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Register a new userThe puzzle of the polar structure in NGC 4650A
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This work presents new surface photometry and two-dimensional modeling of the light distribution of the Polar Ring Galaxy NGC 4650A, based on near-infrared (NIR) observations and high resolution optical imaging acquired during the Hubble Heritage program. The NIR and optical integrated colors of the S0 and the polar ring, and their scale parameters, are compared with those for standard galaxy morphological types. The polar structure appears to be a disk of a very young age, while the colors and light distribution of the host galaxy do not resemble that of a typical early-type system. We compare these observational results with the predictions from different formation scenarios for polar ring galaxies. The peculiarities of the central S0 galaxy, the polar disk structure and stellar population ages suggest that the polar ring galaxy NGC 4650A may be the result of a dissipative merger event, rather than of an accretion process.
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We present the first measurement of the stellar kinematics in the polar ring of NGC 4650A. There is well defined rotation, with the stars and gas rotating in the same direction, and with similar amplitude. The gaseous and stellar kinematics suggest an approximately flat rotation curve, providing further support for the hypothesis that the polar material resides in a disk rather than in a ring. The kinematics of the emission line gas at and near the center of the S0 suggests that the polar disk lacks a central hole. We have not detected evidence for two, equal mass, counterrotating stellar polar streams, as is predicted in the resonance levitation model proposed by Tremaine & Yu. A merger seems the most likely explanation for the structure and kinematics of NGC 4650A.
We present the results of stellar photometry of polar-ring galaxies NGC 2685 and NGC 4650A, using the archival data obtained with the Hubble Space Telescopes Wide Field Planetary Camera 2. Polar rings of these galaxies were resolved into ~800 and ~430 stellar objects in the B, V and Ic bands, considerable part of which are blue supergiants located in the young stellar complexes. The stellar features in the CM-diagrams are best represented by isochrones with metallicity Z = 0.008. The process of star formation in the polar rings of both galaxies was continuous and the age of the youngest detected stars is about 9 Myr for NGC 2685 and 6.5 Myr for NGC 4650A.
Context. The prototype of Polar Ring Galaxies NGC 4650A contains two main structural components, a central spheroid, which is the host galaxy, and an extended polar disk. Both photometric and kinematic studies revealed that these two components co-exist on two different planes within the central regions of the galaxy. Aims. The aim of this work is to study the spectroscopic and kinematic properties of the host galaxy and the polar disk in the central regions of NGC 4650A by disentangling their contributions to the observed galaxy spectrum. Methods. We applied the spectral decomposition technique introduced in previous works to long-slit spectroscopic observations in the CaII triplet region. We focused the analysis along the PA = 152 that corresponds to the photometric minor axis of the host galaxy, where the superimposition of the two components is more relevant and the spectral decomposition is necessary. We aim to disentangle the stars that move in the equatorial plane of the host galaxy from those that move in the meridan plane, which is along the polar disk. Results. We successfully disentangled the spectra of the two structural components of NGC 4650A and measured their line-of-sight velocity and velocity dispersion profiles, and the stellar content along PA = 152. The host galaxy shows significant rotation along its photometric minor axis, indicating that the gravitational potential is not axisymmetric. The polar disk shows a kinematic decoupling: the inner regions counter-rotating with respect the outer-regions and the host spheroid. This suggests a complex formation history for the polar disk, characterised by mass accretion with decoupled angular momenta.
The early-type spiral NGC 4698 is known to host a nuclear disc of gas and stars which is rotating perpendicularly with respect to the galaxy main disc. In addition, the bulge and main disc are characterised by a remarkable geometrical decoupling. Indeed they appear elongated orthogonally to each other. In this work the complex structure of the galaxy is investigated by a detailed photometric decomposition of optical and near-infrared images. The intrinsic shape of the bulge was constrained from its apparent ellipticity, its twist angle with respect to the major axis of the main disc, and the inclination of the main disc. The bulge is actually elongated perpendicular to the main disc and it is equally likely to be triaxial or axisymmetric. The central surface brightness, scalelength, inclination, and position angle of the nuclear disc were derived by assuming it is infinitesimally thin and exponential. Its size, orientation, and location do not depend on the observed passband. These findings support a scenario in which the nuclear disc is the end result of the acquisition of external gas by the pre-existing triaxial bulge on the principal plane perpendicular to its shortest axis and perpendicular to the galaxy main disc. The subsequent star formation either occurred homogeneously all over the extension of the nuclear disc or through an inside-out process that ended more than 5 Gyr ago.
We have obtained optical spectrophotometry of 11 HII regions in the polar ring of NGC 2685 (the Helix galaxy), and have used these data to study the physical characteristics of the polar-ring HII regions. The HII regions have normal spectra with no suggestion of unusual density, temperature, extinction, or composition. Semi-empirical calculations yield high oxygen abundance estimates (0.8--1.1 Z-Solar) in all HII regions. This, along with the observed (B-V) color, H-alpha equivalent width, and molecular gas properties argue against the current picture in which polar rings form from tidally captured dwarf irregular galaxies, and suggests instead that the rings are long-lived, self-gravitating structures as predicted by some dynamical models. This would allow the time required for multiple generations of star formation, and for the retention of the resulting enriched ejecta for inclusion in further generations of star formation.
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