David J. Westpfahl

A Test of the Standard Hypothesis for the Origin of the H I Holes in Holmberg II

The nearby irregular galaxy Holmberg II (Ho II, DDO 50) has been extensively mapped in H I using the Very Large Array, revealing intricate structure in its interstellar gas component, as reported by Puche et al. in 1992. An analysis of these structures shows the neutral gas to contain a number of expanding H I holes. The formation of the H I holes has been attributed to multiple supernova (SN) events occurring within wind-blown shells around young, massive star clusters, with as many as 10–200 SNe required to produce many of the holes. From the sizes and expansion velocities of the holes, Puche et al. assigned ages of ~107 to 108 years. If the SN scenario for the formation of the H I holes is correct, it implies the existence of star clusters with a substantial population of late B, A, and F main-sequence stars at the centers of the holes. Many of these clusters should be detectable in deep ground-based CCD images of the galaxy. To test the SN hypothesis for the formation of the H I holes, we have obtained and analyzed deep broadband BVR and narrowband Hα images of Ho II. We compare the optical and H I data and search for evidence of the expected star clusters in and around the H I holes. We also use the H I data to constrain models of the expected remnant stellar population. Assuming that the H I holes are created by multiple SNe, that the number of SNe required can be determined from the observed energetics of the holes, and that the SNe represent the high-mass population of a cluster with a normal initial mass function, we show that in several of the holes the observed upper limits for the remnant cluster brightness are strongly inconsistent with the SN hypothesis described by Puche et al. Moreover, many of the H I holes are located in regions of very low optical surface brightness that show no indication of recent star formation. Here we present our findings, discuss their implications, and explore possible alternative explanations for the existence of the H I holes in Ho II, including the recent suggestion that some of the holes could be produced by gamma-ray burst events.

Star Formation Triggering Mechanisms in Dwarf Galaxies: The Far-Ultraviolet, Hα, and H I Morphology of Holmberg II

Far-ultraviolet (FUV), Ha, and H I observations of dwarf galaxy Holmberg II are used to investigate the means by which star formation propagates in galaxies lacking global internal triggering mechanisms such as spiral density waves. The observations trace the interaction between sites of massive star forma- tion and the neutral and ionized components of the surrounding ISM in this intrinsically simple system. Both local and large-scale triggering mechanisms related to massive star formation are seen, suggesting that feedback from massive stars is a microscopic process operating in all galaxies to a certain degree. The data emphasize the importance of local conditions in regulating star formation from evidence such as massive stars inside ionized shells, compact H II regions surrounding aging clusters, and stars formed in chains of progressing age. Surface brightness pro—les show that current activity correlates with the time-averaged level of past star formation at a given radius demonstrating a reliance on local conditions. Large-scale triggering by H I shells is supported by observations of progenitor populations as well as secondary sites of star formation associated with their dense rims. Analysis of the energy available from massive stars inside H I shells indicates that energy deposited into the ISM from supernovae and stellar winds is sufficient to account for the H I morphology. Ages of individual star-forming regions are derived using B ,H a, and FUV photometry and show both older, diUuse FUV regions and younger, compact H II regions. The distribution of ages is reconciled with the H I morphology, showing a clear preference of young regions for areas of dense H I and old regions for H I voids. Global kinematical properties may also play a role in the star formation process since diUerences in the rotation characteristics of the neutral gas disk correlate with diUerences in triggering mechanisms. Large-scale feedback from massive stars is shown to operate in regions that lack diUerential shear in the gas disk. Subject headings: galaxies: dwarfgalaxies: ISMgalaxies: individual (DDO 50) ¨ stars: formationultraviolet: galaxies

The Geometry of the H I of Several Members of the M81 Group: The H I Is Fractal

We use the box-counting method on single H I contours and the perimeter-area method on families of contours to show that the projected H I distributions of seven members of the M81 group are fractal. All seven have fractal dimensions that cover a small range, 1.5 ≥ D ≥ 1.2. This is nearly the same dimension as that of CO clouds in the Milky Way and suggests that the ISM may have a fractal dimension that stays within a limited range at all scales. Reliable determination of fractal dimension requires that observations be made with high angular and frequency resolution. If theorems of projection of opaque fractals apply to H I, then the dimension of the deprojected H I distributions may be 1.5 ≥ D ≥ 1.2 as well. Fractal structures have much more surface area than smooth structures, allowing heating, cooling, and material exchange to proceed much more rapidly.

The Speed and Origin of the H I Spiral Pattern in M81

A study of the pattern speeds of disk galaxies is under way using integrated forms of the continuity equation that relate the pattern speed to the velocity and surface density of the pattern tracer. Integration removes the unobservable sky-plane velocities and allows averaging to improve accuracy and spatial resolution. This approach assumes the disk is flat and the pattern rigid but makes no assumption about the dynamics. The method relies on the nonaxisymmetric nature of the velocity field and brightness distribution of a spiral. The speeds of nonrigid patterns may be approximated by the same equations in regions where the shear is small. The method is used to determine the speed of the H I spiral pattern in M81 using intensity and velocity maps from the VLA. The mean pattern speed is 23.4 ± 2.3 km s-1 kpc-1. If the pattern rotates rigidly then corotation is at a radius of 8.4 kpc, the outer Lindblad resonance (OLR) is within the spiral at 12.6 kpc, and the inner Lindblad resonance (ILR) is in the central hydrogen hole and well inside the inner limit of spiral structure. The pattern speed may be lower in the northern half of M81 than the southern, in which case the present high symmetry of M81 is accidental. This lower speed could be explained by the lower H I content in the south arm. Reapplication of the method without the assumption of rigidity shows that the pattern is shearing at a rate of 2.3 km s-1 kpc-2, which is consistent with swing amplification owing to a recent close passage of M82 and NGC 3077. The pattern and the material have nearly the same speed from a radius of about 9 kpc outward. This means there is no OLR in the observed H I galaxy and explains the lack of asymmetric photometric gradients across the arms. The timescale for winding of the pattern and the period of the orbit of NGC 3077 are roughly the same; periodic reamplification of the pattern by a close passage of NGC 3077 can remove the winding dilemma. The rate of shear, the velocity field of the south arm, and the very small amount of interarm H I are inconsistent with a nearly rigid density wave. The models most similar to M81 are the shearing gas models. Stochastic models give similar morphologies but make no prediction about velocities.

Noncircular gas velocities and the radial dependence of mass-to-light ratio in NGC 4594 (the Sombrero Galaxy)

The mass distribution of the Sombrero Galaxy, NGC 4594, is calculated in order to investigate the suspicion that the rotational velocity of the galactic gas does not measure the circular velocity in the galaxy. It is shown that the H II rotation velocities are much less than circular in the central 35 arcsec of the galaxy, and that the suspicion is correct. Thus, the H II rotation velocities cannot be used to measure the mass distribution. The absorption-line rotation curve is used to derive the mass distribution, and it is found that the M/L ratio is nearly constant. It is concluded that the visible matter is self-gravitating at least in the central 180 arcsec. 44 references.

High magnetic shear gain in a liquid sodium stable couette flow experiment A prelude to an alpha - omega dynamo

The Ω phase of the liquid sodium α-Ω dynamo experiment at New Mexico Institute of Mining and Technology in cooperation with Los Alamos National Laboratory has demonstrated a high toroidal field B(ϕ) that is ≃8×B(r), where B(r) is the radial component of an applied poloidal magnetic field. This enhanced toroidal field is produced by the rotational shear in stable Couette flow within liquid sodium at a magnetic Reynolds number Rm≃120. Small turbulence in stable Taylor-Couette flow is caused by Ekman flow at the end walls, which causes an estimated turbulence energy fraction of (δv/v)(2)∼10(-3).

The Pattern Speeds of NGC 3031, NGC 2366, and DDO 154 as Functions of Radius

The pattern speeds of NGC 3031, NGC 2366, and DDO 154 are measured using a solution of the Tremaine-Weinberg equations derived in a previous paper. Four different data sets of NGC 3031 produce consistent results despite differences in angular resolution, spectral resolution, and sensitivities to structures on different scales. The results for NGC 3031 show that the pattern speed is more similar to the material speed than it is to the speed of a rigidly rotating pattern, and that there are no clear indications of unique corotation or Lindblad resonances. Unlike NGC 3031, the results for NGC 2366 and DDO 154 show clear departures from the material speed. The results for NGC 2366 and DDO 154 also show that the solution method can produce meaningful results that are simple to interpret even if there is not a coherent or well-defined pattern in the data. The angular resolution of a data set has the greatest affect on the results, especially for determining the radial behavior of the pattern speed, and whether there is a single, global pattern speed.

THE SHEARING H I SPIRAL PATTERN OF NGC 1365

The Tremaine-Weinberg equations are solved for a pattern speed that is allowed to vary with radius. The solution method transforms an integral equation for the pattern speed to a least-squares problem with well-established procedures for statistical analysis. The method applied to the H I spiral pattern of the barred, grand-design galaxy NGC 1365 produces convincing evidence for a radial dependence in the pattern speed. The pattern speed behaves approximately as 1/r and is very similar to the material speed. There are no clear indications of corotation or Lindblad resonances. Tests show that the results are not selection biased, and that the method is not measuring the material speed. Other methods of solving the Tremaine-Weinberg equations for shearing patterns are found to produce results in agreement with those obtained using the current method. Previous estimates that relied on the assumptions of the density-wave interpretation of spiral structure are inconsistent with the results obtained using the current method. The results are consistent with spiral structure theories that allow for shearing patterns, and contradict fundamental assumptions in the density-wave interpretation that are often used for finding spiral arm pattern speeds. The spiral pattern is winding on a characteristic timescale of {approx}500 Myr.

Investigating Effect of Space Radiation Environment on Piezoelectric Sensors: Cobalt-60 Irradiation Experiment

Piezoelectric sensors are used in many structural health monitoring (SHM) methods to interrogate the condition of the structure to which the sensors are affixed or embedded. Among SHM methods utilizing thin wafer piezoelectric sensors, embedded ultrasonics is seen as a promising approach to assess condition of space structures. If SHM is to be implemented in space vehicles, it is imperative to determine the effects of the extreme space environment on piezoelectric sensors in order to discern between actual structural damage and environmental effects. The near-Earth space environment comprises extreme temperatures, vacuum, atomic oxygen, microgravity, micrometeoroids and debris, and significant amounts of radiation. Gamma radiation can be used to emulate the space radiation environment. In this contribution, the effects of gamma radiation on piezoelectric ceramic sensors are investigated for equivalent gamma radiation exposure of more than a year on low Earth orbit (LEO). Two experiments were conducted in which cobalt-60 was utilized as the source of radiation. Freely supported piezoelectric sensors were exposed to increasing levels of gamma radiation. Impedance data were collected for the sensors after each radiation exposure. The results show that piezoelectric ceramic material is affected by gamma radiation. Over the course of increasing exposure levels to cobalt-60, the impedance frequencies of the free sensors increased with each absorbed dose. The authors propose that the mechanism causing these impedance changes is due to gamma rays affecting piezoelectric, electric, and elastic constants of the piezoelectric ceramic. A theoretical model describing observed effects is presented. [DOI: 10.1115/1.4037684]

The Magdalena Ridge Observatory: a look ahead

The Magdalena Ridge Observatory project has received first- year funding to complete planning and environmental work. The observatory will have three 2.4-meter telescopes that can be used individually for conventional single-telescope projects or linked to do interferometry. The layout of the observatory will allow fixed east-west baselines as long as 75 meters and may include one telescope that can be moved north-south 100 meters or more to improve coverage in the u- v plane.