Asymmetries within Optical Discs of Spiral Galaxies

1. Figure 2 Figure 3 DDO 189 UGC 628 Asymmetries within Optical Discs of Spiral Galaxies P. Amram1, L. Chemin2,3, O. Garrido3,1, C. Carignan2, M. Marcelin1, C. Balkowski3, O. Hernandez1,2 (1) Laboratoire d’Astrophysique de Marseille [F] - (2) Université de Montréal [Ca] - (3) Observatoire de Paris [F] Introduction Non-axisymmetric disturbances, such as bars, oval distortions, spiral structures, triaxial haloes in disk galaxies are often seen in the optical 2D velocity fields but are usually ignored and azimuthally averaged in the rotation curves (RCs). These effects may lead to asymmetric RCs and to an erroneous determination of the galactic mass distribution and of the dark-to-luminous matter distribution. Asymmetries in RCs are more frequent in low mass and LSB galaxies than in high mass and HSB galaxies. The GHASP Survey The GHASP survey (Garrido et al. 2002,2003,2004) consists in mapping the distribution of the Ha of spirals and irregulars using a scanning Fabry-Perot interferometer and a photon counting camera at the 1.93 m telescope of the Observatoire de Haute-Provence. High resolution 2D velocity fields are derived. To cover a large range of luminosities (-15 < Mb < -22) and morphological types (from Sa to Irr) a sample of 200 galaxies has been obtained. This survey provides a homogeneous reference sample at z=0 and is used here to study the asymmetries in RCs (difference between both sides of the RCs). We quantify the asymmetries using the parameter AD from Dale et al (2003) for about half of the whole GHASP sample. AD increases with the asymmetry. We find no clear influence of the presence of a bar on the asymmetry of the RCs but we find that the asymmetry is correlated with the luminosity, the color, the central surface brightness and the maximal velocity of a galaxy. The most asymmetric RCs are for less massive (Fig. 1a), fainter (Fig. 1d),bluer (Fig. 1b)and late-type(Fig. 1c). The inner slope of RCs decreases when the central surface brightness increases (Fig. 1f)thus a large range of asymmetry is observed for the lower inner slope (Fig. 1g). d Figure 1 The LSB sample LSBs are ideal laboratories to study the DM halo properties. LSB DM haloes parameters require 3D spectroscopy at high angular resolution, such as FP interferometry. So far, 20 isolated LSBs have been observed at the ESO and CFH 3.6m telescopes equipped with the same instrumentation than GHASP. The angular resolution of the observations presented here is 0.49" and the spectral resolution at the Ha emission line is ~10000. The exposure time was ~ 3h per galaxy. The RCs are obtained using the rotcur task in the GIPSY package, which fits a tilted ring model to the velocity field. Results for 2 LSB are presented : UGC 628 and DDO 189 (UGC 9211). Ha integrated emission line map (a), velocity field (b), model of the velocity field (d), l-v diagram (c) and residual velocity map (e) are presented for UGC 628 (Fig 2). RCs (filled squares) of UGC 628 and DDO 189 (UGC 9211) are shown in Fig. 3. Blue (red) triangles represent the RC of the approaching (receding) side respectively. Green open circles correspond to Ha measurements from long-slit spectroscopy (de Blok & Bosma 2002). UGC 628: An isophotal analysis indicates that the bar major axis of this SBm galaxy differs by ~25° from the disk major axis. The bar probably induces the strong asymmetry between both sides of the RCs, observed in the first 3 kpc (~10”) of the galaxy (Fig. 3a), which cannot be modeled by axisymmetric radial motions. DDO 189 (UGC 9211): This irregular (Irr), which is not identified as a barred galaxy, displays the same asymmetry as UGC 628, in the first ~1.5 kpc (~25”) of its RC (Fig 3b). Here again, axisymmetric radial motions cannot be used to model the asymmetry. a b UGC 628 c e d a b Nature or Nurture (environmental effects in clusters) ? In long-slit RCs of galaxies in clusters (Rubin et al. 1999, Dale et al. 2001) no relationship has been observed between the asymmetry of the RCs and either, the cluster-centric distance or the morphological type. These conclusions will be checked using RCs derived from 2D velocity fields. Nevertheless, this may indicate that asymmetries in spirals have likely a secular origin. www.astro.umontreal.ca/fantomm