Star Formation Activity Beyond the Outer Arm II: Distribution and Properties of Star Formation

TL;DR

This study investigates star formation activity beyond the Outer Arm of the Galaxy, analyzing the properties of star-forming regions and molecular clouds, and finds that star formation efficiency appears independent of galactic environment within the studied range.

Contribution

It provides a detailed statistical analysis of star-forming regions and molecular clouds in the outer Galaxy, revealing environmental independence of star formation efficiency.

Findings

Molecular clouds with star-forming candidates have a shallower mass function slope.

Star formation efficiency shows no clear dependence on galactocentric radius between 13.5 and 20 kpc.

Decreasing star formation rate with radius may be due to less HI gas converting to H2.

Abstract

The outer Galaxy beyond the Outer Arm represents a promising opportunity to study star formation in an environment vastly different from the solar neighborhood. In our previous study, we identified 788 candidate star-forming regions in the outer Galaxy (at galactocentric radii $R_{\rm G}$ $\ge$ 13.5 kpc) based on Wide-field Infrared Survey Explorer (WISE) mid-infrared (MIR) all-sky survey. In this paper, we investigate the statistical properties of the candidates and their parental molecular clouds derived from the Five College Radio Astronomy Observatory (FCRAO) CO survey. We show that the molecular clouds with candidates have a shallower slope of cloud mass function, a larger fraction of clouds bound by self-gravity, and a larger density than the molecular clouds without candidates. To investigate the star formation efficiency (SFE) at different $R_{\rm G}$, we used two parameters: 1) the fraction of molecular clouds with candidates and 2) the monochromatic MIR luminosities of candidates per parental molecular cloud mass. We did not find any clear correlation between SFE parameters and $R_{\rm G}$ at $R_{\rm G}$ of 13.5 kpc to 20.0 kpc, suggesting that the SFE is independent of environmental parameters such as metallicity and gas surface density, which vary considerably with $R_{\rm G}$. Previous studies reported that the SFE per year (SFE/yr) derived from the star-formation rate surface density per total gas surface density, HI plus H$_2$, decreases with increased $R_{\rm G}$. Our results might suggest that the decreasing trend is due to a decrease in HI gas conversion to H$_2$ gas.