The Volumetric Star Formation Law in the Almost Edge-on Galaxy NGC 4302 Revealed by the Atacama Large Millimeter/submillimeter Array

Abstract

We observe the almost edge-on (i ∼ 90°) galaxy NGC 4302 using the Atacama Large Millimeter/submillimeter Array (CO) and Very Large Array (H i) to measure the gas disk thickness for investigating the volumetric star formation law (SFL). The recent star formation rate (SFR) is estimated based on a linear combination of IR 24 μm and Hα emissions. The measured scale heights of CO and H i increase significantly with radius. Using the scale heights along with the vertically integrated surface densities, we derive the midplane volume densities of the gas (ρ H I, , and ρ gas = ρ H I+) and the SFR (ρ SFR) and compare the volumetric SFL (ρ SFR ∝ ρ gas n) with the vertically integrated SFL (ΣSFR ∝ Σgas N). We find tight power-law correlations between the SFR and the gas (H i, H2, and the total gas) in both volume and surface densities. The power-law indices of the total gas and H i for the volumetric SFL are noticeably smaller than the indices for the vertically integrated SFL while the H2 indices for both cases are similar to each other. In terms of the star formation efficiency (SFE), we find that the molecular and total gas SFEs are roughly constant, while the atomic SFE is clearly decreasing with radius in both cases.