Christos Siopis

THE CENTERS OF EARLY-TYPE GALAXIES WITH HUBBLE SPACE TELESCOPE. V. NEW WFPC2 PHOTOMETRY

We present observations of 77 early-type galaxies imaged with the PC1 CCD of the Hubble Space Telescope (HST) WFPC2. Nuker-law parametric fits to the surface brightness profiles are used to classify the central structure into core or power-law forms. Core galaxies are typically rounder than power-law galaxies. Nearly all power-law galaxies with central ellipticities ? 0.3 have stellar disks, implying that disks are present in power-law galaxies with < 0.3 but are not visible because of unfavorable geometry. A few low-luminosity flattened core galaxies also have disks; these may be transition forms from power-law galaxies to more luminous core galaxies, which lack disks. Several core galaxies have strong isophote twists interior to their break radii, although power-law galaxies have interior twists of similar physical significance when the photometric perturbations implied by the twists are evaluated. Central color gradients are typically consistent with the envelope gradients; core galaxies have somewhat weaker color gradients than power-law galaxies. Nuclei are found in 29% of the core galaxies and 60% of the power-law galaxies. Nuclei are typically bluer than the surrounding galaxy. While some nuclei are associated with active galactic nuclei (AGNs), just as many are not; conversely, not all galaxies known to have a low-level AGN exhibit detectable nuclei in the broadband filters. NGC 4073 and 4382 are found to have central minima in their intrinsic starlight distributions; NGC 4382 resembles the double nucleus of M31. In general, the peak brightness location is coincident with the photocenter of the core to a typical physical scale of <1 pc. Five galaxies, however, have centers significantly displaced from their surrounding cores; these may be unresolved asymmetric double nuclei. Finally, as noted by previous authors, central dust is visible in about half of the galaxies. The presence and strength of dust correlates with nuclear emission; thus, dust may outline gas that is falling into the central black hole. The prevalence of dust and its morphology suggest that dust clouds form, settle to the center, and disappear repeatedly on ~108 yr timescales. We discuss the hypothesis that cores are created by the decay of a massive black hole binary formed in a merger. Apart from their brightness profiles, there are no strong differences between core galaxies and power-law galaxies that demand this scenario; however, the rounder shapes of core, their lack of disks, and their reduced color gradients may be consistent with it.

THE BLACK HOLE MASS AND EXTREME ORBITAL STRUCTURE IN NGC 1399

The largest galaxies, and in particular central galaxies in clusters, offer unique insight into understanding the mechanism for the growth of nuclear black holes. We present Hubble Space Telescopekinematics for NGC 1399, the central galaxy in Fornax. We find the best-fit model contai ns a black hole of (5: 1� 0: 7)� 10 8 M� (at a distance of 21.1 Mpc), a factor of over 2 below the correlation of black hole mass and velocity dispersion. We also find a dramatic signature for central tangential anisotropy. Th e velocity profiles on adjacent sides 0.5 00 away from the nucleus show strong bimodality, and the central spectrum shows a large drop in the dispersion. Both of these observations point to an orbital distribution that is tange ntially biased. The best-fit orbital model suggests a ratio of the tangential to radial internal velocity dispersions o f three. This ratio is the largest seen in any galaxy to date and will provide an important measure for the mode by which the central black hole has grown. Subject headings: galaxies: nuclei — galaxies: statistics — galaxies: genera l

Chemically tagging the Hyades stream: does it partly originate from the Hyades cluster?

The Hyades stream has long been thought to be a dispersed vestige of the Hyades cluster. However, recent analyses of the parallax distribution, of the mass function, and of the actionspace distribution of stream stars have shown it to be rather composed of orbits trapped at a resonance of a density disturbance. This resonant scenario should leave a clearly different signature in the element abundances of stream stars than the dispersed cluster scenario, since the Hyades cluster is chemically homogeneous. Here, we study the metallicity as well as the element abundances of Li, Na, Mg, Fe, Zr, Ba, La, Ce, Nd and Eu for a random sample of stars belonging to the Hyades stream, and compare them with those of stars from the Hyades cluster. From this analysis: (i) we independently confirm that the Hyades stream cannot be solely composed of stars originating in the Hyades cluster; (ii) we show that some stars (namely 2/21) from the Hyades stream nevertheless have abundances compatible with an origin in the cluster; (iii) we emphasize that the use of Li as a chemical tag of the cluster origin of mainsequence stars is very efficient in the range 5500 K ≤ T eff ≤ 6200 K, since the Li sequence in the Hyades cluster is very tight, while at the same time spanning a large abundance range; (iv) we show that, while this evaporated population has a metallicity excess of ∼0.2 dex with respect to the local thin-disc population, identical to that of the Hyades cluster, the remainder of the Hyades stream population has still a metallicity excess of ∼0.06–0.15 dex, consistent with an origin in the inner Galaxy and (v) we show that the Hyades stream can be interpreted as an inner 4:1 resonance of the spiral pattern: this then also reproduces an orbital family compatible with the Sirius stream, and places the origin of the Hyades stream up to 1 kpc inwards from the solar radius, which might explain the observed metallicity excess of the stream population.

Phase-space transport in cuspy triaxial potentials: can they be used to construct self-consistent equilibria?

This paper focuses on the statistical properties of chaotic orbit ensembles evolved in triaxial generalizations of the Dehnen potential which have been proposed recently to model realistic ellipticals that have a strong density cusp and manifest significant deviations from axisymmetry. Allowance is made for a possible supermassive black hole, as well as low-amplitude friction, noise, and periodic driving which can mimic irregularities associated with discreteness effects and/or an external environment. The chaos exhibited by these potentials is quantified by determining (1) how the relative number of chaotic orbits depends on the steepness of the cusp, as probed by γ, the power-law exponent with which density diverges, and MBH, the black hole mass, (2) how the size of the largest Lyapunov exponent varies with γ and MBH, and (3) the extent to which Arnold webs significantly impede phase-space transport, both with and without perturbations. The most important conclusions dynamically are (1) that, in the absence of irregularities, chaotic orbits tend to be extremely‘sticky’, so that different pieces of the same chaotic orbit can behave very differently for times ∼10000tD or more, but (2) that even very low-amplitude perturbations can prove efficient in erasing many – albeit not all – of these differences. The implications of these facts are discussed both for the structure and evolution of real galaxies and for the possibility of constructing approximate near-equilibrium models using Schwarzschilds method. For example, when trying to use Schwarzschilds method to construct model galaxies containing significant numbers of chaotic orbits, it seems advantageous to build libraries with chaotic orbits evolved in the presence of low-amplitude friction and noise, since such noisy orbits are more likely to represent reasonable approximations to time-independent building blocks. Much of the observed qualitative behaviour can be reproduced with a toy potential given as the sum of an anisotropic harmonic oscillator and a spherical Plummer potential, which suggests that the results may be generic.

Chaos and chaotic phase mixing in cuspy triaxial potentials

This paper continues an investigation of chaos and chaotic phase mixing in triaxial generalizations of the Dehnen potential which have been proposed to describe realistic elliptical galaxies that have a strong density cusp and manifest significant deviations from axisymmetry. Earlier work is extended in three important ways, namely by exploring systematically the effects of (1) variable axis ratios, (2) ‘graininess’ associated, for example, with stars and bound substructures, idealized as friction and white noise, and (3) large-scale organized motions within a galaxy and a dense cluster environment, each presumed to induce near-random forces idealized as coloured noise with a finite autocorrelation time. The effects of varying the axis ratio were studied in detail by considering two sequences of models with cusp exponent γ = 1 and, respectively, axis ratios a : b : c = 1.00 : 1.00 − � : 0.50 and a : b : c = 1.00 : 1.00 − � : 1.00 − 2� for variable �. Three important conclusions are that (1) not all the chaos can be attributed to the presence of the cusp, (2) significant chaos can persist even for axisymmetric systems, and (3) the introduction of a supermassive black hole can induce both moderate increases in the relative number of chaotic orbits and substantial increases in the size of the largest Lyapunov exponent. In the absence of any perturbations, the coarse-grained distribution function associated with an initially localized ensemble of chaotic orbits evolves exponentially towards a

Universal Properties of Escape in Dynamical Systems

This paper summarizes a numerical study of the escape properties of three two-dimensional, time-independent potentials possessing different symmetries. It was found, for all three cases, that (i) there is a rather abrupt transition in the behaviour of the late-time probability of escape, when the value of a coupling parameter, ε, exceeds a critical value, ε2. For e > e2, it was found that (ii) the escape probability manifests an initial convergence towards a nearly time-independent value, po(ɛ), which exhibits a simple scaling that may be universal. However, (iii) at later times the escape probability slowly decays to zero as a power-law function of time. Finally, it was found that (iv) in a statistical sense, orbits that escape from the system at late times tend to have short time Lyapounov exponents which are lower than for orbits that escape at early times.

Large-scale environments of binary AGB stars probed by Herschel: II. Two companions interacting with the wind of π 1 Gruis

Context. The Mass loss of Evolved StarS (MESS) sample observed with PACS on board the Herschel Space Observatory revealed that several asymptotic giant branch (AGB) stars are surrounded by an asymmetric circumstellar envelope (CSE) whose morphology is most likely caused by the interaction with a stellar companion. The evolution of AGB stars in binary systems plays a crucial role in understanding the formation of asymmetries in planetary nebulae (PNe), but at present, only a handful of cases are known where the interaction of a companion with the stellar AGB wind is observed. Aims. We probe the environment of the very evolved AGB star π 1 Gruis on large and small scales to identify the triggers of the observed asymmetries. Methods. Observations made with Herschel/PACS at 70 μm and 160 μm picture the large-scale environment of π 1 Gru. The close surroundings of the star are probed by interferometric observations from the VLTI/AMBER archive. An analysis of the proper motion data of Hipparcos and Tycho-2 together with the Hipparcos Intermediate Astrometric Data help identify the possible cause for the observed asymmetry. Results. The Herschel/PACS images of π 1 Gru show an elliptical CSE whose properties agree with those derived from a CO map published in the literature. In addition, an arc east of the star is visible at a distance of 38 �� from the primary. This arc is most likely part of an Archimedean spiral caused by an already known G0V companion that is orbiting the primary at a projected distance of 460 au with a period of more than 6200 yr. However, the presence of the elliptical CSE, proper motion variations, and geometric modelling of the VLTI/AMBER observations point towards a third component in the system, with an orbital period shorter than 10 yr, orbiting much closer to the primary than the G0V star.

Universal Properties of Escapea

This Hamiltonian was chosen because it may perhaps represent the central part of a deformed galaxy. In fact, it describes two harmonic oscillators, coupled via the quartic term EX’Y’, where E is a parameter that determines the strength of the coupling. For a fixed value of E, escape from the system is possible only when the total energy h, is higher than a certain critical escape value. Alternatively, for a fixed value of h,, escape is possible only when E exceeds a critical value E..,. In this work, the energy of the system was held fixed, but the coupling parameter E was allowed to vary. The form of the potential is symmetric with respect to the origin and, for E > E,,, , the equipotential surfaces allow for four channels of escape (FIG. l(a)). The fact that escape is possible energetically gives no information about when an orbit will escape. However, the time of escape can be important for a number of different problems, both in astronomy and other fields. For instance, if a star has enough energy to escape, but it takes more than a Hubble time for this to happen, for most practical purposes it should be considered as being bound to the galaxy. Similar situations are encountered in plasma physics, molecular dynamics, heavy ion physics, and so forth.’-* To better characterize the escape properties of the system, several numerical experiments were performed. Each experiment involved solving the equations of motion for an initially localized ensemble of orbits with E > E , ~ , . The value of E was

LACK OF INTERACTION BETWEEN THE DUST GRAINS AND THE ANOMALOUS RADIO JET IN THE NEARBY SPIRAL GALAXY NGC 4258

We obtained Spitzer/IRAC 3.6-8 μm images of the nearby spiral galaxy NGC 4258 to study possible interactions between dust and the radio jet. In our analysis, we also included high-resolution radio continuum, Hα, CO, and X-ray data. Our data reveal that the 8 μm emission, believed to originate largely from polycyclic aromatic hydrocarbon molecules and hot dust, is an excellent tracer of the normal spiral structure in NGC 4258, and hence it originates from the galactic plane. We investigated the possibility of dust destruction by the radio jet by calculating correlation coefficients between the 8 μm and radio continuum emissions along the jet in two independent ways, namely, (1) from wavelet-transformed maps of the original images at different spatial scales and (2) from one-dimensional intensity cuts perpendicular to the projected path of the radio jet on the sky. No definitive sign of a correlation (or anticorrelation) was detected on relevant spatial scales with either approach, implying that any dust destruction must take place at spatial scales that are not resolved by our observations.

Orbital Complexity, Short-Time Lyapunov Exponents, and Phase Space Transport in Time-Independent Hamiltonian Systemsa

ABSTRACT: This paper compares two alternative characterizations of chaotic orbit segments, one based on the complexity of their Fourier spectra, as probed by the number of frequencies n(k) required to capture a fixed fraction k of the total power, and the other based on the computed values of short‐time Lyapunov exponentsχ. An analysis of orbit ensembles evolved in several different two‐ and three‐dimensional potentials reveals that there is a strong, roughly linear correlation between these alternative characterizations, and that computed distributions of complexities, N[n(k)], and short‐time χ, N[χ], often assume similar shapes. This corroborates the intuition that chaotic segments which are especially unstable should have Fourier spectra with particularly broad‐band power. It follows that orbital complexities can be used as probes of phase space transport and other related phenomena in the same manner as can short‐time Lyapunov exponents.