Star–Gas Surface Density Correlations in 12 Nearby Molecular Clouds. I. Data Collection and Star-sampled Analysis

Abstract

We explore the relation between the stellar mass surface density and the mass surface density of molecular hydrogen gas in 12 nearby molecular clouds that are located at <1.5 kpc distance. The sample clouds span an order-of-magnitude range in mass, size, and star formation rates. We use thermal dust emission from Herschel maps to probe the gas surface density and the young stellar objects from the most recent Spitzer Extended Solar Neighborhood Archive catalog to probe the stellar surface density. Using a star-sampled nearest neighbor technique to probe the star–gas surface density correlations at the scale of a few parsecs, we find that the stellar mass surface density varies as a power law of the gas mass surface density, with a power-law index of ∼2 in all the clouds. The consistent power-law index implies that star formation efficiency is directly correlated with gas column density, and no gas column density threshold for star formation is observed. We compare the observed correlations with the predictions from an analytical model of thermal fragmentation and with the synthetic observations of a recent hydrodynamic simulation of a turbulent star-forming molecular cloud. We find that the observed correlations are consistent for some clouds with the thermal fragmentation model and can be reproduced using the hydrodynamic simulations.

The Single-Cloud Star Formation Relation

“…In this article we re-examine the KS relation within single clouds using a uniformly reduced catalog of the protostars from the Extended Solar Neighborhood Archive (SESNA) and uniformly reduced gas column density maps based on dust emission from ℎ for 12 molecular clouds that cover very different star formation environments 5 . Compared to previous samples, our YSO catalogue and dust maps probe a much larger dynamic range in gas column density and star formation rate and have well-modelled sensitivity and completeness, allowing a much more sensitive test of the single-cloud KS relation.…”

“…It is immediately clear that the scatter is much smaller for this relationship than for the one between Σ SFR and Σ gas alone; quantitatively, the IQR of the data is reduced from 0.36 to 0.14 (computed at Σ gas / ff = 10 2. 5 pc −2 Myr −1 ) by inclusion of the free-fall time. Moreover, the relationship is now linear, with a median best-fit slope of 0.99.…”

“…background galaxies and unresolved molecular hydrogen shock emission) using a series of reddening-safe colour and flux selections 42 . With a few exceptions 5,22 , prior work on the intracloud KS relation employed protostar identifications that required 24 m flux measurements (e.g. 19,30 ).…”

“…The correction procedure is explained in detail in ref. 5 ; all our analysis in this work makes use of the statistically-corrected data.…”

“…The most complete study to date is that of ref. 5 , which uses the same underlying data that we will use here, and which does find a strong correlation between gas and YSO surface densities. However, their analysis technique examines the density of gas around stars on a star-by-star basis, and thus cannot easily determine whether the KS relation is the same from cloud to cloud, or whether there is dependence on additional parameters such as the cloud free-fall time.…”