The Edison infrared space observatory and the universe at high redshifts

0273—1177/91 $0.00 + .50 Copyright © 1991 COSPAR

Ads’. Space Res. Vol. 11, No. 2, pp. (2)341—(2)344. 1991 Printed in Great Britain. Alt rights reserved.

THE EDISON INFRARED SPACE OBSERVATORY AND THE UNIVERSE AT HIGH REDSHIFTS H. A. Thronson, Jr ,“~ T. Hawarden,** J. K. Davies,** T. J. Lee,** C. M. Mountain** and M. Longair** *

Wyoming Infrared Observatory, University of Wyoming, Laramie,

WY 82071, U.S.A. *

*Ropal Observatory, Blackfrod Hill, Edinburgh EH9 3HJ, Scotland

ABSTRACT Edison is a radiatively-cooled, long-lived, large-aperture space observatory optimized for operation in the near- and mid-infrared. Our current design is for a 2.5m telescope launched as an AmericanEuropean collaboration in the NASA Explorer program. The telescope equilibrates at about 40 60 K, depending upon location and structure, which means that the system is celestial backgroundlimited at all wavelengths shortward of about 30 pm. Under these circumstances, the sensitivity of a telescope system depends strongly on aperture size, making Edison by far the most powerful observatory proposed for operation at near- and mid-infrared wavelengths. Here we describe part of our scientific program to study the universe at high redshifts. —.~

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JUSTIFICATION: TELESCOPE APERTURE AND OPERATION AT THE BACKGROUND LIMIT IN SPACE The sky from space is fairly bright at wavelengths longward of about 5 pm and all proposed or planned infrared space telescopes, including Edi8on, reach the celestial background after only short integrations. Emission from the telescope itself makes an insignificant contribution to radiation on the detectors over proposed operating ranges for all future infrared space missions. Therefore, observations of faint sources high-z and primeval galaxies, brown dwarfs, distant novae and supernovae will be dominated by either detector or celestial noise. Under these circumstances, if an object is resolved with an angular size of 9, the system sensitivity, S (the inverse of the integration time to reach a given brightness), varies with telescope diameter D as —

—

S

2. integration time ix (DIe) .

.

(1)

In the event that the object is unresolved, the dependence of sensitivity upon aperture is even more extreme, S~. cxD~, (2) integration time which means that aperture size must be the single most important parameter in infrared space observatory design. In addition to sensitivity, a large aperture also means a smaller diffractionlimited beam, which will be crucial in the confusion-rich environment of, for example, high-z or primeval galaxies, circumstellar dust emission at large distances in the Milky Way, or supernovae in distant galaxies. .

EDISON: THE SECOND GENERATION INFRARED SPACE OBSERVATORY

Objects launched into space naturally radiatively cool to temperatures far below those of the same structure on the surface of the earth. Indeed, it is common to include heaters on spacecraft to keep many sub-systems above a minimum operating temperature. As an example, after the cryogens boiled away, the IRAS telescope equilibrated at about 100 K, although this spacecraft was not at all optimized for radiative cooling and was significantly heated by being in a low earth orbit.

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2.5m ~ Radiation loss to space Radiators and shields Secondary

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EDISON

(off-axis schematic)

Figure 1: A schematic drawing, only roughly to scale, of one off-axis Cassegrain design for Edison. The telescope is cooled by a set of nested shields coupled to large-area radiators and the off-axis design has a very clean beam with no obscuring telescope structures. In addition, it may be possible to easily actively cool the secondary in this design, further reducing telescope emission. We are developing Edison, a large-aperture infrared space observatory using a mixture of radiative and mechanical cooling for the telescope and detector systems. Our philosophy contrasts with that of first-generation planned or proposed cryogenically-cooled infrared space telescopes (e.g., ISO, IRTS, and SIBTF). Although the technique of open-loop cryogenic cooling is very effective, with negligible telescope emission throughout the infrared and sub-millimeter, the bulky tanks of coolant severely constrain both telescope aperture and mission lifetime. Moreover, cryogenic cooling is unnecessary at the shorter near- and mid-infrared wavelengths, where emission from radiatively-cooled telescopes is both less than that from space and below the detector noise. A schematic drawing of our off-axis design is shown in Figure 1. Our current goal is for a —~ 2.5 m telescope which will radiatively cool to 40- 60 K, depending upon choice of location (orbit), radiator design, telescope structure, and heat input from satellite sub-systems. At these temperatures, telescope radiation is less than that from space at all wavelengths shortward of about 20 40 pm. This particular aperture size was chosen as being the largest that can plausibly be carried by launch vehicles available for either the NASA Explorer and ESA Medium Missions. In keeping with the philosophy behind these two programs, we are also proposing very capable, but simple and reliable photometric and spectroscopic instruments. A larger observatory may not be desirable, given its complexity, as well as human and financial cost. -

Although some detectors and elements of the instruments operate well at temperatures achievable by radiative cooling alone, an additional technique of refrigeration will be necessary to achieve the highest sensitivity, particularly at the longest wavelengths of operation. Edison is planned to use electrically-driven, closed-cycle refrigerators: temperatures as low as 4 K are the goal of a variety of mechanical coolers now in advanced stages of development for space applications both in Europe and in the U.S.

The Edison Infrared Space Observatory

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Figure 2: Edison will investigate the infrared universe with high angular resolution imaging limited only by the celestial background at all wavelengths shortward of 20 40 pm, depending upon the equilibrium temperature of the telescope. The bold line in the figure shows the sensitivity achievable by Edison with a temperature of either 40 or 50 K, an integration time of 1000 seconds, a signalto-noise ratio of 10, and R = )t/~.A = 2. An estimated performance for ISOCAM for the same observing parameters is also shown. Normal galaxies, as well as primeval supernovae can be studied at cosmological distances after modest integration times. -

Edison is under study by an American-European team from several institutions as an Explorer-class, second-generation infrared space observatory to be launched around 2000. A SELECTION OF INVESTIGATIONS INTO THE UNIVERSE AT HIGH REDSHIFTS Edison is proposed as a long-lived observatory capable of a wide range of astronomical programs in the wavelength range of about 2.5 30 pm. Studies of the distant universe will be a major part of its scientific program, as indicated by Figures 2 and 3. A partial list of proposed projects include: -

• Sensitive tests of cosmological parameters via deep multi-band imaging of distant galaxies and clusters (Fig. 2). • Search for hypothetical luminous, but low surface brightness galaxies that may have been abundant in the early universe via surveys for distant “isolated” supernovae (Fig. 2). • Study of the formation and morphological evolution of galaxies via high-resolution imaging of the infrared emission from normal galaxies at z = 4+ after modest integration times (Fig. 2). • Star formation and evolution in the first generation of stars using number counts, distribution and location, and light curves of supernovae in high-redshift galaxies. Supernovae light curves at cosmological distances. Are there extinct types of supernovae? (Fig. 2) • Study of the birth of heavy elements via spectra of individual Type II supernovae to distances of 1 Gpc or more (Fig. 3).

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WAVELENGTH (MICRONS)

Figure 3: Near- and mid-infrared spectra of supernovae show the important diagnostic features of hydrogen, CO and SiO bands, and iron peak elements. Edison has sufficient sensitivity to obtain spectra similar to that of the low luminosily SNe 1987A out to distances of many tens of Mpc. Edison’s small beam will be necessary to discriminate between a supernova and, for example, a luminous galaxy nucleus. • Birth and evolution of the ISM: spectra of gas and dust features in galaxies as a function of both redshift and location. • Evolution of the stellar populations within galaxies: spectra of photospheric features both as a function of redshift and location within high-z galaxies.