A Kennicutt-Schmidt relation at molecular cloud scales and beyond

Sergey A. Khoperskov; Evgenii O. Vasiliev

doi:10.1093/mnras/stx532

paper

A Kennicutt-Schmidt relation at molecular cloud scales and beyond

arXiv:1702.08562 · doi:10.1093/mnras/stx532

Abstract

Using N-body/gasdynamic simulations of a Milky Way-like galaxy we analyse a Kennicutt-Schmidt relation, $Σ_{S F R} \propto Σ_{g a s}^{N}$ , at different spatial scales. We simulate synthetic observations in CO lines and UV band. We adopt the star formation rate defined in two ways: based on free fall collapse of a molecular cloud - $Σ_{S F R, c l}$ , and calculated by using a UV flux calibration - $Σ_{S F R, U V}$ . We study a KS relation for spatially smoothed maps with effective spatial resolution from molecular cloud scales to several hundred parsecs. We find that for spatially and kinematically resolved molecular clouds the $Σ_{S F R, c l} \propto Σ_{gas}^{N}$ relation follows the power-law with index $N \approx 1.4$ . Using UV flux as SFR calibrator we confirm a systematic offset between the $Σ_{UV}$ and $Σ_{gas}$ distributions on scales compared to molecular cloud sizes. Degrading resolution of our simulated maps for surface densities of gas and star formation rates we establish that there is no relation $Σ_{SFR, UV} - Σ_{gas}$ below the resolution $\sim 50$ pc. We find a transition range around scales $\sim 50 - 120$ pc, where the power-law index $N$ increases from 0 to 1-1.8 and saturates for scales larger $\sim 120$ pc. A value of the index saturated depends on a surface gas density threshold and it becomes steeper for higher $Σ_{g a s}$ threshold. Averaging over scales with size of $> 150$ pc the power-law index $N$ equals 1.3-1.4 for surface gas density threshold $\sim 5 M_{⊙}$ pc $^{- 2}$ . At scales $> 120$ pc surface SFR densities determined by using CO data and UV flux, $Σ_{SFR, UV} / Σ_{SFR, cl}$ , demonstrate a discrepancy about a factor of 3. We argue that this may be originated from overestimating (constant) values of conversion factor, star formation efficiency or UV calibration used in our analysis.

8 pages, 3 figures, accepted for publication in MNRAS

A Kennicutt-Schmidt relation at molecular cloud scales and beyond

Abstract

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