A Kennicutt–Schmidt relation at molecular cloud scales and beyond

Abstract

Using N -body/gasdynamic simulations of a Milky Way-like galaxy we analyse a Kennicutt-Schmidt relation, Σ SFR ∝ Σ gas N , at different spatial scales. We simulate synthetic observations in CO lines and UV band. We adopt the star formation rate (SFR) defined in two ways: based on free fall collapse of a molecular cloud -Σ SFR, cl , and calculated by using a UV flux calibration -Σ SFR, UV . 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 SFR, cl ∝ Σ gas N relation follows the power-law with index N ≈ 1.4. Using UV flux as SFR calibrator we confirm a systematic offset between the Σ SFR, 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 ∼ 50 pc. We find a transition range around scales ∼ 50 − 120 pc, where the power-law index N increases from 0 to 1 − 1.8 and saturates for scales larger ∼ 120 pc. A value of the index saturated depends on a surface gas density threshold and it becomes steeper for higher Σ gas threshold. Averaging over scales with size of > ∼ 150 pc the power-law index N equals 1.3 − 1.4 for surface gas density threshold ∼ 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.

Coupling local to global star formation in spiral galaxies: the effect of differential rotation

“…Onodera et al (2010) found that the scatter of the KS relation increases on smaller scales until it breaks completely on the scale of molecular clouds (80 pc scales). The breakdown of the KS relation on small scales has also been demonstrated in dwarf galaxies (Krumholz 2013), in spiral galaxies (Khoperskov & Vasiliev 2017), and recently in some observational studies (Salim et al 2020;Kumari, Irwin & James 2020). Additionally an increasing number of studies suggest that this relation varies between different galaxies, and even within different regions of the same galaxy (Roman-Duval et al 2016;Morokuma-Matsui & Muraoka 2017).…”

A panchromatic spatially resolved analysis of nearby galaxies – I. Sub-kpc-scale main sequence in grand-design spirals

“…These scales are at the limit of the assumption on which the balance between star formation laws and energy output still holds (e.g. Kravtsov 2003;Khoperskov & Vasiliev 2017;Schruba et al 2010). In Sec.…”

Unique Tracks Drive the Scatter of the Spatially Resolved Star Formation Main Sequence

“…The star formation law (Σ SFR ∝ Σ N gas ) has received considerable attention (Schmidt 1959;Kennicutt 1998a;Kravtsov 2003;Schaye & Dalla Vecchia 2008). However, only recently has it become possible to study scales down to 30 pc and address on what physical scales the star formation law holds and possibly even breaks down (Kravtsov 2003;Onodera et al 2010;Schruba et al 2010;Kruijssen & Longmore 2014;Khoperskov & Vasiliev 2017).…”