Smithsonian

Contributions to Astrophysics

VOLUME 5, NUMBER 13

NEUTRAL HYDROGEN BETWEEN

GALACTIC LONGITUDES 200° AND 265°

by R. J. DAVIS

am

SMITHSONIAN INSTITUTION

Washington, D.C.

1962
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For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington 25, D.C. - Price 25 cents
Neutral Hydrogen Between Galactic

Longitudes 200° and 265°

By R. J. Davisx

This paper presents the results of an observational program conducted with the George R.

Agassiz radio telescope of Harvard College

Observatory. The purpose of the program was

to determine which OB associations between

galactic longitudes 200° and 265° might contain

sufficient neutral hydrogen gas to be effectively

studied with 21-centimeter techniques. The

present survey covers this entire strip of galactic longitude obtained by taking frequency

scans on rather widely spaced centers between

galactic latitudes +15° and —15°, with additional closely spaced centers covering five OB

associations and the extremely young cluster

NGC 2362. Table 1 lists these centers. Table

2 lists the characteristics of these and similar

optical features in this region.

This paper uses throughout the new galactic

coordinate system as defined by Blaauw, Gum,

Pawsey, and Westerhout (1959). In addition,

it employs the nomenclature for OB associations

as defined by Alter et al. (1958, 1959, 1960),

the most extensive list of associations yet

published and continually kept up to date.

Unfortunately, no list of designations for these

associations is as yet either entirely authoritative or complete. Other authors, as indicated

in table 2, have used different designations for

the associations within this region.

In this region of the sky the primary features

of the spiral structure as seen in the 21-cm

radiation are the Orion Arm, in which the sun

and all features of table 2 (except Puppis II)

i Astrophysicist, Project Celescope, Smithsonian Astrophysical

Observatory, Cambridge, Massachusetts.

are located; and the Perseus Arm, in which

Puppis II lies. The Outer Arm, beyond the

Perseus Arm, is also evident on many of the

scans.

This region of the Milky Way was included

in two extensive 21-centimeter surveys of the

galactic plane: the Leiden survey (Muller and

Westerhout, 1957), and the Commonwealth

Scientific and Industrial Research Organization

(CSIRO) survey (Kerr, Hindman, and Gum,

1959). Both of these were conducted with less

beam width and bandwidth resolution than that

provided by Harvard's radio telescope. Certain

of the OB associations within this region have

already been studied elsewhere: Orion I, Monoceros I and Monoceros H by Menon (1955, 1958);

and Lambda Orionis by Wade (1957a). In addition, van Woerden, at Groningen, using the

higher beamwidth and bandwidth resolution of

the Dwingeloo 25-meter radio telescope, is now

making a much more detailed investigation of

Orion I.

For the Orion associations the present evidence favors a strong correlation between the

neutral hydrogen distribution and the optical

features; the detailed features, however, do not

appear to correlate well with one another.

The evidence for such a strong correlation was

less conclusive for Monoceros I and II. Since

throughout this entire region the continuous

radiation near 21 cm is extremely weak, its

effects can be neglected for each of the centers

of this study.

Table 1 lists the positions of the 90 centers

observed. Each observation consisted of two

frequency scans taken with the antenna track209
210 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

TABLE 1.—Coordinates of centers observed at 21 centimeters; epoch, 1958.0

Center

SO

SI

S2

S3

S4=A4

S5=A6

86

S7

88

S9

S10

Sll

S12

813

814

815

816

817

818

819

S2O=B1

821

822

823

824

S25

826

827

S28

829

830

831

832

S33

834

835

S36

837

838

839

840

841

842

843

844

S45

846

847

848

849

850

S51

852

a

h

20

5

7

7

6

6

6

5

5

6

7

7

7

6

6

6

5

7

7

7

7

6

6

6

8

7

7

7

7

6

6

8

8

8

7

7

7

6

9

8

8

8

7

7

7

9

9

8

8

8

10

9

9

m

38.96

16.00

19. 10

01.00

42.22

24.21

06.48

48.88

31.40

33.64

36.36

18.47

00.64

42.96

25.24

07.47

49.56

54.56

37.04

19.48

01. 72

43.72

25.44

06.80

14.44

57. 12

39.44

21.36

02.80

4a 64

23.88

36.88

19. 64

01.72

43.04

23.52

03.08

41.61

02.92

45.80

27.63

08.20

47.43

25. 17

01.28

33.88

17.23

59.00

38.97

16. 72

15.03

55. 77

38. 40

5

0

+ 42

+ 16

+ 15

+ 13

+ 11

+ 8

+ 6

+ 4

+ 1

+ 4

+ 6

+ 4

+ 2

- 0

- 2

— 4

- 6

— 1

- 4

- 6

- 8

-11

-13

-15

- 9

-12

-15

-17

-20

-22

-24

-17

-20

-23

-26

-28

-31

-33

-25

-28

-31

-34

-37

-39

-42

-32

-36

-39

-42

-45

-38

-42

-46

45.8

28.8

32.3

22. 1

06.5

47.3

24. 6

00.0

34 2

22.2

51.0

34 2

15.4

03.6

22.8

40.8

57.6

40.8

06.6

31.8

54.0

13.2

28.8

39.0

5a 2

37.2

12.0

41.4

04. 2

19.2

26.4

57.6

52. 2

40.2

20.4

50.4

10.2

16.8

29.4

43.8

49.2

45.0

2a 2

56.4

07.8

23.4

01.2

29.4

46.2

4a o

23.4

26.4

22.8

Coordinate

0

82.30

187. 13

202. 32

202. 32

202. 31

202. 31

202. 30

202. 29

202. 29

207. 31

212. 21

212. 24

212. 26

212. 29

212. 32

212. 34

212. 37

222. 13

222. 19

222. 25

222. 30

222. 34

222. 39

222. 44

232. 09

232. 17

232. 23

232. 30

232. 37

232. 44

232. 52

242. 04

242. 13

242. 22

242. 31

242. 40

242. 49

242. 57

251. 99

252. 10

252. 20

252. 30

252. 40

252. 51

252. 61

261. 96

262. 07

262. 18

262. 30

262. 41

272. 57

272. 03

272. 16

0

+ 0.69

-11.96

+ 13.51

+ 8.51

+ 3.51

- 1.49

- 6.49

-11.49

— 16.49

- 1.48

+ 13.52

+ 8.52

+ 3.52

- 1.48

- 6.48

-11.48

-16.48

+ 13.61

+ 8.61

+ 3.61

- 1.38

- 6.39

-11.39

-16.39

+ 13.74

+ & 7 4

+ 3.74

- 1.26

- 6.26

-11.26

-16.26

+13. 89

+ 8.89

+ 3.89

- 1. 11

— 6. 11

-11. 11

— 16. 11

+ 14 08

+ 9.09

+ 4 09

- 0.91

- 5.91

-10.91

-15.91

+ 14 29

+ 9.30

+ 4 30

- 0.70

- 5.69

+ 14 99

+ a 56

+ 4 5 4

o

50

154 9

170

170

170

170

170

170

170

175

180

180

180

180

180

180

180

190

190

190

190

190

190

190

200

200

200

200

200

200

200

210

210

210

210

210

210

210

220

220

220

220

220

220

220

230

230

230

230

230

240. 67

240

240

&

1

o

0

— 10.5

+ 15

+ 10

+ 5

0

- 5

-10

-15

0

+ 15

+ 10

+ 5

0

-10

-15

+ 15

+ 10

+ 5

0

- 5

-10

-15

+ 1 5

+ 10

+ 5

0

- 5

-10

-15

+ 15

+ 10

+ 5

0

- 5

-10

-15

+ 15

+ 10

+ 5

0

- 5

-10

-15

+ 15

+ 10

+ 5

0

- 5

+ 15.41

+ 10

+ 5
NEUTRAL HYDROGEN

TABLE 1.—Coordinates of centers observed at 21 centimeters; epoch, 1958.0—Continued

211

Center

S53

AO

Al

A2

A3

A4=S4

A5

A6=S5

A7

A8

BO

Bl = S20

B2

B3

B4

B5

B6

CO

Cl

C2

C3

C4

C5

C6

C7

C8

C9

DO

D l

D2

D3

D4

D5

D6

D7

D8

D9

D1O

D l l

D12

a

b

10

6

6

6

6

6

6

6

6

6

7

7

7

7

7

6

7

7

7

7

7

7

7

7

7

7

7

7

7

7

7

7

8

7

7

7

7

7

7

7

m

55.52

36.80

29.24

33.04

49.47

42.22

31.40

24 21

40.52

44 24

06.24

01.72

12.68

09. 12

01.92

58.32

10. 12

17.03

06.96

14.92

18.84

14.96

11. 04

07.08

03. 12

19. 17

15.24

47.00

38.84

49.29

43.48

37.64

00.38

52.80

41. 14

33.20

56.48

50.64

44 72

55.64

5

o

- 4 3

+ 10

+ 13

+ 12

+ 12

+ 11

+ 9

+ 8

+ 8

+ 6

- 1 0

- 8

- 9

- 1 0

- 1 1

- 1 1

- 1 2

- 2 4

- 2 4

- 2 3

- 2 4

- 2 5

- 2 5

- 2 6

- 2 6

- 2 5

- 2 7

- 2 8

- 2 5

- 2 6

- 2 7

- 2 8

- 2 6

- 2 7

- 2 9

- 3 0

- 2 9

- 3 0

- 3 0

- 3 1

09.0

25.2

57.6

11.4

01. 2

06.5

43.2

47.3

39.0

52.8

34 2

54 0

43.2

11.4

08.4

36.6

19.8

52.5

03.6

59.4

51.6

21.0

49.2

16.8

44 4

44 4

34 2

43.5

44 4

39.6

27.0

13.2

51.0

56.0

30.6

31.2

12.0

00.0

47.4

41.4

Coordinate

I

11

o

282. 73

202. 31

198. 30

200. 31

202. 28

202. 31

202. 31

202. 31

204 30

206. 30

224 31

222. 30

224 30

224 30

224 32

224 34

226. 31

238. 20

236. 41

237. 18

238. 38

238. 40

238. 42

238. 42

238. 45

239. 20

240. 42

244 82

241. 33

243. 30

243. 27

243. 35

244. 76

244. 80

244. 85

244 89

246. 30

246. 32

246. 34

248. 32

6"

o

+ 14 79

+ 2.04

+ 2.01

+ 2.04

+ 5.50

+ 3.51

+ 0.54

- 1.49

+ 2.04

+ 2.04

- 1. 13

- 1.38

+ 0.66

- 0.33

- 2.34

- 3.34

- 1.08

- 5.50

- 7. 18

- 5.54

- 5. 14

- 6. 15

- 7. 15

- 8. 15

- 9. 15

- 5.42

- 7. 11

- 1.54

- 1.60

- 0.06

- 1.56

- 3.06

+ 1.97

- 0.03

- 3.04

- 5.03

— 0.01

- 1.51

- 3.05

- 1.47

<•

o

250

170

166

168

170

170

170

170

172

174

192

190

192

192

192

192

194

205. 81

204

204 81

206

206

206

206

206

206. 81

208

212.5

209

211

211

211

212.5

212.5

212.5

212.5

214

214

214

216

»«

0

+ 15

+ 3.5

+ 3.5

+ 3.5

+ 7

+ 5

+ 2

0

+ 3.5

+ 3.5

+ 0.2

0

+ 2

+ 1

- 1

— 2

+ 0.2

- 4 36

- 6

- 4 36

— 4

- 5

- 6

- 7

- 8

- 4 36

- 6

- 0.5

- 0.5

+ 1

- 0.5

- 2

+ 3

+ 1

- 2

- 4

+ 1

- 0.5

- 2

- 0.5

ing the center, the receiver being scanned both

ways across the frequency of the hydrogen-line

signal. The resulting profile was the mean of

these two scans. Many of the observations

were repeated on a second day. Centers SO,

Si, and S8 were used as standards; each day's

observations were referred to S8; then S8 was

compared carefully with the other standards.

Center SO has been used as a standard at

Leiden, and Center Si by Wade.

Centers A0 through A8 are in the vicinity of

Monoceros I and Monoceros II; B0 through B6

in that of Canis Major I; CO through C9 in

that of NGC 2362; and DO through D12 in that

of Puppis I and Puppis II. Table 1 gives right

ascension and declination for 1958.0, as well as
212 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS vou 5

TABLE 2.—OB associations and similar

Feature Nomenclature

Alter et al.

(1958, 1959,

1960)

Cyg III

Cygl

Cyg IV

Cyg II

Lac I

Perl

Peril

Orill

Mon I

Oril

Mon II

CMal

NGC 2362

Pupil

Pup I

Veil

Carl

Dieter (1960)

I Cyg

II & III Cvg

VI Cyg

I Lac

II Ori (S)

II Ori (M)

II Mon

I Ori

I Mon

III Mon

ICMa?

Drake (1958)

h & x Per

Howard

(1958)

I Lac

Kassim (1958)

I Cyg

II & III Cyg

VI Cyg

IV Cyg

Matthews

(1956)

I Lac

II Per

I Ori

Menon

(1958b)

II Cyg

VI Cyg

I Lac

h & x Per

II Per

Ori II

NGC 2264

Ori I

NGC 2244

galactic longitude and latitude in both the old

and new coordinate systems. Between December 19, 1956, and June 4, 1959, I obtained 197

satisfactory observations on these 90 centers;

64 were on the secondary standard, Center S8.

Equipment and calibration

Wade (1957b) and Drake (1958) have described

the 60-foot antenna of the George R. Agassiz

radio telescope used for these observations.

This antenna has an efficiency of 55 percent.

Lilley (1955) has described the older radiometer used for all but the last 37 observations.

This instrument was a double-conversion superheterodyne receiver with automatic gain

control.

In the autumn of 1958 the radiometer was

entirely modernized. The local oscillator now

uses a 2C39B lighthouse tube. It is tuned to

oscillate 38.5 mc/sec above the signal frequency.

The first IF amplifier now accepts a pass band

between 34 and 39 mc/sec; the second uses a

pass band between 7 and 12 mc/sec. A 46-

mc/sec crystal-oscillator produces the necessary

mixing signal. A new set of filters enables the

operator to select the signal and comparison

filters independently of one another. There is

a new DC amplifier and comparator, as well as

a new 2-channel Sanborn hot-wire recorder.
NEUTRAL HYDROGEN 213

stellar groups discussed in this paper

Feature Nomenclature—

Continued

Morgan et al.

(1953)

ICyg

II & III Cyg

IV Cyg

I Lac

I Per

II Per

IOri

I Mon

K. H.

Schmidt

(1958)

ICyg

II Cyg

VI Cyg

IV Cyg

I Lac

I Per

II Per

II Ori

II Mon

I Ori

I Mon

I CMa

I Pup

II Pup

I Vel

I Car

I

71° to 75°

74 to 77

80

81 to 84

96 to 98

132 to 136

157 to 163

187 to 189

196

196 to 210

198 to 214

205 to 209

222 to 226

238

242 to 245

242 to 246

262 to 268

283 to 292

b

+ 2° to +4°

Oto +4

+ 2

- 7 to - 5

- 1 7 to -1 4

- 4 to —2

—20 to —12

- 4 to +1

- 1 2

- 2 to +2

- 2 5 to —13

- 3 to +1

- 3 to +1

- 6

- 4 to +2

- 1 to +2

- 2 to +2

-2t o +2

Distance

2260 pc

1930

1850

1000

460

2300

390

3240

600

550

500

1550

950

1450

4200

2500

1450

1100

21-cm study by

Kassim

Kassim

Kassim

Kassim

Matthews,

Howard

Drake

Matthews

Wade

Davis,

Dieter

Menon,

Westerhout,

van Woerden

Menon,

Davis,

Dieter

Davis,

Dieter

Davis,

Dieter

Davis

Davis

(Davis)

Optical features

NGC 6871

P Cyg; M29

<r Cyg

10 Lac

h & x Per

r Per, IC 348

(Cluster).

X Ori, H II region

NGC 2264

(Cluster), IC

2169? (Cluster).

Orion Nebula

(M42), and

many others.

NGC 2244

(Cluster), NGC

2237, 2238, 2239

(Nebulae)

Stromlo 2 (H II

region).

T C Ma, Stromlo

8 (H II region).

AQ Pup? (Cepheid

variable).

i) Car

Each observation consisted of a frequency

scan obtained with the antenna tracking on a

center; the radiometer was scanned by varying

the frequency of the first local oscillator. With

the old radiometer an AFC frequency of 5.3444

mc/sec corresponded to hydrogen at rest relative

to the observer. With the modernized instrument, used for the last 37 observations, 8.9056

mc/sec corresponds to this rest frequency.

I calibrated the intensity scale by comparing,

for 18 centers, my profiles with published

profiles taken at Leiden and at CSIRO. Using

a temperature scale higher by a factor of 1.28

than the Leiden scale, the CSIRO group obtained its intensity calibration from a fluorescent gas-discharge tube. Since both scales

represent an uncertainty of 20 percent, this

difference is not considered significant, even

though in making comparisons it must be taken

into account. Kerr, Hindman, and Gum (1959)

ascribe the difference to the reduction from

antenna temperature to brightness temperature.

As a further check on the calibration, I compared, for Center Si, my profiles with Wade's

(1957b, 1959). He tied in the Harvard temperature scale with that of Leiden by observing

the continuous radiation from Taurus A.

The advantage of calibration by the comparison method lies in the probable uniformity of

available results obtained at different institu-
214

SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

tions and in the greater ease of comparison with

subsequent observations. Until the absolute

calibration of 21-cm intensities is more feasible,

if all results are derived from a basis as nearly

uniform as possible the necessary corrections

will be easier to make.

The principal difficulties involved in such a

comparison stem from the differences in the

equipment used. The Leiden radiometer had a

bandwidth of 33 kc/sec between half-power

points, whereas at Harvard the bandwidth was

15 kc/sec. The Leiden profiles thus had lower

peaks and higher troughs. The integrated

intensity of the line, however, is not affected by

receiver bandwidth.

The Leiden 7.5-meter antenna at Kootwijk

had a beamwidth of 1?7 as compared with 0?8

for the Harvard 60-foot antenna. Complete

correction for antenna patterns requires more

information than is available from my observations. At any point in the galactic plane,

however, the entire profile will in general be

higher for the larger antenna. The lobe pattern of the antenna and the shape of the pass

band can greatly affect the shape of the profile.

Raimond and van Woerden (1959) compare

various Leiden observations of the center at

Z=82?30, b= +0?69 (table 3). These measurements indicate the effects of bandwidth and

beamwidth on peak intensities.

The CSIRO data, obtained from drift curves

with the 36-foot antenna near Sydney, are

presented in the form of velocity versus

galactic latitude curves of constant intensity.

These data also indicate that variations of

intensity with position will probably not cause

difficulty in the region between galactic longitudes 200° and 260°. Additional evidence of

TABLE 3.—Leiden standard center intensity

comparisons

Antenna

I Kootwijk 7.5-meter dish

Dwingeloo 25 meter dish

ii ii it

u << K

Bandwidth

(kc/sec)

33

33

16

7

3.5

Peak

intensity

(units,

by definition)

100

107.7

113.8

115.2

116.4

this fact is provided by the agreement of my

profiles with those of Leiden even for centers

differing somewhat in position.

The Harvard temperature scale is, therefore,

approximately equivalent to that used by the

Leiden group. However, primarily because of

the smaller side-lobe contribution of the 60-foot

antenna and the narrower bandwidth of the

Harvard radiometers, the Harvard profiles are

more nearly representative of the true intensity

profiles. The observational estimates of TK,

the representative mean kinetic temperature of

the hydrogen atoms responsible for the 21-cm

radiation, as given by M. Schmidt (1957) and

the Harvard observers, and as used by Oort,

Kerr, and Westerhout (1958), are all based

upon the Leiden temperature scale.2

Evidence of possible errors in that scale is

not yet quite ready for evaluation. The scale

was chosen for the present paper to permit

easier comparison with the results of other

investigators in the field. Muller and Westerhout (1957) believe the maximum error in the

scale to be ±20 percent.

All scans used a 15-kc/sec signal bandwidth,

a 2-mc/sec comparison bandwidth, and an

integrating time of 10 to 20 seconds. This

study used Center S8 (Z=202?29, 6=-16?49)

as a secondary standard for observation with a

single scan once each day. Except for the last

37 observations, when no such circuit was available, I used automatic gain control.

Muller and Westerhout (1957) discuss the

scale nonlinearities introduced by a radiometer, such as the Harvard radiometer prior to

late 1958, with a linear detector and automatic

gain-control. For such an instrument, the

recorder deflection h is given by

(1)

For a linear detector without automatic gain

control, used for the last 37 observations, h is

given by

(y (2)

In equations (1) and (2), HT is a constant determined by the sensitivity of the receiver; AT is

the intensity of the signal arriving at the radi-

• A further discussion of the parameter Tk Is given on pp. 216-222.
NEUTRAL HYDROGEN 215

ometer; and Tis the effective noise temperature

of the radiometer,

T=2(N'-l)T0. (3)

Here, N' is the noise figure of the radiometer,

and To is very nearly the ambient temperature

of the crystal mixer. For the Harvard radiometer, T=900° K, corresponding (for To=

280° K) to a noise figure of 4.2 db. Both h

and AT are, of course, functions of frequency.

Muller and Westerhout (1957) also discuss

the effects of atmospheric absorption and radiation upon the incoming 21-cm radiation.

They conclude on both theoretical and observational grounds, that

(4)

where Tt is the intensity of the 21-cm signal

beyond the earth's atmosphere; #=0.9915; and

F(z) is the air mass.

For observations taken within 5° of the

horizon, the effects of ground radiation upon

the AGC circuit become comparable to the

direct atmospheric effects, even in the case of

Harvard's 60-foot antenna. When this effect

is included, equation (4) becomes

(5)

Here, y is the efficiency at which the antenna

collects ground radiation when it is pointed at

the center under observation; this efficiency is

0.5 when the antenna is pointed exactly at the

apparent horizon, and 1.0 when depressed 5°

or more below.

For a linear detector without AGC, equations

(4) and (5) become

AT=Ttp

rw

. (6)

Westerhout (1957) discusses the effects of

bandwidth and beamwidth upon the observed

profiles. Using Eddington's approximation, he

derives the equation for bandwidth correction

as equivalent to

and his beamwidth correction is equivalent to

Ti' (6)=T; (6)- (° 2

4

(

2

^

) 2

 AT,' (6). (8)

(7)

Here, W, is the bandwidth of the receiver

(assumed to have a Gaussian bandpass) between

half-power points; v is the frequency; WA is

the beamwidth of the main lobe of the antenna

(also assumed Gaussian in shape) between halfpower points; b is galactic latitude; and A

and A2 represent first and second differences,

respectively.

Figure 1 shows the results of correcting the

profiles on three centers for the instrumental

effects listed above. These three centers are

S27 at Z=232?30, b= —1?26; S48 at Z=262?30,

b=— o?7O; and the center at Z=282?30, 6=

—0?30, which was observed by the CSIRO

group.

From figure 1 it is evident that in the case of

the Harvard radio telescope these corrections

can safely be neglected for those centers where

atmospheric extinction and ground effects are

unimportant, i.e., for all centers observed more

than 10° above the horizon. For Center S48,

however, the correction for atmospheric extinction alone amounts to 14 percent, and the total

correction for nonlinearity amounts to 20 percent. Center S52 at Z=272?16, 6=+4?54

(a=9b 36?40, 5= —46° 22:8 (1958.0)) is probably too near the horizon for reliable observation.

For the CSIRO observers Center S48 passes

nearly through the zenith. My profile, corrected by means of equations (1) through (8),

has very nearly the same integrated intensity

as theirs if one bears in mind the difference by

the factor of 1.28 between the CSIRO and the

Leiden temperature scales; my uncorrected profile has an integrated intensity 18 percent

smaller. The beamwidth correction is important here. Table 4 lists these integrated

intensities. Center S27 is included for comparison. In the last case the corrections amount to

less than the errors and can be neglected.

Observational results and analysis

Through use of the Leiden intensity scales, there

is plotted in figure 2 the mean intensity profile

for each center. Note that 1} and b1 rather than

F

1

 and bn are indicated in the figure.

833310—62 2
216 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

Center S27 Center S48

= 262°3O,Z>=-O?3O)

Davis

1OO°K. -

100 km/sec +100 km/sec

Sydney

0 +100 km/sec

FIGURE 1.—Corrections to profiles on Centers S27; S48; and /=282.°3O, b=±0.°30. Solid line, uncorrected profile; dash-dot line, profile corrected for intensity-scale nonlinearities (equations 1 and 5);

dotted line, profile also corrected for receiver bandwidth (equations 1, 5, and 7); broken line, profile

also corrected for antenna beamwidth (equations 1, 5, 7, and 8).

The profiles of Centers S47 through S53 have

been corrected for the effects of intensity scale

nonlinearity, bandwidth, beamwidth, atmospheric extinction, and horizon effects. These

effects are unimportant for this survey except

for Centers S48 through S53.

The observational results end with the derivation of the intensity profiles. In order to

derive the hydrogen densities from these profiles, we must compute the optical depths.

Heeschen (1955) gives the reduction formulas

for the case where a source of continuous radiation lies behind the source of line radiation; and

AT(v) = (10)

where a source of continuous radiation lies in

front of the source of line radiation. In both

cases, AT(y) is the measured intensity (in tern-
NEUTRAL HYDROGEN 217

perature units) at frequency v; TK is the kinetic

temperature of the interstellar hydrogen (assumed to be constant); Tc is the intensity of the

continuous radiation in temperature units; TC

is the optical depth in the continuum; and T,

is the optical depth in the line at frequency P.

The conversion from intensity units to temperature units is given by the Rayleigh-Jeans law,

(ii)

where c is the velocity of light and k is the

Boltzmann constant.

Westerhout (1958) shows that in this region

Tc and, therefore, TC are essentially zero, except

in the vicinity of discrete sources. The measurements of Denisse, Lequeux, and Le Roux (1957)

at 900 mc/sec substantiate this finding. No

discrete source lies within 1° of any of my

centers.

Since both Tc and TC can be neglected for

each center, inversion of either equation (9) or

equation (10) gives the reduction formula for

converting intensity profiles to optical depth:

(12)

Purcell and Field (1956) have shown that for

the conditions obtaining in the interstellar

medium it is correct to use TK in equations (9)

and (10), although these equations hold rigorously only when TK is replaced by Ts

, a spin

temperature defined by the relation

TABLE 4.—Integrated intensities of selected profiles

/ ko>\ (13)

where nt is the population of the upper (F=l)

level, and n0 is that of the lower (F=0); and

hca is the energy difference between the two.

Van de Hulst, Muller, and Oort (1954) show

that optical depth is connected with NB(V),

the number of hydrogen atoms per square

centimeter in the line of sight with radial

velocity V, by the equation

NB(V)

(1.835X10w)T*

(14)

Profile

Davis uncorrected

profile

Davis corrected

profile

Sydney corrected

profile

Sydney uncorrected

profile

Integrated intensity

(deg km sec"1)

S46

5104

6024

6200X1.28

5720X1.28

S25

4965

5200

4850X1.28

4700X1.28

The most difficult part of the analysis lies in

assigning the correct value to TK. The usual

method is to assume that TK is constant, and

that in the case of the strongest peak intensities

the line is saturated. M. Schmidt (1957) discusses various refinements by which one can

obtain TK from the observed peak intensities.

He agrees with van de Hulst, Muller, and Oort

(1954) in assigning the kinetic temperature of

the interstellar hydrogen, 2^= 125° K. We

must remember, of course, that this value is

based upon the Leiden temperature scale.

The greatest peak temperature (146° K)

found in this study occurs for Center S48 at

Z=262!30, b=— 0?70. This line is evidently

not yet saturated, although it may be approaching that state. This area may possibly be

characterized by hydrogen at a higher kinetic

temperature than is normal for the part of the

galaxy near the sun.

I agree with M. Schmidt (1957) in choosing

a value of TJC=125° K on the Leiden scale as

probably the most representative kinetic temperature for this region as a whole. In the Orion

Arm, the kinetic temperature may perhaps vary

from ^=120 ° K at Z=202° to 7*= 130° K

or 140° K at Z=262°. Such variations, however,

will not appreciably affect the reductions of

centers other than S27, S41, B4, B5, and B6,

where the peak intensity exceeds 100° K. Localized fluctuations of the type probably responsible for the high peak at Z=262° will

introduce greater errors. The best indication

of the kinetic temperature in the Perseus Arm

comes from the Center S27 at Z=232?30, b=

1?26. This profile is definitely not saturated.

The CSIRO observations near this center indi-
218 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

a a

•si

u e

i:-

M

IS

-i 8
NO

. 13 NEUTRAL HYDROGEN 21

9
220 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

Is

A . \. . A

b'-0

0 fcm/HC +80

i'-170*

I'-W

FIGURE 2.—Continued*
NEUTRAL HYDROGEN

C3

FIGURE 2.—Continued.

0 km/sec +100

221

t'-aw

/'-209*

100 "K.

03

0 km/sec +100

,/l ,
222 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL. S

cate that in the region the beam width correction

for Harvard's 60-foot antenna amounts to

about 1° K. Corrected for bandwidth and

beamwidth, the peak corresponding to the

Perseus Arm is 114° K and occurs at a radial

velocity of +51 km/sec.

Although 125° K is a fair estimate of the

kinetic temperature of the neutral hydrogen

throughout the region in both the Orion and

Perseus Arms, serious errors came from the

assumption that TK is constant. Davies (1956,

1958), discussing the observed departures from

this condition, finds indications that TK may

vary from perhaps 60° K to 250° K. Unfortunately, 21-cm data do not yet warrant for my

region an assumption more refined than TK—

125° K. On that assumption, therefore, all

corrected profiles have been reduced to optical

depth. These results appear in figure 5, showing monochromatic optical depths, T(V), as

functions of position, and giving T(F) at 5

km/sec intervals of radial velocity.

As Westerhout (1957) points out, the radial

component of random cloud velocities tends to

widen the 21-cm line. The root-mean-square

value of this component lies between 5 and 10

km/sec (Spitzer, 1954). Blaauw (1952) suggests

a velocity distribution of the form exp (— | V\rj),

basing this suggestion on observations of interstellar absorption lines. For the 21-cm observations, Westerhout (1957) and Davis (1957)

favor a Gaussian form, exp (—V2/2a2). The

optical depths can be corrected for random

cloud velocities by Eddington's second-order

approximation,

where A3T(V) is the second difference of the

optical depth as a function of radial velocity,

and e=c/AF2. If we take AF=4 km/sec and

«=2, we fairly well represent the exact resolving

function for a Gaussian distribution with <r=6

km/sec, or for a Blaauw distribution with 17=5.5

km/sec, provided that the peak-to-valley distances of the profiles are approximately 10 to

15 km/sec. This procedure corresponds almost

exactly to that used by Westerhout (1957).

However, when we set AF=2 km/sec and «=8,

this method causes considerable overcorrection.

The Harvard radiometer, with its 15-kc/sec

bandwidth, is capable of detecting details much

narrower than the assumed velocity distribution. As Westerhout points out, we should

use €=<T2/2AV2=4 when AF=2 km/sec.

This survey does not include correction for

the effects of random cloud velocities. In the

first place, the correction appears to be unstable; small errors in either the assumptions or the

observational data lead to large errors in the

results. Secondly, we can see just as clearly

the over-all galactic structure by plotting the

positions of the peaks of the major features.

We can obtain approximate hydrogen densities

for major features by integrating under the

observed curve between limiting velocities.

Since the quantitative derivation of space

densities for neutral hydrogen depends upon

the correction for random cloud velocities

(which are here omitted) and upon the rather

dangerous assumption that this hydrogen moves

at the circular velocity in accordance with a

prescribed model of galactic rotation, no attempt is made to determine quantitative values

for this space density. Westerhout (1957)

describes the procedures by which one may

derive these data.

Differential galactic rotation must be based

upon observational data. In the region of the

galactic system the relationship between rotational velocity and the distance from the galactic center has been determined primarily from

21-cm observations. Kwee, Muller, and Westerhout (1954) tabulate this relationship for the

galactic plane (z=0).

Optical data are useful mainly for examining

the region near the galactic plane within two

kiloparsecs of the sun. Away from the plane,

where interstellar absorption is not so strong,

this limit does not apply. The commonly

accepted value of the distance from the sun to

the galactic center is i?0=8.2 kpc. Baade

(1955) determined this value from the magnitude distribution of RR Lyrae variable stars

in a field near the galactic center. Baade

(1951) had earlier described in detail the method

he used. Allen (1955, p. 237) lists the same value

for Ro; it is the mean of five values published

in various journals between 1939 and 1952.

It has a greater mean error. However, the

value of Ro should not be considered as really

well established; it may easily be in error by as

much as 10 percent. Weaver (1954), for
NEUTRAL HYDROGEN 223

example, found from observations made on

Cepheid variables that R0=8.8 kpc.

The outer part of the galactic system is best

examined by means of galactic models. M.

Schmidt (1956) constructed a series of such

models, the final one of which was made both

to agree as far as possible with observational

data and to meet certain criteria of continuity

and equilibrium. Although his is the best yet

constructed, it should still be considered only

preliminary.

Figure 3 shows two relationships between

the angular velocity of rotation and the distance

from the galactic center R. The solid curve

(km/sec"1 kpc"1)

+20-

and

R

2=RZ+D*-2R0D cos I, (17)

+ 10

s

\ 10 kpc 20

-10

-20

Schmidt

Weaver

FIGURE 3.—Relationships between the angular velocity of

rotation « and distance from the galactic center R. Solid

curve, M. Schmidt (1956); broken curve, Weaver (1955).

is from Schmidt (1956); for R<CR0, this curve

was made to agree with that derived empirically by Kwee, Muller, and Westerhout (1954).

The dashed curve shows the empirical relationship between these two quantities as derived

by Weaver (1955) from observations of Cepheid

variables.

Once this relationship between &> and R is

assumed, the relationship between the radial

velocity V and the distance from the sun D

follows from the usual equations

V=R0 sin l(a—o?0) (16)

where I is the galactic longitude in the new

coordinate system, and the subscript 0 refers

to the sun's position in the galaxy.

Discussing some observational problems of

galactic structure, Weaver (1956) emphasizes

that none of the basic parameters is as yet

really well determined. Better data are necessary for a really good determination of Ro, «o>

the Oort A and B constants, etc.

Figure 4 plots the average positions of the

21-cm intensity peaks at 10° intervals of galactic longitude. It is based upon Schmidt's

(1956) model. Note that this figure shows Z1,

rather than P. Features A and B belong to

the Orion Arm, feature D to the Perseus Arm,

and feature E to the Outer Arm. Feature Cis

probably on the outer edge of the Orion Ann

but could represent an interarm feature. These

observations indicate that in the galactic system the spiral structure is trailing. This conclusion is not dependent upon the detailed

structure of the distance-scale assumptions; it

holds under any galactic rotation model in

which the three following requirements are met.

First, w must be decreasing with increasing

R for all R>R0; second, w must not be a function of direction from the galactic center (0);

and third, large interstellar neutral hydrogen

complexes must have no systematic motions

other than the circular velocity of rotation. The

first condition is certainly met. There is as

yet no adequate method of evaluating the

degree to which the second condition is met.

The third condition is very definitely not met.

Such absence of ideal requirements represents

the most serious limitations on establishing an

adequate distance scale for neutral hydrogen.

For observations away from the galactic

equator, there is a fourth consideration: « must

decrease with increasing distance from the galactic plane (2). Although this last condition

is actually much more restrictive than is

mathematically necessary, it is certainly met

to the required degree of accuracy.

The strongest 21-cm feature in the region

from J=190° to 290° (Bok's [1959] CarinaCygnus Arm, or the Orion Arm) may be distorted in figure 4 by departures of the hydrogen
224 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

velocity from the circular velocity of galactic

rotation. As plotted in figure 4, ten kilometers

per second near 1=270° would make a significant difference in the appearance of this arm,

which everywhere appears to be farther from

the galactic center than the sun is. Because

of departures of the hydrogen motion from the

assumed model, however, the profiles near

£=285° show the peak from this arm blended

with that from the inner arm.

As Baade (1958) points out, the structure of

spiral galaxies is not simple. Arms branch,

branches are lost, and even entire arms

sometimes disappear, only to reappear farther

on. His description of the Andromeda Galaxy

is very similar in many respects to that of our

own galaxy, as shown in figure 4.

These profiles show that Bok's (1959) Orion

Spur is a 3-dimensional phenomenon, coming out from the Carina-Cygnus Arm at an

angle to and lying considerably below the

galactic plane. The Vela Spur, however, lies

in the plane and seems to be a link between the

Carina-Cygnus Arm and an outer arm that is

probably a continuation of the Perseus Arm,

but that cannot definitely be followed through

the anticenter.

Discussion

Several investigators have observed a definite

concentration of neutral hydrogen near OB

associations, e.g., Matthews (1956) for Lacerta

I and Perseus II. Studying Lacerta I in more

detail, Howard (1957) found it to be surrounded

by an expanding H I region. Kassim (1958)

obtained a definite correlation between the distribution of OB stars and that of neutral

hydrogen in the Cygnus region, specifically

mentioning an expanding H I shell surrounding

Cygnus II, a branching of the Orion Arm at

Cygnus IV, and a concentration of hydrogen

in the vicinity of Cygnus I and III.

Menon (1955) studied associations Monoceros I and II. He found a branching of the

Orion Arm to occur at Monoceros II. At

Monoceros I, he observed a decrease in the

21-cm radiation, which he interpreted as due

to the combination of the atomic hydrogen

into molecules. Menon (1958) also found an

expanding H I shell about Orion I. Wade

(1957a) found a similar expanding shell about

l

I

=260°

A—

.

—•— ^

— • —

—-* .

Galactic

s Center

V

\ A^

Galactic

Rotation

X

mi

< 200° *

\

, 160°T

I

FIGURE 4.—Average positions of 21-cm intensity peaks at 10°

intervals of galactic longitude.

the grouping of extremely early-type stars

associated with Lambda Orionis. As pointed

out in table 2, Kassim's and Menon's nomenclatures differ from those used here.

Drake's (1958) study of the 21-cm radiation

in the vicinity of a number of galactic clusters

showed a very strong relationship between the

age for a cluster and the ratio of hydrogen

mass to stellar mass. The youngest clusters

he dealt with—h and x Persei (3X106 years)

and the Pleiades (150 and 106 years)—contained

the highest preponderance of interstellar neutral

hydrogen. He found somewhat more mass in

the interstellar hydrogen than in the stars.

He specifically mentions the need for studying

NGC 2362 as a further check on his conclusions.

Dieter (1960) used the Leiden results (Muller

and Westerhout, 1957) to study 40 OB associations. Her method was to plot intensity contours at discrete radial velocities in the vicinities of each of these associations. She found

neutral hydrogen to be connected with 31 of

them.
NEUTRAL HYDROGEN 225

Dieter's method has been used in this study

for examining associations Monoceros I, Canis

Major I, Puppis I and II, and the extremelyyoung cluster NGC 2362. Figure 5 plots,

at 5 km/sec intervals of radial velocity, contours

of equal T{V). It also gives the approximate

outlines of each of these optical features

(K. H. Schmidt, 1958). As pointed out in

table 2, both Dieter's and Schmidt's nomenclatures differ from mine.

The total amount of hydrogen in the line of

sight does not depend upon the corrections for

random and systematic cloud velocities, but

only upon the integral of optical depth over

radial velocity (still on the assumption, however, that kinetic temperature is constant).

Figure 6 shows the contours of equal NB, the

total number of hydrogen atoms per square

centimeter in the line of sight. This information was obtained from the relationship (see

Westerhout, 1957; Heeschen, 1955)

NB= (1.835 X1013 deg-1 sec cm3)TK

P r(V)dV. (18)

* / — oo

Table 5, constructed from figure 5, gives for

each of the indicated H I features the names

of the associated optical feature (if any), and

the radial velocity of its stars (F*); the radial

velocity of the neutral hydrogen (VH); the

distance to the feature (d); the approximate

size of the feature in velocity (AF), and angular

extent (r); the approximate mass of the excess

hydrogen {MH'); and the average excess hydrogen density p within the feature. The mass

MB' is, of course, only a very rough approximation, being in error even for the strongest concentrations by perhaps as much as a factor of

three.

At this point one might look for possible

differences between the H I regions connected

with known OB associations in position and

velocity, and those not so connected. It turns

out that the four densest H I regions are those

tentatively connected with Vela I, Canis Major

I, Puppis I, and Monoceros I. In addition,

the H I region tentatively connected with

Puppis II is considerably denser than other H I

regions of similar distance. As expected, the

more distant H I regions tend to be less dense

TABLE 5.—H I regions and OB associations

Feature

Vel I

CMal

NGC 2362

Pup I

Mon I

Mon II

Pupil

I

221°

262

212

225

248

239

245

203

226

207

207

241

232

222

242

261

235

244

245

224

b

-12°

i

- 1

0

- 4

- 5

0

- 1

j

0

+ 2

- 1

- 1

- 1

- 1

- 1

- 2

- 1

0

V*

km/

sec

+ 21

+ 21

+ 23

+ 60

kmvB/

sec

+ 5

+ 10

+ 15

+ 17

+ 20

+ 25

+ 25

+ 32

+ 35

+ 40

+ 45

+ 50

+ 60

+ 65

+ 70

+ 80

+ 85

+ 90

+ 95

AF

km/

sec

5

10

5

17.5

7.5

5

12.5

12.5

2.5

2.5

17.5

10

12. 5

10

7.5

7.5

5

10

22. 5

D (kilo

parsecs)

optical

1.45

0.95

1.45

2.5

0. 55

1. 4

4.2

gal.

rot.

0.2

0.9

1.0

0.8

1.3

1. 5

1.3

2.75

0. 9

4. 5

2.7

2. 8

5

4. 2

5. 3

5. 7

6

6.3

8.8

Im«

atoms cm~J

(km/sec)"1

0. 9X1O20

CO

0.9

2.3

1. 4

1.2

3.0

1. 8

0. 9

0. 9

- 0 . 5

2. 7

1. 4

0.9

1.8

0.9

- 0 . 5

1. 4

0.2

r

deg.

3. 5

3

2.5

2

2

0. 7

0. 8

2.5

0.25

0. 5

1. 1

1.2

2. 5

0. 8

3.5

3

1

1.2

0. 5

pc

12

47

44

33

45

18

35

24

4

27

52

59

218

59

324

298

104

132

77

Excess hydrogen

solar

masses

286

oo

-3600

18500

9050

790

19100

5420

14

710

-9850

38900

349000

13000

593000

252000

-11500

102000

11100

atoms

cm-*

7. 9

oo

-2.02

24 6

4, 74

6.48

21. 3

18.7

10. 5

1. 72

- 3 . 36

9. 05

1. 61

3. 03

0. 83

0. 45

- 0 . 49

2. 12

1. 16
226 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL. s

b" - 2 0 km/tee

nlPm/tee

- 1 5 km/s

MonlVi

1 0 » Ori I

- 1 0 km/sac

MorTI

+35 km/we

G©-.

MoMI

J - 1 0

2gQ° l " 270°

+40 km/*

210' Ori I 200°

_ l L _

+45 km/t«c

200"

+5 km/sec +50 km/we

+ 10 km, sec +55 km, sec

--1 O

280" j " 270° l " 270° 260° 250" 240° 230°

+15 km/we +60 km sec

10" Ori l(_200" 210° W 1 200"

t •• '

+65 km/Me
NEUTRAL HYDROGEN 227

+70 km/sec

230" 220"

+ 115 km, « •

MonI

Man II

210° Ori I 230-

Pup I

CM* I

MC

23S2

250" 240" 2XT

+ 120 km/sec

MonI

Mon I I

210"

 Or i

 I 200°

+80 km/sec + 125 km, *

Pup I

CM. I

2362

250" 240f 230"

niaa r

+85 km s.

CMal

+ 130 km, sec

MonI

Man II

21<T

 Ofl

 ' 200"

230- 220T

230" 220*

+95 lan/f«

MonI

Mon II

in* Ori I mo"

+100 km/sec

FIGURE 5.—Contours of equal optical

depth as a function of position, plotted

at 5 km/sec intervals of radial velocity.
228

SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL. 5

because of the larger linear size required to

make them stand out against the background.

A further difference between the associated

and the other H I regions is the velocity spread

of the feature AV. Those connected show a

considerably greater AV.

Note also that the H I region suggested as

associated with NGC 2362 is in all respects

more nearly similar to the unconnected than

to the H I regions with OB associations.

Table 5 also shows that the denser H I

regions are considerably larger than the associations with which they seem to be connected.

Thus, although there appears to be a statistical

correlation between H I regions and stellar

associations, most of the gas is contained outside the boundaries of the associations themselves. The H I region connected with NGC

2362 appears in this respect to be more closely

related to the optical feature.

The H I region tentatively associated with

Monoceros I may in reality belong to the

Perseus Arm. Although at this galactic longitude radial velocity is a poor indicator of distance, there is a slight indication that the +32

km/sec feature at Z=200° to 210°, which Dieter

(1960) lists as connected with Monoceros II, separates from the Orion Arm feature at 1=215°,

becoming the main Perseus Arm feature at +40

km/sec. At the latter velocity, Dieter lists

this feature as connected with Monoceros II.

These conclusions are based upon data derived from rather widely spaced centers.

Without more intensive investigation we should

not suggest a really strong connection between

the optical features and the neutral hydrogen

regions. The hydrogen complexes are considerably larger than the associations; the

agreement in position and velocity may arise

from the fact that both types of objects are

found primarily in spiral arms.

In addition to these correlations (and lacks

of correlation) between hydrogen distribution

and optical features, certain details of the

hydrogen distribution in this region are of

special interest. Figure 5 shows two large

areas where very little or no hydrogen is found

near V=0. Unless local hydrogen is moving

significantly relative to the local standard of

rest, it must be absent. And in one of these

same areas (from Z=237°, b=— 6° to Z=262°,

FIGURE 6.—Contours of equal NB, the total number of hydrogen atoms per square centimeter in the line of sight, as a

function of position.

b= +4°) the intensity minimum (which, on the

Schmidt model, corresponds to a distance of

about 10 kpc from the galactic center or 3 kpc

from the sun) is very pronounced, approaching

zero.

Peaks at Z=242° and 252°, b= + 14° (Centers S31 and S38) are somewhat higher than

those at the same longitudes, b= +9° . This

TABLE 6.—Recommendations for further study of hydrogen

in association with optical features

Feature

MonI

Monll

Oril

XOri

C Ma i

NGC

2362

fJ PUP I

1 Pupil

I Vela I

Stromlo

12a

and

12b

Excess

Hydrogen?

Possibly

No

Yes

Yes

Probably

Possibly

1f I Possi- 1

1

 bly

I J Possibly

No

Recommendations

More study needed, using

drift curves with 60-

foot or larger antenna.

Studied by Menon. Being studied at Leiden.

Studied in detail by

Menon. Being studied

at Leiden.

Studied in detail by

Wade.

More study needed, using

drift curves with 60-

foot or larger antenna.

More study needed, using

drift curves with 90-

foot or larger antenna.

f More study needed, using

1 drift curves with an1 tenna preferably larger

[ than 60 feet.
NEUTRAL HYDROGEN 229

excess may be associated with a new hydrogen

cloud discovered by Murray and McGee

(1958) in the Pyxis-Hydra region.

No evidence has appeared for association of

neutral hydrogen with Gum's (1955) H II

regions Stromlo 12a and 12b. He gives their

distance as 250 parsecs. Stromlo 12a lies in

one of my areas of very low local hydrogen,

whereas Stromlo 12b lies in an area of relatively

high local hydrogen.

These conclusions are summarized in table 6.

The amounts of hydrogen involved are probably

about an order of magnitude greater than the

mass of the average OB association.

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230 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

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Abstract

The George R. Agassiz radio telescope of Harvard College Observatory was used to obtain profiles of intensity

versus radial velocity at 88 centers in the 21-cm line of neutral hydrogen. These centers were separated by 10° in

galactic longitude between longitudes 200° and 265°, and by 5° in galactic latitudes between +15° and —15°, with

additional centers in the vicinities of OB associations Monoceros I and II, Cam's Major I, Puppis I, and Puppis II,

aod of the very young cluster NGC 2362. These observations indicate that there is probably an excess of neutral

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There is probably no neutral hydrogen associated with Monoceros II. The observations also confirm the conclusions

of previous investigators that the spiral arms in our galaxy are trailing.