Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.atnf.csiro.au/research/multibeam/papers/DeepSouth.ps.gz Äàòà èçìåíåíèÿ: Tue Apr 7 09:37:03 1998 Äàòà èíäåêñèðîâàíèÿ: Sun Dec 23 06:25:31 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: dwingeloo 1

Galaxies behind the deepest extinction layer of
the southern Milky Way
Ren'eeC. Kraan­Korteweg 1 , B¨arbel Koribalski 2 , and SebastianJuraszek 3;2
1 Departamento de Astronomia, Universidad de Guanajuato, Mexico
kraan@norma.astro.ugto.mx
2 Australia Telescope National Facility, CSIRO, P.O. Box 76, Epping, Australia
bkoribal@atnf.csiro.au
3 School of Physics, Univ. of Sydney, NSW 2006, Australia, sjurasze@atnf.CSIRO.AU
Abstract. About 25% of the optical extragalactic sky is obscured by the dust and stars
of our Milky Way. Dynamically important structures might still lie hidden in this zone.
Various approaches are presently being employed to uncover the galaxy distribution in
this Zone of Avoidance (ZOA). Results as well as the different limitations and selection
effects from these multi­wavelengths explorations are being discussed. Galaxies within
the innermost part of the Milky Way --- typically at a foreground obscuration in the
blue of AB ? ¸ 5 m and jbj ! ¸ \Sigma5 ffi --- remain particularly difficult to uncover except
for H i­surveys: the Galaxy is fully transparent at the 21cm line and H i­rich galaxies
are easy to trace. We will report here on the first results from the systematic blind
H i­search (v Ÿ 12700 km s \Gamma1 ) in the southern Zone of Avoidance which is currently
being conducted with the Parkes Multibeam (MB) Receiver.
1 Introduction
To understand the dynamics within the local Universe -- the mass distribution
and the local velocity field with its peculiar and streaming motions -- a detailed
map of the 3­dimensional galaxy distribution is highly desirable. However, the
dust extinction and confusion with stars in the disk of our Galaxy make this very
difficult for ¸25% of the sky, and the following questions remain unanswered:
Could a nearby Andromeda­like galaxy have escaped detection to date, hence
change our understanding of the internal dynamics and mass derivations of the
Local Group (LG), and the present density of the Universe from timing argu­
ments (Peebles 1994)?
Is the dipole in the Cosmic Microwave Background Radiation (direction
and amplitude) entirely explained by the gravity on the LG from the irregu­
lar mass/galaxy distribution? As the nearest galaxies (v ! 300 km s \Gamma1 ) gener­
ate 20% of the total dipole moment (Kraan­Korteweg 1989) nearby individual
galaxies are equally important as massive groups, clusters and voids.
Is the mass overdensity in the Great Attractor (GA) region -- postulated from
a large­scale systematic flow of galaxies towards (`; b; v) ¸ (320 ffi ; 0 ffi ; 4500 km s \Gamma1 )
(Kolatt et al. 1995) -- in the form of galaxies, hence does light trace mass?
Does the Supergalactic Plane, other superclusters, walls and voids connect
across the Milky Way and might other large­scale structures (LSS) have gone
undetected due to this 'zone of avoidance'?

2 Ren'ee C. Kraan­Korteweg, B¨arbel Koribalski , and Sebastian Juraszek
2 Multiwavelengths explorations of the southern ZOA
Various approaches are presently being employed to uncover the galaxy distribu­
tion in the ZOA: deep optical searches, far­infrared (FIR), near­infrared (NIR)
surveys and blind H I searches. All methods produce new results, but all suffer
from (different) limitations and selection effects.
OPTICAL: Nearly the whole southern ZOA has been systematically surveyed
for highly obscured but still visible galaxies using existing sky surveys (cf. Woudt
1998, for a detailed overview). These surveys achieve a considerable reduction
of the ZOA and have uncovered distinct LSS unrelated with the foreground
extinction. Follow­up redshift observations have revealed a number of dynami­
cally important structures such as e.g., the nearby overdensity in Puppis (La­
hav et al. 1993) and the massive cluster A3627 at the core of the GA (Kraan­
Korteweg et al. 1996, cf. Fig. 1). Deep optical surveys are not biased with respect
to any particular morphological type. However, for foreground extinctions above
AB ? ¸ 5 m (H I­column­densities N HI ? ¸ 6 \Delta 10 21 cm \Gamma2 ), the ZOA remains fully
opaque (cf. inner contour in Fig. 1). For the southern Milky Way this corre­
sponds roughly to jbj ! ¸ \Sigma5 ffi .
FIR: The IRAS Point­Source Catalog (PSC) has been exploited in the last
decade to identify galaxy candidates behind the ZOA. Using different colour se­
lection criteria, galaxy candidates were followed up by HI radio surveys (e.g., Lu
et al. 1990) or by inspection of plates (e.g., Takata et al. 1996). To avoid confu­
sion with Galactic sources, K­band snapshots have proved very efficient (Saun­
ders et al. 1994). Confirmed IRAS galaxies can be merged with IRAS galaxy
samples outside the ZOA to produce uniform whole­sky samples for LSS stud­
ies. But bright spiral and starburst galaxies dominate these samples.
NIR: The recent near infrared (NIR) surveys, 2MASS (Skrutskie et al. 1997) and
DENIS (Epchtein 1997), provide complementary data. NIR surveys are sensitive
to early­type galaxies, are tracers of massive groups and clusters missed in IRAS
and H I surveys, have little confusion with Galactic objects and are less affected
by absorption than optical surveys.
In a pilot study, we examined the effeciency of uncovering galaxies at high ex­
tinctions with DENIS images (cf. Schr¨oder et al. 1997 & Kraan­Korteweg et al.
1998): highly obscured, optically invisible galaxies can indeed be traced to lower
latitudes (jbj ? ¸ 1 \Gamma 1: ffi 5) than deep optical surveys. This is not only of interest in
charting early­type galaxies but also with respect to the combination of H I data
of heavily obscured spiral galaxies detected in blind H I surveys (cf. below) with
NIR data, and therewith the possibility to extend the peculiar velocity field into
the ZOA via the NIR Tully -- Fisher relation.
H I: In the regions of highest obscuration and infrared confusion the Galaxy is
fully transparent to the 21­cm line radiation of neutral hydrogen. H I­rich galaxies
can readily be found at lowest latitudes through detection of their redshifted

Galaxies behind the deepest extinction layer of the southern Milky Way 3
21­cm emission. Only low­velocity extragalactic sources (blue­ and redshifted)
within the strong Galactic HI emission will be missed, and -- because of baseline
ripple -- galaxies close to radio continuum sources.
Until recently, radio receivers were not sensitive and efficient enough to at­
tempt large systematic surveys of the ZOA. In a pilot survey with the late 300­ft
telescope of Green Bank, Kerr & Henning (1987) surveyed 1:5% of the ZOA
and detected 16 new spiral galaxies. Since then a systematic shallow search for
nearby, massive galaxies has been completed in the north (Henning et al. 1998),
yielding five objects including Dwingeloo 1 (Kraan­Korteweg et al. 1994).
3 The Parkes Multibeam Survey in the southern ZOA
In March 1997, a systematic blind H I survey began with the Multibeam Receiver
(13 beams in the focal plane array) at the 64m Parkes telescope in the most
opaque region of the southern Milky Way (213 ffi Ÿ ` Ÿ 33 ffi ; jbj Ÿ 5 ffi ). The ZOA
will be surveyed along constant Galactic latitudes in 23 contiguous fields of length
\Delta` = 8 ffi . The ultimate goal is 25 scans per field where adjacent strips will be
offset in latitude by \Deltab = 1: 0 5 for homogeneous sampling. With a total observing
time of 1500h, we will obtain an effective integration time of 25 min/beam with
a 3 oe detection limit of 15mJy. Roughly 3000 detections are predicted for the
covered velocity range of \Gamma1200 ! ¸ v ! ¸ 12700km s \Gamma1 (Staveley­Smith 1997).
This allows the detection of dwarfs with H I­masses as low as 10 6 M fi in the
local neighbourhood, and will be sensitive to normal Sc galaxies well beyond the
Great Attractor region. As a byproduct, the survey will produce a high resolution
integrated column density map of the southern Milky Way and a detailed catalog
of high velocity clouds (cf. Putnam et al. 1998).
3.1 First Results from the Parkes Multibeam survey
At the time of this meeting, the whole southern ZOA survey had been surveyed
twice (\Deltab = 17 0 , rms ¸ 20mJy). The cubes of the Hanning smoothed data
(26 km s \Gamma1 resolution) were inspected visually for 21 of the fields (220 ffi Ÿ ` Ÿ 4 ffi )
and all galaxy candidates with H I fluxes ? ¸ 100mJy were catalogued.
87 galaxies were uncovered in this way. Four were seen in more than one
cube. Though galaxies up to 6500km s \Gamma1 were identified, most of the galaxies
(80%) are quite local (v! 3500km s \Gamma1 ) due to the (yet) low sensitivity. In the
low­extinction Puppis region (cf. Fig. 1), a large fraction of the galaxies and
their velocities were already known. In the remaining ZOA, about 1/3 have a
counterpart in NED or the deep optical surveys.
The distribution of the H I­detected galaxies is shown in the lower panel of
Fig. 1. Here we also display the results by Juraszek et al. (in prep.) in the GA
region. In this high priority area, defined as 310 ffi Ÿ ` Ÿ 330 ffi , jbj Ÿ 10 ffi , four
scans rms (¸ 15mJy) were analyzed and 82 galaxies charted. This area hence
probes deeper and finds -- not unexpectedly -- a peak in the velocity distribution
between 3000 and 4500km s \Gamma1 in the GA direction. The top panel of Fig. 1

4 Ren'ee C. Kraan­Korteweg, B¨arbel Koribalski , and Sebastian Juraszek
GALACTIC LONGITUDE
­40
­20
0
20
40
­40
­20
0
20
40
350 300 250
­40
­20
0
20
40
Fig. 1. Galaxies with v!10000 km s \Gamma1 . Top panel: literature values (LEDA), superim­
posed are extinction levels AB ¸ 1: m 5 and 5 m ; middle panel: follow­up redshifts (ESO,
SAAO and Parkes) from deep optical ZOA survey with locations of clusters and dy­
namically important structures; bottom panel: redshifts from shallow MB­ZOA and
deeper GA survey in H I with the Parkes radio telescope.
shows the distribution of all galaxies with velocities v Ÿ 10000km s \Gamma1 centered
on the southern Milky Way. Note the near full lack of galaxy data for extinction
levels AB ¸ 1: m 5 (outer contour). The middle panel results from the follow­up
observations of the optical galaxy search by Kraan­Korteweg and collaborators.
Various new overdensities become apparent at low latitudes. But the innermost
part of our Galaxy remains obscured (AB ? ¸ 5 m ; jbj ! ¸ 5 ffi ). Here, the blind H I
data (cf. lower panel) finally can provide the missing link for LSS studies.
In Fig. 2, the data of Fig. 1 are combined in redshift slices. The achieved

Galaxies behind the deepest extinction layer of the southern Milky Way 5
GALACTIC LONGITUDE
­40
­20
0
20
40
­40
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0
20
40
350 300 250
­40
­20
0
20
40
Fig. 2. Redshift slices from data in Fig. 1: 500!v!3500 (top), 3500!v!6500 (middle),
6500!v!9500 km s \Gamma1 (bottom). The open circles mark the nearest \Deltav=1000 km s \Gamma1
slice in a panel, then triangles, then the filled dots the 2 more distant ones.
sensitivity in the current MB H I­survey fills in structures all the way across
the ZOA for the upper panel (v ! 3500km s \Gamma1 ) for the first time. Note the
continuity of the thin filamentary sine­wave­like structure that dominates the
whole southern sky, and the prominence of the Local Void. This feature is very
different from the thick, foamy Great Wall­like structure, the GA, in the middle
panel. With the full sensitivity aimed at with the MB­survey, we will be able to
fill in the LSS in the more distant panels of Fig. 2 as well.
ATCA follow­up observations of three very extended (20 0 to ? ¸ 1 ffi ), nearby
(v ! 1500km s \Gamma1 ) sources revealed them to be interesting galaxies/complexes,
with unprecedented low H I column densities (cf. Staveley­Smith et al. 1998).

6 Ren'ee C. Kraan­Korteweg, B¨arbel Koribalski , and Sebastian Juraszek
4 Conclusions
The combination of the complementary multiwavelength surveys allow a new
probing of LSS in the 'former' ZOA. The H I surveys are particularly powerful
at the lowest latitudes. But future merging of ZOA data with catalogs outside
the ZOA will have to be done with care to obtain 'unbiased' whole­sky surveys.
From the sensitivity attained with the first 2 scans of the ZOA MB­survey
it can be maintained that no Andromeda or other H I­rich Circinus galaxy is
lurking undetected behind the extinction layer of the southern Milky Way.
Acknowledgements --- The help of the HIPASS ZOA team members R.D.
Ekers, A.J. Green, R.F. Haynes, P.A. Henning, R.M. Price, E. Sadler, and L.
Staveley­Smith is gratefully acknowledged.
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