Galactic Distribution

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The large-scale distribution of the various components of the interstellar medium is best studied in external galaxies, which can readily be seen as a whole. Photographs of M31, the closest spiral galaxy, show that bright young O stars, the emitting H II regions that surround them, and the dust clouds which obscure starlight generally are all concentrated within the spiral arms. Figure 1.4, photographed with an Ha filter [12], shows conspicuous H II regions along a spiral arm in M31. Between the arms in spiral galaxies there appears to be no conspicuous obscuration nor emission nebulosity. Probably the neutral hydrogen and helium are also concentrated to the spiral arms, although the resolution of most 21-cm H-line

Figure 1.4 H 11 emission regions In M31. This photograph was obtained [12] with the 48-in. Schmidt telescope at the Hale Observatories; a filter with a band-pass of 100 A, centered at Ha, was used. The H II regions along a spiral arm, together with some obscuration by dust particles, are clearly evident.

Figure 1.4 H 11 emission regions In M31. This photograph was obtained [12] with the 48-in. Schmidt telescope at the Hale Observatories; a filter with a band-pass of 100 A, centered at Ha, was used. The H II regions along a spiral arm, together with some obscuration by dust particles, are clearly evident.

surveys is too low to verify this. Since the O stars responsible for the brighter H II regions cannot shine for more than about 106 years, photographs such as Fig. 1.4 are generally taken as evidence that formation of these massive stars occurs in the relatively dense interstellar gas found in spiral arms. It should be noted that the pattern of spiral arms in M31 is somewhat irregular, with more than two arms present, and with some elongated features not clearly related to the overall structure.

The spiral galaxy M51 has a somewhat more regular two-arm pattern than does M31. The continuous emission at 1415 MHz (21.2 cm) from this object, believed to be largely synchrotron radiation, has been measured [13] with a beam width of about 28 arcseconds, corresponding to about 500 pc at the distance of M51. The resultant contours, superposed on an optical photograph, are shown in Fig. 1.5. There is a clear apparent concentration of the synchrotron radiation to the inner edges of the spiral arms.

Figure 1.5 Regions of peak radio emission from M51. Superposed on the photograph of M5l, obtained with the Hale Telescope on Mt. Palomar, are white lines indicating apparent ridges of peak emission observed [13] at 1415 MHz with the Westerbork, Netherlands, Synthesis Radio Telescope. The beam shape at half peak power is indicated in the lower left of the figure.

Within our own Galaxy uncertainty as to the distance of any emitting feature makes it difficult to determine the overall structure of the gas. The difficulty is compounded at some wavelengths by the presence of obscuration which is sometimes difficult to estimate. For H II regions the distances can be obtained with reasonable accuracy from observations of the central stars, whose spectra can be measured; the resultant spectral type yields an absolute magnitude, whereas the measured color index gives a color excess EB_ v, which is proportional to the amount of obscuration by dust grains. The locations determined for H II regions and for other spiral arm "tracers" are assembled together in Fig. 1.6 [14], projected on the galactic plane. Included are data on O-BO star associations and clusters, dark clouds, and a few individual young stars, mostly within about 3000 pc from the Sun. Three rough concentrations seem indicated: the Orion (or Ori-Cyg) local arm, just beyond the Sun, the Perseus (or Per-Cas) arm, some 2000 pc further out, and the Sagittarius arm, some 2000 pc further in.

Figure 1.6 Spiral arm tracers near the Sun [14]. The locations of young stars, gas and dust, which are mostly confined to spiral arms, are shown projected on the galactic plane for distances within about 5000 pc from the Sun. The large open and large filled circles represent O-BO associations and clusters, while Bpe and bright Cepheids are indicated by small open and small filled circles, respectively. H II regions and dark clouds are represented by large filled circles with central holes and by plus signs. The galactic center is by definition in the direction /=0.

Figure 1.6 Spiral arm tracers near the Sun [14]. The locations of young stars, gas and dust, which are mostly confined to spiral arms, are shown projected on the galactic plane for distances within about 5000 pc from the Sun. The large open and large filled circles represent O-BO associations and clusters, while Bpe and bright Cepheids are indicated by small open and small filled circles, respectively. H II regions and dark clouds are represented by large filled circles with central holes and by plus signs. The galactic center is by definition in the direction /=0.

However, an interconnection seems suggested between the Orion and Sagittarius arms, and as in M31, the distribution evidently cannot be described in terms of entirely separate arms that spiral about the galactic center.

Similar plots can be obtained for more distant H II regions [4] and for H I clouds [15], using the observed radial velocities of these regions to determine the distances. In these determinations the gas is assumed to have an equilibrium circular velocity v9 around the galactic center at a distance R, with the centrifugal force vj/R just balanced by the gravitational acceleration gr; from an assumed model for the Galaxy, gr can be computed. This method suffers from two serious uncertainties. The mass distribution assumed for the Galaxy may be incorrect, and the gas velocities may differ appreciably from the circular velocities. Hence these results, which have the advantage of extending to much greater distances, indeed across the entire Galaxy, are of lower weight than the ones presented in Fig. 1.6. In fact, there is general qualitative agreement between these methods for regions within 2500 pc.

Evidence on the distribution of cosmic rays within the galactic disk is provided by the intensity of the y rays observed from different regions of the galactic disk. These data can be fitted with a cosmic ray density which increases with decreasing R, and at R = 5 kpc is between two and four times the measured value near the Sun. The larger increase is obtained [16] if nH, the total particle density of H nuclei in all forms, is assumed proportional to n(H I) obtained from 21-cm observations, while the smaller increase results [17] if an increasing fraction of H is assumed to be molecular in the denser regions near the galactic center, and nH in these regions is assumed proportional to the observed particle density of CO molecules.

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