Spectral photometry of the dark lane of NGC 4594.

Abstract

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spectral photometry of the dark lane of NGC 4594. van de Huist, H. C. Abstract NGC 4594 is a beautiful example of a spiral nebula seen nearly edgewise. Its major axis, of about 10', is in the east-west direction. It has a very bright nucleus and at 16" north of the center a dark lane is seen that has a width of about 8". The light in the minor axis can be traced on either side of the nucleus to about 90" from the center. The dark lane is probably caused by a well defined, flat ring of obscuring matter in the equator plane of the nebula. Photometry of the light in this dark lane at various wave lengths may therefore give information about the properties of the obscuring particles. The photometric problem is difficult, however, because the immediate neighborhood of the bright nucleus tends to introduce serious errors arising from imperfect guiding and from photographic effects, in particular Eberhard effect. Last April I took several spectrograms of this object with the Cassegrain spectrograph of the McDonald Observatory. The slit was a step slit placed along the minor axis. Its length was 20 mm = 148". The part covering the bright nucleus had a width of 0.30 mm, while the parts at either side that covered the faint outer parts of the nebula and the dark lane had a width of 2.55 mm. At the southern part, opposite the dark lane, there was an intermediate step, I .o6 mm wide. The spectrum, taken with the F/I Schmidt camera in 22' hours exposure time, shows the streaks due to the various steps and to the dark lane, all of them very nicely of measurable density. The further reduction follows the usual scheme; its preliminary results may be reported here. By making a microphotometer tracing across the spectrum at a certain wave length, then converting the densities into intensities and, finally, reducing the intensities to one slit width, the intensity curve along the minor axis of the nebula is found. This curve is of the same type as that found by Oort' from direct photographs. Its most striking property is that the northern and southern halves are, except for the dip due to the dark lane, exactly symmetrical. The depth of this dip is found to range from a factor 3 at 4000A to a factor 3.5 at ~oooA, in striking contrast with Oort's value of a factor 12. The latter difference may be due to Eberhard effect on the direct photographs. The further result that the light in the dark lane is slightly bluish would seem to indicate that this is light scattered by the dark clouds (mainly from the nucleus) and not light from the deeper layers coming through these clouds. I.M. N. io6, 159, 1946. Yerkes Observatory, Williams Bay, Wis,. Publication: The Astronomical Journal Pub Date: February 1948 DOI: 10.1086/106078 Bibcode: 1948AJ.....53..118V full text sources ADS |