A wind-blown bubble in the Central Molecular Zone cloud G0.253+0.016

Abstract

G0.253+0.016, commonly referred to as ‘the Brick’ and located within the Central Molecular Zone, is one of the densest (≈103–4 cm−3) molecular clouds in the Galaxy to lack signatures of widespread star formation. We set out to constrain the origins of an arc-shaped molecular line emission feature located within the cloud. We determine that the arc, centred on $\lbrace l_{0},b_{0}\rbrace =\lbrace 0{_{.}^{\circ}} 248,\, 0{_{.}^{\circ}} 018\rbrace$, has a radius of 1.3 pc and kinematics indicative of the presence of a shell expanding at $5.2^{+2.7}_{-1.9}$ $\mathrm{\, km\, s}^{-1}$. Extended radio continuum emission fills the arc cavity and recombination line emission peaks at a similar velocity to the arc, implying that the molecular gas and ionized gas are physically related. The inferred Lyman continuum photon rate is NLyC = 1046.0–1047.9 photons s−1, consistent with a star of spectral type B1-O8.5, corresponding to a mass of ≈12–20 M⊙. We explore two scenarios for the origin of the arc: (i) a partial shell swept up by the wind of an interloper high-mass star and (ii) a partial shell swept up by stellar feedback resulting from in situ star formation. We favour the latter scenario, finding reasonable (factor of a few) agreement between its morphology, dynamics, and energetics and those predicted for an expanding bubble driven by the wind from a high-mass star. The immediate implication is that G0.253+0.016 may not be as quiescent as is commonly accepted. We speculate that the cloud may have produced a ≲103 M⊙ star cluster ≳0.4 Myr ago, and demonstrate that the high-extinction and stellar crowding observed towards G0.253+0.016 may help to obscure such a star cluster from detection.

FEEDBACK from the NGC 7538 H II region

“…To estimate expected expansion velocities, for the thermal expansion, we followed the Spitzer solution (Spitzer 1998), whereas for the wind-driven expansion, the approach by Weaver et al (1977) and its adaption to include radiative cooling by Mac Low & McCray (1988) was adopted. A comprehensive summary of these two approaches is given in Henshaw et al (2022). The expansion velocities depending on time for the thermal expansion v th (t) and the wind-driven expansion v w (t) are:…”

“…Here, c s is the sound speed in the ionized gas (8 km s −1 at 5000 K), L w = 1 2 Ṁv 2 ∞ is the mechanical wind luminosity, ρ the ambient density, and t cool the cooling time following Mac Low & McCray (1988) and Henshaw et al (2022). The wind mass flow rate Ṁ and the escape velocity v ∞ for an O3 star (IRS6, Puga et al 2010) are taken from Muijres et al (2012).…”

The initial conditions for young massive cluster formation in the Galactic Centre: convergence of large-scale gas flows

“…Walker et al (2021) find that this is only a small grouping of 18 low-to-intermediate mass sources, and is contained to a small area rather than widespread throughout the 'Brick'. Henshaw et al (2022b) find that the Brick may have already formed a small (∼10 3 M ) cluster.…”

“…A subset of the dust ridge clouds have been studied in detail, from pc scales down to the scale of individual cores (∼1000 au) (e.g. Lis et al 1994;Immer et al 2012;Longmore et al 2012Longmore et al , 2013bRodríguez & Zapata 2013;Rathborne et al 2014Rathborne et al , 2015Mills et al 2015;Walker et al 2018Walker et al , 2021Barnes et al 2019;Lu et al 2019aLu et al ,b, 2020Lu et al , 2021Henshaw et al 2019Henshaw et al , 2022bBattersby et al 2020;Hatchfield et al 2020). Based on the evolution of dense gas structure and analysis of the gas kinematics, Walker et al (2015Walker et al ( , 2016 and Barnes et al (2019) conclude that YMCs forming from these clouds are more likely to do so in a way that is more consistent with the predictions of a hierarchical conveyor belt mode.…”

“…Lu et al 2019aLu et al , 2020Walker et al 2021). Unfortunately -at least as far as searching for a prestar-forming YMC progenitor cloud is concerned -even the most massive and previously most quiescent of these, G0.253+0.016 (the 'Brick'), has now been shown to be unambiguously forming stars (Walker et al 2021;Henshaw et al 2022b). Walker et al (2021) find that this is only a small grouping of 18 low-to-intermediate mass sources, and is contained to a small area rather than widespread throughout the 'Brick'.…”