La Freccia Rossa: an IR-dark cloud hosting the Milky Way intermediate-mass black hole candidate

Abstract

The dynamics of the high-velocity compact molecular cloud CO-0.40-0.22 have been interpreted as evidence for a ∼10 5 M black hole within 60 pc of Sgr A *. Recently, Oka et al. have identified a compact millimetre-continuum source, CO-0.40-0.22 * , with this candidate black hole. Here we present a collation of radio and infrared data at this location. Australia Telescope Compact Array constraints on the radio spectrum, and the detection of a mid-infrared counterpart, are in tension with an Sgr A *-like model for CO-0.40-0.22 * despite the comparable bolometric to the Eddington luminosity ratios under the intermediate-mass black hole interpretation. A protostellar-disc scenario is, however, tenable. CO-0.40-0.22(*) is positionally coincident with an arrowhead-shaped infrared-dark cloud (which we call the Freccia Rossa). If the V LSR ≈ 70 km s −1 systemic velocity of CO-0.40-0.22 is common to the entire Freccia Rossa system, we hypothesize that it is the remnant of a high-velocity cloud that has plunged into the Milky Way from the Galactic halo.

Weighing the IMBH candidate CO-0.40-0.22* in the Galactic Centre

“…This is the first paper where a robust lower limit is given. However, we cannot exclude that other phenomena explain the observed velocity gradient inside CO-0.40-0.22, such as collisions with other clouds, bipolar outflows from young stellar objects or supernova explosions (Tanaka et al 2014;Ravi et al 2018;Yalinewich & Beniamini 2017).…”

“…In contrast with the assumptions of Mastrobuono-Battisti et al (2014), and Fragione et al (2018) expect the parent globular cluster to have dissolved before reaching the observed position of CO-0.40-0.22* and have left a "naked" black hole. From the IR observations of CO-0.40-0.22 (Ravi et al 2018), it is almost impossible to understand whether the IMBH candidate is naked or surrounded by a star cluster, because of the high absorption along the line of sight.…”

“…In particular, concerning thermodynamics, we can exclude that the observed velocity gradient is produced by a strong unbalance between the internal cloud pressure and the external pressure of the surrounding interstellar medium. In fact, such a high velocity gradient would mean either an "explosion" or an "implosion" of the cloud and requires another physical process (such as those invoked by Tanaka et al 2014;Tanaka 2018;Ravi et al 2018;Yalinewich & Beniamini 2017) to explain such an out-of-equilibrium configuration. However, we can easily test whether internal pressure gradients induced by radiation and hydrodynamical cooling/heating can become as important as tides.…”

Supernovae-generated high-velocity compact clouds

“…One option that has recently been proposed is the collision of two clouds (Ravi et al 2017). In order to distinguish between the two models, more observations are needed to catch these clouds at a younger age.…”

“…This inconsistency can be due to fluctuations that exceed the error margin, as can be seen in the observations of the Crab pulsar spectrum (Arendt et al 2011). An examination of archival data favors a protostellar disk (Ravi et al 2017), but more observations are required to shed light on the nature of the continuum source.…”