The Luminosity Distribution in the Spiral Galaxy NGC 5055.

Abstract

view Abstract Citations (15) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Luminosity Distribution in the Spiral Galaxy NGC 5055. Fish, Robert A. Abstract The luminosity distribution of the spiral galaxy NGC 5055 is studied in the three colors of the U, B, V system, using photographic photometry calibrated photoelectrically. On comparison of the luminosity distribution with the mass distribution found by Burbidge, Burbidge, and Prendergast (1960), the massto-light ratios are determined in each of the colors as a function of distance from the nucleus. They are found to rise at first, then decrease steadily on proceeding outward from the nucleus The over-all M/L ratio in photographic light out to 150" of arc from the nucleus is 5 46. The expected and observed dependence of luminosity on position angle are discussed for a spheroidal galaxy under the conditions of (a) no obscuration within the galaxy, (b) obscuration by pure absorption, and (c) obscuration by conservative scattering. In the first case the isophotes must be ellipses (as Contopoulos has shown), but the photometric information does not determine the three-dimensional luminosity distribution in the galaxy. The observed isophotes are far from elliptical. Nonetheless, except near the nucleus, NGC 5055 gives evidence of having nearly an axially symmetrical three-dimensional luminosity distribution. Unless the axis ratio of equidensity spheroids is a rapidly varying quantity, the deviation of the isophotes from ellipses must be due mainly to obscuration. For the pure absorption case, the observed asymmetry and the predicated asymmetry along the minor axis of the galaxy can be reconciled by a model axisymmetric spheroidal galaxy of approximately constant M/L ratio, mean axial ratio of 0.250.30 (out to 4000 pc from the nucleus), and with the dust having absorption cross-sections for ultraviolet, blue, and yellow light near the Galactic values. While no observations in NGC 5055 rule out the possibility that the dust is an absorbing medium, emissive only in the far infrared, the case of conservative scattering would more nearly coincide with data on our own Galaxy. The effect of scattering would be to redirect photons preferentially out of the light- path of the observer and toward normal emergence from the plane of the galaxy. In this case the geometrical picture of the asymmetry based on pure absorption would still be applicable, if a correction were made for diffuse scattering and transmission. The interpretation of the reddening and dimming of galaxies with decreasing inclination to the line of sight should be different for scattering than it would be for true absorption. For conservative scattering all photons reappear, and the mean apparent brightness and color of spiral galaxies of all inclinations must equal the true mean brightness and color. The face-on spiral galaxies of types Sa, Sb-, and Sb+ are then measured 0.42 mag. brighter and 0.04 mag. bluer, judging from Holmberg's (1958) photometry, than they would be if no dust at all were present. The assumption of conservative scattering would mean that the spiral galaxies of these types are, on the average, 0.9 mag. fainter than they are taken to be under the postulate of absorption according to a csc 0 law. Holmberg's measures suggest that galaxies with the inclination angle of NGC 5055 should, on the average, be slightly brightened if light were scattered conservatively, but the high asymmetry of NGC 5055 suggests that the albedo of the dust particles is not higher than about 0.9. The uncertainties caused by light-scattering mean that the steepness of the gradient of M/L is not known to better than a factor of 2. The combination of information from mass-to-light ratios and photometric colors is discussed, and it is found that the sensitivity of the observed colors to changes in the proportionate mass of the constituent stellar populations is related to the parameter K = (J3 - J)/(J - fi), where f is the measured mass-to- light ratio in the blue color and fi and f are blue M/L ratios of the constituent populations. The high color and low M/L ratio in the outer regions of NGC 5055 imply that if early-type stars are the principal luminous constituent of the stellar population 3000 pc from the nucleus, they are considerably reddened. U, B, and V colors for a group of diffuse luminous regions in the spiral arms of the galaxy are plotted in the U - B, B - V plane. The colors of all the regions are clearly related and give evidence of 0.3-04 mag of reddening Publication: The Astrophysical Journal Pub Date: November 1961 DOI: 10.1086/147216 Bibcode: 1961ApJ...134..880F full text sources ADS | data products NED (3)