Pericenter passage of the gas cloud G2 in the Galactic Center

TL;DR

This study tracks the evolution of the gas cloud G2 as it approaches the Galactic Center's black hole, revealing its physical stretching, stable luminosity, and ballistic orbit, challenging some existing models of its nature.

Contribution

It provides detailed observational evidence of G2's physical and orbital evolution, supporting a ballistic cloud model over star-related hypotheses.

Findings

G2's head is increasingly stretched along its orbit.

Luminosity and line ratios of G2 remain constant over 9 years.

G2's motion is consistent with a ballistic orbit, not hydrodynamical interactions.

Abstract

We have further followed the evolution of the orbital and physical properties of G2, the object currently falling toward the massive black hole in the Galactic Center on a near-radial orbit. New, very sensitive data were taken in April 2013 with NACO and SINFONI at the ESO VLT . The 'head' of G2 continues to be stretched ever further along the orbit in position-velocity space. A fraction of its emission appears to be already emerging on the blue-shifted side of the orbit, past pericenter approach. Ionized gas in the head is now stretched over more than 15,000 Schwarzschild radii RS around the pericenter of the orbit, at ~ 2000 RS ~ 20 light hours from the black hole. The pericenter passage of G2 will be a process stretching over a period of at least one year. The Brackett-{\gamma} luminosity of the head has been constant over the past 9 years, to within +- 25%, as have the line ratios Brackett-{\gamma} / Paschen-{\alpha} and Brackett-{\gamma} / Helium-I. We do not see any significant evidence for deviations of G2's dynamical evolution, due to hydrodynamical interactions with the hot gas around the black hole, from a ballistic orbit of an initially compact cloud with moderate velocity dispersion. The constant luminosity and the increasingly stretched appearance of the head of G2 in the position-velocity plane, without a central peak, is not consistent with several proposed models with continuous gas release from an initially bound zone around a faint star on the same orbit as G2.