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There are significant differences among old stellar systems which may partly be a function of luminosity. One such difference is the presence of nuclei in many high luminosity spiral galaxies but their absence in low luminosity irregular systems. Other differences are seen in the spectroscopic and photometric properties.
A general review is given of the content and distribution of interstellar gas within galaxies. The constancy of the ratio N(He)/N(H), independent of galaxy type (spirals and irregulars), is discussed and the possible mechanisms for this constancy are considered. The helium abundance does not vary across the disk of spirals, although nitrogen and possibly other elements do.
The gross features of the neutral hydrogen distribution in our Galaxy and other systems are described. In spirals, the peak of the radial distribution of Hi is located well away from the optical center. This is not the case for irregular-type systems. A possible correlation of the relative location of the maxima of Hi and Hii distributions with galaxy type is described. Many spirals studied with high enough relative angular resolution show concentrations of Hi in their outermost regions. These may be due to hydrogen companions or warps in the hydrogen plane. Hydrogen ‘bridges’ are described and a new example for the triple system M81–M82–NGC 3077 is given. This latter case may be an extreme example of distortion by companion galaxies of the Hi associated with a massive galaxy.
The neutral hydrogen content of a galaxy and its correlation with other integral properties is discussed. The absorption profile due to hydrogen associated with the radio galaxy Centaurus A is given. Comparison of optical and 21-cm measurements of galaxian redshifts shows excellent agreement over the radical velocity range −400 to + 5200 km s−1. There is, however, a systematic difference between 21 cm and optical redshifts over the range ∼ 1200 to ∼ 2400 km s−1 for optical values based on blue-sensitive spectra. The difference, ∼ 100 km s−1, is most likely due to blending of galaxian and night sky H and K absorption lines. The Hubble Constant is derived from a redshift-21 cm flux relation. Values in the range 78 to 109 km s−1 Mpc−1 are derived. A value of 97 kms−1 Mpc−1 is favored.
New observational data (Spinrad, 1970; Van den Bergh, 1970; Rubin and Ford, 1970) are used to determine structural and kinematic parameters of the nucleus, the subsystem of globular clusters, and interstellar hydrogen in M31.
The mass derived for the nucleus from the new spectrophotometric data is in good agreement with the virial mass 6 × 108M⊙. Model calculations show that there is no appreciable exchange of stars between the nucleus and the bulge. The rotation energy of the nucleus is only 7.5% of the total kinetic energy; the central density is 2 × 106M⊙ pc−3.
The mean radius of the subsystem of globular clusters is 4.5 kpc. This indicates that the subsystem of old stars is not identical with the spheroidal component of the galaxy, whose mean radius is only 1 kpc. Radial velocity dispersion of globular clusters is only half of that of the nucleus. This shows a strong dependence of the velocity dispersion on distance to the center of the galaxy and a bias in mass determination of a galaxy from velocity dispersion near the nucleus.
On the basis of data on rotation two mass distribution models have been found, differing from each other in respect of the mass concentration to the center. Spectrophotometric data on the stellar content of the bulge are urgently needed to solve the mass distribution problem.
Twenty years ago Walter Baade pointed out the remarkable fact that two of the four elliptical galaxy companions of M31 (namely NGC 185 and 205) contained apparent Population I in their central areas, evidenced by the presence of dust clouds and a dozen or so OB stars. I wish to report here on some observations of these stars and absorption regions, made in an attempt to understand the origin of this anomalous material and its relevance, if any, to the pattern of galaxy development.
Due to the linearity, high quantum efficiency, and high storage capacity of electronographic image intensifies, their application to astronomical photometry constitutes an advance equal in importance to those that resulted from the introduction of the photographic plate and the photomultiplier. With existing electronographic image tubes and telescopes of 60-in. aperture stellar photometry is possible to about magnitude 23. Consequently a major advance in the study of the stellar contents of galaxies in the Local Group may now be made.
During 1968–69, the spectracon image intensifier was used on the 60-in. reflector of the Cerro Tololo Observatory for B, V observations of 14 globular clusters in the Magellanic Clouds. Details of the observing program and discussions of the observations so far reduced will appear shortly in papers in the Astrophysical Journal and in Sky and Telescope.
Image tube spectra of emission regions in M31 have been studied to determine relative line strengths and abundances. Lines of H, He, [NII], [SII], [Oi], [Oii], [Oiii] are observed. The analysis indicates: (1) that there are ions whose line strengths, relative to Hα, are a function of the position of the emission region in M31; (2) abundance differences or different excitation mechanisms exist in regions separated by only a few hundred parsecs.
The bright Hii regions of a galaxy are always found near or in regions of high obscuration. It is also generally true that the dust lanes of a galaxy better define a spiral pattern than do Hii regions. There appears to be a correlation between the size of the central region and the number of Hii regions in a galaxy.
Multicolor observations of galaxies are being carried out at the prime focus of the 2.6-m Schajn telescope using an image intensifier and 6–9 color filters. The effective wavelengths of the bandpasses used are approximately 3600, 3730, 4400, 4680, 5090, 5280, 6090, 6600 and 7400 Å. The filters for 3730, 5090 and 6600 Å are centered on emission lines.
A spectral survey of Hii regions in the galaxies M33, M51, M101, NGC 2403 and NGC 1232 has been carried out. Absolute fluxes have been measured photo-electrically. The energy distribution of the embedded O-association and the intensities of the emission lines have been obtained. The emission spectra can be classified in a one-parameter sequence which is the same for all the galaxies studied. The spectral type is independent of size, shape, surface brightness and density of the Hii region, but does depend on the location of the region in its galaxy.
In the galaxies surveyed there is a close correlation between the distance of an Hii region from the center of the galaxy and the appearance of its spectrum. The line ratios [OIII]/Hβ, Hα/[NII], and [OII]/[NII] all increase by large factors as one passes from regions in the inner spiral arms to those in the outermost arms. The average value of these ratios in a galaxy increases on going from early Sc galaxies to late Sc galaxies. The results show that the N/O abundance ratio (and probably also the O/H abundance ratio) is lowest in irregular galaxies and in the outer parts of late Sc galaxies. It increases towards the center of the spirals and it is highest in the inner spiral arms of the early Sc galaxies. In contrast, the He/H abundance ratio is constant across galactic disks and along the morphological sequence.
An investigation of several nearby and irregular galaxies shows significant changes in the neutral hydrogen distribution with morphological type. M31, an Sb, has a central hole while M33, an Sc, has relatively a much smaller hole or no hole at all and NGC 6822, an irregular galaxy, is centrally concentrated. The Hii regions in all these morphological types are found in the regions of highest neutral hydrogen density.
A neutral hydrogen survey of the irregular galaxy M82 has been carried out with the transit radio telescope at Nançay, France. The resolving power was 4′ in right ascension and 34′ in declination. The velocity resolution was 59 km s−1.
Drift scans covering 58′ in right ascension were taken across the center of the galaxy where the radio source is located; fourteen scans were averaged.
Line profiles were derived every 2′ in right ascension. The profiles at ±4′ from the radio source are similar within the measurement errors. In particular, they do not show any rotation effect within ±15 km s−1. They were averaged in order to provide an estimate of the expected emission profile at the radio source position.
Subtraction of this average from the line profile measured in front of the radio source yielded significant negative temperatures at all velocities from 180 to 360 km s−1 and no positive temperatures at other velocities.
These negative temperatures were assigned to absorption of the radiation from the radio source by neutral hydrogen in M82.
Absorption is running from 3 to 6% in depth. The average velocity of the absorption profile is lying between the central emission velocity and the optically-determined velocity which are known to show a large disagreement.
The width of the absorption profile shows a velocity gradient of 200 km s−1 across the 35′ × 20′ radio source. Such a large velocity spread across this angular extent is only shown by the excited gas showing emission lines at optical wavelengths (Burbidge et al., 1964).
Since then, the absorption in M82 at the neutral hydrogen wavelength has been confirmed by measurements done with the Owens Valley Radio interferometer at 2 spacings where any emission is completely resolved (interfringes were 3′ and 1′.5).
In addition to the conclusions of the single dish observations done at Nançay, the interferometer data have shown that the steeper velocity gradient occurs along the major axis of the galaxy which coincides with the major axis of the radio source.
Thermal radiation from normal spiral galaxies may be detectable at centimeter and millimeter wavelengths. Predictions have been made assuming free-free radiation from Hii regions at Te = 7000 K, and a range of mean electron densities and radii.
It is suggested that some 5C2 radio sources previously identified with normal spiral galaxies, are associated with supernova remnants in these galaxies. This hypothesis is tested by the relative radio and optical positions and by luminosity estimates. It could be further tested by possible coincidences of Ohio radio sources with known supernovae, and also by predicted decreases in radio emission.
Observations of velocity dispersion and of light distribution at the centers of 22 galaxies, mostly ellipticals give M/L values of 15 to 40 for giants of all luminosities with smaller values for intrinsically faint galaxies. Core radii within which surface brightness drops by a factor of two are typically 50–100 pc with a range from 2 to several hundred parsecs. Elliptical galaxies fit the modified isothermal models used for clusters, while later types have a flatter density gradient. Complete results will be published elsewhere.
Statistical mass-to-light ratios are found for spiral and irregular galaxies, and E and S0 galaxies, using 57 multiple systems including 43 pairs. The results are consistent with those of the earlier study by Page. Preliminary conclusions point out the great uncertainties in such mass-to-light ratios due to inaccurate magnitudes and relative radial velocities. The derived masses are relatively insensitive to the choice of the distribution function of projected separations.
The stellar contents of Sb and Sc galaxies do not appear to be related to their rotation properties. This lack of correlation suggests that the position of a galaxy on the R – ω sequence is determined to a large extent by the environment in which the galaxy formed, rather than by subsequent evolution.