While a broad profile of the Fe K$alpha$ emission line is frequently found in the X-ray spectra of typical Seyfert galaxies, the situation is unclear in the case of Narrow Line Seyfert 1 galaxies (NLS1s)---an extreme subset which are generally though
t to harbor less massive black holes with higher accretion rates. In this paper, the ensemble property of the Fe K$alpha$ line in NLS1s is investigated by stacking the X-ray spectra of a large sample of 51 NLS1s observed with {it XMM-Newton}. The composite X-ray spectrum reveals a prominent, broad emission feature over 4--7 keV, characteristic of the broad Fe K$alpha$ line. In addition, there is an indication for a possible superimposing narrow (unresolved) line, either emission or absorption, corresponding to Fe XXVI or Fe XXV, respectively. The profile of the broad emission feature can well be fitted with relativistic broad-line models, with the line energy consistent either with 6.4 keV (i.e., neutral Fe) or with 6.67 keV (i.e., highly ionized Fe), in the case of the narrow line being emission and absorption, respectively. Interestingly, there are tentative indications for low or intermediate values of the average spins of the black holes ($a<0.84$), as inferred from the profile of the composite broad line. If the observed feature is indeed a broad line rather than resulting from partial covering absorption, our results suggest that a relativistic Fe line may in fact be common in NLS1s; and there are tentative indications that black holes in NLS1s may not spin very fast in general.
Observations reveal that the peaks of the X-ray map and the Sunyaev-Zeldovich (SZ) effect map of some galaxy clusters are offset from each other. In this paper, we perform a set of hydrodynamical simulations of mergers of two galaxy clusters to inves
tigate the spatial offset between the maxima of the X-ray and the SZ surface brightness of the merging clusters. We find that significantly large SZ-X-ray offsets (>100kpc) can be produced during the major mergers of galaxy clusters. The significantly large offsets are mainly caused by a `jump effect occurred between the primary and secondary pericentric passages of the two merging clusters, during which the X-ray peak may jump to the densest gas region located near the center of the small cluster, but the SZ peak remains near the center of the large one. Our simulations show that merging systems with higher masses and larger initial relative velocities may result in larger offset sizes and longer offset time durations; and only nearly head-on mergers are likely to produce significantly large offsets. We further investigate the statistical distribution of the SZ-X-ray offset sizes and find that (1) the number distribution of the offset sizes is bimodal with one peak located at low offsets ~0 and the other at large offsets ~350-450kpc/h, but the objects with intermediate offsets are scarce; and (2) the probabilities of the clusters in the mass range higher than 2x10^{14}Msun/h that have offsets larger than 20, 50, 200, 300, and 500kpc/h are 34.0%, 11.1%, 8.0%, 6.5%, and 2.0% respectively at z=0.7. The probability is sensitive to the underlying pairwise velocity distribution and the merger rate of clusters. Future observations on the offsets for a large number of clusters may put strong constraints on the cosmic velocity fields on the cluster scale and the cluster merger rate. (Abridged)
In this paper, we study the chemical properties of the stars in the dwarf satellites around the MW-like host galaxies, and explore the possible effects of several baryonic processes, including supernova (SN) feedback, the reionization of the universe
and H$_2$ cooling, on them and how current and future observations may put some constraints on these processes. We use a semi-analytical model to generate MW-like galaxies, for which a fiducial model can reproduce the luminosity function and the stellar metallicity--stellar mass correlation of the MW dwarfs. Using the simulated MW-like galaxies, we focus on investigating three metallicity properties of their dwarfs: the stellar metallicity--stellar mass correlation of the dwarf population, and the metal-poor and metal-rich tails of the stellar metallicity distribution in individual dwarfs. We find that (1) the slope of the stellar metallicity--stellar mass correlation is sensitive to the SN feedback strength and the reionization epoch; (2) the extension of the metal-rich tails is mainly sensitive to the SN feedback strength; (3) the extension of the metal-poor tails is mainly sensitive to the reionization epoch; (4) none of the three chemical properties are sensitive to the H$_2$ cooling process; and (5) comparison of our model results with the current observational slope of the stellar metallicity--stellar mass relation suggests that the local universe is reionized earlier than the cosmic average and local sources may have a significant contribution to the reionization in the local region, and an intermediate to strong SN feedback strength is preferred. Future observations of metal-rich and metal-poor tails of stellar metallicity distributions will put further constraints on the SN feedback and the reionization processes.
We explore a cyclic universe due to phantom and quintessence fields. We find that, in every cycle of the evolution of the universe, the phantom dominates the cosmic early history and quintessence dominates the cosmic far future. In this model of univ
erse, there are infinite cycles of expansion and contraction. Different from the inflationary universe, the corresponding cosmic space-time is geodesically complete and quantum stable. But similar to the Cyclic Model, the flatness problem, the horizon problem and the large scale structure of the universe can be explained in this cyclic universe.
Pulsars, if existing and detectable in the immediate vicinity of the massive black hole (MBH) in the Galactic center (GC), may be used as a superb tool to probe both the environment and the metric of the central MBH. The recent discovery of a magneti
zed pulsar in the GC suggests that many more pulsars should exist near the MBH. In this paper, we estimate the number and the orbital distribution of pulsars in the vicinity of the MBH in the GC by assuming that the pulsar progenitors, similar to the GC S-stars, were captured to orbits tightly bound to the MBH through the tidal breakup of stellar binaries. We use the current observations on both the GC S-stars and the hypervelocity stars to calibrate the injection rate(s) of and the dynamical model(s) for the stellar binaries. By including the relaxation processes, supernova kicks, and gravitational wave radiation in our simulations, we estimate that ~97-190 (9-14) pulsars may presently orbit the central MBH with semimajor axes <=4000AU (<=1000AU), which is compatible with the current observational constraints on the number of the GC pulsars. The semimajor axis and the pericenter distance of the pulsar closest to the central MBH are probably in the range of ~120-460AU and ~2-230AU, respectively. Future telescopes, such as the SKA, may be able to detect a significant number of pulsars with semimajor axis smaller than a few thousand AU in the GC. Long-term monitoring of these pulsars would be helpful in constraining both the environment and the metric of the central MBH. Our preferred model also results in about ten hyperfast pulsars with velocity >~1500km/s moving away from the Milky Way.
Growth of massive black holes (MBHs) in galactic centers comes mainly from gas accretion during their QSO/AGN phases. In this paper we apply an extended Soltan argument, connecting the local MBH mass function with the time-integral of the QSO luminos
ity function, to the demography of MBHs and QSOs from recent optical and X-ray surveys, and obtain robust constraints on the luminosity evolution (or mass growth history) of individual QSOs (or MBHs). We find that the luminosity evolution probably involves two phases: an initial exponentially increasing phase set by the Eddington limit and a following phase in which the luminosity declines with time as a power law (with a slope of -1.2--1.3) set by a self-similar long-term evolution of disk accretion. Neither an evolution involving only the increasing phase with a single Eddington ratio nor an exponentially declining pattern in the second phase is likely. The period of a QSO radiating at a luminosity higher than 10% of its peak value is about (2-3)x10^8 yr, during which the MBH obtains ~80% of its mass. The mass-to-energy conversion efficiency is $0.16pm0.04 ^{+0.05}_{-0}$, with the latter error accounting for the maximum uncertainty due to Compton-thick AGNs. The expected Eddington ratios in QSOs from the constrained luminosity evolution cluster around a single value close to 0.5-1 for high-luminosity QSOs and extend to a wide range of lower values for low-luminosity ones. The Eddington ratios for high luminosity QSOs appear to conflict with those estimated from observations (~0.25) by using some virial mass estimators for MBHs in QSOs unless the estimators systematically over-estimate MBH masses by a factor of 2-4. We also infer the fraction of optically obscured QSOs ~60-80%. Further applications of the luminosity evolution of individual QSOs are also discussed.
The hypervelocity stars recently found in the Galactic halo are expelled from the Galactic center through interactions between binary stars and the central massive black hole or between single stars and a hypothetical massive binary black hole. In th
is paper, we demonstrate that binary stars can be ejected out of the Galactic center with velocities up to 10^3 km/s, while preserving their integrity, through interactions with a massive binary black hole. Binary stars are unlikely to attain such high velocities via scattering by a single massive black hole or through any other mechanisms. Based on the above theoretical prediction, we propose a search for binary systems among the hypervelocity stars. Discovery of hypervelocity binary stars, even one, is a definitive evidence of the existence of a massive binary black hole in the Galactic center.
Within a half-parsec from the Galactic center (GC), there is a population of coeval young stars which appear to reside in a coherent disk. Surrounding this dynamically-cool stellar system, there is a population of stars with a similar age and much la
rger eccentricities and inclinations relative to the disk. We propose a hypothesis for the origin of this dynamical dichotomy. Without specifying any specific mechanism, we consider the possibility that both stellar populations were formed within a disk some 6 Myr ago. But this orderly structure was dynamically perturbed outside-in by an intruding object with a mass ~10^4 Msun, which may be an intermediate-mass black hole (IMBH) or a dark stellar cluster hosting an IMBH. We suggest that the perturber migrated inward to ~0.15-0.3pc from the GC under the action of dynamical friction. Along the way, it captured many stars in the outer disk region into its mean-motion resonance, forced them to migrate with it, closely encountered with them, and induced the growth of their eccentricity and inclination. But stars in the inner regions of the disk retain their initial coplanar structure. We predict that some of the inclined and eccentric stars surrounding the disk may have similar Galactocentric semimajor axis. Future precision determination of their kinematic distribution of these stars will not only provide a test for this hypothesis but also evidences for the presence of an IMBH or a dark cluster at the immediate proximity of the massive black hole at the GC. (abridged)
