[Paper Review] MASSIV: Mass Assembly Survey with SINFONI in VVDS - II. Kinematics and close environment classification

B. Épinat, L. Tasca+13 more|Jan 16, 2012

Astronomy and Astrophysical ResearchPhysics and Astronomy64 references85 citations

TL;DR

This study analyzes 50 high-redshift galaxies (0.9 < z < 1.6) using SINFONI/VLT integral field spectroscopy to classify their kinematics and close environment. It finds that ~44% are rotation-dominated, ~35% are non-rotating, and at least 29% are interacting or have close companions, indicating a transition epoch at z ~ 1–1.5 where mergers dominate mass assembly over cold gas accretion.

ABSTRACT

(Abridged) Processes driving mass assembly are expected to evolve on different timescales along cosmic time. A transition might happen around z ~ 1 as the cosmic star formation rate starts its decrease. Identifying the dynamical nature of galaxies on a representative sample is necessary to infer and compare the mass assembly mechanisms across cosmic time. We present an analysis of the kinematics properties of 50 galaxies with 0.9 &lt; z &lt; 1.6 from the MASSIV sample observed with SINFONI/VLT with 4.5x10^9 Msun &lt; M &lt; 1.7x10^11 Msun and 6 Msun/yr &lt; SFR &lt; 300 Msun/yr. This is the largest sample with 2D-kinematics in this redshift range. We provide a classification based on kinematics as well as on close galaxy environment. We find that 29% of galaxies are experiencing merging or have close companions that may be gravitationally linked. This is placing a lower limit on the fraction of interacting galaxies. We find that at least 44% of the galaxies display ordered rotation whereas at least 35% are non-rotating objects. All rotators except one are compatible with rotation-dominated (Vmax/sigma &gt; 1) systems. Non-rotating objects are mainly small objects (Re &lt; 4 kpc). Combining our sample with other 3D-spectroscopy samples, we find that the local velocity dispersion of the ionized gas component decreases continuously from z ~ 3 to z = 0. The proportion of disks also seems to be increasing in star-forming galaxies when the redshift decreases. The number of interacting galaxies seems to be at a maximum at z ~ 1.2. These results draw a picture in which cold gas accretion may still be efficient at z ~ 1.2 but in which mergers may play a much more significant role at z ~ 1.2 than at higher redshift. From a dynamical point of view, the redshift range 1 &lt; z &lt; 2 therefore appears as a transition period in the galaxy mass assembly process.

Motivation & Objective

• To understand the dynamical state of star-forming galaxies at z ~ 1 by analyzing their 2D kinematics.
• To classify galaxies based on kinematic properties (rotation vs. non-rotation) and close environment (interacting or isolated).
• To investigate the evolution of mass assembly mechanisms across cosmic time, particularly around z ~ 1.
• To determine whether non-rotating galaxies are transient mergers, unstable gaseous systems, or spheroids.
• To compare the fraction of disks and interacting systems with other high-redshift 3D spectroscopy samples to identify trends in galaxy evolution.

Proposed method

• Acquired deep SINFONI/VLT integral field spectroscopy of 50 galaxies in the redshift range 0.9 < z < 1.6.
• Produced kinematic maps from data cubes to derive velocity fields and velocity dispersion maps.
• Classified galaxies based on alignment between morphological and kinematic position angles and fit quality of rotating disk models.
• Identified interacting systems via kinematic signatures (e.g., tidal features, asymmetric velocity fields) and close companions in broad-band images.
• Combined the MASSIV sample with other 3D spectroscopy surveys (SINS, LSD/AMAZE, IMAGES, GHASP) to compare velocity dispersion and disk fractions across redshifts.
• Used the Vmax/σ ratio to assess rotational support, defining rotation-dominated systems as those with Vmax/σ > 1.

Experimental results

Research questions

• RQ1 What fraction of high-redshift galaxies (z ~ 1) exhibit ordered rotation versus non-rotating kinematics?
• RQ2 How does the frequency of interacting or companion-hosting galaxies vary in this redshift range?
• RQ3 What is the relationship between galaxy size, velocity dispersion, and dynamical state in non-rotating systems?
• RQ4 How does the fraction of disk-like galaxies evolve from z ~ 3 to z ~ 0, and when is the peak of merger activity?
• RQ5 To what extent do kinematic properties suggest a transition from cold gas accretion to merger-driven mass assembly at z ~ 1–1.5?

Key findings

• 44% of the sample are classified as rotation-dominated systems, with Vmax/σ > 1, indicating significant rotational support.
• 35% of the galaxies show no dominant ordered rotation, with velocity dispersions of ~60 km s⁻¹, similar to high-redshift galaxies but lower than at z ~ 3.
• At least 29% of the galaxies are interacting or have close companions, suggesting a lower limit on the merger fraction at z ~ 1.2.
• Non-rotating galaxies are predominantly small (Re < 4 kpc) and show an anti-correlation between velocity dispersion and effective radius.
• The local velocity dispersion of ionized gas decreases continuously from z ~ 3 to z = 0, indicating a secular decline in turbulence.
• The fraction of disk-like galaxies increases with decreasing redshift, while the fraction of interacting galaxies peaks at z ~ 1.2, indicating a transition epoch in mass assembly mechanisms.