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SCIENTIFIC ABSTRACT We propose to observe a set of Ne lines at various ionization stages from infrared luminous galaxies to reveal the origin of their luminosity and their evolutionary sequence. The IRAS mission revealed a new population of galaxies, infrared luminous galaxies, which are up to 100 time more luminous in the far-IR than in the optical band, are radiating more than 10^12 solar luminosity in the far-IR, and are comparable to quasars in their luminosity. Strong optical line emission emanating from their nuclei suggests two kinds of ionizing sources: (1) active galactic nuclei (AGN) and (2) huge starburst activity. Since these activities are deeply buried within a huge amount of molecular gas, their direct observations are impossible in the optical band. Hence the true origin of the luminosity of infrared luminous galaxies is still a mystery. One of the most promising way to discern AGNs from starburst and to reveal the origin of the huge luminosity will be through infrared-fine structure line observations, since (1) there are various kinds of lines at various ionization stages in the infrared, and (2) the infrared lines can penetrate deep into the dust-rich infrared luminous galaxies. For this purpose, we selected a set of Ne lines at various stages of ionization. AGN activities are definitely required to excite highly ionized lines, while low ionization lines will trace starburst activities. Hence the intensity ratios of the lines at various stages of ionization will reveal the relative importance of AGNs and starbursts. By revealing the relative importance of AGNs and starbursts activities in various kinds of infrared luminous galaxies, we can test a hypothesis in which quasars evolved from infrared luminous galaxies and, during the evolutionary sequence, the dominant energy sources shift from starburst to AGNs. OBSERVATION SUMMARY We propose to observe 13 galaxies for the autumn launch and 7 galaxies for spring launch. Most of them are infrared-luminous galaxies (far-infrared luminosities > 10^11 solar luminosity) with possible AGN activities. We also include some star-burst galaxies for comparison. For the galaxies with possible AGN activities, we observe the six Ne ionic lines listed in the following table. Among them, the [NeII] line traces star-forming activities, while highly ionized Ne ionic lines ([NeV] and [NeVI]) are attributed only to active galactic nuclei. Hence the detection of the highly ionized lines is clear evidence of AGNs, and their intensities are good measures of their activities. On the other hand, we observe only [NeII] and [NeIII] lines for starburst galaxies. From these lines, we estimate the intensity of current star-forming activity. Our selected lines are of the same element and are free from the effect of differences of elemental abundances among galaxies. Moreover, since these lines are at mid-IR ranges, they do not suffer from interstellar extinction very much. The latter point is especially important to reveal the hidden AGNs in dust-rich infrared luminous galaxies. Most of the selected lines (except for [NeII]) are not observable from the ground, and all the lines are at the wavelengths where thermal emission from ambient temperature telescopes are the strongest. Hence the ISO is the only instrument by which we can make sensitive observations of this sets of lines. Target Lines [NeII] 12.8 micron [NeIII] 15.6 micron [NeIII] 36 micron [NeV] 14.3 micron [NeV] 24.2 micron [NeVI] 7.6 micron