X‐Ray Emission from the Hot Interstellar Medium and Southwest Radio Lobe of the Nearby Radio Galaxy Centaurus A

Abstract

We present results from two Chandra/ACIS-I observations and one XMM-Newton observation of X-ray emission from the ISM and the inner radio lobes of the nearby radio galaxy Centaurus A. The ISM has an average radial surface brightness profile that is well described by a β-model profile with index β=0.40±0.04 and a temperature of k B T ISM ∼0.29 keV beyond 2 kpc from the nucleus. We find that diffuse X-ray emission is coincident with the outer half of the southwest radio lobe, and a bright X-ray enhancement is detected along the edge of the lobe. On the basis of energetic and lifetime arguments, we reject a nonthermal explanation for this emission. We model this emission as a thin, hot shell or cap of X-ray emitting plasma surrounding the radio lobe that was created by the supersonic inflation of the lobe. This plasma shell is both hotter than (k B T SH ∼2.9 keV) and greatly overpressurized relative to the ambient ISM indicating supersonic expansion. We estimate that the lobe is expanding into the ISM at approximately Mach 8.5 or 2400 km s −1 . We are not directly observing the bow shock, but rather the cooler, denser material that is accumulating ahead of the contact discontinuity. The thermal energy in the shell is a significant fraction of the thermal energy of the hot ISM, demonstrating the possibility that the hot ISM of early galaxies can be re-energized by outflows from nuclear activity.

The Planetary Nebula System and Dynamics in the Outer Halo of NGC 5128

“…Van Gorkom et al (1990) reached a similar conclusion when comparing H i dynamical mass estimates with the X-ray estimates. However, Kraft et al (2003) recently used Chandra and XMM data to measure the total gravitating mass with 15 kpc and determined a value of $2 Â 10 11 M , which is statistically indistinguishable from the values we obtain from our mass models within the same radius. This independent mass determination implies that our assumptions of orbital isotropy and a high inclination for the stellar disk are not likely to be grossly wrong, at least within 15 kpc.…”

Dynamical constant mass-to-light ratio models of NGC 5128

“…This is the largest galactocentric distance in an early-type galaxy for which dark matter is not necessary to explain its internal kinematics. The obtained value of the massto-light ratio (6.24 < ∼ M/L B < ∼ 7.00) agrees with the value inferred from the X-rays (Kraft et al 2003). In the outer region of NGC 5128, 7 < r < 14R e (= 78 arcmin) we encountered an increasing velocity dispersion, which implies an increase of the mass-to-light ratio (or tangential anisotropies) implying certain amounts of dark matter in these regions (if any).…”

“…They found that the ISM has an average radial surface brightness profile that is well described by a β-model profile with the index β = 0.40 ± 0.04 and a temperature of k B T ISM = 0.29 keV beyond 2 kpc from the galactic nucleus. Kraft et al (2003) calculated that within 15 kpc of the nucleus the total mass of NGC 5128 is ∼2 × 10 11 M . This value corresponds to the massto-light ratio M/L B = 6.70 at 15 kpc (= 2.44R e , which is a region which contains 75 per cent of the total light for de Vaucouleurs law).…”

ChandraandXMM‐NewtonObservations of a Sample of Low‐Redshift FR I and FR II Radio Galaxy Nuclei

“…As the only FR I-type radio galaxy whose X-ray emission is heavily obscured and dominated by an accretion flow, its nuclear X-ray emission is more similar to that of the FR II-type sources. One possible resolution of this problem is that the recent merger that has occurred in Cen A may have provided additional material to accrete onto the supermassive black hole, triggering heightened nuclear activity and a new phase of radio activity, causing the supersonic reinflation of the lobes (Kraft et al 2003).…”