Blowing in the Milky Way wind: neutral hydrogen clouds tracing the Galactic nuclear outflow

TL;DR

This study uses a sensitive hydrogen survey to identify high-velocity clouds near the Galactic Center, supporting a model of a large-scale nuclear outflow driven by star formation feedback, with implications for galactic wind dynamics.

Contribution

It provides the first detailed survey of neutral hydrogen clouds above and below the Galactic Center, modeling their kinematics as part of a bipolar outflow from the Milky Way's nucleus.

Findings

Discovery of high-velocity clouds up to 1.5 kpc from the Galactic Plane.

Modeling suggests an outflow velocity of ~330 km/s with a bi-cone structure.

Estimated outflow luminosity and cold gas mass-loss rate consistent with star formation feedback.

Abstract

We present the results of a new sensitive survey of neutral hydrogen above and below the Galactic Center with the Green Bank Telescope. The observations extend up to Galactic latitude | b | < 10 deg with an effective angular resolution of 9.5' and an average rms brightness temperature noise of 40 mK in a 1 km/s channel. The survey reveals the existence of a population of anomalous high-velocity clouds extending up to heights of about 1.5 kpc from the Galactic Plane and showing no signature of Galactic rotation. These clouds have local standard of rest velocities | Vlsr | < 360 km/s and, assuming a Galactic Center origin, they have sizes of a few tens of parsecs and neutral hydrogen masses spanning $10-10^5 \, M_\odot$. Accounting for selection effects, the cloud population is symmetric in longitude, latitude, and Vlsr. We model the cloud kinematics in terms of an outflow expanding from the Galactic Center and find the population consistent with being material moving with radial velocity Vw ~ 330 km/s distributed throughout a bi-cone with opening angle $\alpha>140$ deg. This simple model implies an outflow luminosity $Lw > 3 \times 10^{40}$ erg/s over the past 10 Myr, consistent with star formation feedback in the inner region of the Milky Way, with a cold gas mass-loss rate $\lesssim 0.1 \, M_\odot$/yr. These clouds may represent the cold gas component accelerated in the nuclear wind driven by our Galaxy, although some of the derived properties challenge current theoretical models of the entrainment process.