The ALMA-CRISTAL survey. Discovery of a 15 kpc-long gas plume in a z=4.54 Lyman-alpha blob

Abstract

Massive star-forming galaxies in the high-redshift universe host large reservoirs of cold gas in their circumgalactic medium (CGM). Traditionally, these reservoirs have been linked to diffuse Lyman-alpha ( emission extending beyond approx parsec scales. In recent years, millimeter and submillimeter observations have started to identify even colder gas in the CGM through molecular and/or atomic tracers such as the transition. In this context, we studied the well-known system at $z=4.54$ that hosts a massive dusty star-forming galaxy (DSFG), a UV-bright companion, and a blob. We combined new ALMA line observations taken by the CRISTAL survey with data from previous programs targeting the system, and achieved a deep view into a DSFG and its rich environment at a $ parsec $ resolution. We identified an elongated structure with a projected size of parsec stemming from the bright DSFG at the center of the field, with no clear counterpart at any other wavelength. The plume is oriented approx away from the minor axis of the DSFG, and shows significant spatial variation of its spectral parameters. In particular, the emission shifts from second to second between the bottom and top of the plume, relative to the DSFG's systemic velocity. At the same time, the line width starts at $400- second $ but narrows down to second at the top end of the plume. We discuss four possible scenarios to interpret the plume: a conical outflow, a cold accretion stream, ram pressure stripping, and gravitational interactions. While we cannot strongly rule out any of these with the available data, we disfavor the ram pressure stripping scenario due to the requirement of special hydrodynamic conditions.