GCNEWS
A Newsletter for Galactic Center Research
Currently edited by Sera Markoff, Loránt Sjouwerman, Joseph Lazio, Cornelia Lang, Rainer Schödel, Masaaki Sakano & Feng Yuan
email: gcnews@aoc.nrao.edu
GCNEWS
In China, recently, the Shanghai Astronomical Observatory (SHAO) has become a center for research on the Galactic center (GC). There are also faculty interested in this research area at the Institute of High Energy Physics (IHEP) and at Shanghai Jiaotong University. Both SHAO and IHEP belong to the Chinese Academy of Sciences (CAS), a national organization composed of over eighty institutes, aiming at conducting research in basic and technological sciences (similar to the Max-Planck Society in Germany). At SHAO, the three faculty working on GC research are Dr. Tao An, Dr. Feng Yuan, and Dr. Zhiqiang Shen.
The following are highlights of Galactic Center research in China:
Furthermore, Dr. Shen and his collaborators detected larger apparent source sizes in one epoch of 7 mm VLBA observation. This indicates a temporal variation in the structure of Sgr A^*. The derived structure is relatively big and intrinsically symmetrical within the error bars. Assuming this was caused by an outburst, they estimated the location of such a flare to be about 40 Schwarzschild radii from the central supermassive black hole (Shen et al. 2006).
Dr. Fangjun Lu at IHEP and collaborators observed two elongated X-ray features, G359.89-0.08 and G359.54+0.18, in the Galactic center (GC) region using the Chandra X-Ray Observatory (Lu et al. 2003). The former is an elongated X-ray feature partially coincident with a slightly curved nonthermal radio source. It is best interpreted as the synchrotron emission from a ram pressure-confined pulsar wind nebula. The later is one of the most prominent radio nonthermal filaments in the GC region located 30' in projection from Sgr A*. Dr. Fangjun Lu is now contributing most of his time to the HXMT (Hard X-ray Modulation Telescope) project in China (http://www.hxmt.org).
References
An, T., et al. 2005, ApJ, 634, L49
Baganoff, F. K.et al., 2003,
ApJ, 591, 891
Lu, F. J., Wang, Q. D. & Lang, C. C. 2003, AJ, 126, 319
Shen, Z.-Q. et al. 2005, Nature, 438, 62
Shen, Z.-Q. et al. 2006, in preparation
Xu, Y.D. et al. 2006, \apj,
640, 319
Yuan, F. 2006, astro-ph/0607123
Yuan, F. et al. 2003, ApJ,598, 301
Yuan, F. et al. 2004, ApJ, 606, 894
Yuan, F. et al. 2006, \apj, 642, L45
On July 6th, 2006, there was a Galactic center mini-workshop held at the National Astronomical Observatory of Japan (NAOJ). This meeting focused on the the arc-like and circular structures of molecular gas discovered in the Galactic center (GC).
Discovery of Molecular Loops in the GC
Molecular loops in the GC by FUKUI, Yasuo (Nagoya Univ.) Molecular loops were discovered in the Galactic center by the NANTEN telescope. They presented a magnetic floatation model in which the loops are formed due to the magnetic buoyancy caused by the Parker instability.
Molecular loops in the GC by KUDO, Natsuko (Nagoya Univ.) They discussed the fourth quadrant loops known as loop 1 and loop 2. The kinetic energy involved in a loop is estimated to be 1051 erg for a velocity dispersion of 30 km -1. The position-velocity diagram showed that the loop foot points have a large velocity span and are connected by a bridge.
Molecular loops in the Galactic center - comparison with CO and HI by TORII, Kazufumi (Nagoya Univ.) They compared the distribution of CO with HI gas in loop 1 and loop 2. They found a good correlation between CO and HI.
Observations of the GC Molecular Loops in Other Wavelengths
Vertical structure in the edge-on galaxy NGC891 by HANDA, Toshihiro (Univ. of Tokyo) These authors found that a prominent spur structure of molecular gas is emerging vertically from the disk in NGC 891. They suggested that this spur may be ejected from the galactic disk due to a superbubble or formed by the Parker instability.
Distribution of high excitation molecular gas in CMZ by OKA, Tomoharu (Univ. of Tokyo) They discovered a vast amount of high-temperature and low-density gas with a large velocity dispersion in the central molecular zone of the Galaxy. These high excitation gas and wide velocity dispersion indicate that local expansions are the origin of compact molecular gas structures.
Magnetic field structure of SNRs with hard X-ray observation by BAMBA, Aya (RIKEN) Supernova remnants (SNRs) are one of the efficient accelerators of cosmic rays. Hard X-ray observations show that the acceleration efficiency at shock fronts of SNRs is very high. From TeV gamma-ray observations, they suggested that a few hundred undiscovered SNRs may exist.
Magnetic field structure observed with infrared by NAKAGAWA, Takao (JAXA) According to the infrared observations, the magnetic field in the Galaxy is aligned with the galactic plane. In CMZ, the magnetic field strength is estimated to be milli-Gauss.
Theories and Numerical simulations of the Parker instability and Phenomena in the GC and the Galactic Disk
The origin of molecular loop structures by MATSUMOTO, Ryoji (Chiba Univ.) Here, these authors considered that molecular loop structures are created by the Parker instability. If the strength of the magnetic field is about 100 mu Gauss, the molecular gas sliding down along the magnetic fields can generate a velocity dispersion of as high as 50 km/sec in the molecular gas.
Local two-dimensional simulations of galactic loops by NOZAWA, Satoshi (Ibaraki Univ.) This work reports on two-dimensional MHD simulations of the Parker instability. It successfully reproduced the two loop-like structures observed in the Galactic center region.
Lobal three-dimensional simulations of galactic disks by MACHIDA, Mami (NAOJ) They carried out three-dimensional MHD simulations of galactic gas disks. They showed that the magnetic flux buoyantly rise from the disk.
Magnetic field amplification by Magneto-rotational instability Speaker: SANO, Takayoshi (Osaka Univ.) Due to Galactic rotation, the Galactic gas disk is subject to a magneto-rotational instability and and becomes turbulent. When the gas is isothermal, magnetic flux tubes can be created.
Formation of molecular gas cloud by KOYAMA, Hiroshi (Kobe Univ.) These authors studied the conditions for molecular cloud formation by the Parker instability.
The mechanism of particle acceleration inside the Galaxy by ASANO, Katsuaki (NAOJ) These authors investigated whether a shock formed by the Parker instability can be the origin of the particle acceleration for the energy range between 1015 and 1018 eV detected inside the Galaxy. They conclud from the rough estimation that the Parker instability is likely to be the origin of cosmic-ray acceleration.
At the General Assembly XXVI of the International Astronomical Union, a session entitled "New Results on the Galactic Centre" was held under the auspices of Division VII (Galactic Systems). The meeting had two purposes. The first was to summarize recent work on the Galactic center to the larger IAU audience. Invited talks highlighted a number of the recent observational advances, such as in the very high energy and infrared, or theoretical puzzles, such as the existence of young stars and a stellar disk (or two!) in the central parsec. There was also an effort made to place the center of the Milky Way into a larger context with respect to galactic centers in general.
The second purpose was to gauge the interest in forming an IAU Working Group on the Galactic center. The aim of such a working group would be to enhance the visibility of Galactic center science within the IAU and to increase the interest in the Milky Way Galactic center as an exemplar for low-luminosity galactic nuclei.
There was significant interest in forming such a working group. An Organizing Committee is being identified, with Joseph Lazio as the chair.