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IV - STRUCTURE A GRANDE ÉCHELLECARBON
MONOXIDE IN THE GALACTIC DISK AND
NUCLEUS
W. Burton, H. Liszt, M. Gordon
To cite this version:
V. — STRUCTURE A GRANDE ECHELLE
CARBON MONOXIDE IN THE GALACTIC DISK AND NUCLEUS
W. B. BURTON, H. S. LISZT and M. A. GORDON National Radio Astronomy Observatory*
Green Bank, West Virginia 24944, U. S. A.
Abstract. — Most of the interstellar gas in our Galaxy is concentrated in dense, optically opaque clouds in which hydrogen is principally molecular. As traced in the J = 1 -> 0 rotational transition of CO, these clouds are found to lie mostly in an annulus between 4 and 8 kpc from the galactic centre, and to have a spatial distribution similar to that of young stars, supernovae, pulsars, etc. This distribution is, however, quite different from that of the more diffuse gas in which hydrogen is principally atomic. In the innermost kpc of the Galaxy, there is a prominent component of mole-cular gas having a large velocity with respect to the galactic centre. This gas has a spatial distribution which is tilted with respect to the fundamental plane of the larger-scale galactic morphology.
We give here a brief overview of characteristics of the Galaxy and its nucleus derived from radio astro-nomical observations of the A 2.6 mm J = 1 -+ 0 spectral line of 1 2CO.
The principal characteristic which distinguishes spiral galaxies from other galaxies is the large amount of gas and dust found in their disks and distributed in more or less regular patterns. In external galaxies dust lanes often appear as obscuring bands
silhouett-ed against the bright stellar background. Although the bulk of the interstellar gas occurs in star-forming dark clouds, their opacity at optical wavelengths renders all but some 10 % of the volume of the disk of our own Galaxy inaccessible at optical wavelengths. Because almost all of the hydrogen in the cold, dark environments of these clouds is in the molecular form, the much-studied spectral line at wavelength 21 cm (resulting from the hyperfine transition in atomic hydrogen) is not a suitable tracer. Molecular hydrogen itself provides no generally observable transition.
Although the carbon monoxide molecule is some 4 orders of magnitude less abundant than molecular hydrogen, it is an ideal galactic tracer for both phy-sical and practical reasons. The CO molecule consists of a strong bond between relatively abundant elements. In the dark environment of cold clouds it is shielded from ultra-violet photo-dissociation. The upper level of the J = 1 -* 0 rotational transition is populated through inelastic collisions with other molecules. A Boltzmann distribution in the upper level
correspond-ing to the excitation temperature of 15 K typically observed requires particle densities of order 500 c m- 3
or somewhat larger. Because only molecular hydrogen occurs at such large densities, the relationship between the CO and H2 molecules is intimate.
The amount of energy released in the J = 1 -» 0 transition corresponds to a photon of wavelength 2.6 mm. The interstellar medium is generally trans-parent at this wavelength, so that transgalactic paths are accessible. Because 2.6 mm lies in a favorable atmospheric window, ground-based observations are possible. The short wavelength also provides high angular resolution ; e. g., the 11-meter radio telescope on Kitt Peak, in Arizona, has at X 2.6 mm an angular resolution of 1 arc minute. Doppler shifts of spectral features from the rest frequency of 115.27 GHz pro-vide valuable kinematic information. Because of the low temperatures and modest turbulence in the cold clouds, the line is only moderately broadened by mechanisms other than those governing the overall galactic kinematics.
Thus observations of CO provide an excellent probe of the kinematic and spatial distribution of the cold dense portions of the interstellar gas. Advances in receiver technology have recently allowed general investigation of the galactic distribution of CO. Now experience shows that essentially any galactic dark cloud with particle density greater than 500 c m- 3
and not much smaller than the telescope beam can give a measurable CO spectral line.
Figure 1 gives the arrangement of CO emission in longitude-velocity coordinates along the galactic equator over the longitude range 10° < / < 82°.
(*) Operated by Associated Universities, Inc., under contract with the National Science Foundation.
Résumé. — Le gaz interstellaire de notre galaxie est concentré en nuages denses et optiquement épais où l'hydrogène est principalement moléculaire. D'après les observations de la transition J = 1 -*- 0 de CO, la plupart de ces nuages se trouve dans un anneau de 4 à 8 kpc du centre galac-tique. Leur distribution spatiale ressemble à celle des étoiles jeunes, des supernovae, des pulsars, etc. et diffère de la distribution du gaz diffus et essentiellement atomique. Dans un rayon de 1 kpc du centre galactique, on observe du gaz moléculaire à grande vitesse (relative au centre), incliné par rapport au plan galactique.
C1-104 W. B. BURTON, H. S. LISZT AND M. A. GORDON
FIG. I. - Grey-scale representation of emission from the 2.6-mm spectral line of 12C160 in longitude-velocity coordinates
along the galactic equator [4].
These data are from a paper by Burton and Gordon (1977) [4] which also contains references to earlier galactic CO observations and to related work. Although the distribution of the observed intensities is strikingly clumpy, it is confined within rather definite kinematic borders. Thus little CO emissions observed at the negative velocities corresponding (in terms of the known differential galactic rotation) to the outer regions of the Galaxy at distances R from the galactic centre greater than the 10-kpc distance of the solar neighbourhood. The CO emission is also confined for the most part to I < 60°, corresponding to regions at R < 9 kpc.
Figure 2 shows the radial abundance distribution of CO emission intensities and also, for comparison, the radial abundance distribution of atomic hydrogen as derived from radio observations at
A
21 cm. The overall extent of the galactic disk defined by atomic His fully twice the extent defined by the CO molecule. Sixty-five percent of the CO intensities in the galactic equator emanate from the annular region 4 < R < 8 kpc, whereas only 36
%
of the atomic Hdistribution lies there.
The CO radial distribution is undoubtedly that of compressed dark material in general, because outside the high density opaque regions CO would form very slowly and would be destroyed very rapidly. Within the observational uncertainties, the CO distribution is roughly equivalent to those measured indepen- dently for the recently formed 0 and B-type stars for ionized H, for supernovae remnants, for pulsars, for y-radiation, and for galactic synchrotron radiation. Instead of being the prototype for the distribution of most constituents of the galactic Population I (as until recently seemed plausible), the fundamental dis- tribution of atomic hydrogen seems unique.
The Sun is located near the periphery of the region containing the cold, dark, compressed material and thus is located near the outer extent of the region of currently active star formation. The concentration in our Galaxy of star-forming material to a disk less extensive than the disk of unprocessed atomic hydrog- en is a situation now commonly observed in external galaxies. Much of the light on a photograph of an external galaxy is contributed by recently formed stars. The visual dimensions of a spiral galaxy are, however, usually smaller than the dimensions measured in the
A
21-cm radio line of atomic H. (That substantial mass exists beyond the optical structure is evident from the large deviations from Keplerian rotation measured in the outermost parts of the atomic hydrog- en disk.)Derivation of the total distribution of material in the Galaxy requires that the thickness of the galactic disk be known, in addition to its extent. Figure 3
shows the comparative latitude distributions of CO and atomic H, measured on lines of sight which tra- verse long paths through the galactic layer. CO is more confined to the galactic equator than atomic H. The scale height of the molecular layer is 50-pc, compared with the 120-pc scale height of the atomic hydrogen layer. Both tracers deviate systematically from the plane b = 0".
CARBON MONOXIDE I N GALACTIC DISK AND NUCLEUS C1-105
FIG. 2.
-
(Left) Radial distribution of 12C160 emission, expressed as the intensity accumulated in galactocentric annuli. (Right) Radial distribution of H I volume densities [4]. The cold compressed molecular material is more confined t o the innerGalaxy than is the warmer, more diffuse, atomic hydrogen gas.
FIG. 3. - Comparison of the latitude-velocity distributions of CO and H I emission observed in the direction 1 = 21" [3].
The z-thickness of the atomic hydrogen layer is more than twice that of the molecular layer.
the average separation between the major CO clouds is 1000 pc. They have random components to their motions of
-
4 km. s-l. That the clouds typically fill the 11-m telescope's beam implies diameters2
5 pc. CO clouds in the Galaxy number some lo6. The combination of the rarity of the clouds with the large velocity shifts between clouds resulting from galactic kinematics produces an assemblage of clouds which is transparent, even though the clouds are individually opaque. Thus almost all the galactic molecular clouds are accessible to observation. The total masses of the dark clouds, estimated following the rather uncertain conversion of CO intensities to molecular H densities, range up to lo4 solar masses. Together with the stellar globular clusters, thesegaseous clouds are the most massive entities observed in the Galaxy.
The stochastic model which provides these para- meter values is azimuthially symmetric. Most of its shortcomings result from the imposed symmetry. Clearly the figure 1 observations show some non- random ordering of the dark clouds. Although we have found no compelling, straightforward evidence for a large-scale design, more detailed observations and analysis might reveal for our Galaxy a situation similar to that found in external spiral galaxies, in which the dark material traces spiral arms [2], [7]. Although the interstellar gas is generally depleted in the inner 4 kpc of the Galaxy, large densities are found within 400 pc of the galactic centre. This nuclear gas is much more smoothly distributed than the galactic disk material, and displays several easily recognized kinematic patterns.
The central regions of many galaxies, including our own, show signs of disruptive activity. The subject of the nucleus of the Galaxy has been reviewed recently by Oort (1977) [6] ; also see Bania (1977) [I]. Because of the large fluxes of disrupted material, the energies involved are large. Although the mechanisms asso- ciated with this activity are unknown, it is clear that an understanding of the activity in galactic nuclei is crucial to an understanding of phenomena observed throughout galaxies. Because of the detailed spectral information available at high angular resolution in millimeter-wave observations of CO, such observa- tions are especially useful for investigation of the central region of the Galaxy.
Recently Liszt and Burton (1977) [5] have observed CO emission in a longitude strip covering the range
-
120' d A1 G 148' at b = 0'. The displacements A1and Ab are measured with respect to the peak in
the 2-pm radiation in the direction of the source Sgr A (West) at the galactic coordinate 1 = - 3 ' 2 0 ,
C1-106 W. B. BURTON, H. S. LISZT AND M. A. GORDON
FIG. 4.
-
Grey-scale representation of 12C160 emission andabsorption intensities in longitude-velocity coordinates at Ab = 0' between A1 =
-
120' and 148' [ 5 ] .of the observed CO intensities. Inspection of this figure reveals 4 separate kinematic patterns.
1. Near v = 0 km. s- l, vertical bands are contri- buted by gas clouds lying somewhere on the line of sight in the inner 8 kpc of the Galaxy. This line-of- sight accumulation of material, which is not asso- ciated with the galactic nucleus, appears both in emission and in absorption against the hotter nuclear background material.
2. A vertical band near v =
-
53 km. s-' (atA1 = 0') is contributed by molecular material in the
3-kpc arm, a large-scale expanding feature well- known from 21-cm observations of atomic hydrogen.
Two prominent kinematic patterns are associated with nuclear phenomena :
3. The molecular ring displays a remarkable
regularity over an almost complete circle of galactic azimuth. Where it is seen tangentially it crosses v = 0 km.s-I at A l =
-
50 km.sP1 and+
120 km.s-l, indicating a diameter of 500 pc. At A1 = Or its velocity extent is 300 km. s-l. However, the centroid of the pattern lies at A1 = 35',Av = 50 km. s-l. The non-circular, presumably expansion, component of the motion is 150 km.sP1 ;
the skewness of the pattern indicates a transverse velocity of some 50 km.s-l. Observations in a strip perpendicular to the galactic equator indicate that the plane of the molecular ring is tilted with respect to the plane b = 0". The thickness of the feature projected onto the plane of the sky is 100 pc. Addi- tional observations sufficient to determine the two angles specifying the pole of the ring's fundamental plane are given by Liszt and Burton (1977) [5].
4. Most of the high-intensity material in figure 4 is confined to the two quadrants AZ > 0', v > 0 km. s-I and AZ < Or, v
<
0 km.s-', permitted in the sense of simple galactic rotation. Identification of patterns consistent with pure rotation is important because they can provide the mass density in the inner Galaxy. Although the kinematic pattern of the high-intensity material crosses AZ = Or at v = 0 km.s-l, it is not symmetric about AZ = 0'. This lack of symmetry implies either an asymmetric gravitational field or that other forces on the gas contribute also to this pattern.At present the major obstacles to a unified study of the molecular gas in the nucleus are the almost complete lack of information in latitude and the still heavily undersampled nature of the longitude data.
References
[I] BANIA, T. M., Astrophys. J., 216 (1 977) 38 1. [5] LISZT, H. S., BURTON, W. B. (1977), in preparation.
[2] BASH, F. N., PETERS, W. L., Astrophys. J. 198 (1976) 281. [6] OORT, J. H., Annu. Rev. Astron. Astrophys. 15 (1977) [3] BURTON, W. B., GORDON, M. A., Astrophys. J. Lett. 207 295.
(1976) L 189. [7] ROBERTS, W. W., BURTON, W. B., Topics in Interstellar [4] BURTON, W. B., GORDON, M. A., Astron. Astrophys. (1977) Matter (H. van Woerden (ed.), Reidel, Dordrecht)
