Structure Of NGC Research Articles

OPTICAL STRUCTURE AND EVOLUTION OF THE Arp 104 INTERACTING GALAXY SYSTEM

Arp 104 is a pair of luminous interacting galaxies consisting of NGC 5216, an elliptical, and NGC 5218, a disturbed disk galaxy and joined by a stellar bridge. We obtained optical imaging to support photometric and color studies of the system. NGC 5216 lies on the red sequence, while the unusual distribution of stellar population properties in combination with intense central star formation in a dusty region result in NGC 5218 being a nearby example of an intermediate color (green valley) system. The stellar bridge has remarkably uniform optical surface brightness, with colors consistent with its stars coming from the outskirts of NGC 5218, but is relatively gas-poor while the northern tidal tail is rich in HI. While both galaxies contain shells, the shell structures in NGC 5218 are pronounced, and some appear to be associated with counter-rotating gas. This combination of features suggests that Arp 104 could be the product of distinct multiple interactions in a small galaxy group, possibly resulting from a hierarchical merging process, and likely leading to the birth of a relatively massive and isolated early-type galaxy.

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 micron 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, H-alpha, CO, and X-ray data. Our data reveal that the 8 micron emission, believed to originate largely from PAH 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 micron and radio continuum emissions along the jet in two independent ways, namely (i) from wavelet-transformed maps of the original images at different spatial scales, and (ii) 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.

OBSERVATIONAL STUDY OF THE MULTISTRUCTURED PLANETARY NEBULA NGC 7354

We present an observational study of the planetary nebula (PN) NGC 7354 consisting of narrowband Halpha and [NII]6584 imaging as well as low- and high-dispersion long-slit spectroscopy and VLA-D radio continuum. According to our imaging and spectroscopic data, NGC 7354 has four main structures: a quite round outer shell and an elliptical inner shell, a collection of low-excitation bright knots roughly concentrated on the equatorial region of the nebula, and two symmetrical jet-like features, not aligned either with the shells' axes, or with each other. We have obtained physical parameters like electron temperature and electron density as well as ionic and elemental abundances for these different structures. Electron temperature and electron density slightly vary throughout the nebula. The local extinction coefficient c_Hbeta shows an increasing gradient from south to north and a decreasing gradient from east to west consistent with the number of equatorial bright knots present in each direction. Abundance values show slight internal variations but most of them are within the estimated uncertainties. In general, abundance values are in good agreement with the ones expected for PNe. Radio continuum data are consistent with optically thin thermal emission. We have used the interactive three-dimensional modeling tool SHAPE to reproduce the observed morphokinematic structures in NGC 7354 with different geometrical components. Our SHAPE model is in very good agreement with our imaging and spectroscopic observations. Finally, after modeling NGC 7354 with SHAPE, we suggest a possible scenario for the formation of the nebula.

Kinematic and Excitation Structure of the NGC 1068 Narrow-Line Region

Abstract We investigated the kinematic and excitation structure of the NGC 1068 narrow-line region (NLR). We obtained profiles of several emission lines ([O iii]$\lambda$5007, H$\beta$, [O i]$\lambda$6300, and [Fe vii]$\lambda$6087) at high-velocity resolution ($R$$\sim$ 7500–11000), and confirmed that they showed different profiles. These profiles are useful for understanding the NLR structure, since they cover a wide ionization-potential range. By comparing the results with a photoionization model, we found that: (1) blueshifted components at the center are very dense, (2) those in the northeast region have slightly lower densities than those in the center, and (3) the ionization parameters of the blueshifted components increase with increasing velocity with respect to the systemic velocity. We investigated the NLR structure in NGC 1068 based on these results. We show that both the observed velocity dependence of the ionization parameter and the gradually increasing velocity field can be reproduced by varying the ionizing continuum attenuation, assuming a hollowed biconical geometry and varying the column densities of outflowing clouds.

The magnetic field structure in NGC 253 in presence of a galactic wind

AbstractWe present radio continuum polarimetry observations of the nearby edge-on galaxy NGC 253 which possesses a very bright radio halo. Using the vertical synchrotron emission profiles and the lifetimes of cosmic-ray electrons, we determined the cosmic-ray bulk speed as 300±30 km s−1, indicating the presence of a galactic wind in this galaxy. The large-scale magnetic field was decomposed into a toroidal axisymmetric component in the disk and a poloidal component in the halo. The poloidal component shows a prominent X-shaped magnetic field structure centered on the nucleus, similar to the magnetic field observed in other edge-on galaxies. Faraday rotation measures indicate that the poloidal field has an odd parity (antisymmetric). NGC 253 offers the possibility to compare the magnetic field structure with models of galactic dynamos and/or galactic wind flows.

Super Star Cluster NGC 1705‐1: A Local Analog to the Birth Site of Long‐Duration γ‐Ray Bursts

Recent observations suggest that global properties of the host galaxies for long-duration γ-ray bursts (GRBs) are particularly well suited for creating the massive star progenitors of these GRBs. Motivated by the hypothesis that massive young star clusters located in metal-poor, low-metallicity galaxies are a natural birth site for GRB progenitors, we present a comparison study of the ISM properties along the sight line toward the super star cluster NGC 1705-1 and those in distant GRB hosts. Using the same set of metal transitions in the UV and applying known ISM structures in NGC 1705, we find that NGC 1705-1 resembles distant GRB host galaxies in its high neutral gas column, low molecular gas fraction, low metallicity, α-element enhancement, and low dust depletion. The lack of molecular gas is due to the enhanced UV radiation field in the starburst environment, consistent with the expectations for GRB progenitors. In addition, the known presence of dense neutral gas clouds at r 500 pc from NGC 1705-1 provides a natural reservoir of C+, Si+, and Fe+ ions that may be subsequently excited by the afterglow UV radiation field to produce excited lines commonly seen in GRB host ISM. We further argue that the apparent offset in the velocity profiles of low- and high-ionization transitions from absorption-line observations alone already offers important clues for related starburst episodes in GRB host galaxies. Our study shows that a statistical comparison between the ISM around star clusters and high-redshift GRB progenitors is important for identifying the key physical parameters that facilitate the formation of GRBs.

Quantifying Resonant Structure in NGC 6946 from Two-dimensional Kinematics

We study the two-dimensional kinematics of the Hα-emitting gas in the nearby barred Scd galaxy NGC 6946, in order to determine the pattern speed of the primary m = 2 perturbation mode. The pattern speed is a crucial parameter for constraining the internal dynamics, estimating the impact velocities of the gravitational perturbation at the resonance radii, and setting up an evolutionary scenario for NGC 6946. Our data allow us to derive the best-fitting kinematic position angle and the geometry of the underlying gaseous disk, which we use to derive the pattern speed using the Tremaine-Weinberg method. We find a main pattern speed Ω = 22 km s-1 kpc-1, but our data clearly reveal the presence of an additional pattern speed Ω = 47 km s-1 kpc-1 in a zone within 1.25 kpc of the nucleus. Using the epicyclic approximation, we deduce the location of the resonance radii and confirm that inside the outer inner Lindblad resonance radius of the main oval, a primary bar has formed rotating at more than twice the outer pattern speed. We further confirm that a nuclear bar has formed inside the inner Lindblad resonance radius of the primary bar, coinciding with the inner inner Lindblad resonance radius of the large-scale m = 2 mode oval.

On the Spiral Structure of NGC 5248: An Analytic Approach

An improved nonlinear asymptotic theory of spiral density waves is developed. It extends the theory developed by Yuan & Kuo to a global theory, not just one limited to the vicinity of the resonance. It can then be applied to the recent high-resolution observations of CO and HST NICMOS of nearby spiral galaxies. As an example, we apply the theory to the nearby grand-design spiral galaxy NGC 5248, which has excellent CO observations in the central regions and exhibits a continuous optical spiral pattern from about 7 kpc to about 100 pc. Using the improved nonlinear asymptotic theory, we confirm the earlier results that the inner two-arm spiral structure of NGC 5248 is driven by a slowly rotating bar potential at the outer inner Lindblad resonance and is a continuation of the outer optical spiral arms. Using the new theory, we calculate the streamlines, surface density, and line-of-sight velocity contours of the bar-driven flow in the central gas disk, and these results are in good agreement with the observations.

New observations of the halo radial temperature structure in NGC 7662

We report on our studies of the physical structure of the planetary nebula (PN) NGC7662. Using (3D) Integral Field Spectroscopy we have been able to measure the electron temperature more accurately and at a larger number of radial locations than before. Here we briefly present our method by which we find a strong positive temperature gradient with increas- ing radius. According to hydrodynamic models a hot halo, when compared to the central star, can be the product of the passage of an ionization front (e.g. Marten 1993). Such a gradient is not found in equilibrium models, and this finding - when confirmed for other objects - strongly advocates the use of hydrodynamic models when modeling PN halos.

The Physical Structure of NGC 3242

The Physical Structure of NGC 3242

A relativistic model of the radio jets in NGC 315

We apply our intrinsically symmetrical, decelerating relativistic jet model to deep VLA imaging of the inner 140 arcsec of the giant low-luminosity radio galaxy NGC 315. An optimized model accurately fits the data in both total intensity and linear polarization. We infer that the velocity, emissivity and field structure in NGC 315 are very similar to those of the other low-luminosity sources we have modelled, but that all of the physical scales are larger by a factor of about 5. We derive an inclination to the line of sight of 38 degrees for the jets. Where they first brighten, their on-axis velocity is approximately v/c = 0.9. They decelerate to v/c = 0.4 between 8 and 18 kpc from the nucleus and the velocity thereafter remains constant. The speed at the edge of the jet is roughly 0.6 of the on-axis value where it is best constrained, but the transverse velocity profile may deviate systematically from the Gaussian form we assume. The proper emissivity profile is split into three power-law regions separated by shorter transition zones. In the first of these, at 3 kpc (the flaring point) the jets expand rapidly at constant emissivity, leading to a large increase in the observed brightness on the approaching side. At 10 kpc, the emissivity drops abruptly by a factor of 2. Where the jets are well resolved their rest-frame emission is centre-brightened. The magnetic field is modelled as random on small scales but anisotropic and we rule out a globally ordered helical configuration. To a first approximation, the field evolves from a mixture of longitudinal and toroidal components to predominantly toroidal, but it also shows variations in structure along and across the jets, with a significant radial component in places. Simple adiabatic models fail to fit the emissivity variations.

Dense and Warm Molecular Gas between Double Nuclei of the Luminous Infrared Galaxy NGC 6240

Abstract High spatial resolution observations of the ${{12\atop} \mathrm{CO}}$ (1–0), HCN(1–0), $\mathrm{HCO}^{\mkern-1.26421mu{+}\mkern-2.52842mu}$(1–0), and ${{13\atop} \mathrm{CO}}$(1–0) molecular lines toward the luminous infrared merger NGC 6240 have been performed using the Nobeyama Millimeter Array and the RAINBOW Interferometer. All of the observed molecular emission lines are concentrated in the region between the double nuclei of the galaxy. However, the distributions of both HCN and $\mathrm{HCO}^{{\mkern-1.26421mu{+}\mkern-2.52842mu}}$ emissions are more compact compared with that of ${{12\atop} \mathrm{CO}}$, and they are not coincident with the star-forming regions. The $\mathrm{HCN} / {{12\atop} \mathrm{CO}}$ line intensity ratio is 0.25; this suggests that most of the molecular gas between the double nuclei is dense. A comparison of the observed high $\mathrm{HCN}/{{13\atop} \mathrm{CO}}$ intensity ratio, 5.9, with large velocity gradient calculations suggests that the molecular gas is dense [$n(\mathrm{H}_2) = 10^{4 \hbox{--} 6} \,\mathrm{cm}^{-3}$] and warm ($T_{\mathrm{kin}} \gt 50 \,\mathrm{K}$$T_{\mathrm{kin}} \gt 50 \,\mathrm{K}$). The observed structure in NGC 6240 may be explained by time evolution of the molecular gas and star formation, which was induced by an almost head-on collision or very close encounter of the two galactic nuclei accompanied with the dense gas and star-forming regions.

Hubble Space TelescopeImaging of Extraplanar Dust Structures in the Edge-On Spiral NGC 4217

We present high-resolution (~ 01) B, V, and I images of the edge-on Sb galaxy NGC 4217 obtained with the Wide Field Planetary Camera 2 (WFPC2) on-board the Hubble Space Telescope (HST). We make use of these images to study extraplanar (z > 0.4 kpc) dust structures visible in absorption against the background stellar light of the galaxy, similar to those observed previously in NGC 891 and other spiral galaxies by Howk & Savage. The HST images of NGC 4217 reveal extensive high-z dust to both sides of its midplane. The dust structures have a range of morphologies, and some of the more tenuous features are traceable to heights ~2 kpc from the midplane. We investigate the physical properties of 12 individual features. We measure the apparent optical extinctions of the features and use these to infer the hydrogen column densities and masses of the structures. The extraplanar dust structures in NGC 4217 typically have NH 4 × 1020 cm-2, with associated gas masses estimated to be 2 × 105 M⊙, assuming Milky Way gas-to-dust relationships apply to this spiral galaxy. The energy requirements for the expulsion of such massive structures into the lower halo are large; the gravitational potential energies of similar individual structures in NGC 891 (>1052 ergs) represent the energy input of at least tens of supernovae. The morphologies of some of the extraplanar dust structures in NGC 4217 suggest supernova-driven galactic fountain or chimney phenomena may be responsible for their production. Other features at high-z are not readily linked to energetic processes in the disk.

Physical Structure of Planetary Nebulae. II. NGC 7662

We present a study of the structure and kinematics of the triple-shell planetary nebula NGC 7662 based on long-slit echelle spectroscopic observations and Hubble Space Telescope archival narrowband images. The structure of the main nebula consists of a central cavity surrounded by two concentric shells. The bright inner shell has a nonnegligible thickness and the highest density in the nebula. The outer shell is filled with nebular material whose density profile shows a ∝r-1 drop-off in the inner regions of the shell and flattens in the outermost regions. Both the inner and the outer shells can be described as prolate ellipsoids: the inner shell is more elongated, while the outermost layer of the outer shell expands faster. The physical structure of NGC 7662 is qualitatively consistent with the predictions of hydrodynamical simulations based on the interacting stellar wind models for planetary nebulae with a 0.605 M⊙ central star. Models with a higher asymptotic giant branch wind velocity are needed for detailed comparisons.

The isolated interacting galaxy pair NGC 5426/27 (Arp 271)

We present H alpha observations of the isolated interacting galaxy pair NGC 5426/27 using the scanning Fabry-Perot interferometer PUMA. The velocity field, various kinematical parameters and rotation curve for each galaxy were derived. The FWHM map and the residual velocities map were also computed to study the role of non-circular motions of the gas. Most of these motions can be associated with the presence of spiral arms and structure such as central bars. We found a small bar-like structure in NGC 5426, a distorted velocity field for NGC 5427 and a bridge-like feature between both galaxies which seems to be associated with NGC 5426. Using the observed rotation curves, a range of possible masses was computed for each galaxy. These were compared with the orbital mass of the pair derived from the relative motion of the participants. The rotation curve of each galaxy was also used to fit different mass distribution models considering the most common theoretical dark halo models. An analysis of the interaction process is presented and a possible 3D scenario for this encounter is also suggested.

A VLBI Study of Free-Free Absorbed Compact Radio Sources in the NGC 253 Starburst

The first wide-field, very long baseline interferometry (VLBI) observations of the nearby starburst galaxy NGC 253, obtained with the Australian Long Baseline Array (LBA), have produced a 1.4 GHz image with angular resolution matched to the Very Large Array (VLA) at 22 GHz. Two compact radio sources have been detected with these LBA observations, neither of which corresponds to the assumed nucleus of the galaxy, which is prominent in VLA 22 GHz images. A multifrequency comparison of the high-resolution radio structure in NGC 253 has been made. In particular, the radio spectra of the two detected sources and the assumed nucleus are discussed. A case is made that the radio emission from the compact sources in NGC 253 is absorbed by partially ionized gas with a free-free optical depth ranging at least between 1 and 4 at 1.4 GHz, in broad agreement with previous low-resolution optical and radio studies of NGC 253 and high-resolution studies of a similar starburst galaxy, M82.

X‐Ray Arc Structures inChandraImages of NGC 5128 (Centaurus A)

We describe results from our initial study of the diffuse X-ray emission structures in NGC 5128 (Centaurus A) using the Chandra X-ray Observatory observations. The high angular resolution Chandra images reveal multiscale X-ray structures with unprecedented detail and clarity. The large-scale structures suggest complex symmetry, including a component possibly associated with the inner radio lobes, and a separate component with an orthogonal symmetry that may be associated with the galaxy as a whole. We detect arclike soft X-ray structures, extending to ~8 kpc in the direction perpendicular to the jet. These arcs appear to be tracing an ellipse, or a ring seen in projection, and are enclosed between the dust and H I-emitting gas in the central region of the galaxy and the optical and H I shell fragments at a distance of about 8-10 kpc. The diffuse X-ray and the optical emission in the arcs could originate in a region of interaction (possibly a shock) between the infalling material from the outer regions of the galaxy and the cool dust and H I-emitting material in the center or from an equatorial outflow resulting from an outburst of nuclear activity ~107 yr ago.

Multiwavelength Structural Manifestations of Fast Magnetohydrodynamic Density Waves in Spiral Galaxies

In a spiral galaxy, disks of stars and of magnetized interstellar medium (ISM) are dynamically coupled by gravity. The theory of magnetohydrodynamic (MHD) density waves in such a composite rotating disk system provides a basic framework for studying large-scale dynamics and multiwavelength diagnostics of galactic structures. As swirllike, trailing spiral structures with broad, fuzzy arms (regular or irregular) manifest in neutral hydrogen H I disks that are usually larger than optical spiral patterns in disk galaxies, spiral MHD density waves in H I gas disks should persist within and beyond optical patterns. Sporadic diffusions of relativistic cosmic-ray electrons beyond optical patterns may cause radio continuum arms to occasionally extend across optical spiral patterns and correlate with H I spiral arms at larger radii. We here report the first H I observations of the southern spiral galaxy NGC 2997 to support the fast MHD density wave (FMDW) scenario and to confirm the prior prediction that the isolated, polarized radio continuum arm discovered in the southeast quadrant is indeed associated with a broad segment of the H I arm. For multiwavelength observations of spiral galaxies, we emphasize the important perspective that large-scale magnetic fields of MHD density waves play the key role in organizing correlated spiral structures of the various underlying ISM components. Meanwhile, we briefly discuss the nature of circumnuclear structures of NGC 2997 in reference to those of NGC 1097.

Unusual magnetic fields in the interacting spiral NGC 3627

By observing the interacting galaxy NGC 3627 in radio polarization we try to answer the question to which degree the magnetic field follows the galactic gas flows. We obtained total power and polarized intensity maps at 8.46 GHz and 4.85 GHz using the VLA in its compact D-configuration. In order to overcome the zero-spacing problems, the interferometric data were combined with single-dish measurements obtained with the Effelsberg 100-m radio telescope. The observed magnetic field structure in NGC 3627 suggests that two field components are superposed. One component smoothly fills the interarm space and shows up also in the outermost disk regions, the other component follows a symmetric S-shaped structure. In the western disk the latter component is well aligned with an optical dust lane, following a bend which is possibly caused by external interactions. However, in the SE disk the magnetic field crosses a heavy dust lane segment, apparently being insensitive to strong density-wave effects. We suggest that the magnetic field is decoupled from the gas by high turbulent diffusion, in agreement with the large \ion{H}{i} line width in this region. We discuss in detail the possible influence of compression effects and non-axisymmetric gas flows on the general magnetic field asymmetries in NGC 3627. On the basis of the Faraday rotation distribution we also suggest the existence of a large ionized halo around this galaxy.

Circumnuclear kinematics in NGC 5248: the origin of nuclear spiral arms

We present for the first time a two-dimensional velocity field of the central region of the grand-design spiral galaxy NGC 5248, at 0.9-arcsec spatial resolution. The Hα velocity field is dominated by circular rotation. While no systematic streaming motions are seen in the area of the nuclear grand-design spiral or the circumnuclear ring, the amplitude of residual velocities, after subtracting a model circular velocity field, reaches 20 km s−1 in projection. The rotation curve levels out at around 140 km s−1, after a well-resolved and rather shallow rise. We have generated an analytical model for the nuclear spiral and fitted it to our observations to obtain estimates of the pattern speed of the spiral and the speed of sound in the central region of NGC 5248. Our results are consistent with a low pattern speed, suggesting that the nuclear spiral rotates with the same rate as the main spiral structure in NGC 5248, and thus that the spiral structure is coupled from scales of a few hundred parsecs to several kiloparsecs. We have also compared the observed structure and kinematics between the nuclear regions of NGC 5248 and M100. Several similarities and differences are discussed, including the location of the peak emission regions on major and minor axes, and the spiral arm streaming motions. We find no kinematic evidence for the presence of a nuclear bar in NGC 5248.