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

G0.253+0.016, commonly referred to as "the Brick" and located within the Central Molecular Zone, is one of the densest ($\approx10^{3-4}$ 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 $\{l_{0},b_{0}\}=\{0.248^{\circ}, 0.18^{\circ}\}$, has a radius of $1.3$ pc and kinematics indicative of the presence of a shell expanding at $5.2^{+2.7}_{-1.9}$ 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 and ionised gas are physically related. The inferred Lyman continuum photon rate is $N_{\rm LyC}=10^{46.0}-10^{47.9}$ photons s$^{-1}$, consistent with a star of spectral type B1-O8.5, corresponding to a mass of $\approx12-20$ M$_{\odot}$. 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; 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 $\lesssim10^{3}$ M$_{\odot}$ star cluster $\gtrsim0.4$ 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

Robust estimation of bacterial cell count from optical density

Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data

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

G0.253+0.016, commonly referred to as "the Brick" and located within the Central Molecular Zone, is one of the densest ($\approx10^{3-4}$ 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 $\{l_{0},b_{0}\}=\{0.248^{\circ}, 0.18^{\circ}\}$, has a radius of $1.3$ pc and kinematics indicative of the presence of a shell expanding at $5.2^{+2.7}_{-1.9}$ 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 and ionised gas are physically related. The inferred Lyman continuum photon rate is $N_{\rm LyC}=10^{46.0}-10^{47.9}$ photons s$^{-1}$, consistent with a star of spectral type B1-O8.5, corresponding to a mass of $\approx12-20$ M$_{\odot}$. 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; 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 $\lesssim10^{3}$ M$_{\odot}$ star cluster $\gtrsim0.4$ 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.Comment: 17 pages, 7 figures. Accepted for publication in MNRAS (October 15, 2021