ALMA observations of the interaction of a radio jet with molecular gas in Minkowski's Object

Abstract: We use ALMA to detect and image CO (1-0) emission from Minkowski's Object, a dwarf galaxy that is interacting with a radio jet from a nearby elliptical galaxy. These observations are the first to detect molecular gas in Minkowski's Object. We estimate the range in the mass of molecular gas in Minkowski's Object assuming two different values of the ratio of the molecular gas mass to the CO luminosity, $\alpha_{\rm CO}$. For the Milky Way value of $$\alpha_{\rm CO}=4.6~M_{\odot}{\rm (K~km~s<sup>{-1}~pc<sup>2)<sup>{-1}}$$ we obtain a molecular gas mass of $M_{\rm H_2} =3.0 \times 10<sup>7~M_{\odot}$, 6% of the HI gas mass. We also use the prescription of Narayanan et al. (2012) to estimate an $$\alpha_{\rm CO}=27~M_{\odot}{\rm (K~km~s<sup>{-1}~pc<sup>2)<sup>{-1}}$$, in which case we obtain $M_{\rm H_2} =1.8 \times 10<sup>8~M_{\odot}$, 36% of the HI mass. The observations are consistent with previous claims of star formation being induced in Minkowski's Object via the passage of the radio jet, and it therefore being a rare local example of positive feedback from an AGN. In particular, we find highly efficient star formation, with gas depletion timescales $\sim 5\times 10⁷ - 3\times 10<sup>8$yr (for assumed values of $\alpha_{\rm CO}=4.6$ and $$27~M_{\odot}{\rm (K~km~s<sup>{-1}~pc<sup>2)<sup>{-1}}$$, respectively) in the upstream regions of Minkowski's Object that were struck first by the jet, and less efficient star formation downstream. We discuss the implications of this observation for models of jet induced star formation and radio mode feedback in massive galaxies.