Introduction

1
The Magellanic Mopra Molecular Line Survey
(MaMMLS)
The MaMMLS Observing team E. Muller1, A.
Hughes2, J. Ott1, T. Wong3, and J. Pineda4
1 Bolton Fellow, Australia Telescope National Fa
cility (ATNF), 2ATNF/Swinburne, 3ATNF/University
of NSW, 4 Bonn University
Introduction The Magellanic System, comprising
the Small and Large Magellanic Clouds (SMC,
LMC), is a superb laboratory for observational
studies of the relationship between different
phases of the interstellar medium (ISM). The
proximity of these two star-forming dwarf
galaxies (50-60 kpc) allows studies of galactic
structure and kinematics at high spatial
resolution without the confusion encountered in
the Milky Way. In contrast to other phases of the
ISM, the molecular gas in the Magellanic Clouds
has not been studied in detail. Yet there are
compelling reasons to study the molecular gas
the formation and evolution of molecular clouds
and the relationship between molecular cloud
properties and star formation processes remain
highly uncertain. The intense interstellar
radiation field and low metallicity of the
Magellanic Clouds (LMC 0.3 solar, SMC 0.1
solar) provide further motivation for such
studies, since the physical conditions in the ISM
of the Magellanic Clouds may resemble the
conditions in the early Universe.
In an international collaboration, including tea
m members from the ATNF, the University of Nagoya
(Japan), the University of Bonn and the MPIfR
(Germany), we are conducting a survey of the
molecular gas in the Magellanic System with the
ATNF Mopra Telescope. The project targets regions
of known molecular emission in order to obtain
sensitive (RMS 0.4 K), high angular resolution
(? 33, corresponding to 9 pc) and good
spectral resolution (?v 0.16 km s-1)
observations of the molecular clouds within the
LMC and SMC. The Mopra observations improve on
the spatial resolution of existing maps by an
order of magnitude, fully resolving molecular
clouds within the Magellanic System for the first
time (see Figure 1). In 2005, we completed a map
of the 12CO(1-0) emission from the molecular
ridge complex, which is situated south of 30
Doradus in the LMC (see Figure 1 and talk by
Jorge Pineda). Preliminary observations of the
12CO(1-0) emission from several molecular clouds
in the SMC and the inner LMC were also conducted
(Figures 2 and 3).
Future Work 1. Molecular Line Surveys Follow
ing the completion of 12CO(1-0) observations of
the LMC molecular ridge, we are embarking on a
targeted, multi-transition experiment in order to
characterize the physical and chemical conditions
in the molecular ridge region. These measurements
will be made with the MOPS spectrometer, a newly
installed 8 GHz broadband backend system on the
Mopra Telescope. 2. The Small Magellanic Cloud
The interstellar medium of the SMC is the clos
est truly metal-poor environment (0.1 solar).
Previous studies (Rubio et al. 1999, Mizuno et
al. 2001) have shown that the 12CO(1-0) emission
in the SMC is much weaker than in the LMC. In
2006, we will completely map the south-western
region of the SMC where the most massive
molecular clouds are located (Figure 3). Our
observations will provide the first complete
survey of molecular gas in this active
star-forming region, allowing us to probe the
mass spectrum of SMC molecular clouds down to
104 M?.
The ATNF Mopra Telescope
Figure 1 The LMC Molecular Ridge. Left to right
Greyscale maps of the Ha (SHASSA), 100 µm (IRAS),
HI (ATCAParkes), 12CO(1-0) (NANTEN) and
12CO(1-0) (Mopra) emission. In each panels, the
contours indicate the Mopra 12CO(1-0) data (Tmb
range 0.5-3.5 K km s-1) which is also displayed
as a color map to the very right.
30 Dor
Figure 3 First Mopra maps of the SMC south-west
molecular region. Left to right Greyscale maps
show the Ha (SHASSA), 100 µm (IRAS), HI
(ATCAParkes) and 12CO(1-0) (NANTEN) emission. In
each panel, contours indicate the 12CO(1-0)
emission observed by Mopra. As traced by the 12CO
emission, the molecular gas appears to encircle a
local peak of Ha emission (left panel), and is
coincident with a ring of warm dust (second to
left). The contour levels indicate 10-90 levels
of the maximum integrated 12CO brightness (2.4 K
Kms-1)
3. The Inner Large Magellanic Cloud
In 2006, our 12CO(1-0) observations of the LMC w
ill focus on molecular clouds in the inner 1.4
kpc of the LMC (Figure 4). This region has been
selected to include molecular clouds with and
without signs of star formation, clouds
associated with the stellar bar, clouds
associated with expanding HI shells, and clouds
situated near the LMCs kinematic centre. With
the exception of the clouds associated with N44,
our observations target molecular clouds that
have not previously been mapped at high
resolution. Single-pointing observations,
however, suggest that the physical conditions in
these inner molecular clouds are quite distinct
to those in the molecular gas associated with the
well-known massive star-forming complexes that
are located in the outer arms of the LMC (e.g. 30
Dor and N11, Sorai et al. 2001). In combination
with our molecular ridge data, these new
observations will cover 70 of the molecular gas
in the LMC, enabling precise statistical
descriptions such as the mass spectrum and the
size-linewidth relation for molecular clouds in
the LMC. Our planned observations have
complementary spatial and velocity resolution to
the ParkesATCA HI survey of the LMC (Kim et al.
2003). The two datasets will be used to identify
interfaces between the molecular and atomic gas
phases and search for spatial and kinematic
correlations between the atomic and molecular
emission, in order to shed light on the processes
of molecular cloud formation and evolution.
The Molecular Ridge The LMC is host to a massiv
e, linear filament of molecular gas that extends
over 2 kpc south from the active star-forming
region 30 Doradus (see Fig. 4). The presence of
this unusual feature was established in the
surveys of Cohen et al. (1988) and Fukui et al.
(1999) but the Mopra observations presented here
are the first to resolve the knotted and clumped
molecular clouds that make up the molecular
ridge. This feature is unique insofar as the
radiation field, indicated by the brightness of
the Ha emission in Figure 1, varies along the
length of the filament. The molecular ridge
clouds thus provide a unique opportunity to
measure the properties of molecular clouds as a
function of the radiation field.
In a first analysis of our molecular ridge data,
we find that N(H2)/ICO (the so-called
X-factor) varies along the filament, suggesting
that the X-factor varies with the intensity of
the ambient radiation field. This result is
particularly important for studies of molecular
gas at high redshift, which use the integrated
intensity of the 12CO(1-0) transition to estimate
the molecular mass of objects in the early
Universe. (For more details, see the talk by
Jorge Pineda). Figure 2 shows a pixel-by-pixel
comparison of the 12CO(1-0) integrated intensity
and the column density of neutral hydrogen (HI)
in the molecular ridge region. 12CO(1-0) emission
rarely occurs below N(HI) 3 x 1021 cm-2,
suggesting that there may be a threshold in N(HI)
that is required for the formation of molecular
gas. In the Milky Way, the threshold for
molecular gas formation is estimated to be around
N(HI) 5 x 1020 cm-2 (Federman et al. 1979).
This minimum gas column density may reflect the
level of self and dust grain shielding necessary
to protect the molecular gas from
photodissociation by the interstellar radiation
field. It is not unexpected that the N(HI)
threshold should be higher in the LMC molecular
ridge than in the Milky Way, because the LMCs
low dust and molecular contents reduce shielding
of the molecular gas. In addition, the intense
interstellar radiation field in the 30 Dor region
increases the photodissociation and
Molecular ridge
30 Doradus
photoionization rate, further hampering the
formation of molecules. We also observe a
maximum N(HI) threshold, 7x1021 cm-2, all
molecular gas is found below this value. This
threshold may indicate that once the conversion
of atomic into molecular gas has commenced, it
proceeds very rapidly and efficiently creating
local minima of the HI distribution in which the
molecular gas is stored (see also Fig. 1).
Figure 4 Left NANTEN 12CO(1-0) map of the LMC
(Fukui et al 1999), overlaid by rectangles that
outline the regions that we will observe with
Mopra in 2006. The asterisk marks the dynamical
centre of the LMCs gas disk, as determined by
Kim et al. (2003). The circle has a radius of 1.4
kpc. Right. Preliminary observations of a
molecular cloud complex in the inner LMC. The
left part of the blow-up shows contours of the
12CO integrated intensity overlaid on an
ATCAParkes HI integrated intensity map. At the
very right we show the integrated 12CO emission
from the same molecular cloud complex in
colorscale.
Figure 2 Pixel-by-pixel correlation of 12CO
integrated intensity against HI column density
for the molecular ridge region in the LMC.
Molecular material is rarely associated with HI
column densities threshold gas column density for molecular gas
formation. There is some indication that 12CO
emission is not seen for very high HI column
densities above 7 x1021 cm-2.
For further information about the MaMMLS project,
please contact Juergen.Ott_at_csiro.au. Poster
title background shows a map of the integrated HI
emission from the Magellanic System by the Parkes
Telescope.