* Astronomy

Title: The nature of filamentary cold gas in the core of the Virgo Cluster
Authors: N. Werner, J. B. R. Oonk, R. E. A. Canning, S. W. Allen, A. Simionescu, J. Kos, R. J. van Weeren, A. C. Edge, A. C. Fabian, A. von der Linden, P. E. J. Nulsen, C. S. Reynolds, M. Ruszkowski

We present a multi-wavelength study of the emission-line nebulae located southeast of the nucleus of M87, the central dominant galaxy of the Virgo Cluster. We report the detection of far-infrared (FIR) [CII] line emission from the nebulae using observations made with Herschel PACS. The infrared line emission is extended and cospatial with optical H{\alpha}+[NII], far-ultraviolet CIV lines, and soft X-ray emission. The filamentary nebulae evidently contain multi-phase material spanning a temperature range of at least 5 orders of magnitude, from ~100 K to ~10^7 K. This material has most likely been uplifted by the AGN from the center of M87. The thermal pressure of the 10^4 K phase appears to be significantly lower than that of the surrounding hot intra-cluster medium (ICM) indicating the presence of additional turbulent and magnetic pressure in the filaments. If the turbulence in the filaments is subsonic then the magnetic field strength required to balance the pressure of the surrounding ICM is B~30-70 µG. The spectral properties of the soft X-ray emission from the filaments indicate that it is due to thermal plasma with kT~0.5-1 keV, which is cooling by mixing with the cold gas and/or radiatively. Charge exchange can be ruled out as a significant source of soft X-rays. Both cooling and mixing scenarios predict gas with a range of temperatures. This is at first glance inconsistent with the apparent lack of X-ray emitting gas with kT<0.5 keV. However, we show that the missing very soft X-ray emission could be absorbed by the cold gas in the filaments with an average absorption column density of ~10^21 cm^-2, providing a natural explanation for the apparent temperature floor to the X-ray emission at kT~0.5 keV. The FIR through ultra-violet line emission is most likely primarily powered by the ICM particles penetrating the cold gas following a shearing induced mixing process.

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Title: Hundred Thousand Degree Gas in the Virgo Cluster of Galaxies
Authors: W.B. Sparks (1), J.E. Pringle (1,2), R.F. Carswell (2), M. Donahue (3), R. Martin (1), M. Voit (3), M. Cracraft (1), N. Manset (4), J.H. Hough (5) (1. Space Telescope Science Institute, 2. Institute of Astronomy, University of Cambridge, 3. Dept. of Physics and Astronomy, Michigan State University, 4. CFHT, 5. University of Hertfordshire)

The physical relationship between low-excitation gas filaments at ~10^4 K, seen in optical line emission, and diffuse X-ray emitting coronal gas at ~10^7 K in the centers of many galaxy clusters is not understood. It is unclear whether the ~10^4 K filaments have cooled and condensed from the ambient hot (~10^7 K) medium or have some other origin such as the infall of cold gas in a merger, or the disturbance of an internal cool reservoir of gas by nuclear activity. Observations of gas at intermediate temperatures (~10^5-10^6 K) can potentially reveal whether the central massive galaxies are gaining cool gas through condensation or losing it through conductive evaporation and hence identify plausible scenarios for transport processes in galaxy cluster gas. Here we present spectroscopic detection of ~10^5 K gas spatially associated with the H-alpha filaments in a central cluster galaxy, M87 in the Virgo Cluster. The measured emission-line fluxes from triply ionised carbon (CIV 1549 A) and singly ionised helium (HeII 1640 A) are consistent with a model in which thermal conduction determines the interaction between hot and cold phases.

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Title: Spitzer IRAC Low Surface Brightness Observations of the Virgo Cluster
Authors: J. E. Krick, C. Bridge, V. Desai, J. C. Mihos, E. Murphy, C. Rudick, J. Surace, J. Neill

We present 3.6 and 4.5 micron Spitzer IRAC imaging over 0.77 square degrees at the Virgo cluster core for the purpose of understanding the formation mechanisms of the low surface brightness intracluster light features. Instrumental and astrophysical backgrounds that are hundreds of times higher than the signal were carefully characterized and removed. We examine both intracluster light plumes as well as the outer halo of the giant elliptical M87. For two intracluster light plumes, we use optical colours to constrain their ages to be greater than 3 & 5 Gyr, respectively. Upper limits on the IRAC fluxes constrain the upper limits to the masses, and optical detections constrain the lower limits to the masses. In this first measurement of mass of intracluster light plumes we find masses in the range of 5.5 x 10^8 - 4.5 x 10^9 and 2.1 x 10^8 - 1.5 x 10^9 solar masses for the two plumes for which we have coverage. Given their expected short lifetimes, and a constant production rate for these types of streams, integrated over Virgo's lifetime, they can account for the total ICL content of the cluster implying that we do not need to invoke ICL formation mechanisms other than gravitational mechanisms leading to bright plumes. We also examined the outer halo of the giant elliptical M87. The colour profile from the inner to outer halo of M87 (160 Kpc) is consistent with either a flat or optically blue gradient, where a blue gradient could be due to younger or lower metallicity stars at larger radii. The similarity of the age predicted by both the infrared and optical colours (> few Gyr) indicates that the optical measurements are not strongly affected by dust extinction.

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Stripped down: Hubble highlights 2 galaxies that are losing it
A newly released set of images, taken by the NASA/ESA Hubble Space Telescope before the recent Servicing Mission, highlight the ongoing drama in two galaxies in the Virgo Cluster affected by a process known as "ram pressure stripping", which can result in peculiar-looking galaxies. An extremely hot X-ray emitting gas known as the intra-cluster medium lurks between galaxies within clusters. As galaxies move through this intra-cluster medium, strong winds rip through galaxies distorting their shape and even halting star formation.
Both images were taken by the Advanced Camera for Surveys on Hubble before it suffered from a power failure in 2007. Astronauts on Servicing Mission 4 in May 2009 were able to restore ACS during their 13-day mission.

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