AN X-RAY AND OPTICAL STUDY OF THE DWARF GALAXY NGC-1569 - EVIDENCE FOR A STARBURST-DRIVEN OUTFLOW

Supernova-driven outflows produced by intense bursts of star formation can drastically affect the structure and subsequent evolution of dwarf galaxies. Extensive mass loss from such systems may also provide an important source of chemical enrichment for the intergalactic medium. Despite the potential importance of these outflows, there is a very limited amount of direct observational evidence for their existence. One of the clearest signatures of a starburst-driven outflow is the X-ray emission from the hot gas that drives the outflow. We have therefore undertaken an X-ray imaging and optical spectroscopic investigation of the nearest and best-studied starbursting dwarf galaxy NGC 1569 using the HRI on ROSAT and the Ritchey-Chretien spectrograph on the KPNO 4 m telescope, respectively. We find that at least half the keV X-ray emission of NGC 1569 is associated with a diffuse halo that is some 3.'8 x 2.'2 similar to 2.4 x 1.5 kpc in size. Kiloparsec-scale ''spurs'' of diffuse X-ray emission extend outward along or near the optical minor axis of the galaxy. These diffuse X-ray spurs are morphologically associated with the well-known system of H alpha filaments. Previous kinematic studies have suggested that the H alpha filament system is a bipolar outflow. Our new optical spectroscopic data show that the H alpha emission comprises two kinematically distinct components: a quiescent system about 1 kpc in size responsible for about 75% of the H alpha emission and a fainter and more complex system over 2 kpc in size with radial velocities of as much as +/-200 km s(-1) relative to v(sys). The latter appears to define expanding hollow structures consisting of several kiloparsec-scale ''superbubbles.'' The dynamical age of the high-velocity system (similar to 10(7) yr) is similar to various estimates of the age of the starburst. We also see very broad wings (full width at zero intensity of 1400-2300 km s(-1)) on the H alpha emission-line profile at the location of super starcluster A, suggestive of recent supernova activity there. We argue that the starburst in NGC 1569 is driving an outflow of the interstellar medium on a global scale. Our simple models suggest that the X-ray emission is too bright to come primarily from the hot tenuous supernova-heated gas in the interior of a superbubble, unless this gas is evaporating or ablating cool dense material. This latter material may be the outer shell of the superbubble or interstellar clouds that have been engulfed by the superbubble. The high-velocity H alpha filaments probably correspond to the shocked and accelerated ambient material in the superbubble's outer walls, while the low-velocity material may be massive photoionized clouds in the starbursting galactic disk. The observed expansion speeds probably exceed the galactic escape velocity. It seems likely that the expanding material will ultimately ''blow out'' of the interstellar medium, and in so doing may allow most of the metals created by the starburst to escape from the galaxy. It is less clear whether the outflow will lead to the ejection of most or ah of this galaxy's interstellar medium, though it appears at least energetically feasible. We briefly discuss the significance of these results for ideas about the evolution of dwarf galaxies.