Evolution of structure in late-type spiral galaxies^*,**

I. Ionized gas kinematics in NGC 628

¹ Instituto de Astrofísica de Canarias, C/ Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain e-mail: fathi@iac.es
² Stockholm Observatory, AlbaNova University Center, 106 91 Stockholm, Sweden
³ Consejo Superior de Investigaciones Científicas (SCIC), Spain
⁴ Departamento de Física Teórica y del Cosmos, Campus de Fuentenueva, Universidad de Granada, 18071 Granada, Spain
⁵ Centre for Astrophysics Research, University of Hertfordshire, Hatfield, Herts AL10 9AB, UK
⁶ Université de Montréal, C.P. 6128 succ. centre ville, Montréal, QC, H3C 3J7 Canada

Received: 20 December 2006
Accepted: 26 February 2007

Abstract

Aims.We study two dimensional Fabry-Perot interferometric observations of the nearby face-on late-type spiral galaxy, NGC 628, in order to analyse the ionized gas component of the interstellar medium. Covering the galaxy out to a radius larger than 12 kpc, and with a spatial sampling of 16, we aim to investigate the large-scale dynamics as well as feedback from individual H ii regions into their surrounding medium.

Methods.The observed Hα emission distribution and kinematics are compared with auxiliary data from molecular and atomic gas observations, which display many similarities. We decompose the observed line-of-sight velocities into rotational and higher-order harmonic components, and study the role of gravitational perturbations along with that of external triggers which can disturb the kinematics and morphology of NGC 628. We calculate radial profiles of the emission-line velocity dispersion which we use to study the role of feedback from individual H ii regions.

Results.We verify the presence of an inner rapidly rotating disc-like component in NGC 628, which we interpret as caused by slow secular evolution of the large-scale spiral arms and oval structure. In combination with auxiliary data, we find indication for that gas is falling in from the outer parts towards the central regions, where a nuclear ring has formed at the location of the inner Lindblad resonance radius of an an perturbation. Complementary continuum subtracted narrow band images in Hα have been used to identify 376 H ii regions with calibrated luminosities. The mean velocity dispersion for the ionized gas (even when excluding pixels belonging to H ii regions) is almost constant out to 12 kpc, although it varies from 14 to 20 , with a steady decline in the outer parts.

Conclusions.We have found kinematic signatures of radial motions caused by an perturbation. Such a perturbation may well be responsible for the inflow of material forming the nuclear ring and the inner rapidly rotating disc-like structure. The latter, in turn, could help build a pseudo-bulge in NGC 628. The current paper demonstrates a number of tools that we have developed for building a solid frame work for studying the evolution of structure in spiral galaxies using two dimensional kinematic observations.