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Steven Diehl

Publications and source records attributed to Steven Diehl.

26 records · Page 2Linked to original sources
arXiv2008

The Hot Interstellar Medium in Normal Elliptical Galaxies. II. Morphological Evidence for AGN Feedback

We report on the discovery of a new quantitative relationship between X-ray gas morphology and radio and X-ray AGN luminosities in normal elliptical galaxies. This is the second paper in a series using data on 54 objects from the Chandra public archive and builds on the findings of Paper I, which demonstrated that hydrostatic equilibrium in elliptical galaxies holds, at best, only approximately and that the shape of the X-ray isophotes is unrelated to the shape of the gravitational potential. Instead, the gas is almost always asymmetrically disturbed. In this paper, we quantify the amount of asymmetry and study its correlation with other galaxy properties. We also determine radio powers and derive X-ray AGN luminosities for our galaxy sample. We find that the amount of asymmetry in the gas is correlated with both measures of AGN activity, in the sense that the hot gas is more disturbed in galaxies with higher radio and X-ray AGN luminosities. We find no evidence that galaxy density has significant effects on gas morphology. We do however find evidence for a correlation between gas asymmetry and the presence of hot ambient gas, which we interpret as a signature of hydrodynamic interactions with an external ambient medium. Surprisingly, the AGN-morphology connection persists all the way down to the weakest AGN luminosities in rather X-ray faint galaxies. This is strong morphological evidence that supports the general importance of AGN feedback, even in normal elliptical galaxies.

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arXiv2007

On the Road to Understanding Type Ia Progenitors: Precision Simulations of Double Degenerate Mergers

We review the current state of the art in double degenerate merger simulations to better understand the role this phenomenon plays in type Ia progenitors. Because the fate of a merged system may well depend on the exact evolution of the matter temperature (as well as mixing of the merged system), precision simulations are required to determine the true fate of these systems. Unfortunately, if we compare the results of current simulations, we find many-order of magnitude differences in quantities like mass-transfer rates in the merger process. We discuss these differences and outline an approach using verification and validation that should allow us to achieve a level of precision sufficient to determine the true fate (thermonuclear vs. collapse) of double degenerate mergers. Understanding the fate of lower-mass systems (e.g. R Coronae Borealis stars) may be key in our final testing phase.

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arXiv2007

X-ray Observables of Magnetically Dominated Cavities in Clusters

We present results from magneto-hydrodynamic jet simulations in the magnetically dominated regime and find that they inflate cavities into the intracluster medium. Mock Chandra X-ray observations of our simulations show a strong resemblance to real cavities observed in X-ray images of clusters, both in terms of their morphology and thermodynamic structure. An analysis of the evolution of bubble sizes in the multi-cavity system Hydra A, as well as in a large sample of 64 cavities in 32 clusters shows that bubbles tend to expand much faster than expected in the purely adiabatic regime. Instead, we find that the bubbles follow more closely a trend predicted by our current-carrying jet models.

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arXiv2007

Numerical Modeling of AGN Jets: Formation of Magnetically Dominated Lobes and Stability Properties of Current-carrying Jets

We argue the behavior of Poynting flux-dominated outflows from AGN in the galactic cluster systems by performing three-dimensional MHD simulations within the framework of the "magnetic tower" model. Of particular interests are the structure of MHD waves, the cylindrical radial force balance, the (de)collimation, and the stability properties of magnetic tower jets. Transition between the jet/lobe and the formation of wiggling jet by growing current-driven instability are discussed.

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arXiv2007

The Hot Interstellar Medium of Normal Elliptical Galaxies. I. A Chandra Gas Gallery and Comparison of X-ray and Optical Morphology

We present an X-ray analysis of 54 normal elliptical galaxies in the Chandra archive and isolate their hot gas component from the contaminating point source emission, allowing us to conduct, for the first time, a morphological analysis on the gas alone. A comparison with optical images and photometry shows that the hot gas morphology has surprisingly little in common with the shape of the stellar distribution. We observe no correlation between optical and X-ray ellipticities in the inner regions where stellar mass dominates over dark matter. A shallow correlation would be expected if the gas had settled into hydrostatic equilibrium with the gravitational potential. Instead, observed X-ray ellipticities exceed optical ellipticities in many cases. We exclude rotation as the dominant factor to produce the gas ellipticities. The gas appears disturbed, and hydrostatic equilibrium is the exception rather than the rule. Nearly all hydrostatic models can be ruled out at 99% confidence, based of their inability to reproduce the optical-X-ray correlation and large X-ray ellipticities. Hydrostatic models not excluded are those in which dark matter either dominates over stellar mass inside the inner half-light radius or has a prominently cigar-shaped distribution, both of which can be ruled out on other grounds. We conclude that, even for rather X-ray faint elliptical galaxies, the gas is at least so far out of equilibrium that it does not retain any information about the shape of the potential, and that X-ray derived radial mass profiles may be in error by factors of order unity.

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arXiv2005

Adaptive Binning of X-ray data with Weighted Voronoi Tesselations

We present a technique to adaptively bin sparse X-ray data using weighted Voronoi tesselations (WVTs). WVT binning is a generalisation of Cappellari & Copin's (2001) Voronoi binning algorithm, developed for integral field spectroscopy. WVT binning is applicable to many types of data and creates unbiased binning structures with compact bins that do not lead the eye. We apply the algorithm to simulated data, as well as several X-ray data sets, to create adaptively binned intensity images, hardness ratio maps and temperature maps with constant signal-to-noise ratio per bin. We also illustrate the separation of diffuse gas emission from contributions of unresolved point sources in elliptical galaxies. We compare the performance of WVT binning with other adaptive binning and adaptive smoothing techniques. We find that the CIAO tool csmooth creates serious artefacts and advise against its use to interpret diffuse X-ray emission.

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arXiv2005

A Fundamental Plane Relation for the X-Ray Gas in Normal Elliptical Galaxies

We report on the discovery of a new correlation between global parameters of the hot interstellar gas in elliptical galaxies. We reanalyze archival Chandra data for 30 normal early-type systems, removing the contributions of resolved and unresolved point sources to reveal the X-ray morphology of the hot gas. We determine the half-light radius, R_X, and the mean surface brightness, I_X, from the gas surface brightness profiles. A spectral analysis determines the temperature, T_X, of the gas within 3 optical effective radii. We find that the galaxies lie on an X-ray Gas Fundamental Plane (XGFP) of the form T_X ~ R_X^{0.28} I_X^{0.22}. This is close to, but distinct from, a simple luminosity-temperature relation. The intrinsic width of the XGFP is only 0.07dex, nearly identical to that of the stellar (optical) fundamental plane (SFP). This is surprising since X-ray gas masses are typically ~10^{-2} of the stellar masses. We show that the XGFP is not a simple consequence of the virial theorem or hydrostatic equilibrium, and that it is essentially independent of the SFP. The XGFP thus represents a genuinely new constraint on the hydrodynamical evolution of elliptical galaxies.

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