EURD OBSERVATIONS OF INTERSTELLAR RADIATION JERRY EDELSTEIN, STUART BOWYER, ERIC J. KORPELA and MICHAEL LAMPTON Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450 JOAQUÍN TRAPERO ⋆ , JOSÉ F. GÓMEZ, CARMEN MORALES and VERONICA OROZCO Laboratorio Astrofísica Espacial y Física Fundamental, INTA, Apdo. Correos 50727, E-28080 Madrid, Spain Abstract. The hot interstellar medium (ISM) has far-reaching effect upon the structure of galaxies. Although ISM heating processes are fairly well understood, after decades of study, the processes that cool the hot interstellar medium remain obscure. The EURD spectrograph was designed to measure the diffuse cosmic background from 350 to 1100 Å in order to study the hot ISM and the mechanisms by which it sheds its energy. We present the first analysis of EURD observations of the cosmic background. These EURD observations have proven to be far more sensitive than previous work; compared to previous results, we have improved the limits to the intensity of 450 to 900 Å line emission from the ISM by one to two orders of magnitude. Our limit to OVI 1032 Å/ 1038 Å doublet of 7900 ph s -1 cm -2 str -1 is the lowest yet reported. The EURD limits to line emission are less intense than predicted by a variety theoretical models of the local ISM. 1. Introduction The extreme ultraviolet (EUV) diffuse background is the most poorly known of any of the diffuse astronomical backgrounds. Because of the observational complexity of background measurements in the EUV bandpass, only upper limits to this flux exist. The limits have been obtained with spectrometers with very crude (from ≈ 15 to 30 Å) resolution (Holberg, 1986; Labov and Bowyer, 1991) and are generally one to two orders of magnitude larger than the expected sources of cosmic flux. A variety of source mechanisms have been postulated to radiate in this bandpass; the most discussed is the poorly understood hot gas in the interstellar medium. The actual lines observed from a hot ISM will be strongly dependent upon the temperature and thermal history of this material (Breitschwerdt and Schmutzler, 1994). Several temperatures have been suggested for this phase. Soft X-ray data suggest 10 6 K gas (Cox and Reynolds, 1987). Absorption line data showing O VI (Jenkins, 1978a,b) is often cited in combination with the soft X-ray data as further evidence for a 10 6 K gas, but the peak of the emission curve for O VI is at the substantially lower temperature of ∼ 3 × 10 5 K. High ionization absorption lines ⋆ Present address: Universidad SEK, Cardenal Z´ uñiga s/n, Segovia, Spain Astrophysics and Space Science is the original source of this article. Please cite this article as: Astrophysics and Space Science 276: 177–185, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.