INVESTIGATION OF THE LARGE-SCALE NEUTRAL HYDROGEN NEAR THE SUPERNOVA REMNANT W28 P. F. Vela ´ zquez 1 Instituto de Ciencias Nucleares, Universidad Nacional Auto ´ noma de Me ´xico, Apdo. Postal 70-543, 04510 Me ´xico; D.F., Mexico pablo@nuclecu.unam.mx G. M. Dubner 2 Instituto de Astronomı ´a y Fı ´sica del Espacio, C.C. 67, 1428 Buenos Aires, Argentina; gdubner@iafe.uba.ar W. M. Goss National Radio Astronomy Observatory, P.O. Box O, Socorro, NM 87801; mgoss@nrao.edu and A. J. Green School of Physics, University of Sydney, A28, NSW 2006, Australia; agreen@physics.usyd.edu.au Received 2002 May 29; accepted 2002 July 15 ABSTRACT The distribution and kinematics of neutral hydrogen have been studied in a wide area around the super- nova remnant (SNR) W28. A 2=5  2=5 field centered at l =6=5, b =0  was surveyed using the Parkes 64 m radio telescope (half-power beamwidth of 14<7 at !21 cm). Even though W28 is located in a complex zone of the Galactic plane, we have found different H i features, which are evidence of the interaction between W28 and its surrounding gas. An extended cold cloud with about 70 M  of neutral hydrogen was detected at the location of W28 as a self-absorption feature, near the local standard of rest velocity of +7 km s 1 . This H i feature is the atomic counterpart of the molecular cloud shown by previous studies to be associated with W28. From this detection, we can independently confirm a kinematical distance of about 1.9 kpc for W28. In addition, the neutral hydrogen observed in emission around the SNR displays a ringlike morphology in sev- eral channel maps over the velocity interval [25.0, +38.0] km s 1 . We propose that these features are part of an interstellar H i shell that has been swept up by the supernova shock front. Emission from this shell is con- fused with unrelated gas. Hence, we derive an upper limit for the shell mass of 1200–1600 M  , a maximum radius of the order of 20 pc, an expansion velocity of 30 km s 1 , an initial energy of about 1.4–1.8  10 50 ergs, and an age of 3.3  10 4 yr. The preexisting ambient medium has a volume density on the order of 1.5–2 cm 3 . W28 is probably in the radiative evolutionary phase, although it is not possible to identify the recombined thin neutral shell expected to form behind the shock front with the angular resolution of the present survey. Key words: ISM: H i — ISM: individual (W28) — ISM: structure — supernova remnants 1. INTRODUCTION Each supernova remnant (SNR) is the unique product of its own history (the progenitor and the explosion mecha- nism) and the characteristics of the environs in which it evolves. The study of the interstellar medium around SNRs can be used to understand the appearance of a remnant in different spectral regimes (distorted shapes, local brightness enhancements, filamentary emission, etc.). Such studies also allow the analysis of the temporal evolution of SNRs. In addition, the investigation of the gaseous matter around SNRs can lead to an understanding of the Galactic interstel- lar medium. These studies are important in understanding the response of the interstellar gas to the large injection of energy and momentum that a supernova (SN) explosion represents. Numerous investigations of interaction of SNRs with the surrounding interstellar medium (ISM) have been made using atomic and molecular lines (Routledge et al. 1991; Pineault et al. 1993; Wallace, Landecker, & Taylor 1994; Frail, Goss, & Slysh 1994; Frail et al. 1996; Frail & Mitchell 1998; Reynoso et al. 1995; Dubner et al. 1998a, 1998b). These investigations show the manner in which the expansion of a SN shock front modifies the surrounding environment and the effect that the surrounding gas has, in turn, on the shape and dynamics of the SNR. In the present study, we report the results of an H i study around the SNR W28 (G6.40.1). The SNR W28 is located in a very complex region of the Galaxy, near the large H ii regions M8 and M20 and the young clusters NGC 6530, NGC 6514, and Bo 14. It has a number of prominent morphological characteristics. In the radio continuum, there is diffuse emission, together with thin filaments and small bright regions, as seen in Figure 1. This image is the result of combining 50 Very Large Array (VLA) pointings into a 20 cm mosaic (Dub- ner et al. 2000). In X-rays, diffuse thermal emission fills the interior of W28, although ear-shaped segments of a limb-brightened shell can also be observed toward the northeast and northwest (Rho & Petre 1996). In the opti- cal, there are bright narrow filaments strongly correlated with radio features and diffuse H nebulosities, appar- ently anticorrelated with the radio synchrotron emission (Long et al. 1991; Dubner et al. 2000). 1 Fellow of CONICET, Argentina. 2 Member of the Carrera del Investigador Cientı ´fico, CONICET, Argentina. The Astronomical Journal, 124:2145–2151, 2002 October # 2002. The American Astronomical Society. All rights reserved. Printed in U.S.A. 2145