Near-infrared H2 and Continuum Survey of Extended Green Objects. II. Complete Census for the Northern Galactic Plane

TL;DR

This study provides a comprehensive, unbiased survey of 94 Extended Green Objects in the northern Galactic plane, analyzing their near-infrared H2 emission and continuum to understand outflow activity and emission origins.

Contribution

It offers the first complete census of EGOs in the northern Galactic plane using near-infrared H2 and continuum observations, revealing new insights into their morphologies and outflow associations.

Findings

Only 28% of EGOs with H2 emission have similar 4.5 μm and H2 morphologies.

About half of EGOs are associated with H2 outflows, indicating active star formation.

H2 outflow detection rate is higher in EGOs with K-band detections, suggesting a link with outflow activity.

Abstract

We discuss 94 Extended Green Objects (EGOs) in the northern Galactic plane cataloged by Cyganowski et al, based on near-infrared narrowband H2 (2.122 {\mu}m and continuum observations from the United Kingdom Infrared Telescope. This data set is three times larger than our previous study, and is unbiased by preselection. As discussed in the previous paper, the morphologies of the 4.5 {\mu}m emission generally resemble those of the near-infrared continuum, but are different from those of the H2 emission. Of our sample, only 28% of EGOs with H2 emission show similar morphologies between 4.5 {\mu}m and H2 emission. These results suggest that the 4.5 {\mu}m emission mainly comes from scattered continuum from the embedded young stellar object (YSO), and partially from H2 emission. About half of EGOs are associated with H2 outflows, if the H2 outflow incompleteness is considered. The H2 outflow detection rate for EGOs with K-band detections (61%) is significantly higher than for those without K-band detections (36%). This difference may be due to the fact that both H2 and K-band emissions are associated with outflows, i.e., H2 emission and K-band continuum are associated with shocks and outflow cavities, respectively. We also compared the correlation between the H2 outflows and Class I 44 GHz methanol masers from literature. The methanol masers can be located upstream or downstream of the H2 outflows and some bright H2 spots or outflows are not associated with methanol masers, suggesting that methanol masers and H2 emission trace different excitation conditions.