EPoS Contribution
EPoS Contribution
Star Formation Efficiency in Low Surface Gas Density and Low Metallicity Environment

Natsuko Izumi
NAOJ, Tokyo, JP
How does the star-formation process change as a function their surrounding environment such as gas density and metallicity? is key question in understanding star formation. For example, the star-formation rate (SFR) and constant star-formation efficiency (cSFE: SFR per total (atomic and molecular) surface gas density) are known to decrease significantly in low surface gas density environment from the Kennicutt-Schmidt law. However, the mechanisms for this change of star-formation have not been understood, because detailed observations of star-forming regions have been impossible even in nearby galaxies, not to mention in high-z galaxies. Thus, we study SFE in the outer Galaxy, which has very different environment from the solar neighborhood with lower surface gas density (~10%) and lower metallicity (~20%). Because of the relative proximity compared to nearby dwarf galaxies such as LMC and SMC (~10 kpc vs. 50 kpc), the outer Galaxy serves as an ideal "laboratory" to study the details of star formation process in such interesting environment. Using the WISE all-sky survey data in MIR and the FCRAO outer Galaxy survey data in CO, we searched young star-forming region beyond the outer arm (Rg > 13.5 kpc), where surface gas density, SFR, and cSFE are known to decrease significantly. The metallicity also decreases with increasing Rg, and is about 30% to 10% of that in the solar neighborhood at Rg = 13.5-20 kpc. We examined 466 molecular clouds and identified 788 candidate young star-forming regions in 252 clouds at up to Rg ~ 20 kpc. Among the 788 candidates, 711 in 240 clouds were newly identified. We constructed two empirical SFE-indicies of molecular cloud for converting molecular (H2) gas to stars: 1) the number ratio of clouds with star-forming region to all clouds, and 2) MIR luminosities per cloud mass as a first step. We found that both indices are roughly constant throughout the outer Galaxy (Rg = 13.5-20 kpc), although a large variation of environment is anticipated. These results suggest that star formation processes inside molecular clouds do not heavily depend on environmental parameters. Thus, the low cSFE found in the outer Galaxy may simply reflect the lower number of molecular clouds per area, in other words, SFR and cSFE simply depends on the amount of H2 gas in the outer Galaxy as in the inner Galaxy
Caption: (a): Distribution of molecular clouds with star-forming regions in the second and third quadrants (adapted from the image of Milky Way Galaxy from NASA's Spitzer Space Telescope by NASA/JPL-Caltech). The red circles and white squares show newly identified star-forming regions and known star-forming regions, respectively. The white fan-shaped region shows the survey area of FCRAO CO outer Galaxy survey (102.49° < l < 141.54°, -3.03° < b < 5.41°), and thus also that of the our survey of star-forming region. (b): Galactocentric variation of the empirical SFE-index 1: the ratio of clouds with star-forming regions to the total number of clouds with cloud mass (Mc) of larger than 100 Msun (c): Galactocentric variation of the empirical SFE-index 2: MIR luminosities per cloud mass. The size of these markers indicate the mass of molecular cloud (Mc).
Collaborators:
N. Kobayashi, U Tokyo, JP
C. Yasui, NAOJ, JP
M. Saito, NAOJ, JP
Key publication

Suggested Session: Star formation "laws" and IMF