Abstract
ABSTRACT The SEDIGISM (Structure, Excitation and Dynamics of the Inner Galactic Interstellar Medium) survey used the APEX telescope to map 84 deg2 of the Galactic plane between ℓ = −60° and +31° in several molecular transitions, including 13CO (2 – 1) and C18O (2 – 1), thus probing the moderately dense (∼103 cm−3) component of the interstellar medium. With an angular resolution of 30 arcsec and a typical 1σ sensitivity of 0.8–1.0 K at 0.25 km s−1 velocity resolution, it gives access to a wide range of structures, from individual star-forming clumps to giant molecular clouds and complexes. The coverage includes a good fraction of the first and fourth Galactic quadrants, allowing us to constrain the large-scale distribution of cold molecular gas in the inner Galaxy. In this paper, we provide an updated overview of the full survey and the data reduction procedures used. We also assess the quality of these data and describe the data products that are being made publicly available as part of this First Data Release (DR1). We present integrated maps and position–velocity maps of the molecular gas and use these to investigate the correlation between the molecular gas and the large-scale structural features of the Milky Way such as the spiral arms, Galactic bar and Galactic Centre. We find that approximately 60 per cent of the molecular gas is associated with the spiral arms and these appear as strong intensity peaks in the derived Galactocentric distribution. We also find strong peaks in intensity at specific longitudes that correspond to the Galactic Centre and well-known star-forming complexes, revealing that the 13CO emission is concentrated in a small number of complexes rather than evenly distributed along spiral arms.
Highlights
Over the last few decades, many systematic continuum surveys of the Galactic plane have been carried out over the full electromagnetic spectrum, from the infrared (e.g. GLIMPSE, Churchwell et al 2009; MIPSGAL, Carey et al 2009; Hi-GAL, Molinari et al 2010) to themillimetre (ATLASGAL, Schuller et al 2009; BGPS, Aguirre et al 2011; JPS, Moore et al 2015; Eden et al 2017), and radio range (CORNISH, Hoare et al 2012; Purcell et al 2013; THOR, Beuther et al 2016, Wang et al 2020; GLOSTAR, Medina et al 2019)
It is worth noting that there is a wealth of intricate features that emerge in individual channel maps, but that do not appear or are washed out in the integrated intensity map. This implies that, even if most of the molecular gas is associated with a few major complexes as discussed above, there are plenty of other smaller features detected in the SEDIGISM data that are not associated with known complexes
It is possible to examine the intensity distribution as a function of the Galactocentric distance. This is accomplished by calculating the kinematic distance for each pixel in the v map above a 3σ rms threshold using the three-component rotation curve of Eilers et al (2019) and the Reid et al (2019) values for solar position and velocity, 8.15 kpc and 236 km s−1. This produces two distances for sources located within the solar circle spaced on either side of the tangent distance, it provides a unique distance from the Galactic Centre, which, allows us to investigate the distribution of the integrated intensity as a function of Galactocentric distance
Summary
Over the last few decades, many systematic continuum surveys of the Galactic plane have been carried out over the full electromagnetic spectrum, from the infrared (e.g. GLIMPSE, Churchwell et al 2009; MIPSGAL, Carey et al 2009; Hi-GAL, Molinari et al 2010) to the (sub)millimetre (ATLASGAL, Schuller et al 2009; BGPS, Aguirre et al 2011; JPS, Moore et al 2015; Eden et al 2017), and radio range (CORNISH, Hoare et al 2012; Purcell et al 2013; THOR, Beuther et al 2016, Wang et al 2020; GLOSTAR, Medina et al 2019). Recent results from the THOR survey reveal an increase by a factor of 6 of atomic to molecular gas ratio from the arms to inter-arm regions It is not clear whether this is due to the collection of molecular clouds that fall into their gravitational potential The spectroscopic data provide essential information on the distribution of interstellar matter along the line of sight, complementing the existing continuum surveys These data allow us to achieve an unbiased view of the moderately dense ISM over a large fraction of the Galactic disc. The survey has been described in Schuller et al (2017, hereafter Paper I) It consists of spectroscopic data covering the inner Galactic plane in the frequency range 217–221 GHz, which includes the 13CO (2 – 1) and C18O (2 – 1) molecular lines, at 30-arcsec angular resolution.
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