ATLASGAL-selected massive clumps in the inner Galaxy

Abstract

Aims. Observations of millimeter wavelength radio recombination lines (mm-RRLs) are used to search for Hii regions in an unbiased way that is complementary to many of the more traditional methods previously used (e.g., radio continuum, far-infrared colors, maser emission). The mm-RRLs can be used to derive physical properties of Hii regions and to provide velocity information of ionized gas. Methods. We carried out targeted mm-RRL observations (39 ≤ principal quantum number (n) ≤ 65 and ∆n = 1, 2, 3, and 4, named Hnα, Hnβ, Hnγ, and Hnδ) using the IRAM 30m and Mopra 22m telescopes. In total, we observed 976 compact dust clumps selected from a catalog of ∼10,000 sources identified by the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). The sample was selected to ensure a representative mix of star-forming and quiescent clumps such that a variety of different evolutionary stages is represented. Approximately half of the clumps are mid-infrared quiet while the other half are mid-infrared bright. Results. We detected Hnα mm-RRL emission toward 178 clumps; Hnβ, Hnγ, and Hnδ were also detected toward 65, 23, and 22 clumps, respectively. This is the largest sample of mm-RRLs detections published to date. Comparing the positions of these clumps with radio continuum surveys we identified compact radio counterparts for 134 clumps, confirming their association with known Hii regions. The nature of the other 44 detections is unclear, but 8 detections are thought to be potentially new Hii regions while the mm-RRL emission from the others may be due to contamination from nearby evolved Hii regions. Broad linewidths are seen toward nine clumps (linewidth > 40 km s −1 ) revealing significant turbulent motions within the ionized gas; in the past, such wide linewidths were found toward very compact and dense Hii regions. We find that the systemic velocity of the associated dense molecular gas, traced by H 13 CO + (1−0), is consistent with the mm-RRL velocities and confirms them as embedded Hii regions. We also find that the linewidth of the H 13 CO + (1−0) emission is significantly wider than those without mm-RRL detection, indicating a physical connection between the embedded Hii region and their natal environments. We also find a correlation between the integrated fluxes of the mm-RRLs and the 6 cm continuum flux densities of their radio counterparts (the correlation coefficient, ρ, is 0.70). By calculating the electron densities we find that the mm-RRL emission is associated with Hii regions with n e <10 5 cm −3 and Hii region diameter > 0.03 pc. Conclusions. We detected mm-RRLs toward 178 clumps and identified eight new Hii region candidates. The broad mm-RRL from nine clumps may indicate that they arise in very young hyper-compact Hii regions. The mm-RRLs trace the radio continuum sources detected by high-resolution observations and their line parameters show associations with the embedded radio sources and their parental molecular clumps.

ATLASGAL-selected massive clumps in the inner Galaxy

“…In Giannetti et al (2014) we study the CO content of the clumps in the TOP100, showing that the abundance of CO is lower than expected in the cold and dense gas typical of clumps in early stages of evolution, and increases to canonical values in more evolved sources, closely mimicking the behaviours of low-mass cores. The TOP100 sample is included in the work of Kim et al (2017) as well, where mm-recombination lines are used to characterise the ionised gas and identify potentially young compact Hii regions. Mid-J CO lines, observed with CHAMP + at APEX in a region of 1 × 1 around the clumps of the TOP100, are analysed in Navarete et al (subm.…”

ATLASGAL-selected massive clumps in the inner Galaxy

“…Previous work on this sample addressed SiO emission (for parts of the sample, Csengeri et al 2016), dust continuum characterization , millimetre hydrogen recombination lines (for more evolved (i.e. H II regions) parts of the sample; Kim et al 2017), and temperature structure . The TOP100 is an ideal sample to study the physical and chemical parameters of the potentially massive star-forming regions at various evolutionary stages.…”

ATLASGAL-selected massive clumps in the inner Galaxy

“…In this paper, we investigate the properties of mid-J CO lines for a sub-sample of the original TOP100 as part of our effort to observationally establish a solid evolutionary sequence for highmass star formation. In addition to the dust continuum analysis of König et al (2017), we further characterised the TOP100 in terms of the content of the shocked gas in outflows traced by SiO emission (Csengeri et al 2016) and the ionised gas content (Kim et al 2017), the CO depletion (Giannetti et al 2014), and the progressive heating of gas due to feedback of the central objects (Giannetti et al 2017;Tang et al 2018). These studies confirm an evolution of the targeted properties with the original selection criteria and strengthen our initial idea that the TOP100 sample constitutes a valuable inventory of high-mass clumps in different evolutionary stages.…”