Chorng-Yuan Hwang

In-situ acceleration of subrelativistic electrons in the Coma halo and the halo's influence on the Sunyaev-Zeldovich effect

Aims. The stochastic acceleration of subrelativistic electrons from a background plasma is studied in order to find a possible explanation of the hard X-ray emission detected from the Coma cluster. Methods. We calculate the necessary energy supply as a function of the plasma temperature and of the electron energy, and we show that, for the same value of the hard X-ray flux, the energy supply changes gradually from its high value for the case when emitting particle are non-thermal to lower values when the electrons are thermal. The kinetic equations we use include terms describing particle thermalization as well as momentum diffusion due to the Fermi II acceleration. Results. We show that the temporal evolution of the particle distribution function has, at its final stationary stage, a rather specific form. This distribution function cannot be described by simple exponential or power-law expressions. A broad transfer region is formed by Coulomb collisions at energies between the Maxwellian and power-law parts of the distribution functions. In this region the radiative lifetime of a single quasi-thermal electron differs greatly from the lifetime of the distribution function as a whole. For a plasma temperature of 8 keV, the particles emitting bremsstrahlung at 20 − 80 keV lie in this quasi-thermal regime. We show that the energy supply required by quasi-thermal electrons to produce the observed hard X-ray flux from Coma is one or two orders of magnitude smaller than the value derived from the assumption of a nonthermal origin of the emitting particles. This result may solve the problem of rapid cluster overheating by nonthermal electrons raised by Petrosian (2001): while Petrosian’s estimates are correct for nonthermal particles, they are inapplicable in the quasi-thermal range. We finally analyze the change in Coma’s Sunyaev-Zeldovich effect caused by the implied distortions of the Maxwellian spectrum of electrons, and we show that evidence for the acceleration of subrelativistic electrons can, in principle, be derived from detailed spectral measurements.

The Evolution of Diffuse Radio Sources in Galaxy Clusters

We investigate the evolution and number distribution of radio halos in galaxy clusters. Without reacceleration or regeneration, the relativistic electrons responsible for the diffuse radio emission will lose their energy via inverse Compton and synchrotron losses in a rather short time, and radio halos will have lifetimes ~0.1 Gyr. Radio halos could last for ~Gyr if a significant level of reacceleration is involved. The lifetimes of radio halos would be comparable with the cosmological time if the radio-emitting electrons are mainly the secondary electrons generated by pion decay following proton-proton collisions between cosmic-ray protons and the thermal intracluster medium within the galaxy clusters. Adopting both observational and theoretical constraints for the formation of radio halos, we calculate the formation rates and the comoving number density of radio halos in the hierarchical clustering scheme. Comparing with observations, we find that the lifetimes of radio halos are ~Gyr. Our results indicate that a significant level of reacceleration is necessary for the observed radio halos and the secondary electrons may not be a dominant origin for radio halos.

PROBING CIRCUMNUCLEAR ENVIRONMENTS WITH THE HCN(J = 3-2) AND HCO+(J = 3-2) LINES: CASE OF NGC 1097

We present the first interferometric HCN(J = 3-2) and HCO+(J = 3-2) maps in the circumnuclear region of NGC 1097, obtained with the Submillimeter Array. The goal is to study the characteristics of the dense gas associated with the starburst ring/Seyfert nucleus. With these transitions, we suppress the diffuse low density emission in the nuclear region. We detect and resolve the individual compact giant molecular cloud associations (GMAs) in the 1.4 kpc circumnuclear starburst ring and within the 350 pc nuclear region. The nucleus is brighter than the ring in both lines, and contributes ~20% and ~30% to the total detected HCO+(J = 3-2) and HCN(J = 3-2) flux, within the central 1.4 kpc. The intensity ratios of HCN(J = 3-2)/HCO+(J = 3-2) are roughly unity in the GMAs of the starburst ring. However, this ratio is up to ~2 in the nuclear region. From the HCN(J = 3-2)/HCN(J = 1-0) ratio of ≤0.2 in the nucleus, we infer that the nuclear HCN(J = 3-2) emission might be optically thin. The HCO+(J = 3-2) and HCN(J = 3-2) show correlations with 12CO(J = 3-2) and the 24 μm emission. The tight correlations of HCN(J = 3-2), HCO+(J = 3-2), and 24 μm emission in the starburst ring suggest that the dense molecular gas and the dust are from the same origins of star-forming regions. On the other hand, the HCN(J = 3-2) emission of the nucleus is significantly enhanced, indicating mechanisms other than star formation, such as active galactic nucleus activities. A self-consistent check of the fractional abundance enhanced by X-ray ionization chemistry of the nucleus is possible with our observations.

Detection of an X-ray pulsation for the Gamma-ray pulsar centered in CTA 1

We report the detection of X-ray pulsations with a period of ~315.87 ms from the 2009 XMM-Newton observation for the radio-quiet γ-ray pulsar, LAT PSR J0007+7303, centered in the supernova remnant CTA 1. The detected pulsed period is consistent with the γ-ray periodicity at the same epoch found with the Fermi Gamma-ray Space Telescope. The broader sinusoidal structure in the folded light curve of the X-ray emission is dissimilar to that of the γ-ray emission, and the phase of the peak is about 0.5 shifting from the peak in the γ-ray bands, indicating that the main component of the X-rays originates from different sites of the pulsar. We conclude that the main component of the X-ray pulsation is contributed by the thermal emission from the neutron star. Although with a significantly different characteristic age, PSR J0007+7303 is similar to Geminga in emission properties of X-rays and γ-rays; this makes PSR J0007+7303 the second radio-quiet γ-ray pulsar with detected X-ray pulsations after Geminga.

The Contribution to the Extragalactic Gamma-Ray Background by Hadronic Interactions of Cosmic Rays Producing Extreme-Ultraviolet Emission in Clusters of Galaxies

A substantial number of processes have been suggested as possible contributors to the extragalactic γ-ray background (EGRB). Yet another contribution to this background will be emission produced in hadronic interactions of cosmic-ray protons with the cluster thermal gas; this class of cosmic rays (CRs) has been shown to be responsible for the EUV emission in the Coma Cluster of galaxies. In this paper we estimate this contribution. We first derive an improved estimate of the γ-ray emission produced by this population of CRs in the Coma Cluster. We consider the impact of different strength shocks in the production of this flux. We then assume the that CRs in the Coma Cluster are prototypical of all clusters and derive the contribution to the EGRB from all clusters over time. We examine two different possibilities for the scaling of the CR flux with cluster size: the number density of the CRs scales with the number density of the thermal plasma, and alternatively, the energy density of the CRs scales with the energy density of the plasma. We find that in all scenarios the EGRB produced by this process is sufficiently low that it will not be observable in comparison with other mechanisms that are likely to produce an EGRB. Nonetheless, it is quite likely that future missions will be able to detect emission produced by this process in large clusters of galaxies. In particular, a signature of this process should be observable in the Coma Cluster with instruments on the GLAST satellite. We show that the spectral shape of this emission in the Coma Cluster may well allow a determination as to whether strong or mild shocks are primarily responsible for the production of CRs in this cluster.

Constraints Imposed by a High Magnetic Field on Models for the Extreme-Ultraviolet Emission in the Coma Cluster

A variety of models have been explored in regard to the origin of the excess extreme-ultraviolet (~0.1 keV) emission in the Coma cluster. It is now established that the flux is nonthermal, and the only nonthermal source mechanism that appears viable is inverse Compton emission produced by ~100 MeV electrons interacting with the cosmic microwave background photons. All but one of the models that have been proposed require a cluster magnetic field less than 1 μG. However, recent observations strongly suggest that the magnetic field in the Coma cluster is ~5 μG. We investigate the constraints on models imposed by a 5 μG cluster field and find a limited class of models that are compatible with this constraint. We also investigate the possibility that the excess hard (40-60 keV) X-ray emission in the cluster is produced by inverse Compton emission with the same electron population that produces the EUV excess. We find no scenarios that are compatible with a large cluster magnetic field, and consequently, in this case, these two components must be unrelated.

IONIZATION MECHANISMS OF HBLR AND NON-HBLR SEYFERT 2 GALAXIES

We investigate the ionization mechanisms for hidden broad-line region (HBLR) and non-HBLR Seyfert 2 galaxies by comparing some optical emission line ratios. We note that the [N II] {lambda}6583/H{alpha} ratio of the non-HBLR Seyfert 2 galaxies is significantly higher than that of the HBLR Seyfert 2 galaxies while other line ratios, such as [O III]/H{beta} and [O I]/H{alpha}, are similar. To probe the origin of this difference, we explore theoretical results of different ionization models, such as photoionization, starburst, and shock models. We find that none of these models can explain the high [N II] {lambda}6583/H{alpha} ratio of the non-HBLR Seyfert 2 galaxies with solar abundance; the high [N II] {lambda}6583/H{alpha} must be reproduced from enhanced nitrogen abundance. Since nitrogen overabundance can be achieved from the dredge-up of red supergiants in the post-main-sequence stage, we suggest that the observed nitrogen overabundance of the non-HBLR Seyfert 2 galaxies might be caused by stellar evolution, and there could be an evolutionary connection between the HBLR and non-HBLR Seyfert 2 galaxies.

DUST MORPHOLOGY OF HIDDEN BROAD-LINE REGION AND NON-HIDDEN BROAD-LINE REGION SEYFERT 2 GALAXIES

We investigate the nuclear dust properties of hidden broad-line region (HBLR) and non-HBLR Seyfert 2 galaxies. Optical images obtained from the Hubble Space Telescope for a selected sample of HBLR and non-HBLR Seyfert 2 galaxies are fitted with the Galfit package to probe the inner structures of these galaxies within the central 1 kpc regions. Most of the galaxies show complicated dust features in these regions. However, the dust morphology shows no significant difference between the HBLR and non-HBLR Seyfert 2 galaxies. Dust masses inside the 1 kpc nuclear regions (M1 kpc) are estimated from the obscuration levels in the central regions of these galaxies. We compare our results with other observed properties, including [O III], far-infrared, and radio emission. We find that the HBLR and non-HBLR Seyfert 2 galaxies show different near-infrared colors and M1 kpc-FIR correlations, indicating that these two classes of Seyfert 2 galaxies are dominated by different emission mechanisms. We suggest that they are intrinsically different and cannot be explained by the standard unification model.

TESTING THE EVOLUTIONARY SEQUENCE BETWEEN HIDDEN BROAD-LINE REGION (HBLR) AND NON-HBLR SEYFERT 2 GALAXIES WITH THE 4000 A BREAK STRENGTHS

We compare the 4000 A break (Dn (4000)) strength in the central kpc of hidden broad-line region (HBLR) and non-HBLR Seyfert 2 galaxies to investigate the origin of these galaxies. Our results show that the Dn (4000) strengths in the nuclear regions of the non-HBLR Seyfert 2 galaxies are larger than those in the HBLR galaxies. We also show that the Dn (4000) strength is not related to the morphology of host galaxies. These results imply that the non-HBLR Seyfert 2 galaxies have an older stellar population in nuclear regions than the HBLR galaxies. This suggests that an evolutionary connection might exist between non-HBLR and HBLR Seyfert 2 galaxies. We propose a potential evolutionary scenario and a modified unification model for Seyfert galaxies. In this scheme, Seyfert 1 and HBLR Seyfert 2 galaxies evolve into unabsorbed and absorbed non-HBLR Seyfert 2 galaxies. We also discuss the implications of our results in the hydrogen column density distribution of the non-HBLR Seyfert 2 galaxies.

A CATALOG OF MORPHOLOGICALLY IDENTIFIED MERGING GALAXIES

We present a new catalog and images of interacting and merging galaxies obtained with morphological identification from Canada-France-Hawaii Telescope observations. We first used a morphological pattern recognition searching routine to select possible candidates, and then visually inspected the images of these candidates to identify true merging/interacting sources. We discover 15,147 new pairs of merging galaxies within 422 deg{sup 2} of Red Sequence Cluster Survey 2. Furthermore, we also find nine new candidates of galaxy clusters by searching for regions with significant density enhancements of merging galaxies. This catalog has the largest number of morphologically identified interacting and merging galaxies based on consistent searching criteria. These sources provide a uniform sample of merging galaxies for further photometric and spectroscopic studies of galaxy evolutio000.