A cool accretion disk around the Galactic Centre black hole

Abstract

A supermassive black hole Sagittarius A* (SgrA*) with the mass M SgrA * 4 × 10 6 M resides at the centre of our galaxy [1,2]. A large reservoir of hot (10 7 K) and cooler (10 2 − 10 4 K) gas surrounds it within few pc [3]. Building up such a massive black hole within the ∼ 10 10 year lifetime of our galaxy would require a mean accretion rate of ∼ 4 × 10 −4 M yr −1 . At present, X-ray observations constrain the rate of hot gas accretion at the Bondi radius (10 5 R Sch = 0.04 pc at 8 kpc) tȯ M Bondi ∼ 3 × 10 −6 M yr −1 [4,5,6], and polarization measurements [7] constrain it near the event horizon toṀ horizon ∼ 10 −8 M yr −1 . A range of models was developed to describe the accretion gas onto an underfed black hole [8,9,10]. However, the exact physics still remains to be understood. One challenge with the radiation inefficient accretion flows (RIAFs) is that even if one understands the dynamics there is no accepted prescription for associating emissivity (and absorption) with the flow. The other issue is the lack of model-independent probes of accretion flow at intermediate radii (between few and ∼ 10 5 R s ), i.e. the constraints that do not assume a model of accretion flow as an input parameter. Here we report a detection and imaging of the 10 4 K ionized gas disk within 2×10 4 R Sch in a millimetre hydrogen recombination line H30α : n = 31 → 30 at 231.9 GHz [11,12] using the Atacama Large Millimeter/submillimeter Array (ALMA). The emission was detected with a double-peaked line profile spanning full width of 2, 200 km s −1 with the approaching and the receding components straddling Sgr A*, each offset from it by 0.11 arcsec = 0.004 pc. The red-shifted side is displaced to the north-east, while the blue-shifted side is displaced to the south-west. The limit on the total mass of ionized gas estimated from the emission is 10 −4 − 10 −5 M at a mean hydrogen density 10 5 − 10 6 cm −3 , depending upon whether or not we assume the presence of a uniform density disk or an ensemble of orbiting clouds, and the amplification factor of the mm radiation due to the strong background source which is Sgr A* continuum.Black hole accretion and feedback are crucial to understanding the evolution of galaxies, the origin of relativistic jets, and to study physics near black hole horizons. Sgr A* is our nearest supermassive black hole. As such it offers an excellent opportunity to study accretion and outflow processes close to a black hole. X-ray measurements of gas density and temperature at the outer edge of the accretion flow, together with the assumptions of spherical adiabatic and constant in time accretion [13], can be used to estimate the mass supply to the black hole aṡ M Bondi ∼ 3×10 −6 M yr −1 [4,5,6]. If the radiative efficiency were ∼ 10% [14,15], the luminosity would be roughly 30,000 times the bolometric luminosity of ∼ 3 × 10 36 erg s −1 [16]. Extensive

Monitoring dusty sources in the vicinity of Sagittarius A*

“…1 and 6), hydrogen gets excited and the depletion of [FeIII] from the dust grains of the dusty sources is increased. Upcoming observations with MIRI and long term monitoring with SIN-FONI on larger scales of these sources and the GC will probably reveal more interesting details about the connection of the Sgr A* bubble, the wind, the in Murchikova et al (2019) presented Doppler-shifted Brγ emission, and the presented north-to south Brγ-bar. These observations becoming more important in the future since some dusty sources, e.g., D2 and D3 are moving in projection towards regions, where no wind features are present.…”

A Hidden Friend for the Galactic Center Black Hole, Sgr A*

“…where M d is the mass of the disk. Thus, for the cold disk, with M d ∼ 10 −4 − 10 −5 M (Murchikova et al 2019) the timescale due to accretion is rather long, depicted in Figure 4 as a grey band. For the hot component we adopt M d ∼ 10 −9 M .…”

“…This accretion model seems to be consistent with the observed spectrum of Sgr A* and may yield high temperature for the plasma, (∼ 10 12 K and ∼ 10 9 K for the ions and electrons, respectively, e.g., Narayan et al 1998). Recently, Murchikova et al (2019) reported the detection of a cool (∼ 10 4 K) rotationally supported disk, embedded within the hot plasma. Below we adopt a two component model in order to investigate the consequences of a companion on SgrA* under a broad range of external conditions.…”

Wideband 67−116 GHz receiver development for ALMA Band 2

“…The second key science driver proposed in Mroczkowski et al (2019a) is to probe the origins of chemical complexity, one of the key science goals identified in the ALMA 2030 Development These include the ground state transitions of deuterated dense gas probing species which trace the cold, quiescent regions at the cusp of star formation and complex organic molecules that can trace building blocks of life (including amino acids such as glycene), which at these lower frequencies are easier to identify due to the lower spectral-crowding. In addition, this frequency range is rich in recombination lines of hydrogen (see Murchikova et al 2019 and the discussion in Mroczkowski et al 2019a), along with tracers of the chemical evolution of stars and stellar envelopes, to name a few. As with the above case, the molecular lines found in Band 2 are often due to ground state transitions, meaning they are brighter and that they leverage abundances directly.…”