Estimating the Star Formation Rate at 1 kpc Scales in Nearby Galaxies

TL;DR

This study refines methods to estimate star formation rates at 1 kpc scales in nearby galaxies using combined Hα, UV, and IR data, highlighting calibration uncertainties and systematic differences among tracers.

Contribution

It introduces a new IR spectral energy distribution-based approach and re-calibrates hybrid SFR tracers for 1 kpc resolution in galaxy disks.

Findings

Calibration of IR-based SFR tracers remains uncertain within a factor of two.

Systematic differences among SFR tracers are less than a factor of two above 10^-3 M_sun/yr/kpc^2.

Uncertainties in Hα and FUV-based SFRs are approximately 0.3 and 0.5 dex, respectively.

Abstract

Using combinations of H\alpha, ultraviolet (UV), and infrared (IR) emission, we estimate the star formation rate (SFR) surface density, \Sigma_SFR, at 1 kpc resolution for 30 disk galaxies that are targets of the IRAM HERACLES CO survey. We present a new physically-motivated IR spectral energy distribution-based approach to account for possible contributions to 24\mum emission not associated with recent star formation. Considering a variety of "reference" SFRs from the literature, we revisit the calibration of the 24\mum term in hybrid (UV+IR or H\alpha+IR) tracers. We show that the overall calibration of this term remains uncertain at the factor of two level because of the lack of wide-field, robust reference SFR estimates. Within this uncertainty, published calibrations represent a reasonable starting point for 1 kpc-wide areas of star-forming disk galaxies but we re-derive and refine the calibration of the IR term in these tracers to match our resolution and approach to 24\mum emission. We compare a large suite of \Sigma_SFR estimates and find that above \Sigma_SFR \sim 10^-3 M_\odot yr^-1 kpc^-2 the systematic differences among tracers are less than a factor of two across two orders of magnitude dynamic range. We caution that methodology and data both become serious issues below this level. We note from simple model considerations that focusing on a part of a galaxy dominated by a single stellar population the intrinsic uncertainty in H\alpha and FUV-based SFRs are \sim 0.3 and \sim 0.5 dex.