Partial surveys in the far infrared in the Northern and Southern Hemispheres have covered 40% of the galactic equator and assorted regions away from the galactic plane. Approximately 120 100-micron objects are known. These are distributed extensively in galactic longitude and concentrated within + or - two degrees in galactic latitude. From this information, some general conclusions can be drawn about the sensitivity and coverage required for a general sky survey in the far infrared

Aannestad, P. A.

Hoffmann, W. F.

English

NASA Technical Reports Server

N75 12 "7 3 7
PAPER 1.2
100 MICRON SURVEYS IN THE NORTHERN AND SOUTHERN HEMISPHERES*
William F. Hoffmann
Per A. Aannestad**
Steward Observatory
University of Arizona
ABSTRACT
Partial surveys in the far infrared in the Northern and Southern Hemi-
spheres have covered 40% of the galactic equator and assorted regions away
from the galactic plane. Approximately 120 100-micron objects are known.
These are distributed extensively in galactic longitude and concentrated
within ± two degrees in galactic latitude. From this information, some
general conclusions can be drawn about the sensitivity and coverage required
for a general sky survey in the far infrared.
INTRODUCTION
As new techniques of detection open up new parts of the spectrum to
astronomical observation, a survey of the sky at the new wavelength is ap-
propriate and valuable. Sometimes, although not always, this is a means
for making unexpected new discoveries. In general, such surveys are of
value for providing a general picture of the sky at the new wave length,
making possible classification and statistical analysis, and providing a
guide to detailed studies.
In the infrared and submillimeter part of the spectrum, there are the
2.2 micron Cal Tech sky survey (Neugebauer and Leighton 1969) and the 10 mi-
cron Air Force Cambridge Research Laboratory rocket survey (Walker and Price
1974). The shortest wavelength continuum radio survey of the galactic plane
are the surveys at 6 cm of Altenhoff et al (1970) and Goss and Shaver (1970).
The shortest wavelength radio survey of substantial portion of the entire
sky is at 21 cm. Thus, there is a substantial gap in our systematic know-
ledge of the sky between the wavelengths of ten microns and 6 cm (60,000
microns).
The phenomena which dominate the emission processes over this large
spectral range vary sufficiently so that knowledge gained in the infrared
at 10 microns or in the radio region at 6 cm does not provide much of a
clue as to the objects to be found at the intermediate wave length of 100
microns. The stellar photospheres and circumstellar dust envelopes which'
dominate the 10 micron sky provide little contribution at 100 microns. Sim-
ilarly, the hot ionized gas which accounts for the thermal radio-continuum
emission in HII regions provides very little flux in the far infrared. On
the other hand, interstellar dust clouds, dust in HII regions, and dust in
galactic nucleii, because of its low temperatures, emits thermally primarily
in the far infrared.
For this reason, a balloon-borne far infrared astronomy program was
initiated at Goddard Institute for Space Studies and is now being continued
*Much of this work was carried out at the Goddard Institute for Space
Studies. It is currently partially supported by NASA grant NCR 03-002-371
at Steward Observatory.
**P.A.A. gratefully acknowledges support from the Smithsonian Astro-
physical Observatory.
.1.2-1
7
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1.2-2
8
at the Steward Observatory In order to survey the sky at 100 microns. This
survey is not complete. In this paper we would like to discuss what has
been observed, what can be said statistically about the spatial and bright-
ness distribution of known sources, and what level of performance is re-
quired for a more detailed and more complete survey. While the largest
number of sources analyzed has come from the Goddard/Arizona balloon sur-
veys, we have attempted to include a number of other objects not observed
in that program but reported by other groups. The sources for the data
analyzed are: Harper and Low (1971), Hoffmann, Frederick, and Emery (1971),
Soifer, Pipher, and Houck (1972), Emerson, Jennings, and Moorewood (1973a),
Emerson, Jennings, and Moorewood (1973b), Harper and Low (1973), Olthof
(1974), and Hoffmann, Frederick, Emery, and Aannestad (1974).
OBSERVATIONS
Figure 1 shows the regions of the sky that have been surveyed in the
balloon experiment. The *'s give the positions of 100 micron sources ob-
served in the balloon survey. The open circles give additional sources ob-
served in other experiments. 140° or 40% of the galactic plane has been
covered in a band varying from 4° to 8° wide. 700 sq. degrees of sky along
the plane and 250 sq. degrees elsewhere have been surveyed. This represents
about 2.4% of the entire celestial sphere. Some of the interesting areas
are: to the extreme left, the Orion infrared nebula; at RA 45 min DEC-260,
the galaxy NGC 253; at 20 hrs. 40 min., + 42° the Cygnus region; a stretch
centered on 19 hours including W51, W49, and M17; another stretch centered
on the galactic center at 18 hours including M8 and the galactic center
complex; in the region of 16 hrs. 30 min., - 20 , dark clouds in the Ophiu-
chus region; centered on 13 hrs., - 63° the Coal Sac Nebula; near 13 hrs.
20 min., - 43° the Centaurus A region; at 11 hours, - 59° the Carina Nebula;
and at 10 hrs., + 70 , the galaxy M82. As can be seen from Figure 1, this
is a patchy and highly selected sample of the sky.
With some caution to avoid extracting more than is present from this
limited sample, we have attempted to learn something about the distribution
of the known 100 micron objects in galactic coordinates. Figure 2 shows a
histogram of the sources in galactic longitude. The lower part gives the
number of sources in 10 blocks of longitude. Since this distribution can
be heavily biased by the spottiness of the sample of the galactic plane, we
have shown in the upper part a histogram indicating the regions of the
galactic longitude which have been scanned. The full value indicates that
the scans cover 4 or more across the galactic plane. The half value
indicates a narrower region less than 4° wide. The lower part of Figure 2
shows a slight concentration of sources toward the galactic center, although
this concentration is not nearly as much as might have been expected. In
addition, since the region between 320 and 345 degrees galactic longitude
has not been covered in the Goddard surveys, this apparent peaking may be a
selection effect. The results of Ricker, Lewen, and Low (1974), when avail-
able, should partially cover this region. The relatively small number of
bright sources between M17 and the Cygnus region, however, does appear to be
a real effect rather than an observational selection effect. Although there
remains a substantial gap in the data along the galactic equator in the
northern hemisphere, it does appear that the preponderance of sources are
in the southern hemisphere. From left to right, the main peaks occur in
the areas of the Carina Nebula, the Coal Sac Nebula, the Galactic Center,
and the Cygnus Region.
' 1.2-3
NUMBER OF OBJECTS' 105
REGIONS SCANNED (< OR* 4° WIDE)
D_ nyin.
100 MICRON SIGHTINGS
250 300 50 100 150 200
GALACTIC LONGITUDE I
Figure 2 - Lower portion: A histogram of lOOpi objects as a function of
galactic longitude. Upper portion: A diagram indicating the regions in
galactic longitude which have been scanned. The full value indicates that
the scans covered 4° or more in the direction normal to the galactic plane,
the half value corresponds to scans less than 4 wide.
Figure 3 gives a histogram in galactic latitude with intervals of 1
from the galactic equator. As in figure 2, the upper part indicates the
distribution of scans in galactic latitude. The three levels represent
the portion of scans at various widths. Approximately half were 6 wide
or wider. Of the remaining, most were at least 4 wide and a small number
were as small as 3° in width. The histogram in the lower half is substan-
tially more concentrated toward zero galactic latitude than the distribu-
tion of scan widths. We, therefore, conclude that the distribution of 100
micron sources is very much concentrated within ±2 or less of the galac-
tic equator. For the sample over a 6° wide band, 62% are within ± 1° galac-
tic latitude and 93% are within ± 2°. While the fall-off at the edge of
this histogram is biased by the finite width sample, we conclude from this
that the far infrared sources within 3° of the galactic equator are sub-
stantially concentrated toward the equator with over half within ±1 of it.
From the present survey information, it is not possible to tell what the
wings of the distribution look like with increasing galactic latitude.
1.2-4
10
SCAN WIDTH NUMBER OF OBJECTS « 97
100 MICRON OBJECTS
-10 -2 0 +2
GALACTIC LATITUDE b1
Figure 3 - Lower portion: A histogram of lOOu. objects as a function of
galactic latitude. Upper portion: A diagram indicating the portion of
scans of various widths across the galactic plane. See text for details.
It is also interesting to analyze the known sources in terms of their
brightness (flux density), as a guide to the sensitivity that should be
achieved in a more extensive survey. Figure 4 shows a histogram of the
sources as a function of 100 micron flux density in units of 10"22 W/m Hz.
The intervals are logarithmic. The rapid falloff for the faint sources is
representative of the sensitivity limit of the apparatus near 1 x 10~22
W/m2Hz. The steepness of the rise in numbers from the brightest sources
toward weaker ones is much slower than one would expect from a group of
sources of identical intrinsic brightness uniformly distributed in volume.
This is consistent with the picture that the bulk of the sources are neither
solar neighborhood sources nor extragalactic sources but are primarily con-
fined to the Milky Way and hence, constrained to a limited range of dis-
tance. It appears from Figure 4, that as the sensitivity of a survey is
increased from 10,000 flux units (1 flux unit = 10"26 W/m2Hz) to 1,000 flux
units, the number of sources along the plane should increase substantially.
As the sensitivity is further increased to 100 flux units, one would expect
a continuous mapping of 100 micron emission along the galactic plane simi-
lar to the radio maps where the "sources" are peaks in a continuous distri-
bution flux.
1.2-5
11
NUMBER OF SOURCES' III
1 2 1
13
33
27
23
5
.1 .22 .46 I 2.2 4.6 10 22
100 MICRON FLUX DENSITY (K)22 W/m2 Hz)
46 100
Figure 4 - A histogram of lOOjj, objects as a function of their lOOp. flux
density.
FUTURE SURVEYS
Two balloon programs now being carried out to substantially improve the
sensitivity of survey type instruments are the 8-inch low emissivity off-
axis Hershellian telescope of Frank Low, at the University of Arizona, and
the Cornell/Arizona 16-inch cryogenically cooled telescope. Both of these
instruments are aimed at high throughput (product of solid angle in sky and
telescope aperture) with the thermal emission of the instrument reduced to,
or below, the atmospheric emissivity in order to make possible the use of
more sensitive detectors optimized for very low background flux. These
two instruments are described elsewhere in these proceedings. In order to
provide a set of numbers from which to start, we have considered two pos-
sible survey objectives, (a) a survey of the galactic plane, and (b) a
survey of most of the celestial sphere. The first goal is compatible with
a number of short duration balloon flights. The entire celestial sphere
survey is ideally suited to a long duration super pressure balloon flight.
Table 1 gives a set of parameters for the short duration balloon flights.
Some of these are taken from our experience with the current Goddard Insti-
tute for Space Studies/Arizona balloon gondola. It would appear that with
reasonable scan rates and overlapping coverage, the galactic plane can be
covered redundantly in five flights. The resolution given is 3 arc minutes.
The location of all but the faintest sources should be obtained to approxi-
mately 1 arc minute. While this can be achieved with a rather larger beam
by careful determination of the center point of the scan signal, large beams
(greater than 12 min. of arc) will probably suffer substantially from source
confusion at these high sensitivity levels along the galactic plane. The
integration time of 0.12 seconds is derived from the 3 min. resolution and
the 48 hours of flight. While this resolution is required for adequately
1.2-6
12
determining the center of sources to 1 arc minute, very faint sources can
be analyzed at lower resolution to take advantage of a larger beam size.
For this reason, we have shown sensitivity figures for both 3 arc minute
resolution and 12 minute resolution. The latter is 1 x 10"^  W/m Hz. The
detector NEP assumed, 10 W/V"Hz appears to be the best that is currently
available.
TABLE 1
SHORT DURATION BALLOON FLIGHTS
o
Coverage: Galactic Plane 3600 deg
Rate: 75 deg /hr (48 hours)
Number Flights: 5
Resolution: 3'
Integration Time: .12 sec
2
Telescope Area: .13m (16")
Wavelength: 100^
Detector NEP: 10 Watt/VHz
Overall Efficiency: 10%
Sensivity (3<?) in 3': 4 x 10"24 W/m2Hz
Sensivity in 12': 1 x 10"24 W/m2Hz
A survey of the entire celestial sphere would be aimed at discovering
extragalatic sources of far infrared radiation. As such, to be of use,
it would have to be even more sensitive than a survey of the galactic
plane and, at the same time, cover a substantially larger area. Assuming
the same detector sensitivity, this objective can be realized by only more
observing hours, hence a long duration balloon flight. In table 2, we
have assumed a two-week flight with an ambient temperature low-emissivity
telescope of 25 centimer diameter (and a liquid helium-cooled detector).'
This system offers a sensitivity of only 1 x 10-23 w/m2Hz (1,000 flux units)
at the 3 arc minute resolution. At 12 minutes, the sensitivity is 250 flux
units. If the throughput is increased sufficiently to provide a 50' (1 )
beam, the sensitivity could be as good as 50 flux units. Since the density
of infrared objects at high galactic altitudes is expected to be substan-
tially less than that along the galactic plane, the problem of source con-
fusion should be much less than along the galactic plane. For this reason,
a compromise favoring greater throughput and sensitivity at the expense of
resolution and position accuracy would probably be desirable.
1.2-7
13
Coverage
Area;
Resolution:
Data Points:
Time:
Integration Time:
Telescope Area:
Wavelength:
Detector NEP:
Overall Efficiency:
Sensivity in 3' (3a):
Sensivity in 12':
Sensivity in 60':
TABLE 2
LONG DURATION BALLOON FLIGHT
Declination +30° to -90°
31000 deg2
3'
12 x 106
2 weeks = 1.2 x 10 seconds
0.1 second
2
.05 m (25 cm diameter)
(80u. - 120u.)
10 Watts/V"Hz
10%
1 x 10"23 W/m2Hz
2.5 x 10" 24 W/m2Hz
5 x 10"25 W/m2Hz
CONCLUSION
With a little over one hundred objects known from far infrared sky
surveys to date, some characteristics of these objects in the Galaxy are
beginning to become known. Two extragalactic objects have been measured
in directed (non-survey) efforts (Harper and Low 1973). It is now possible
to define with reasonable confidence the surveys required for detailed
mapping of the galactic plane and for an all-sky survey for galactic and
extragalactic objects.
REFERENCES
Altenhoff, W. J., Downes, D., Goad, L., Maxwell, A., and Rinehart, R.,
1970. Astr. and Ap. Suppl., 1: 319.
Emerson, J. P., Jennings, R. E. and Moorwood, A. F. M., 1973a. Nature
(Phys. Sci.) 241: 108.
Emerson, J. P., Jennings, R. E., and Moorwood, A. F. M., 1973b. Ap. J.
184: 401.
Goss, W. M., and Shaver, P. A., 1970. Au. J. Phys., Ap. Suppl., 14: 1.
Harper, D. A., and Low, F. J. , 1971. Ap. J. (Letters), 165: L9.
Harper, D. A., and Low, F. J., 1973. Ap. J. (Letters),182: L89.
Hoffmann, W. F., Frederick, C. L., and Emery, R. J., 1971. Ap. J. (Letters),
170: L89.
Hoffmann, W. F., Frederick, C. L., Emery, R. J., and Aannestad, P. A., 1974.
To be published.
1.2-8
14
Neugebauer, G., and Leighton, R. B., 1969. NASA SP-3047
Olthof, H. 1974. To be published.
Ricker, G., Lewen, W., and Low, F. J., 1974. To be published.
Soifer, B. T., Pipher, J. L., and Houck, J. R., 1972. Ap. J., 177: 315.
Walker, R. G., and Price, S. D., 1974. To be published.
1.2-9
15
V
f
DISCUSSION SUMMARY - PAPER 1.2
It was pointed out from the floor that a collaborative investigation
involving scientists from the University of Arizona and from MIT had produced
a partial survey of the Milky Way. This investigation used a system with a
field-of-view of 3/4 of a degree and a sensitivity threshold of 700 to 1000
flux units at 100 microns. With this system the Milky Way appears as a bright
band several degrees wide. Observations were made from south of the galactic
center northward to W3. It is not known if this is radiation from a continu-
ous emitting volume such as the interstellar dust, or if it is from unresolved
discrete sources.
This was observed,as well by Dr. Hoffmann when surveying the galactic
plane in the vicinity of the galactic center.
The discussion continued with consideration given to the relative merits
of low resolution and medium resolution surveys. Higher resolution surveys
of the galactic plane are in order since the initial low resolution surveys
have been performed. A survey of the entire sky can be done adequately with
a field-of-view of 12 to 15 arcminutes if the number of sources expected can
be resolved on the average with this field-of-view.
It was asked if any evidence had been detected of water vapor associated
with the balloon. None had.
1.2-10
16


The 100 micron surveys in the Northern and Southern Hemispheres

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