A&A 651, A85 (2021) Astronomy https://doi.org/10.1051/0004-6361/202039856 & c A. Brunthaler et al. 2021 Astrophysics A global view on star formation: The GLOSTAR Galactic plane survey I. Overview and first results for the Galactic longitude range 28◦ < l < 36◦ A. Brunthaler1 , K. M. Menten1, S. A. Dzib1, W. D. Cotton2,3, F. Wyrowski1, R. Dokara1;?, Y. Gong1, S.-N. X. Medina1, P. Müller1, H. Nguyen1;?, G. N. Ortiz-León1, W. Reich1, M. R. Rugel1, J. S. Urquhart4, B. Winkel1, A. Y. Yang1, H. Beuther5, S. Billington4, C. Carrasco-Gonzalez6, T. Csengeri7, C. Murugeshan8, J. D. Pandian9, and N. Roy10 1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: [email protected] 2 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA 3 South African Radio Astronomy Observatory, 2 Fir St, Black River Park, Observatory 7925, South Africa 4 Centre for Astrophysics and Planetary Science, University of Kent, Ingram Building, Canterbury, Kent CT2 7NH, UK 5 Max Planck Institute for Astronomy, Koenigstuhl 17, 69117 Heidelberg, Germany 6 Instituto de Radioastronomía y Astrofísica (IRyA), Universidad Nacional Autónoma de México Morelia, 58089 Morelia, Mexico 7 Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France 8 Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia 9 Department of Earth & Space Sciences, Indian Institute of Space Science and Technology, Trivandrum 695547, India 10 Department of Physics, Indian Institute of Science, Bengaluru 560012, India Received 5 November 2020 / Accepted 5 May 2021 ABSTRACT Aims. Surveys of the Milky Way at various wavelengths have changed our view of star formation in our Galaxy considerably in recent years. In this paper we give an overview of the GLOSTAR survey, a new survey covering large parts (145 square degrees) of the northern Galactic plane using the Karl G. Jansky Very Large Array in the frequency range 4−8 GHz and the Effelsberg 100-m telescope. This provides for the first time a radio survey covering all angular scales down to 1.5 arcsecond, similar to complementary near-IR and mid-IR galactic plane surveys. We outline the main goals of the survey and give a detailed description of the observations and the data reduction strategy. Methods. In our observations we covered the radio continuum in full polarization, as well as the 6.7 GHz methanol maser line, the 4.8 GHz formaldehyde line, and seven radio recombination lines. The observations were conducted in the most compact D configura- tion of the VLA and in the more extended B configuration. This yielded spatial resolutions of 1800 and 1.500 for the two configurations, respectively. We also combined the D configuration images with the Effelsberg 100-m data to provide zero spacing information, and we jointly imaged the D- and B-configuration data for optimal sensitivity of the intermediate spatial ranges. Results. Here we show selected results for the first part of the survey, covering the range of 28◦ < l < 36◦ and jbj < 1◦, including the full low-resolution continuum image, examples of high-resolution images of selected sources, and the first results from the spectral line data. Key words. surveys – ISM: general – H ii regions – ISM: supernova remnants – radio lines: ISM – radio continuum: general 1. Introduction Galactic Plane Survey (BGPS; Aguirre et al. 2011), JCMT Plane Survey (JPS; Moore et al. 2015; Eden et al. 2017), the Millime- Stars with more than about ten solar masses dominate galactic tre Astronomy Legacy Team 90 GHz (MALT90; Jackson et al. ecosystems. Understanding the circumstances of their formation 2013); and radio wavelength ranges (e.g., the Multi-Array Galac- is one of the great challenges of modern astronomy. In recent tic Plane Imaging Survey (MAGPIS; Becker 1990; Becker et al. years, our view of massive star-forming regions has been dramat- 1994), the Sino-German 6 cm survey (Han et al. 2015)1, the ically changed by Galactic plane surveys covering the infrared Coordinated Radio and Infrared Survey for High-Mass Star For- (e.g., Galactic Legacy Infrared Mid-Plane Survey Extraordinaire mation (CORNISH; Hoare et al. 2012; Purcell et al. 2013), The (GLIMPSE; Churchwell et al. 2009), MIPSGAL; Carey et al. H i, OH and Radio Recombination line survey of the Milky Way 2009, Hi-GAL; Molinari et al. 2010); (sub-)millimeter (e.g., (THOR; Beuther et al. 2016; Wang et al. 2020), the Southern APEX Telescope Large Area Survey of the Galaxy (ATLAS- Galactic Plane Survey (McClure-Griffiths et al. 2005), the VLA GAL; Schuller et al. 2009; Csengeri et al. 2014), Bolocam ? Member of the International Max Planck Research School (IM- 1 Data from this and various other large-scale centimeter-wavelength PRS) for Astronomy and Astrophysics at the Universities of Bonn and surveys can be accessed via the MPIfR Survey Sampler at https:// Cologne. www.mpifr-bonn.mpg.de/survey.html A85, page 1 of 18 Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Open Access funding provided by Max Planck Society. A&A 651, A85 (2021) Galactic Plane Survey (VGPS; Stil et al. 2006), the Canadian was realized early on by Menten(2007), who described a project Galactic Plane Survey (Taylor et al. 2003), the H2O Southern that has essentially evolved into the GLOSTAR survey. Galactic Plane Survey (HOPS; Walsh et al. 2011), the Methanol The GLOSTAR survey has been designed to address the Multibeam Survey (MMB; Green et al. 2009), the Galactic Ring limitation placed on these previous surveys using the vastly Survey (GRS; Jackson et al. 2006)). These surveys allow us for improved capabilities of the upgraded VLA. Our goal was to the first time to study all the evolutionary stages of massive star use the extremely wide-band (4–8 GHz) C-band receivers of formation (MSF) in an unbiased way (e.g., König et al. 2017; the VLA for an unbiased survey to find and characterize star- Elia et al. 2017; Urquhart et al. 2018). forming regions in the Galaxy. All fields are observed in the With the exciting results of the submillimeter and far- most compact D configuration of the VLA with a resolution of infrared (FIR) surveys from the ground (ATLASGAL) and space ∼1800 to have good surface brightness sensitivity for extended (Hi-GAL), the massive and cold dust clumps from which mas- structures and in the more extended B configuration that pro- sive clusters form are now being detected galaxy-wide. Com- vides a ten times higher spatial resolution to investigate compact plementary to these surveys, the centimeter studies using the sources. The VLA data are then complemented by observations VLA allow very powerful and comprehensive radio-wavelength with the Effelsberg 100-m telescope to provide zero spacings surveys of the ionized and the molecular tracers of star forma- for the D-configuration data. Finally, the data from the D and tion in the Galactic plane. There have been a number of VLA B configurations are combined (D+B) to provide the best pos- surveys of the inner part of the first quadrant of the Galac- sible images of sources with intermediate sizes by combining tic plane starting with the pioneering 20 cm (Becker 1990) and the surface brightness sensitivity of the D configuration with the 6 cm surveys (Becker et al. 1994) that are now combined in high resolution of the B configuration. the MAGPIS project2. More recently the CORNISH team have This survey of the Galactic plane detects tell-tale tracers of mapped the northern GLIMPSE region at 5 GHz with higher star formation: compact, ultra-compact, and hyper-compact H ii resolution and sensitivity than MAGPIS. CORNISH was tai- regions and molecular masers that trace different stages of early lored to specifically focus on identifying and parameterizing the stellar evolution and pinpoints the very centers of the early phase ultra-compact (UC) H ii region stage of MSF (e.g., Purcell et al. of star-forming activity. Combined with the submillimeter and 2013; Kalcheva et al. 2018), although it has also been used to infrared surveys, and our ongoing work in the Bar and Spiral investigate the Galactic population of planetary nebulae (PNe; Structure Legacy (BeSSeL) Survey4 to measure distances by Irabor et al. 2018). These studies have identified many thou- trigonometric parallaxes to most of the dominant star-forming sands of radio sources located towards the Galactic plane, and regions in the Galaxy, it offers a nearly complete census of the follow-up work has allowed many hundreds of Galactic radio number, luminosities, and masses of massive star-forming clus- sources to be identified and studied (e.g., Urquhart et al. 2013; ters in a large range of evolutionary stages and provides a unique Cesaroni et al. 2015; Kalcheva et al. 2018; Irabor et al. 2018). data set with true legacy value for a global perspective on star However, these interferometric snapshot surveys have been lim- formation in our Galaxy. In addition to information on high-mass ited by the historically small bandwidths available (∼50 MHz) star formation, the GLOSTAR survey allows efficient identifica- and sensitivity to emission on larger angular scales. These sur- tion and imaging of supernova remnants, planetary nebulae, and veys have therefore been restricted to continuum studies of rel- numerous extragalactic background sources. atively compact structures (i.e., <2000 for CORNISH) and are therefore unable to provide us with a global view of star forma- tion in the Galaxy.