Polar Disk Galaxies as new way to study galaxy formation: the case of NGC4650A
NGC4650A is a polar disk galaxy: this is a peculiar object composed by a central spheroidal component, the host galaxy (HG), and an extended disk made up by gas, stars and dust, which orbits nearly perpendicular to the plane of the central galaxy. The existence of two decoupled components of the angular momentum let this object the ideal laboratory i) to study gravitational interactions and merging and ii) to constrain the 3D shape of its dark matter halo. In view of these applications, I will present two ongoing projects which aim to constrain i) the formation scenario for polar disks and ii) the dark halo content and shape, through a detailed analysis of the observed structure, metallicity and dynamics of NGC4650A.
Keywords:galaxies: formation — galaxies: kinematics and dynamics
:98.62.Ai, 98.62.Bj, 98.62.Ck, 98.62.Dm, 98.62.Mw
1 The puzzling structure of NGC4650A
In the latest years, our group has given a big effort to study the formation and evolution of Polar Ring Galaxies (PRGs). NGC4650A is one of the best-investigated object among the class of PRGs, in the South hemisphere. Its luminous components, inner spheroid and polar structure, have been studied with optical and near-infrared (NIR) photometry and spectroscopy, and in the radio, HI 21 cm line and continuum. The polar structure in NGC 4650A has been shown to be a disk, rather than a ring. Its stars and dust can be reliably traced inward within the stellar spheroid to kpc radius from the galaxy nucleus (Iodice et al. (2002); Gallagher et al. (2002)). Emission and absorption line optical spectra are also consistent with an extended stellar disk rather than a narrow ring (Swaters and Rubin (2003)): both rotation curves show a linear inner gradient and a plateau, as expected for a disk in differential rotation. Furthermore, the HI 21 cm observations (Arnaboldi et al. (1997)) show that the gas is 5 times more extended than the luminous polar structure, with a position-velocity diagram very similar to those observed for edge-on gaseous disks. The polar disk is very massive, since the total HI mass in this component is about , which added to the mass of stars is comparable with the total mass in the central spheroid (Iodice et al. (2002)). New high resolution spectroscopy in NIR, obtained with [email protected], along the main axes of the central HG suggests that it resembles a nearly-exponential oblate spheroid supported by rotation (Iodice et al. (2006)). These new spectroscopic data set important constraints, both on current models for the formation scenarios and on the mass models proposed till now for NGC4650A, which were the starting points for the two ongoing works presented in the following sections. They aimed i) to test the formation of polar disks by cold accretion of gas through a ”cosmic filament”, and ii) to constrain the dark halo (DH) content and shape.
2 Chemical abundances in the polar disk: implications for the cold accretion
According to simulations of galaxy mergers (e.g. Bournaud et al. (2005)), polar merging of two disk galaxies fails to form a massive polar disk around a spheroid with rotation velocities as large as observed along the HG major axis (Iodice et al. (2006)); the tidal accretion scenario fails in reproducing disk-like polar structures as extended as observed in NGC4650A (Iodice et al. (2002)). Very recently, a new formation mechanism has been proposed within the scenario proposed for the build-up of high redshift disk galaxies: a long-lived polar structure will form through cold gas accretion along a filament, extended for Mpc, into the virialized DH (Macciò et al. (2006); Brook et al. (2008)). In this formation scenario, there is no limits to the mass of the accreted material, thus a very massive polar disk may develop either around a stellar disk or a spheroid: the morphology and kinematics of one simulated object are quite similar to those observed for NGC4650A. In order to test the cold accretion scenario for this object, Spavone et al. (2009) (see also Spavone et al. in this book) have studied the abundance ratios and metallicities of the HII regions associated to the polar disk in NGC4650A, by using new deep longslit spectroscopy obtained with [email protected] Main results show i) that NGC4650A has metallicity lower than spiral galaxy disks of the same total luminosity (see Fig.1 left panel), where , which is consistent with values predicted for the formation of disks by cold accretion processes; ii) the absence of any metallicity gradient along the polar disk (see Fig.1 right panel), which suggests that the metal enrichment is not influenced by the stellar evolution of the older central spheroid and, thus, the disk was formed later by the infall of metal-poor gas from outside which is still forming the disk.
3 Dynamical model for NGC 4650A: solving the enigma of the flattening of its Dark Halo
The existence of two orthogonal components of the angular momentum let the PRGs the ideal laboratory to put limits on the 3-D shape of the DH. The question of the DH shape is important because cosmological simulations (Navarro et al. (1997)) predict the distribution of the DH shapes and the universal radial dependence (NFW). NFW density profiles are shown to describe the DH in many morphological types of galaxies, but divergences have been found (de Blok et al. (2003); Napolitano et al. (2005)). In view of these applications, if one derives the most likely DH flattening distribution from the PRGs dynamics, tests can be performed on the likelihood of the different cosmological models. The latest dynamical models developed for NGC4650A were by Sackett et al. (1994), that found a flattened (E6-E7) DH, whose axes are aligned with those of the central galaxy, and by Combes and Arnaboldi (1996) that derived a best fit with a DH flattened in the plane of the polar disk itself. The biggest uncertainties in the mass models proposed until 1996 are due to the low resolution of the HI data and to the difficulty in measuring the velocity dispersion along the spheroid major axis. The new high resolution CaT spectra (Iodice et al. (2006)) are better tracers of the kinematics for the NGC4650A spheroid than was available before: they allow us to measure a flat profile along the spheroid major axis, while previous measurements were too scattered to reliably establish any trend with radius. The new profiles show that both the linear decreasing fit proposed by Sackett et al. (1994) and the exponential empirical law proposed by Combes and Arnaboldi (1996) do not reproduce the observed trend with radius. Previous conclusions that the same authors drew were based on data that are no longer valid, thus we aim to develop a new dynamical model for NGC4650A (Iodice, Napolitano, Arnaboldi et al. in preparation) by using the new spectroscopy available for the HG (Iodice et al. (2006)) and rotation curves of stars and gas in the polar disk (Arnaboldi et al. (1997); Swaters and Rubin (2003); Iodice et al. (2008)). As a first step we fitted the kinematics of the two components (HG and polar disk) separately: 1) we have assumed a centrifugal equilibrium along the polar disk, , where the circular velocity ; 2) we have performed an axisymmetric Jeans analysis on the equatorial plane of the spheroid. The best fit for the polar disk (see Fig.2 left panel) was obtained with (K band) by adding to an exponential disk a DH with a NFW radial profile, with a scale radius kpc and a DH mass of about 5.9 inside a radius of 25 kpc. To fit the rotation velocity and velocity dispersion along the spheroid equatorial plane, we assumed a NFW DH, with virial mass and the concentrations fixed from the polar axis, and isotropy of the velocity ellipsoid: the best fit was obtained with a scale radius kpc for the DH (see Fig.2 right panel). By such a simple analysis we have obtained that the two scale radii are different which suggests a DH flattened along the polar axis with a flattening of the potential (E7). This preliminary result, together with the DH mass, are consistent with the DH content and flattening given by Combes and Arnaboldi (1996). We are going to perform a more accurate dynamical model to fit at the same time the kinematics along the equatorial and meridian plane, by introducing the shape parameter in the DH profile.
- Iodice et al. (2002) E. Iodice, M. Anaboldi, G. De Lucia, J. Gallagher, L. Sparke, and K. Freeman, ApJ 123, 195–206 (2002).
- Gallagher et al. (2002) J. Gallagher, L. Sparke, L. Matthews, L. Frattare, J. English, A. Kinney, E. Iodice, and M. Arnaboldi, AJ 568, 199–209 (2002).
- Swaters and Rubin (2003) R. Swaters, and V. Rubin, ApJ 587, L23–L26 (2003).
- Arnaboldi et al. (1997) M. Arnaboldi, T. Oosterloo, F. Combes, K. Freeman, and B. Koribalski, AJ 113, 585–598 (1997).
- Iodice et al. (2006) E. Iodice, M. Arnaboldi, R. Saglia, L. Saprke, O. Gerhard, J. Gallagher, F. Combes, F. Bournaud, M. Capaccioli, and K. Freeman, ApJ 643, 200–209 (2006).
- Bournaud et al. (2005) F. Bournaud, C. Jog, and F. Combes, A&A 437, 69–85 (2005).
- Macciò et al. (2006) A. Macciò, B. Moore, and J. Stadel, ApJL 636, L25–L28 (2006).
- Brook et al. (2008) C. Brook, F. Governato, T. Quinn, J. Wadsley, A. Brooks, B. Willman, A. Stilp, and P. Jonsson, ApJ 689, 678–686 (2008).
- Spavone et al. (2009) M. Spavone, E. Iodice, M. Arnaboldi, O. Gerhard, R. Saglia, and G. Longo, ApJ submitted (2009).
- Navarro et al. (1997) J. Navarro, C. Frenk, and S. White, ApJ 490, 493 (1997).
- de Blok et al. (2003) W. de Blok, A. Bosma, and M. S., MNRAS 340, 657–678 (2003).
- Napolitano et al. (2005) N. Napolitano, M. Capaccioli, A. Romanowsky, N. Douglas, M. Merrifield, K. Kuijken, M. Arnaboldi, O. Gerhard, and K. Freeman, MNRAS 357, 691–706 (2005).
- Sackett et al. (1994) P. Sackett, H. Rix, and K. Jarvis, B.J.and Freeman, ApJ 436, 629–641 (1994).
- Combes and Arnaboldi (1996) F. Combes, and M. Arnaboldi, A&A 305, 763 (1996).
- Iodice et al. (2008) E. Iodice, M. Arnaboldi, N. Napolitano, T. Oosterloo, and G. Józsa, ASPC 396, 483 (2008).
- Pèrez-Montero et al. (2009) E. Pèrez-Montero, R. García-Benito, A. Díaz, E. Pèrez, and C. Kehrig, A&A 497, 53–63 (2009).
- Pilyugin et al. (2006) L. S. Pilyugin, T. X. Thuan, and J. M. Vílchez, MNRAS 367, 1139–1146 (2006).