I would like to start by bringing the good wishes of Australian radio astronomers to this new group which is joining us in the radio study of the southern sky. The nuclear regions of galaxies are generally considered as regions of high density, with population II stars in a "nuclear bulge". In some galaxies, the presence of gas in the nucleus is shown by the appearance of the OII 3727 doublet, sometimes with a high velocity dispersion. Some nearby galaxies (e.g. M31, M33, M51) show a small very bright spot, right at the nucleus, of the order of 10 pc in diameter. Non-circular velocities have been found near the centre in some galaxies.Asociación Argentina de Astronomí

Kerr, F.J.

English

Servicio de Difusión de la Creación Intelectual

- 3-THE GALACTIC CENTRE F.J.Kerr(CSIRO Radlophvslcs Laboratory« Australia)I would like to start by bringing the good wishes of Australian radio astronomers to this new group which is Joining us in the ra­dio study of the southern sky.The nuclear regions of galaxies are generally considered as re­gions of high density, with population II stars in a nnuclear bul­ge". In some galaxies, the presence of gas in the nucleus is shown by the appearance of the Oil 3727 doublet, sometimes with a high velocity dispersion. Some nearby galaxies (e.g. M 31» M 33» M 51) show a small very bright spot, right at the nucleus, of the order of 10 pc in diameter. Non-circular velocities have been found near the centre in some galaxies.Optical observations of our own galactic nucleus are very diffi­cult, as there is at least 9m of absorption between us and the ce£ tre. Baade has studied a star cluster near the centre (in the bul­ge), and Courtes has observed ionized hydrogen patches with high velocities, which are presumably in the nucleus. But there are ve­ry few optical observations, and the main study must be done by r& dio. We in the south are very well placed for work on the central region.There are several radio approaches, involving the continuum and various radio lines. The region is found to be very complex, and many problems remain.THE CONTINUUMThe most interesting (that is the most informative) region of the continuum is 1-100 cm in wavelength. Over all of this range, the main radiation is from the Milky Way belt, with a concentration in the region of the centre.The first striking conclusion about the central part is the small angular width of the emitting layer, which must be quite thin, with no relation to the stellar bulge.Two types of continuum emission processes take place in the Ga­laxy: (i) thermal emission from HII regions, through free-free tran sltions, and (ii) non-thermal emission from the synchrotron process,from electrons spiralling in a magnetic field. The two types can be best distinguished by their spectra. The thermal component b£ comes relatively more important as the wavelength is reduced.The strong central source is known as Sagittarius A. The cen­tral complex was first resolved into separate components by Drake (I960). Subsequently the details of the regions have been studied in the following published surveys:Authors WavelengthcmBeamwidthCooper and Price (196*0 11 6.7Broten et al (1965) Downes, Maxwell and6 *f.OMeeks (1965) 3.6 »+.2Hollinger (1965) Downes, Maxwell and2.1 5.9Meeks (1965) 1.9 2.2These surveys all show a number of components, one to the south, and three to the north of the central peak. (The papers should be consulted for details). Spectral studies have been carried out by Cooper and Price (196*0, Burke (1965)» and Downes (1965)« By sepa­rating the components as well as possible, and then comparing the data at different wavelengths all agree that the main Sgr-A compo­nent is non-thermal in origin, whereas the other components all appear to be thermal.There is no direct evidence from the continuum observations themselves that all the components are actually located in the vici. nity c>f the centre; however our hydrogen-line absorption observations indicate that the three strongest components, and probably the whole complex, are physically associated, and located at the centre.The angular size of the strong central component is ** min arc, equivalent to 12 pc at the distance of the galactic centre. The best position for Sgr A, from all the available observations, is:0( = 17h **2* 28s (19?0) l 11 = 359° 58'6  = -28fi 58» 5 & 11 = -00 03»We have surveyed a larger area of 60 square degrees around the- 5-galactic centre (Kerr and Sinclair, in preparation). The results for the whole region are not yet available, but the pattern of the "rldgelines" shows some interesting features (Figure 1). This pa­ttern is extremely symmetrical about the position of Sgr A, and in fact shows a higher degree of symmetry than any other aspect of the galactic centre observations. It also includes features which are steeply Inclined to the galactic equator and strongly suggest ejection of material. The main galactic ridge shows a pronounced tilt over several degrees of longitude, which is in the same sense as that found for high-velocity neutral hydrogen.21-cm HYDROGEN LINEVery strong absorption effects are found In the spectrum of Sa­gittarius A in the frequency range of the 21-cm hydrogen line (Fi­gure 2). There are deep absorption components associated with hy­drogen in the nearby spiral arms (near zero velocity) and in the ?3-kpc" arm (near -50 km sec). Absorption is found over a wide ran­ge of negative and positive velocity.In the vicinity of the centre, both absorption and emission show considerable fine structure (Rougoor 196*+, Kerr 1964-, Kerr and Va- llak 1966). Large outward motions are indicated for much of the hydrogen. An example of the detail available is shown in Figure 3» which gives the hydrogen distribution along the equator in the lon­gitude range 355D-5C»One of thè most Interesting features in the central region is the way that the high velocity meterial is inclined to the galactic plane.Figure 4- shows the latitude of the high velocity ( W  ^ 100 km/sec) peak as a function of longitude; the diagram is based on positive- velocity material for 3600, and negative-velocity gas for3600. The pattern is roughly symmetrical, with a long extension out of the galactic plane on each side. The southern feature proba­bly Joins on to the 3-kpc arm, which is itself well north of the plane over a considerable range of longitude; however the northern section appears to fade out at the northernmost longitudes.The major '’expanding'' arm, usually known as the 3-kpc arm, can be followed from = 4-2 to 336®, but it is not clear whether it Is a single continuous feature. There is a major complexity at 34-82,and another at about 34-2Q, and the overall arrangement Is not clear. There are probably several branches, and also cross-links to other parts of the spiral pattern.A possible structure for the whole central region Is shown In Figure 5« In the centre Is a rapidly-rotating feature, possibly a disk as proposed by the Dutch, with some expansion motion in its outer parts. From this disk there may be a rudimentary bar, incli­ned to the galactic plane, wnich contains much of the high-veloci­ty material. Further out, and possibly connected to the bar, is the "3 kpc arm”, with a less regular counterpart on the opposite side.Minor features are certalriLy present, but cannot be located in space. There is a general appearance of quadrantal symmetry. The picture proposed here is consistent with the observations, but cannot be regarded as well established, as there is no way of establishing distances for most of the hydrogen in the central regions,OR LIMESThe OH group of lines near 18 cm has been observed in the galac­tic nucleus by the Australian group (Robinson et al 1964-$ Bolton et al 1964-). The OH abundance in some parts of this region is 100 times that observed near the Sun. The relative strengths of the various lines are quite different from those expected theoretically and the ratios vary greatly from point to point. The OH observations in this region have all been in absorption so far, except for a small comp.2, nent of emission at one point.The OH absorption spectrum for^Sgr A is shown in Figure 6 for the 16^7 Mc/s line. The two strongest*'eomponents are at velocities around + 4-0 and -120 km/sec. The great strength of the former compo­nent indicated that a considerable proportion of the OK has a compo­nent of motion towards the Sgr-A source, i.e. presumably towards the "centre". Components at the OH velocities can be seen in the hydro­gen absorption profile (Figure 2), but they are much less important in the hydrogen pattern.In discussing the distribution and motions of the OH sources, Bolton et al (1964-) stress some minor features which appear to have a constant velocity over a few degrees of longitude. However, the greatest kinetic energy appears to be associated with a slow rota­tional motion.- 7-The present author considers that the OH Is located in the outer part of the nuclear "disk”, because It extends over several degrees In longitude, and also It exhibits a low rotational velocity. Rougoor (196*0 has already suggested that the rotational velocity of the hydrogen falls off In this region.HYDROGEN RECOMBINATION LINEMezger (1966) has recently detected a hydrogen line near 6 cm, which corresponds to the transition from n= 109 to 108, in two components of the galactic centre source. The line was not obser­vable in the main Sgr-A component. As this line is only emitted in K II regions, this results supports the view that the central com­ponent is nonthermal in origin, while the other components are thermal.X-RAYSA number of X-ray sources have recently been found in observa­tions from rockets * The only one identified so far is associated with the Crab nebula. There Is a cluster of other sources in Sagi­ttarius and Scorpius. None of these coincides with the radio source Sgr A, but the clustering in this direction suggests a relation with the nuclear disk (Johnson 1966). However it should be emphazized that no individual correlations have been found with, nuclear objects and it should be noted that the X-ray sources show a greater apread in latitude than the radio-emitting region in the nucleus.CONCLUSIONThe central region has many complexities. We have some idea of its structure, but our interpretation must still be inconclusive. There is a forthcoming series of lunar occultations of the galactic centre, from 1967 to 1970, which should yield more information on the fine structure of the region. Unfortunately, parallax moves the apparent position of the Moon northwards for a southern observatory, and consequently Parkes and Pereyra will not see any occultations of Sgr A. In this case, we must leave the main work to northern observatories.I conclude this review about one part of the galaxy by saying that there is still plenty to be discovered, as in fact there is in the whole Galaxy. I air sure that this new telescope, which is so well placed geographically for galactic studies, will play an important part in attacking the many problems that are to be found in the Galaxy.REFERENCESBolton, J.G., Gardner, F.F., McGee, R.X., and Robinson, B.J. (196*0. Nature T 2^ ,  30.Broten, N.W., Cooper, B.F.C., Gardner, F.F., Minnett, H.C. Price,R.M., Tonking, F.C. and Yabsley, T.E. (1965)» AUst.J.Physics, 1 8 , 65.Burke, B.F. (1965). Ann. Rev. Astron. Astrophvs, 1, 275.Cooper, B.F.C. and Price, R.M. (196*0. Svmp. ItTA. 20. l68.(nThe Galaxy and the Magallanlc Clouds", Kerr, F.J. and Rodgers, A.W.E. Aust. Acad. Scl., Canberra), 'n.Townes, D.N. (1965). In press.Townes, D.N., Maxwell, A. and Meeks, M.L. (1965). Nature, 208, H 89. Trake, F.D. (I960). NRAO Annual Report, 1, 2.Hollinger, J.P. (1965). Ap .J. 1*»2, 6O9.Johnson, H.C. (1966). Ap .J. 1*^, 261.Kerr, F.J. (196*0. Svmp. I£0~, 20, 187.Kerr, F.J. and Vallak, R. (1966). Aust. J. Phvslcs, (In press)Mezger, P. (1966). In press.Robinson, B.J., Gardner. F.F., van Damme, K.J. and Bolton, J.G. (196*0 Nature t 202, 989Rougoor, G.W. (196*0. B.A.N. !£. 381.- 10-LEGENDS TO FIGURESFig. 1 Location of 20-cm continuum "ridge lines" In the galactic centre region.Fig. 2 21-cm absorption profile for Sgr A, together with theestimated emission profile in the absence of absorption.Fig. 3 Hydrogen contour diagram along galactic equator (scale ] ufcit = l.'fflK in T^).Fig. ** Latitude of high-velocity hydrogen peak.Fig. 5 A possible structure for the galactic centre region.Fig. 6 OH absorption profile for Sgr A (1667 Mc/s).The 21-cm profile is drawn as a dashed line, on an arbitrary scale. (Robinson et al. 196*0.■11coLOCO00LOCOOCMtl P7o p720cm continuum ridge lines- 12-profil«!.- 13-50403020TO0 I359358357356355-14.oSun■15OCMâ§m350 0 10A possible structure for the galactic centre region- 16-Depth of Absorption -°Kgo o) roo  o  o  o  oRadial Velocity - Km/sec-200 -100 0 +.100

The galactic centre

SEDICI - Repositorio de la UNLP

- 3-
THE GALACTIC CENTRE 
F.J.Kerr
(CSIRO Radlophvslcs Laboratory« Australia)
I would like to start by bringing the good wishes of Australian 
radio astronomers to this new group which is Joining us in the ra­
dio study of the southern sky.
The nuclear regions of galaxies are generally considered as re­
gions of high density, with population II stars in a nnuclear bul­
ge". In some galaxies, the presence of gas in the nucleus is shown 
by the appearance of the Oil 3727 doublet, sometimes with a high 
velocity dispersion. Some nearby galaxies (e.g. M 31» M 33» M 51) 
show a small very bright spot, right at the nucleus, of the order 
of 10 pc in diameter. Non-circular velocities have been found near 
the centre in some galaxies.
Optical observations of our own galactic nucleus are very diffi­
cult, as there is at least 9m of absorption between us and the ce£ 
tre. Baade has studied a star cluster near the centre (in the bul­
ge), and Courtes has observed ionized hydrogen patches with high 
velocities, which are presumably in the nucleus. But there are ve­
ry few optical observations, and the main study must be done by r& 
dio. We in the south are very well placed for work on the central 
region.
There are several radio approaches, involving the continuum and 
various radio lines. The region is found to be very complex, and 
many problems remain.
THE CONTINUUM
The most interesting (that is the most informative) region of 
the continuum is 1-100 cm in wavelength. Over all of this range, 
the main radiation is from the Milky Way belt, with a concentration 
in the region of the centre.
The first striking conclusion about the central part is the small 
angular width of the emitting layer, which must be quite thin, with 
no relation to the stellar bulge.
Two types of continuum emission processes take place in the Ga­
laxy: (i) thermal emission from HII regions, through free-free tran 
sltions, and (ii) non-thermal emission from the synchrotron process,
from electrons spiralling in a magnetic field. The two types can 
be best distinguished by their spectra. The thermal component b£ 
comes relatively more important as the wavelength is reduced.
The strong central source is known as Sagittarius A. The cen­
tral complex was first resolved into separate components by Drake 
(I960). Subsequently the details of the regions have been studied 
in the following published surveys:
Authors Wavelength
cm
Beamwidth
Cooper and Price (196*0 11 6.7
Broten et al (1965) 
Downes, Maxwell and
6 *f.O
Meeks (1965) 3.6 »+.2
Hollinger (1965) 
Downes, Maxwell and
2.1 5.9
Meeks (1965) 1.9 2.2
These surveys all show a number of components, one to the south, 
and three to the north of the central peak. (The papers should be 
consulted for details). Spectral studies have been carried out by 
Cooper and Price (196*0, Burke (1965)» and Downes (1965)« By sepa­
rating the components as well as possible, and then comparing the 
data at different wavelengths all agree that the main Sgr-A compo­
nent is non-thermal in origin, whereas the other components all 
appear to be thermal.
There is no direct evidence from the continuum observations 
themselves that all the components are actually located in the vici. 
nity c>f the centre; however our hydrogen-line absorption observations 
indicate that the three strongest components, and probably the whole 
complex, are physically associated, and located at the centre.
The angular size of the strong central component is ** min arc, 
equivalent to 12 pc at the distance of the galactic centre. The best 
position for Sgr A, from all the available observations, is:
0( = 17h **2* 28s (19?0) l 11 = 359° 58'6  = -28fi 58» 5 & 11 = -00 03»
We have surveyed a larger area of 60 square degrees around the
- 5-
galactic centre (Kerr and Sinclair, in preparation). The results 
for the whole region are not yet available, but the pattern of the 
"rldgelines" shows some interesting features (Figure 1). This pa­
ttern is extremely symmetrical about the position of Sgr A, and in 
fact shows a higher degree of symmetry than any other aspect of 
the galactic centre observations. It also includes features which 
are steeply Inclined to the galactic equator and strongly suggest 
ejection of material. The main galactic ridge shows a pronounced 
tilt over several degrees of longitude, which is in the same sense 
as that found for high-velocity neutral hydrogen.
21-cm HYDROGEN LINE
Very strong absorption effects are found In the spectrum of Sa­
gittarius A in the frequency range of the 21-cm hydrogen line (Fi­
gure 2). There are deep absorption components associated with hy­
drogen in the nearby spiral arms (near zero velocity) and in the 
?3-kpc" arm (near -50 km sec). Absorption is found over a wide ran­
ge of negative and positive velocity.
In the vicinity of the centre, both absorption and emission show 
considerable fine structure (Rougoor 196*+, Kerr 1964-, Kerr and Va- 
llak 1966). Large outward motions are indicated for much of the 
hydrogen. An example of the detail available is shown in Figure 3» 
which gives the hydrogen distribution along the equator in the lon­
gitude range 355D-5C»
One of thè most Interesting features in the central region is 
the way that the high velocity meterial is inclined to the galactic 
plane.
Figure 4- shows the latitude of the high velocity ( W  ^ 100 km/sec) 
peak as a function of longitude; the diagram is based on positive- 
velocity material for 3600, and negative-velocity gas for
3600. The pattern is roughly symmetrical, with a long extension 
out of the galactic plane on each side. The southern feature proba­
bly Joins on to the 3-kpc arm, which is itself well north of the 
plane over a considerable range of longitude; however the northern 
section appears to fade out at the northernmost longitudes.
The major '’expanding'' arm, usually known as the 3-kpc arm, can 
be followed from = 4-2 to 336®, but it is not clear whether it 
Is a single continuous feature. There is a major complexity at 34-82,
and another at about 34-2Q, and the overall arrangement Is not clear. 
There are probably several branches, and also cross-links to other 
parts of the spiral pattern.
A possible structure for the whole central region Is shown In 
Figure 5« In the centre Is a rapidly-rotating feature, possibly a 
disk as proposed by the Dutch, with some expansion motion in its 
outer parts. From this disk there may be a rudimentary bar, incli­
ned to the galactic plane, wnich contains much of the high-veloci­
ty material. Further out, and possibly connected to the bar, is the 
"3 kpc arm”, with a less regular counterpart on the opposite side.
Minor features are certalriLy present, but cannot be located in space. 
There is a general appearance of quadrantal symmetry. The picture 
proposed here is consistent with the observations, but cannot be 
regarded as well established, as there is no way of establishing 
distances for most of the hydrogen in the central regions,
OR LIMES
The OH group of lines near 18 cm has been observed in the galac­
tic nucleus by the Australian group (Robinson et al 1964-$ Bolton et 
al 1964-). The OH abundance in some parts of this region is 100 times 
that observed near the Sun. The relative strengths of the various 
lines are quite different from those expected theoretically and the 
ratios vary greatly from point to point. The OH observations in this 
region have all been in absorption so far, except for a small comp.2, 
nent of emission at one point.
The OH absorption spectrum for^Sgr A is shown in Figure 6 for 
the 16^7 Mc/s line. The two strongest*'eomponents are at velocities 
around + 4-0 and -120 km/sec. The great strength of the former compo­
nent indicated that a considerable proportion of the OK has a compo­
nent of motion towards the Sgr-A source, i.e. presumably towards the 
"centre". Components at the OH velocities can be seen in the hydro­
gen absorption profile (Figure 2), but they are much less important 
in the hydrogen pattern.
In discussing the distribution and motions of the OH sources, 
Bolton et al (1964-) stress some minor features which appear to have 
a constant velocity over a few degrees of longitude. However, the 
greatest kinetic energy appears to be associated with a slow rota­
tional motion.
- 7-
The present author considers that the OH Is located in the outer 
part of the nuclear "disk”, because It extends over several degrees 
In longitude, and also It exhibits a low rotational velocity. 
Rougoor (196*0 has already suggested that the rotational velocity 
of the hydrogen falls off In this region.
HYDROGEN RECOMBINATION LINE
Mezger (1966) has recently detected a hydrogen line near 6 cm, 
which corresponds to the transition from n= 109 to 108, in two 
components of the galactic centre source. The line was not obser­
vable in the main Sgr-A component. As this line is only emitted in 
K II regions, this results supports the view that the central com­
ponent is nonthermal in origin, while the other components are 
thermal.
X-RAYS
A number of X-ray sources have recently been found in observa­
tions from rockets * The only one identified so far is associated 
with the Crab nebula. There Is a cluster of other sources in Sagi­
ttarius and Scorpius. None of these coincides with the radio source 
Sgr A, but the clustering in this direction suggests a relation with 
the nuclear disk (Johnson 1966). However it should be emphazized 
that no individual correlations have been found with, nuclear objects 
and it should be noted that the X-ray sources show a greater apread 
in latitude than the radio-emitting region in the nucleus.
CONCLUSION
The central region has many complexities. We have some idea of 
its structure, but our interpretation must still be inconclusive. 
There is a forthcoming series of lunar occultations of the galactic 
centre, from 1967 to 1970, which should yield more information on 
the fine structure of the region. Unfortunately, parallax moves the 
apparent position of the Moon northwards for a southern observatory, 
and consequently Parkes and Pereyra will not see any occultations 
of Sgr A. In this case, we must leave the main work to northern 
observatories.
I conclude this review about one part of the galaxy by saying 
that there is still plenty to be discovered, as in fact there is 
in the whole Galaxy. I air sure that this new telescope, which is 
so well placed geographically for galactic studies, will play an 
important part in attacking the many problems that are to be found 
in the Galaxy.
REFERENCES
Bolton, J.G., Gardner, F.F., McGee, R.X., and Robinson, B.J. (196*0. 
Nature T 2^ ,  30.
Broten, N.W., Cooper, B.F.C., Gardner, F.F., Minnett, H.C. Price,
R.M., Tonking, F.C. and Yabsley, T.E. (1965)» AUst.
J.Physics, 1 8 , 65.
Burke, B.F. (1965). Ann. Rev. Astron. Astrophvs, 1, 275.
Cooper, B.F.C. and Price, R.M. (196*0. Svmp. ItTA. 20. l68.(nThe 
Galaxy and the Magallanlc Clouds", Kerr, F.J. and 
Rodgers, A.W.E. Aust. Acad. Scl., Canberra), 'n.
Townes, D.N. (1965). In press.
Townes, D.N., Maxwell, A. and Meeks, M.L. (1965). Nature, 208, H 89. 
Trake, F.D. (I960). NRAO Annual Report, 1, 2.
Hollinger, J.P. (1965). Ap .J. 1*»2, 6O9.
Johnson, H.C. (1966). Ap .J. 1*^, 261.
Kerr, F.J. (196*0. Svmp. I£0~, 20, 187.
Kerr, F.J. and Vallak, R. (1966). Aust. J. Phvslcs, (In press)
Mezger, P. (1966). In press.
Robinson, B.J., Gardner. F.F., van Damme, K.J. and Bolton, J.G. (196*0 
Nature t 202, 989
Rougoor, G.W. (196*0. B.A.N. !£. 381.
- 10-
LEGENDS TO FIGURES
Fig. 1 Location of 20-cm continuum "ridge lines" In the galactic 
centre region.
Fig. 2 21-cm absorption profile for Sgr A, together with the
estimated emission profile in the absence of absorption.
Fig. 3 Hydrogen contour diagram along galactic equator (scale ] 
ufcit = l.'fflK in T^).
Fig. ** Latitude of high-velocity hydrogen peak.
Fig. 5 A possible structure for the galactic centre region.
Fig. 6 OH absorption profile for Sgr A (1667 Mc/s).
The 21-cm profile is drawn as a dashed line, on an 
arbitrary scale. (Robinson et al. 196*0.
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- 13-
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The galactic centre

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