John E. Everett

DUSTY WIND-BLOWN BUBBLES

Spurred by recent observations of 24 ?m emission within wind-blown bubbles (WBBs), we study the role that dust can play in such environments and build an approximate model of a particular WBB, N49. First, we model the observations with a dusty WBB, and then ask whether dust could survive within N49 to its present age (estimated to be 5 ? 105 to 106 yr). We find that dust sputtering and especially dust-gas friction would imply relatively short timescales (t ~ 104 yr) for dust survival in the wind-shocked region of the bubble. To explain the 24 ?m emission, we postulate that the grains are replenished within the WBB by destruction of embedded, dense cloudlets of interstellar medium gas that have been overrun by the expanding WBB. We calculate the ablation timescales for cloudlets within N49 and find approximate parameters for the embedded cloudlets that can replenish the dust; the parameters for the cloudlets are roughly similar to those observed in other nebula. Such dust will have an important effect on the bubble: including simple dust cooling in a WBB model for N49, we find that the luminosity is higher by approximately a factor of 6 at a bubble age of about 104 yr. At ages of 107 yr, the energy contained in the bubble is lower by about a factor of 8 if dust is included; if dust must be replenished within the bubble, the associated accompanying gas mass will also be very important to WBB cooling and evolution. While more detailed models are certainly called for, this work illustrates the possible strong importance of dust in WBBs, and is a first step toward models of dusty WBBs.

THE ROLE OF THE MAGNETIC FIELD IN THE INTERSTELLAR MEDIUM OF THE POST-STARBURST DWARF IRREGULAR GALAXY NGC 1569

NGC 1569 is a nearby dwarf irregular galaxy which underwent an intense burst of star formation 10-40 Myr ago. We present observations that reach surface brightnesses 2-80 times fainter than previous radio continuum observations and the first radio continuum polarization observations of this galaxy at 20 cm, 13 cm, 6 cm, and 3 cm. These observations allow us to probe the relationship of the magnetic field of NGC 1569 to the rest of its interstellar medium (ISM). We confirm the presence of an extended radio continuum halo at 20 cm and see for the first time the radio continuum feature associated with the western Hα arm at wavelengths shorter than 20 cm. Although, in general, the spectral indices derived for this galaxy steepen as one moves into the halo of the galaxy, there are filamentary regions of flat spectral indices extending to the edge of the galaxy. The spectral index trends in this galaxy support the theory that there is a convective wind at work in this galaxy. There is strong polarized emission at 3 cm and 6 cm and weak polarized emission at 20 cm and 13 cm. We estimate that the thermal fraction is 40%-50% in the center of the galaxy and falls off rapidly with height above the disk. Using this estimate, we derive a total magnetic field strength of 38 μG in the central regions and 10-15 μG in the halo. The magnetic field is largely random in the center of the galaxy; the uniform field is ~3-9 μG and is strongest in the halo. Using our total magnetic field strength estimates and the results of previous observations of NGC 1569, we find that the magnetic pressure is the same order of magnitude but, in general, a factor of a few less than the other components of the ISM in this galaxy. The uniform magnetic field in NGC 1569 is closely associated with the Hα bubbles and filaments. We suggest that a supernova-driven dynamo may be operating in this galaxy. Based on our pressure estimates and the morphology of the magnetic field, the outflow of hot gas from NGC 1569 is clearly shaping the magnetic field, but the magnetic field in turn may be aiding the outflow by channeling gas out of the disk of the galaxy. Dwarf galaxies with extended radio continuum halos like that of NGC 1569 may play an important role in magnetizing the intergalactic medium.

Synchrotron Constraints on a Hybrid Cosmic-ray and Thermally Driven Galactic Wind

Cosmic rays and magnetic fields can substantially impact the launching of large-scale galactic winds. Many researchers have investigated the role of cosmic rays; our group previously showed that a cosmic-ray and thermally driven wind could explain soft X-ray emission toward the center of the Galaxy. In this paper, we calculate the synchrotron emission from our original wind model and compare it to observations; the synchrotron data show that earlier assumptions about the launching conditions of the wind must be changed: we are required to improve that earlier model by restricting the launching region to the domain of the inner Molecular Ring, and by decreasing the magnetic field strength from the previously assumed maximum strength. With these physically motivated modifications, we find that a wind model can fit both the radio synchrotron and the X-ray emission, although that model is required to have a higher gas pressure and density than the previous model in order to reproduce the observed X-ray emission within the smaller footprint. The drop in magnetic field also decreases the effect of cosmic-ray heating, requiring a higher temperature at the base of the wind than the previous model.

THE INTERACTION OF COSMIC RAYS WITH DIFFUSE CLOUDS

We study the change in cosmic-ray pressure, the change in cosmic-ray density, and the level of cosmic-ray-induced heating via Alfven-wave damping when cosmic rays move from a hot ionized plasma to a cool cloud embedded in that plasma. The general analysis method outlined here can apply to diffuse clouds in either the ionized interstellar medium or in galactic winds. We introduce a general-purpose model of cosmic-ray diffusion building upon the hydrodynamic approximation for cosmic rays (from McKenzie and Voelk and Breitschwerdt and collaborators). Our improved method self-consistently derives the cosmic-ray flux and diffusivity under the assumption that the streaming instability is the dominant mechanism for setting the cosmic-ray flux and diffusion. We find that, as expected, cosmic rays do not couple to gas within cool clouds (cosmic rays exert no forces inside of cool clouds), that the cosmic-ray density does not increase within clouds (it may decrease slightly in general, and decrease by an order of magnitude in some cases), and that cosmic-ray heating (via Alfven-wave damping and not collisional effects as for {approx}10 MeV cosmic rays) is only important under the conditions of relatively strong (10 {mu}G) magnetic fields or high cosmic-ray pressure ({approx}10{sup -11} erg cm{sup -3}).

Acceleration and substructure constraints in a quasar outflow

We present observations of probable line-of-sight acceleration of a broad absorption trough of C IV in the quasar SDSS J024221.87+004912.6. We also discuss how the velocity overlap of two other outflowing systems in the same object constrains the properties of the outflows. The Si IV doublet in each system has one unblended transition and one transition that overlaps with absorption from the other system. The residual flux in the overlapping trough is well fit by the product of the residual fluxes in the unblended troughs. For these optically thick systems to yield such a result, at least one of them must consist of individual subunits, rather than being a single structure with velocity-dependent coverage of the source. If these subunits are identical, opaque, spherical clouds, we estimate the cloud radius to be r 3.9 × 1015 cm. If they are identical, opaque, linear filaments, we estimate their width to be w 6.5 × 1014 cm. These subunits are observed to cover the Mg II broad emission-line region of the quasar, at which distance from the black hole the above filament width is equal to the predicted scale height of the outer atmosphere of a thin accretion disk. Insofar as that scale height is a natural size scale for structures originating in an accretion disk, these observations are evidence that the accretion disk can be a source of quasar absorption systems. This paper is based on data from ESO program 075.B-0190(A).

Environments for Magnetic Field Amplification by Cosmic Rays

We consider a recently discovered class of instabilities, driven by cosmic ray streaming, in a variety of environments. We show that although these instabilities have been discussed primarily in the context of supernova-driven interstellar shocks, they can also operate in the intergalactic medium and in galaxies with weak magnetic fields, where, as a strong source of helical magnetic fluctuations, they could contribute to the overall evolution of the magnetic field. Within the Milky Way, these instabilities are strongest in warm ionized gas and appear to be weak in hot, low density gas unless the injection efficiency of cosmic rays is very high.

A SEARCH FOR NEUTRON STAR PRECESSION AND INTERSTELLAR MAGNETIC FIELD VARIATIONS VIA MULTIEPOCH PULSAR POLARIMETRY

In order to study precession and interstellar magnetic field variations, we measured the polarized position angle of 81 pulsars at several-month intervals for four years. We show that the uncertainties in a single-epoch measurement of position angle are usually dominated by random pulse-to-pulse jitter of the polarized subpulses. Even with these uncertainties, we find that the position angle variations in 19 pulsars are significantly better fitted (at the 3σ level) by a sinusoid than by a constant. Such variations could be caused by precession, which would then indicate periods of ~(200-1300) days and amplitudes of ~(1-12) degrees. We narrow this collection to four pulsars that show the most convincing evidence of sinusoidal variation in position angle. Also, in a handful of pulsars, single discrepant position angle measurements are observed which may result from the line of sight passing across a discrete ionized, magnetized structure. We calculate the standard deviation of position angle measurements from the mean for each pulsar and relate these to limits on precession and interstellar magnetic field variations.

Hubble Space Telescope Ultraviolet Spectroscopy of 14 Low-Redshift Quasars

We present low-resolution ultraviolet spectra of 14 low-redshift (z_(em) ≲ 0.8) quasars observed with the Hubble Space Telescope STIS as part of a Snapshot project to understand the relationship between quasar outflows and luminosity. By design, all observations cover the C IV emission line. Ten of the quasars are from the Hamburg-ESO catalog, three are from the Palomar-Green catalog, and one is from the Parkes catalog. The sample contains a few interesting quasars, including two broad absorption line (BAL) quasars (HE 0143-3535 and HE 0436-2614), one quasar with a mini-BAL (HE 1105-0746), and one quasar with associated narrow absorption (HE 0409-5004). These BAL quasars are among the brightest known (although not the most luminous) since they lie at z_(em) 1.4 Large Bright Quasar Survey samples. By design, our objects sample luminosities in between these two surveys, and our four absorbed objects are consistent with the v ~ L^(0.62) relation derived by Laor & Brandt (2002). Another quasar, HE 0441-2826, contains extremely weak emission lines, and our spectrum is consistent with a simple power-law continuum. The quasar is radio-loud but has a steep spectral index and a lobe-dominated morphology, which argues against it being a blazar. The unusual spectrum of this quasar resembles the spectra of the quasars PG 1407+265, SDSS J1136+0242, and PKS 1004+13, for which several possible explanations have been entertained.

Blowing Away the “Torus”: Dusty Winds in AGN

The “torus” is a central component of Active Galactic Nuclei models that produces the observed mid‐IR emission, and obscures a large fraction of accreting black holes from direct observation. We explore whether the torus is an outflowing dusty wind driven by both magnetocentrifugal forces and radiation pressure. We have developed a model that includes these driving mechanisms and utilizes 3D Monte Carlo simulations to produce SEDs and images of the wind. We find that, in initial tests, the model is able to approximately reproduce the luminosity of the IR composite Spectral Energy Distribution when the wind has a base column of NH∼1025 cm−2 and Ṁout∼1 M⊙yr−1.

Cosmic ray production and emission in M82

Starting from first principles, we construct a simple model for the evolution of energetic particles produced by supernovae in the starburst galaxy M82. The supernova rate, geometry, and properties of the interstellar medium are all well observed in this nearby galaxy. Assuming a uniform interstellar medium and constant cosmic-ray injection rate, we estimate the cosmic-ray proton and primary & secondary electron/positron populations. From these particle spectra, we predict the gamma ray flux and the radio synchrotron spectrum. The model is then compared to the observed radio and gamma-ray spectra of M82 as well as previous models by Torres (2004), Persic et al . (2008), and de Cea del Pozo et al . (2009). Through this project, we aim to build a better understanding of the calorimeter model, in which energetic particle fluxes reflect supernova rates, and a better understanding of the radio-FIR correlation in galaxies.