Measuring 60-pc-scale Star Formation Rate of the Nearby Seyfert Galaxy NGC 1068 with ALMA, HST, VLT/MUSE, and VLA

TL;DR

This study calibrates the star formation rate in NGC 1068 using high-resolution ALMA and HST data, addressing uncertainties in traditional tracers and revealing a total SFR of about 3.2 solar masses per year.

Contribution

It introduces a new SFR calibration method combining ALMA and HST data, accounting for diffuse ionized gas and other factors, improving accuracy over previous methods.

Findings

Total SFR of NGC 1068 is approximately 3.2 M_sun/yr.

Accounting for diffuse ionized gas reduces SFR estimates by about one-third.

Total SFR exceeds dust-based estimates by a factor of 1.5.

Abstract

Star formation rate (SFR) is a fundamental parameter for describing galaxies and inferring their evolutionary course. HII regions yield the best measure of instantaneous SFR in galaxies, although the derived SFR can have large uncertainties depending on tracers and assumptions. We present an SFR calibration for the entire molecular gas disk of the nearby Seyfert galaxy NGC 1068, based on our new high-sensitivity ALMA 100GHz continuum data at 55pc (=0."8) resolution in combination with the HST Pa{\alpha} line data. In this calibration, we account for the spatial variations of dust extinction, electron temperature of HII regions, AGN contamination, and diffuse ionized gas (DIG) based on publicly available multi-wavelength data. Especially, given the extended nature and the possible non-negligible contribution to the total SFR, a careful consideration of DIG is essential. With a cross-calibration between two corrected ionized gas tracers (free-free continuum&Pa{\alpha}), the total SFR of the NGC 1068 disk is estimated to be 3.2\pm0.5 Msol/yr, one-third of the SFR without accounting for DIG (9.1\pm1.4 Msol/yr). We confirmed high SFR around the southern bar-end and the corotation radius, which is consistent with the previous SFR measurements. In addition, our total SFR exceeds the total SFR based on 8{\mu}m dust emission by a factor of 1.5. We attribute this discrepancy to the differences in the young stars at different stages of evolution traced by each tracer and their respective timescales. This study provides an example to address the various uncertainties in conventional SFR measurements and their potential to lead to significant SFR miscalculations.