[Paper Review] The Properties of the Star-Forming Interstellar Medium at z=0.84-2.23 from HiZELS - I: Mapping the Internal Dynamics and Metallicity Gradients in High-Redshift Disk Galaxies

A. M. Swinbank, David Sobral+6 more|arXiv (Cornell University)|Sep 6, 2012

Astrophysics and Star Formation Studies1 references3 citations

TL;DR

This study uses adaptive optics spectroscopy of 9 Hα-selected galaxies at z=0.84–2.23 to map ionized gas dynamics and metallicity gradients, revealing that high-redshift disk galaxies exhibit strong rotational motions and negative metallicity gradients. Key findings show a 3.5-fold increase in B-band mass-to-light ratio from z≈1.5 to z=0, with stellar mass Tully-Fisher evolution indicating fixed baryonic conversion efficiency and strong evolution in luminosity-driven scaling relations.

ABSTRACT

We present adaptive optics assisted, spatially resolved spectroscopy of a sample of nine H-alpha-selected galaxies at z=0.84--2.23 drawn from the HiZELS narrow-band survey. These galaxies have star-formation rates of 1-27Mo/yr and are therefore representative of the typical high-redshift star-forming population. Our ~kpc-scale resolution observations show that approximately half of the sample have dynamics suggesting that the ionised gas is in large, rotating disks. We model their velocity fields to infer the inclination-corrected, asymptotic rotational velocities. We use the absolute B-band magnitudes and stellar masses to investigate the evolution of the B-band and stellar mass Tully-Fisher relationships. By combining our sample with a number of similar measurements from the literature, we show that, at fixed circular velocity, the stellar mass of star-forming galaxies has increased by a factor 2.5 between z=2 and z=0, whilst the rest-frame B-band luminosity has decreased by a factor ~6 over the same period. Together, these demonstrate a change in mass-to-light ratio in the B-band of Delta(M/L_B)/(M/L_B)_(z=0) \sim 3.5 between z=1.5 and z=0, with most of the evolution occurring below z=1. We also use the spatial variation of [NII]/Halpha to show that the metallicity of the ionised gas in these galaxies declines monotonically with galacto-centric radius, with an average Delta(log O/H)/DeltaR=-0.027+/-0.005dex/kpc. This gradient is consistent with predictions for high-redshift disk galaxies from cosmologically based hydrodynamic simulations.

Motivation & Objective

• To measure the internal kinematics and metallicity structure of high-redshift star-forming disk galaxies at z=0.84–2.23.
• To test the evolution of the Tully-Fisher relation in both rest-frame B-band luminosity and stellar mass over cosmic time.
• To assess the consistency of observed metallicity gradients with cosmological hydrodynamic simulations and galaxy formation models.
• To determine whether high-redshift disks evolve along scaling relations or undergo significant structural change.
• To constrain the role of gas accretion and feedback in shaping disk scaling relations and chemical abundance profiles.

Proposed method

• Acquired spatially resolved, adaptive optics-assisted SINFONI spectroscopy of 9 Hα-emitting galaxies from the HiZELS survey at z=0.84–2.23.
• Mapped ionized gas kinematics using Hα and [N ii] emission lines to derive rotation curves and velocity fields.
• Modelled velocity fields to infer inclination-corrected asymptotic rotational velocities (V_circ) and dynamical masses.
• Combined with literature data to construct rest-frame B-band and stellar mass Tully-Fisher relations across redshifts.
• Used spatially resolved [N ii]/Hα ratios to measure radial metallicity gradients in the ionized gas, assuming O/H abundance proxies.
• Compared observed scaling relations and metallicity gradients with predictions from cosmological hydrodynamic simulations and semi-analytic models.

Experimental results

Research questions

• RQ1 Do high-redshift star-forming disk galaxies exhibit large-scale, ordered rotation, and what are their dynamical masses?
• RQ2 How do the B-band and stellar mass Tully-Fisher relations evolve from z≈2 to z=0, and what does this imply for star formation efficiency?
• RQ3 What is the radial metallicity gradient of the ionized interstellar medium in high-redshift disks, and how does it compare to local galaxies and simulations?
• RQ4 Is the observed evolution in the Tully-Fisher relation consistent with gas accretion and feedback-driven disk evolution?
• RQ5 Do high-redshift disks follow the same scaling relations as local spirals, or do they require distinct formation mechanisms?

Key findings

• Approximately half of the sample galaxies exhibit large, rotating ionized gas disks, with inclination-corrected rotational velocities derived from kinematic modeling.
• The rest-frame B-band Tully-Fisher relation shows strong evolution: at fixed circular velocity, B-band luminosity decreases by a factor of ~6 from z≈2 to z=0.
• The stellar mass Tully-Fisher relation shows only modest evolution, with stellar mass increasing by a factor of 2.5 between z=2 and z=0, indicating a nearly constant baryonic conversion efficiency.
• The B-band mass-to-light ratio evolves by a factor of Δ(M/L_B)/(M/L_B)_z=0 ≈ 3.5 from z≈1.5 to z=0, with most evolution occurring below z=1.
• The average radial metallicity gradient in ionized gas is Δlog(O/H)/ΔR = −0.027 ± 0.005 dex kpc⁻¹, consistent with negative gradients seen in local disks and cosmological simulations.
• High-redshift disks are 1.5× smaller than local disks at fixed circular velocity, suggesting angular momentum evolution via outflows or late-time accretion.