Chaotic electron diffusion through stochastic webs enhances current flow in superlattices (original) (raw)
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Galaxies and Star Clusters - From the Computer to the Real World
Astronomische Nachrichten, 2005
Recent X-ray observations and theoretical modelling have made it plausible that ultraluminous X-ray sources (ULX) are connected to young stellar populations in star forming galaxies and that they could be powered by intermediate-mass black holes (IMBHs). We have performed N -body simulations of the runaway merging of stars in young, dense star clusters and the later dynamical evolution of the IMBHs in the centers of these clusters. We found that the IMBHs have a high chance of capturing passing stars through tidal heating, thereby explaining the ultraluminous X-ray sources. Our results strengthen the case for some ULX being powered by intermediatemass black holes.
THE DYNAMICAL STATE OF THE GLOBULAR CLUSTER M10 (NGC 6254)
The Astrophysical Journal, 2010
Studying the radial variation of the stellar mass function in globular clusters (GCs) has proved a valuable tool to explore the collisional dynamics leading to mass segregation and core collapse. Recently, Pasquato et al. (2009) used the mass segregation profile to investigate the presence of an intermediate-mass black hole (IMBH) in NGC 2298. As a relaxed cluster with a large core, M 10 (NGC 6254) is suitable for a similar investigation. In order to study the radial dependence of the luminosity and mass function of M 10, we used deep high resolution archival images obtained with the Advanced Camera for Survey (ACS) on board the Hubble Space Telescope (HST), reaching out to approximately the cluster's half-mass radius (r hm ), combined with deep Wide Field and Planetary Camera 2 (WFPC2) images that extend our radial coverage to more than 2 r hm . From our photometry, we derived a radial mass segregation profile and a global mass function that we compared with those of simulated clusters containing different energy sources (namely hard binaries and/or an IMBH) able to halt core collapse and to quench mass segregation. A set of direct N-body simulations of GCs, with and without an IMBH of mass 1% of the total cluster mass, comprising different initial mass functions (IMFs) and primordial binary fractions, was used to predict the observed mass segregation profile and mass function.
The Astrophysical …, 2011
We present a wide field study of the Globular Clusters/Low Mass X-ray Binary (LMXB) connection in the giant elliptical NGC1399. The large FOV of the ACS/WFC, combined with the HST and Chandra high resolution, allow us to constrain the LMXB formation scenarios in elliptical galaxies. We confirm that NGC1399 has the highest LMXB fraction in GCs of all nearby elliptical galaxies studied so far, even though the exact value depends on galactocentric distance due to the interplay of a differential GC vs galaxy light distribution and the GC color dependence. In fact LMXBs are preferentially hosted by bright, red GCs out to > 5 R eff of the galaxy light. The finding that GC hosting LMXBs follow the radial distribution of their parent GC population, argues against the hypothesis that the external dynamical influence of the galaxy affects LMXB formation in GCs. On the other hand field LMXBs closely match the host galaxy light, thus indicating that they are originally formed in situ and not inside GCs. We measure GC structural parameters, finding that the LMXB formation likelihood is influenced independently by mass, metallicity and GCs structural parameters. In particular the GC central density plays a major role in predicting which GC host accreting binaries. Finally our analysis shows that LMXBs in GCs are marginally brighter than those in the field, and in particular the only color-confirmed GC with L X > 10 39 erg s −1 shows no variability, which may indicate a superposition of multiple LMXBs in these systems.
Astron Astrophys, 2009
Context. Classical novae (CNe) have been found to represent the major class of supersoft X-ray sources (SSSs) in our neighbour galaxy M 31. Aims. We determine the properties and evolution of the two first SSSs ever discovered in the M 31 globular cluster (GC) system. Methods. We have used XMM-Newton, Chandra and Swift observations of the centre region of M 31 to discover both SSSs and to determine their X-ray light curves and spectra. We performed detailed analysis of XMM-Newton EPIC PN spectra of the source in Bol 111 (SS1) using blackbody and NLTE white dwarf (WD) atmosphere models. For the SSS in Bol 194 (SS2) we used optical monitoring data to search for an optical counterpart. Results. Both GC X-ray sources were classified as SSS. We identify SS1 with the CN M31N 2007-06b recently discovered in the M 31 GC Bol 111. For SS2 we did not find evidence for a recent nova outburst and can only provide useful constraints on the time of the outburst of a hypothetical nova. Conclusions. The only known CN in a M 31 GC can be identified with the first SSS found in a M 31 GC. We discuss the impact of our observations on the nova rate for the M 31 GC system.
Extreme AGN feedback in the MAssive Cluster Survey: a detailed study of X-ray cavities at z>0.3
Monthly Notices of the Royal Astronomical Society, 2012
We present the first statistical study of X-ray cavities in distant clusters of galaxies (z > 0.3). With the aim of providing further insight into how AGN feedback operates at higher redshift, we have analysed the Chandra X-ray observations of the Massive Cluster Survey (MACS) and searched for surface-brightness depressions associated with the Brightest Cluster Galaxy (BCG). The MACS sample consists of the most X-ray luminous clusters within 0.3 z 0.7 (median L X,RASS = 7 × 10 44 erg s −1 ), and out of 76 clusters, we find 13 with "clear" cavities and 7 with "potential" cavities (detection rate ∼ 25 per cent). Most of the clusters in which we find cavities have a short central cooling time below 3 − 5 Gyrs, consistent with the idea that cavities sit predominantly in cool core clusters. We also find no evidence for evolution in any of the cavity properties with redshift, up to z ∼ 0.6. The cavities of powerful outbursts are not larger (or smaller) at higher redshift, and are not able to rise to further (or lesser) distances from the nucleus. The energetics of these outbursts also remain the same. This suggests that extreme "radio mode" feedback (L mech > 10 44 erg s −1 ) starts to operate as early as 7 − 8 Gyrs after the Big Bang and shows no sign of evolution since then. In other words, AGNs lying at the centre of clusters are able to operate at early times with extreme mechanical powers, and have been operating in such a way for at least the past 5 Gyrs.
We present the first statistical study of X-ray cavities in distant clusters of galaxies (z > 0.3). With the aim of providing further insight into how AGN feedback operates at higher redshift, we have analysed the Chandra X-ray observations of the Massive Cluster Survey (MACS) and searched for surface-brightness depressions associated with the Brightest Cluster Galaxy (BCG). The MACS sample consists of the most X-ray luminous clusters within 0.3 z 0.7 (median L X,RASS = 7 × 10 44 erg s −1 ), and out of 76 clusters, we find 13 with "clear" cavities and 7 with "potential" cavities (detection rate ∼ 25 per cent). Most of the clusters in which we find cavities have a short central cooling time below 3 − 5 Gyrs, consistent with the idea that cavities sit predominantly in cool core clusters. We also find no evidence for evolution in any of the cavity properties with redshift, up to z ∼ 0.6. The cavities of powerful outbursts are not larger (or smaller) at higher redshift, and are not able to rise to further (or lesser) distances from the nucleus. The energetics of these outbursts also remain the same. This suggests that extreme "radio mode" feedback (L mech > 10 44 erg s −1 ) starts to operate as early as 7 − 8 Gyrs after the Big Bang and shows no sign of evolution since then. In other words, AGNs lying at the centre of clusters are able to operate at early times with extreme mechanical powers, and have been operating in such a way for at least the past 5 Gyrs.
The Astronomical Journal, 2002
We study the space distribution of Abell and X-ray selected clusters of galaxies from the ROSAT Bright Source Catalog, and determine correlation functions for both cluster samples. On small scales the correlation functions depend on the cluster environment: clusters in rich superclusters have a larger correlation length and amplitude than all clusters. On large scales correlation functions depend on the distribution of superclusters. On these scales correlation functions for both X-ray and Abell clusters are oscillating with a period of ∼ 115 h −1 Mpc. This property shows the presence of a dominating scale in the distribution of rich superclusters.
Unveiling the Galaxy Cluster - Cosmic Web Connection with X-ray Observations in the Next Decade
BAAS 51, 2019
In recent years, the outskirts of galaxy clusters have emerged as one of the new frontiers and unique laboratories for studying the growth of large scale structure in the universe. Modern cosmological hydrodynamical simulations make firm and testable predictions of the thermody-namic and chemical evolution of the X-ray emitting intracluster medium. However, recent X-ray and Sunyaev-Zeldovich effect observations have revealed enigmatic disagreements with theoretical predictions, which have motivated deeper investigations of a plethora of astrophysical processes operating in the virialization region in the cluster outskirts. Much of the physics of cluster out-skirts is fundamentally different from that of cluster cores, which has been the main focus of X-ray cluster science over the past several decades. A next-generation X-ray telescope, equipped with sub-arcsecond spatial resolution over a large field of view along with a low and stable instrumental background, is required in order to reveal the full story of the growth of galaxy clusters and the cosmic web and their applications for cosmology.
Monthly Notices of the Royal Astronomical Society, 2012
Using a sample of 123 X-ray clusters and groups drawn from the XMM Cluster Survey first data release, we investigate the interplay between the brightest cluster galaxy (BCG), its black hole and the intracluster/group medium (ICM). It appears that for groups and clusters with a BCG likely to host significant active galactic nuclei (AGN) feedback, gas cooling dominates in those with T X > 2 keV while AGN feedback dominates below. This may be understood through the subunity exponent found in the scaling relation we derive between the BCG mass and cluster mass over the halo mass range 10 13 < M 500 < 10 15 M and the lack of correlation between radio luminosity and cluster mass, such that BCG AGN in groups can have relatively more energetic influence on the ICM. The L X -T X relation for systems with the most massive BCGs, or those with BCGs co-located with the peak of the ICM emission, is steeper than that for those with the least massive and most offset, which instead follows self-similarity. This is evidence that a combination of central gas cooling and powerful, well fuelled AGN causes the departure of the ICM from pure gravitational heating, with the steepened relation crossing self-similarity at T X = 2 keV. Importantly, regardless of their black hole mass, BCGs are more likely to host radio-loud AGN if they are in a massive cluster (T X 2 keV) and again co-located with an effective fuel supply of dense, cooling gas. This demonstrates that the most massive black holes appear to know more about their host cluster than they do about their host galaxy. The results lead us to propose a physically motivated, empirical definition of 'cluster' and 'group', delineated at 2 keV.