A Galactic-scale Magnetized Wind around a Normal Star-forming Galaxy

Abstract

Galaxy formation theory identifies superwinds as a key regulator of star formation rates, galaxy growth, and chemical enrichment. Thermal and radiation pressure are known to drive galactic-scale winds in dusty starbursting galaxies (e.g., M82), but modern numerical simulations have recently highlighted that cosmic-ray (CR)–driven winds may be especially important in normal galaxies with modest star formation rate surface densities. However, CR-driven winds have yet to be conclusively observed—leaving significant uncertainty in their detailed microphysics. We present MeerKAT radio continuum and H i spectral-line observations of one such normal galaxy, NGC 1532; a nearby ( D ~ 15 Mpc) and nearly edge-on ( i ≳ 80°) spiral galaxy tidally interacting with its smaller elliptical companion, NGC 1531. We find magnetized, highly ordered radio-continuum loops extending ~10 kpc above and below the disk, visibly connecting discrete star-forming regions in the disk to the center. The deep MeerKAT H i observations place an upper limit on the column density of neutral gas coincident with the outflow of N _H I ≲ 3 × 10 ^19 cm ^−2 . Unlike previously observed outflows—for which ejected gas and dust can be traced across multiple wavelengths—the loops in NGC 1532 show no detectable signs of dust or gas coincident with the radio emission far from the disk. We explore multiple possible mechanisms for driving this magnetic wind and favor an explanation where CR pressure plays a significant role in launching these outflows.

Keywords

• Cosmic rays
• Galactic winds
• Radio continuum emission