A gas cloud on its way towards the supermassive black hole at the Galactic Centre

Abstract

Measurements of stellar orbitsAs part of our NACO 9 and SINFONI 10,11 Very Large Telescope (VLT) observation programmes studying the stellar orbits around the Galactic Centre super-massive black hole, Sgr A*, we have discovered an object moving at about 1,700 km s -1 along a trajectory almost straight towards Sgr A* (Fig. 1). The object has a remarkably low temperature (about 550 K, Supplementary Fig. 2) and a luminosity about five times that The Brγ emission is elongated along its direction of motion with a spatially resolved velocity gradient (Fig. 2). Together these findings show that the object is a dusty, ionized gas cloud.The extinction of the ionized gas is typical for the central parsec times greater than that of the surrounding hot gas in the accretion flow 15 ; extrapolating to pericentre its density contrast will then still be about 60f V -1/2 . Similarly, the cloud's 4 4 ram pressure by far exceeds that of the hot gas throughout the orbit. In contrast, the thermal pressure ratio will quickly decrease from unity at apocentre and the hot gas is expected to drive a shock slowly compressing the cloud. Whereas the external pressure compresses the cloud from all directions, the black hole's tidal forces shear the cloud along the direction of its motion, because the Roche density for self-gravitational stabilization exceeds the cloud density by nine orders of magnitude 3 . In addition, the ram pressure compresses the cloud parallel to its motion. The interaction between the fast-moving cloud and the surrounding hot gas should also lead to shredding and disruption, owing to the Kelvin-Helmholtz and Rayleigh-Taylor instabilities 17-20 . Rayleigh-Taylor instabilities at the leading edge should in fact break up the cloud within the next few years if it started as a spheroidal, thick blob (Supplementary Information section 3). A thin, dense sheet would by now already have fragmented anddisintegrated, suggesting that f V is of the order of unity.We are witnessing the cloud's disruption happening in our spectroscopic data (Fig. 2). The intrinsic velocity width more than tripled over the last eight years, and we see between 2008 and 2011 a growing velocity gradient along the orbital direction. Test particle calculations implementing only the black hole's force show that an initially spherical gas cloud placed on the orbit (Table 1) is stretched along the orbit and compressed perpendicular to it, with increasing velocity widths and velocity gradients reasonably matching our observations (Fig. 3, Supplementary Fig. 4). There is also a tail of gas with lower surface brightness on approximately the same orbit as the cloud, which cannot be due to tidal disruption alone. It may be stripped gas, or lower-density, lower-filling-factor gas on the same orbit. The latter explanation is more plausible given that the integrated Brγ and L'-band luminosities did not drop by more than 30% The disruption and energy deposition processes in the next years until and after pericentre are powerful probes of the physical conditions in th...

Coupled jet-disk model for Sagittarius A*: explaining the flat-spectrum radio core with GRMHD simulations of jets

“…OurṀ = 4.5 × 10 −8 M yr −1 (see Table 1) is consistent withṀ = 6 × 10 −8 M yr −1 found in models where the BH is fed by stellar winds (Shcherbakov & Baganoff 2010, but see Quataert 2004, and with estimates by Sharma et al (2007) based on local collisionless plasma models. A higherṀ may also change expectations for the recently discovered cloud-like object G2 that moves toward Sgr A* (Gillessen et al 2012), which is expected to be accreted onto the BH soon. For a higher pre-impact accretion rate the increase inṀ due to the cloud would be much less dramatic than predicted (e.g., in Mościbrodzka et al 2012;or Sadowski et al 2013).…”

A cold-gas reservoir to fuel the M 31 nuclear black hole and stellar cluster

“…Lauer et al 2012;Li et al 2009). In the Galaxy, SgrA * has experienced X-ray flares, attributed to the infall of gas, while a cloud of gas identified by Gillessen et al (2012) is expected to fall onto the black hole in 2013. M 31 * is experiencing a similar murmur according to Li et al (2011), suggesting some gas infall.…”

Dust-enshrouded star near supermassive black hole: predictions for high-eccentricity passages near low-luminosity galactic nuclei

“…Gillessen et al (2012Gillessen et al ( , 2013a studied the evolution of such a cloud of particles in the field of the Sgr A* SMBH. We revisited their simulations by including the interaction with different prescriptions for the ambient wind.…”

“…Recently, an infrared-excess source named G2/DSO has been discovered (Gillessen et al 2012) and subsequently detected in L-and K-bands (Gillessen et al 2013a;Phifer et al 2013;Eckart et al 2013b). It may indeed be a manifestation of a common mechanism of material transport in low-luminosity nuclei.…”