Editorial - GCNEWS, Vol. 4, February 1997


A Newsletter for Galactic Center Research
This Volume was edited by Angela Cotera & Heino Falcke
email: gcnews@aoc.nrao.edu

Volume 4, February 1997 - EDITORIAL

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Where is the Galatic Center?

Angela Cotera & Heino Falcke

Although important to understanding the relation of objects at different wavelenghts, rarely do astrometric papers knock you off your feet: but it has just happened in Galactic Center research! In the last issue we reported about the headline-making determination of the black hole mass of Sgr A* from stellar proper motions. This issue contains another equally important paper published by Menten et al. in the ApJ Letters, using stellar SiO masers in the central cluster to pinpoint the position of Sgr A* in the 2.2 micron difraction limited reference frame to an estimated accuracy of 30 milli-arcseconds. Therefore, we may now know where the exact position of the physical center of the Galaxy is, as it is generally thought that Sgr A* marks the location of the supermassive, central black hole. The observations also reveal that this black hole candidate is actually black in the NIR as the derived position points right at ... nothing! There are no NIR sources coincident with the determined position of Sgr A*. Certainly this remarkable result will stimulate further work in this area in order to confirm and possibly improve the position of the Galactic Center.

Perhaps, within the next few years we as a community will seriously think about using the position of Sgr A* as the l=0.0, b=0.0 reference point for the Galaxy. This would not be the first time that the center of a galaxy was defined by the presence of a radio source. There is, however, a possible caveat occasionally raised mainly by Leo Blitz: is Sgr A* really in the dynamical center of the Galaxy? If so, how could a 2*106 Mo black hole be responsible for such a wide spread gravitational effect? Sgr A* may mark the center but it probably does not cause it to be there. We know from other galaxies that sometimes the physical center seems to be dislocated from the center of the outer isophotes and the center of rotation of the outer parts of the galaxy. In this respect ``Center'' may always be a relative term and one may always need to distinguish between the physical and dynamical centers. How certain are we that they are identical in the Galaxy and to which precision? The position of Sgr A* will probably always be the best known quantity in this equation. No matter what, Sgr A* has certainly captured our full attention in the past few months, with additional information on this source and its surroundings to be presented at the upcoming IAU Symposium No. 184, "The Central Regions of the Galaxy and Galaxies" (see the Preliminary Program in this issue).

Besides this interesting paper on one of the central questions in GC research, the current issue of GCNEWS brings you a number of other interesting abstracts, primarily on the nature of the interstellar matter in the Center of the Galaxy. Interestingly, two contributions in this ongoing discussion come from high-energy experiments which find extended pair annihilation radiation (Cheng et al.) and 26Al emission (Naya et al.). Heretofore, high-energy observations (e.g. beyond x-rays) have been unable to make the vast contributions to Galactic Center research that radio and infrared studies have made because of the low resolution of high energy surveys and the consequent large positional errors. Indeed at previous Galactic Center conferences and in papers published in the available literature, high-energy research has been, unfortunately, somewhat marginalized. We have therefore asked John Mattox, who first reported an EGRET detection of the Galactic Center a few years ago to summarize in a very compact form the results of EGRET observations of the Galactic Center --- and indeed this is an area of research that deserves more attention.

You will find his article and the new abstracts on the following pages and we hope you find this Newsletter a helpful and stimulating tool for your research.

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