The Central Molecular Zone of the Galaxy

Overview

We have undertaken a multi-molecular line mapping survey of the Central Molecular Zone (or CMZ) of the Galaxy. This is a unique region for astrophysical investigations, the molecular environment in the inner few hundred parsecs of our Galaxy, containing approximately 10% of its molecular gas. It is rich in organic molecules, whose distribution is far more widespread than found in giant molecular clouds elsewhere in the Galaxy. The advent of the UNSW-MOPS spectrometer on Mopra, combined with on-the-fly mapping and the MMIC receiver, has made this extensive study of the molecular content of the CMZ possible.

HCN in the CMZ
HCO+ in the CMZ
HNC in the CMZ
Peak line brightness images from the J=1-0 lines of HCN (top), HCO+ (middle) and HNC (bottom) in the Central Molecular Zone, part of the Mopra data set from the 3mm band survey. The principal regions of molecular emission are, from left to right, G1.3 (l~1.3°), Sgr B2 (l~0.7°), Sgr A (l~0°) and Sgr C (l~359.5°). The colour bar is TA* in units of K.

 

There are 6 separate surveys of the CMZ, and of the Sgr B2 core within it, for which data cubes are available:

1. Sgr B2 3mm Band Survey

2. Sgr B2 7mm Band Survey

3. CMZ 3mm Band Survey

4. CMZ 7mm Band Survey

5. CMZ 12mm Band Survey – the HOPS Project

6. CMZ CO J=1-0 Survey

This website provides an overview of this project, together with links to the fits files for the data cubes from this extensive data set.

Timeline

During the commissioning phase of the MOPS in 2006 we conducted a pilot study to test the feasibility of this project. A paper from this work, presenting maps of 38 molecules in the Sgr B2 region of the CMZ emitting in the 3mm (82-114 GHz) band, and detecting ~170 lines, has been published in Jones et al. (2008). A further paper presenting maps of 47 lines from Sgr B2 in the 7mm (30-50 GHz) band from a subsequent study appears in Jones et al. (2011).

In 2007 we began mapping the full CMZ in the 85-93 GHz portion of the 3mm band, and completed this in 2010. We present the maps from this work, of 18 lines across a 2.5° x 0.5° region, in Jones et al. (2012). The CMZ was also surveyed as part of the HOPS project in the 12mm (18-26 GHz) band by Walsh et al. (2008, 2011). In the 7mm band the CMZ was mapped in 2010-11, and this data has been submitted for publication (Jones et al. 2013). Mapping of the main isotopologue lines from CO 1-0 from 109-115 GHz is nearly complete.

CMZ in the mid-IR
Spitzer infrared space telescope image of the central regions of the Galaxy, encompassing the region surveyed with Mopra, and extending ~3° along the Galactic plane (~450 parsecs). The colour scheme ranges from blue (3.6µm), green (4.5µm), orange (5.8µm) to red (8.0µm), and the shows the emission from massive stars, hot dust and PAHs across the CMZ. Prominent dark regions are obscuration from cold dust. The Sgr A cluster at the centre of the Galaxy is in the centre of the picture. The Sgr B2 dust core is the prominent dark cloud just to left of centre. Image produced by NASA (Susan Stolovy).

The Central Molecular Zone

90cm (330 MHz) radio image of the Galactic Centre from Kassim et al. (1999) covering the region of the Central Molecular Zone. The data was taken with the VLA and covers ~2°x2° at 45'' angular resolution and is orientated as RA/Dec. Its shows a variety of thermal and non-thermal emission structures. Examples of the latter include supernova remnants (SNRs) and magnetically-enhanced threads and filaments, and are the most prominent features in at centimetre wavelengths.

The Central Molecular Zone extends approximately 3° along the Galactic plane (though is not centred on it) and 0.3° out of the plane (i.e. an extent of ~400 x 80 pc). It is prominent in molecular emission (hence the name), as well as in far-IR dust emission, indicating the presence of extensive star formation throughout. It contains ~3 x 107 Msun of molecular gas, about 5% of the total molecular content of the Galaxy. The IR emission accounts for a similar fraction of the Galaxy's IR luminosity. The CMZ is quite different in nature to molecular clouds found elsewhere in the Galaxy, such as in the 3-5 kiloparsec Galactic Ring. Temperatures are in general significantly higher (typically 30-60K, though up to 200K, compared to 10-20K in giant molecular clouds [GMCs]), densities exceed 104 cm-3 throughout, and turbulent velocities are high (~15-50 km/s vs. ~5 km/s in GMCs). Elsewhere in the Galaxy such densities are only found in cloud cores, not extended over entire cloud complexes, but this must be necessitated by the need to withstand tidal shearing in the central regions. The CMZ is thus a very different environment to that in which star formation has been studied in either clouds nearby to the Sun or in the molecular clouds of the Galactic Ring.

Widespread Organic Molecules

Perhaps most surprisingly the CMZ is rich in organic molecules, and these appear to be widespread throughout it. Yet, while the prominent cloud of SgrB2 in the CMZ is well studied (containing about 10% of the CMZ's mass, the largest column density along any sight line in the Galaxy, and virtually every exotic molecular species so far found in the ISM), the rest of the CMZ has been little studied outside the CO lines. However, it is clear from CH3OH (methanol) measurements made at 834 MHz with a 40' beam (~100 pc), that the emission is extended even on this scale (Gottlieb et al. 1979). From HNCO (isocyanic acid) mapping at 110 GHz with a 9' beam by Dahmen et al. (1997), made fortuitously at the same time as a C18O map of the CMZ (the line fell in the same bandpass), the organic species was seen to be extended on much the same scale as the far more abundant CO molecule. This was completely unexpected and still remains unexplained. While organic reservoirs are common in molecular clouds, they are confined to compact hot molecular cores (HMCs; warm [~100K], dense [~ 106-7 cm-3] knots inside GMCs), where massive star formation has recently started, ~104-5 years ago (see, for instance, Purcell et al.'s 2006 Mopra survey). Hot molecular cores are of order 0.1pc in size, and not the ~10pc scales associated with GMC-complexes, let alone the ~100pc scale associated with the CMZ. The CMZ appears to be behaving like a hot molecular core writ-large?! We know of no other region like this in the universe, though possibly it may represent the typical environment found in the central regions of rich, star forming galaxies (e.g. see Menten 2004)?

A Unique Region in the Galaxy Requiring New Capabilities for its Study

The morphology of the region is complex, characterised by expanding arcs, shells and filaments. Furthermore, the molecular line profiles are exceedingly broad across the entire CMZ, typically 15-30 km/s wide, but reaching up to 100 km/s in places. In addition, SiO emission is widespread, and this requires large-scale shocks or cloud-cloud collisions for its occurrence (Martin-Pintado et al., 1997). These may possibly be the result of the violent release of kinetic energy by some hundreds (?) of supernova explosions in the past 104 years, for the CMZ region also contains extensive hot (~108K) plasma (Koyama et al., 1989) and gamma-ray emission from the relatively short-lived 26Al (von Ballmoos et al., 1987).

This clearly is a unique region of the Galaxy, the environment ranging from the coldest and most quiescent regions required for complex organic molecules to exist, to the hottest for the X-ray emitting plasma. The distributions of all the molecules within the CMZ needs to be determined before their existence there can be understood. Why are organic molecules so widespread throughout it? Which ones are present? There were no facilities capable of tackling such a broad investigation until the advent of the UNSW-MOPS on Mopra, as it requires a combination of large areal coverage, multi-line capability, good spatial resolution and extensive observing time.

Key Science Questions

Two driving science questions are:

  1. what allows the distribution of organic molecules to be so widespread in the CMZ, compared to their limited distribution within the hot molecular cores in GMCs elsewhere in the Galaxy, and
  2. how have their complex dynamical motions been driven?

These questions could not be adequately addressed until the distributions of the different organic species were known, in comparison to that traced by CO. How different are these for each species present? For instance, alcohol-related molecules like CH3OH (methanol), C2H5OH (ethanol), CH2H2O (dimethyl ether), HCOOCH3 (methyl formate), CH3COOH (acetic acid) and H2CO (formaldehyde), which are now known to be widespread through the region due to pointed observations in the CMZ with the IRAM telescope (see Requena-Torres et al. 2006). Such molecules are also commonly found in hot molecular cores, associated with the cradles where infant massive stars are found. Does this imply widespread massive star formation has recently been initiated across the central few hundred parsecs of the Galaxy, as it has inside the 0.1 pc-sized hot molecular cores?

These organic species are the result of gas phase, alcohol-chemistry occurring after ices have been evaporated from grain mantles (e.g. Charnley et al. 1995), before the radiation field from the incipient star has become strong enough to destroy the molecules and form an UCHII region. This chemistry is now is an open debate as it seems to be necessary to have a rich chemistry on the grain mantles, as well the gas phase, in order to obtain some of the more complex species (Garrod & Herbst 2006). Whether this same environment can be extrapolated to the entire CMZ is unknown, particularly in view of the much broader line profiles. Is the chemistry through the CMZ sustained by the ejection of ices from grain mantles through the shock waves that are widespread throughout the region? Are these sustained through continual SN-events? Is this related to ongoing massive star formation there? Thus, is the CMZ a transient feature of our Galaxy, created by the feeding of the central region with gas (the fuel for star formation), or can the CMZ be maintained over galactic timescales of ~106 years? These are questions that can be better addressed through an understanding of the dynamics, content and physical conditions within the molecular gas of the CMZ, using the data we make available on this web site.

References

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