Interesting Astrophysics: Apr 19 to Apr 30

There really haven't been many papers or preprints that have caught my eye over the last two weeks or so, so this edition of Interesting Astrophysics is pretty short. The most interesting (to me) of the bunch are Nobukawa et al - on origin of K shell X-ray line emission in the Galactic Center - and Parkin & Pittard's work on the effects of numerical heat conduction on the dynamics of colliding winds.

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Galaxies and Starbursts

The Buried Starburst in the Interacting Galaxy II Zw 096 as Revealed by the Spitzer Space Telescope
Hanae Inami, et al, arXiv:1004.3543 [pdf, ps, other]
Comments: 46 pages, 10 figures, accepted for publication in AJ

Discovery of K-Shell Emission Lines of Neutral Atoms in the Galactic Center Region
Masayoshi Nobukawa, Katsuji Koyama, Takeshi Go Tsuru, Syukyo G Ryu, Vincent Tatischeff, arXiv:1004.3891 [pdf, ps, other]
Comments: 7 pages, 5 figures, accepted for publication in PASJ (Vol.62, No.2, pp.423--429)

This is potentially important. If the interpretation is correct then we don't have to worry so much about unusual (non-Maxwellian) electron energy distributions creating exotic X-ray spectra, and can also still trust and use more traditional models/processes in interpreting diffuse X-ray emission in galaxies other than the Milky Way. This comes at the price of invoking an explanation that posits substantially higher X-ray luminosities for the Milky Way's central black hole in the past than is observed now.

Their abstract: "The K-shell emission line of neutral irons from the Galactic center (GC) region is one of the key for the structure and activity of the GC. The origin is still open question, but possibly due either to X-ray radiation or to electron bombarding to neutral atoms. To address this issue, we analyzed the Suzaku X-ray spectrum from the GC region of intense neutral iron line emission, and report on the discovery of Kalpha lines of neutral argon, calcium, chrome, and manganese atoms. The equivalent widths of these Kalpha lines indicate that the metal abundances in the GC region should be ~1.6 and ~4 of solar value, depending on the X-ray and the electron origins, respectively. On the other hand, the metal abundances in the hot plasma in the GC region are found to be ~1-2 solar. These results favor that the origin of the neutral Kalpha lines are due to X-ray irradiation."

Note that what they observe is K-alpha emission from neutral Fe, Ar, Ca, Mg and Chrome(!). This is unlike M82 (Griffiths et al 2000, Strickland & Heckman 2007, 2009), where we see Helium-like emission from S, Ar, Ca and Fe, i.e. direct emission from highly ionized gas, not X-ray fluorescence from irradiated neutral atoms.

Relating dust, gas and the rate of star formation in M31
F. S. Tabatabaei, E. M. Berkhuijsen, arXiv:1004.4306 [pdf, ps, other]
Comments: 22 pages accepted for publication in A&A

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Black Holes and AGN

X-ray and multiwavelength view of NGC 4278. A LINER-Seyfert connection?

G. Younes, D. Porquet, B. Sabra, N. Grosso, J.N. Reeves, M.G. Allen, arXiv:1004.5134 [pdf, other]

Comments: 14 pages, 15 figures, accepted for publication in Astronomy and Astrophysics

Mid-Infrared Properties of the Swift Burst Alert Telescope Active Galactic Nuclei Sample of the Local Universe. I. Emission-Line Diagnostics
K. A. Weaver, M. Meléndez, R. F. Mushotzky, S. Kraemer, K. Engle, E. Malumuth, J. Tueller, C. Markwardt, C.T. Berghea, R. P. Dudik, L. M. Winter, L. Armus, arXiv:1004.5321 [pdf, ps, other]
Comments: 54 pages, 9 Figures. Accepted for publication in The Astrophysical Journal.

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Numerical Astrophysics and Hydrodynamics

Numerical heat conduction in hydrodynamical models of colliding hypersonic flows
E. R. Parkin, J. M. Pittard, arXiv:1004.3753 [pdf, ps, other]
Comments: 14 pages, 10 figures, accepted for publication in MNRAS

Their abstract: "Hydrodynamical models of colliding hypersonic flows are presented which explore the dependence of the resulting dynamics and the characteristics of the derived X-ray emission on numerical conduction and viscosity. For the purpose of our investigation we present models of colliding flow with plane-parallel and cylindrical divergence. Numerical conduction causes erroneous heating of gas across the contact discontinuity which has implications for the rate at which the gas cools. We find that the dynamics of the shocked gas and the resulting X-ray emission are strongly dependent on the contrast in the density and temperature either side of the contact discontinuity, these effects being strongest where the postshock gas of one flow behaves quasi-adiabatically while the postshock gas of the other flow is strongly radiative. Introducing additional numerical viscosity into the simulations has the effect of damping the growth of instabilities, which in some cases act to increase the volume of shocked gas and can re-heat gas via sub-shocks as it flows downstream. The resulting reduction in the surface area between adjacent flows, and therefore of the amount of numerical conduction, leads to a commensurate reduction in spurious X-ray emission, though the dynamics of the collision are compromised. The simulation resolution also affects the degree of numerical conduction. A finer resolution better resolves the interfaces of high density and temperature contrast and although numerical conduction still exists the volume of affected gas is considerably reduced. However, since it is not always practical to increase the resolution, it is imperative that the degree of numerical conduction is understood so that inaccurate interpretations can be avoided. This work has implications for the dynamics and emission from astrophysical phenomena which involve high Mach number shocks."