Recent surveys of 2.6 mm CO emission and 100 MeV gamma-radiation in the galactic plane reveal a striking correlation suggesting that both emissions may be primarily proportional to the line-of-sight column density of H2 in the inner galaxy. Both the gamma ray and CO data suggest a prominent ring or arm consisting of cool clouds of H2 at a galactocentric distance of approximately 5 kpc with a mean density of approximately 4 atoms/cu cm. The importance of H2 in understanding galactic gamma ray observations is also reflected in the correlation of galactic latitude distribution of gamma rays and dense dust clouds. A detailed calculation of the gamma ray flux distribution in the 0 deg to 180 deg range using the CO data to obtain the average distribution of molecular clouds in the galaxy shows that most of the enhancement in the inner galaxy is due to pion-decay radiation and the 5 kpc ring plays a major role. Detailed agreement with the gamma ray data is obtained with the additional inclusion of contributions from bremsstrahlung and Compton radiation of secondary electrons and Compton radiation from the intense radiation field near the galactic center

Ryter, C. E.

Scoville, N. Z.

Solomon, P. M.

Stecker, F. W.

English

NASA Technical Reports Server

Molecular hydrogen in the galaxy and galactic gamma rays

Estimates made of column densities of H_2 at galactic longitude equals 0° indicate that H_2 is far more abundant than HI in the inner galaxy and is the key to an explanation of the gamma-ray observations. This is also reflected in the correlation of galactic longitude and latitude distributions of gamma-rays and molecular clouds. Particularly strong evidence is found from the galactic survey of CO emission at 2.64 mm. The cosmic-ray distribution inferred from the calculations is not uniform but only weakly dependent on the total gas distribution in the inner galaxy

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MOLECULAR HYDROGEN IN THE GALAXY AND GALACTIC GAMMA RAYS 
F. w. Stecker 
Theoretical Studies Group, NASA Goddard Space Flight Center, 
Greenbelt, Maryland 20771 
P.. M- Solomon. 
Department of Earth and Space Sciences, State University of New York, 
Stony Brook, New York 11794 
N. z. Scoville 
OWens Valley Radio Observatory, California Institute of Technology, 
Pasadena, California 91109 
and 
C. E. Ryter* 
Laboratory for High Energy Astrophysics, NASA Goddard Space Flight Center, 
Greenbelt, Maryland 20771 
Estimates made of colwnn densities of H2 at i 11 = o0 indicate 
that H2 is far more abundant than HI in the inner galaxy and is the 
key to an explanation of the y-ray observations. This is also 
reflected in the correlation of galactic longitude and latitude 
distributions of y-rays and molecular clouds. Particularly strong 
evidence is found from the galactic survey of CO emission at 
2.64 nun. The cosmic-ray distribution inferred from the calculations 
is not uniform but only weakly dependent on the total gas distribution 
in the inner galaxy. 
1. Introduction. Molecular hydrogen has long been suspected to be an important 
component of interstellar gas because it is the most stable low-temperature form of 
the most abundant element in the galaxy. H2 is expected to be the predominant form 
of hydrogen in cool clouds of sufficient density. However, despite its abui,dance, it 
is difficult to measure its galactic distribution directly. Results from two pro-
mising methods for indirectly studying the galactic distribution of H2 arc discussec~ 
here, viz., recent galactic surveys of 100 MeV y-radiation and 2.6 mm radio line 
emission from the J = 1 + O transition of CO molecules. To these surveys, which 
reflect the extent and distribution of H2 in the plane of galaxy, we will add 
corroborating information on the amount and latitude distribution of gas in the 
direction of the galactic center supplied by X-ray, opticalr and infrared 
absorption measurements. 
2. The Recent SAS-2 Gamma-Ray Galactic Longitude Observations. Fichtel et al. 
(1975) have recently reported the results of a sky survey made of 100 MeV y-rays 
using a spark chamber aboard the SAS-2 satellite. The similar galactic longitude 
distribution observed by the OS0-3 detector was also noted to be distinctly uncor-
related with the 21 cm distribution by Clark et al. (1970). The OS0-3 result 
implied an increase in cosmic-rays, unseen gas or both in the inner galaxy, and it 
was suggested by Stecker (1969, 1971) and Stecher and Stecker (1970) that molecular 
*NAS/NRC Postdoctoral Resident Research Associate. Permanent Address: Centre 
d'Etudes Nucleaires de Saclay, 91190 Gif-sur-Yvette, France 
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hydrogen unseen in 21 cm surveys could account for a large part of the y-ray 
enhancement in the inner galaxy. A model has been proposed by Bignami and Fichtel 
(1974) and Bignami et al. (1975) based on producing large enhancements at the 
locations of arms mapped by 21 cm surveys by postulating higher gas densities than 
are seen in 21 cm and proportionately higher cosmic-ray intensities in the arms. 
However, Burton et al. (1975) indicate that the arms outside of 25°from tlie gale;._,± 
center are optically thin in 21 cm and significantly higher amounts of IU :in the ar!:'.ls 
than those deduced in previous surveys appear to be ruled out. We, therefore, feel 
that the galactic y-ray observations can be better understood by using other 
observations in addition to 21 cm surveys to determine the role of 52 clouds :inv:is1l-i .cc 
in 21 cm emission. 
3. The Galactic CO Distribution and the Molecular Ring at ~5kpc. A survey of the 
galactic longitude distribution of CO emission in the galactic plane has recently 
been made by Scoville and Solomon (1975). The importance of this survey in under-
standing the distribution of H2 in the galaxy lies in the fact that CO is an 
excellent tracer of H2 . 
Scoville and Solomon (1975) have used the velocity profile data obtained in 
their CO survey in conjunction with the Schmidt (1965) rotational model of the 
galaxy to determine the mean distribution of CO in the galaxy as a function of 
galactocentric distance for distances greater than 2.6 kpc. This distribu~~on 
shows a broad peak with a maximum near 5 kpc which they have concluded indicates a 
ring of H2 clouds in this region. The general form of the mdecular cbud diotrii:ut:i::Jr. 
obtained by Scoville and Solomon has recently been confirmed in an independent CO 
survey by Burton et al. (1975). The connection between this feature and they-ray 
emission ring at ~s kpc (Puget and Stecker, 1974) led to the suggestion that the 
y-ray data also provide evidence for the molecular cloud ring near 5 kpc (Solomon 
and Stecker, 1974). Coincidently, there is also a similar distribution and peak 
in the giant HII regions of the galaxy (Metzger 1970). This may be understood to 
be the effect of hot young stars in OB associations being formed out of dense 
molecular clouds in this ring. The formation of such a prominent molecular ring 
poses an intriguing problem for galactic structure theory. 
4. Molecular Hydrogen and Total Column Densities in the Direction i=0°. Various 
methods can be used to estimate the amount of gas in the direction of the galactic 
center (Stecker et al. 1975). The results are smnmarized in Table 1. 
Table 1. Column Densities of Hydrogen at t = o 0 Excluding the Galactic Nucleus 
( xlo-22) tcm- 2> (NG.C. ,0> 
<NHI> 
from 21 cm radio 
<2NH > 
frem co 
<2NH2 + NHI> 
from SAS-2 Y-ray flux 
<2NH2 + Ne1> 
from X-ray absorption 
< 2NH2 + NHI> 
from IR absorption 
ito.6 to 1. 5 
l to 2 
itL 2 
3 to 10 
6. 5 to 9 
5 to 7.5 
Daltabuit and Meyer (1972) 
Kerr and Westerhout (1965) 
Clark (1965) 
Scoville and Solomon (1975) 
Off2/2o81 ~ 1.7 (Kaplan and Markin 
1973) as verified by the measure-
ments of Craseman~ et al. (1974). 
Ryter, et al. (1975) 
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Scoville, Solomon and Jefferts (1974) estimate the total mass of the molecular 
disk near the galactic center to be 
4 x 107 ~ !Mc;cfM0 ) ~ 108 
This yields an estimated flux from n° decay in the range 
o.6xlo-5 5 2 1 -1 ~ ~I GC ~ l.5xl0- cm- s- rad y, 
which is only about one tenth of the observed flux at i=o0 • 
5. Galactic Gannna Ray Flux as a Function of Longitude. In performing numerical 
calculations of the longitude distribution of the galactic y-ray flux from n° 
decay, we have used the survey of Scoville and Solomon (1975) to obtain the 
relative distribution of molecular hydrogen in the galaxy as a function of galacto-
centric distancew and have normalized to a total column density of 7xlo22 cm-2 
in the direction of the galactic center using the X-ray and infrared absorption 
measurements (Table 1) which are consistent with, but presumably more accurate 
than the column densities deduced from the y-ray or CO results. The contribution 
from atomic hydrogen was estimated based on the numbers given by Kerr and 
Westerhout (1965) and Westerhout (1970). 
For the purpose of the calculations to estimate the effect of cosmic ray 
enhancements in the galaxy, it was assumed that such enhancements may be correlated 
with the gas distribution so that 
HI(W) + 
nHI,0 
(7) 
The results are shown in Figure 1 for NGC ~ = 7 x lo22 cm-2 and c;=O. 3 corresponding 
to an increase of about a factor of 2 in t~e cosmic ray flux in the 5kpc region as 
indicated by studies of the supernova 14 .-..-..--...--r--.--,--.-""""T""---r-r--.--.--.--,.--..-...-.,...-, 
remnant distribution in the galaxy 
(Ilovaisky and Lequeux 1972, Kodaira 
1974) and the synchrotron radiation 
measurements (Daniel and Stephens 
1975). This corresponds to a mean· 
value for the total gas density of ~l 
atom/cm3 at 10 kpc of which '\40 per-
cent is in molecular form. This 
1.2 
j 0.8 
agrees with the values given by Spitzer, ~ 
et al. (1973) from measurements of 1, o.6 
rotational UV absorption lines of H2 
and those given by Jenkins and Savage 
(1974) for Lyman a lines of HI. In the 
5 kpc region, this corresponds to a 
0_4 
I RANGE OF SAS-2 LONGITUDE DATA 
L, TOTAL CALCULATED Y-RAY FLUX 
' DISTRIBUTION 
volume-averaged total density of "-'5 o2 
atoms/cm3 of which "'80 percent would be 
in molecular form. 
The results of the numerical cal-
culations are shown in Figure 1 together 
with the flux distribution given by 
Fichtel et al. (1975) from the SAS-2 
20 4Q 60 80 100 12C J40 160 180° 
f 
Figure 1. Compariso·- of the total 
calculated y-ray flu,: C::is"tribution 
with the SAS-2 longitude data. 
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observations for the half-plane from o0 to 180° over which the CO measurements of 
Scoville and Solomon (1975) can be applied. Effects of s=condary electron uroductis~. 
bremsstrahlung and Compton interactions are included. These results indicate that 
most of the observed y-ray enhancement at low longitudes is primarily a result of 
increased gas density and that the molecular ring near 5 kpc plays an important 
role in accounting for this increase. 
Cosmic rays are limited to a small variation over the galactic disk given 
by 0. 2~ a~ 0. 5 as derived from the limits on the amount and distribution of gas 
implied by the observations discussed in section 4. We thus conclude that cosmic 
rays cannot vary linearly with gas density over all segments of the galaxy nor 
do they appear to be uniformly distributed. 
The results indicate that the character of the longitude distribution of 
galactic Y-radiation corresponds well with the overall density distribution in 
the galaxy implied by the CO and 21 cm measurements and that this distribution has 
a broad maximum in the 5 to 6 kpc region. Higher frequency modulations by spiral 
arms do not appear to us to play a significant role in determining the galactic 
Y-ray disribution within the statistical errors and 5° resolution of the SAS-2 
data, at least for the half plane analyzed here using the CO data. In the other 
half plane, 180°~ £ 2 3600, three apparently sharp features have been identified 
by Fichtel et al. (1975) with the Scutum, Norma and "3 kpc" arm features designated 
in 21 cm surveys. The two inner features at 330°-335° and 340°-345° may correspond 
to a bifurcation of the broad moleuclar ring near 5 kpc on the other half-plane 
of the galaxy; the feature at 310°-315° is perhaps somewhat more puzzling since 
this should be associated with a correspondingly strong feature at Ji, "' so0 which 
appears to be absent in both the Y-ray and CO data. There are, however, large 
error bars associated with the 310°-315° observation due to the fact that the 
SAS-2 spark-chamber telescope only viewed this direction obliquely. Also, Puget 
et al. (in prep.) have indicated that large corrections due to nearby features are 
necessary in the 310°-360° region. Future CO observations from the southern 
hemisphere could help increase our understanding of the matter distribution in 
this region. 
Thus, the galactic gas seems to have a large-scale superstructure modulated 
by spiral arm perturbations similar to that seen in M31 in 21 cm emission (Guibert 
1974, Emerson 1974) and in our own galaxy in nonthermal radio emission (Price 197~ 
and it appears to be this superstructure which determines the character of the 
general central enhancement in the Y-ray longitude distribution. 
Thus, it would appear that in both external galaxies and our own Galaxy, the 
density gradient of the Hz distribution is steeper than that of the HI distribution, 
resulting in a decrease in the ratio nu2/nHI with distance outward from the region 
of maximum density. 
The total contribution from bremsstrahlung and Compton interactions of both 
primary and secondary electrons to the galactic Y-ray flux in the central region 
is of the order of 30 percent, in agreement with the estimates made by Stecker 
et al. (1974) using the observed Y-ray energy spectrum obtained by SAS-2. 
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6. Gamma-Ray Latitude Distribution and Associated Line of Sight Reddening. In 
addition to the Y-ray longitude distribution measurements reported by Fichtel 
et al. (1975), a galactic latitude distribution was also obtained. This 
distribution shows an asymmetry with respect to the galactic plane with more 
flux coming from positive galactic latitudes in the case of moderate latitudes 
~ :::-- tb I :::_ 30°. This can be understood in the context of section 4 where it 
was pointed out that UV, X-ray infrared absorption measurements indicate that the 
total column density of gas is proportional to the reddening in a given direction 
and that in the direction of highly reddened objects most of the hydrogen may well 
be in molecular form as expected in dense dust clouds and as is indicated by only 
a partial correlation with 21 cm emission. The data of Knapp and Kerr (1974) in 
the region of the sky located between i = 345° and 30° and between b = 10° and 3<f' 
ca.IL--1le used to evaluate the average reddening and HI column density at b = 200. 
We obtain <E8 _v(200)> = 0.26 mag and <NH(20°)> = 9.7 x 1020 H atoms cm-2. 
Postulating the same cosmic ray density as observed locally we find from the 
reddening and relation (2) a gamma-ray flux of 2.2 x lo-5 cm-2 s-1 sr-l which com-
pares favorably with the flux reported by Fichtel et al. (1975) observed at 
b = 2cP of 2 to 2.5 x lo-5 x lo-5 cm-2 s-1 sr-1 . However, the flux estimated from 
the HI-column density is only 1.4 x lo-5 cm-2 sec-l sr-1 which is particularly 
significant in this case because at high galactic latitudes the 21 cm emission line 
is not optically thick (Knapp and Kerr 1974). Thus, the situation with regard to 
the latitude distribution of galactic gamma-rays is analogous to that of the 
longitude distribution and can be better understood by taking account of H2 unseen 
in 21 cm surveys of HI gas. 
References 
Bignami, G. F. and Fichtel, C. E. 1974; Astrophys. J. 189, L65. 
Bignami, G. F., Fichtel, C. E.; Kniffen, D. A. and Thompson, D. 
phys. J., in press. 
,. 
u. , 1975; Astrc-
Burton, W.B.; Gordon, M.A.; Bania, T. M. and Lockman, F. J. 1975; Astrophys. J., 
in press. 
Clark, B. G. 1965; Astrophys. J. 142, 1398. 
Clark, G. W.; Garmire, G. P. and Kraushaar, w. L. 1970 in Proc. l~Q symposium 
#37 £!!.Non-Solar !- and y-ray Astronomy (ed. L. Gratton) Reidel, Dordrecht, 
Holland, 239. 
Crasemann, B.; Koblas, P. E.; Wang, T.; Birdseye, H. E. and Chen, M. H. 1974; 
Phys. Rev. A9, 1143. 
Daltabuit, E. and Meyer, S. 1972; Astron. and Astrophys. 20, 415. 
Daniel, R. R. and Stephens, S. A. 1975; Space Sci. Rev. 17, 45. 
Fichtel, C. E.; Hartman, R. C.; Kniffen, D. A.; Thompson, D. J.; Bignami, G. F.; 
Qgelman, H.; Ozel, M. F. and Tlliner, T. 1975; NASA preprint X-662-74-304, 
Astrophys. J., in press. 
Ilovaisky, A. cind Lepueux, J. 1972; Astron. anu Astrophys. 20, 347. 
Jenkins, E. B. and Savage, B. D. 1974; Astrophys. J. 187, 243. 
Kaplan, I. G.; and Markin, A. P. 1973; Sov. Phys. JETP. ~· 216. 
Kerr, F. J. 1969; Ann. Rev. Astron. and Astrophys. !__, 39. 
Kerr, F. J. and Westerhout, G. 1965 in ~~l~ctic Structure, Stars and Stellar 
Systu"s 5, 167 (ed. A. Blaauw and '1. Schmidt) U. of Chicago Press. 
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Knapp, G.R. and Kerr, F.J. 1974; Astron. and Astrophys. 35, 361. 
Kodaira, K. 1974; Puhl. Astr. Soc. Japan 26, 255. 
Mezger, P. G. 1970 in Proc. IAU Symposium #38, Basel, Switzerland (ed. W. Becker 
and G. Contopoulos) Reidel, Holland, 107. 
Parker, E. N. 1969; Space Sci. Rev. -2.1 651. 
Puget, J. L. and Stecker, F. W. 1974; Astrophys. J. 191, 323. 
Schmidt, M. 1965; Galactic Structure (ed. A. Blaauw and M. Schmidt) U. of Chicago 
Press, Chicago, Ill. 513. 
Scoville, N. z. and Solomon, P. M. 1975; Astrophys. J. Lett., in press. 
Scoville, N. Z.; Solomon, P. M. and Jefferts, K. B. 1974; Astrophys. J. 187, L63. 
Solomon, P. M. and Stecker, F. W. 1974; Proc. ESLAB Symp. on the Context and 
Status of Gamma Ray Astronomy, Frascati, ESRO SP-106, 253. 
Spitzer, L.; Drake, J.F.; Jenkins, E.B.; Morton, D.C.; Rogerson, J.B. and York, 
D.G. 1973; Astrophys. J. 181, Lll6. 
Stecher, T.P., and Stecker, F.W. 1970; Nature 226, 1234. 
Stecker, F.W. 1969; Nature 222, 865. 
Stecker, F. W.; Puget, J.L.; Strong, A. W. and Bredekamp, J. H. 1974; Astrophys. 
J. 188, L59. 
Stecker, F. W.; Solomon, P. M.; Scoville, N.Z. and Ryter, C.E. 1975; Astrophys. 
J. in press. 
Strong, A. W. 1975; J. Phys. A. ~, 617. 
© MPE • Provided by the NASA Astrophysics Data System 


Recent surveys of 2.6-cm CO emission and 100-MeV γ-radiation in the galactic plane reveal a striking correlation suggesting that both emissions may be primarily proportional to the line-of-sight column density of H_2 in the inner Galaxy. Both the γ-ray and CO data suggest a prominent ring or arm consisting of cool clouds of H_2 at a galactocentric distance of ~5 kpc with a mean total hydrogen density equivalent to ~5 atoms cm^(-3). Estimates are made of column densities of H_2 at 0° galactic longitude. The estimates are compared with estimates from infrared and X-ray absorption measurements. These estimates are all consistent, indicating that H_2 is far more abundant than H I in the inner Galaxy and is the key to a more satisfactory explanation of the γ-ray observations than previous suggestions. The importance of H_2 in understanding galactic γ-ray observations is also reflected in the correlation of galactic-latitude distribution of γ-rays and dense dust clouds. The deduced cosmic-ray distribution inferred from the calculations is similar to that of galactic supernova remnants, suggesting a galactic origin for most cosmic rays. 



A detailed calculation of the γ-ray flux distribution in the 0°-180° longitude range using the CO data to obtain the average distribution of molecular clouds in the Galaxy shows that most of the enhancement in the inner Galaxy is due to π^0-decay radiation and the 5-kpc ring plays a major role. Detailed agreement with the γ-ray data is obtained with the additional inclusion of contributions from bremsstrahlung and Compton radiation of secondary electrons and Compton radiation from the intense radiation field near the galactic center

Molecular hydrogen in the Galaxy and galactic gamma rays

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MOLECULAR HYDROGEN IN THE GALAXY AND GALACTIC GAMMA RAYS 
F. w. Stecker 
Theoretical Studies Group, NASA Goddard Space Flight Center, 
Greenbelt, Maryland 20771 
P.. M- Solomon. 
Department of Earth and Space Sciences, State University of New York, 
Stony Brook, New York 11794 
N. z. Scoville 
OWens Valley Radio Observatory, California Institute of Technology, 
Pasadena, California 91109 
and 
C. E. Ryter* 
Laboratory for High Energy Astrophysics, NASA Goddard Space Flight Center, 
Greenbelt, Maryland 20771 
Estimates made of colwnn densities of H2 at i 11 = o0 indicate 
that H2 is far more abundant than HI in the inner galaxy and is the 
key to an explanation of the y-ray observations. This is also 
reflected in the correlation of galactic longitude and latitude 
distributions of y-rays and molecular clouds. Particularly strong 
evidence is found from the galactic survey of CO emission at 
2.64 nun. The cosmic-ray distribution inferred from the calculations 
is not uniform but only weakly dependent on the total gas distribution 
in the inner galaxy. 
1. Introduction. Molecular hydrogen has long been suspected to be an important 
component of interstellar gas because it is the most stable low-temperature form of 
the most abundant element in the galaxy. H2 is expected to be the predominant form 
of hydrogen in cool clouds of sufficient density. However, despite its abui,dance, it 
is difficult to measure its galactic distribution directly. Results from two pro-
mising methods for indirectly studying the galactic distribution of H2 arc discussec~ 
here, viz., recent galactic surveys of 100 MeV y-radiation and 2.6 mm radio line 
emission from the J = 1 + O transition of CO molecules. To these surveys, which 
reflect the extent and distribution of H2 in the plane of galaxy, we will add 
corroborating information on the amount and latitude distribution of gas in the 
direction of the galactic center supplied by X-ray, opticalr and infrared 
absorption measurements. 
2. The Recent SAS-2 Gamma-Ray Galactic Longitude Observations. Fichtel et al. 
(1975) have recently reported the results of a sky survey made of 100 MeV y-rays 
using a spark chamber aboard the SAS-2 satellite. The similar galactic longitude 
distribution observed by the OS0-3 detector was also noted to be distinctly uncor-
related with the 21 cm distribution by Clark et al. (1970). The OS0-3 result 
implied an increase in cosmic-rays, unseen gas or both in the inner galaxy, and it 
was suggested by Stecker (1969, 1971) and Stecher and Stecker (1970) that molecular 
*NAS/NRC Postdoctoral Resident Research Associate. Permanent Address: Centre 
d'Etudes Nucleaires de Saclay, 91190 Gif-sur-Yvette, France 
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hydrogen unseen in 21 cm surveys could account for a large part of the y-ray 
enhancement in the inner galaxy. A model has been proposed by Bignami and Fichtel 
(1974) and Bignami et al. (1975) based on producing large enhancements at the 
locations of arms mapped by 21 cm surveys by postulating higher gas densities than 
are seen in 21 cm and proportionately higher cosmic-ray intensities in the arms. 
However, Burton et al. (1975) indicate that the arms outside of 25°from tlie gale;._,± 
center are optically thin in 21 cm and significantly higher amounts of IU :in the ar!:'.ls 
than those deduced in previous surveys appear to be ruled out. We, therefore, feel 
that the galactic y-ray observations can be better understood by using other 
observations in addition to 21 cm surveys to determine the role of 52 clouds :inv:is1l-i .cc 
in 21 cm emission. 
3. The Galactic CO Distribution and the Molecular Ring at ~5kpc. A survey of the 
galactic longitude distribution of CO emission in the galactic plane has recently 
been made by Scoville and Solomon (1975). The importance of this survey in under-
standing the distribution of H2 in the galaxy lies in the fact that CO is an 
excellent tracer of H2 . 
Scoville and Solomon (1975) have used the velocity profile data obtained in 
their CO survey in conjunction with the Schmidt (1965) rotational model of the 
galaxy to determine the mean distribution of CO in the galaxy as a function of 
galactocentric distance for distances greater than 2.6 kpc. This distribu~~on 
shows a broad peak with a maximum near 5 kpc which they have concluded indicates a 
ring of H2 clouds in this region. The general form of the mdecular cbud diotrii:ut:i::Jr. 
obtained by Scoville and Solomon has recently been confirmed in an independent CO 
survey by Burton et al. (1975). The connection between this feature and they-ray 
emission ring at ~s kpc (Puget and Stecker, 1974) led to the suggestion that the 
y-ray data also provide evidence for the molecular cloud ring near 5 kpc (Solomon 
and Stecker, 1974). Coincidently, there is also a similar distribution and peak 
in the giant HII regions of the galaxy (Metzger 1970). This may be understood to 
be the effect of hot young stars in OB associations being formed out of dense 
molecular clouds in this ring. The formation of such a prominent molecular ring 
poses an intriguing problem for galactic structure theory. 
4. Molecular Hydrogen and Total Column Densities in the Direction i=0°. Various 
methods can be used to estimate the amount of gas in the direction of the galactic 
center (Stecker et al. 1975). The results are smnmarized in Table 1. 
Table 1. Column Densities of Hydrogen at t = o 0 Excluding the Galactic Nucleus 
( xlo-22) tcm- 2> (NG.C. ,0> 
<NHI> 
from 21 cm radio 
<2NH > 
frem co 
<2NH2 + NHI> 
from SAS-2 Y-ray flux 
<2NH2 + Ne1> 
from X-ray absorption 
< 2NH2 + NHI> 
from IR absorption 
ito.6 to 1. 5 
l to 2 
itL 2 
3 to 10 
6. 5 to 9 
5 to 7.5 
Daltabuit and Meyer (1972) 
Kerr and Westerhout (1965) 
Clark (1965) 
Scoville and Solomon (1975) 
Off2/2o81 ~ 1.7 (Kaplan and Markin 
1973) as verified by the measure-
ments of Craseman~ et al. (1974). 
Ryter, et al. (1975) 
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Scoville, Solomon and Jefferts (1974) estimate the total mass of the molecular 
disk near the galactic center to be 
4 x 107 ~ !Mc;cfM0 ) ~ 108 
This yields an estimated flux from n° decay in the range 
o.6xlo-5 5 2 1 -1 ~ ~I GC ~ l.5xl0- cm- s- rad y, 
which is only about one tenth of the observed flux at i=o0 • 
5. Galactic Gannna Ray Flux as a Function of Longitude. In performing numerical 
calculations of the longitude distribution of the galactic y-ray flux from n° 
decay, we have used the survey of Scoville and Solomon (1975) to obtain the 
relative distribution of molecular hydrogen in the galaxy as a function of galacto-
centric distancew and have normalized to a total column density of 7xlo22 cm-2 
in the direction of the galactic center using the X-ray and infrared absorption 
measurements (Table 1) which are consistent with, but presumably more accurate 
than the column densities deduced from the y-ray or CO results. The contribution 
from atomic hydrogen was estimated based on the numbers given by Kerr and 
Westerhout (1965) and Westerhout (1970). 
For the purpose of the calculations to estimate the effect of cosmic ray 
enhancements in the galaxy, it was assumed that such enhancements may be correlated 
with the gas distribution so that 
HI(W) + 
nHI,0 
(7) 
The results are shown in Figure 1 for NGC ~ = 7 x lo22 cm-2 and c;=O. 3 corresponding 
to an increase of about a factor of 2 in t~e cosmic ray flux in the 5kpc region as 
indicated by studies of the supernova 14 .-..-..--...--r--.--,--.-""""T""---r-r--.--.--.--,.--..-...-.,...-, 
remnant distribution in the galaxy 
(Ilovaisky and Lequeux 1972, Kodaira 
1974) and the synchrotron radiation 
measurements (Daniel and Stephens 
1975). This corresponds to a mean· 
value for the total gas density of ~l 
atom/cm3 at 10 kpc of which '\40 per-
cent is in molecular form. This 
1.2 
j 0.8 
agrees with the values given by Spitzer, ~ 
et al. (1973) from measurements of 1, o.6 
rotational UV absorption lines of H2 
and those given by Jenkins and Savage 
(1974) for Lyman a lines of HI. In the 
5 kpc region, this corresponds to a 
0_4 
I RANGE OF SAS-2 LONGITUDE DATA 
L, TOTAL CALCULATED Y-RAY FLUX 
' DISTRIBUTION 
volume-averaged total density of "-'5 o2 
atoms/cm3 of which "'80 percent would be 
in molecular form. 
The results of the numerical cal-
culations are shown in Figure 1 together 
with the flux distribution given by 
Fichtel et al. (1975) from the SAS-2 
20 4Q 60 80 100 12C J40 160 180° 
f 
Figure 1. Compariso·- of the total 
calculated y-ray flu,: C::is"tribution 
with the SAS-2 longitude data. 
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observations for the half-plane from o0 to 180° over which the CO measurements of 
Scoville and Solomon (1975) can be applied. Effects of s=condary electron uroductis~. 
bremsstrahlung and Compton interactions are included. These results indicate that 
most of the observed y-ray enhancement at low longitudes is primarily a result of 
increased gas density and that the molecular ring near 5 kpc plays an important 
role in accounting for this increase. 
Cosmic rays are limited to a small variation over the galactic disk given 
by 0. 2~ a~ 0. 5 as derived from the limits on the amount and distribution of gas 
implied by the observations discussed in section 4. We thus conclude that cosmic 
rays cannot vary linearly with gas density over all segments of the galaxy nor 
do they appear to be uniformly distributed. 
The results indicate that the character of the longitude distribution of 
galactic Y-radiation corresponds well with the overall density distribution in 
the galaxy implied by the CO and 21 cm measurements and that this distribution has 
a broad maximum in the 5 to 6 kpc region. Higher frequency modulations by spiral 
arms do not appear to us to play a significant role in determining the galactic 
Y-ray disribution within the statistical errors and 5° resolution of the SAS-2 
data, at least for the half plane analyzed here using the CO data. In the other 
half plane, 180°~ £ 2 3600, three apparently sharp features have been identified 
by Fichtel et al. (1975) with the Scutum, Norma and "3 kpc" arm features designated 
in 21 cm surveys. The two inner features at 330°-335° and 340°-345° may correspond 
to a bifurcation of the broad moleuclar ring near 5 kpc on the other half-plane 
of the galaxy; the feature at 310°-315° is perhaps somewhat more puzzling since 
this should be associated with a correspondingly strong feature at Ji, "' so0 which 
appears to be absent in both the Y-ray and CO data. There are, however, large 
error bars associated with the 310°-315° observation due to the fact that the 
SAS-2 spark-chamber telescope only viewed this direction obliquely. Also, Puget 
et al. (in prep.) have indicated that large corrections due to nearby features are 
necessary in the 310°-360° region. Future CO observations from the southern 
hemisphere could help increase our understanding of the matter distribution in 
this region. 
Thus, the galactic gas seems to have a large-scale superstructure modulated 
by spiral arm perturbations similar to that seen in M31 in 21 cm emission (Guibert 
1974, Emerson 1974) and in our own galaxy in nonthermal radio emission (Price 197~ 
and it appears to be this superstructure which determines the character of the 
general central enhancement in the Y-ray longitude distribution. 
Thus, it would appear that in both external galaxies and our own Galaxy, the 
density gradient of the Hz distribution is steeper than that of the HI distribution, 
resulting in a decrease in the ratio nu2/nHI with distance outward from the region 
of maximum density. 
The total contribution from bremsstrahlung and Compton interactions of both 
primary and secondary electrons to the galactic Y-ray flux in the central region 
is of the order of 30 percent, in agreement with the estimates made by Stecker 
et al. (1974) using the observed Y-ray energy spectrum obtained by SAS-2. 
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6. Gamma-Ray Latitude Distribution and Associated Line of Sight Reddening. In 
addition to the Y-ray longitude distribution measurements reported by Fichtel 
et al. (1975), a galactic latitude distribution was also obtained. This 
distribution shows an asymmetry with respect to the galactic plane with more 
flux coming from positive galactic latitudes in the case of moderate latitudes 
~ :::-- tb I :::_ 30°. This can be understood in the context of section 4 where it 
was pointed out that UV, X-ray infrared absorption measurements indicate that the 
total column density of gas is proportional to the reddening in a given direction 
and that in the direction of highly reddened objects most of the hydrogen may well 
be in molecular form as expected in dense dust clouds and as is indicated by only 
a partial correlation with 21 cm emission. The data of Knapp and Kerr (1974) in 
the region of the sky located between i = 345° and 30° and between b = 10° and 3<f' 
ca.IL--1le used to evaluate the average reddening and HI column density at b = 200. 
We obtain <E8 _v(200)> = 0.26 mag and <NH(20°)> = 9.7 x 1020 H atoms cm-2. 
Postulating the same cosmic ray density as observed locally we find from the 
reddening and relation (2) a gamma-ray flux of 2.2 x lo-5 cm-2 s-1 sr-l which com-
pares favorably with the flux reported by Fichtel et al. (1975) observed at 
b = 2cP of 2 to 2.5 x lo-5 x lo-5 cm-2 s-1 sr-1 . However, the flux estimated from 
the HI-column density is only 1.4 x lo-5 cm-2 sec-l sr-1 which is particularly 
significant in this case because at high galactic latitudes the 21 cm emission line 
is not optically thick (Knapp and Kerr 1974). Thus, the situation with regard to 
the latitude distribution of galactic gamma-rays is analogous to that of the 
longitude distribution and can be better understood by taking account of H2 unseen 
in 21 cm surveys of HI gas. 
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© MPE • Provided by the NASA Astrophysics Data System 


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Molecular hydrogen in the galaxy and galactic gamma rays

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