Systematic difference between ionized and molecular gas velocity dispersion in $z\sim 1-2$ disks and local analogues

Abstract

We compare the molecular and ionized gas velocity dispersion of 9 nearby turbulent disks, analogues to high-redshift galaxies, from the DYNAMO sample using new ALMA and GMOS/Gemini observations. We combine our sample with 12 galaxies at $z\sim$0.5-2.5 from the literature. We find that the resolved velocity dispersion is systematically lower by a factor $2.45\pm 0.38$ for the molecular gas compared to the ionized gas, after correcting for thermal broadening. This offset is constant within the galaxy disks and indicates the co-existence of a thin molecular and thick ionized gas disks. This result has a direct impact on the Toomre $Q$ and pressure derived in galaxies. We obtain pressures $\sim 0.22$ dex lower on average when using the molecular gas velocity dispersion, ${\sigma }_{0,mol}$. We find that ${\sigma }_{0,mol}$ increases with gas fraction and star formation rate. We also obtain an increase with redshift and show that the EAGLE and FIRE simulations overall overestimate ${\sigma }_{0,mol}$ at high redshift. Our results suggest that efforts to compare the kinematics of gas using ionized gas as a proxy for the total gas may overestimate the velocity dispersion by a significant amount in galaxies at the peak of cosmic star formation. When using the molecular gas as a tracer, our sample is not consistent with predictions from constant efficiency star formation models, even when including transport as a source of turbulence. Feedback models with variable star formation efficiency, ${\epsilon }_{ff}$, and/or feedback efficiency, $p_{\ast }/m_{\ast }$, better predict our observations.

Accepted for publication in ApJ. 19 pages, 6 figures

References in corpus (22)

• Cosmic Star Formation History
• The EAGLE project: Simulating the evolution and assembly of galaxies and their environments
• The EAGLE simulations of galaxy formation: calibration of subgrid physics and model variations
• Formation of Massive Galaxies at High Redshift: Cold Streams, Clumpy Disks and Compact Spheroids
• Combined CO & Dust Scaling Relations of Depletion Time and Molecular Gas Fractions with Cosmic Time, Specific Star Formation Rate and Stellar Mass
• The KMOS^3D Survey: design, first results, and the evolution of galaxy kinematics from 0.7<z<2.7
• The SAMI Galaxy Survey: Shocks and Outflows in a normal star-forming galaxy
• Modelling the supernova-driven ISM in different environments
• High star formation rates as the origin of turbulence in early and modern disk galaxies
• PHIBSS2: survey design and z=0.5-0.8 results. Molecular gas reservoirs during the winding-down of star formation
• The KMOS Redshift One Spectroscopic Survey (KROSS): the origin of disk turbulence in z~0.9 star-forming galaxies
• Molecular gas content in strongly-lensed z~1.5-3 star-forming galaxies with low IR luminosities
• DYNAMO-HST Survey: Clumps in Nearby Massive Turbulent Disks and the Effects of Clump Clustering on Kiloparsec Scale Measurements of Clumps
• Spatially resolved kinematics in the central 1 kpc of a compact star-forming galaxy at z=2.3 from ALMA CO observations
• Thermal Pressure in the Cold Neutral Medium of Nearby Galaxies
• Thick Disks in the Hubble Space Telescope Frontier Fields
• Extreme gas fractions in clumpy, turbulent disk galaxies at z~0.1
• The kiloparsec-scale gas kinematics in two star-forming galaxies at $z\sim 1.47$ seen with ALMA and VLT-SINFONI
• ALMA shows that gas reservoirs of star-forming disks over the past 3 billion years are not predominantly molecular
• The connection between the peaks in velocity dispersion and star-forming clumps of turbulent galaxies
• Gas Content and Kinematics in Clumpy, Turbulent Star-forming Disks
• Momentum Injection by Clustered Supernovae: Testing Subgrid Feedback Prescriptions