Do active galaxies have a massive halo component?

We present a dynamical model for an active galaxy. Our model is a mass model with a disk, nucleus, and halo components. Numerical calculations and theoretical evidence show, that for a fixed value of mass of the galaxy the stellar velocities in the central region decrease as the mass of halo increases. Furthermore, the motion tends to be regular while, when the halo component is absent, the majority of orbits are chaotic. The dynamical evolution of the system is also studied when mass is transported from the halo to the disk and the nucleus. Our results are compared to the recently obtained observation data for active galaxies.     

A chemically decoupled nucleus and inner polar ring of the SBb galaxy NGC 4548

Our inzvestigation of the central region in NGC 4548, a bright Sb galaxy with a large-scale bar, using the Multipupil Field Spectrograph of the 6-m telescope revealed a chemically decoupled compact stellar nucleus with [Fe/H]=+0.6 and [Mg/Fe]=+0.1...+0.2 and with a mean stellar-population age of 5 Gyr. This nucleus, a probable circumnuclear disk coplanar with the global galactic disk, is embedded in the bulge whose stars are generally also young, T≈4 Gyr, although they are a factor of 2.5 more metal-poor. The bulge of NGC 4548 is triaxial and has a de Vaucouleurs surface-brightness profile; the unusual characteristics of its stellar population suggest the bulge formation or completion in the course of secular evolution in the triaxial potential of the global bar. The ionized gas within 3″ of the NGC 4548 nucleus rotates in a plane inclined to the principal symmetry plane of the galaxy, possibly, even in its polar plane, which may also result from the action of the large-scale bar.     

Vilnius photometry of the red horizontal branch stars in the Galactic thick disk

The photometric observations and three-dimensional classification in the Vilnius system have been carried out for 13 of the red horizontal branch (RHB) stars in the Galactic thick disk identified by Rose (1985). The photometric spectral types, metallicities [Fe/H], effective temperatures, surface gravities and absolute magnitudes are determined. The age of about 10–12 Gyr is evaluated for this group of stars from comparison with model isochrones, indicating that the age of the thick disk is intermediate between the disk globular and the oldest open clusters.     

Spectroscopic study of the peculiar galaxy IC 883

We analyze new optical spectroscopic observations obtained at the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences with the SCORPIO focal reducer (in the modes of a Fabry–Perot interferometer (FPI) and long-slit spectroscopy) and the Multi-Pupil Fiber Spectrograph for the galaxy IC 883. We have confirmed that the main body of the galaxy rotates around its minor axis. The positions of the dynamical axes of the stellar and gaseous components have been found to differ by ~10°. The velocities in the SE tail do not correspond to the circular rotation around the galaxy’s minor axis. This structure is probably a fragment of an unwound curved spiral arm. Regions with high velocity dispersions and peculiarities in the velocity fields have been found along the minor axis. Our study of the age and metallicity of the galaxy’s stellar population has shown that the mean values of these parameters in the stellar disk, except for the central region (r ≤ 5?), are ≈1 Gyr and ≈?0.4 dex, respectively. Both young (2?5 × 10⁸ yr) and old (5?10 × 109 yr) stellar populations are present in the circumnuclear region. Our analysis of the spectroscopic data for the bright feature 8? south of the nucleus coincident in position with a compact X-ray source has shown that this is apparently a dwarf galaxy or a remnant of a companion galaxy. Our FPI observations in the Hα emission line and direct images have revealed a region of ionized gas that together with the already known structures along the minor axis forms a clumpy tidal structure of ionized gas pulled from the companion galaxy. The results of our study confirm the previously proposed hypothesis that the observed peculiar structures were formed by the merger of two galaxies. However, it can be said that IC 883 does not belong to the class of polar-ring galaxies.     

Modelling the formation of individual galaxies: A morphology problem for CDM?

We use a semi-analytic model of halo formation to study the dynamical history of giant field galaxies like the Milky Way. We find that in a concordance LCDM cosmology, most isolated disk galaxies have remained undisturbed for 8–10 Gyr, such that the age of the Milky Way's thin disk is unremarkable. Many systems also have older disk components which have been thickened by minor mergers, consistent with recent observations of nearby field galaxies. We do have a considerable problem, however, reproducing the morphological mix of nearby galaxies. In our fiducial model, most systems have disk-to-bulge mass ratios of order 1, and look like S0s rather than spirals. This result depends mainly on merger statistics, and is unchanged for most reasonable choices of our model parameters. We discuss two possible solutions to this morphology problem in LCDM. This revised version was published online in August 2006 with corrections to the Cover Date.     

Recent investigations on disk galaxies in massive halos

Numerical experiments undertaken to investigate the longevity and behavior of dark-lane elliptical galaxies are described. This is dynamically the same problem as a disk galaxy in a massive halo. Spiral galaxies are disks from a dynamical point of view. A disk of particles embedded in a self-consistent galaxy provides the basic model used for the experiments. This model is applicable to ordinary disk galaxies if the disk is interpreted as the visible galaxy and the galaxy is interpreted as the massive halo thought to be present around disk galaxies. Fully three-dimensional fully self-consistentn-body computer programs that can handle 100,000 particles are used for the experiments. The background galaxy is oblate, and the disk is inclined to the axis of the oblate galaxy, so the disk precesses differentially to produce a warp. A surprising result is that the galaxy center shifted, leaving the disk center orbiting around the galaxy center. This produces interesting phenomena reminiscent of observations in the region of the Galactic center.     

Modelling gaseous and stellar kinematics in the disc galaxies NGC 772, 3898 and 7782

We present V -band surface photometry and major-axis kinematics of stars and ionized gas of three early-type spiral galaxies, namely NGC 772, 3898 and 7782. For each galaxy we present a self-consistent Jeans model for the stellar kinematics, adopting the light distribution of bulge and disc derived by means of a two-dimensional parametric photometric decomposition. This allows us to investigate the presence of non-circular gas motions, and derive the mass distribution of luminous and dark matter in these objects.
NGC 772 and 7782 have apparently normal kinematics with the ionized gas tracing the gravitational equilibrium circular speed. This is not true in the innermost region (| r |≲8 arcsec) of NGC 3898, where the ionized gas is rotating more slowly than the circular velocity predicted by dynamical modelling. This phenomenon is common in the bulge-dominated galaxies for which dynamical modelling enables us to make the direct comparison between the gas velocity and the circular speed, and it poses questions about the reliability of galaxy mass distributions derived by the direct decomposition of the observed ionized-gas rotation curve into the contributions of luminous and dark matter.     

The effect of projection on the observed gas velocity fields in barred galaxies

The problem of determining the pattern of gas motions in the central regions of disk spiral galaxies is considered. Two fundamentally different cases—noncircular motions in the triaxial bar potential and motions in circular orbits but with orientation parameters different from those of the main disk—are shown to have similar observational manifestations in the line-of-sight velocity field of the gas. A reliable criterion is needed for the observational data to be properly interpreted. To find such a criterion, we analyze two-dimensional nonlinear hydrodynamic models of gas motions in barred disk galaxies. The gas line-of-sight velocity and surface brightness distributions in the plane of the sky are constructed for various inclinations of the galactic plane to the line of sight and bar orientation angles. We show that using models of circular motions for inclinations i>60° to analyze the velocity field can lead to the erroneous conclusions of a “tilted (polar) disk” at the galaxy center. However, it is possible to distinguish bars from tilted disks by comparing the mutual orientations of the photometric and dynamical axes. As an example, we consider the velocity field of the ionized gas in the galaxy NGC 972.     

Evolutionary history of the elliptical galaxy NGC 1052

We have obtained Keck spectra for 16 globular clusters (GCs) associated with the merger remnant elliptical NGC 1052, as well as a long-slit spectrum of the galaxy. We derive ages, metallicities and abundance ratios from simple stellar population models using the recently published methods of Proctor & Sansom , applied to extragalactic GCs for the first time. A number of GCs indicate the presence of strong blue horizontal branches that are not fully accounted for in the current stellar population models. We find all of the GCs to be ∼13 Gyr old according to simple stellar populations, with a large range of metallicities. From the galaxy spectrum we find NGC 1052 to have a luminosity-weighted central age of ∼2 Gyr and metallicity of  [Fe/H]∼+0.6  . No strong gradients in either age or metallicity were found to the maximum radius measured  (0.3  r _e≃ 1 kpc)  . However, we do find a strong radial gradient in α-element abundance, which reaches a very high central value. The young central starburst age is consistent with the age inferred from the H  i tidal tails and infalling gas of ∼1 Gyr. Thus, although NGC 1052 shows substantial evidence for a recent merger and an associated starburst, it appears that the merger did not induce the formation of new GCs, perhaps suggesting that little recent star formation occurred. This interpretation is consistent with 'frosting' models for early-type galaxy formation.     

The formation of a disk galaxy within a slowly growing dark halo

The formation of a disk galaxy within a slowly growing dark halo is simulated with a new chemo-dynamical model. The model describes the evolution of the stellar populations, the multi-phase ISM and all important interaction. I find, that the galaxy forms radially from inside-out and vertically from top-to-bottom. The derived stellar age distributions show that the inner halo is the oldest component, followed by the outer halo, the triaxial bulge, the halo-disk transition region and the disk. Despite the still idealized model, the final galaxy resembles present-day disk galaxies in many aspects. In particular, the stellar metallicity distribution in the halo of the model resembles the one of M31. The bulge in the model shows, at least two stellar subpopulations, an early collapse population and a population that formed later out of accreted disk mass. In the stellar metallicity distribution of the disk, I find a pronounced ‘G-dwarf problem’ which is the result of a pre-enrichment of the disk ISM with metal-rich gas from the bulge. This revised version was published online in September 2006 with corrections to the Cover Date.     

Dynamical friction on a satellite of a disk galaxy: The circular orbit

In a previous paper, we have studied dynamical friction during a parabolic passage of a companion galaxy past a disk galaxy. This paper continues with the study of satellites in circular orbits around the disk galaxy. Simulations of orbit decay in a self gravitating disk are compared with estimates based on two-body scattering theories; the theories are found to give a satisfactory explanation of the orbital changes. The disk friction is strongly dependent on the sense of rotation of the companion relative to the rotation of the disk galaxy as well as on the amount of mass in a spherical halo. The greatest amount of dynamical friction occurs in direct motion if no spherical halo is present. Then the infall time from the edge of the disk is about one half of the orbital period of the disk edge. A halo twice as massive as the disk increases the infall time four fold. The results of Quinn and Goodman, obtained with a non-self-gravitating method, agree well with our experiments with massive halos (Q ₀ 1.5), but are not usable in a more general case. We give analytic expressions for calculating the disk friction in galaxies of different disk/halo mass ratios.     

Photometry of the low-luminosity spiral galaxy NGC 4136

Multicolor BVRI surface photometry of the low-luminosity (M _V ≈?18^m) spiral galaxy NGC 4136 is presented. The photometric parameters of its components and the color distribution over the galactic disk are estimated. The color indices and the corresponding effective ages are determined for the brightest star-forming regions. The disk-to-dark halo mass ratio is derived from the measured rotation curve of the galaxy. The disk mass dominates within the optical boundaries of the galaxy, so its disk can be considered as a self-gravitating system.     

The Sagittarius Dwarf Galaxy Survey (SDGS) – II. The stellar content and constraints on the star formation history

A detailed study of the star formation history of the Sagittarius dwarf spheroidal galaxy is performed through the analysis of data from the Sagittarius Dwarf Galaxy Survey (SDGS). Accurate statistical decontamination of the SDGS colour–magnitude diagrams (CMDs) allows us to obtain many useful constraints on the age and metal content of the Sgr stellar populations in three different regions of the galaxy.
A coarse metallicity distribution of Sgr stars is derived, ranging from [Fe/H]∼−2.0 to [Fe/H]∼−0.7, the upper limit being somewhat higher in the central region of the galaxy. A qualitative global fit to all the observed CMD features is attempted, and a general scheme for the star formation history of the Sgr dSph is derived. According to this scheme, star formation began at a very early time from a low metal content interstellar medium and lasted for several  Gyr, coupled with progressive chemical enrichment. The star formation rate (SFR) had a peak from 8 to 10  Gyr ago, when the mean metallicity was in the range −1.3≤[Fe/H]≤−0.7. After that maximum, the SFR rapidly decreased and a very low rate of star formation took place until ∼1–0.5  Gyr ago.     

Imaging of the protoelliptical NGC 1700 and its globular cluster system

An excellent candidate for a young elliptical, or 'protoelliptical' galaxy is NGC 1700. Here we present new B -, V - and I -band imaging using the Keck telescope, and reanalyse existing V - and I -band images from the Hubble Space Telescope . After subtracting a model of the galaxy from the Keck images, NGC 1700 reveals two symmetric tidal tail-like structures. If this interpretation is correct, it suggests a past merger event involving two spiral galaxies. These tails are largely responsible for the 'boxiness' of the galaxy isophotes observed at a radius of ∼13 kpc.
We also show that the B − I colour distribution of the globular cluster system is bimodal. The mean colour of the blue population is consistent with that of old Galactic globular clusters. Relative to this old, metal-poor population, we find that the red population is younger and more metal-rich. This young population has an age and metallicity similar to that inferred for the central stars, suggesting that both populations are associated with an episode of star formation triggered by the merger that may have formed the galaxy. We find that, although they have large errors, the majority of the age estimates of NGC 1700 are reasonably consistent and we adopt a 'best estimate' for the age of 3.0±1.0 Gyr. This relatively low age places NGC 1700 within the age range where there is a notable lack of obvious candidates for protoellipticals. The total globular cluster specific frequency is rather low for a typical elliptical, even after taking into account the fading of the galaxy over the next 10 Gyr. We speculate that NGC 1700 will eventually form a relatively 'globular cluster poor' elliptical galaxy.     

Two MASS photometry of open star clusters: King 13 and Berkeley 53

In the present work, we used the near-infrared JHK_s photometric data from the 2-Micron All Sky Survey (2MASS) to determine the morphological and photometric parameters for two rarely studied open star clusters; King 13 and Berkeley 53. Luminosity function, mass function and dynamical relaxation time have been determined for the two clusters. We estimated the distances of 2.11±0.25 Kpc and 3.51±0.21 Kpc for King 13 and Berkeley 53 respectively, and both clusters have the same age 1.00±0.12 Gyr at solar metallicity; z=0.019.     

Numerical simulations of interacting gas-rich barred galaxies: vertical impact of small companions

We investigate the dynamical effects of an interaction between an initially barred galaxy and a small spherical companion using an N -body/smoothed-particle-hydrodynamics algorithm. In the models described here the small companion passes through the disc of the larger galaxy nearly perpendicular to its plane. The impact positions and times are varied with respect to the phase of the bar and the dynamical evolution of the disc.
The interactions produce expanding ring structures, offset bars, spokes and other asymmetries in the stars and gas. These characteristic signatures of the interaction are present in the disc for about 1 Gyr. We find that in some cases it is possible to destroy the bar while keeping the disc structure. In general, the central impacts cause larger damage to the bar and the disc than the peripheral ones. The interaction tends to accelerate the transition from a strongly-barred galaxy to a weakly- or non-barred galaxy. The final disc morphology is determined more by the impact position relative to the bar rather than the impact time.     

Stochastic stellar orbits in a pair of interacting galaxies

A dynamical model composed of a disk galaxy with an elliptic companion, moving in a circular orbit, is used in order to study the stellar orbits in a binary galaxy. Using the Poincare surface of section we study the evolution of the stochastic regions in the primary galaxy considering the mass of the companion or the value of the Jacobi’s integral as a parameter. Our numerical calculations suggest that the regions of stochasticity increase, as the mass of the companion or the value of the Jacobi’s integral increase. An interesting observation is that only direct orbits become stochastic.     

The globular cluster system of the young elliptical NGC 6702

We study the globular cluster (GC) system of the dust-lane elliptical galaxy NGC 6702, using B -, V - and I -band imaging observations carried out at the Keck telescope. This galaxy has a spectroscopic age of ≈2 Gyr suggesting recent star formation. We find strong evidence for a bimodal GC colour distribution, with the blue peak having a colour similar to that of the Galactic halo GCs. Assuming that the blue GCs are indeed old and metal-poor, we estimate an age of 2–5 Gyr and supersolar metallicity for the red GC subpopulation. Despite the large uncertainties, this is in reasonable agreement with the spectroscopic galaxy age. Additionally, we estimate a specific frequency of S _N =2.3±1.1 for NGC 6702. We predict that passive evolution of NGC 6702 will further increase its specific frequency to S _N ≈2.7 within 10 Gyr, in closer agreement to that of typical present-day ellipticals. We also discuss evidence that the merger/accretion event that took place a few Gyr ago involved a high gas fraction.     

The SFR and IMF of the galactic disk

There is a long term dynamical heating of stellar populations with age observed in the age – velocity dispersion – relation (AVR). This effect allows a determination of the star formation history SFR(t) from local kinematical data of main sequence stars. Using a self-consistent disk model for the vertical structure of the disk, we find from the kinematics of the stars in the solar neighbourhood that the SFR shows a moderate star burst about 10 Gyr ago followed by a continuous decline to the present day value consistent with the observed number of OB stars. The gravitational potential of the gas component and of the Dark Matter Halo is included and the effect of chemical enrichment, finite lifetime of the stars and mass loss of the stellar component are taken into account. The scale heights for main sequence stars together with the SFR is then used to determine constistently the IMF from the observed local luminosity function. The main new result is that the power law break in the present day mass function (PDMF) around 1 M _⊙ is entirely due to evolutionary effects of the disk and does not appear in the IMF. This revised version was published online in August 2006 with corrections to the Cover Date.     

The early evolution of the star cluster mass function

Several recent studies have shown that the star cluster initial mass function (CIMF) can be well approximated by a power law, with indications for a steepening or truncation at high masses. This contribution considers the evolution of such a mass function due to cluster disruption, with emphasis on the part of the mass function that is observable in the first ∼1 Gyr. A Schechter type function is used for the CIMF, with a power-law index of −2 at low masses and an exponential truncation at M _*. Cluster disruption due to the tidal field of the host galaxy and encounters with giant molecular clouds flattens the low-mass end of the mass function, but there is always a part of the 'evolved Schechter function' that can be approximated by a power law with index −2. The mass range for which this holds depends on age, τ, and shifts to higher masses roughly as  τ^0.6  . Mean cluster masses derived from luminosity-limited samples increase with age very similarly due to the evolutionary fading of clusters. Empirical mass functions are, therefore, approximately power laws with index −2, or slightly steeper, at all ages. The results are illustrated by an application to the star cluster population of the interacting galaxy M51, which can be well described by a model with   M _*= (1.9 ± 0.5) × 10⁵ M_⊙  and a short (mass-dependent) disruption time destroying M _* clusters in roughly a Gyr.