The Central Molecular Zone of the Milky Way
The central ~500pc of the Milky Way, the center being marked by the supermassive black hole Sgr A*, is called the Central Molecular Zone (CMZ) due to the large amounts of molecular gas found there. It is a peculiar environment that differs significantly from typical disk ISM and poses additional challenges but also a unique opportunity to understand star formation in high spatial resolution.
The figure above shows the inner region on the CMZ, approximately 250 pc across, traced by ammonia (3,3) emission. Several landmarks emit this spectral line such as Sgr B2, one of the most massive GMCs in the Milky Way, that is thought to be the precursor of a massive stellar cluster (Walker et al., 2015). Some smaller molecular clouds at positive longitudes were named by Lis et al. (1999) along a coherent structure (Brick, cloud d, e, f, Sgr B2) that was dubbed “dust ridge” due the large amount of dust detected there. The asymmetry in gas distribution with Galactic longitude is immediately visible, only small and comparatively thin clouds are detected east of the 20 km/s cloud. Most of the projected emission is placed at Galactic Center distances and can be assumed to be part of the CMZ with a few exceptions, like a cloud at l ~ 0.4 (Longmore et al., 2013). Two stellar clusters close to the Galactic Center (Arches and Quintuplett) are invisible in the figure above but located close to the gas streams in projection. Their 3D position is debated (e.g. Stolte et al., 2008) but might prove important for testing the kinematic star formation model by Longmore et al. (2013) and Kruijssen et al. (2015) that I studied during my Master's thesis. The large reservoir of molecular gas in the CMZ adds up to M ~ 5*107 M☉ (Ferrière et al., 2007; Morris & Serabyn, 1996) at densities much higher than in the Milky Way's disk with a mean of ~ 104 cm3 compared to 102 cm3 in the disk (Longmore et al., 2013). Such high densities imply a star formation rate of ~ 0.7-0.9 M☉/yr according to the star formation law (Kennicutt, 1998) while only 0.08-0.14 M☉/yr is observed (Immer et al., 2012; Yusef-Zadeh et al., 2009). The reason is suspected to be the highly turbulent medium with Mach numbers up to M ~ 30 and significantly higher temperatures than in disk of 50 to > 100 K (Tdisk ~ 10-30 K in the CNM). These conditions are similar to high-z star forming and star bursting galaxies at z ~ 2-3 which is why the CMZ was given the nickname of “nearest high redshift galaxy”. Understanding the physics of the CMZ at moderate distance therefore enable us to understand how most of today’s stars formed. The peak of the cosmic star formation history occurs around z = 2-3 and 50% of stars formed earlier than z ~ 1.3 (Madau & Dickinson, 2014).
Overall, gas and dust in the CMZ is structured in a ring-like feature of ~100pc radius that follows the dust ridge (l+, b+ stream), turns to the far side near Sgr B2 (l+, b-) to pass behind Sgr A* and close the apparent “ring” through the negative longitude streams l-, b+, the GMC Sgr C, and l-, b-. Recent observational hints (Longmore et al., 2013) brought up the idea of a triggered star formation sequence in the dust ridge and two kinematic models to describe gas motions (Kruijssen et al., 2015; Krumholz & Kruijssen, 2015; Molinari et al., 2011). Understanding the thermal properties of the dense gas is key to understanding star formation in this peculiar environment as temperature sets sound speed which in turn sets the Mach number M that is used to characterize turbulence which inhibits star formation.
With the SWAG survey as described below, we can study the Galactic Center in general and this model of triggered star formation in particular.
Survey of Water and Ammonia in the Galactic Center
The Galactic Center contains more than 10% of the Milky Way’s molecular gas in the CMZ that is eventually converted into stars. This gas is shaped by the extreme environment by strong shocks, cosmic rays, high energy photons, turbulence, strong magnetic and tidel fields. To gain a better understanding of how gas cools, collapses and forms stars, we are carrying out the “Survey of Water and Ammonia in the Galactic Center”. In this survey the whole CMZ is imaged in more than 42 spectral lines at 1pc resolution from data obtained at the Australia Telescope Compact Array (ATCA) including several species of Ammonia, Radio Recombination lines, masers and shock tracers. ~20 of these lines have never been sampled before in the Galactic Center to offer a new and more detailed insight then before. SWAG will enable us to better understand the small scale gas physics of galactic nuclei and sets a benchmark in this field.
The wealth of this survey allows us to map the structure and kinematics of the gas at 1pc resolution, obtain a large scale temperature map, accurate up to hundreds of K, derive a map of opacity, obtain an "archaeological" map of gas formation temperatures, collate a water maser survey to trace actual star formation, obtain maps of shock and photon- dominated region tracers, derive maps of radio recombination lines to mark ionizing sources, compute maps of continuum, spectral index and curvature, derive a molecular clump catalog with properties for ~10000 objects. Together these data will allow us to disentangle the processes that control the state of the gas and capability to form stars in the CMZ.