Full text of DOI:10.1071/AS00001

Publ. Astron. Soc. Aust., 2000, 17, 1–5
.
The Magellanic System’s Interactive Formations
M. E. Putman
Research School of Astronomy & Astrophysics, Australian National University,
Private Bag, Weston Creek PO, ACT 2611, Australia
putman@mso.anu.edu.au
Received 1999 November 17, accepted 2000 January 7
Abstract: The interaction between the Galaxy and the Magellanic Clouds has resulted
in several high-velocity complexes which are connected to the Clouds. The complexes
are known as the Magellanic Bridge, an HI connection between the Large and Small
Magellanic Clouds, the Magellanic Stream, a 10
100 HI lament which trails the
Clouds, and the Leading Arm, a di use HI lament which leads the Clouds. The
mechanism responsible for these features formation remains under some debate, with
the lack of detailed HI observations being one of the limiting factors in resolving the
issue. Here I present several large mosaics of HI Parkes All-Sky Survey (HIPASS)
data which show the full extent of the three Magellanic complexes at almost twice
the resolution of previous observations. These interactive features are connected, but
unique in their spatial and velocity distribution. The di erences may shed light on
their origin and present environment. Dense clumps of HI along the sightline to the
Sculptor Group, which may or may not be associated with the Magellanic complexes,
are also discussed.
Keywords: intergalactic medium — Local Group — Magellanic Clouds —
galaxies: HI
1 Introduction
this by providing a marked improvement in spatial
resolution compared to previous HVC surveys (see
Putman & Gibson 1999), and revealing previously
hidden structure and small high-velocity clumps
about the Magellanic System.
Almost every galaxy in the Universe is either
currently, or was at some time, part of an interacting
system. This includes our Galaxy, but until the
recent discovery of the Sagittarius dwarf, HI features
about the Magellanic Clouds were the only clear
sign of our interactive past. Giant neutral hydrogen
structures, known as the Magellanic Stream, the
Magellanic Bridge and the Leading Arm, depict
the Clouds’ interaction with each other and the
Galaxy. These Magellanic HI complexes make up a
large fraction of the high-velocity HI concentrations
in the southern sky which are known as high-
velocity clouds (HVCs). High-velocity clouds are
objects which do t into the classical picture of
2 Magellanic Complexes
The three HVCs which are classi ed as Magellanic
debris are the Magellanic Bridge, the Magellanic
Stream and the Leading Arm (see Putman 2000 for
a review). The Magellanic Bridge (see Figure 1)
is a
108
M
lament of neutral hydrogen which
joins the two Magellanic Clouds and has a velocity
1
gradient which proceeds from 125 km s at the tail
1
Galactic HI rotation and fall in the velocity range
of |vlsr| = 90 400 km s
of the SMC or Shapley’s Wing, to 300 km s at
1
(see Wakker & van
the extended arm of the LMC. Tidal models predict
that the Bridge was pulled from the SMC during
a close encounter between the two Clouds 0 2 Gyr
ago (Gardiner & Noguchi 1996). The HIPASS data
reveal new structure along the Bridge, as well as an
extension from the LMC which suggests that the
Bridge is made of both SMC and LMC material.
The Bridge is the only HVC known to contain stars,
and the age of many of the stellar concentrations
(10–25 Myr) indicates that the Bridge is a star
forming region (Grondin, Demers & Kunkel 1992).
Woerden 1997 for a review). The overall HVC
population has had a number of possibilities put
forth for its origin: from superbubbles in the
Galactic disk, to tidal debris, to building blocks of
the Local Group. To distinguish between the range
of possibilities and determine if more HVCs are
related to the interaction of our Galaxy with the
Magellanic Clouds, we must better understand the
overall HI distribution of HVCs. Maps from the
HI Parkes All Sky Survey (HIPASS) have enabled
᭧ Astronomical Society of Australia 2000
10.1071/AS00004
1323-3580/00/010001$ 05.00

2
M. E. Putman
Figure 1—Neutral hydrogen column density map of the Magellanic Bridge which joins the
Large and Small Magellanic Clouds. The projection was chosen based on previous maps of the
2
system and the main features are labelled. The contours are 1, 2, 4, 8, 16 and 32 1020 cm
.
The HIPASS map of the Magellanic Stream is
tidal forces are the dominant mechanism responsible
for forming the Magellanic Stream. This complex
was only recently con dently de ned as Magellanic
debris (Putman et al. 1998), as the course resolution
and observing grid used in the previous surveys
missed the laments between the clumps and the
shown in Figure 2. The Stream trails the Magellanic
Clouds for over 100 and is not a con ned lament
as previous maps depict (Mathewson, Cleary &
Murray 1974), but a complex network of laments
and clumps. The beginning of the Stream consists
of multiple laments and head-tail structures as it
spews from the northern side of the SMC and Bridge
at vlsr = 90 240 km s 1. Most of these laments
connection to the Magellanic Clouds.
Further
evidence that this feature is of Magellanic origin is
the metallicity determination of Lu et al. (1998),
using the background galaxy NGC 3783.
end at declination
60 , but the main lament
of the Stream continues to march northward, its
bifurcated structure diminishing towards the northern
tip.1 Dense clumps follow the Stream in position
and velocity, except for the region of the Sculptor
Group which will be discussed in the next section.
There are no stars in the Stream (e.g. Guhathakurta
& Reitzel 1998), but H appears to be detectable
at every location along the Stream with a column
3 Is it all Magellanic Debris?
The ability of HIPASS to uncover the Leading Arm
leads to speculation about the origin of other southern
HI clouds. An important class of HVC that HIPASS
is successful in revealing are the compact HVCs
(CHVCs) which surround the Magellanic Stream
and Leading Arm (see Figures 2 and 3). Are these
objects additional interactive debris or possibly
distant remnants of the Local Group (Blitz et al.
1999; Braun & Burton 1999)? The majority of the
CHVCs follow the Magellanic complexes in position
and velocity, suggesting they are related to the Stream
or Leading Arm; however, between declinations 30
to 40 the CHVCs have a much larger velocity range
density greater than 1019
M
(Weiner & Williams
1996; Bland-Hawthorn & Maloney 1999b). These
detections, together with the fact that no [OIII] has
been detected from the Stream, indicates that the
Stream is being ionised by photons escaping from
the Galaxy (Bland-Hawthorn & Maloney 1999a).
The third Magellanic complex is a natural tidal
counterpart to the Stream, and is called the Leading
Arm (see Figure 3). This feature is more di use than
the Stream and its leading position indicates that
( 500 km s 1; from vlsr
250 to 250 km s 1) than
1
Figure 2 does not show the last 15 of the Stream—see Wayte (1989) for detailed maps of this region.

Magellanic System’s Interactive Formations
3
MS IV
MS III
MS II
MS I
SMC
Bridge
Figure 2—Integrated intensity map of the Magellanic Stream (vlsr
=
400 to +400 km s 1),
which starts at the SMC and Bridge and extends to declination +02 . The location of the
clumps MS I to MS IV, which were originally classi ed by Mathewson et al. (1979), are labelled
for reference. The Stream passes through the velocity of Galactic emission at approximately
declination 35 , making it somewhat di cult to trace. At these velocities, Galactic emission
has been blanked as much as possible.
the narrow range of the Stream material ( 80 km
1). This declination region also encompasses the
into the normal distribution of Magellanic Stream
material, and could lie anywhere along this lament
of galaxies. At a distance of 2 Mpc most of the
s
southern and near side (at 2 Mpc) of the Sculptor
Group of galaxies. Figure 4 shows the complexity of
this region, with the positive velocity CHVCs mixed
in amongst the galaxies while the Magellanic Stream
continues southwards. These results are particularly
interesting when one considers the recent distance
determination work of Jerjen, Freeman & Binggeli
(1998) which indicates that the Sculptor Group is
not actually a separate group, but an extension of
the Local Group, forming a 6 2 Mpc lament
of galaxies with M31 and the Milky Way at one
clouds would have HI masses of
107
M
and
diameters of 20 kpc. The clouds may be leftover
galactic building blocks and the local equivalent of
the Ly-limit absorber (e.g. Stocke et al. 1995). They
also t nicely into hierarchical structure formation
simulations which predict that there should be more
satellites associated with the Local Group galaxies
than are currently observed (e.g. Moore et al. 1999).
The Sculptor Group sightline may be a nearby
tracer of the cosmic web and should be investigated
more closely. H observations are in progress and
end.2 The CHVCs along this sightline do not
t
2
This prolate cloud of galaxies is actually part of an even larger structure known as the Coma–Sculptor cloud which stretches
out to the Coma I cluster at 15 Mpc, in the direction of the Virgo cluster.

4
M. E. Putman
here are truly extragalactic objects at Mpc distances
they should not be detectable in H .
* NGC 3783
4 Conclusions
HIPASS provides a monumental amount of infor-
mation on the distribution of neutral hydrogen in
the southern sky. The interaction of the Magellanic
Clouds with the Milky Way has created striking
features, and the structure revealed here will o er
important clues as to the exact mechanism respon-
sible for their formation. The resolution and spatial
sampling of HIPASS also allow the rst thorough
investigation of the population of compact HVCs and
their possible extragalactic nature. The sightline
to the Sculptor Group is an ideal place to begin
this investigation. Further studies in progress of
both the Magellanic and compact HVCs include
sensitive H observations, higher velocity resolution
HI observations, and optical and UV measurements
to determine metallicities.
Leading
Arm
LMC
Acknowledgments
I thank the multibeam working group for their
help in the data collection and development of the
instrumentation and software, in particular Lister
Staveley-Smith.
References
SMC
Bland-Hawthorn, J., & Maloney, P. 1999a, ApJ, 510, L33
Bland-Hawthorn, J., & Maloney, P. 1999b, in Stromlo
Workshop on High-velocity Clouds, ASP Conf. Series,
Vol. 166 (San Francisco: ASP), p. 212
Stream
Blitz, L., et al. 1999, ApJ, 514, 818
Braun, R., & Burton, W. B. 1999, A&A, 341, 437
Gardiner, L., & Noguchi, M. 1996, MNRAS, 278, 191
Grondin, L., Demers, S., & Kunkel, W. E. 1992, AJ, 103,
1234
Guhathakurta, P., & Reitzel, D. B. 1998, in Galactic Halos:
UC Santa Cruz Workshop, ed. D. Zaritsky, ASP Conf.
Series, Vol. 136 (San Francisco: ASP)
Jerjen, H., Freeman, K., & Binggeli, B. 1998, AJ, 116, 2873
Lu, L., et al. 1998, AJ, 115, 162.
Mathewson, D. S., et al. 1979, in The Large-scale Structure
of the Galaxy (Dordrecht: Kluwer), p. 556
Mathewson, D. S., Cleary, M. N., & Murray, J. D. 1974,
ApJ, 190, 291.
Figure 3—A HIPASS peak intensity map which shows the
full extent of the Leading Arm, as well as the Magellanic
Clouds, the Bridge and the beginning of the Stream (as
labelled). The position of the background galaxy, NGC
3783, is also noted (see text). To avoid the emission from
1
the Galactic Plane (which extends out to 120 km s in this
1
direction), only velocities between 130 and 400 km s
were
used. (Thus the truncated appearance of the SMC which
begins at 80 km s 1.)
Moore, B., et al. 1999, ApJ, 524, L19
Putman, M. E. 2000, in High-velocity Clouds, ed. H.
van Woerden et al. (Kluwer: Dordrecht) (see astro-
ph/9909080)
Putman, M. E., & Gibson, B. K. 1999, PASA, 16, 70
Putman, M. E., Gibson, B. K., Staveley-Smith, L., et al.
1998, Nature, 394, 752
Stocke, J. T., Shull, J. M., Penton, S., Donahue, M., &
Carilli, C. 1995, 451, 24
Tufte, S. L., Reynolds, R. J., & Ha ner, L. M. 1998, ApJ,
504, 773
will provide signi cant insight into the origin of
the CHVCs. If the CHVCs are within a 100 kpc
radius from the Galactic Plane they should be
detectable, as ionising photons from the Milky Way
can easily reach this radius (Bland-Hawthorn &
Maloney 1999a). This is shown through the H
detections of the Magellanic Stream and other HVCs
which have been detected at the 0 2–0 4 Rayleigh
level (Weiner & Williams 1996; Tufte, Reynolds &
Ha ner 1998). If the population of CHVCs shown
Wakker, B. P., & van Woerden, H. 1997, ARA&A, 35, 21
Wayte, S. R. 1989, PASA, 8, 195
Weiner, B. J., & Williams, T. B. 1996, AJ, 111, 1156

Magellanic System’s Interactive Formations
5
NGC 247
NGC 253
NGC 7793
Scl dSph
NGC 300
NGC 55
Phoenix
Figure 4—The Magellanic Stream meets the Sculptor Group. A peak HI intensity map including
velocities from 73 to 450 km s 1. Several galaxies of the Sculptor Group are labelled, as well
as the positions of two nearby dwarf galaxies. The bifurcation of the Magellanic Stream and
the clumps which follow the Stream are also depicted. The contours are 0 4, 0 8, 1 6, 3 2
and 6 4 K.