Smith's Cloud: A High-velocity Cloud Colliding with the Milky Way

TL;DR

Smith's Cloud is a massive high-velocity cloud interacting with the Milky Way, providing insights into galactic gas accretion processes and the dynamics of high-velocity clouds.

Contribution

This study presents new 21cm HI observations revealing the structure, velocity, and interaction of Smith's Cloud with the Milky Way, highlighting its role in galactic gas accretion.

Findings

Smith's Cloud has a cometary shape and interacts with the Galactic ISM.

The cloud is located about 12.4 kpc away with a mass over 10^6 solar masses.

It is moving toward the Galactic plane at ~300 km/s and will cross in ~27 million years.

Abstract

New 21cm HI observations made with the Green Bank Telescope show that the high-velocity cloud known as Smith's Cloud has a striking cometary appearance and many indications of interaction with the Galactic ISM. The velocities of interaction give a kinematic distance of 12.4 +/-1.3 kpc, consistent with the distance derived from other methods. The Cloud is >3 x 1 kpc in size and its tip at (l,b)=(39 deg,-13 deg) is 7.6 kpc from the Galactic center and 2.9 kpc below the Galactic plane. It has greater than 10^6 M solar masses in HI. Its leading section has a total space velocity near 300 km/s, is moving toward the Galactic plane with a velocity of 73+/-26 km/s, and is shedding material to the Galaxy. In the absence of drag the Cloud will cross the plane in about 27 Myr. Smith's Cloud may be an example of the accretion of gas by the Milky Way needed to explain certain persistent anomalies in Galactic chemical evolution.