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Dynamical effects of interactions and the Tully-Fisher relation for Hickson compact groups C. Mendes de Oliveira 1,2,3, P. Amram 4, H. Plana 5 and C. Balkowski 6 [email protected] ABSTRACT We investigate the properties of the B-band Tully-Fisher (T-F) relation for 25 compact group galaxies, using Vmax derived from 2-D velocity maps. Our main result is that the majority of the Hickson Compact Group galaxies lie on the T-F relation. However, about 20% of the galaxies, including the lowest-mass systems, have higher B luminosities for a given mass, or alternatively, a mass which is too low for their luminosities. We favour a scenario in which outliers have been brightened due to either enhanced star formation or merging. Alternatively, the T-F outliers may have undergone truncation of their dark halo due to interactions. It is possible that in some cases, both effects contribute. The fact that the B-band T-F relation is similar for compact group and field galaxies tells us that these galaxies show common mass-to-size relations and that the halos of compact group galaxies have not been significantly stripped inside R25. We find that 75% of the compact group galaxies studied (22 out of 29) have highly peculiar velocity fields. Nevertheless, a careful choice of inclination, position angle and center, obtained from the velocity field, and an average of the velocities over a large sector of the galaxy enabled the determination of fairly well-behaved rotation curves for the galaxies. However, two of the compact group galaxies which are the most massive members in M51–like pairs, HCG 91a and HCG 96a, have very asymmetric rotation curves, with one arm rising and the other one falling, indicating, most probably, a recent perturbation by the small close companions. Subject headings: Galaxies: individual ( galaxies: kinematics and dynamics — galaxies: interactions — galaxies: ISM — galaxies: intergalactic medium — instrumentation: interferometers) arXiv:astro-ph/0311433v1 18 Nov 2003 1. Introduction 1Universidade de S˜ao Paulo, IAG, Departamento de As- tronomia, Rua do Mat˜ao 1226, 05508-900, S˜ao Paulo, SP, The influence of environmental effects on the in- Brazil ternal dynamics and matter distribution of com- 2 Max-Planck-Institut f¨ur Extraterrestrische Physik, pact group galaxies has not yet been clearly es- Giessenbachstrasse, D-85748 Garching b. M¨unchen, Ger- many tablished, mostly due to lack of reliable kinematic 3Universit¨ats-Sternwarte der LMU, Scheinerstrasse 1, data. A recent study of the internal kinematics D-81679 M¨unchen, Germany of 30 galaxies by Nishiura et al. (2000, hereafter 4Observatoire Astronomique Marseille-Provence & Lab- N2000) found that asymmetric and peculiar rota- oratoire d’Astrophysique de Marseille, 2 Place Le Verrier, tion curves are more frequently seen in the Hick- 13248 Marseille Cedex 04, France son Compact Groups (HCG) spiral galaxies than 5Universidade Estadual de Santa Cruz, Dept. Ciencias Exatas e Tecnologicas, Rodovia Ilheus-Itabuna, km. 16, in field or cluster spirals and the dynamical prop- 45650-000, Ilh´eus, BA, Brazil erties of the galaxies do not seem to correlate with 6Observatoire de Paris, GEPI, CNRS and Universite any group or galaxy parameter. An older but very Paris 7, 5 Place Jules Janssen, F-92195 Meudon Cedex, influential study is that by Rubin et al. (1991, France 1 hereafter R1991), who analyzed rotation curves 2. Data for 32 Hickson compact group galaxies and found that 2/3 of them had peculiar rotation curves. For The data used in this paper are gathered from the subsample of galaxies for which rotation curves three publications which studied the kinematics were possible to be derived, they found a large off- of galaxies in nine compact groups (HCG 10, 16, set of the T-F relation with respect to the field re- 19, 87, 88, 89, 90, 91, 96, 100). A detailed descrip- lation in the sense that galaxies in compact groups tion of the observations and data reduction can be have “too low velocities for their luminosities or, found in Mendes de Oliveira et al. (1998), Plana alternatively, luminosities which are overbright for et al. (2003) and Amram et al. (2003). In sum- their rotation velocities.” This suggested to the mary, our dataset consists of Hα emission-line ve- authors that spiral galaxies in compact groups locity maps obtained with a Fabry-Perot system, have low mass-to-light ratios compared to field from which rotation curves were obtained. For galaxies by about 30% which, in turn, could be the T-F study we excluded from the sample all explained if compact group galaxies have smaller the elliptical galaxies. We also excluded galaxies dark halos than their field counterparts. Given HCG 10a, 10c, 16d and 100a because their rota- that compact groups are environments where tidal tion curves have a very short extension, well short encounters are common, it may be expected that of R25 and galaxies HCG 16b and HCG 100b due interactions have stripped or disrupted the galaxy to their extremely peculiar rotation curves. We dark halos at some level. These conclusions have included HCG 19a in our sample, although Hick- important consequences for the determination of son (1993) classifies it as an E2 galaxy, because we group lifetimes and understanding how compact have reclassified it as an S0 based on its kinematic groups evolve and eventually merge. properties. In addition, we included in our sample unpublished data for two other galaxies (HCG 07c We revisit this important problem using a new and HCG 79d). dataset for 25 galaxies, of which 13 are in common with either the sample of R1991 or N2000 or both. We show in Figs. 1a and 1b the rotation curves In this paper we re-examine the T-F relation for for the galaxies in the sample studied in this paper. compact group galaxies. Our study differs from The x-axis plots r/R25, the galaxy radius along the previous work in that it uses rotation curves its major axis normalized by R25, the length of −2 obtained from 2-dimensional velocity fields. In a the major axis at the 25 mag arcsec isophote number of cases this allows a more accurate de- (as given by Hickson 1993; note, however that in termination of the rotation curves than was pos- Plana et al. 2003 and Amram et al. 2003, the sible with slit spectroscopy. Additionally, a fuller value for R25 was taken from de Vaucouleurs et characterization of the kinematics of each galaxy al. 1991, the RC3, resulting in slightly different is possible. We show that, with these new data, numbers from those used here). In order to obtain the T-F relation for compact group galaxies is sim- a Vmax for use in the T-F relation (represented by ilar to that for galaxies in less dense environments. a black dot in each subpannel of Figs. 1a and 1b) The exceptions to this conclusion are seen for some we have computed the maximum velocity of the of the least massive galaxies in our sample. average velocity curve (single average of the values for the receding and approaching sides). V is This paper is organized as follows. In Section max not the best kinematic parameter to be used in 2 we present the set of rotation curves that are the T-F relation, since V , the velocity of the used. In Section 3 we illustrate and discuss com- flat flat portion of the curve or V , the velocity at parisons of rotation curves for several galaxies in 2d two times the galaxy scale length are known to common with R1991 and N2000 and we show that yield less scatter in the T-F relation. However, the 2-D spectroscopy is needed if one wants to accu- use of V allowed us to use the largest possible rately describe the kinematics of interacting galax- max number of compact group galaxies. ies. Sections 4 and 5 show the results on the T-F relation and a discussion respectively. The control sample used for the T-F compar- ison was the sample of Sb and Sc galaxies from Courteau (1997), where we used their “Vmax” as the equivalent to the maximum velocity deter- 2 mined by us. We also used the sample of galaxies ticular, if the kinematic parameters are used and in the Ursa Major cluster (Verheijen 2001), in or- velocities over a large sector of the galaxies are av- der to fill in the lower end of the mass sequence in eraged, the bifurcation present in Fig. 2, at r ∼ the T-F diagram. We note that our values of Vmax 10 arcsec, disappears, and the galaxy has normal are adjusted by the cosmological correction (1+z) kinematics, with both sides of the curve matching. as are also the values given for the galaxies in the In order to understand the magnitude of the control samples. All velocities were corrected for variations of the parameters measured from the the same Virgocentric infall (Paturel et al. 1997) kinematic and photometric data we list in Table 2 when distances were obtained using the Hubble values for the inclination, PA and differences be- law (for our sample and Courteau’s sample). tween centers measured from the continuum im- We note that in Fig. 1 the rotation of HCG ages and velocity fields. Detailed discussion on the 87a is one-sided because half of this almost edge- method used to obtain the center, PA of the ma- on galaxy is not observed due to a strong dust jor axis, and inclination of the galaxies are given lane. in Amram et al.

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