Bruce Balick

Galaxy collisions and mergers - the genesis of very powerful radio sources

Results are reported from optical, long-slit interferometry, narrow-band imaging and VLA radiotelescope scans of 43 radio galaxies. The strongest radio galaxies (log P at 408 MHz exceeding 25.5 W/Hz for a Hubble parameter of 75 km/sec per MPC) displayed the most distinct optical morphologies, e.g., bridges, fans, dust features, etc. The sources also emitted continuum lines and other line features which were morphologically distinct from the source regions, conditions which suggest that collisions between galaxies occurred. Other characteristics of the strong radio galaxies are discussed, including lowered optical luminosities, reduced galactic densities, and an absence of edge-darkened radio morphologies. 101 references.

The luminosity, mass, and age distributions of compact star clusters in M83 based on Hubble Space Telescope/Wide Field Camera 3 observations

The newly installed Wide Field Camera 3 (WFC3) on the Hubble Space Telescope has been used to obtain multiband images of the nearby spiral galaxy M83. These new observations are the deepest and highest resolution images ever taken of a grand-design spiral, particularly in the near-ultraviolet, and allow us to better differentiate compact star clusters from individual stars and to measure the luminosities of even faint clusters in the U band. We find that the luminosity function (LF) for clusters outside of the very crowded starburst nucleus can be approximated by a power law, dN/dL ∝ L α , with α =− 2.04 ± 0.08, down to MV ≈− 5.5. We test the sensitivity of the LF to different selection techniques, filters, binning, and aperture correction determinations, and find that none of these contribute significantly to uncertainties in α. We estimate ages and masses for the clusters by comparing their measured UBVI ,H α colors with predictions from single stellar population models. The age distribution of the clusters can be ··· · ·· · ·· )

FLIERs and Other Microstructures in Planetary Nebulae. IV. Images of Elliptical PNs from the Hubble Space Telescope

?????We report new results from high spatial resolution Wide Field Planetary Camera 2 imaging studies of FLIERs and other microstructures in the planetary nebulae NGC 3242, 6826, 7009, and 7662. Most FLIERs have head-tail morphologies, with the tails pointing outward from the nucleus. Ionization gradients that decrease with distance from the nebular center are ubiquitous. These are consistent with an ionization front in neutral knots of density ?104 cm-3. Can neutral knots account for the properties of FLIERs? We compare two broad classes of possible explanations for FLIERs with the new images: high-speed bullets ramming through the shells of planetary nebulae, and photoevaporated gas swept by winds into head-tail shapes. Both classes of models fail basic consistency tests. Hence an entirely new conceptual paradigm is needed to account for the phenomenology of FLIERs.

The evolution of planetary nebulae. II - Dynamical evolution of elliptical PNs and collimated outflows

Spatial and kinematic features found in mappings of the velocity of the gas in H-alpha and forbidden N II of the planetary nebulae NGC 40, 2392, 3242, 6543, 6826, 7009, 7354, and 7662 suggest the existence of highly collimated fast outflows characterized by velocities in the 10-60 km/s range. A hydrodynamic mechanism that collimates the gas as it passes outward through a prolate shock front is proposed to explain these observations. The usual fast-wind model is modified under the assumption that the slow-wind envelope that surrounds the star is denser near its equator than near the poles. In the present model, the collimated flow consists of a two-phase gas of cool weakly ionized or neutral knots passing through the surrounding highly ionized material. 35 references.

ABUNDANCES OF GALACTIC ANTICENTER PLANETARY NEBULAE AND THE OXYGEN ABUNDANCE GRADIENT IN THE GALACTIC DISK

We have obtained spectrophotometric observations of 41 anticenter planetary nebulae (PNe) located in the disk of the Milky Way. Electron temperatures and densities, as well as chemical abundances for He, N, O, Ne, S, Cl, and Ar were determined. Incorporating these results into our existing database of PN abundances yielded a sample of 124 well-observed objects with homogeneously determined abundances extending from 0.9 to 21 kpc in galactocentric distance. We performed a detailed regression analysis which accounted for uncertainties in both oxygen abundances and radial distances in order to establish the metallicity gradient across the disk to be 12 + log(O/H) = (9.09 ± 0.05) – (0.058 ± 0.006) × Rg , with Rg in kpc. While we see some evidence that the gradient steepens at large galactocentric distances, more objects toward the anticenter need to be observed in order to confidently establish the true form of the metallicity gradient. We find no compelling evidence that the gradient differs between Peimbert Types I and II, nor is oxygen abundance related to the vertical distance from the galactic plane. Our gradient agrees well with analogous results for H II regions but is steeper than the one recently published by Stanghellini & Haywood over a similar range in galactocentric distance. A second analysis using PN distances from a different source implied a flatter gradient, and we suggest that we have reached a confusion limit which can only be resolved with greatly improved distance measurements and an understanding of the natural scatter in oxygen abundances.

On the nature of QSO host galaxies

Results of an analysis of deep images of 31 low-redshift QSOs and lower luminosity QSO/AGNs obtained with the prime focus CCD system on the CTIO 4 m telescope are presented. The host galaxies of QSOs are generally luminous systems populating the exponential tail of the galaxy luminosity function. Seyfert galaxies are apparently less luminous than the hosts of radio-quiet QSOs. The radio-loud QSOs are more luminous than the radio-quiet QSO hosts. For the sample of QSOs and QSO/AGNs, there is a trend for the radio-loud hosts to be better fitted by elliptical galaxy models and for the radio-quiet hosts to be fitted by disk galaxies, suggesting that the dichotomy known to exist between Seyfert and radio galaxies extends to much higher levels of nuclear luminosity. These results strengthen the empirical basis for a continuity in properties between Seyfert galaxies and radio-quite QSOs and between radio galaxies and radio-loud QSOs. 51 references.

STAR FORMATION IN 30 DORADUS

Using observations obtained with the Wide-Field Camera 3 on board the Hubble Space Telescope, we have studied the properties of the stellar populations in the central regions of 30 Dor in the Large Magellanic Cloud. The observations clearly reveal the presence of considerable differential extinction across the field. We characterize and quantify this effect using young massive main-sequence stars to derive a statistical reddening correction for most objects in the field. We then search for pre-main-sequence (PMS) stars by looking for objects with a strong (>4σ) Hα excess emission and find about 1150 of them over the entire field. Comparison of their location in the Hertzsprung-Russell diagram with theoretical PMS evolutionary tracks for the appropriate metallicity reveals that about one-third of these objects are younger than ~4 Myr, compatible with the age of the massive stars in the central ionizing cluster R 136, whereas the rest have ages up to ~30 Myr, with a median age of ~12 Myr. This indicates that star formation has proceeded over an extended period of time, although we cannot discriminate between an extended episode and a series of short and frequent bursts that are not resolved in time. While the younger PMS population preferentially occupies the central regions of the cluster, older PMS objects are more uniformly distributed across the field and are remarkably few at the very center of the cluster. We attribute this latter effect to photo-evaporation of the older circumstellar disks caused by the massive ionizing members of R 136.

UV-DROPOUT GALAXIES IN THE GOODS-SOUTH FIELD FROM WFC3 EARLY RELEASE SCIENCE OBSERVATIONS

We combine new high sensitivity ultraviolet (UV) imaging from the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) with existing deep HST/Advanced Camera for Surveys optical images from the Great Observatories Origins Deep Survey (GOODS) program to identify UV-dropouts, which are Lyman break galaxy (LBG) candidates at z 1-3. These new HST/WFC3 observations were taken over 50 arcmin2 in the GOODS-South field as a part of the Early Release Science program. The uniqueness of these new UV data is that they are observed in three UV/optical (WFC3 UVIS) channel filters (F225W, F275W, and F336W), which allows us to identify three different sets of UV-dropout samples. We apply Lyman break dropout selection criteria to identify F225W-, F275W-, and F336W-dropouts, which are z 1.7, 2.1, and 2.7 LBG candidates, respectively. We use multi-wavelength imaging combined with available spectroscopic and photometric redshifts to carefully access the validity of our UV-dropout candidates. Our results are as follows: (1) these WFC3 UVIS filters are very reliable in selecting LBGs with z 2.0, which helps to reduce the gap between the well-studied z 3 and z ~ 0 regimes; (2) the combined number counts with average redshift z 2.2 agree very well with the observed change in the surface densities as a function of redshift when compared with the higher redshift LBG samples; and (3) the best-fit Schechter function parameters from the rest-frame UV luminosity functions at three different redshifts fit very well with the evolutionary trend of the characteristic absolute magnitude, M*, and the faint-end slope, ?, as a function of redshift. This is the first study to illustrate the usefulness of the WFC3 UVIS channel observations to select z 3 LBGs. The addition of the new WFC3 on the HST has made it possible to uniformly select LBGs from z 1 to z 9 and significantly enhance our understanding of these galaxies using HST sensitivity and resolution.

Disk Formation by Asymptotic Giant Branch Winds in Dipole Magnetic Fields

We present a simple, robust mechanism by which an isolated star can produce an equatorial disk. The mechanism requires that the star have a simple dipole magnetic field on the surface and an isotropic wind acceleration mechanism. The wind couples to the field, stretching it until the field lines become mostly radial and oppositely directed above and below the magnetic equator, as occurs in the solar wind. The interaction between the wind plasma and magnetic field near the star produces a steady outflow in which magnetic forces direct plasma toward the equator, constructing a disk. In the context of a slow (10 km s-1) outflow (10-5 M☉ yr-1) from an asymptotic giant branch star, MHD simulations demonstrate that a dense equatorial disk will be produced for dipole field strengths of only a few Gauss on the surface of the star. A disk formed by this model can be dynamically important for the shaping of planetary nebulae.

The Morphology of Planetary Nebulae: Simulations with Time-evolving Winds

We have carried out simulations of planetary nebulae (PNs) within the general framework of the interacting stellar winds models, wherein the fast wind from a central star sweeps up the slow wind emitted in a previous epoch. Our two-dimensional simulations take into account the evolution of the velocity and mass-loss rate of the fast wind. This leads to considerably more structure on smaller scales than was seen in those nebulae where the fast wind velocity is held constant. The nebula evolves through an initial momentum-conserving phase before entering the more commonly encountered energy-conserving stage. Both stages are prone to instabilities, which may be the precursor of knots, filaments, and other microstructures that are now commonly seen in Hubble Space Telescope images of PNs. In particular, we note the occurrence of the nonlinear thin shell instability in the early stages, and the formation of Rayleigh-Taylor filaments in the energy-conserving stage. The growth of small-scale structure in the momentum-conserving stage is sensitive to the ratio of the initial wind momenta; the lifetime of the nebula in this stage depends also on the evolution of the wind properties. The overall size scale of the system is determined mainly by the evolution of the fast wind properties. If the evolution is not taken into account when computing kinematic ages, then the ages may be underestimated.