A Global View on Star Formation: The GLOSTAR Galactic Plane Survey: II Supernova Remnants in the first quadrant of the Milky Way

Context. The properties of the population of the Galactic Supernova Remnants (SNRs) are essential to our understanding of the dynamics of the Milky Way’s interstellar medium (ISM). However, the completeness of the catalog of Galactic SNRs is expected to be only ∼30%, with on order 700 SNRs yet to be detected. Deep interferometric radio continuum surveys of the Galactic plane help in rectifying this apparent deficiency by identifying low surface brightness SNRs and compact SNRs that have not been detected in previous surveys. However, SNRs are routinely confused with H ii regions, which can have similar radio morphologies. Radio spectral index, polarization, and emission at mid-infrared (MIR) wavelengths can help distinguish between SNRs and H ii regions. Aims. We aim to identify SNR candidates using continuum emission from the Karl G. Jansky Very Large Array Global view of the Star formation in the Milky Way (GLOSTAR) survey. Methods. GLOSTAR is a C-band (4–8 GHz) radio wavelength survey of the Galactic plane covering 358◦ ≤ l ≤ 60◦, |b| ≤ 1◦. The continuum images from this survey that resulted from observations in the array’s most compact configuration have an angular resolution of 18″. We searched for SNRs in these images to identify known SNRs, previously-identified SNR candidates and new SNR candidates. We study these objects in MIR surveys and the GLOSTAR polarization data to classify their emission as thermal or nonthermal. Results. We identify 157 SNR candidates, out of which 80 are new. Polarization measurements provide evidence of nonthermal emission from 9 of these candidates. We find that two previously identified candidates are filaments. We also detect emission from 91 out of 94 known SNRs in the survey region. Four of these are reclassified as H ii regions following detection in MIR surveys. Conclusions. The better sensitivity and resolution of the GLOSTAR data have led to the identification of 157 SNR candidates, along with the reclassification of several misidentified objects. We show that the polarization measurements can identify nonthermal emission, despite the diffuse Galactic synchrotron emission. These results underscore the importance of higher resolution and higher sensitivity radio continuum data in identifying and confirming SNRs

GLOSTAR — Radio Source Catalog II: 28◦ < l < 36◦ and |b| < 1◦,VLA B-configuration

As part of the Global View on Star Formation (GLOSTAR) survey we have used the Karl G. Jansky Very Large Array (VLA) in its B-configuration to observe the part of the Galactic plane between longitudes of 28◦ and 36◦ and latitudes from −1◦ to +1◦ at the C-band (4–8 GHz). To reduce the contamination of extended sources that are not well recovered by our coverage of the (u, v)-plane we discarded short baselines that are sensitive to emission on angular scales < 4′′. The resulting radio continuum images have an angular resolution of 1.′′0, and a sensitivity of ∼ 60 μJy beam−1; making it the most sensitive radio survey covering a large area of the Galactic plane with this angular resolution. An automatic source extraction algorithm was used in combination with visual inspection to identify a total of 3325 radio sources. A total of 1457 radio sources are ≥ 7σ and comprise our highly reliable catalog; 72 of these are grouped as 22 fragmented sources, e.g., multiple components of an extended and resolved source. To explore the nature of the catalogued radio sources we searched for counterparts at millimeter and infrared wavelengths. Our classification attempts resulted in 93 H ii region candidates, 104 radio stars, 64 planetary nebulae, while most of the remaining radio sources are suggested to be extragalactic sources. We investigated the spectral indices (α, S ν ∝ να) of radio sources classified as H ii region candidates and found that many have negative values. This may imply that these radio sources represent young stellar objects that are members of the star clusters around the high mass stars that excite the H ii regions, but not these H ii regions themselves. By comparing the peak flux densities from the GLOSTAR and CORNISH surveys we have identified 49 variable radio sources, most of them with an unknown nature. Additionally, we provide the list of 1866 radio sources detected within 5 to 7σ levels