Where is the western part of the Galactic Center Lobe located really?

The Galactic Center Lobe (GCL) is a peculiar object widely protruding from the Galactic plane toward the positive Galactic latitude, which had been found toward the Galactic Center (GC) in the early days of the radio observation. The peculiar shape has suggested a relation with historical events, star burst, large explosion, and so on in the GC. However, the issue of whether the GCL is a single large structure located in the GC region is not yet settled conclusively. In the previous observations, the silhouette against the low-frequency emission was found in the western part of the GCL (WPGCL); this suggests that the part is located in front of the GC region. On the other hand, the Local Standard of Rest (LSR) velocity of the radio recombination line toward it was found to be as low as 0 km s−1. However, these observations cannot determine the exact position on the line-of-sight. There is still another possibility that it is in the near-side area of the GC region. In this analysis, we compare these results with the visual extinction map toward the GC. We found that the distribution of the visual extinction larger than 4 mag clearly corresponds to the silhouette of the WPGCL. The WPGCL must be located at most within a few kpc from us and not in the GC region. This would be a giant H ii region in the Galactic disk.

Do dense molecular cores with broad emission spectra at |l| ≈ 5.4○, |b| ≈ 0.4○ trace the Galactic bar? A multi molecular line study from HOPS

We used NH$\rm _3$(1,1) data from the H$\rm _2$O Southern Galactic Plane Survey (HOPS) between −60○ < l < −2.7○ and 3.9○ < l < 30○ and |b| < 0.5○ to identify dense molecular regions/clumps of the Milky Way Galaxy that are likely to form high-mass stars. We identified ∼500 such clumps and in this paper we report on 14 of these clumps near the Central Molecular Zone that show broad emission spectral lines (with observed velocity widths between 19.8 and 47.6 km/s corresponding to intrinsic velocity widths between 7.1 and 25.2 km/s). We find that these clumps are grouped into three clusters of dense molecular cores centred at l ≈ 5.4○, −5.4○ and −10○. We name them ‘Cluster-1’, ‘Cluster-2’ (also known as ‘Bania’s Clump 1’) and ‘Cluster-3’. We find that the same clumps exhibit broad emission spectra for other molecular lines - NH$\rm _3$(2,2), (3,3) and HC3N(3−2). The anti-symmetry of Cluster-1 and Cluster-2 in the l − b plane, and the large velocity dispersion of each clump in these two clusters suggest that clusters 1 and 2 may be associated with the Galactic bar potential x-1 orbits. Our assessments show that the clumps of these three clusters host hot gases and their emission line broadening are associated with shock heating. We find that Cluster-3 is likely to be undergoing high-mass star formation, as suggested by the presence of emission from H$\rm _2$O maser and radio recombination line (H69α) from HOPS.

Radio recombination line observations towards Spitzer infrared bubbles with the TianMa radio telescope

ABSTRACT Many of the Spitzer infrared bubbles identified by the Milky Way Project (MWP) are suggested to be $\rm{H \small {II}} $ regions in nature. More than 70 per cent of the ∼5000 known bubbles do not have radio recombination line (RRL) observations, hence have not been confirmed as $\rm{H \small {II}} $ regions. A systematic RRL survey should be helpful to identify the nature of the bubbles. With the Shanghai TianMa 65-m radio telescope, we searched for RRLs towards 216 selected Spitzer bubbles by simultaneously observing 19 RRLs in the C band (4–8 GHz). RRLs are detected in the directions of 75 of the 216 targets. 31 of the 75 RRL sources are classified as new detections, which are possibly from new $\rm{H \small {II}} $ regions or diffuse warm ionized medium; 36 of them are probably from the outskirts of nearby bright $\rm{H \small {II}} $ regions, rather than bubble-encircled ionized gas; and the detected RRLs towards 8 bubbles are identified from known $\rm{H \small {II}} $ regions. For 58 of the 75 RRL sources, we obtained their distances after resolving the kinematic distance ambiguity by combining the results of the H2CO absorption method, the $\rm{H \small {I}} $ emission/absorption method, and the $\rm{H \small {I}} $ self-absorption method. The low detection rate of new $\rm{H \small {II}} $ regions implies that a number of MWP bubbles in the DR1 catalogue are too faint if they are $\rm{H \small {II}} $ regions.

Substructures in the Keplerian disc around the O-type (proto-)star G17.64+0.16

We present the highest angular resolution (∼20 × 15 mas–44 × 33 au) Atacama Large Millimeter/sub-millimeter Array (ALMA) observations that are currently possible of the proto-O-star G17.64+0.16 in Band 6. The Cycle 5 observations with baselines out to 16 km probe scales < 50 au and reveal the rotating disc around G17.64+0.16, a massive forming O-type star. The disc has a ring-like enhancement in the dust emission that is especially visible as arc structures to the north and south. The Keplerian kinematics are most prominently seen in the vibrationally excited water line, H2O 55, 0−64, 3 ν2 = 1 (Eu = 3461.9 K). The mass of the central source found by modelling the Keplerian rotation is consistent with 45 ± 10 M⊙. The H30α (231.9 GHz) radio-recombination line and the SiO (5-4) molecular line were detected at up to the ∼10σ level. The estimated disc mass is 0.6 − 2.6 M⊙ under the optically thin assumption. Analysis of the Toomre Q parameter in the optically thin regime indicates that the disc stability is highly dependent on temperature. The disc currently appears stable for temperatures > 150 K; this does not preclude that the substructures formed earlier through disc fragmentation.

The Galactic center lobe filled with thermal plasma

An observational result of a radio continuum and H92α radio recombination line of the Galactic center lobe (GCL), using the Yamaguchi 32 m radio telescope, is reported. The obtained spatial intensity distribution of the radio recombination line shows two distinctive ridge-like structures extending from the Galactic plane vertically to the north at the eastern and western sides of the Galactic center, which are connected to each other at a latitude of ${1{^{\circ}_{.}}2}$ to form a loop-like structure as a whole. This suggests that most of the radio continuum emission of the GCL is free–free emission, and that the GCL is filled with thermal plasma. The east ridge of the GCL observed with the radio recombination line separates 30 pc from the radio arc, which has been considered a part of the GCL, but coincides with a ridge of the radio continuum at a Galactic longitude of 0°. The radial velocity of the radio recombination line is found to be between −4 and +10 km s−1 across the GCL. This velocity is much smaller than expected from the Galactic rotation, and hence indicates that the GCL exists apart from the Galactic center. These characteristics of the GCL suggest that the long-standing hypothesis that the GCL was created by explosive activity in the Galactic center is unlikely, but favor that the GCL is a giant H ii region.

New detections of (sub)millimeter hydrogen radio recombination lines towards high-mass star-forming clumps

Aims. Previous radio recombination line (RRL) observations of dust clumps identified in the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) have led to the detection of a large number of RRLs in the 3 mm range. Here, we aim to study their excitation with shorter wavelength (sub)millimeter radio recombination line (submm-RRL) observations. Methods. We made observations of submm-RRLs with low principal quantum numbers (n ≤ 30) using the APEX 12 m telescope, toward 104 H II regions associated with massive dust clumps from ATLASGAL. The observations covered the H25α, H28α, and H35β transitions. Toward a small subsample the H26α, H27α, H29α, and H30α lines were observed to avoid contamination by molecular lines at adjacent frequencies. Results. We have detected submm-RRLs (signal-to-noise (S∕N)≥ 3 σ) from compact H II regions embedded within 93 clumps. The submm-RRLs are approximately a factor of two brighter than the mm-RRLs and consistent with optically thin emission in local thermodynamic equilibrium (LTE). The average ratio (0.31) of the measured H35β/H28α fluxes is close to the LTE value of 0.28. No indication of RRL maser emission has been found. The Lyman photon flux, bolometric, and submm-RRL luminosities toward the submm-RRL detected sources present significant correlations. The trends of dust temperature and the ratio of bolometric luminosity to clump mass, Lbol ∕Mclump, indicate that the H II regions are related to the most massive and luminous clumps. By estimating the production rate of ionizing photons, Q, from the submm-RRL flux, we find that the Q(H28α) measurements provide estimates of the Lyman continuum photon flux consistent with those determined from 5 GHz radio continuum emission. Six RRL sources show line profiles that are a combination of a narrow and a broad Gaussian feature. The broad features are likely associated with high-velocity ionized flows. Conclusions. We have detected submm-RRLs toward 93 ATLASGAL clumps. Six RRL sources have high-velocity RRL components likely driven by high-velocity ionized flows. Their observed properties are consistent with thermal emission that correlates well with the Lyman continuum flux of the H II regions. The sample of H II regions with mm/submm-RRL detections probes, in our Galaxy, luminous clumps (Lbol > 104 L⊙) with high Lbol∕Mclump. We also provide suitable candidates for further studies of the morphology and kinematics of embedded, compact H II regions with the Atacama Large Millimeter/submillimeter Array (ALMA).