ISOGAL is a 15 &#956;m ISOCAM survey of &#8764; 12 deg2

in the Galactic Plane interior to |&#x2113;| = 45&#176;. In combination with IJK data from the near-infrared southern sky survey DENIS, the ISO images allow the first detailed study of stellar populations throughout the inner Galaxy. We present preliminary results from a test observation at &#x2113; = 45&#176; with 6" pixels and completeness limit 8 mJy. Of the &#8764; 3000 sources deg2 detected, about half are KM giants, seen through extinction of up to Av, ∼, 30,while most of the remainder are probably dusty young stars. Although away from bright IRAS regions, the field displays spectacular emission features, and, unexpectedly, a number of regions which are optically thick at 15 &#956;m. The dark regions are presumably dense filaments with Av, >, 25

Blommaert, J.

Cesarsky, C.

de Batz, B.

Elbaz, D.

Epchtein, N.

Felli, M.

Fouque, P.

Gilmore, G.

Guest, S.

Guglielmo, F.

Habing, H.

Levine, D.

Ojha, D.

Omont, A.

Perault, M.

Pollock, A.

Price, S.

Prusti, T.

Robin, A.

Seguin, P.

Siebenmorgen, R.

Simon, G.

Testi, L.

Tiphene, D.

English

Publications of the IAS Fellows

Astron. Astrophys. 315, L165–L168 (1996) ASTRONOMY
AND
ASTROPHYSICS
First ISOCAM images of the Milky Way?
M. Pe´rault1;2, A. Omont3, G. Simon4, P. Se´guin1, D. Ojha3, J. Blommaert5, M. Felli6, G. Gilmore7, F. Guglielmo3,
H. Habing8, S. Price9, A. Robin10, B. de Batz4, C. Cesarsky11, D. Elbaz11, N. Epchtein4, P. Fouque´12, S. Guest5, D. Levine5,
A. Pollock5, T. Prusti5, R. Siebenmorgen5, L. Testi6, and D. Tiphe`ne4
1 Laboratoire de Radioastronomie Millime´trique, ENS and CNRS, Paris, France
2 Institut d’Astrophysique Spatiale, CNRS, Orsay, France
3 Institut d’Astrophysique de Paris, CNRS, France
4 Observatoire de Paris, France
5 ISO Science Operation Center, ESOC, Villafranca, Spain
6 Osservatorio Astronomico, Arcetri, Italy
7 Institute of Astronomy, Cambridge, UK
8 Sterrewacht, Leiden, The Netherlands
9 Air Force Geophysics Laboratory, Bedford MA, USA
10 Observatoire de Besanc¸on, France
11 Service d’Astrophysique, CEA, Saclay, France
12 European South Observatory, La Silla, Chile, on leave from Observatoire de Paris
Received 16 September 1996 / Accepted 20 September 1996
Abstract. ISOGAL is a 15m ISOCAM survey of  12 deg2
in the Galactic Plane interior to j`j = 45. In combination with
IJK data from the near-infrared southern sky survey DENIS, the
ISO images allow the first detailed study of stellar populations
throughout the inner Galaxy. We present preliminary results
from a test observation at ` =  45with 6" pixels and complete-
ness limit 8 mJy. Of the  3000 sources deg 2 detected, about
half are KM giants, seen through extinction of up to A
v
 30,
while most of the remainder are probably dusty young stars.
Although away from bright IRAS regions, the field displays
spectacular emission features, and, unexpectedly, a number of
regions which are optically thick at 15m. The dark regions are
presumably dense filaments with A
v
> 25.
Key words: Galaxy: stellar content – interstellar medium: gen-
eral – interstellar medium: dust, extinction – infrared: galaxies
– infrared: stars
1. Introduction
In the wavelength range 3-20 m ISOCAM, the camera of the
Infrared Space Observatory, is able to detect point sources more
than 100 times fainter than IRAS. Indeed the 1000 pixels cov-
ering about the area of a single IRAS detector, give access to
Send offprint requests to: M. Pe´rault, michel.perault@ens.fr
? Based on observations with ISO, an ESA project with instruments
funded by ESA Member States (especially the PI countries: France,
Germany, the Netherlands and the United Kingdom) and with the par-
ticipation of ISAS and NASA; and on DENIS observations collected
at the European Southern Observatory.
major stellar populations at galactic scale. Interstellar extinc-
tion and intrinsic colors can be derived from the combination
of mid-infrared data with ground-based near-infrared surveys.
Dusty pre-main-sequence stars down to 1 M

will trace the for-
mation of low mass stars several kpc from the Sun, while em-
bedded OB stars and HII regions will be detected throughout the
Galaxy, as well as supergiants, asymptotic giant branch (AGB)
stars and Planetary Nebulae. Individual late-M giants, visible
beyond the Galactic Center are expected to dominate the deep
source counts, tracing the old disk population and thereby al-
lowing determination of a reliable picture of the structure of the
inner obscured Galaxy (central star cluster, bar, inner old disk,
molecular ring, young disk and the spiral arms).
With these goals in mind, we have initiated a 15m ISO-
CAM survey (ISOGAL, Omont et al. 1994) of 12 deg2 dis-
tributed along the inner Galactic disk. Our default camera pa-
rameters are a 6" pixel field-of-view and the LW3 ISOCAM
filter (12-18m). The fields selected to optimise the range of
studies outlined above are the following: jbj < 0:5 and with
` = 0:5, centered at j`j = 2.75, 4.25, 5.75, 7.25, 9.75, 12.25,
15.75, 19.25, 22.75, 26.25, 30.0 and 45.15, and a central field
with jbj < 0:5 and j`j < 1:5. Because of the need to avoid
strong sources potentially saturating the detectors, we devised
rasters within these fields which avoid IRAS point sources
stronger than 6 Jy at 12m. These sources were identified us-
ing the improved reduction of the Galactic plane IRAS data by
Price (1994). Point sources and resolved compact sources de-
tected in the ISOCAM data are being cross-identified with the
L166 M. Pe´rault et al.: First ISOCAM images of the Milky Way
Fig. 1. Views of the field observed. a) 12m finding chart (credit: IRAS
project and IPAC). The limits of the ISO field are outlined. b) ISOCAM
image at 15m.
IJK DENIS survey (Epchtein 1994, 1995, Copet 1996), which
is currently also in progress.
In this paper we provide the results from the first ISOGAL
field, observed with ISO in January 1996 as a part of commis-
sioning of ISOCAM (Cesarsky et al. 1996). The complexity of
the data reduction is such that we are presently limited to a
preliminary analysis. Even so, the results already available il-
lustrate the characteristics of the ISOGAL data and provide an
indication of the scientific achievements that one can expect
from this survey.
2. Observations
A 12 arcmin by 1 field, centered at l = -45.26, b = 0.0 was
imaged with a (l,b) raster of 7 by 24 pointings, allowing half
an array overlap of the 6" pixel, so that each piece of sky is
observed twice. 12 second exposures were obtained with the
LW3 filter (12-18m), with 0.28 second elementary integration
time. End-of-integrationminus reset, and 4 by 4 co-adding were
performed on-board.
The data have undergone the following processing steps:
data were extracted from the raw telemetry file, deglitched and
linearly scaled with optimized offset and gain frames for the
entire raster. Detector remanence ghosts of bright sources were
then deleted from each pixel time series. The readout time series
was sliced and mosaiced. The result is compared in Fig.1 to the
IRAS Sky Survey Atlas at 12m. The rms fluctuation estimated
from close to empty background positions is 2 mJy per pixel.
The main difficulties in point source extraction are: accu-
rate deletion of glitches and ghosts; automatic detection of point
source candidates against a much structured background; point
source photometry from spatially undersampled data; allowance
for the time- and signal-dependance of the pixel sensitivity when
deriving photometry (Cesarsky et al. 1996). A catalogue has
nevertheless been extracted through the following steps: low
spatial frequency subtraction from the mosaiced image, allow-
ing an easy detection of point-like spots; aperture photometry
and theoretical PSF fitting in the mosaiced image at the pre-
viously determined candidate positions; aperture photometry
at these positions in each of the (nominally 2) exposures; ex-
trapolation of the time sequence of aperture photometry from
individual readouts and elimination of candidates significantly
more compact or more extended than the PSF, as well as of can-
didates in the close neighbourhood of bright extended sources.
The consistency of the various flux derivations was then checked
and the source list produced with flux, position, size, errors and
quality flags. The flux error is presently estimated to be 30%.
M. Pe´rault et al.: First ISOCAM images of the Milky Way L167
Fig. 2. Histogram of the 15 m sources found in the ` '-45 field in
0.144 deg2. The completeness limiting flux is approximately 8 mJy.
Fig. 3. Colour-colour diagram of the ISOGAL sources associated with
bright DENIS sources (K11) for the l'-45 field. For most of sources,
the estimated error rms is0.2 mag for (J-K) and0.3 mag for (K-15).
3. Stellar sources
We restrict the analysis to a central area of 0.144 deg2 in which
each sky point was observed twice. This contains some 395
point sources, with a completeness limit  8 mJy (m15  8.5),
as shown in Fig.2. In the same 0.144 deg2 field, observed in
1996 with DENIS at ESO La Silla, down to completeness lim-
its K=12.5, J=14.5 and I=16.5 we found 2650, 3000 and 4300
sources respectively.
About 180 of the 15m sources have a clear identification
with a bright K source (K<11). Among them 160 have a J
counterpart. Our preliminary ISOGAL astrometry still requires
a matching radius of  8" for the cross-identifications with
DENIS. This is not a problem for the strong DENIS sources,
but it severely plagues the association with weaker sources.
The two-colour diagram (Fig.3) allows identification of the
main astrophysical classes of the ISO sources, in spite of the
large scatter from our preliminary photometry. The sources dis-
tributed along the reddening vector (Rieke and Lebovsky 1985)
are predominantely red giants and supergiants, with very few
earlier-type foreground stars and luminous OB stars. Fig.3 pro-
vides clear identification of stars with an intrinsic 15 m excess.
These are a mix of bright, dust-enshrouded, young stars, and
dusty AGB stars.
Fig. 4. ISOCAM 15 m image of the brightest star forming region in
the field.
The nature and number of the other reddest stars is of con-
siderable interest. Within a 6" coincidence radius, 52 ISOGAL
sources are associated with DENIS sources with 11 < K <
12:5. This is twice the number of expected chance associations,
assuming uniform distributions of sources. Of these 52, 27 are
also matched at J. Most of these sources have colours typical of
Ae, Be and T Tauri stars. In the remaining cases, and for a total
of 160 ISOGAL sources, no K source is detected by DENIS.
The actual number of 15 m sources without real K associa-
tion is thus almost half the total number (1300/deg 2). The
expected numbers of detectable galaxies and asteroids in our
field are less than 1 and 5 respectively. Thus, the only plausible
candidates for such a large number of cold sources are dusty
young stars.
The observed density of such sources is in the range of the
expected number of young sources in a giant molecular cloud
(GMC) a few kpc away, inferred from the IRAS results in nearby
clouds. For instance in the L1641 cloud in Orion (d 500 pc),
there are  80 IRAS 12m sources identified as young stars
with a dust disk or cocoon, in a projected area400 pc2 (Strom
et al. 1989). Among these sources 35 are stronger than 0.8 Jy
( 7/deg2). Roughly scaling this estimate to the CO surface
brightness towards our field (Bronfman et al. 1989), dominated
by molecular concentrations around the tangent point at 5 kpc,
we would expect an object density of1400/deg2 brighter than
8 mJy at 15m. This is the order of magnitude detected by
ISOGAL.
4. Diffuse emission and absorption at 15m
The ISOGAL fields deliberately avoid the brightest IRAS
sources and bright star forming regions, as traced by far-infrared
and radio continuum peaks, or warm CO clouds, in order to
avoid detector saturation and the extreme confusion that would
result between highly contrasted star forming region and back
and foreground populations. Lower luminosity regions however
L168 M. Pe´rault et al.: First ISOCAM images of the Milky Way
Fig. 5. Images of one of the dark
areas detected by ISO. a) ISO-
CAM image at 15 m. b) Image
synthetized from the DENIS K
catalog
present a rich structure which can be studied with a relatively
high level of detail.
For instance the brightest extended source in the field, that
barely shows up in the IRAS plate, and is not detectable in
optical plates, most likely corresponds to a moderately active
star forming region associated to the concentration of molecular
clouds at the tangent point, 5 to 6 kpc away from the Sun. The
extent (roughly one arcmin) and infrared luminosity inferred
from the image shown in Fig.4 ( 3 105 L

) make it comparable
to the Orion Nebula, 10 times further away.
ISOGAL images present numerous dark features, which are
likely due to absorption. The example shown in Fig.5 from the
same field presents a convincing correlation with a depletion in
2 m source counts. If 5 kpc away, such features would cor-
respond to very dense filaments of width 0.2 pc, that absorb
more than half of the backgrounddiffuse emission at 15m, im-
plying A
v
> 25. These would be good candidates for precursors
of star formation sites.
5. Conclusions
The first  1% of the ISOGAL survey illustrates the consid-
erable potential of ISOCAM for analyses of Galactic structure.
ISOGAL, complemented by near-IR, radio and mm data, can
provide detailed analyses of star formation rates, the distribu-
tion of both the ionised and the cold interstellar medium, the
Galactic distribution of normal stars, and quantification of the
number and distribution of both young stellar objects and AGB
stars.
Acknowledgements. The DENIS project is operated by CNRS/INSU
and partly funded by the SCIENCE and the Human Capital and Mo-
bility plans of the European Commision under grants CT920791 and
CT940627 and by various grants of the DENIS consortium partners.
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First ISOCAM images of the Milky Way

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