 5 Centimeter OH Masers
Recent studies of methanol and ground-state OH masers at very high spectral
resolution have shed new light on small-scale maser processes. The nearby
source W3(OH), which contains numerous bright masers in several different
transitions, provides an excellent laboratory for high spectral resolution
techniques. We present a model of W3(OH) based on EVN observations of the
rotationally-excited 6030 and 6035 MHz OH masers taken at 0.024 km/s spectral
resolution. The 6.0 GHz masers are becoming brighter with time and show
evidence for tangential proper motions. We confirm the existence of a region of
magnetic field oriented toward the observer to the southeast and find another
such region to the northeast in W3(OH), near the champagne flow. The 6.0 GHz
masers trace the inner edge of a counterclockwise rotating torus feature.
Masers at 6030 MHz are usually a factor of a few weaker than at 6035 MHz but
trace the same material. Velocity gradients of nearby Zeeman components are
much more closely correlated than in the ground state, likely due to the
smaller spatial separation between Zeeman components. Hydroxyl maser peaks at
very long baseline interferometric resolution appear to have structure on
scales both smaller than that resolvable as well as on larger scales.Comment: 21 pages using emulateapj.cls including 16 figures and 2 tables,
accepted to Ap
A Very High Spectral Resolution Study of Ground-State OH Masers in W3(OH)
We present VLBA observations of the ground-state hydroxyl masers in W3(OH) at 0.02 km s-1 spectral resolution. Over 250 masers are detected, including 56 Zeeman pairs. Lineshapes are predominantly Gaussian or combinations of several Gaussians, with normalized deviations typically of the same magnitude as in masers in other species. Typical FWHM maser linewidths are 0.15 to 0.38 km s-1 and are larger in the 1665 MHz transition than in the other three ground-state transitions. The satellite-line 1612 and 1720 MHz masers show no evidence of sigma^+/-2,3 components. The spatial positions of most masers are seen to vary across the line profile, with many spots showing clear, organized positional gradients. Equivalent line-of-sight velocity gradients in the plane of the sky typically range from 0.01 to 1 km s-1 AU-1 (i.e., positional gradients of 1 to 100 AU (km s-1)-1). Small velocity gradients in the 1667 MHz transition support theoretical predictions that 1667 MHz masers appear in regions with small velocity shifts along the amplification length. Deconvolved maser spot sizes appear to be larger in the line wings but do not support a spherical maser geometry
Two New X-ray/Optical/Radio Supernova Remnants in M31
We compare a deep (37 ks) Chandra ACIS-S image of the M31 bulge to Local
Group Survey narrow-band optical data and Very Large Array (VLA) radio data of
the same region. Our precisely registered images reveal two new optical shells
with X-ray counterparts. These shells have sizes, [S II]/H-alpha flux ratios,
and X-ray spectral properties typical of supernova remnants (SNRs) with ages of
9 and 17 kyr. Analysis of complementary VLA data
reveals the radio counterparts, further confirming that they are SNRs. We
discuss and compare the properties and morphologies of these SNRs at the
different wavelengths.Comment: 18 pages, 5 figures, accepted for publication in Ap
18-cm VLA observations of OH towards the Galactic Centre : Absorption and emission in the four ground-state OH lines
The OH distribution in the Sgr A Complex has been observed in the 1612-, 1665-, 1667- and 1720-MHz OH transitions with the Very Large Array (VLA) in BnA configuration. Maps are presented with a channel velocity resolution of about 9 km s -1 and with angular resolutions of . Some clear results are highlighted here, such as absorption from the Circumnuclear Disk (CND) and the OH-Streamer inside the CND near Sgr , strong absorption towards most of the eastern and western parts of the Sgr A East shell, lack of absorption towards both Sgr A West and the compact H II-regions to the east of Sgr A East, and double-lobed structure of the High Negative Velocity Gas (HNVG) oriented northeast and southwest of Sgr , and finally compact, point-like maser emission in all four transitions, in particular a 1720-MHz maser at -132 km s -1 in the CND as counterpart to a 1720-MHz maser at +132 km s -1 in the CND