Distances from Stellar Kinematics for Peculiar Virgo Cluster Spiral Galaxies

Distances from Stellar Kinematics for Peculiar Virgo Cluster Spiral Galaxies Juan R. Cortés Departamento de Astronom´ıa, Universidad de Chile Casilla 36-D, Santiago, Chile [email protected] National Astronomical Observatory of Japan 2-21-1 Osawa, Mitaka, Tokyo, 181-8588 Jeffrey D. P. Kenney Department of Astronomy, Yale University P.0. Box 208101, New Haven, CT 06520-8101 [email protected] Eduardo Hardy1 National Radio Astronomy Observatory Casilla El Golf 16-10, Las Condes, Santiago, Chile [email protected] Departamento de Astronom´ıa, Universidad de Chile2 Casilla 36-D, Santiago, Chile ABSTRACT We present distance estimates for eleven peculiar Virgo cluster spiral galaxies based on mea- surements of the stellar kinematics of their central 2 kpc. Stellar circular velocities were obtained using two-integral dynamical models. Distances were obtained by comparing, at each radius, the stellar circular velocities with synthetic Hα rotation curves derived from NIR Tully-Fisher relations. The results show that most of our galaxies are located within 4 Mpc of the core arXiv:0803.3638v2 [astro-ph] 14 Apr 2008 of the cluster. Three of these galaxies, previously classified as “low rotator galaxies” or with “Truncated/Compact” Hα radial distributions, have stellar kinematics-based distances that are discrepant with HI-based distances by at least 60%, and are likely to be located within the virial radius of the cluster. These discrepancies appear due to very truncated gas distributions plus non-circular gas motions or gas motions not in the plane of the stellar disk, perhaps as the result of gravitational interactions. Our results show that environmental effects can significantly reduce the measured HI linewidths for some disturbed cluster galaxies, thus affecting the accurate determination of distances based on gas kinematics methods. Subject headings: galaxies: clusters: individual (Virgo) — galaxies: distances and redshifts — galaxies: fundamental parameters (classification, colors, luminosities, masses, radii, etc.) — galaxies: ISM —— galaxies: stars —- galaxies: kinematics and dynamics galaxies: nuclei — galaxies: evolution —- galaxies: interactions —- galaxies: peculiar 1 1. Introduction group (Sanchis et al. 2002, Mamon et al. 2004, and Sanchis et al. 2004), the possibility was For many years, the Tully-Fisher relation (Tully analyzed that these 13 HI-deficient objects had & Fisher, 1977) has been used to estimate dis- crossed the core of the cluster, losing their gas tances to the Virgo Cluster. These studies were content as a result of ICM-ISM stripping. They based on optical or near infrared photometry concluded these galaxies lie farther away than (Gavazzi et al. 1999), together with gas kine- the maximum radius to which these galaxies can matics from HI linewidths (e.g., Yasuda et al. bounce out, making core-crossing improbable, and 1997) or Hα rotation curves (Rubin et al. 1999). thus difficult for ICM-ISM stripping to be the These techniques have been useful in the determi- cause of the observed HI-deficiency. These au- nation of distances to individual galaxies within thors suggested alternative scenarios including the the cluster, allowing attempts to determine its 3- possibility of erroneous distance estimations, HI- D structure (i.e. Yasuda et al. 1997, Gavazzi et deficiency caused by tidal-interactions, heating of al. 1999, and more recently Solanes et al. 2002). the gas by mergers, and even questioned the claims Recent results show that there is a population for HI-deficiency. of galaxies in Virgo with HI line–widths narrower The application of the techniques discussed than those expected from the mean Tully-Fisher above to HI deficient galaxies suffers from the relation derived for the cluster. We refer to these possibility that environmental effects can reduce as “Tully-Fisher deviant” galaxies. Either these the spatial extent of the atomic gas, cause non- galaxies are foreground to the Virgo cluster, or circular motions or other disturbances, thus re- they have peculiarities which make their observed ducing the measured line-widths and therefore gas line–widths poor tracers of galaxy mass, and causing derived distances to be underestimated hence luminosity. (Guhathakurta et al. 1988 and Teerikorpi et al. The Rubin et al. (1999) study of Hα rotation 1992). Indeed these small line-widths could be curves of 89 Virgo disk galaxies identified a popu- caused either by truncation of the gas disks by lation ( 6% of total sample) which appears to de- ICM-ISM stripping, or because the gas velocities ∼ viate significantly from the Tully-Fisher relation, are intrinsically small at every radius (e.g. Rubin showing anomalously “low gas rotation” velocities et al. 1999), which can happen if the gas motion at apparently every radius, which they called “low is non-equilibrium or non-circular, or the gas ge- rotators galaxies”. Solanes et al. (2002) studied ometry is not understood. the 3-D structure of the cluster using the Tully– These conclusions raise obvious questions about Fisher relation based on HI linewidths and, most these objects, such as: Could these “Tully-Fisher recently, 13 HI deficient spiral galaxies with appar- deviant” galaxies be the result of an underestima- ently large 3-D barycentric distances located far tion of the distances produced by environmental away in front and behind the core of Virgo (San- effects, leading to an artificial distortion of the true chis et al. 2004). Seven galaxies were identified as cluster shape? being in the foreground of the cluster core (D < We have carried out an extensive study of the 13 Mpc), supposedly in areas where the ICM is kinematics and morphology of 13 peculiar Virgo too tenuous to be responsible for the stripping of Cluster galaxies (Cortés et al. 2006; Cortés et the ISM gas from these galaxies. Of these galaxies al. 2008 in prep). Six galaxies within our sample, only three had large projected distances (θ > 7◦) which includes four tentative foreground galaxies, , whereas the rest had small projected distance are objects with small HI line-widths and therefore (θ < 4◦). Some of the Solanes’s small HI line– “Tully-Fisher deviant”. In order to answer these width galaxies are also Hα Rubin “low rotator” questions, in this paper distances to these galaxies galaxies. are estimated through an approach that relies on In subsequent studies by the Sanchis et al. stellar kinematics1 and is therefore less affected by 1 The National Radio Astronomy Observatory is a fa- 1 Unfortunately, two of the original sample galaxies; NGC cility of the National Science Foundation operated under 4457 and NGC 4698, are unsuitable for deriving their dis- cooperative agreement by Associated Universities, Inc. tances using stellar kinematics since NGC 4457 is almost 2Adjoint Professor 2 environmental effects. Normal galaxies: NGC 4651. • This paper is structured as follows. We describe Truncated/Normal galaxies: NGC 4351, in 2 the galaxy sample. In 3, the method for es- • NGC 4569, NGC 4580, and NGC 4694. timating§ distances via stellar§ kinematics is intro- duced. The detailed application of this method to Truncated/Compact galaxies: NGC 4064, our data, the results on the distances to individ- • NGC 4424, and NGC 4606. ual galaxies of the sample, and the nature of the Anemic or Truncated/Anemic galaxies: NGC “Tully-Fisher deviant” galaxies is studied in 4. • A discussion about the existence of the putative§ 4293, NGC 4429, and NGC 4450. foreground group of galaxies in Virgo as well as of the causes for these galaxies HI-deficiency is pre- Although our main goal is to study galaxy mor- sented in 5. Finally, suggested improvements to phological evolution in the cluster, we have at least the technique,§ summary and concluding remarks six galaxies in the sample with small HI linewidths are found in 8. (NGC 4064, NGC 4351, NGC 4424, NGC 4580, § NGC 4606, and NGC 4694), including four galax- 2. The galaxy sample ies that Sanchis et al. (2004) suggest may belong to a foreground group. The sample consists of eleven peculiar Virgo cluster spiral galaxies, spanning a variety of op- 3. Distance estimations by using stellar tical morphologies (Table 1). Our sample includes kinematics more early type than late type galaxies, since most of the strongly disturbed cluster galaxies are early The determination of distances to HI-deficient types. Morphological selection was made using the galaxies using HI line-widths is vulnerable to R and Hα atlas of Virgo cluster galaxies of Koop- the reduction in the spatial extension of the HI mann et al. (2001), whereas the kinematical selec- disks resulting from environmental effects, and tion made use of the published Hα kinematics on by the possibility that gas velocities be intrinsi- 89 Virgo cluster spirals by Rubin et al. (1999). cally smaller at every radius. On the other hand, the use of stellar kinematics for distance deter- The Virgo cluster has about 110 spiral and mination (“stellar kinematics based distances”, lenticular galaxies brighter than 0.1 L∗ (Bingelli ⊙ hereafter SKB distances) has the advantage that et al. 1985, BST), so we have observed about 12% it is in principle unaffected by physical processes of the brightest Virgo cluster spiral galaxies. The other than gravitational processes, whereas as we sample galaxies are spread around the cluster core, have seen, the kinematics of the gaseous content spanning projected distances ranging from a few can be affected by a number of non-gravitational hundred kiloparsecs to 2.5 Mpc (Fig. 1). physical processes resulting from environmental While the sample selection is not uniform, it interactions (e.g. ICM-ISM stripping). However, is designed to include bright Virgo spirals whose stellar kinematics are far more difficult to measure peculiarities are most poorly understood, and to and analyze than gas velocities, on account of the include representatives of the different Hα types faintness of the stellar component as well as the identified by Koopmann & Kenney (2004).

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