A Search For Star Formation in the Smith Cloud

TL;DR

This study searches for star formation in the Smith Cloud using ultraviolet and infrared data, finding no statistically significant evidence but suggesting the cloud's star formation rate aligns with its gas density.

Contribution

It introduces a method to detect star formation in high-velocity clouds and compares the Smith Cloud's properties to similar star-forming systems like Leo P.

Findings

No significant excess of young stars detected in the Smith Cloud.

Star formation rate consistent with gas density expectations.

Cannot rule out the Smith Cloud being a dwarf galaxy.

Abstract

Motivated by the idea that a subset of HVCs trace dark matter substructure in the Local Group, we search for signs of star formation in the Smith Cloud, a nearby ~2x10^6 Msun HVC currently falling into the Milky Way. Using GALEX NUV and WISE/2MASS NIR photometry, we apply a series of color and apparent magnitude cuts to isolate candidate O and B stars that are plausibly associated with the Smith Cloud. We find an excess of stars along the line of sight to the cloud, but not at a statistically significant level relative to a control region. The number of stars found in projection on the cloud after removing an estimate of the contamination by the Milky Way implies an average star formation rate surface density of 10^(-4.8 +/- 0.3) Msun yr^(-1) kpc^(-2), assuming the cloud has been forming stars at a constant rate since its first passage through the Milky Way ~70 Myr ago. This value is consistent with the star formation rate expected based on the average gas density of the cloud. We also discuss how the newly discovered star forming galaxy Leo P has very similar properties to the Smith Cloud, but its young stellar population would not have been detected at a statistically significant level using our method. Thus, we cannot yet rule out the idea that the Smith Cloud is really a dwarf galaxy.