Internal kinematics of spiral galaxies in distant clusters^*,**

IV. Gas kinematics of spiral galaxies in intermediate redshift clusters and in the field

¹ Kapteyn Astronomical Institute, PO BOX 800, 9700 AV Groningen, The Netherlands e-mail: kutdemir@astro.rug.nl
² European-Southern Observatory, Karl-Schwarzschild Str. 2, 85748 Garching, Germany
³ Núcleo de Astrofísica Teórica, Universidade Cruzeiro do Sul, R. Galvão Bueno 868, 01506-000 São Paulo, SP, Brazil
⁴ Institute of Astro- and Particle Physics, Technikerstrasse 25/8, 6020 Innsbruck, Austria
⁵ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching be München, Germany

Received: 7 September 2009
Accepted: 21 June 2010

Abstract

Aims. We trace the interaction processes of galaxies at intermediate redshift by measuring the irregularity of their ionized gas kinematics, and investigate these irregularities as a function of the environment (cluster versus field) and of morphological type (spiral versus irregular).

Methods. We obtain the gas velocity fields by placing three parallel and adjacent VLT/FORS2 slits on each galaxy. To quantify irregularities in the gas kinematics, we use three indicators: the standard deviation of the kinematic position angle (σ_PA), the mean deviation of the line of sight velocity profile from the cosine form which is measured using high order Fourier terms (k_3,5/k₁) and the average misalignment between the kinematical and photometric major axes (Δϕ). These indicators are then examined together with some photometric and structural parameters (measured from HST and FORS2 images in the optical) such as the disk scale length, rest-frame colors, asymmetry, concentration, Gini coefficient and M₂₀. Our sample consists of 92 distant galaxies. 16 cluster (z ~ 0.3 and z ~ 0.5) and 29 field galaxies (0.10 ≤ z ≤ 0.91, mean z = 0.44) of these have velocity fields with sufficient signal to be analyzed. To compare our sample with the local universe, we also analyze a sample from the SINGS survey.

Results. We find that the fraction of galaxies that have irregular gas kinematics is remarkably similar in galaxy clusters and in the field at intermediate redshifts (according to σ_PA ≈ 10%, k_3,5/k₁ ≈ 30%, Δϕ ≈ 70%). The distribution of the field and cluster galaxies in (ir)regularity parameters space is also similar. On the other hand galaxies with small central concentration of light, that we see in the field sample, are absent in the cluster sample. We find that field galaxies at intermediate redshifts have more irregular velocity fields as well as more clumpy and less centrally concentrated light distributions than their local counterparts. Comparison with a SINS sample of 11 z ~ 2 galaxies shows that these distant galaxies have more irregular gas kinematics than our intermediate redshift cluster and field sample. We do not find a dependence of the irregularities in gas kinematics on morphological type. We find that two different indicators of star formation correlate with irregularity in the gas kinematics.

Conclusions. More irregular gas kinematics, also more clumpy and less centrally concentrated light distributions of spiral field galaxies at intermediate redshifts in comparison to their local counterparts indicate that these galaxies are probably still in the process of building their disks via mechanisms such as accretion and mergers. On the other hand, they have less irregular gas kinematics compared to galaxies at z ~ 2.