437 research outputs found
Proper Motions in the Andromeda Subgroup
This article presents results of VLBI observations of regions of H2O maser
activity in the Local Group galaxies M33 and IC10. Since all position
measurements were made relative to extragalactic background sources, the proper
motions of the two galaxies could be measured. For M33, this provides this
galaxy's three dimensional velocity, showing that this galaxy is moving with a
velocity of 190 +/- 59 km\s relative to the Milky Way. For IC10, we obtain a
motion of 215 +/- 42 km/s relative to the Milky Way. These measurements promise
a new handle on dynamical models for the Local Group and the mass and dark
matter halo of Andromeda and the Milky Way.Comment: 4 pages 1 figures, to appear in the proceedings of "Galaxies in the
Local Volume", Astrophysics and Space Science, editors B. Koribalski and H.
Jerjen also available at
http://www.mpifr-bonn.mpg.de/staff/abrunthaler/pub.shtm
Jet precession in the active nucleus of M81. Ongoing VLBI monitoring
In a recent publication, we reported results of a multi-frequency VLBI
campaign of observations of the Active Galactic Nucleus (AGN) in galaxy M\,81,
phase-referenced to the supernova SN\,1993J. We were able to extract precise
information on the relative astrometry of the AGN radio emission at different
epochs and frequencies. We found strong evidence of precession in the AGN jet
(i.e., a systematic evolution in the jet inclination at each frequency) coupled
to changes in the overall flux density at the different frequencies. In these
proceedings, we summarise the main contents of our previous publication and we
report on (preliminary) new results from our follow-up VLBI observations, now
phase-referenced to the young supernova SN2008iz. We also briefly discuss how
these results match the picture of our previously-reported precession model.Comment: 6 pages, 3 figures. Proceedings of EVN meeting 201
The innermost region of the water megamaser radio galaxy 3C403
The standard unified scheme of active galactic nuclei requires the presence
of high column densities of gas and dust potentially obscuring the central
engine. So far, few direct subarcsecond resolution studies of this material
have been performed toward radio galaxies. The goal of this paper is to
elucidate the nuclear environment of the prototypical X-shaped Fanaroff-Riley
type II radio galaxy 3C403, the only powerful radio galaxy known to host a
water megamaser. Very Large Array A-array and single-dish Green Bank and
Effelsberg 1.3 cm measurements were performed to locate and monitor the water
maser emission. Very Long Baseline Interferometry 6 cm continuum observations
were taken to analyze the spatial structure of the nuclear environment at even
smaller scales, while the CO J=1-0 and 2-1 transitions were observed with the
IRAM 30-m telescope to search for thermal emission from a spatially extended,
moderately dense gas component.[abridged]Comment: 11 pages, 12 figures, accepted by A&A. For a version with high
quality figures, see http://erg.ca.astro.it/~atarchi/3C403
Discovery of a bright radio transient in M82: a new radio supernova?
In this Letter, we report the discovery of a new bright radio transient in
M82. Using the Very Large Array, we observed the nuclear region of M82 at
several epochs at 22 GHz and detected a new bright radio source in this
galaxy's central region. We find a flux density for this flaring source that is
~300 times larger than upper limits determined in previous observations. The
flare must have started between 2007 October 29 and 2008 March 24. Over the
last year, the flux density of this new source has decreased from ~100 mJy to
~11 mJy. The lightcurve (based on only three data points) can be fitted better
with an exponential decay than with a power law. Based on the current data we
cannot identify the nature of this transient source. However, a new radio
supernova seems to be the most natural explanation. With it's flux density of
more than 100 mJy, it is at least 1.5 times brighter than SN1993J in M81 at the
peak of its lightcurve at 22 GHz.Comment: accepted Astronomy & Astrophysics, 4 pages, 3 figures, final version
& corrected abstract, also available at
http://www.mpifr-bonn.mpg.de/staff/abrunthaler/pub.shtm
Towards Proper Motions in the Local Group
Key and still largely missing parameters for measuring the mass content and
distribution of the Local Group are the proper motion vectors of its member
galaxies. The problem when trying to derive the gravitational potential of the
Local Group is that usually only radial velocities are known, and hence
statistical approaches have to be used. The expected proper motions for
galaxies within the Local Group, ranging from 20 to 100 as/yr, are
detectable with VLBI using the phase-referencing technique. We present
phase-referencing observations of bright masers in IC~10 and M33 with respect
to background quasars. We observed the HO masers in IC10 three times over a
period of two months to check the accuracy of the relative positions. The
relative positions were obtained by modeling the interferometer phase data for
the maser sources referenced to the background quasars. The model allowed for a
relative position shift for the source and a single vertical atmospheric delay
error in the correlator model for each antenna. The rms of the relative
positions for the three observations is only 0.01 mas, which is approximately
the expected position error due to thermal noise. Also, we present a method to
measure the geometric distance to M33. This will allow re-calibration of the
extragalactic distance scale based on Cepheids. The method is to measure the
relative proper motions of two HO maser sources on opposite sides of M33.
The measured angular rotation rate, coupled with other measurements of the
inclination and rotation speed of the galaxy, yields a direct distance
measurement.Comment: 4 pages, Proceedings of the 6th European VLBI Network Symposium, Ros,
E., Porcas, R.W., Zensus, J.A. (eds.), MPIfR, Bonn, Germany (2002); Also
availabe http://www.mpifr-bonn.mpg.de/staff/abrunthaler/brunthal01.p
A Parallax-based Distance Estimator for Spiral Arm Sources
The spiral arms of the Milky Way are being accurately located for the first
time via trigonometric parallaxes of massive star forming regions with the
BeSSeL Survey, using the Very Long Baseline Array and the European VLBI
Network, and with the Japanese VERA project. Here we describe a computer
program that leverages these results to significantly improve the accuracy and
reliability of distance estimates to other sources that are known to follow
spiral structure. Using a Bayesian approach, sources are assigned to arms based
on their (l,b,v) coordinates with respect to arm signatures seen in CO and HI
surveys. A source's kinematic distance, displacement from the plane, and
proximity to individual parallax sources are also considered in generating a
full distance probability density function. Using this program to estimate
distances to large numbers of star forming regions, we generate a realistic
visualization of the Milky Way's spiral structure as seen from the northern
hemisphere.Comment: 25 pages with 16 figures; to appear in Ap
The Proper Motion of Sagittarius A*: III. The Case for a Supermassive Black Hole
We report measurements with the Very Long Baseline Array of the proper motion
of Sgr A* relative to two extragalactic radio sources spanning 18 years. The
apparent motion of Sgr A* is -6.411 +/- 0.008 mas/yr along the Galactic plane
and -0.219 +/- 0.007 mas/yr toward the North Galactic Pole. This apparent
motion can almost entirely be attributed to the effects of the Sun's orbit
about the Galactic center. Removing these effects yields residuals of -0.58 +/-
2.23 km/s in the direction of Galactic rotation and -0.85 +/- 0.75 km/s toward
the North Galactic Pole. A maximum-likelihood analysis of the motion, both in
the Galactic plane and perpendicular to it, expected for a massive object
within the Galactic center stellar cluster indicates that the radiative source,
Sgr A*, contains more than about 25% of the gravitational mass of 4 x 10^6 Msun
deduced from stellar orbits. The intrinsic size of Sgr A* is comparable to its
Schwarzschild radius, and the implied mass density of >4 x 10^23 Msun/pc^-3
very close to that expected for a black hole, providing overwhelming evidence
that it is indeed a super-massive black hole. Finally, the existence of
"intermediate-mass" black holes more massive than 3 x 10^4 Msun between
approximately 0.003 and 0.1 pc from Sgr A*are excluded.Comment: 19 pages, 4 figures and 4 table
- …