The KMOS^3D Survey: design, first results, and the evolution of galaxy kinematics from 0.7<z<2.7

Abstract: We present the KMOS^3D survey, a new integral field survey of over 600 galaxies at 0.7<z\<2.7 using KMOS at the Very Large Telescope (VLT). The KMOS^3D survey utilizes synergies with multi-wavelength ground and space-based surveys to trace the evolution of spatially-resolved kinematics and star formation from a homogeneous sample over 5 Gyrs of cosmic history. Targets, drawn from a mass-selected parent sample from the 3D-HST survey, cover the star formation-stellar mass ($M_*$) and rest-frame $(U-V)-M_*$ planes uniformly. We describe the selection of targets, the observations, and the data reduction. In the first year of data we detect Halpha emission in 191 $M_*=3\times10^{9}-7\times10^{11}$ Msun galaxies at z=0.7-1.1 and z=1.9-2.7. In the current sample 83% of the resolved galaxies are rotation-dominated, determined from a continuous velocity gradient and $v_{rot}/\sigma\>1$, implying that the star-forming 'main sequence' (MS) is primarily composed of rotating galaxies at both redshift regimes. When considering additional stricter criteria, the Halpha kinematic maps indicate at least ~70% of the resolved galaxies are disk-like systems. Our high-quality KMOS data confirm the elevated velocity dispersions reported in previous IFS studies at z>0.7. For rotation-dominated disks, the average intrinsic velocity dispersion decreases by a factor of two from 50 km/s at z~2.3 to 25 km/s at z~0.9 while the rotational velocities at the two redshifts are comparable. Combined with existing results spanning z~0-3, disk velocity dispersions follow an approximate (1+z) evolution that is consistent with the dependence of velocity dispersion on gas fractions predicted by marginally-stable disk theory.

• The paper demonstrates that 83% of resolved galaxies are rotationally-supported, emphasizing the dominance of disk galaxies at high redshift.
• The paper finds that intrinsic velocity dispersion in rotation-dominated disks decreases by roughly a factor of two from z~2.3 to z~0.9.
• The paper aligns elevated velocity dispersions with high gas fractions in marginally stable disks, supporting equilibrium models of galaxy evolution.

Analysis of "The KMOS$^{3D}$ Survey: Design, First Results, and the Evolution of Galaxy Kinematics"

The study presented by Wisnioski et al. documents the initiation and preliminary findings of the KMOS$^{3D}$ survey—a comprehensive integral field survey targeting over 600 galaxies within the redshift range $0.7 \leq z \leq 2.7$. Leveraging the K-band Multi-Object Spectrograph (KMOS) at the Very Large Telescope (VLT), the survey delivers a novel investigation into the evolution of spatially-resolved galactic kinematics and star formation histories over a 5 billion year span of the universe's history.

Methodology and Data

The KMOS$^{3D}$ survey utilizes a synergy of ground and space-based multi-wavelength surveys, including the 3D-HST. Its mass-selected parent sample ensures a representative selection of galaxies, cutting across a range of star formation rates and stellar masses uniformly across the rest-frame optical and near-infrared wavelengths. The first-year survey data record 191 galaxies with detected H$\alpha$ emissions, emphasizing the diversity and richness of the dataset in capturing both low- and high-mass galaxies at the defined epochs.

Key Results

1. Kinematic Classification:
   • The survey identifies that a significant majority (83%) of resolved galaxies exhibit rotational support, exemplified by continuous velocity gradients and $v_\mathrm{rot}/\sigma_0 > 1$. This suggests that the star-forming main sequence at these redshifts predominantly comprises rotating disk galaxies.
2. Intrinsic Velocity Dispersion:
   • The study confirms high intrinsic velocity dispersions reported in earlier IFS studies for galaxies at $z > 0.7$. For rotation-dominated disks, a notable trend is observed where the average intrinsic velocity dispersion decreases approximately by a factor of two from 50 km s$^{-1}$ at $z \sim 2.3$ to 25 km s$^{-1}$ at $z \sim 0.9$. This is a keystone finding indicating a decrease in turbulence and velocity dispersion over cosmic time.
3. Stability and Gas Fractions:
   • The elevated velocity dispersions align well with theoretical predictions concerning gas fractions in marginally-stable disks under equilibrium models of galaxy growth. This proposes that the elevated dispersions reflect the physical conditions and stability thresholds of the galaxies under study.

Implications and Future Prospects

The results from KMOS$^{3D}$ provide concrete evidence supporting the paradigm of gas-rich, turbulent disk galaxies which undergo significant stabilization and kinematic quiescence over cosmic time. This work demonstrates the essential role of intrinsic velocity dispersions and gas dynamics in framing the star-forming histories and morphological transformations of galaxies since the peak of cosmic star formation.

Future prospects for the KMOS$^{3D}$ survey will explore the high-redshift frontier, unraveling the dynamics of gas accretion and the regulatory mechanisms that control star formation efficiency and feedback processes. As the survey advances, a more substantial statistical sample will enable disentangled analyses of factors affecting galaxy structure and evolution, catering to refined models of galaxy formation in cosmological contexts.

Ultimately, the progression of the KMOS$^{3D}$ survey sets a foundation for upcoming observational campaigns with next-generation facilities, benefitting from enhanced spatial resolution and sensitivity to further resolve the fine-grained kinematic structures of galaxies shaping their evolutionary pathways across cosmic history.