A global view on star formation: The GLOSTAR Galactic plane survey. VII. Supernova remnants in the Galactic longitude range 28∘<l<36∘28^\circ<l<36^\circ

Context. While over 1000 supernova remnants (SNRs) are estimated to exist in the Milky Way, only less than 400 have been found to date. In the context of this apparent deficiency, more than 150 SNR candidates were recently identified in the D-configuration Very Large Array (VLA-D) continuum images of the 4--8 GHz global view on star formation (GLOSTAR) survey, in the Galactic longitude range $-2^\circ<l<60^\circ$. Aims. We attempt to find evidence of nonthermal synchrotron emission from 35 SNR candidates in the region of Galactic longitude range $28^\circ<l<36^\circ$, and also to study the radio continuum emission from the previously confirmed SNRs in this region. Methods. Using the short-spacing corrected GLOSTAR VLA-D+Effelsberg images, we measure ${\sim}6$ GHz total and linearly polarized flux densities of the SNR candidates and the SNRs that were previously confirmed. We also attempt to determine the spectral indices by measuring flux densities from complementary Galactic plane surveys and from the temperature-temperature plots of the GLOSTAR-Effelsberg images. Results. We provide evidence of nonthermal emission from four candidates that have spectral indices and polarization consistent with a SNR origin, and, considering their morphology, we are confident that three of these (G28.36+0.21, G28.78-0.44, and G29.38+0.10) are indeed SNRs. However, about $25\%$ of the candidates have spectral index measurements that indicate thermal emission, and the rest of them are too faint to have a good constraint on the spectral index yet. Conclusions. Additional observations at longer wavelengths and higher sensitivities will shed more light on the nature of these candidates. A simple Monte-Carlo simulation reiterates the view that future studies must persist with the current strategy of searching for SNRs with small angular size to solve the problem of the Milky Way's missing SNRs.Comment: To be published in A&A. 21 pages, 15 figure

A global view on star formation: The GLOSTAR Galactic plane survey VIII. Formaldehyde absorption in Cygnus~X

Cygnus X is one of the closest and most active high-mass star-forming regions in our Galaxy, making it one of the best laboratories for studying massive star formation. As part of the GLOSTAR Galactic plane survey, we performed large scale simultaneous H$_{2}$CO (1$_{1,0}$-1$_{1,1}$) spectral line and radio continuum imaging observations toward Cygnus X at $\lambda\sim$6 cm with the Karl G. Jansky Very Large Array and the Effelsberg-100 m radio telescope. Our Effelsberg observations reveal widespread H$_{2}$CO (1$_{1,0}$-1$_{1,1}$) absorption with a spatial extent of $\gtrsim$50 pc in Cygnus~X for the first time. On large scales of 4.4 pc, the relative orientation between local velocity gradient and magnetic field tends to be more parallel at H$_{2}$ column densities of $\gtrsim$1.8$\times 10^{22}$~cm$^{-2}$. On the smaller scale of 0.17 pc, our VLA+Effelsberg combined data reveal H$_{2}$CO absorption only toward three bright H{\scriptsize II} regions. Our observations demonstrate that H$_{2}$CO (1$_{1,0}$-1$_{1,1}$) is commonly optically thin. Kinematic analysis supports the assertion that molecular clouds generally exhibit supersonic motions on scales of 0.17-4.4 pc. We show a non-negligible contribution of the cosmic microwave background radiation in producing extended absorption features in Cygnus X. Our observations suggest that H$_{2}$CO ($1_{1,0}-1_{1,1}$) can trace molecular gas with H$_{2}$ column densities of $\gtrsim 5 \times 10^{21}$ cm$^{-2}$. The ortho-H$_{2}$CO fractional abundance with respect to H$_{2}$ has a mean value of 7.0$\times 10^{-10}$. A comparison of velocity dispersions on different linear scales suggests that the dominant $-3$ km s$^{-1}$ velocity component in the prominent DR21 region has nearly identical velocity dispersions on scales of 0.17-4.4 pc, which deviates from the expected behavior of classic turbulence.Comment: 27 pages, 23 figures, accepted for publication in A&

A global view on star formation: the GLOSTAR Galactic plane survey III. 6.7 GHz methanol maser survey in Cygnus X

The Cygnus X complex is covered by the Global View of Star Formation in the Milky Way (GLOSTAR) survey, an unbiased radio-wavelength Galactic plane survey, in 4–8 GHz continuum radiation and several spectral lines. The GLOSTAR survey observed the 6.7 GHz transition of methanol (CH$_3$OH), an exclusive tracer of high-mass young stellar objects. Using the Very Large Array in both the B and D configurations, we observed an area in Cygnus X of 7° × 3° in size and simultaneously covered the methanol line and the continuum, allowing cross-registration. We detected thirteen sources with Class II methanol maser emission and one source with methanol absorption. Two methanol maser sources are newly detected; in addition, we found four new velocity components associated with known masers. Five masers are concentrated in the DR21 ridge and W75N. We determined the characteristics of the detected masers and investigated the association with infrared, (sub)millimeter, and radio continuum emission. All maser sources are associated with (sub)millimeter dust continuum emission, which is consistent with the picture of masers tracing regions in an active stage of star formation. On the other hand, only five masers (38 ± 17%) have radio continuum counterparts seen with GLOSTAR within ~1″, testifying to their youth. Comparing the distributions of the bolometric luminosity and the luminosity-to-mass ratio of cores that host 6.7 GHz methanol masers with those of the full core population, we identified lower limits L$_{Bol}$ ~ 200 L$_⊙$ and L$_{Bol}$/M$_{core}$ ~ 1 L$_⊙$M$_⊙^{−1}$ for a dust source to host maser emission

A global view on star formation: The GLOSTAR Galactic plane survey I. Overview and first results for the Galactic longitude range 28° < l < 36°

Aims. Surveys of the Milky Way at various wavelengths have changed our view of star formation in our Galaxy considerably in recentyears. In this paper, we give an overview of a new survey covering large parts (145 square degrees) of the Northern Galactic planeusing the Karl G. Jansky Very Large Array (JVLA) in the frequency range from 4 − 8 GHz and the Effelsberg 100-m telescope. Thisprovides for the first time a radio survey covering all angular scales down to 1.5 arcsec, similar to complementary NIR/MIR galacticplane surveys. We outline the main goals of the survey and give a detailed description about the observations and the data reductionstrategy.Methods. In our observations we cover the radio continuum in full polarization, as well as the 6.7 GHz methanol maser line, the4.8 GHz formaldehyde line, and 7 radio recombination lines. The observations were conducted in the most compact D-configuration ofthe VLA, as well as in the more extended B-configuration. This yielded spatial resolutions of 18” and 1.5” for the two configurations,respectively. Additionally, we also combined the D-configuration images with the Effelsberg 100-m data to provide zero spacinginformation, 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 |b| < 1◦, including thefull low-resolution continuum image, examples of high-resolution images of selected sources, and first results from the spectral line data

A global view on star formation: The GLOSTAR Galactic plane survey IV. Radio continuum detections of young stellar objects in the Galactic Centre region

Context. The Central Molecular Zone (CMZ), a ∼200 pc sized region around the Galactic Centre, is peculiar in that it shows a star formation rate (SFR) that is suppressed with respect to the available dense gas. To study the SFR in the CMZ, young stellar objects (YSOs) can be investigated. Here we present radio observations of 334 2.2 μm infrared sources that have been identified as YSO candidates. Aims. Our goal is to investigate the presence of centimetre wavelength radio continuum counterparts to this sample of YSO candidates which we use to constrain the current SFR in the CMZ. Methods. As part of the GLObal view on STAR formation (GLOSTAR) survey, D-configuration Very Large Array data were obtained for the Galactic Centre, covering −2° < l < 2° and −1° < b < 1° with a frequency coverage of 4–8 GHz. We matched YSOs with radio continuum sources based on selection criteria and classified these radio sources as potential H II regions and determined their physical properties. Results. Of the 334 YSO candidates, we found 35 with radio continuum counterparts. We find that 94 YSOs are associated with dense dust condensations identified in the 870 μm ATLASGAL survey, of which 14 have a GLOSTAR counterpart. Of the 35 YSOs with radio counterparts, 11 are confirmed as H II regions based on their spectral indices and the literature. We estimated their Lyman continuum photon flux in order to estimate the mass of the ionising star. Combining these with known sources, the present-day SFR in the CMZ is calculated to be ∼0.068 M⊙ yr−1, which is ∼6.8% of the Galactic SFR. Candidate YSOs that lack radio counterparts may not have yet evolved to the stage of exhibiting an H II region or, conversely, are older and have dispersed their natal clouds. Since many lack dust emission, the latter is more likely. Our SFR estimate in the CMZ is in agreement with previous estimates in the literature

A global view on star formation: The GLOSTAR Galactic plane survey. VII. Supernova remnants in the Galactic longitude range 28° < l < 36°

Context. While over 1000 supernova remnants (SNRs) are estimated to exist in the Milky Way, fewer than 400 have been found to date. In the context of this apparent deficiency, more than 150 SNR candidates were recently identified in the D-configuration Very Large Array (VLA-D) continuum images of the 4–8 GHz Global View on Star Formation (GLOSTAR) survey, in the Galactic longitude range −2° < l < 60°. Aims. We attempt to find evidence of nonthermal synchrotron emission from 35 SNR candidates in the Galactic longitude range 28° < l < 36°, and to study the radio continuum emission from the previously confirmed SNRs in this region. Methods. Using the short-spacing-corrected GLOSTAR VLA-D+Effelsberg images, we measured the ∼6 GHz total and linearly polarized flux densities of the SNR candidates and the confirmed SNRs. We also attempted to determine the spectral indices by measuring flux densities from complementary Galactic plane surveys and from the temperature-temperature plots of the GLOSTAREffelsberg images. Results. We provide evidence of nonthermal emission from four candidates that have spectral indices and polarization consistent with a SNR origin, and, considering their morphology, we are confident that three of these (G28.36+0.21, G28.78-0.44, and G29.38+0.10) are indeed SNRs. However, about 25% of the candidates (8 out of 35) have spectral index measurements that indicate thermal emission, and the rest of them are so faint that is not possible to place a good constraint on the spectral index. Conclusions. Additional observations at longer wavelengths and higher sensitivities will shed more light on the nature of these candidates. A simple Monte Carlo simulation reiterates the view that future studies must persist with the current strategy of searching for SNRs with small angular sizes to solve the problem of the Milky Way’s missing 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

Confirmation Of Two Galactic Supernova Remnant Candidates Discovered by THOR

Anderson et al. identified 76 candidate supernova remnants (SNRs) using data from the H I/OH/Recombination line survey of the Milky Way. The spectral index and polarization properties can help distinguish between SNRs and H II regions, which are often confused. We confirm two SNR candidates using spectral index data and morphology. However, we observe that the fractional linear polarization cannot distinguish between SNRs and H II regions, likely due to contamination by diffuse Galactic synchrotron emission. We also comment on the association of SNR candidates with pulsars through geometric and age considerations

A Global View on Star Formation: The GLOSTAR Galactic PlaneSurvey: V: 6.7 GHz Methanol Maser Catalogue

Class II methanol (CH3OH) masers are amongst the clearest signposts of recent high-mass star formation (HMSF). A complete catalogue outlines the distribution of star formation in the Galaxy, the number of young star-forming cores, and the physical conditions of their environment. The Global View on Star Formation (GLOSTAR) survey, which is a blind survey in the radio regime of 4–8 GHz, maps the Galactic mid-plane in the radio continuum, 6.7 GHz methanol line, the 4.8 GHz formaldehyde line, and several radio recombination lines. We present the analysis of the observations of the 6.7 GHz CH3OH maser transition using data from the D-configuration of the Very Large Array (VLA). We analyse the data covering Galactic longitudes from −2 ◦ < l < 60◦ and Galactic latitudes of |b| < 1 ◦ . We detect a total of 554 methanol masers, out of which 84 are new, and catalogue their positions, velocity components, and integrated fluxes. With a typical noise level of ∼18 mJy beam−1 , this is the most sensitive unbiased methanol survey for methanol masers to date. We search for dust continuum and radio continuum associations, and find that 97% of the sources are associated with dust, and 12% are associated with radio continuum emission

A global view on star formation: The GLOSTAR Galactic plane survey. XI. Radio source catalog IV: 2∘<ℓ<28∘, 36∘<ℓ<60∘ and |b|<1∘

Context. The GLOSTAR survey studies star formation with the Very Large Array (VLA) and the Effelsberg 100 meter radio telescope in the Galactic plane between −2 ◦ < ℓ < 60◦ and |b| < 1 ◦ , and the Cygnus X region (76◦ < ℓ < 83◦ and −1 ◦ < b < 2 ◦ ) with unprecedented sensitivity in both flux density (1σ ∼50 µJy beam−1 ) and the capability of detecting emission with angular scales in the range from 1.′′0 to the largest radio structures in the Galaxy on the order of a few degrees size. Aims. Here, we provide a complete GLOSTAR-VLA D-configuration radio source catalog for the covered part of the Galactic disk. A catalog for the “pilot region” between 28◦ < ℓ < 36◦ has been published in a previous paper and here we present the complementary catalog for the area within 2◦ < ℓ < 28◦ , 36◦ < ℓ < 60◦ and |b| < 1 ◦ . Methods. Observations were taken with the VLA in a 4 – 8 GHz band to image 100 square degrees of the inner Galactic disk at a reference frequency of 5.8 GHz, using a total of 260 hours of telescope time. We determined spectral indices (α; S ν ∝ ν α ) inside the observed band and in the frequency range 1.4 – 5.8 GHz by complementing our results with those from the THOR survey, which covers 1 – 2 GHz. Results. The final images have an angular resolution of 18′′ and an average sensitivity of 123 µJy beam−1 . The sensitivity is better (∼ 60 µJy beam−1) in areas free of extended emission. The complementary Galactic disk catalog, presented in this work, consists of 11 211 radio sources. Of these, 1 965 are known large-scale structure sources such as star-forming region complexes, well-known supernova remnants (SNRs), SNR candidates or parts thereof. The remaining 9 227 are discrete individual sources. Source parameters, namely flux densities, sizes, spectral indices, and classifications are reported. We identify 769 H ii region candidates, 359 of which are newly classified as such. The mean value of spectral indices of 225 H ii regions is +0.14 ± 0.02, consistent with most of them emitting optically thin thermal radio emission. Combining our results with the previously published catalog of the pilot region, the final GLOSTAR-VLA D-configuration catalog contains 12 981 radio sources