Most cirrus point sources are associated with interstellar gas. A subset of these was isolated, together with other sources showing large band 4 to 3 flux density ratios, that are not associated with interstellar gas. Most of the point sources are associated with diffuse cirrus emissions. The sources appear to be distributed randomly on the sky, with the exception of six clusters, one of which is not associated with any known object. Six sources out of seventeen that were observed for redshifted H I at Arecibo were found to be associated with relatively nondescript external galaxies. Most of the sources do not appear on the Palomar Sky Survey. Deep optical observations of eight fields revealed some fairly distant galaxies, one object with a very peculiar optical spectrum, and several blank fields

Heiles, Carl

Mccarthy, Patrick J.

Reach, William

Strauss, Michael A.

English

NASA Technical Reports Server

N87-
WHAT ARE 'CIRRUS' POINT SOURCES? 1
CARL HEILES
PATRICK J. MCCARTHY
WILLIAM REACH
MICHAEL A. STRAUSS
Astronomy Department
University of California, Berkeley
24345
ABSTRACT. Most 'cirrus' point sources are associated with interstellar gas. We have isolated a subset of
these, together with other sources showing large band 4 to band 3 flux density ratios, that are not associated
with interstellar gas. Most of the point sources are associated with diffuse cirrus emission. The sources appear
to be distributed randomly on the sky, with the exception of six clusters, one of which is not associated with
any known astronomical object.
Six sources out of seventeen that were observed for redshifted H I at Arecibo were found to be associated
with relatively nondescript external galaxies. Most of the sources do not appear on the Palomar Sky Survey.
Deep optical observations of eight fields revealed some fairly distant galaxies, one object with a very peculiar
optical spectrum, and several blank fields.
I. INTRODUCTION
IRAS 'cirrus' point sources (i.e., sources detected only in band 4, A _ 100 #m) are suspected to
arise from the same stuff as the diffuse cirrus background, namely interstellar dust. Since dust and gas
are intermixed, we expect the presence of clumps of 21-cm emission at the positions of 'cirrus' point
sources. However, our first cursory inspection of the distribution of these sources (Figure I.C.4 in the
IRAS Explanatory Supplement, 1984) revealed that a small fraction are located in regions containing
very little interstellar gas. By 'little' we mean HI column densities smaller than 102o cm -2, which is
very low--typical of values toward the Galactic poles, and in other regions containing the very lowest
column densities as measured with HI surveys using angular resolutions of 36 arcmin or more. This
made us suspect that they are, in fact, not associated with interstellar gas.
We developed a source list for further study using a two-stage selection process. In the first
stage, we included only those sources satisfying the following criteria: (1) reliable (quality 3) band 4
flux densities (F4); (2) F4/F3 > 5; (3) F4 > 2 Jy; (4) Galactic latitude Ib] > 10 °. This provided
more than 20000 sources. In the second stage, this was reduced to 293 sources by requiring F4 to be
larger than the limit determined from the H I column density described by equation (1) below.
From their locations in the sky, many of these sources are obviously associated with inter-
stellar gas. Some are positionally coincident with known astronomical objects, many of which are
galaxies. Some have no obvious association with known astronomical objects. Most sources have no
corresponding image on the Palomar Sky Survey (PSS).
II. STATISTICS
Most of the 20000-odd sources chosen from the first stage selection are associated with inter-
stellar gas. This is apparent from the scatter diagram of F4 vs. N(H I) presented in Figure 1: for
N(H I) < 150, the upper envelope of the densely-populated portion of the F4-N(H I) plane has a
tThis paper is based in part on observations performed at the Lick Observatory, operated by the University of California
at Santa Cruz.
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies 553
https://ntrs.nasa.gov/search.jsp?R=19870014912 2020-03-20T11:07:20+00:00Z
C. HEILES ET AL.
4O
3O
"¢ 20
10
0
I
IIIII
i
i
0 50 100 150 200 250 300
N(H I)
Figure 1. Scatter diagram of sources satisfying the first-stage selection process. F4 is the IRAS band
4 flux density in Jy; N(H I) is the H I column density in units of 9.96 x 10 is cm -2, taken from Stark
et al. (1986) for the Northern sky and from Cleary, Heiles, and Haslam (1974) for the Southern sky.
This diagram does not include the full range of each variable, but does include the vast majority of
the points. The solid line is the second-stage criterion of equation (1).
positive slope. For F4's much larger than the majority, there is no obvious correlation with N(H I).
We assumed that all sources with F4 exceeding a linear function of N(H I) should be considered as
possible non-cirrus candidates, and determined the linear function by eye. The function, drawn in
Figure 1, is
F4 ----0.1116 N(H I) - 0.625 Jy, (1)
where the units of N(H I) are 9.96 x 10 is cm -2, as on Figure 1.
The total number of sources satisfying the above criterion is 293. A bit fewer than half have
positional coincidences with objects in astronomical catalogs other than the IRAS point source catalog.
The distribution of the 293 sources on the sky is shown in Figure 2. The points tend to avoid low
Galactic latitudes; this is a result of equation (1), because high H I column densities tend to lie at low
latitudes.
554
CIRRUS POINT SOURCES
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Figure 2. Distribution on the sky of sources above the solid line in Figure 1.
...a._
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Some of the points are highly clustered. Five clusters can be identified with specific Galactic dust
regions or extragalactic objects, as follows: (l, b) .., (122 °, -22°), M31; (161 °, - 19°), Perseus region;
(208 °, -19°), Orion region; (280 °, -34°), Large Magellanic Cloud; (353 °, 17°), Ophiuchus region. Two
clusters have no obvious identifications: (/, b) _ (92 °, 38°), and the very sparse and possibly spurious
cluster covering the large area centered near (140 ° ,50°). The former is located near the nondescript
external galaxies NGC 6223, 6226, 6238, and 6244, and near the nondescript Abell cluster 2232. It
is also located in a region containing high-velocity H I, but because other high-velocity H I regions
do not exhibit clustered sources there is probably no real association. Sources with the largest F4's
(> 50 Jy) are associated either with dense clouds of Galactic interstellar matter, the Large Magellanic
Cloud, or the external galaxy NGC 891.
Are these 293 sources unique or do they simply occupy th6 tails of the distributions of F4 and
F4/F3? We are unable to tell. Our selected sources represent only a tiny fraction of all sources with
interesting IR properties; the lion's share was discarded using equation (1). Answering the question
would require generating a statistically unbiased list of IRAS point sources that are not associated with
interstellar gas on arcminute scales. This might be done using high-resolution 21-cm line observations
of a large unbiased sample, but this would be a tremendous undertaking.
III. GROUND-BASED OBSERVATIONS
A. 21-crn Observations
Equation (1) discriminates against regions of overall high H I column density, as measured with
the large beamwidths (... 1 °) used in the H I surveys. However, the possibility remains that small,
arcminute-size clumps of H I exist within such regions. We used the NAIC Arecibo telescope I with
1The NAIC is a national facility operated by Cornell University under contract with the National Science Foundation.
555
C.HEILESETAL.
its angular resolution of 3 arcminutes to check this possibility.
Out of 17 sources observed, 13 showed an H I column density excess relative to adjacent positions
smaller than 1019 cm -2, and the rest smaller than 3 x 1019 cm -z. These are strict, absolute, and in
many cases conservative upper limits. A point source with F4 = 2 Jy would produce an H I column
density of 1.2 x 1021 cm -2 if the relation of Boulanger, Baud, and van Albada (1985) applies--100
times more H I than the observed upper limits. The most straightforward conclusion is the expected
one, that these sources have nothing whatsoever to do with the interstellar gas.
We also used Arecibo to search for redshifted H I emission out to -_ 6000 km s -1. Of the sample
of 17 observed, 6 were detected. All 6 were positionally associated with ordinary nearby galaxies and
were so designated in the iRAS point source catalog. For these 6 galaxies, the ratios of H I mass
to IR luminosity were typically much higher than those of the IR-bright sample studied by Young et
al. (1986), by factors ranging from ,-_ 10 to -_ 1000. These large factors result from the combined
effects of our galaxies being overly abundant in H I and underluminous in the IR, with respect to
the optical brightnesses of that sample. Our 6 galaxies are very easily visible on the PSS, and are
therefore intrinsically different from the PSS blank-field objects discussed below.
B. Optical Observations
We used the CCD spectrograph in the direct imaging mode on Lick Observatory's Shane 3-m
telescope (Miller et al., 1983) to take optical images of fields centered on 8 PSS blank-field sources.
One and possibly one other showed the streaks characteristic of interstellar cirrus. Two fields clearly
contained external galaxies within the IRAS error ellipse: one a pair of galaxies, and one a compact
galaxy. One field, 1645+37, contained three very faint galaxies within the error ellipse. The other
three fields were blank with R > 22.5 mag. The observations are summarized in Table 1, where the
optical flux densities are compared to the IR flux densities.
TABLE 1
PROPERTIES OF OPTICALLY-OBSERVED SOURCES
SOURCE IDENTIFICATION R MAG F4(Jy)
0917+69 cirrus - 3.5
F4/(R-BAND FLUX)
1148+58 compact galaxy 18 2.4 1.3E4
1149+61 pair of galaxies 16,19 2.0 1.8E3,2.8E4
1150-09 cirrus? 21 2.9 2.6E5
1231-05 9 >22.5 2.2 >7.8E5
1324+16 9 >22.5 2.8 >I.0E6
1325+16 9 >22.5 2.1 >7.4E5
1645+37 three galaxies 21.0-23.5 2.4 2.6E5-3.4E6
We also used the CCD spectrograph to obtain low-dispersion spectra of the three galaxies
in the 1645+37 field. These data were taken very recently and are currently being analyzed. The
easternmost galaxy exhibits a clear emission line. If this emission line is [OII] A3727, then the redshift
is 0.444 + 0.002. With this redshift, the optical luminosity of the galaxy is roughly typical of L,
galaxies and the galaxy exhibits small internal reddening. The westernmost galaxy has a broadband
556
CIRRUS POINT SOURCES
spectrum similar to that of the easternmost galaxy, but exhibits no emission line. The central object
exhibits an unusual broadband spectrum, becoming spectacularly bright towards the blue.
There is a good a priori probability of finding the galaxies described in the above paragraph by
observing eight randomly-selected fields to the depth that we observed. However, the central object in
the 1645+37 field is very unusual, and the a priori probability of finding it in eight randomly-selected
fields would appear to be small. Further observations and analysis are needed to determine just how
unusual this object is.
C. 1._ mm Continuum Observations
We used the NRAO 12-m telescope at Kitt Peak x to observe a selection of sources at 1.2 mm
wavelength. Our goal was restricted to determining whether the sources are nonthermal emitters,
i.e. similar to ordinary continuum radio sources with flux density rising toward lower frequencies.
Sensitivity was low because of power outages, hardware failures, basic errors in system software, and
(least of all) weather; the average 3a flux density limit was 5.5 Jy. No source was detected. Thus
no source was significantly stronger at 1.2 mm than at 100 #urn. Any typical power-law synchrotron
source would have been easily detectable. The most straightforward conclusion is that the sources are
thermal emitters, perhaps modified by the wavelength-dependence of the emissivity of dust particles.
IV. DISCUSSION
Our 1.2 mm observations imply that the IRAS point-source emission is thermal. Nevertheless,
it would be remiss not to mention the unlikely alternative that the IR emission is produced by an as-
yet unspecified powerful process, making the situation analagous to the early days of radio astronomy
when virtually nothing was known about the basic characteristics of nonthermal radio sources. If the
radiation has a blackbody spectrum, (F4/F3) > 5 corresponds to T < 30.5 K; if the emission comes
from dust, which radiates less efficiently at long wavelengths, then the physical temperature of the
dust is smaller. For a blackbody emitter at T=30.5 K to produce F4 -- 2 Jy, its angular diameter
> 0.6 arcsec; if the radiation is from dust, this is a lower limit.
With such angular diameters, most of the objects can hardly be anything but diffuse matter. If
Galactic, their lack of concentration toward the Galactic plane would imply that they are nearby, with
typical distances smaller than their z-heights above the Galactic plane. If extragalactic, they would
be truly spectacular because the ratios of IR to optical flux densities are enormous. For example, for
objects fainter than 22.5 mag, the ratio of F4 to optical R-band flux density is > 106. This is about
50 times larger than the corresponding ratio for the extremely far-IR bright starburst/Seyfert galaxy
Arp 220 (Rieke et al., 1985; Soifer et hi., 1984) and about 10 times larger than the ratio for the most
extreme example in the lists of Aaronson and Olszewski (1984) and Houck et al. (1985).
Finally, we mention ripe areas for future observational work. Most intriguing is the uniden-
tified cluster of sources, associated with diffuse cirrus, centered near (l,b) N (92o,38 o) or (c_,6) ,,_
(16h47"_,62°). IR observations longward of 100 pm should be successful because of the rapid rise
of flux density with decreasing frequency, unless the radiation is line emission; very low-resolution
spectroscopy is the best approach for this test. If these sources obey the same statistical relationships
as external galaxies they should be detectable as radio continuum sources, typically with flux densities
> 3 mJy at 1.4 GHz (Helou, Soifer, and Rowan-Robinson, 1985) and as CO emitters, typically at
the level TAr > 0.5 °K-km/s with the FCRAO 14-m telescope (Young et hi., 1986). Finally, we will
continue our optical observations.
1The N_O is operated by Associated Universities, Inc., under contract with the National Science Foundation.
557
C.HEILESETAL.
We gratefully acknowledge the suppport of the Lick Observatory staff for their assistance in
performing the optmed observations. It is a pleasure to thank Prof. Frank Low for pointing out to us
the preponderance of diffuse cirrus emission, and for providing the stimulus to write this paper in a
timely fashion. This research was supported in part by a National Science Foundation grant to CH,
and also under the IRAS extended mission program by a JPL contract to CH. MAS is supported by
a Berkeley Graduate Fellowship.
REFERENCES
Aaronson, M., and Olszewski, E.W. 1984, Nature, 309, 414.
Boulanger, F., Baud, B., and van Albada, G.D. 1985, Astron. Ap., 144, Lg.
Cleary, M.N., Heiles, C. and Haslam, C.G.T. 1979, Astron. Ap. Suppl., 36, 95.
Helou, G., Soifer, B.T., and Rowan-Robinson, M. 1985, Ap. J., 298, L7.
Houck, J.R., Schneider, D.P., Danielson, G.E., Beichman, C.A., Lonsdale, C.J., Neugebauer, G., and
Soifer, B.T. 1985, Ap. J., 290, LS.
IRAS Catalogs and Atlases, Explanatory Supplement 1985, edited by Beichman, C.A., Neugebauer,
G., Habing, H.J., Clegg, P.E., and Chester, T.J. (Washingon D.C.: U..S. Government Printing Office).
IRAS Point Source Catalog 1985, Joint IRAS Science Working Group (Washington, D.C.: U.S. Gov-
ernment Printing Office).
Miller, J.S. 1983, The Lick 3-m Cassegrain Spectrograph, Lick Observatory, University of California,
Santa Cruz, CA.
Rieke, G.H., Cutri, R.M., Black, J.H., Kailey, W.F., McAlary, C.W., Lebofsky, M.J., and Elston, R.
1985, Ap. J., 290, 116.
Soifer, B.T., Helou, G., Lonsdale, C.J., Neugebauer, G., Hacking, P., Houck, J.R., Low, F.J., Rice,
W., and Rowan-Robinson, M. 1984, Ap. J., 283, L1.
Stark, A.A., Bally, J., Linke, R.A., and Heiles, C. 1986, in preparation.
Young, J.S., Schloerb, F.P., Kenney, J.D., and Lord, S.D. 1986, Ap. J., 304, 443.
558


What are cirrus point sources?

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