NEW INFRARED EMISSION OF THE MILKY WAY

In the autumn of 1950, S. F. Rodionov and I. G. Frishman¹ discovered a new infrared emission of the Milky Way in the region of \(1\mu\). This emission, observed at any point of the northern Milky Way from the constellation Orion to the constellation Cepheus, was measured by the authors against the background of the powerful infrared emission of the upper layers of the atmosphere, thanks to the use of a highly sensitive electrophotometer.

The study of the emission of the Milky Way in the infrared region is of extraordinary interest, since it makes it possible to reveal features of the structure of the Milky Way hidden from us by powerful masses of interstellar matter that absorbs visible rays but is transparent to infrared rays.

The first attempt to measure the infrared emission of star clouds was made in 1945 by Stebbins and Whitford², who studied the region of the galactic center with an electrophotometer equipped with a light filter placed at the focus of the large reflector of the Mount Wilson Observatory. These measurements revealed, in the region of the galactic center, a light body not detected in visible rays. In 1948 A. A. Kalinyak, B. I. Krasovsky, and V. B. Nikonov³, using an electron-optical converter, undertook photography in infrared rays—

FROM CURRENT LITERATURE

... of a more extensive region near the center of the Galaxy, with the aim of determining the actual dimensions of the galactic nucleus. The photographs obtained showed the existence of an invisible stellar cloud, symmetrical with the Large Cloud in the constellation Sagittarius and, together with it, forming part of the general system of the galactic nucleus. On the basis of a photometric analysis of the photographs, an attempt was made to estimate the dimensions of the galactic nucleus. In 1949 the same authors continued their investigations, obtaining photographs of a somewhat wider region of the galactic center and confirming their conclusions.^4

A study of the entire visible region of the northern Milky Way in infrared rays was undertaken in 1950 by S. F. Rodionov and I. G. Frishman. This work was a definite stage in a series of studies on the electrophotometry of the radiation of the night sky in various parts of the spectrum, carried out on Elbrus. The purpose of the work was: 1) to detect possible fluctuations in the distribution of the brightness of the glow of the upper layers of the atmosphere over the celestial vault, and 2) to investigate the glow in the region of the Milky Way in the infrared part of the spectrum.

In the measurements, carried out at an altitude of 2200 m above sea level, a photometer with a light filter was used, developed earlier by S. F. Rodionov and his collaborators and consisting of a secondary-electron tube with electrostatic focusing and a direct-current amplifier. The effective wavelength of the spectral region recorded by the photometer was \(1\ \mu\). As the authors report, the sensitivity of the photometer was considerably increased in comparison with the instrument (sensitivity \(8 \cdot 10^{4}\ k/\text{cal}\)) previously used by Rodionov and his collaborators in measuring the infrared glow of the night sky, as a result of which it became possible to reduce the angular aperture of the instrument.

The method consisted in measuring the brightness of the celestial vault as a function of zenith and azimuthal angles, the Milky Way being crossed at various points. It turned out that each passage through the Milky Way gives a maximum on the measured curve; outside the Milky Way the brightness-distribution curve observed was one already obtained earlier by S. F. Rodionov for the glow of the upper layers of the atmosphere, with a minimum of brightness at the zenith and with a symmetrical increase of brightness toward the horizon (no fluctuations of brightness over the celestial vault were found for the glow of the upper atmospheric layers). The authors give brightness curves measured in various cross sections of the Milky Way; all the curves have a maximum corresponding to passage through the Milky Way, and in some of them (for example, in the constellation Cygnus) a structure is noticeable which reflects the bifurcation of the Milky Way.

The magnitude of the detected infrared radiation could be easily determined for any of the investigated regions of the Milky Way, since, with the sensitive electrophotometric method used by the authors, the background glow of the atmosphere presents no difficulty. Indeed, the infrared radiation at a given point of the Milky Way is equal to the excess of the measured brightness above the background curve, obtained practically simultaneously. The brightness values of the Milky Way given by the authors amount to from 2 to 10% of the background; the absolute infrared brightness of the Milky Way, measured at the Earth’s surface, proved to be from \(5 \cdot 10^{-4}\) to \(5 \cdot 10^{-3}\) erg/sec·cm² steradian (this quantity is determined with some inaccuracy, since the distribution of the emitted brightness in the spectral region under investigation is unknown; the authors approximately regard it as independent of wavelength in the given spectral interval).

Of great interest is the curve of the distribution of infrared brightness along the Milky Way, in which the results of the measurements are summed (see the figure); in constructing the curve all brightness values for

various points of the Milky Way have been reduced to the zenith, i.e., the difference in the quantities of brightness for places close to the zenith and close to the horizon, arising from the attenuation of light in the Earth’s atmosphere, has been taken into account. The curve clearly shows a rapid increase in infrared brightness in the region of the constellation Ophiuchus, i.e., in the direction toward the center of the Galaxy; it is possible that the brightness values in the region of Ophiuchus are somewhat exaggerated: on the curve in this region there is only one point, and comparison with the data given by the authors for \(z = 55^\circ\) gives smaller brightness values for the region of Ophiuchus. It is to be regretted that the authors were unable to investigate the region of the constellation Sagittarius, which throughout the entire period of measurements was below the horizon. However, from the curve it is clear that the effect

Distribution of infrared brightness along the Milky Way.
\(I\)—\(\alpha\)-Ophiuchi, \(II\)—\(\delta\)-Aquilae, \(III\)—\(\eta\)-Aquilae,
\(IV\)—\(\beta\)-Cygni, \(V\)—\(\gamma\)-Cygni, \(VI\)—\(\alpha\)-Cygni,
\(VII\)—\(\gamma\)-Cassiopeiae, \(VIII\)—\(\alpha\)-Persei, \(IX\)—\(\beta\)-\(\zeta\)-Tauri,
\(X\)—\(\varepsilon\)-Orionis.

in this region must be very large; it is possible that the sensitivity of the method, already in the form in which it was applied, will prove sufficient for studying the detailed structure of the galactic nucleus.

The decline in infrared brightness from the constellation Ophiuchus toward greater galactic longitudes (if the maximum in the constellation Cygnus is excluded) corresponds to a decrease in the density of the stellar population with distance from the center of the Galaxy. The considerable infrared radiation of the Milky Way that has been discovered possibly indicates the existence, within the limits of our Galaxy, of a larger number than had previously been assumed of “cold” stars emitting infrared rays. The maximum of infrared radiation in the constellation Cygnus reflects the presence in this region of a stellar cloud, observed, as is well known, also in visible rays. It is still difficult to say whether the rise of the curve in the region of \(\varepsilon\)-Orionis corresponds to the existence here of invisible star clusters.

It is possible that further detailed measurements of the distribution curve will prove the presence in this region of stellar clouds hidden from us by dark interstellar matter.

The existence of considerable infrared radiation throughout the entire extent of the Northern Milky Way is a very noteworthy fact. Whether it is explained by the existence, in the plane of the Galaxy, of a large number of low-temperature stars, or is connected with the presence of masses of opaque-

for visible-light matter, will be shown by further measurements. However, it is already clear that measurement of the newly discovered infrared radiation of the Milky Way will be a powerful method for studying the structure of the Galaxy.

The further application of the highly sensitive electrophotometric methods developed by Soviet investigators opens new prospects for astrophysics. There is no doubt that these methods will be widely used in the work of our observatories.

E. P.

CITED LITERATURE

1. S. F. Rodionov and I. G. Frishman, DAN 77, 998 (1951).
2. G. Stebbins and Whitford, Astrophys. Journ. 106, 235 (1947).
3. A. A. Kalinyak, V. I. Krasovsky and V. B. Nikonov, DAN 66, 1 (1949).
4. A. A. Kalinyak, V. I. Krasovsky and V. B. Nikonov, Izv. Kr. Astr. Obs., issue 6, 119 (1951).