Baltic Astronomy, vol. 6, 183-191, 1997.
SURVEYING THE GALACTIC PLANE
T. Mahoney, P. L. Hammersley, F. Garzón, X. Calbet and M. López
Instituto de Astrofisica de Canarias, La Laguna, Tenerife E-38200, Spain Received August 8, 1996.
Abstract. The Galactic plane is discussed as a case history of the application of different kinds of databases to the particularly difficult problem of the large-scale Galactic structure. Key words: Galaxy: disk - surveys
1. INTRODUCTION
Surveys can be driven by curiosity, the development of new tech- nology, results obtained (or not obtained) from previous surveys, and by philosophical hypotheses, such as a belief in a "Copernican" (non- heliocentric) Galaxy. The successful application of any astronomical database depends greatly on the assumptions made concerning the types of sources whose detection is anticipated. A knowledge of the large scale structures in the Galactic plane is important for a number of reasons: (1) most of the known matter in the Galaxy is located in the Galactic plane (the existence of "dark" matter has yet to be confirmed and its true nature ascertained), (2) the most active regions of the Galaxy are located in the plane, (3) the plane is the least known region of the Galaxy (with the possible exception of a supposed "dark halo"), (4) the large-scale geometry of the plane is a fossil bed of past interactions between the Galaxy and other members of the Local Group (LG), (5) there are other members of the LG still to be discovered behind the Galactic plane (e.g. Dwingeloo I and the Sgr dwarf galaxy), (6) a considerable part of the Universe (up to a quarter at microwave frequencies) is hidden from view by the Milky Way and (7) we still do not know what type of Galaxy we live in. 184 T. Mahoney, P. L. Hammersley, F. Garzón et al.
2. SHAPE OF THE GALAXY
A full, but polemical, account of philosophical speculations con- cerning the nature of the Milky Way is given by Jaki (1972). The history of Galactic research, seen as a stepping stone to more cosmo- logical concerns, is dealt with by Whitney (1971), Berendzen, Hart & Seeley (1976) and Smith (1982). Useful historical background is given by Hoskin (1985), Paul (1985) and Smith (1985). By far the most thorough investigation into the history of Galactic astronomy to date is given by Paul (1993). A useful nineteenth-century per- spective of "Galactic" astronomy is to be found in Clerke (1905). Mihalas & Binney (1981) and Gilmore, King & van der Kruit (1989) provide much useful background to Galactic astronomy. Mihalas & Binney (1981) is especially useful for its full references. Reviews are to be found in the ever increasing number of conference proceedings (e.g. Holt & Verter 1993, Majewski 1993). Croswell (1995) offers a fine popular account of modern Galactic astronomy. In the 5th century B.C., Democritus suggested that the Milky Way was composed of faint stars, a hypothesis that was confirmed observationally by Galileo in 1609. The earliest extant detailed de- scription of the surface brightness distribution of the Milky Way was given by Ptolemy in Book VIII of the Almagest (Toomer 1984). Herschel undertook a probing of the large-scale structure of the Universe by means of "star gages" (i.e. star counts) in selected boxes measuring 15' x 15'. He assumed: (a) a uniform intrinsic brightness for all stars, (b) that the apparent brightness of the stars varied in inverse proportion to the square of their distance (i.e. that there was no interstellar medium), (c) a uniform stellar distribution, (d) no stellar multiplicity and (e) that his survey was "complete". Herschel's "Universe" was a spheroidal distribution of stars with an axial ratio of about 5:1, with the Sun occupying a more or less central position. He later realized that most of his assumptions were false. In fact, it turned out that they all were. Herschel himself discovered stellar multiplicity and realized that the stars in multiple systems were not all of the same intrinsic brightness, so, that the stellar distribution could not be considered uniform. He also found that a larger aperture revealed more stars, so that his initial survey could not be complete. The inverse square law continued to confound astronomers until 1930, when Trumpler (1930) proved conclusively the existence of interstellar extinction. Surveying the Galactic plane 185
Harlow Shapley (1918 a,b; 1919 a,b,c) used the distribution of globular clusters as a probe of the large scale structure of the Galaxy. Globulare are intrinsically bright, and many are located well away from the Galactic plane; hence Shapley was able to determine their spatial distribution. He used a hierarchical sequence of distance de- terminations (Cepheids, RR Lyrae stars, brightest stars and appar- ent cluster size) to probe ever further out. His model placed the center of the Galaxy 15 kpc distant in the direction of Sagittarius, almost twice the currently accepted value of 8.5 kpc, and gave a di- ameter for the entire Galaxy of 100 kpc. Shapley's distances were inflated for two reasons: (1) he had mis-identified W Vir stars as Cepheids (which are brighter) and (2) he ruled out the possibility of ail absorbing interstellar medium. Shapley showed that, even where no adequate database exists, problems can still be solved with the correct assumptions. The existence of the interstellar medium merely inflated Shapley's distances, but did not fundamentally affect his model. With this assumption and a bold interpretation of an extremely limited database, Shapley had finally Copernicized the Galaxy. Jacobus Kapteyn used photographic sky surveys (the Carte du Ciel and Gill's Cape Photographic Durchmusterung), together with proper-motion, spectral-type and parallax databases to make a full statistical analysis of 206 Selected Areas of the sky. Again, the effects of interstellar extinction were ignored to the detriment of the final model (Kapteyn 1922), which produced a spheroidal Galaxy with an axial ratio of approximately 5:1, with the Sun at 650 pc from the Galactic center - all highly reminiscent of Herschel's model. Kapteyn's databases were enormously sophisticated, but they were inappropriate for the problem of Galactic structure. Their use was completely undermined by a single wrong assumption, the sup- posed non-existence of an interstellar medium. Although pioneering radio survey work on the HI content of the Galactic plane was carried out in the 1950s, no further advance on the stellar content of the deep disk was possible until the advent of near-infrared techniques. In the late 1970s and early 1980s Japanese balloon experiments (NIR) provided the first view, in diffuse 2.4-μιη emission, of the Milky Way as an edge-on spiral galaxy. This work is reviewed by Garzón et al. (1993 and references therein). The diffuse emission infrared images of the Galactic plane are from the Diffuse Infrared Background Experiment (DIRBE) on board of the COBE satellite. The first results are summarized by 186 T. Mahoney, P. L. Hammersley, F. Garzón et al.
Holt & Verter (1993) and they are unlikely to be bettered in the foreseeable future. The first infrared survey to delineate the Galactic plane in terms of point source objects (mainly stars) was made by IRAS. Earlier infrared surveys were too insensitive to map the deep plane. Habing (1988) used the IRAS Point Source Catalogue with the selection criterion 0.82^2 < i*25 < F12 to identify long period variables with a high mass loss rate. He concluded that: (1) 80% of sources were located in the thin disk (FWHM 440 pc), the disk having a cut-off at 9.5 kpc and scale length of about 4.5 kpc, and (2) 20% of sources were found in a "thick disk" that he identified with Gilmore's thick disk (see Kuijken & Gilmore 1989). The first spiral nebula was discovered in 1845 by Lord Rosse. A spiral structure for the Milky Way was suggested by Alexander (1852), Proctor (1869) and Easton (1900, 1913), and Bok looked unsuccessfully in the 1930s for evidence of a spiral structure in star counts. However, the first real demonstration of the existence of spiral arms was made by Morgan (Morgan, Sharpless & Osterbrock 1952) with his discovery of two spiral arms using HII regions as tracers. The following objects, in varying degrees of usefulness, may be used as visible tracers of spiral arms: (1) Ο, Β supergiants (and other supergiants), (2) HII regions, (3) O associations, (4) young open clusters and (5) longer period Cepheids. The requirements for tracers of the spiral arm are that they must be: (1) young (i.e. close to their birthplace), (2) very luminous and (3) of known intrinsic brightness (for an accurate distance determi- nation). The main problem with this method is its limitation (by interstellar extinction) to distances of a few kpc from the Sun. Radio wavelengths penetrate the entire Galactic plane, but there are severe problems in using radio sources as spiral arm tracers. Van de Hulst's prediction of the neutral hydrogen emission at 21 cm due to the reorientation of its electron spin vector (van de Hulst 1945) and the subsequent confirmation of this prediction by Ewen & Purcell (1951) revolutionized Galactic astronomy by pro- viding the first full penetration of the Galactic plane. The 21-cm maps give a clear impression of spiral structure in the Galactic plane, but not very precisely. The method is to use the radial velocity VR = (Ω — LÚQ)R0 sin/, where Ω = Θ/R and Q(R) vs. R is derived from the rotation curve. It is assumed that (1) the HI is optically thin, (2) Tgpin is uniform, and the gas is in circular motion. The Surveying the Galactic plane 187 problems are: (1) the diffuse distribution of HI throughout the plane, spiral arms being merely localized concentrations in this distribution, (2) the presence of non-circular motion due to (a) random motions within the gas, (b) possible non-circular streaming motions of cer- tain features and (c) possible acceleration or deceleration due to passing density waves, (3) non-unique R values within the solar cir- cle (R > #0,-90° < I < 90°) and (4) two "blind" wedges centered on the Sun in the Sun-Galactic Center radial, where the modeling technique is inapplicable. The net result is that, although a general impression of spirals of HI in the Galactic plane is clearly evident in the maps, no precise information can be obtained on any specific spiral arm. Another, preferably discrete, tracer is required. The hydrogen 109a recombination line in HII regions can be used to obtain radial velocities, from which distances can be derived using the rotation curve of the Galaxy. Georgelin L· Georgelin (1976) used both optical and Η 109α studies of HII regions to produce what is probably still the best representation of the spiral structure of our Galaxy to date. However, their method suffers the usual restrictions on all optical work in the Galactic plane. Beyond a few kpc they were forced to rely solely on the radio data. While their delineation of the Norma and Scutum-Crux arms looks very reasonable, there is clearly something wrong for the inner sections of the Perseus and Sagittarius-Carina arms (for 0° < I < 50°, R > RQ). Molecular clouds, being compact, would seem to be the ideal candidates for delineating the spiral arms of the Galaxy. However, as Combes (1991) has convincingly demonstrated, even when the po- sitions of molecular clouds are known for a given galaxy (she chooses M 51), the CO I — ν plot cannot be used to reconstruct this distri- bution by means of deriving kinematic distances, no matter how the rotation curve is varied. The culprit is the assumed circular motion.
3. TILTS AND WARPS IN THE PLANE To discuss the geometry of the Galactic plane (GP), it is nec- essary to decide: (1) the orientation of the GP (pole + reference direction in plane), (2) the Sun's position with respect to the GP, (3) which objects define the GP and (4) a wavelength with deep penetration. The present IAU (/n,fen) system (Blaauw et al. 1960) is based on the so-called HI Principal Plane (Gum, Kerr & Westerhout 1960), defined by: (1) the North Galactic Pole (NGP) at 12h49m, +27°24' 188 T. Mahoney, P. L. Hammersley, F. Garzón et al.
(1950.0), (2) the plane is flat for R < 7 kpc and (3) warping outside the solar circle towards I = 90° and I - 270° (R > 7 kpc). There are also systematic offsets (discussed by Hammersley et al. 1995) due to: • The Sun's height above the GP (zo)· In HI it amounts to 2o = +4± 12 pc (Gum et al. 1960); in the infrared, it is about 15 pc (Cohen 1995, Hammersley et al. 1995), and in the visible, around 20 pc (Blaauw 1960, Humphreys & Larsen 1995). • A tilt of 0.4° ± 0.30° at b = 0° of the old disk with respect to the Galactic plane (Hammersley et al. 1995) by rationing the flux at negative and positive b from the 2.2-μτη DIRBE maps. • Warps. Gum, Kerr & Westerhout (1960) report the well- known warping of the HI Principal Plane outside the solar circle. Freudenreich et al. (1993) find warping from the DIRBE maps, and Djorgowski & Sosin (1989) use the IRAS Point Source Catalogue.
4. IS THE MILKY WAY A BARRED SPIRAL? The presence of a central bar would be a major component of the Galactic plane. Various models of such a central bar have been proposed by de Vaucouleurs (1964, to explain "expanding arms"), Binney et al. (1991, to explain CO gas kinematics), Blitz & Spergel (1991, in fact a triaxial bulge extending outside the solar circle with a small inner bar to explain large scale asymmetries in HI / — υ dia- grams), Weinberg (1992, using AGB stars as tracers), Nakai (1992, using radial distribution of CO) and Hammersley et al. (1994, using NIR star counts and DIRBE maps). Also, Calbet et al. (1996) claim near infrared evidence of a dust lane preceding the approaching arm of the bar at negative Galactic longitudes (as for NGC 1300).
5. WHAT KIND OF GALAXY ?
It is well established that: (1) the Galaxy has spiral arms (al- though their number and spatial distribution are still unknown), (2) the bulge is of intermediate type, (3) there is a thin (comprising a young and old) disk and (4) the Galaxy's luminosity is of intermedi- ate type. Still being debated are (1) the existence of a bar and the triaxiality of the bulge and (2) the existence of a thick disk. Using the de Vaucouleurs' system of galaxy classification, we def- initely have S(l)(2)bcII, where (1) and (2) have yet to be decided. With the slender evidence such as it is, however, we could hazard a Surveying the Galactic plane 189 guess that the Milky Way is of de Vaucouleurs type SAB(rs)bc II, with the proviso that many astronomers would have severe reserva- tions about the "B". But it is only a guess. To complicate matters even further, the evidence to date sug- gests a "flocculent" (i.e. irregular) rather than a "grand design" spiral structure, which implies that it may be many years before a reliable picture of the Galactic plane structure is achieved.
6. TWO NEW NIR ALL-SKY SURVEYS
Two new near infrared surveys will contribute enormously to our knowledge of the Galactic plane. DENIS (Deep Near Infrared Sky Survey) is already under way and 2MASS (Two-Micron All-Sky Survey) is about to come into operation in the near future. DENIS is surveying the entire sky as seen from La Silla (Chile), and 2MASS will consist of two telescopes (one in the northern and one in the southern hemisphere). The telescopes are of 1 m and the limiting magnitude at Κ (2.2 μπι) will be about 13.5. DENIS has J, J, and Κ bands, and 2MASS will have J, Η and K, so infrared color information will be available for many sources. Although there are also NIR satellites either in operation now (i.e. ISO) or planned for the future (e.g. SIRTF), none will be survey instruments (although ISO is capable of limited surveying when it is in slew mode, it will not be of much use in the Galactic plane because of saturation).
REFERENCES
Alexander S. 1852, AJ, 2, 101 Berendzen R., Hart R., Seeley D. 1976, Man Discovers the Galaxies, Science History Publications, New York Binney J., Gerhard O. E., Stark Α. Α., Bally J.,Uchida Κ. 1.1991, MNRAS, 252, 210 Blaauw Α., Gum C.S., Pawsey J.L., Westerhout G. 1960, MNRAS, 121, 123 Blitz L., Spergel D.N. 1991, ApJ, 370, 205 Calbet X., Mahoney T., Hammersley P. L., Garzón F., Lopez-Corredoira M. 1996, ApJ, 457, L27 Clerke A. 1905, The System of the Stars, 2nd ed., A $¿ C Black, London Cohen M. 1995, ApJ, 444, 874 Combes F. 1991, ARA&A, 29, 195 Croswell K. 1995, Alchemy of the Heavens, Oxford University Press 190 T. Mahoney, P. L. Hammersley, F. Garzón et al. de Vaucouleurs G. 1964, in The Galaxy and the Magellanic Clouds (IAU Symp. No. 20), eds. F. J. Kerr & A. W. Rodgers, Canberra Aust. Acad. Sci., p. 195 Djorgovski S., Sosin C. 1989, ApJ, 341, L13 Easton C. 1900, ApJ, 12, 136 Easton C. 1913, ApJ, 37, 105 Ewen H.I., Purcell E. M. 1951, Nature, 168, 356 Freudenreich et al. 1993, in Back to the Galaxy (AIP Conf. Proc. v. 278), eds. S.S. Holt L· F. Verter, Amer. Inst, of Physics, New York, p. 485 Garzón F., Hammersley P. L., Mahoney T., Calbet X., Selby M.J., Hepburn I.D. 1993, MNRAS, 264, 773 Georgelin Y.M., Georgelin Y.P. 1976, A&A, 49, 57 Gilmore G., King I., van der Kruit 1989, The Milky Way as a Galaxy, eds. R. Buser L· I. King, Geneva Observatory Gum C.S., Kerr F. J., Westerhout G. 1960, MNRAS, 121, 132 Habing H. 1988, A&A, 200, 40 Hammersley P. L., Garzón F., Mahoney T., Calbet X. 1994, MNRAS, 269, 753 Hammersley P.L., Garzón F., Mahoney T., Calbet X. 1995, MNRAS, 273, 206 Holt S.S., Verter F. (eds.) 1993, Back to the Galaxy (AIP Conf. Proc. v. 278), Amer. Inst, of Physics, New York Hoskin M. A. 1985, in The Milky Way Galaxy (IAU Symp. No. 106), eds. H. van Woerden, R.J. Allen L· W. Butler Burton, Reidel Publishing Company, Dordrecht, p. 11 Humphreys R.M., Larsen J. A. 1995, AJ, 110, 2183 Jaki S.L. 1972, The Milky Way: An Elusive Road for Science, Science History Publications, New York Kapteyn J. 1922, ApJ, 55, 314 Kuijken K., Gilmore G. 1989, MNRAS, 239, 605 Majewski S.R. (ed.) 1993, Galaxy Evolution: The Milky Way Perspective (A. S. P. Conf. Ser. 49), San Francisco Mihalas D., Binney J. 1981, Galactic Astronomy, 2nd ed., W. H. Freeman L· Co., San Francisco Morgan W. W., Sharpless S., Osterbrock D. 1952, AJ, 57, 3 Nakai N. 1992, PAS J, 44, L27 Paul E.R. 1985, in The Milky Way Galaxy (IAU Symp. No. 106), eds. H. van Woerden, R.J. Allen h W. Butler Burton, Reidel Publishing Company, Dordrecht, p. 25 Paul E.R. 1993, The Milky Way Galaxy and Statistical Cosmology, 1890- 1924, Cambridge University Press Surveying the Galactic plane 191
Proctor R. 1869, MNRAS, 30, 50 Shapley H. 1918a, ApJ, 48, 154 Shapley H. 1918b, PASP, 30, 42 Shapley H. 1919a, ApJ, 49, 249 Shapley H. 1919b, ApJ, 49, 311 Shapley H. 1919c, ApJ, 50, 107 Smith R. W. 1982, The Expanding Universe, Cambridge University Press Smith R.W. 1985, in The Milky Way Galaxy (IAU Symp. No. 106), eds. H. van Woerden, R.J. Allen & W. Butler Burton, Reidel Publishing Company, Dordrecht, p. 43 Toomer G.J. 1984, Ptolemy's Almagest, Duckworth, London Trumpler R.J. 1930, PASP, 42, 267 van de Hulst H. C. 1945, Ned. Tydschrift Natuurkunde, 11, 201 Weinberg S. 1992, ApJ, 384, 81 Whitney C. A. 1971, The Discovery of Our Galaxy, Knopf, New York