Very Extended X-ray and H-alpha Emission in M82: Implications for the Superwind Phenomenon

We discuss the properties and implications of a 3.7x0.9 kpc region of spatially-coincident X-ray and H-alpha emission about 11.6 kpc to the north of the galaxy M82 previously discussed by Devine and Bally (1999). The PSPC X-ray spectrum is fit by thermal plasma (kT=0.80+-0.17 keV) absorbed by only the Galactic foreground column density. We evaluate the relationship of the X-ray/H-alpha ridge to the M82 superwind. The main properties of the X-ray emission can all be explained as being due to shock-heating driven as the superwind encounters a massive ionized cloud in the halo of M82. This encounter drives a slow shock into the cloud, which contributes to the excitation of the observed H-alpha emission. At the same time, a fast bow-shock develops in the superwind just upstream of the cloud, and this produces the observed X-ray emission. This interpretation would imply that the superwind has an outflow speed of roughly 800 km/s, consistent with indirect estimates based on its general X-ray properties and the kinematics of the inner kpc-scale region of H-alpha filaments. The gas in the M82 ridge is roughly two orders-of-magnitude hotter than the minimum "escape temperature" at this radius, so this gas will not be retained by M82. (abridged)Comment: 24 pages (latex), 3 figures (2 gif files and one postscript), accepted for publication in Part 1 of The Astrophysical Journa

The Twin-Jet of NGC1052 at Radio, Optical, and X-ray Frequencies

We present results from a combined radio, optical, and X-ray study of the jet-associated emission features in NGC1052. We analyse the radio-optical morphology and find a good positional correlation between the radio jet and the optical emission cone. Two optical emission knots are directly associated with radio counterparts exhibiting a radio to X-ray broadband spectrum not compatible with synchrotron emission. We discuss the possibility that the thermal soft spectrum of the extended X-ray emission originates from jet driven shocks produced in the interaction between the jet-plasma and its surrounding medium.Comment: 4 pages, 1 figure, needs elsart.cls, to be published in ''The Physics of Relativistic Jets in the CHANDRA and XMM Era'', G. Brunetti, D.E. Harris, R.M. Sambruna, G. Setti (eds.

Magnetic Reconnection Triggered by the Parker Instability in the Galaxy: Two-Dimensional Numerical Magnetohydrodynamic Simulations and Application to the Origin of X-Ray Gas in the Galactic Halo

We propose the Galactic flare model for the origin of the X-ray gas in the Galactic halo. For this purpose, we examine the magnetic reconnection triggered by Parker instability (magnetic buoyancy instability), by performing the two-dimensional resistive numerical magnetohydrodynamic simulations. As a result of numerical simulations, the system evolves as following phases: Parker instability occurs in the Galactic disk. In the nonlinear phase of Parker instability, the magnetic loop inflates from the Galactic disk into the Galactic halo, and collides with the anti-parallel magnetic field, so that the current sheets are created in the Galactic halo. The tearing instability occurs, and creates the plasmoids (magnetic islands). Just after the plasmoid ejection, further current-sheet thinning occurs in the sheet, and the anomalous resistivity sets in. Petschek reconnection starts, and heats the gas quickly in the Galactic halo. It also creates the slow and fast shock regions in the Galactic halo. The magnetic field ($B\sim 3 \mu$G), for example, can heat the gas ($n\sim 10^{-3}$ cm$^{-3}$) to temperature of $\sim 10^6$ K via the reconnection in the Galactic halo. The gas is accelerated to Alfv\'en velocity ($\sim 300$ km s$^{-1}$). Such high velocity jets are the evidence of the Galactic flare model we present in this paper, if the Doppler shift of the bipolar jet is detected in the Galactic halo. Full size figures are available at http://www.kwasan.kyoto-u.ac.jp/~tanuma/study/ApJ2002/ApJ2002.htmlComment: 13 pages, 12 figures, uses emulateapj.sty, accepted by Ap

Jet emission in NGC1052 at radio, optical, and X-ray frequencies

We present a combined radio, optical, and X-ray study of the nearby LINER galaxy NGC 1052. Data from a short (2.3 ksec) {\it CHANDRA} observation of NGC 1052 reveal the presence of various jet-related X-ray emitting regions, a bright compact core and unresolved knots in the jet structure as well as an extended emitting region inside the galaxy well aligned with the radio synchrotron jet-emission. The spectrum of the extended X-ray emission can best be fitted with a thermal model with $kT = (0.4-0.5)$ keV, while the compact core exhibits a very flat spectrum, best approximated by an absorbed power-law with $N_{\rm H} = (0.6-0.8) \times 10^{22} {\rm cm^{-2}}$. We compare the radio structure to an optical ``structure map'' from a {\it Hubble Space Telescope} ({\it HST}) observation and find a good positional correlation between the radio jet and the optical emission cone. Bright, compact knots in the jet structure are visible in all three frequency bands whose spectrum is inconsistent with synchrotron emission.Comment: 8 pages, 5 figures (figure 2 in color), image resolution degraded wrt journal version, needs aa.cls. Accepted for publication in A&

A Very Sensitive 21cm Survey for Galactic High-Velocity HI

Very sensitive HI 21cm observations have been made in 860 directions at dec >= -43deg in search of weak, Galactic, high-velocity HI emission lines at moderate and high Galactic latitudes. One-third of the observations were made toward extragalactic objects. The median 4-sigma detection level is NHI = 8x10^{17} cm^-2 over the 21' telescope beam. High-velocity HI emission is detected in 37% of the directions; about half of the lines could not have been seen in previous surveys. The median FWHM of detected lines is 30.3 km/s. High- velocity HI lines are seen down to the sensitivity limit of the survey implying that there are likely lines at still lower values of NHI. The weakest lines have a kinematics and distribution on the sky similar to that of the strong lines, and thus do not appear to be a new population. Most of the emission originates from objects which are extended over several degrees; few appear to be compact sources. At least 75%, and possibly as many as 90%, of the lines are associated with one of the major high-velocity complexes. The Magellanic Stream extends at least 10 deg to higher Galactic latitude than previously thought and is more extended in longitude as well. Although there are many lines with low column density, their numbers do not increase as rapidly as NHI^-1, so most of the HI mass in the high-velocity cloud phenomenon likely resides in the more prominent clouds. The bright HI features may be mere clumps within larger structures, and not independent objects.Comment: 88 pages includes 22 figures Accepted for Publication in ApJ Suppl. June 200

Spectral Analysis of the Accretion Flow in NGC 1052 with Suzaku

We present an analysis of the 101 ks, 2007 Suzaku spectrum of the LINER galaxy NGC 1052. The 0.5-10 keV continuum is well-modeled by a power-law modified by Galactic and intrinsic absorption, and it exhibits a soft, thermal emission component below 1 keV. Both a narrow core and a broader component of Fe K emission centered at 6.4 keV are robustly detected. While the narrow line is consistent with an origin in material distant from the black hole, the broad line is best fit empirically by a model that describes fluorescent emission from the inner accretion disk around a rapidly rotating black hole. We find no evidence in this observation for Comptonized reflection of the hard X-ray source by the disk above 10 keV, however, which casts doubt on the hypothesis that the broad iron line originates in the inner regions of a standard accretion disk. We explore other possible scenarios for producing this spectral feature and conclude that the high equivalent width (EW ~ 185 keV) and full-width-half-maximum velocity of the broad iron line (v ~ 0.37c) necessitate an origin within d ~ 8 gravitational radii of the hard X-ray source. Based on the confirmed presence of a strong radio jet in this galaxy nucleus, the broad iron line may be produced in dense plasma near the base of the jet, implying that emission mechanisms in the centralmost portions of active galactic nuclei are more complex than previously thought.Comment: 33 pages, 8 figures, accepted for publication in Ap

Two-Dimensional MHD Numerical Simulations of Magnetic Reconnection Triggered by A Supernova Shock in Interstellar Medium, Generation of X-Ray Gas in Galaxy

We examine the magnetic reconnection triggered by a supernova (or a point explosion) in interstellar medium, by performing two-dimensional resistive magnetohydrodynamic (MHD) numerical simulations with high spatial resolution. We found that the magnetic reconnection starts long after a supernova shock (fast-mode MHD shock) passes a current sheet. The current sheet evolves as follows: (i) Tearing-mode instability is excited by the supernova shock, and the current sheet becomes thin in its nonlinear stage. (ii) The current-sheet thinning is saturated when the current-sheet thickness becomes comparable to that of Sweet-Parker current sheet. After that, Sweet-Parker type reconnection starts, and the current-sheet length increases. (iii) ``Secondary tearing-mode instability'' occurs in the thin Sweet-Parker current sheet. (iv) As a result, further current-sheet thinning occurs and anomalous resistivity sets in, because gas density decreases in the current sheet. Petschek type reconnection starts and heats interstellar gas. Magnetic energy is released quickly while magnetic islands are moving in the current sheet during Petschek type reconnection. The released magnetic energy is determined by the interstellar magnetic field strength, not energy of initial explosion nor distance to explosion. We suggest that magnetic reconnection is a possible mechanism to generate X-ray gas in Galaxy.Comment: 17 pages using emulateapj.sty, 24 figures (4colors), submitted to ApJ, mpeg simulations and psfiles are available at http://stesun8.stelab.nagoya-u.ac.jp/~tanuma/apj2000/apj2000.htm

Atomic and molecular gas properties during cloud formation

Context: Molecular clouds, which harbor the birthplaces of stars, form out of the atomic phase of the interstellar medium (ISM). To understand this transition process, it is crucial to investigate the spatial and kinematic relationships between atomic and molecular gas. Aims: We aim to characterize the atomic and molecular phase of the ISM and set their physical properties into the context of cloud formation processes. Methods: We study the cold neutral medium (CNM) by means of H$\tiny{I}$ self absorption (HISA) toward the giant molecular filament GMF20.0-17.9 (distance=3.5 kpc, length ∼170 pc) and compare our results with molecular gas traced by $^{13}$CO emission. We fit baselines of HISA features to H$\tiny{I}$ emission spectra using 1st and 2nd order polynomial functions. Results: The CNM identified by this method spatially correlates with the morphology of the molecular gas toward the western region. However, no spatial correlation between HISA and $^{13}$CO is evident towards the eastern part of the filament. The distribution of HISA peak velocities and line widths agrees well with $^{13}$CO within the whole filament. The column densities of the CNM probed by HISA are on the order of 1020 cm$^{−2}$ while those of molecular hydrogen traced by $^{13}$CO are an order of magnitude higher. The column density probability density functions (N-PDFs) of HISA (CNM) and H$\tiny{I}$ emission (tracing both the CNM and the warm neutral medium, WNM) have a log-normal shape for all parts of the filament, indicative of turbulent motions as the main driver for these structures. The H$_2$N-PDFs show a broad log-normal distribution with a power-law tail suggesting the onset of gravitational contraction. The saturation of H$\tiny{I}$ column density is observed at ∼25 M_\bigodotpc$^{−2}$. Conclusions: We conjecture that different evolutionary stages are evident within the filament. In the eastern region we witness the onset of molecular cloud formation out of the atomic gas reservoir while the western part is more evolved as it reveals pronounced H2 column density peaks and signs of active star formation