Cold molecular gas in the hot nuclear wind of the Milky Way

¹ Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
² Dipartimento di Fisica e Astronomia, Universitá degli Studi di Firenze, via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
³ Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autonóma de México, Apartado Postal 3-72, Morelia 58090, Michoacán, Mexico
⁴ Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
⁵ David Rockefeller Center for Latin American Studies, Harvard University, 1730 Cambridge Street, Cambridge, MA 02138, USA
⁶ National Radio Astronomy Observatory, Green Bank, WV 24944, USA
⁷ Research School of Astronomy and Astrophysics, Australian National University, ACT 2611, Australia

^★ Corresponding author; heyer@umass.edu

Received: 4 November 2024
Accepted: 6 February 2025

Abstract

Using the Large Millimeter Telescope and the SEQUOIA 3 mm focal plane array, we have searched for molecular line emission from two atomic clouds associated with the Fermi Bubble of the Milky Way. Neither ¹²CO nor ¹³CO J=1–0 emission is detected from the H I cloud, MW-C20. ¹²CO J=1–0 emission is detected from MW-C21 that is distributed within 11 clumps with most of the CO luminosity coming from a single clump. However, we find no ¹³CO emission to a 3σ brightness temperature limit of 0.3 K. Using this limit and RADEX non-local thermodynamic equilibrium (non-LTE) excitation models, we derive H₂ column density upper limits of (0.4–3)×10²¹ cm⁻² for a set of physical conditions and a H₂ to ¹²CO abundance ratio of 10⁴. Model CO-to-H₂ conversion factors are derived for each set of physical conditions. We find the maximum value is 1.6×10²⁰ cm⁻²/(K km s⁻¹). Increasing [H₂/¹²CO] to 10⁵ to account for photodissociation and cosmic ray ionization increases the column density and X(CO) upper limits by a factor of 10. Applying these X(CO) limits to the CO luminosities, the upper limit on the total molecular mass in MW-C21 is 132±2 M_⊙, corresponding to <27% of the neutral gas mass. For the three clumps that are fully resolved, lower limits to the virial ratios are 288±32, 68±28, and 157±39, which suggest that these structures are bound by external pressure to remain dynamically stable over the entrainment time of 2×10⁶ years or are being disrupted by shear and expansion over the clump crossing times of 3–8×10⁵ years. The observations presented in this study add to the growing census of cold gas entrained within the Galactic Center wind.