The Volumetric Star Formation Law in the Almost Edge-on Galaxy NGC 4302 Revealed by ALMA

TL;DR

This study uses ALMA and VLA observations of NGC 4302 to analyze the volumetric star formation law, revealing tight correlations between gas densities and star formation rates in an almost edge-on galaxy.

Contribution

It introduces a method to derive the volumetric star formation law in an edge-on galaxy by measuring gas scale heights and comparing volumetric and surface density relations.

Findings

Tight power-law correlations between SFR and gas densities in volume and surface.

Volumetric SFL indices for total gas and H I are smaller than surface SFL indices.

Molecular and total gas star formation efficiencies are roughly constant, atomic SFE decreases with radius.

Abstract

We observe the almost edge-on (i $\sim$ 90 degrees) galaxy NGC 4302 using ALMA (CO) and VLA (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 micron and H$\alpha$ 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 mid-plane volume densities of the gas ($\rho_{\rm HI}$, $\rho_{\rm H_2}$, and $\rho_{\rm gas}$ = $\rho_{\rm HI}$+$\rho_{\rm H_2}$) and the SFR ($\rho_{\rm SFR}$) and compare the volumetric SFL ($\rho_{\rm SFR} \propto \rho_{\rm gas}^n$) with the vertically integrated SFL ($\Sigma_{\rm SFR} \propto \Sigma_{\rm gas}^N$). We find tight power-law correlations between the SFR and the gas (H I, H$_2$, 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 H$_2$ 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.