B. Pichardo

Formation of a Tidal Dwarf Galaxy in the Interacting System Arp 245 (NGC 2992/93)

Original article can be found at: http://www.journals.uchicago.edu/AJ/--Copyright American Astronomical Society

Understanding the spiral structure of the Milky Way using the local kinematic groups

We study the spiral arm influence on the solar neighbourhood stellar kinematics. As the nature of the Milky Way (MW) spiral arms is not completely determined, we study two models: the Tight-Winding Approximation (TWA) model, which represents a local approximation, and a model with self-consistent material arms named sPiral arms potEntial foRmed by obLAte Spheroids (PERLAS). This is a mass distribution with more abrupt gravitational forces. We perform test particle simulations after tuning the two models to the observational range for the MW spiral arm properties. We find that some of the currently observed MW spiral arm properties are not in obvious agreement with the TWA model. We explore the effects of the arm properties and find that a significant region of the allowed parameter space favours the appearance of kinematic groups. The velocity distribution is mostly sensitive to the relative spiral arm phase and pattern speed. In all cases the arms induce strong kinematic imprints for pattern speeds around 17 km s-1 kpc-1 (close to the 4:1 inner resonance) but no substructure is induced close to corotation. The groups change significantly if one moves only similar to 0.6 kpc in galactocentric radius, but similar to 2 kpc in azimuth. The appearance time of each group is different, ranging from 0 to more than 1 Gyr. Recent spiral arms can produce strong kinematic structures. The stellar response to the two potential models is significantly different near the Sun, both in density and in kinematics. The PERLAS model triggers more substructure for a larger range of pattern speed values. The kinematic groups can be used to reduce the current uncertainty about the MW spiral structure and to test whether this follows the TWA. However, groups such as the observed ones in the solar vicinity can be reproduced by different parameter combinations. Data from velocity distributions at larger distances are needed for a definitive constraint.

STELLAR KINEMATIC CONSTRAINTS ON GALACTIC STRUCTURE MODELS REVISITED: BAR AND SPIRAL ARM RESONANCES

We study the phase space available to the local stellar distribution using a Galactic potential consistent with several recent observational constraints. We find that the induced phase space structure has several observable consequences. The spiral arm contribution to the kinematic structure in the solar neighborhood may be as important as the one produced by the Galactic bar. We suggest that some of the stellar kinematic groups in the solar neighborhood, such as the Hercules structure and the kinematic branches, can be created by the dynamical resonances of self-gravitating spiral arms and not exclusively by the Galactic bar. A structure coincident with the Arcturus kinematic group is developed when a hot stellar disk population is considered, which introduces a new perspective on the interpretation of its extragalactic origin. A bar-related resonant mechanism can modify this kinematic structure. We show that particles in the dark matter disk-like structure predicted by recent lambda cold dark matter galaxy formation experiments, with similar kinematics to the thick disk, are affected by the same resonances, developing phase space structures or dark kinematic groups that are independent of the Galaxy assembly history and substructure abundance. We discuss the possibility of using the stellar phase space groups as constraints to nonaxisymmetric models of the Milky Way structure.

Nonlinear Effects in Models of the Galaxy. I. Midplane Stellar Orbits in the Presence of Three-dimensional Spiral Arms

With the aim of studying the nonlinear stellar and gaseous response to the gravitational potential of a galaxy such as the Milky Way, we have modeled three-dimensional Galactic spiral arms as a superposition of inhomogeneous oblate spheroids and added their contribution to an axisymmetric model of the Galactic mass distribution. Three spiral loci are proposed here, based in different sets of observations. A comparison of our model with a tight-winding approximation shows important differences in the middle and outer Galactic regions. A preliminary self-consistency analysis taking Ωp = 15 and 20 km s-1 kpc-1 for the angular speed of the spiral pattern seems to favor the value Ωp = 20 km s-1 kpc-1. As a first step to the full three-dimensional calculations for which the model is suitable, we have explored the stellar orbital structure in the midplane of the Galaxy. We present the standard analysis in the pattern rotating frame and complement this analysis with orbital information from the Galactic inertial frame. Prograde and retrograde orbits are defined unambiguously in the inertial frame, then labeled as such in the Poincare diagrams of the noninertial frame. In this manner, we found a sharp separatrix between the two classes of orbits. Chaos is restricted to the prograde orbits, and its onset occurs for the higher spiral perturbation considered plausible in our Galaxy. An unrealistically high spiral perturbation tends to destroy the separatrix and make chaos pervasive. This might be relevant in other spiral galaxies.

The Orbits of 48 Globular Clusters in a Milky Way-like Barred Galaxy

The effect of a barred potential (such as the one of the Milky Way) on the Galactic orbits of 48 globular clusters for which absolute proper motions are known is studied. The orbital characteristics are compared with those obtained for the case of an axisymmetric Galactic potential. Tidal radii are computed and discussed for both the better known axisymmetric case and that including a bar. The destruction rates due to bulge and disk shocking are calculated and compared in both Galactic potentials.

Two distinct halo populations in the solar neighborhood - III. Evidence from stellar ages and orbital parameters

Context. In Papers I and II of this series, we have found clear indications of the existence of two distinct populations of stars in the solar neighborhood belonging to the metal-rich end of the halo metallicity distribution function. Based on high-resolution, high S/N spectra, it is possible to distinguish between “high-alpha” and “low-alpha” components using the [α/Fe] versus [Fe/H] diagram. Aims. Precise relative ages and orbital parameters are determined for 67 halo and 16 thick-disk stars having metallicities in the range −1.4 < [Fe/H]< −0.4 to better understand the context of the two halo populations in the formation and evolution of the Galaxy. Methods. Ages are derived by comparing the positions of stars in the logTeff–log g diagram with isochrones from the Y 2 models interpolated to the exact [Fe/H] and [α/Fe] values of each star. The stellar parameters have been adopted from the preceding spectroscopic analyses, but possible systematic errors in Teff and log g are considered and corrected. With space velocities from Paper I as initial conditions, orbital integrations have been carried out using a detailed, observationally constrained Milky Way model including a bar and spiral arms. Results. The “high-alpha” halo stars have ages 2–3 Gyr larger than the “low-alpha” ones, with some probability that the thick-disk stars have ages intermediate between these two halo components. The orbital parameters show very distinct differences between the “high-alpha” and “low-alpha” halo stars. The “low-alpha” ones have rmax’s to 30–40 kpc, zmax’s to ≈18 kpc, and emax’s clumped at values greater than 0.85, while the “high-alpha” ones, rmax’s to about 16 kpc, zmax’s to 6–8 kpc, and emax values more or less uniformly distributed over 0.4–1.0. Conclusions. A dual in situ-plus-accretion formation scenario best explains the existence and characteristics of these two metal-rich halo populations, but one remaining defect is that this model is not consistent regarding the rmax’s obtained for the in situ “high-alpha” component; the predicted values are too small. It appears that ω Cen may have contributed in a significant way to the existence of the “low-alpha” component; recent models, including dynamical friction and tidal stripping, have produced results consistent with the present mass and orbital characteristics of ω Cen, while at the same time including extremes in the orbital parameters as great as those of the “low-alpha” component.

On galaxy spiral arms' nature as revealed by rotation frequencies

High-resolution N-body simulations using different codes and initial condition techniques reveal two different behaviours for the rotation frequency of transient spiral arms like structures. Whereas unbarred discs present spiral arms nearly corotating with disc particles, strong barred models (bulged or bulgeless) quickly develop a bar-spiral structure dominant in density, with a pattern speed almost constant in radius. As the bar strength decreases the arm departs from bar rigid rotation and behaves similar to the unbarred case. In strong barred models, we detect in the frequency space other subdominant and slower modes at large radii, in agreement with previous studies, however, we also detect them in the configuration space. We propose that the distinctive behaviour of the dominant spiral modes can be exploited in order to constraint the nature of Galactic spiral arms by the astrometric survey Gaia and by 2D spectroscopic surveys like Calar Alto Legacy Integral Field Area Survey (CALIFA) and Mapping Nearby Galaxies at APO (MANGA) in external galaxies.

EMISSIVITY STATISTICS IN TURBULENT COMPRESSIBLE MAGNETOHYDRODYNAMIC FLOWS AND THE DENSITY-VELOCITY CORRELATION

In this paper we test the results of a recent analytical study by Lazarian and Pogosyan on the statistics of emissivity in velocity channel maps, in the case of realistic density and velocity —elds obtained from numerical simulations of magnetohydrodynamic turbulence in the interstellar medium. To compensate for the lack of well-developed inertial ranges in the simulations owing to the limited resolution, we apply a procedure for modifying the spectral slopes of the —elds while still preserving the spatial structures. We —nd that the density and velocity are moderately correlated in space, and we prove that the analytical results by Lazarian and Pogosyan hold in the case when these —elds obey the —uid conservation equations. Our results imply that the spectra of velocity and density can be safely recovered from the position-position-velocity (PPV) data cubes available through observations and con—rm that the relative contributions of the velocity and density —uctuations to those of the emissivity depend on the velocity resolution used and on the steepness of the density spectral index. Furthermore, this paper supports previous reports that an interpretation of the features in the PPV data cubes as simple density enhancements (i.e., ii clouds ˇˇ) can often be erroneous, as we observe that changes in the velocity statistics substantially modify the emissivity statistics within the velocity data cubes. & ,

INTRINSIC, OBSERVED, AND RETRIEVED PROPERTIES OF INTERSTELLAR TURBULENCE

We generate synthetic observations of three-dimensional, self-gravitating MHD simulations of the interstellar medium (ISM) to evaluate the ability of principal component analysis (PCA) to measure the scale dependence of turbulent velocity fluctuations in molecular clouds. Scaling exponents, α, observationally obtained from the coupled characteristic scales for line profile variability in velocity, δv, and in space, L, where δv ∝ Lα, are compared with the intrinsic scaling exponents of the MHD velocity fields. We determine the approximate structure function order at which PCA operates in order to then verify a previously established calibration of the PCA method. We also analyze the statistical properties of projected velocity line centroid fields, including effects of intermittent velocity fluctuations, density inhomogeneity, and opacity, and examine the relationship of the projected two-dimensional statistics to the intrinsic three-dimensional statistics. Using PCA, we infer steep three-dimensional energy spectra in the molecular ISM, generally steeper than can be accounted for by Kolmogorov turbulence or possibly even shock-dominated turbulence.

On the Effects of Projection on Morphology

We study the eUects of projection of three-dimensional data onto the plane of the sky by means of numerical simulations of turbulence in the interstellar medium including the magnetic —eld, param- eterized cooling and diUuse and stellar heating, self-gravity, and rotation. We compare the physical-space density and velocity distributions with their representation in position-position-velocity (PPV) space (ii channel maps ˇˇ), noting that the latter can be interpreted in two ways: either as maps of the column densitys spatial distribution (at a given line-of-sight (LOS) velocity) or as maps of the spatial distribu- tion of a given value of the LOS velocity (weighted by density). This ambivalence appears related to the fact that the spatial and PPV representations of the data give signi—cantly diUerent views. First, the mor- phology in the channel maps more closely resembles that of the spatial distribution of the LOS velocity component than that of the density —eld, as measured by pixel-to-pixel correlations between images. Second, the channel maps contain more small-scale structure than three-dimensional slices of the density and velocity —elds, a fact evident both in subjective appearance and in the power spectra of the images. This eUect may be due to a pseudorandom sampling (along the LOS) of the gas contributing to the structure in a channel map: the positions sampled along the LOS (chosen by their LOS velocity) may vary signi—cantly from one position in the channel map to the next. Subject headings: ISM: generalISM: magnetic —eldsmethods: numericalturbulence