Are "Bondi-Hoyle Wakes" detectable in clusters of galaxies?

In clusters of galaxies, the reaction of the intracluster medium (ICM) to the motion of the co-existing galaxies in the cluster triggers the formation of unique features, which trace their position and motion. Galactic wakes, for example, are an apparent result of the ICM/galaxy interactions, and they constitute an important tool for deciphering the motion of the cluster galaxies. In this paper we investigate whether Bondi-Hoyle accretion can create galactic wakes by focusing the ICM behind moving galaxies. The solution of the equations that describe this physical problem provide us with observable quantities along the wake at any time of its lifetime. We also investigate which are the best environmental conditions for the detectability of such structures in the X-ray images of clusters of galaxies. We find that significant Bondi-Hoyle wakes can only be formed in low temperature clusters, and that they are more pronounced behind slow-moving, relatively massive galaxies. The scale length of these elongated structures is not very large: in the most favourable conditions a Bondi-Hoyle wake in a cluster at the redshift of z=0.05 is 12 arcsec long. However, the wake's X-ray emission is noticeably strong: the X-ray flux can reach ~30 times the flux of the surrounding medium. Such features will be easily detectable in Chandra's and XMM-Newton's X-ray images of nearby, relatively poor clusters of galaxies.Comment: 8 pages with 11 Postscript figures, accepted for publication in MNRA

XMM-Newton observations of the binary cluster system Abell 399/401

Abell 399 and Abell 401 are both rich clusters of galaxies, at temperatures 7.2keV and 8.5keV respectively. They lie at a projected separation of ~3Mpc, forming a close pair. We have observed the system with the XMM-Newton satellite. The data of each cluster show significant departures from our idealised picture of relaxed rich clusters. There is also evidence for enhanced X-ray flux in the region between the two, where the temperature is higher than our expectations. Although tidal or compression effects might affect the large scale structure of the two clusters, we show that these cannot account for the distortions seen in the inner regions. We argue that the reasonably relaxed morphology of the clusters, and the absence of major temperature anomalies, argues against models in which the two have already experienced a close encounter. The properties of the intermediate region suggests that they are at an early stage of merging, and are currently interacting mildly, because their separation is still too large for more dramatic effects. The substructure we find in their inner regions seems to point to their individual merging histories. It seems likely that in the Abell 399/401 system, we are witnessing two merger remnants, just before they merge together to form a single rich cluster. This picture is consistent with recent numerical simulations of cluster formation. (abridged)Comment: 20 pages, 14 figures, to be published in MNRA

XMM-Newton observations of Abell 2255 : a test case of a merger after `core-crossing'

It has been known that Abell 2255 is not a relaxed cluster, but it is undergoing a merger. Here, we report on the analysis of the XMM-Newton observations of this cluster. The X-ray data give us the opportunity to reveal the complexity of the cluster, especially its temperature distribution. The integrated spectrum is well fitted by a single temperature thermal model, indicating a mean temperature of ~7 keV. However, the cluster is not isothermal at this temperature: its eastern regions are significantly cooler, at ~5.5 keV, whilst towards the West the temperature reaches ~8.5 keV. These temperature asymmetries can be explained if Abell 2255 has been assembled recently by the merging of smaller subunits. It is now in the phase after the cores of these subunits have collided (the `core-crossing' phase) some 0.1-0.2 Gyr ago. A comparison with numerical simulations suggests that it will settle down into a single relaxed cluster in ~(2-3) Gyr.Comment: 9 pages, 6 figures, accepted by MNRA

X-Ray Wakes in Abell 160

`Wakes' of X-ray emission have now been detected trailing behind a few (at least seven) elliptical galaxies in clusters. To quantify how widespread this phenomenon is, and what its nature might be, we have obtained a deep (70 ksec) X-ray image of the poor cluster Abell 160 using the ROSAT HRI. Combining the X-ray data with optical positions of confirmed cluster members, and applying a statistic designed to search for wake-like excesses, we confirm that this phenomenon is observed in galaxies in this cluster. The probability that the detections arise from chance is less than 0.0038. Further, the wakes are not randomly distributed in direction, but are preferentially oriented pointing away from the cluster centre. This arrangement can be explained by a simple model in which wakes arise from the stripping of their host galaxies' interstellar media due to ram pressure against the intracluster medium through which they travel.Comment: 7 pages, 7 figures, accepted for publication in MNRA

Simulations of the Effects of Stripping and Accretion on Galaxy Haloes in Clusters

We present results from a series of hydrodynamic simulations investigating ram pressure stripping of galactic haloes as the host galaxy falls radially into a cluster. We perform a parameter study comprising of variations in initial gas content, gas injection rate (via stellar mass loss processes), galaxy mass and amplitude of infall. From the simulation results we track variations in both physical quantities (e.g. gas mass) and directly observable quantities (e.g. X-ray luminosities). The luminosity of the galaxy's X-ray halo is found to compare favourably with the observationally determined correlation with optical blue band luminosity (L_X:L_B) relation. Factors affecting the X-ray luminosity are explored and it is found that the gas injection rate is a dominant factor in determining the integrated luminosity. Observational properties of the material stripped from the galaxy, which forms an X-ray wake, are investigated and it is found that wakes are most visible around galaxies with a substantial initial gas content, during their first passage though the cluster. We define a statistical skewness measure which may be used to determine the direction of motion of a galaxy using X-ray observations. Structures formed in these simulations are similar to the cold fronts seen in observation of cluster mergers where a sharp increase in surface brightness is accompanied by a transition to a cooler region.Comment: Accepted for publication in MNRAS. 19 pages, 21 figure

Jet speeds in wide angle tailed radio galaxies

The definitive version is available at www.blackwell-synergy.com. Copyright Blackwell Publishing DOI : 10.1111/j.1365-2966.2006.10155.xPeer reviewe

Galaxy-cluster interactions : bent jets, wakes, and Ram pressure stripping

The work in this thesis is concerned with the physical processes that take place in clusters of galaxies, and in particular the manifestations of the galaxy-intracluster medium (ICM) interactions.The effects of such interactions are seen in wide-angle tailed radio galaxies (WATs), which are radio galaxies whose jets are bent in a wide C-shape. Their appearance can be attributed to the combined action of ram pressure and buoyancy forces. A new method is developed for measuring the galaxy velocity independently from the jet velocity, by solving the detailed hydrodynamical equations for a specific point on the radio jets. The technique is applied to two radio galaxies 4C 34.16 and 3C 354. The jet velocity is also calculated: for both sources it is found to be in the range of (103 - 104) km s-1.The first quantitative attempt is made to explain the sudden flaring and subsequent deceleration of radio jets from relativistic to subrelativistic velocities. According to theoretical models such jet structures could arise when the jets cross the interface between the hot, X-ray emitting interstellar medium (ISM) of the host galaxy and the ICM of any surrounding cluster. Using ROSAT PSPC and HRI data, the underlying, extended X-ray emission is detected from the galaxy which hosts 3C 354. It is shown in order to reach firm conclusions for the position and strength of the ISM/ICM interface in this system X-ray data of higher spectral resolution are required.A sample of WAT sources is compiled in order to investigate the dynamics of their host clusters. The direction in which their jets are bent is used to discover their orbits in the clusters. X-ray observation retrieved from the ROSAT archive are used to determine their properties (accurate position of the cluster centre, size and morphology). WATs are found to lie preferentially on radial orbits heading away from or towards the cluster centre. The results are consistent with the idea that WATs are radio galaxies that originated in poor groups, but are now in the process of falling into richer clusters.Another result of galaxy-ICM interactions is the creation of X-ray wakes, down-stream from each moving galaxy. This phenomenon is detected in the ICM around the WAT source 4C 34.16. Its properties revealed by X-ray observations (density and temperature) are used to measure the projection of the galaxy velocity on the plane of the sky.</p

Jet termination in wide-angle tailed radio sources

Peer reviewe