The VIRUS-P Exploration of Nearby Galaxies (VENGA): Spatially Resolved Gas-Phase Metallicity Distributions in Barred and Unbarred Spirals

TL;DR

This study uses high-resolution integral field spectroscopy to map gas excitation and metallicity in nearby spiral galaxies, revealing shallow metallicity gradients that challenge previous assumptions about bar-driven flattening and aligning with cosmological simulations.

Contribution

It provides the highest resolution maps of ionization parameters, star formation rates, and metallicity distributions in nearby spirals, and compares these with high-redshift galaxy gradients and simulations.

Findings

Shallow metallicity gradients in both barred and unbarred spirals.

Presence of LINER excitation at large radii associated with diffuse ionized gas.

Shallower gradients at z ~ 0 compared to z ~ 1.5-2, consistent with certain hydrodynamical models.

Abstract

We present a study of the excitation conditions and metallicity of ionized gas (Z_gas) in eight nearby barred and unbarred spiral galaxies from the VIRUS-P Exploration of Nearby Galaxies (VENGA) survey, which provides high spatial sampling and resolution (median ~ 387 pc), large coverage from the bulge to outer disc, broad wavelength range (3600-6800 Ang.), and medium spectral resolution (~ 120 km s^-1 at 5000 Ang.). Our results are: (1) We present high resolution gas excitation maps to differentiate between regions with excitation typical of Seyfert, LINER, or recent star formation. We find LINER-type excitation at large distances (3-10 kpc) from the centre, and associate this excitation with diffuse ionized gas (DIG). (2) After excluding spaxels dominated by Seyfert, LINER, or DIG, we produce maps with the best spatial resolution and sampling to date of the ionization parameter q, star formation rate, and Z_gas using common strong line diagnostics. We find that isolated barred and unbarred spirals exhibit similarly shallow Z_gas profiles from the inner kpc out to large radii (7-10 kpc or 0.5-1.0 R_25). This implies that if profiles had steeper gradients at earlier epochs, then the present-day bar is not the primary driver flattening gradients over time. This result contradicts earlier claims, but agrees with recent IFU studies. (3) The Z_gas gradients in our z ~ 0 massive spirals are markedly shallower, by ~ 0.2 dex kpc^-1, than published gradients for lensed lower mass galaxies at z ~ 1.5-2.0. Cosmologically-motivated hydrodynamical simulations best match this inferred evolution, but the match is sensitive to adopted stellar feedback prescriptions.