Editorial - GCNEWS, Vol. 18, July 2004
A Newsletter for Galactic Center Research
This Volume was edited by Sera Markoff, Loránt Sjouwerman,
Joseph Lazio, Cornelia Lang, Rainer Schödel & Robin Herrnstein
Volume 18, July 2004
- In the News
In the News - Joseph Lazio & Cornelia Lang
Welcome to the second GCNEWS of 2004. Since the last GCNEWS, we've
seen a number of new results on the size and nature of Sgr A* itself and
ongoing studies of the interstellar medium in this unique region.
Many GC folks have also taken a pause to commemorate the (almost accidental!)
discovery of Sgr A* which occurred only a little over 30 years ago.
In other news, we welcome Robin Hernnstein to our editorial staff.
As always, please feel free to contact any of us with
GC newsworthy items or future article ideas. We also remind our readers
to please submit their abstracts to GCNEWS. As
you know, GCNEWS promises rapid delivery of your results to fellow GC
researchers. While astro-ph is a valuable resource, GCNEWS
directly targets those readers with an acknowledged interest in GC science.
In this Volume
In this volume, we report on a conference
devoted to Sgr A* and its environment, and also provide an
update of GC research presented at the summer AAS meeting in Denver.
We are especially excited to have a comprehensive, invited review
article by Sheperd Doeleman and Geoff Bower. In their article, they
demonstrate how far we have come in understanding the structure
of Sgr A* since its discovery. Their article describes the
methods used to model and constrain the size of the radio source
from radio interferometric data and also highlights future observational
prospects - they point out that we are poised on the edge of being able
to use such techniques to image the shadow of Sgr A*.
A number of GC abstracts have come in this spring and summer (and
as mentioned before, we could always use more!) and are listed at the
end of the issue.
Happy 30th Sgr A*! - Cornelia Lang
A small workshop commemorating the 30th anniversary of the
discovery of Sgr A*, the radio source associated with the
supermassive black hole at the center of our Galaxy, was
held in Green Bank, West Virginia, on March 25 and 26. There were
60 participants from a variety of international and national
institutes and many of the NRAO scientific staff. The program
of a day and a half of scientific talks, a banquet, and a ceremony and
dedication of a plaque on the 45-foot telescope that commemorates the original
discovery. A reception for all participants and
Green Bank staff followed at the Green Bank Science Center.
The discoverers of Sgr A*, Bruce Balick (right) and Bob Brown (left) receive a
commemorative poster from NRAO director Fred Lo (far left). In the
background the commemorative plaque attached to the 45-foot
telescope. (Photo by Bill Saxton)
The discovery of the compact radio source at the center of
the Milky Way was made by Bruce Balick and Bob Brown in 1974 February.
They had originally been searching for compact regions of
star formation in the vicinity of the Galactic Center.
The placement of the 45-foot telescope near Huntersville, West Virginia,
as the 35 km outstation for the NRAO Green Bank radio link
interferometer (consisting of three, 85-foot antennas at Green Bank),
was crucial for resolving out the extended confusion from Sgr A West
and providing spatial resolutions of 0.3'' (at 3.7 cm)
and 0.7'' (at 11 cm) and detecting the very compact radio source, Sgr A*.
An opening review talk on massive objects at the centers of galaxies
was given by Roger Blandford, followed by a series of
historical talks given by the original observers, Bruce Balick and Bob Brown,
and the Green Bank site director
at that time, Dave Hogg. A number of fascinating letters from
Balick to Brown were shown which provided additional insight
into the details of the discovery (also summarized recently by Goss,
Lo 2003, Astron. Nachr., S1, 1). Roy Booth, K. Y. Lo, and
Ron Ekers also presented accounts of early work on the size and
structure of Sgr A* based on some of the first aperture synthesis
studies at Jodrell Bank, Owens Valley Radio Observatory, and the VLA.
Following the historical talks on the discovery of Sgr A*, a series of talks
results were presented. Don Backer gave an overview of
interstellar scattering properties toward Sgr A*, and Geoff Bower and
Zhi-Qiang Shen reported on
VLBA closure phase techniques used to constrain the size
of this compact source. Results
on the linear and circular polarization of Sgr A* were given by
Geoff Bower. Andrea Ghez, Rainer Schödel, Jun Hui-Zhao and Fred Baganoff
confirmed that variability of Sgr A* on a number of timescales has
been detected across the spectrum from radio to near-IR to X-rays.
Progress reports by Ramesh Narayan, Sera Markoff, Eliot Quataert, and Fulvio Melia
followed, describing the various models to explain the spectrum,
polarization, and variability of Sgr A*. Perspectives on the current
state and characteristics of the modeling were given by Heino Falcke.
The stellar and interstellar environment surrounding Sgr A* was discussed
in a series of talks on Friday morning. The scientific talks concluded at noon
with a thoughtful review by Mark Morris.
Following the scientific program, a dedication ceremony
was held at the 45-foot antenna, where NRAO director
Fred Lo presented Bruce Balick and Bob Brown with commemorative
framed posters (see picture). A plaque on the 45-foot antenna and
an informational adjacent sign for visitors were unveiled. An informal
scientific session was held on Friday evening to discuss prospects
for future millimeter and sub-millimeter VLBI observations of Sgr A*.
The next generation of telescopes will provide an excellent
opportunity for imaging on the scale of the event horizon, allowing
for detailed tests of accretion models and general relativity.
The scientific program, PDF and PPT versions of the talks, and
photographs are available at
GC appearances at Denver AAS in June 2004 - Cornelia Lang
A number of GC results were presented at the summer
American Astronomical Society meeting in Denver, Colorado
(May 30-June 3). We describe briefly those papers with a GC
component at the AAS meeting, with apologies to anybody whose work we
might have missed. The AAS paper number is in parentheses at the end
of each summary.
The enigmatic, magnetic ``non-thermal filaments'' (NTFs) in the
GC made an appearance in a number of presentations and posters this
- F. Yusef-Zadeh et al. presented a talk entitled ``A
Radio Survey of the GC Region: Detection of Numerous Linear Filaments'' in
which he presented a 20 cm survey of a 300 pc region around the GC
and a several-degree region toward low Galactic longitudes along the
Galactic plane. A number of NTFs were discovered in this
- J. Hewitt et al. followed up on these results and
presented high resolution images from this 20 cm survey, highlighting
a number of the new filamentary features and their associations with compact,
star forming regions. (59.03)
- T. Freismuth et al. presented recent work on the peculiar
NTF known as the ``Snake.'' For the first time, the orientation of the
intrinsic magnetic field in this object has been revealed to lie along
the NTF, even in the region where the Snake undergoes substantial
``kinks.'' Large rotation measures (> 4000 rad m-2) are also observed
toward much of the Snake. (59.05)
The combination of Green Bank single dish data with Very Large Array
interferometric data was a highlight of the poster by C. Law et al.
(and also the previously mentioned 59.03 poster):
- Law et al. focused on understanding the large-scale
radio continuum emission in the GC. In particular, they are interested
in one of the Galactic center ``Lobes'' of extended emission oriented
perpendicularly to the plane of the Galaxy. They presented the
of spectral index along and across the lobes to quantify the
of this feature. (59.02)
There were two press releases by NRAO on the combination of GBT and
datasets (59.02, 59.03). See http://www.nrao.edu/pr/2004/filaments/
and http://www.nrao.edu/pr/2004/GBTlobe/ for more details.
Several posters were focused on the interstellar medium in the GC
on large and small scales:
- C. Martin et al. showed results from the submillimeter
AST/RO, on several unusual molecular clumps in the GC region. He had
fully-sampled maps in the lines of CO (7->6), CO (4->3) and
C (3P1 -> 3P0)
which suggest that these clouds are tidally shocked through
with the bar potential in the Milky Way. (59.06)
- I. Azcárate et al.
investigated the departure from LTE in the GC region by observing nine positions
around the GC in H159 alpha and H200 beta radio recombination lines. They
employed the 30-m telescope of the Instituto Argentino de Radioastronomia. (59.01)
- Lang et al. presented their findings from a large-scale
survey of HI absorption toward the central 250 pc of the GC in order to
clarify the nature of the physical associations of the atomic, ionized and
molecular components. In addition, line of sight distances to continuum features
can be constrained as well as column densities estimated. Specific
results on Sgr B2 and the Radio Arc were shown. (59.04)
Other GC highlights:
- S. Liu et al. demonstrated that nonthermal electrons
accelerated by plasma wave turbulence in the central concentration of
hot gas may be responsible for portions of the spectrum of Sgr A* and
also its variability in the infrared and X-ray wavebands. (17.01)
- N. Weinberg et al. outlined the prospects for observing
stellar dynamics in the neighborhood of Sgr A* using the proposed
Thirty Meter Telescope (TMT). The TMT will allow astronomers to
measure the mass of the black hole and the distance to the GC with accuracies
better than about 0.1%. Some relativistic effects (such as prograde
orbital procession) will also be detectable. (17.02)
The GCNEWS Logo
at the top of this page shows a 20cm radio map of the GC (Sgr A) made by Yusef-Zadeh & Morris.
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File last modified on Tuesday 27 July 2004 [09:13 MDT].