Keck Observations of the Galactic Center Source G2: Gas Cloud or Star?

Abstract

We present new observations and analysis of G2 -the intriguing red emission-line object which is quickly approaching the Galaxy's central black hole. The observations were obtained with the laser guide star adaptive optics systems on the W. M. Keck I and II telescopes and include spectroscopy (R ∼ 3600) centered on the Hydrogen Brγ line as well as K ′ (2.1 µm) and L ′ (3.8 µm) imaging. Analysis of these observations shows the Br-γ line emission has a positional offset from the L ′ continuum. This offset is likely due to background source confusion at L ′ . We therefore present the first orbital solution derived from Br-γ line astrometry, which when coupled with radial velocity measurements, results in a later time of closest approach (2014.21 ± 0.14), closer periastron (130 AU, 1900R s ), and higher eccentricity (0.9814 ± 0.0060) compared to a solution using L ′ astrometry. The new orbit casts doubt on previous associations of G2 and a low surface brightness "tail." It is shown that G2 has no K ′ counterpart down to K ′ ∼20 mag. G2's L ′ continuum and the Br-γ line-emission is unresolved in almost all epochs; however it is marginally extended in our highest quality Br-γ data set from 2006 and exhibits a clear velocity gradient at that time. While the observations altogether suggest that G2 has a gaseous component which is tidally interacting with the central black hole, there is likely a central star providing the self-gravity necessary to sustain the compact nature of this object.

The Post-periapsis Evolution of Galactic Center Source G1: The Second Case of a Resolved Tidal Interaction with a Supermassive Black Hole

“…In this paper, the primary data sets are WMKO images that have been acquired through the L′ (λ 0 =3.8 μm) broadband filter over a 13 yr period with NIRC2, the facility near-infrared camera (PI: K. Matthews) fed by the Keck II laser guide star adaptive optics system (LGSAO; van Dam et al 2006;Wizinowich et al 2006). Ten of the 12 epochs have been previously reported by us and are part of our group's archive of fully calibrated data sets (Ghez et al 2004(Ghez et al , 2005bHornstein et al 2007;Phifer et al 2013;Witzel et al 2014). Two additional epochs of observation, 2013 August and 2016 May, are reported here for the first time.…”

“…The new L′ data sets were observed and calibrated using the same techniques described in the papers reporting our other L′ data sets (Stolte et al 2010;Phifer et al 2013;Witzel et al 2014). This followed standard techniques with the exception of the treatment of the sky exposures, which were taken for each field rotator mirror position within the same range as the science data in steps of ∼2°.…”

Clump formation through colliding stellar winds in the Galactic Centre

“…The tidal disruption of this cloud is being monitored by different groups (e.g., Eckart et al 2013;Gillessen et al 2013;Phifer et al 2013) and provides a unique opportunity to test accretion physics, due to both its proximity and the timescale on which it happens. Gillessen et al (2012) estimated the mass of G2 to be ∼ 3 M⊕ from its line emission.…”

No asymmetric outflows from Sagittarius A* during the pericenter passage of the gas cloud G2

“…The second possibility under discussion is that G2 is a circumstellar cloud around a star that provides stabilizing gravity and continuously replenishes the gas. Studies supporting this scenario highlighted that the tail might only be a fore-or background feature and might not be physically connected to G2 (Phifer et al 2013;Valencia et al 2015). Recent observations have also shown that G2 survived its pericenter passage as a compact source, which might be a clue that G2 is a star enshrouded by gas and/or dust (Witzel et al 2014;Valencia et al 2015).…”