Color-indices of 155 extragalactic nebulae / Король - индексы 155 внегалактических туманностей

Vashakidze, M.A. and ვაშაკიძე, მ. and Вашакидзе, М.А. (1942) Color-indices of 155 extragalactic nebulae / Король - индексы 155 внегалактических туманностей. აბასთუმნის ასტროფიზიკური ობსერვატორიის ბიულეტენი / Bulletin of the Abastumani Astrophysical Observatory (6). pp. 57-70.


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The determination of color-indices of extragalactic nebulae is a very important problem of astrophysics. Using the color-indices of extragalactic nebulae we arc able to define the selective absorption for the whole space of our Galaxy as the nebular rays penetrate through the whole Galaxy. Until now the selective absorption within our Galaxy has been determined, mostly, by means of color-indices of the stars. Therefore the color-excesses have been derived at best for distances of about 3000 parsecs and the results, then, extrapolated on the whole thickness of the Galaxy. Though we can derive color-excesses at greater distances by using the globular dusters of the stars and the planetary nebulae,—they are not characteristic enough for the absorption in the vicinity of the galactic plane, because these globular clusters are to be found in high galactic latitudes and their color-excesses are, to a certain extent, unreliable. In spite of that, it should be noted that the color-excesses obtained by means of the globular clusters of the stars and by extragalactic nebulae coincide almost fully, as it has been stated by M. S. Eigenson in his work. But the above mentioned defect holds on for the extragalactic nebulae. As it is known, the zone of avoidance of extragalactic nebulae lies near the galactic plane and owing to their scarcity in the vicinity of the galactic equator it is rather difficult to deduce the law of diminution of color-excesses with the extension of galactic latitude. In order to get a reliable picture of variation of color-excesses the color-indices of extragalactic nebulae near the galactic plane should be determined with the utmost care. Such determination secures more accurate' data on the nebulae of low galactic latitudes than those derived from numerous nebulae lying near the galactic poles. As to the so-called 'galactic windows', i. e. such places in the zone of avoidance, where cxtragalactic nebulae can be found, we have to assume that there the interstellar medium is comparatively transparent, and, therefore, both selective and general absorption ere small. To characterize these regions it would be of interest to determine the color-indices of these extragalactic nebulae, or of the stars. General absorption derived by E- P. Hubble on counting the nebulae, is better characterized by the data 011 extragalactic nebulae. While offering in this work the color-indices of 155 extragalactic nebulae we, at the same time, make an attempt to discuss the data obtained by us. For this purpose we have also used the color-indices of 140 extragalactic nebulae, given in the preceding issue of our Bulletin. It should be noted that these data are not sufficient enough for this purpose, but we intend to determine the color-indices of all extragalactic nebulae, (or the majority of them), given in the Catalogue by Shaplev and Ames; after that the interpretation will be more reliable. Thus, the present work is the continuation of the preceding one. The difference between them lies in the color-indices having been determined in the present work for nebulae scattered all over the sky, from 0 —20° to 0 —+90°, while the nebulae, mentioned in the preceding work, arc, for the most part, in the constellation of V i r g o. In order to obtain more precise determinations of the nebular color-indices, we decided to apply, in addition to the polar sequence, the tube photometer, as a scale of comparison. The correction for the zenith distance was reckoned in the same way as it had been done in the preceding work, therefore, we shall not dwell upon it now- We photographed by means of the Schmidt Coma-free camera 45 areas distributed •according to the plan described in the preceding work. We shall not describe the photometric method now, as it had been done in the preceding work; but we shall elucidate the application of the tube photometer. We printed the tube photometer on the film taken from the same packet which had served for photographing the extragalactic nebulae. The calibration has been accomplished separately through the light-filters BG3 and GGII (Schott) which have been used for photographing the nebulae as well. In order to establish the value of the constant, we have used the stars of the polar sequence. The polar sequence has been photographed extrafocally for every night, therefore, we found the value of the zero-point for every area separately. We have generally used $ or 8 red stars of polar sequence and from them derived the arithmetical mean. Thus, the color-indices of nebulae were obtained by two methods: directly from the polar sequence and by means of the tube photometer. The arithmetical means from these two series of values of color-indices are given in Table I. This Table contains the values of color-indices of 155 extragalactic nebulae. Their numbers arc given after the Catalogue NGC. In the second column are given the integral photographic magnitudes after Shapley and Ames. The visual magnitudes obtained on the basis of our determinations of color-indices arc given in the third column. Thus, these. visual magnitudes are given in the same system of integral magnitudes, a the photographic ones. The color-index and the type of nebulae are given in the fourth and fifth columns, respectively. The intervals of diameters and their logarithms arc given in the first and second columns of the Table. As it is evident from this Table, the color-indices increase with the increase of the nebular diameter. This phenomenon should be explained in the same way, as it was done when speaking of bright nebulae. It may be said that both statements confirm one another. Again, the dependence of color-indices on diameters is more evident in spiral nebulae. The similar table was obtained by Shapley and Ames; it shows the saute dependence as in our case. From the determined color-indices of extragalactic nebulae, we have attempted to derive the variation of color-excess along the galactic latitudes. The mean color-indices. corresponding to different latitudes for the Northern and Southern hemispheres of the Galaxy are given in Tables VII and VIII. Before constructing the Table VII which includes all the nebulae with the northern galactic latitude, we have reduced the values of color-indices, correcting them for the magnitude according to the data of the Table V, and also for the type. The latter corrections were made in the following way: at first, we grouped all nebulae according to the type, placing into the first group: Sa. SBa. Sb. SBb: into the second I; into the third -Sc, SBc, S, S: and into the fourth—I and nebulae without classification; then we derived the mean color-indices for these groups. These values were corrected for the type according to the curve in Fig. I. For the Southern hemisphere we have used corrections for the magnitude only, owing to the small number of nebulae. On the basis of these data we are enabled to derive the optical thickness of selective absorption in the direction of the Northern Pole of the Galaxy. This value coincides, almost exactly, with the value obtained by M. Shigenson. In this case the optical thickness for the Southern hemisphere is greater than for the Northern hemisphere. Later on when we have more material at our disposal, we shall try to compute optical thickness on the ground of data at several latitudes. In future we shall make another determination of the color-indices of some extragalactic nebulae, given in the present work, with the purpose of greater accuracy. The color-indices corrected for these nebulae will be listed in the catalogue of the color-indices of extragalactic nebulae, which is now in preparation. July, 1941.

Item Type: Article
Subjects: Q Science > QB Astronomy
Divisions: Institutes > Evgeny Kharadze Abastumani National Astrophysical Observatory
Depositing User: თამარ ჭაღიაშვილი
Date Deposited: 16 Jul 2018 10:11
Last Modified: 16 Jul 2018 10:11

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