arXiv subjects

Alexander Y. Wagner

Publications and source records attributed to Alexander Y. Wagner.

27 records · Page 2Linked to original sources

arXiv2019

Jets blowing bubbles in the young radio galaxy 4C 31.04

We report the discovery of shocked molecular and ionized gas resulting from jet-driven feedback in the compact radio galaxy 4C 31.04 using near-IR imaging spectroscopy. 4C 31.04 is a $\sim 100$ pc double-lobed Compact Steep Spectrum source believed to be a very young AGN. It is hosted by a giant elliptical with a $\sim 10^{9}~\rm M_\odot$ multi-phase gaseous circumnuclear disc. We used high spatial resolution, adaptive optics-assisted $H$- and $K$-band integral field Gemini/NIFS observations to probe (1) the warm ($\sim 10^3~\rm K$) molecular gas phase, traced by ro-vibrational transitions of $\rm H_2$, and (2), the warm ionized medium, traced by the [Fe II]$_{1.644~\rm μm}$ line. The [Fe II] emission traces shocked gas ejected from the disc plane by a jet-blown bubble $300-400~\rm pc$ in diameter, whilst the $\rm H_2$ emission traces shock-excited molecular gas in the interior $\sim 1~\rm kpc$ of the circumnuclear disc. Hydrodynamical modelling shows that the apparent discrepancy between the extent of the shocked gas and the radio emission can occur when the brightest regions of the synchrotron-emitting plasma are temporarily halted by dense clumps, whilst less bright plasma can percolate through the porous ISM and form an energy-driven bubble that expands freely out of the disc plane. This bubble is filled with low surface-brightness plasma not visible in existing VLBI observations of 4C 31.04 due to insufficient sensitivity. Additional radial flows of jet plasma may percolate to $\sim \rm kpc$ radii in the circumnuclear disc, driving shocks and accelerating clouds of gas, giving rise to the $\rm H_2$ emission.

astro-ph.GA↗

arXiv2018

How frequent are close supermassive binary black holes in powerful jet sources?

Supermassive black hole binaries may be detectable by an upcoming suite of gravitational wave experiments. Their binary nature can also be revealed by radio jets via a short-period precession driven by the orbital motion as well as the geodetic precession at typically longer periods. We have investigated Karl G. Jansky Very Large Array (VLA) and MERLIN radio maps of powerful jet sources for morphological evidence of geodetic precession. For perhaps the best studied source, Cygnus A, we find strong evidence for geodetic precession. Projection effects can enhance precession features, for which we find indications in strongly projected sources. For a complete sample of 33 3CR radio sources we find strong evidence for jet precession in 24 cases (73 per cent). The morphology of the radio maps suggests that the precession periods are of the order of 10^6 - 10^7 yr. We consider different explanations for the morphological features and conclude that geodetic precession is the best explanation. The frequently observed gradual jet angle changes in samples of powerful blazars can be explained by orbital motion. Both observations can be explained simultaneously by postulating that a high fraction of powerful radio sources have sub-parsec supermassive black hole binaries. We consider complementary evidence and discuss if any jetted supermassive black hole with some indication of precession could be detected as individual gravitational wave source in the near future. This appears unlikely, with the possible exception of M87.

astro-ph.HE↗

arXiv2018

Relativistic jet feedback III: feedback on gas disks

We study the interactions of a relativistic jet with a dense turbulent gaseous disk of radius $\sim 2$ kpc. We have performed a suite of simulations with different mean density, jet power and orientation. Our results show that: A) the relativistic jet couples strongly with the gas in the inner kpc, creating a cavity and launching outflows. B) The high pressure bubble inflated by the jet and its back-flow compresses the disk at the outer edges, driving inflows within the disk. C) Jets inclined towards the disk couple more strongly with the disk and launch sub-relativistic, wide-angle outflows along the minor axis. D) Shocks driven directly by the jet and the jet-driven energy bubble raise the velocity dispersion throughout the disk by several times its initial value. E) Compression by the jet-driven shocks can enhance the star formation rate in the disk, especially in a ring-like geometry close to the axis. However, enhanced turbulent dispersion in the disk also leads to quenching of star formation. Whether positive or negative feedback dominates depends on jet power, ISM density, jet orientation with respect to the disc, and the time-scale under consideration. Qualitatively, our simulations compare favourably with kinematic and morphological signatures of several observed galaxies such as NGC 1052, NGC 3079, 3C 326 and 3C 293.

astro-ph.HE↗

arXiv2018

Relativistic jet feedback II: Relationship to gigahertz peak spectrum and compact steep spectrum radio galaxies

We propose that Gigahertz Peak Spectrum (GPS) and Compact Steep Spectrum (CSS) radio sources are the signposts of relativistic jet feedback in evolving galaxies. Our simulations of relativistic jets interacting with a warm, inhomogeneous medium, utilize cloud densities and velocity dispersions in the range derived from optical observations, show that free-free absorption can account for the $\sim \rm GHz$ peak frequencies and low frequency power laws inferred from the radio observations. These new computational models replace the power-law model for the free-free optical depth in the \citep{bicknell97a} model by a more fundamental model involving disrupted log-normal distributions of warm gas. One feature of our new models is that at early stages, the low frequency spectrum is steep but progressively flattens as a result of a broader distribution of optical depths, suggesting that the steep low frequency spectra discovered by \citet{callingham17a} may possibly be attributed to young sources. We also investigate the inverse correlation between peak frequency and size and find that the initial location on this correlation is determined by the average density of the warm ISM. The simulated sources track this correlation initially but eventually fall below it, indicating the need for a more extended ISM than presently modelled. GPS and CSS sources can potentially provide new insights into the phenomenon of AGN feedback since their peak frequencies and spectra are indicative of the density, turbulent structure and distribution of gas in the host galaxy.

astro-ph.GA↗

arXiv2017

Ionized gas outflows in infrared-bright dust-obscured galaxies selected with WISE and SDSS

We present the ionized gas properties of infrared (IR)-bright dust-obscured galaxies (DOGs) that show an extreme optical/IR color, $(i - [22])_{\rm AB} > 7.0$, selected with the Sloan Digital Sky Survey (SDSS) and Wide-field Infrared Survey Explorer (WISE). For 36 IR-bright DOGs that show [OIII]$λ$5007 emission in the SDSS spectra, we performed a detailed spectral analysis to investigate their ionized gas properties. In particular, we measured the velocity offset (the velocity with respect to the systemic velocity measured from the stellar absorption lines) and the velocity dispersion of the [OIII] line. We found that the derived velocity offset and dispersion of most IR-bright DOGs are larger than those of Seyfert 2 galaxies (Sy2s) at $z < 0.3$, meaning that the IR-bright DOGs show relatively strong outflows compared to Sy2s. This can be explained by the difference of IR luminosity contributed from active galactic nucleus, $L_{\rm IR}$ (AGN), because we found that (i) $L_{\rm IR}$ (AGN) correlates with the velocity offset and dispersion of [OIII] and (ii) our IR-bright DOGs sample has larger $L_{\rm IR}$ (AGN) than Sy2s. Nevertheless, the fact that about 75% IR-bright DOGs have a large ($>$ 300 km s$^{-1}$) velocity dispersion, which is a larger fraction compared to other AGN populations, suggests that IR-bright DOGs are good laboratories to investigate AGN feedback. The velocity offset and dispersion of [OIII] and [NeIII]$λ$3869 are larger than those of [OII]$λ$3727, which indicates that the highly ionized gas tends to show more stronger outflows.

astro-ph.GA↗

arXiv2017

The formation and physical origin of highly ionized cooling gas

We present a simple model that explains the origin of warm diffuse gas seen primarily as highly ionized absorption line systems in the spectra of background sources. We predict the observed column densities of several highly ionized transitions such as OVI, OVIII, NeVIII, NV, and MgX; and present a unified comparison of the model predictions with absorption lines seen in the Milky Way disk, Milky Way halo,starburst galaxies, the circumgalactic medium and the intergalactic medium at low and high redshifts. We show that diffuse gas seen in such diverse environments can be simultaneously explained by a simple model of radiatively cooling gas. We show that most of such absorption line systems are consistent with being collisionally ionized, and estimate the maximum likelihood temperature of the gas in each observation. This model satisfactorily explains why OVI is regularly observed around star-forming low-z L* galaxies, and why NV is rarely seen around the same galaxies. We further present some consequences of this model in quantifying the dynamics of the cooling gas around galaxies and predict the shock velocities associated with such flows. A unique strength of this model is that while it has only one free (but physically well-constrained) parameter, it nevertheless successfully reproduces the available data on O VI absorbers in the interstellar, circumgalactic, intra-group, and intergalactic media, as well as the available data on other absorption-line from highly ionized species.

astro-ph.GA↗

arXiv2016

Outflows Driven by Quasars in High-Redshift Galaxies with Radiation Hydrodynamics

The quasar mode of Active Galactic Nuclei (AGN) in the high-redshift Universe is routinely observed in gas-rich galaxies together with large-scale AGN-driven winds. It is crucial to understand how photons emitted by the central AGN source couple to the ambient interstellar-medium to trigger large-scale outflows. By means of radiation-hydrodynamical simulations of idealised galactic discs, we study the coupling of photons with the multiphase galactic gas, and how it varies with gas cloud sizes, and the radiation bands included in the simulations, which are ultraviolet (UV), optical, and infrared (IR). We show how a quasar with a luminosity of $10^{46}$ erg/s can drive large-scale winds with velocities of $10^2-10^3$ km/s and mass outflow rates around $10^3$ M$_\odot$/yr for times of order a few million years. Infrared radiation is necessary to efficiently transfer momentum to the gas via multi-scattering on dust in dense clouds. However, IR multi-scattering, despite being extremely important at early times, quickly declines as the central gas cloud expands and breaks up, allowing the radiation to escape through low gas density channels. The typical number of multi-scattering events for an IR photon is only about a quarter of the mean optical depth from the center of the cloud. Our models account for the observed outflow rates of $\sim$500-1000 M$_\odot$/yr and high velocities of $\sim 10^3$ km/s, favouring winds that are energy-driven via extremely fast nuclear outflows, interpreted here as being IR-radiatively-driven winds.

astro-ph.GA↗

arXiv2011

Stereo pairs in Astrophysics

Stereoscopic visualization is seldom used in Astrophysical publications and presentations compared to other scientific fields, e.g., Biochemistry, where it has been recognized as a valuable tool for decades. We put forth the view that stereo pairs can be a useful tool for the Astrophysics community in communicating a truer representation of astrophysical data. Here, we review the main theoretical aspects of stereoscopy, and present a tutorial to easily create stereo pairs using Python. We then describe how stereo pairs provide a way to incorporate 3D data in 2D publications of standard journals. We illustrate the use of stereo pairs with one conceptual and two Astrophysical science examples: an integral field spectroscopy study of a supernova remnant, and numerical simulations of a relativistic AGN jet. We also use these examples to make the case that stereo pairs are not merely an ostentatious way to present data, but an enhancement in the communication of scientific results in publications because they provide the reader with a realistic view of multi-dimensional data, be it of observational or theoretical nature. In recognition of the ongoing 3D expansion in the commercial sector, we advocate an increased use of stereo pairs in Astrophysics publications and presentations as a first step towards new interactive and multi-dimensional publication methods.

astro-ph.IM↗

arXiv2010

Relativistic Jet Feedback in Evolving Galaxies

Over cosmic time, galaxies grow through the hierarchical merging of smaller galaxies. However, the bright region of the galaxy luminosity function is incompatible with the simplest version of hierarchical merging, and it is believed that feedback from the central black hole in the host galaxies reduces the number of bright galaxies and regulates the co-evolution of black hole and host galaxy. Numerous simulations of galaxy evolution have attempted to include the physical effects of such feedback with a resolution usually exceeding a kiloparsec. However, interactions between jets and the interstellar medium involve processes occurring on less than kiloparsec scales. In order to further the understanding of processes occurring on such scales, we present a suite of simulations of relativistic jets interacting with a fractal two-phase interstellar medium with a resolution of two parsecs and a largest scale of one kiloparsec. The transfer of energy and momentum to the interstellar medium is considerable, and we find that jets with powers in the range of 10^43-10^46 erg s^-1 can inhibit star formation through the dispersal of dense gas in the galaxy core. We determine the effectiveness of this process as a function of the ratio of the jet power to the Eddington luminosity of the black hole, the pressure of the interstellar medium and the porosity of the dense gas.

astro-ph.CO↗