Approaching hell’s kitchen: Molecular daredevil clouds in the vicinity of Sagittarius A*

Abstract

We report serendipitous detections of line emission with the Atacama Large Millimeter/submillimeter Array (ALMA) in bands 3, 6, and 7 in the central parsec down to within 1 around Sgr A* at an up to now highest resolution (<0.5 ) view of the Galactic center (GC) in the submillimeter (sub-mm) domain. From the 100 GHz continuum and the H39α emission we obtain a uniform electron temperature around T e ∼ 6000 K for the minispiral. The spectral index (S ∝ ν α ) of Sagittarius A* (Sgr A*) is ∼ 0.5 at 100 -250 GHz and ∼ 0.0 at 230 -340 GHz. The bright sources in the center show spectral indices around -0.1 implying Bremsstrahlung emission, while dust emission is emerging in the minispiral exterior. Apart from CS, which is most widespread in the center, H 13 CO + , HC 3 N, SiO, SO, C 2 H, CH 3 OH, 13 CS and N 2 H + are also detected. The bulk of the clumpy emission regions is at positive velocities and in a region confined by the minispiral northern arm (NA), bar, and the sources IRS 3 and 7. Although partly spatially overlapping with the radio recombination line (RRL) emission at same negative velocities, the relation to the minispiral remains unclear. A likely explanation is an infalling clump consisting of denser cloud cores embedded in diffuse gas. This central association (CA) of clouds shows three times higher CS/X (X: any other observed molecule) ratios than the circumnuclear disk (CND) suggesting a combination of higher excitation, by a temperature gradient and/or infrared (IR) pumping, and abundance enhancement due to UV and/or X-ray emission. Hence, we conclude that this CA is closer to the center than the CND is to the center. Moreover, we find molecular line emission at velocities up to 200 km s −1 . Apart from the CA, we identified two intriguing regions in the CND. One region shows emission in all molecular species and higher energy levels tested in this and previous observations and contains a methanol class I maser. The other region shows similar behavior of the line ratios such as the CA. Outside the CND, we find the traditionally quiescent gas tracer N 2 H + coinciding with the largest IR dark clouds (IRDC) in the field. Methanol emission is found at and around previously detected methanol class I masers in the same region. We propose to make these particular regions subject to further studies in the scope of hot core, cold core, and extreme photon and/or X-ray dominated region (PDR/XDR) chemistry and consequent star formation in the central few parsecs.

The Nuclear Filaments inside the Circumnuclear Disk in the Central 0.5 pc of the Galactic Center

“…However, our wide-field map combining interferometric and single-dish data recovers the extended emission, especially in the inner edge of the CND. This newly detected structural connection of the CND with the nuclear filaments appears clearer and more robust than in previous maps (Moser et al 2017;Tsuboi et al 2018). Moreover, the high-excitation CS(7-6) line also better discriminates against lower-excitation material and reveals the nuclear filaments more clearly than in CO(3-2) (Goicoechea et al 2018).…”

“…The lengths and widths of these filamentary structures are ∼ 0.2 pc ×0.1 pc. In general, the CS(7-6) line morphology is similar to the CS(5-4) in Moser et al (2017). However, our wide-field map combining interferometric and single-dish data recovers the extended emission, especially in the inner edge of the CND.…”

“…Figure 1 displays the integrated CS(7-6) intensity maps (MOM0) in the inner region of the CND. The detection of molecular gas within a projected diameter of 0.5 pc around Sgr A* is called the central association in Moser et al (2017). This central association extends mostly along a north-south orientation, centered on Sgr A*.…”

A common origin for the circumnuclear disc and the nearby molecular clouds in the Galactic Centre

“…These young stars are surrounded by a circumnuclear disc (CND) of molecular and neutral gas and dust, sitting at 1.5-4 pc from SgrA* (e.g., Gatley et al 1986;Guesten et al 1987;Jackson et al 1993;Herrnstein & Ho 2002;Shukla et al 2004;Christopher et al 2005;Montero-Castaño et al 2009;Oka et al 2011;Martín et al 2012;Lau et al 2013;Mills et al 2013). The CND also contains several tens of M of ionized gas in clumpy filaments/arms observable in radio and infrared continuum and emission lines (i.e., the so-called "minispiral"; see, e.g., Lo & Claussen 1983;Serabyn & Lacy 1985;Paumard et al 2004;Zhao et al 2009Zhao et al , 2010Lau et al 2013;Tsuboi et al 2016;Moser et al 2017).…”

A stellar fly‐by close to the Galactic center: Can we detect stars on highly relativistic orbits?

“…The inner edge of the neutral and molecular circum‐nuclear disk (CND) coincides with the radius of the SMBH sphere of influence, (Duschl ), inside which the ionized hot plasma is located that emits thermal X‐ray bremsstrahlung (Mossoux & Eckart ). Three ionized denser streamers of Sgr A West or the Minispiral are thought to have dynamically originated in the CND via the loss of the angular momentum due to mutual collisions of CND clumps or the interaction with fast stellar winds of massive OB stars (Moser et al ; Vollmer & Duschl ).…”