Farhad Yusef-Zadeh

Structural details of the Sagittarius A complex - evidence for a large-scale poloidal magnetic field in the Galactic center region

Detailed radio observations of the Sagittarius A complex are presented and discussed. The evidence is consistent with a large-scale poloidal magnetic field lying at the Galactic center. Arguments are made in favor of the suggestion that ionized gas is flowing out isotropically from the nucleus, and that Sgr A East is located behind Sgr A West. The question whether Sgr East is a supernova remnant or the result of energetic activity related to the Galactic nucleus is addressed. 126 references.

The thermal, arched filaments of the radio arc near the Galactic center - magnetohydrodynamic-induced ionization

This paper presents a detailed observational study of the most prominent thermal component of the radio Arc located near the Galactic center, the arched filaments. Images produced from data gathered at 6 and 20 cm reveal that the arched filaments have a wispy, flocculent character that is quite different from the uniform, continuous, and almost straight filaments of the nonthermal portion of the Arc. The two systems of filaments show clear signs of interaction at their intersection. There is no evidence from the radio maps that the arched filaments are linked to the Galactic nucleus. Various ideas for the ionization mechanism of the arched filaments are considered. It is suggested that the ionization is caused by a magnetohydrodynamic interaction between a molecular cloud with a large peculiar velocity and an ambient poloidal magnetic field of milligauss strength. 50 refs.

A windswept cometary tail on the Galactic center supergiant IRS 7

High-resolution VLA observations provide evidence of optically thick radio emission from IRS 7, a cool red supergiant star, located at a projected distance of roughly 1 1t-yr from the Galactic center. IRS 7 shows a remarkable tail of ionized gas pointing directly away from the compact nonthermal radio source at the Galactic center, Sgr A(asterisk). Given previous evidence for a strong source of UV emission and for a strong circumnuclear wind emanating from the Galactic center, the free-free emission from IRS 7 and its associated tail are interpreted in terms of the ionization and removal of the circumstellar envelope of the red supergiant either by the ram pressure of the nuclear wind or by the pressure of radiation arising from the immediate vicinity of Sgr A(asterisk). The wind mechanism is preferred because: (1) the force it can potentially exert is much greater; and (2) Sgr A(asterisk) is clearly not a known source of luminous energy in the near-IR, whereas it remains a plausible source of a hot, high-velocity wind. Also considered is the potential effect of a nuclear wind upon the atmospheres of red giants in the inner parsec. 24 refs.

A dense molecular ring surrounding the nonthermal Galactic center radio shell G359.1-0.5

A new 12 CO survey of the Galactic center region reveals a nearly continuous ring of molecular gas that is centered on and concentric with the nonthermal radio continuum shell G359.1-0.5. The molecular feature has high radial velocities, between -60 and -190 km/s, indicating that it is likely to be located at the distance of the Galactic center; its diameter is then about 75 pc. The mass of the ring, based on its integrated CO line emission, is about 2.5 x 10 6 M ⊙

Nonthermal radio emission from the galactic center arc

Observations at three frequencies - 160 MHz, 327 MHz, and 4.8 GHz - of the continuum arc near the galactic center are presented and discussed. A map of the 160 MHz radio continuum and another of the polarized 4.8 GHz emission indicate clearly that nonthermal processes are responsible for the emission arising from one limited segment of the arc: that arising from G0.16-0.15, a broad intensity maximum situated on the southern half of the system of parallel filaments oriented perpendicular to the galactic plane. Other portions of this vertical system of filaments have radio characteristics consistent with thermal emission. A model is developed in which the filaments forming the core of the arc are uniformly nonthermal emitters, but are surrounded by a nonuniform distribution of thermal plasma which preferentially absorbs low-frequency radiation, thus flattening the nonthermal spectrum, and depolarizes the nonthermal emission from the filaments everywhere except in the vicinity of G0.16-0.15. 23 references.

A low-energy jet emanating from the galactic nucleus?

On a observe une crete etroite demission radio basse frequence le long de laxe de rotation de la galaxie, jusqua ∼30 pc du noyau vers les latitudes negatives. On suppose quelle resulte de lejection continue ou frequente de particules relativistes par le noyau galactique