The relation between color index and red shift for extragalactic nebulae.

Abstract

view Abstract Citations (4) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The relation between color index and red shift for extragalactic nebulae. Stebbins, Joel ; Whitford, A. E. Abstract Observations of the magnitudes and colors of nebulae were made at Mount Wilson from March to June, 1947, with a 1P21 photomultiplier tube on the large reflectors. The program was arranged by Messrs. Edwin Hubble and Walter Baade who also took part in the observations. In addition to many field nebulae, measures were made of members of the clusters in Virgo, Coma Berenices, Corona Borealis, and Bootes. The nebula in Bootes with apparent velocity 38,900 km/sec and photographic magnitude 18.2 was about at the limit for useful measures of magnitude and color with the installation on the i0o-inch. TABLE I. COLORS OF E NEBULAE Cluster Pgp c~ V No. m km/sec Virgo 9.5-15.6 o.86 1,240 18 Coma 13.2-17.4 0.99 6,~8o 3 Corona 16.8-17.3 1.135 22,000 4 Bootes 18.2 1.36 38,900 1 In Table I the magnitude Pg~ and color index Cv are on the International scale, deduced from the photoelectri~ measures. F is from M. L. Humason. The standard deviation of a Cv in the Virgo cluster is +0.06 mag.; the mean error of the Cv in Bootes is +0.05 mag. from six observations on three nights. From the quantities in the table we derive the relation Cp = 0.84 + 0.0133 X I0-1V where F is in km/sec. That this increase of color index with red shift is not caused by an instrumental error is shown by the facts that our colors agree within a few hundredths with the North Polar Sequence down to mag. 16.5; that there is no increase of reddening in stars in Selected Areas down to mag. 17.0; and, finally, that the nebula in Coma at mag. 17.4 has the same color as the brighter nebulae in the same cfuster. From our six-color photometry of the E nebula M32, assuming dG6 stars to radiate like black bodies at 55000, we find for a red shift like that of the Bootes nebulae, I + dN/X = 1.13, a predicted reddening ACp = o.o8; the observed value is ~Cv = 0.52. If the color excess is to be explained by absorption and scattering in intergalactic space by a mixture of gas and dust like that in our galaxy near the sun, we can compare a selective absorption of 0.5 mag. at 70,000 kpc for Bootes with, say, 0.25 mag. a~ I kpc in the local region. These figures give 3 X I0-~ for the ratio of the densities, intergalactic to interstellar material. Taking the latter as 3 X 10-24 g/cm1, we have 10-28 g/cm1 for intergalactic material, or 100 times the smoothed density of luminous nebular material according to Hubble. Such a figure could perhaps be accepted, but to explain the reddening as a consequence of Rayleigh scattering by gas alone would require an impossibly high density of about the same order as the gas density in the solar neighborhood. Moreover, if the reddening is a distance effe~t the correction to the observed magnitudes for total loss of photographic intensity would be at least I to 2 mag. for an uncorrected 70,000 kpc. Any such value leads to drastic reductions in photometric distances, which, when considered with the smooth run of the nebular counts, could only be interpreted as a rise in the nebular density function as we proceed outward. The absorption would then mean the virtual impenetrability of space at a distance less than the previously assigned range of existing telescopes. The possibility of a time effect was discussed but discarded namely, that the E nebulae were formerly redder than now, and that what we observe is simply their progressive change in color in the interval of 2 X I0~ years from the Bootes nebulae inward. But in the discussion at the meeting in Columbus Dr. Martin Schwarzschild suggested that these nebulae of population II may once have contained large numbers of high luminosity red giants which have consumed their store of energy and faded in the time since light left the distant ones. Also if the total luminosities of these nebulae were greater formerly than now, owing to the fading of the red giants, the difficulty of reconciling the nebular counts with the reddening would be diminished. If the reddening is a combination of both distance and time effects, their unravelling will not be easy. Whatever the cause of the reddening it is evident that the observational data on faint nebulae, including red shifts, magnitudes, colors of all types, counted numbers, and the inferences therefrom will have to be re-examined. Washburn Observatory, Madison, Wis. Publication: The Astronomical Journal Pub Date: 1948 DOI: 10.1086/106139 Bibcode: 1948AJ.....53R.204S full text sources ADS |