The bulk of the diffuse gamma-ray flux comes from cosmic ray interactions in the interstellar medium. A knowledge of the large scale spatial distribution of the Galactic gamma-rays and the cosmic rays enables the distribution of the target gas to be examined. An approach of this type is used here to estimate the total mass of the molecular gas in the galaxy. It is shown to be much less than that previously derived, viz., approximately 6 x 10 to the 8th power solar masses within the solar radius as against approximately 3 x 10 to the 9th power based on 2.6 mm CO measurements

Bhat, C. L.

Mayer, C. J.

Wolfendale, A. W.

English

NASA Technical Reports Server

OG3.1-8
336
RELEVANCE OF COSMIC GAMMA RAYS TO THE MASS OF
GAS IN THE GALAXY
Bhat, C.L., Mayer, C.J., and Wolfendale, A.W.
Physics Department, University of Durham, Durham, U.K.
(CLB is on leave from Bhabha Atomic Research Centre,
Nuclear Research Laboratory, Srinagar, Kashmir)
ABSTRACT
The bulk of the diffuse y-ray flux comes from cosmic ray
interactions in the interstellar medium. A knowledge of
the large-scale spatial distribution of the Galactic y-rays
and the cosmic rays enables the distribution of the target
gas to be examined. An approach of this type is used here
to estimate the total mass of the molecular gas in the
Galaxy. It is shown to be much less than that previously
derived, viz., _ 6 x 108 solar masses within the solar
radius as against % 3 x 109 based on 2.6mm CO measure-
ments.
i. Introduction. Hitherto, the standard way of determining the mass
of the molecular gas (H2) in the Galaxy has been by way of the derivation
of the integral fTl2dv for the J = 1 . O transition of 12CO and its
subsequent conversion to the column density of the molecular hydrogen
N(H 2) through the relation N(H2) = eg0 fTl2dv, where the conversion
factor e20 is in units 1020atoms cm-_K-ikm-ls. Many workers have
employed this procedure, most notably Sanders et al. (1984, to be
denoted SSS). The constant value of 7.2 adopted by SSS for _20 over the
whole galaxy lead, to a mass of 3 x 109MQ for Galactocentric radii
R < i0 kpc, to be compared with = 1 x 109M 0 for the atomic hydrogen,
HI.
The actual value to be used for e20 has been the subject of a con-
siderable contemporary argument. Here we enter the fray by way of the
cosmic y-ray evidence.
2. The method and the results theref_-om. The y-ray method relies on
(i) a demonstration that cosmic ray particles can penetrate the bulk of
the gas in the interstellar medium (ISM), and (ii) a reliable knowledge
of the local y-ray emissivity of gas locally. The first has recently
been shown to be valid by Houston and Wolfendale (1984) in the case of
the local Orion molecular clouds, and should as such hold generally,
since the Orion clouds appear to be representative of most other H 2
clouds in the Galaxy.
Referring specifically to the Orion clouds, we assume that the
cosmic ray intensity is the same as that locally. We further take the
corresponding y-ray emissivity to be identical with that estimated for
high Galactic latitudes (Ibl>10°). The result (Bhat et al., 1985) is
that we find a value for e20 at R _ i0 kpc which is only 40-50% of
that adopted by SSS.
3.1-8 
3  
EVANCE  SMIC G A S   S O  
 I  T  XY 
hat, .L., ayer, .J., and olfendale, . . 
ysics epartment, u iversity f urham, rham, . . 
(CLB is on leave fro  habha tomic esearch entre, 
uclear esearch aboratory, rinagar, ashmir) 
STRACT 
The bulk of the diffuse y-ray flux co es fro  cos ic ray 
i teractions i  t e i terstellar edium.  knowledge f 
the large-scale spatial distribution of the alactic y-rays 
and the cos ic rays enables the istribution of the tar et 
s t   amined.  proach f t is t e i  sed re 
t  esti ate t e t tal ass f t e olecular as i  t e 
alaxy. It is sho n to be uch less than that previously 
rived, iz., ~   8 lar a ses ithin  lar 
ius  ainst ~   9 sed  m  easure-
ents. 
1. Introduction. itherto, the standard ay of determining the ass 
of the olecular gas (H ) in the Galaxy has been by way of the derivation 
f t e i tegral JrT12dv f r t  J   + 0 t nsition f O d  
subsequent conversion to the column densit1 of the olecular hydrogen N(H ) through the relation N(H 2 ) = u~O [  2dv , where the conversion 
factor u20 is in units 1020atoms c - K-lkm-ls. any orkers have 
e ployed t is rocedure, ost otably anders t l. ( 84, t  e 
denoted SSS). The constant value of 7.2 adopted by SSS for u20 over the 
whole galaxy leadtto a mass of 3 x 109MQ for Galactocentric radii 
R < 10 kpc, to be compared with ~ 1 x 109MO for the atomic hydrogen, 
I. 
The actual value to be used for u20 has been the subject of a con-
si erable contemporary argument. ere e enter t e fray by ay f t e 
smic -ray vidence. 
. he ethod and t e results t erefl'om. he -ray ethod r lies on 
(i) a emonstration t at smic r y articles n enetrate t e ulk f 
t e gas i  t e i terstellar edium (.ISM) , and (ii) a reliable knowledge 
of the local y-ray emissivity of gas locally. The first has recently 
en s own t  e alid y ouston d olfendale ( 984) i  t e ase f 
t e l cal rion olecular clouds, and should as such old enerally, 
since the Orion clouds appear to be representative of most other H2 
clouds i  e alaxy. 
eferring specifically t  t e rion clouds, e assume t at t e 
cosmic ray i tensity is the sa e as t at l cally. e further take t e 
o respo ding -ray missivity t  e i entical ith t at stimated f r 
igh lactic l titudes (l l lOO). he result ( hat t l., 1985) i  
that we find a value for u20 at R ~ 10 kpc hich is only 40-50% of 
t at adopted y S. 
https://ntrs.nasa.gov/search.jsp?R=19850025683 2020-03-20T18:06:10+00:00Z
3 3 7 OG3.1-8
HI H2 Furthermore, we argue that
_(Mep62} there is clear evidence for a
cosmic-ray (CR) electron
_0 _ gradient in the inner Galaxy.
30 _-- IRangequ_ed These electrons are responsible
20 "___T_._.__ S for y-rays in the SAS II energy
10 -16 range 35-i00 MeV and the (m_o_i-fi_)
5 :M electron intensity is found to
3 _____/needed be a factor _ 2.3 higher at _C _&_
2 Mr [R Ks R % I0 kpc. The consequence
of this on the surface density
I o of gas in the inner Galaxy
(Fig. I) is that it is a factor
Fig. i. Surface densities of % 5 lower than that given by
HI and H2 at R=6 kpc. The range SSS using a constant e20 % 7.2.
of quoted values from CO obs-
ervations (using various e values) As is indicated in Fig. i,
is indicated, as is the value from the reduced value of a(R=6kpc)
SSS. The value needed to satisfy also follows naturally by
the cosmic y-ray results is shown, another route, involving the
Also indicated is the effect of use of a metallicity correction
reducing the surface density of H2 factor M for deriving _20 in
by 0.37 times (to allow for the the inner Galaxy from its
change in _) and by 0.5 (to allow local value (Bhat et al.
for metallicity). 1985).
There is also a wealth of other supplementary data involving ab-
sorption studies at the X-ray and infra-red wavelengths and virial mass
estimates of molecular clouds, which we have used and which we find to
be consistent with our new estimates of e20,both locally as also in the
inner Galaxy.
3. Conclusions. Figure 2 gives our results on the distribution of H 2
1000 mass as a function of R.
- Here we have used various
_(R) methods for determining e20,
100 and the Galactic centre region
(MoPe"2) 1 .j_2(SSS) has been treated separatel_.
We find H2 masses _ 2 × I0 MQ
10 _. for the Galactic Centre re_ien
(R<0.5 kpc) and _ 6.5 x IO°MQ
I for the inner Galaxy (R<IO kpc).
0 :Z & I_ 8 _ i2 _ References
R(kDc) Bhat, C.L. et al. (1985), Nature
Fig. 2. Dependence of H2 and HI 314, 511.
on R. SSS denotes a widely quoted Houston, B.P., and Wolfendale,
estimate from CO alone, by Sanders A.W. (1984), J. Phys. G., ii,
et al. (1984). 407.
Sanders, D.B., Solomon, P.M.,
and Scoville, N.Z. (1984), Astrophys. J., 276, 182.
 
I 2 
o -2 (M0pC I 
40 
30 I }. Range quoted 
20 10/.--- -SS  
10 
 1M 
=r=..  
~needed 
 for CR ¥s 
Fig. 1. Surface densities of 
HI and H2 at R=6 kpc. The range 
f oted l es   s-
r ations ( si  ri us a l es) 
is indicated, as is the value fro  
S. he l e eeded t  tisfy 
the cos ic y-ray results is sho n. 
lso i icated is t e ffect f 
reducing the surface density of H2 
by 0.37 ti es (to allo  for the 
ange  a  d  .   l  
f r etallicity) . 
OG3.1-8 
Furthermore, we argue that 
there is clear evidence for a 
s ic-ray (C ) l tr n 
gradient in the inner Galaxy. 
These electrons are responsible 
for y-rays in the S S II energy 
range 35-100 MeV and the (m.e-alel- fi t-) 
electron intensity is found to 
be a factor'\, 2.3 higher at b Pore fh~n 
R '\, 10 kpc. The consequence 
f t is on the surface density 
a of gas in the inner alaxy 
( i . 1) i  t t it i  a f t r 
'\, 5 lo er than that given by 
SSS using a constant a20 '\, 7.2. 
s is i icated i  i . 1, 
the reduced value of a (R=6kpc) 
als  f ll s aturally by 
another route, involving the 
use of a etallicity correction 
factor M for deriving a20 in 
the inner alaxy fro  its 
cal l e at t l. 
85) . 
There is also a ealth of other supplementary data involving ab-
s rption st dies t t e -ray and i fra-red avelengths and i rial ass 
sti ates f olecular l uds, hich e ave sed a d hich e fi  t  
be consistent with our new estimates of a20,both locally as also in the 
er alaxy. 
3. Conclusions. Figure 2 gives our results on the distribution of H2 
 ass s  nction f . 
o 2 4 6  10 1  14 
( p l 
~ ere e ave sed arious 
methods for determining a20' 
and the alactic centre region 
has been treated separate17. We find H2 masses '\, 2 y. 10 ~ 
for the Galactic Centre re2ion 
(R<O.S kpc) and'\, 6.5 x 10 Mg 
for the inner Galaxy (R<lO kpc) . 
References 
Bhat, C.L. et al. (1985), Nature 
314, 511. 
Houston, B.P., and Wolfendale, 
A.W. (1984), J. Phys. G., 11, 
407. 
Fig. 2. Dependence of H2 and HI 
on . SSS denotes a idely quoted 
sti ate fr   l ne, y anders 
t l. 984). 
Sanders, .B., Solomon, P. ., 
and coville, .Z. ( 984), strophys. ., 76, 82. 


Relevance of cosmic gamma rays to the mass of gas in the galaxy

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