GALACTIC STRUCTURE, SPIRAL, OBSERVATIONS

BRIAN ROBINSON

  The bright band  of the Milky Way  across the sky has long suggested
that we live inside  a disk of   stars, gas, and dust.  Flat, rotating
disks of  stars can be   seen in photographs  of  other galaxies. When
edge-on they reveal  a thin, flat,  dusty disk with rotational motions
of some hundreds  of kilometers  per  second. When face-on, the  young
stars,  ionized hydrogen regions,  giant   molecular clouds, and  dust
delineate  a   large-scale structure    with several  trailing  spiral
arms. Since the 1950s evidence  has been accumulating that young stars
and gas  clouds  in  our  galaxy have  an arm-like   distribution. But
determining  the connection  of the fragments   of arms into  a unique
spiral pattern remains a challenge.

  From the position of the solar system inside the disk of the Galaxy,
and well out from the center, the appearance is essentially that of an
edge-on galaxy and  it is difficult to  infer the overall structure. A
further problem is  the large amount  of dust in the  disk (as in  any
spiral galaxy), which attenuates starlight  rapidly. Between the solar
system and the  galactic center blue  light  suffers 25  magnitudes of
absorption. So, in the   disk, optical tracers   can only be  seen  at
distances corresponding to about one tenth of the  overall size of the
Galaxy.

  Infrared, millimeter, and radio waves are not attenuated by the fine
dust particles, and at   these  wavelengths measurements can  be  made
right across the  Galaxy. In the disk  a line of sight  will intercept
several arm fragments. To determine an overall  pattern, we need to be
able  to recognize  arms at different  distances  and to measure those
distances.  Making reliable measurements of the  distances is the most
difficult observational problem.

THE DISK OF THE GALAXY

Within a radius of about 8 kpc from the  center of the galaxy the dust
and molecular clouds lie in a thin, flat  disk, with the same ratio of
thickness to  diameter    as a black  vinyl   phonograph  record.  The
thinness of the disk is most clearly seen in the infrared observations
by the Infrared Astronomical Satellite (IRAS)  satellite (Fig.  1) and
in observations of  the 2.6-mm spectral line  of carbon  monoxide. The
molecular disk has  a thickness of 130  pc between its  half-intensity
points.   Further than  10 kpc  from  the  center,  the disk thickness
increases and its centroid deviates by up to 1.6 kpc from the plane of
the inner disk.

  The thin disk of dust and molecules is prima facie evidence that our
galaxy is a member  of the class of  spiral galaxies classified as Sb,
Sc, or Sd. Elliptical, SO, Sa and  irregular galaxies are not so flat,
while E, SO, and Sa galaxies are not so dusty.  ,

  In our Galaxy the long-lived stellar populations and atomic hydrogen
clouds occupy a thicker   disk. In Fig. 1  the  bright areas show  the
distribution   of 12-25-**  emission     from older stars  and   their
envelopes, as measured  by IRAS satellite .  The  thin dark band is  a
superimposed  negative of  the IRAS 60-   and 100-** emission from the
cool dusty matter in which young, massive stars have formed.

THE CENTRAL BULGE

The IRAS infrared observations show  that there is  a central bulge to
our galaxy.  The  bulge  is  clearly seen on  the  composite  image in
Fig. 1.   Long-period  variable stars   in  the bulge have  also  been
studied at optical and radio wavelengths.

  The extent of the bulge is very similar to those of Sb galaxies like
NGC 891  or NGC 4565. Sc galaxies  have  smaller nuclear bulges, while
most Sa galaxies have large, amorphous central regions without dust.

ASSOCIATIONS OF O AND B STARS

Close to  the solar system   the young O  and  B stars in clusters  or
associations are the best spiral tracers. Such stars have ages of only
a few million   years.  Their   ultraviolet radiation   ionizes    the
surrounding gas to form HII regions. The  distance of an OB cluster or
association is  derived from known  absolute magnitudes  and colors of
the embedded stars, and from an estimate of the intervening absorption
by  dust. The arm fragments near  the sun derived from OB associations
are shown in Fig. 4.

DIFFERENTIAL GALACTIC ROTATION

Spiral  galaxies  rotate rapidly, ensuring  the  stability of the flat
disk.  Our galaxy shows  clear evidence for rotation, with  rotational
speeds  of about 200 km s***  at and beyond  the distance of the solar
system  from the galactic  center.  Velocities  near 200  km s***  are
Typical of  Sc galaxies; Sb  galaxies have  rotation  speeds more like
250km s***, and Sa galaxies have speeds above 300 km s**1.

  Atomic hydrogen (H) and carbon  monoxide (CO) spectral line emission
show  a marked  change in  Doppler shift as  the galactic  longitude 1
increases (1 is  the angle of  the line of sight  in the  plane of the
Galaxy measured from the direction of  the galactic center). Figures 2
and 3 show the Doppler shifts for H and CO, respectively, as functions
of 1. For  gas, moving in a  circle  of radius  R the maximum  Doppler
shift is observed  at a galactic longitude  where the line of sight is
tangential to the circle. If  Ro is the distance  of the solar  system
from  the center, the tangential  direction  has galactic longitude  1
given  by  sin  1*****.   The  Doppler shift  (expressed  as  a radial
velocity) is the vector sum of  the motion of the gas  at radius R and
the orbital motion of the solar system at Ro.

  Figures 2 and 3 show a  high degree of symmetry  between the side of
the Galaxy observed from the northern hemisphere (0๘ <1 <90๘) and that
observed from the  southern  hemisphere (270๘ <1 <360๘).  The symmetry
shows that to  a  first approximation  the  gas is moving in  circular
orbits  with the orbital  angular   velocity increasing closer to  the
galactic   center, reflecting an increase of   stellar  density in the
central regions.

KINEMATIC DISTANCES

The change  of  angular  velocity with radius   provides   a means  of
measuring the  normalized   radius R/Ro from  the   radial velocity of
atomic hydrogen, carbon monoxide, or recombination-line emission. This
can provide a "kinematic distance" for the gas.

  For R<Ro, the radial velocity V(R) for motion  on a circle of radius
R varies as
              *************************

where V*** is the maximum radial velocity at the tangent point, observed
at galactic longitude ****(********).

  For R<Ro the  line of sight cuts the  same  radius at two  different
distances.  Often the  observed  width of the  gas  layer helps decide
whether the  gas is at  the "near"  or  "far"  point and  permits  the
assignment     of   an   unambiguous      kinematic  distance.     For
recombination-line  observations   an association   with  OB stars  or
optical HII  regions would indicate the  near distance, and absorption
of the continuum radiation from the HII region at velocities near V***
would indicate a far distance.

  When R>Ro, there is no tangent point. To a first approximation the
gas is found to be moving at a constant velocity Vo=200 km s*** at all
R>Ro. Then
         *******************************

For directions within 10๘  or so of the  galactic center or anticenter
the sin 1 term in Eqs.  (1)  or (2) crowds  the loci of constant R and
kinematic distances cannot be found.

TANGENTIAL DIRECTIONS

From  the offset position of the  solar system we  would expect to see
more spiral  arm tracers when the line  of sight passes along a spiral
arm than when it traverses an interarm region.

  Observations in the     radio continuum at  meter   wavelengths have
located marked jumps  in  the strength  of the integrated  synchrotron
radiation  as the Galaxy    is  scanned  in galactic  longitude.   The
directions  of  these   jumps   define  edges  in   the   longitudinal
distribution   of  high-energy    electrons,  and are   identified  as
directions   where  the telescope   looks   lengthwise along  a spiral
arm.  These edge directions are  found  at approximately 1=283๘, 310๘,
328๘, 339๘, 31๘, and 50๘.

  The distribution   of ionized-hydrogen   regions along  the galactic
plane, based  on   observations at 6-cm  wavelength, shows  tangential
concentrations near 1=283๘, 310๘, 328๘, 24๘, 30๘, and 49๘.

  Carbon monoxide   observations show that  the  warm  giant molecular
clouds are about  five times more abundant within  spiral arms than in
interarm regions. (The    less-massive, colder  molecular clouds   are
distributed homogeneously throughout  the zone 0.4<R/Ro <1.0.)  In the
northern hemisphere the CO tangent  points are at  1=33๘ and 1=50๘. In
the southern hemisphere they are at 1=282๘, 309๘,  326๘, and 336๘. The
so-called  "3-kpc   expanding arm"  is   tangential at   1=342๘. These
tangential points  can be seen  clearly as  peak in  the CO brightness
along  the  tangential   velocity locus  in  Fig.   3.   Between these
longitudes  there   are   pronounced holes   in   the  CO  emissivity,
particularly between the 1=282๘, 1=309๘, and 1=326๘ tangent points.

A PLANE VIEW OF THE GALAXY

Figure 4 presents an artist's impression of  our galaxy as it might be
seen by an observer   in another galaxy.  The  crosses near the  solar
system show the position  of OB star  clusters and associations, based
on  photometric distances. We lie  on the inside of  a spur termed the
Orion arm. About 2 kpc further out the OB stars define a portion of an
outer arm, the Perseus arm. About 2 kpc  closer to the galactic center
than the  Orion arm there  is evidence for a section  of an inner arm,
the Sagittarius arm.

  The dots   show  the location    of ionized-hydrogen   regions  with
kinematic   distances   deduced     from   recombination-line   radial
velocities. The majority of the points lie on our side of the galactic
center,   as a  result   of constraints   on sensitivity   and angular
resolution in surveys at radio  wavelengths. However, some very bright
regions can be detected on the far side of the Galaxy.

  Within  the circle R=Ro (the distance  of the  solar system from the
nucleus) the  fuzzy bands given an  impression of  the distribution of
giant molecular clouds derived from the 2.6-mm CO line observations in
Fig. 3.  The  distances are  again kinematic  distances  based  on the
observed tangential velocities [Eq.(1)]. The bands  drawn for R>Ro are
derived    from     21-cm   hydrogen-line   observations,   using    a
constant-velocity rotation curve [Eq. 2)].

  The straight  lines   radiating   from  the solar   system  are  the
tangential directions discussed  in the previous section.  From simple
geometry these are perpendicular to a radius from the center at points
which lie  on   a  circle drawn   through the  center   and the  solar
system.  The tangential directions   define a series  of benchmarks to
which any pattern of spiral structure must be fitted.

  The observations sketched in  Fig. 4 can  be fitted with equiangular
spiral arms with  pitch  angles between  10๘   and 15๘.  However,  the
overall pattern depends  on how the  various arm  fragments are joined
together,  particularly between   the  left side  (southern-hemisphere
observations)   and    the    right     side      (northern-hemisphere
observations). Kinematic distances  cannot be  defined near  1=0๘  and
1=180๘, whereas noncircular  motions (see next section) are comparable
with    or exceed  the    projected   rotational velocities  in  these
directions. Some authors argue for a pitch  angle as small as 6๘ (like
an Sa galaxy), whereas others argue for a pitch  angle as large as 25๘
(like an Sc galaxy).

NONCIRCULAR MOTIONS

The artist's impression in Fig. 4 is based  on the first approximation
of circular orbits derived from  the locus of maximum radial  velocity
in Fig. 2   and 3 (observed  for  R<Ro)  and  a constant   velocity of
rotation for  R>Ro. Observations in certain areas  reveal a variety of
noncircular motions described   as   shocks, warps, shears,    rolling
motions, and streaming  motions,  as well  as  a component of   random
motions  of the order of 8  kms s**. The  magnitude of the noncircular
velocities make kinematic distances uncertain and make it difficult to
link features from one side of the Galaxy to the other.

SUMMARY

The basic observational data lead to the following picture. Our galaxy
has a flat, thin  disk of dust and giant  molecular clouds  similar to
those of Sb  or Sc galaxies. The  disk is  warped about 1๘in  galactic
latitude in its outer parts. Within the disk the gas, young stars, and
ionized-hydrogen regions near and beyond  the solar system rotate to a
first approximation in circular orbits with velocities of about 200 km
s**. This velocity is typical of Sc galaxies. There is a central bulge
of evolved stars  typical of an Sb   galaxy. Spiral arm  fragments are
shown by various tracers, the  separation between arms being about 25%
of the distance from the Sun  to the center. Kinematic distances based
on circular orbits suggest an equiangular  spiral structure with pitch
angles of 10๘-15๘, like most Sc galaxies; Sb galaxies are more tightly
wound.   Shocks,  warps,  shears,   and    streaming motions lead   to
uncertainties  in kinematic distances  and some authors have suggested
pitch angles as low as 6๘ or as large as 25๘.

    Additional Reading

Beichman, C.A.(1987). The IRAS view of the Galaxy and the
   solar system. Ann. Rev. Astron. Ap. 25 521.

Blitz, L.(1982). Giant molecular cloud complexes in the Galaxy.
   Scientific American 246 (No. 4) 84.

Bok, B.J. and Bok, P.F.(1981). The Milky Way, 5th ed. Harvard
   University Press, Cambridge, MA.

Burton, W.B.(1976). The morphology of hydrogen and of other
   tracers in the Galaxy. Ann. Rev. Astron. Ap. 14 275.

Kerr, F.J.(1969). The large-scale distribution of hydrogen in the
   Galaxy. Ann. Rev. Astron. Ap. 7 39.

Sandage, A.(1961). The Hubble Atlas of Galaxies.
   Carnegie Institution of Washington, Washington, DC.

Scoville, N. and Young, J.S.(1984). Molecular clouds, star formation
   and galactic structure. Scientific American 250 (No. 4) 42.

See also Galactic Structure, Interstellar Clouds; Galactic Structure,
   Large Scale; Galaxies, Spiral, Structure.