arXiv subjects

J. L. Han

Publications and source records attributed to J. L. Han.

At least 145 records · Page 8Linked to original sources

arXiv2011

Stellar populations of ultraluminous infrared galaxies

Ultraluminous infrared galaxies (ULIRGs) have several types according to dominance of starburst or AGN component. We made stellar population analysis for a sample of 160 ULIRGs to study the evolution of ULIRGs. We found that the dominance of intermediate-age and old stellar populations increases along the sequence of HII-like ULIRGs, Seyfert-HII composite ULIRGs, and Seyfert 2 ULIRGs. Consequently the typical mean stellar age and the stellar mass increase along the sequence. Comparing the gas mass estimated from the CO measurements with the stellar mass estimated from the optical spectra, we found that gas fraction is anti-correlated with the stellar mass. HII-like ULIRGs with small stellar masses do not possess enough gas and the total mass, and therefore have no evolution connections with massive Seyfert 2 ULIRGs. Only massive ULIRGs may follow the evolution sequence toward AGNs, and massive HII-like ULIRGs are probably in an earlier stage of the sequence.

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arXiv2011

A Sino-German 6cm polarization survey of the Galactic plane IV. The region from 60deg to 129deg longitude

Linear polarization of diffuse Galactic emission is a signature of magnetic fields in the interstellar medium of our Galaxy. Observations at high frequencies are less affected by Faraday depolarization than those at lower frequencies and are able to detect polarized emission from more distant Galactic regions. We attempt to perform a sensitive survey of the polarized emission from the Galactic disk at 6 cm wavelength. We made polarization observations of the Galactic plane using the Urumqi 25-m telescope at 6 cm covering the area of 60deg< l <129deg and |b|<5deg. Missing large-scale structures in polarization were restored by extrapolation of the WMAP polarization data. We present the 6cm total intensity and linear polarization maps of the surveyed region. We identify two new extended HII regions G98.3-1.6 and G119.6+0.4 in this region. Numerous polarized patches and depolarization structures are visible in the polarization maps. Depolarization along the periphery of a few HII complexes was detected and can be explained by a Faraday screen model. We discuss some prominent depolarization HII regions, which have regular magnetic fields of several $μ$G. Structure functions of U, Q and PI images of the entire 6cm survey region of 10deg< l < 230deg exhibit much larger fluctuation power towards the inner Galaxy, which suggests a higher turbulence in the arm regions of the inner Galaxy. The Sino-German 6cm survey reveals new properties of the diffuse magnetized interstellar medium. The survey is also very useful for studying individual objects such as HII regions, which act as Faraday screens and have high rotation measures and therefore strong regular magnetic fields inside the regions.

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arXiv2011

Gamma-ray Emission from the Vela Pulsar Modeled with the Annular Gap and Core Gap

The Vela pulsar represents a distinct group of γ-ray pulsars. Fermi γ-ray observations reveal that it has two sharp peaks (P1 and P2) in the light curve with a phase separation of 0.42 and a third peak (P3) in the bridge. The location and intensity of P3 are energy-dependent. We use the 3D magnetospheric model for the annular gap and core gap to simulate the γ-ray light curves, phase-averaged and phase-resolved spectra. We found that the acceleration electric field along a field line in the annular gap region decreases with heights. The emission at high energy GeV band is originated from the curvature radiation of accelerated primary particles, while the synchrotron radiation from secondary particles have some contributions to low energy γ-ray band (0.1 - 0.3 GeV). The γ-ray light curve peaks P1 and P2 are generated in the annular gap region near the altitude of null charge surface, whereas P3 and the bridge emission is generated in the core gap region. The intensity and location of P3 at different energy bands depend on the emission altitudes. The radio emission from the Vela pulsar should be generated in a high-altitude narrow regions of the annular gap, which leads to a radio phase lag of ~ 0.13 prior to the first γ-ray peak.

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arXiv2010

A Sino-German 6 cm polarization survey of the Galactic plane III. The region from 10 degree to 60 degree longitude

(Abridged) We have conducted a total intensity and polarization survey of the Galactic plane at 6 cm using the Urumqi 25 m telescope for the Galactic longitude range of 10 deg<l<60 deg and the Galactic latitude range of |b|<5 deg. Missing absolute zero levels of Stokes U and Q maps were restored by extrapolating the WMAP five-year K-band polarization data. For total intensities we recovered missing large-scale components by referring to the Effelsberg 11 cm survey. Total intensity and polarization maps are presented with an angular resolution of 9.5 arcmin and a sensitivity of 1 mK and 0.5 mK in total and polarized intensity, respectively. The 6 cm polarized emission in the Galactic plane originates within about 4 kpc distance, which increases for polarized emission out of the plane. The polarization map shows "patches", "canals" and "voids" with no correspondence in total intensity. We attribute the patches to turbulent magnetic field cells. Canals are caused by abrupt variation of polarization angles at the boundaries of patches rather than by foreground Faraday Screens. The superposition of foreground and Faraday Screen rotated background emission almost cancels polarized emission locally, so that polarization voids appear. By modelling the voids, we estimate the Faraday Screen's regular magnetic field along the line-of-sight to be larger than about 8 microG. We separated thermal (free-free) and non-thermal (synchrotron) emission according to their different spectral indices. The spectral index for the synchrotron emission was based on WMAP polarization data. The fraction of thermal emission at 6 cm is about 60% in the plane.

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arXiv2010

Estimating black hole masses of blazars

Estimating black hole masses of blazars is still a big challenge. Because of the contamination of jets, using the previously suggested size -- continuum luminosity relation can overestimate the broad line region (BLR) size and black hole mass for radio-loud AGNs, including blazars. We propose a new relation between the BLR size and $H_β$ emission line luminosity and present evidences for using it to get more accurate black hole masses of radio-loud AGNs. For extremely radio-loud AGNs such as blazars with weak/absent emission lines, we suggest to use the fundamental plane relation of their elliptical host galaxies to estimate the central velocity dispersions and black hole masses, if their velocity dispersions are not known but the host galaxies can be mapped. The black hole masses of some well-known blazars, such as OJ 287, AO 0235+164 and 3C 66B, are obtained using these two methods and the M - $σ$ relation. The implications of their black hole masses on other related studies are also discussed.

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arXiv2010

Radio sources with ultra-high polarization

A sample of 129 unresolved radio sources with ultrahigh linear polarization (>30 per cent) has been selected from the NRAO VLA Sky Survey. Such high average linear polarization is unusual in extragalactic sources. Higher resolution Australia Telescope Compact Array and Very Large Array observations confirm the high average polarization but find that most of these sources are extended. The Sloan Digital Sky Survey spectroscopy, where available, shows that the optical counterparts are elliptical galaxies with no detectable emission lines. The optical spectra, radio luminosity, linear size and spectral index of these sources are typical of radio-loud active galactic nuclei. Galaxy counts within a 1 Mpc radius of the radio sources show that these highly polarized sources are in environments similar to their low polarization (<2 per cent) counterparts. Similarly, the line-of-sight environments of the ultrahigh polarization sources are on average indistinguishable from those of the low-polarization sources. We conclude that the extraordinarily high average polarization must be due to intrinsic properties of the sources, such as an extremely ordered source magnetic field, low internal thermal plasma density or a preferential orientation of the source magnetic field perpendicular to the line of sight.

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arXiv2010

Lower bounds of altitudes for pulsar $γ$-ray radiation

Determining radiation location observationally plays a very important role in testing the pulsar radiation models. One-photon pair production in the strong magnetic field, $γ-e^{+}e^{1}$, is one of the important physical processes in pulsar radiation mechanisms. Photons near pulsar surface with sufficient energy will be absorbed in the magnetosphere and the absorption optical depth for these GeV $γ$-ray photons is usually large. In this paper, we include the aberrational, rotational and general relativistic effects and calculate the $γ$-B optical depth for $γ$-ray photons. Then we use the derived optical depth to determine the radiation altitude lower bounds for photons with given energies. As a case study, we calculate the lower bounds of radiation altitudes of Crab pulsar for photons with energy from 5 GeV to 1 TeV.

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arXiv2010

Do Ultrahigh Energy Cosmic Rays Come from Active Galactic Nuclei and Fermi $γ$-ray Sources?

We study possible correlations between ultrahigh energy cosmic rays (UHECRs), observed by Auger, AGASA and Yakutsk, and nearby active galactic nuclei (AGNs) and $Fermi$ sources. We consider the deflection effects by a Galactic magnetic field (GMF) model constrained by the most updated measurements. We found that the average deflection angles of UHECRs by the Galactic magnetic fields are less than $4^\circ$. A correlation between the Auger cosmic-ray events and nearby AGNs with a significance level of $\sim 4σ$ was found for the Auger UHECR data sets with or without deflection correction. No correlation was found between the AGASA/Yakutsk events with nearby AGNs. Marginal correlations between the Auger events and the $Fermi$ sources, and between AGASA events and $Fermi$ AGNs were found when the deflections calculated by the GMF model were considered. However, no correlation was found between the Yakutsk data and $Fermi$ sources. Some $Fermi$ sources are close to the arrival directions of UHECR events detected by Auger, AGASA, and Yakutsk, most of which are probably chance coincidence rather than objects producing UHECRs in the nearby Universe. Four $Fermi$ sources, NGC 4945, ESO 323-G77, NGC 6951, and Cen A, within 100~Mpc have UHECR events within $3.1^{\circ}$ from their positions, which could potentially be cosmic ray accelerators. However, the association can only be confirmed if more UHECRs are preferably detected in these directions.

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arXiv2010

Mass Function of Rich Galaxy Clusters and Its Constraint on sigma_8

The mass function of galaxy clusters is a powerful tool to constrain cosmological parameters, e.g., the mass fluctuation on the scale of 8 h^{-1} Mpc, sigma_8, and the abundance of total matter, Omega_m. We first determine the scaling relations between cluster mass and cluster richness, summed r-band luminosity and the global galaxy number within a cluster radius. These relations are then used to two complete volume-limited rich cluster samples which we obtained from the Sloan Digital Sky Survey (SDSS). We estimate the masses of these clusters and determine the cluster mass function. Fitting the data with a theoretical expression, we get the cosmological parameter constraints in the form of sigma_8(Omega_m/0.3)^{alpha}=beta and find out the parameters of alpha=0.40-0.50 and beta=0.8-0.9, so that sigma_8=0.8-0.9 if Omega_m=0.3. Our sigma_8 value is slightly higher than recent estimates from the mass function of X-ray clusters and the Wilkinson Microwave Anisotropy Probe (WMAP) data, but consistent with the weak lensing statistics.

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arXiv2010

A Sino-German 6 cm polarization survey of the Galactic plane II. The region from 129 degree to 230 degree longitude

Linearly polarized Galactic synchrotron emission provides valuable information about the properties of the Galactic magnetic field and the interstellar magneto-ionic medium, when Faraday rotation along the line of sight is properly taken into account. We aim to survey the Galactic plane at 6 cm including linear polarization. At such a short wavelength Faraday rotation effects are in general small and the Galactic magnetic field properties can be probed to larger distances than at long wavelengths. The Urumqi 25-m telescope is used for a sensitive 6 cm survey in total and polarized intensities. WMAP K-band (22.8 GHz) polarization data are used to restore the absolute zero-level of the Urumqi U and Q maps by extrapolation. Total intensity and polarization maps are presented for a Galactic plane region of 129 degree < l < 230 degree and |b| < 5 degree in the anti-centre with an angular resolution of 9'5 and an average sensitivity of 0.6 mK and 0.4 mK Tb in total and polarized intensity, respectively. We briefly discuss the properties of some extended Faraday Screens detected in the 6 cm polarization maps. The Sino-German 6 cm polarization survey provides new information about the properties of the magnetic ISM. The survey also adds valuable information for discrete Galactic objects and is in particular suited to detect extended Faraday Screens with large rotation measures hosting strong regular magnetic fields.

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arXiv2010

The annular gap model for gamma-ray emission from young and millisecond pulsars

Pulsed high energy radiation from pulsars is not yet completely understood. In this paper, we use the 3D self-consistent annular gap model to study light curves for both young and millisecond pulsars observed by the Fermi Gamma-ray Space Telescope. The annular gap can generate high energy emission for short-period pulsars. The annular gap regions are so large that they have enough electric potential drop to accelerate charged particles to produce gamma-ray photons. For young pulsars, the emission region is from the neutron star surface to about half of the light cylinder radius, and the peak emissivity is in the vicinity of the null charge surface. The emission region for the millisecond pulsars is located much lower than that of the young pulsars. The higher energy gamma-ray emission comes from higher altitudes in the magnetosphere. We present the simulated light curves for three young pulsars (the Crab, the Vela, the Geminga) and three millisecond pulsars (PSR J0030+0451, PSR J0218+4232, PSR J0437-3715) using the annular gap model. Our simulations can reproduce the main properties of observed light curves.

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arXiv2010

Nature of W51e2: Massive Cores at Different Phases of Star Formation

We present high-resolution continuum images of the W51e2 complex processed from archival data of the Submillimeter Array (SMA) at 0.85 and 1.3 mm and the Very Large Array (VLA) at 7 and 13 mm. We also made line images and profiles of W51e2 for three hydrogen radio recombination lines (H26α, H53α, and H66α) and absorption of two molecular lines of HCN(4-3) and CO(2-1). At least four distinct continuum components have been detected in the 3" region of W51e2 from the SMA continuum images at 0.85 and 1.3 mm with resolutions of 0.3"x0.2" and 1.4"x0.7", respectively. The west component, W51e2-W, coincides with the UC HII region reported from previous radio observations. The H26αline observation reveals an unresolved hyper-compact ionized core (<0.06" or <310 AU) with a high electron temperature of 1.2x10^4 K, with corresponding emission measure EM>7x10^{10} pc cm^{-6} and electron density N_e>7x10^6 cm^{-3}. The inferred Lyman continuum flux implies that the HII region W51e2-W requires a newly formed massive star, an O8 star or a cluster of B-type stars, to maintain the ionization. The east component, W51e2-E, has a total mass of ~140 M_{\sun} according to our SED analysis and a large infall rate of > 1.3x10^{-3} M_{\sun}yr^{-1} inferred from the absorption of HCN. W51e2-E appears to be the accretion center in W51e2 and to host one or more growing massive proto-stars. Located 2" northwest from W51e2-E, W51e2-NW is not detected in the continuum emission at λ>=7 mm. Along with the maser activities previously observed, our analysis suggests that W51e2-NW is at an earlier phase of star formation. W51e2-N is located 2" north of W51e2-E and has only been detected at 1.3 mm with a lower angular resolution (~1"), suggesting that it is a primordial, massive gas clump in the W51e2 complex.

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arXiv2009

Galaxy clusters identified from the SDSS DR6 and their properties

Clusters of galaxies in most previous catalogs have redshifts z<0.3. Using the photometric redshifts of galaxies from the Sloan Digital Sky Survey Data Release 6 (SDSS DR6), we identify 39,668 clusters in the redshift range 0.05< z <0.6 with more than eight luminous (M_r<-21) member galaxies. Cluster redshifts are estimated accurately with an uncertainty less than 0.022. The contamination rate of member galaxies is found to be roughly 20%, and the completeness of member galaxy detection reaches to ~90%. Monte Carlo simulations show that the cluster detection rate is more than 90% for massive (M_{200}>2\times10^{14} M_{\odot}) clusters of z<0.42. The false detection rate is ~5%. We obtain the richness, the summed luminosity, and the gross galaxy number within the determined radius for identified clusters. They are tightly related to the X-ray luminosity and temperature of clusters. Cluster mass is related to the richness and summed luminosity with M_{200}\propto R^{1.90\pm0.04} and M_{200}\propto L_r^{1.64\pm0.03}, respectively. In addition, 685 new candidates of X-ray clusters are found by cross-identification of our clusters with the source list of the ROSAT X-ray survey.

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arXiv2009

Mergers of luminous early-type galaxies

We have made a careful selection of a large complete volume-limited sample (1209) of projected close pairs (7<r_p<50 kpc) of luminous early-type galaxies (M_r<-21.5) in the local universe (z<0.12) from the SDSS data. 249 (21%) of them show interaction features, which suggests that about 0.8% of the galaxies are merging. We derived a comoving volume merger rate of ~(1.0+/-0.4)\times 10^{-5} Mpc^{-3} Gyr^{-1} for luminous early-type galaxies. This is a direct observational determination of the merger rate of luminous galaxies in the local universe. We also obtained the chirp mass distribution of supermassive black hole (SMBH) binary following log[Phi(log M /M_{odot})]=(21.7+/-4.2)-(3.0+/-0.5)log M/M_{odot}. With less assumptions than previous works, we estimated the strain amplitude of the gravitational wave (GW) background from coalescence of SMBH binaries at frequency 10^{-9}-10^{-7} Hz.

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arXiv2009

Ultra-luminous Infrared Galaxies in Sloan Digital Sky Survey Data Release 6

Ultra-luminous infrared galaxies (ULIRGs) are interesting objects with dramatic properties. Many efforts have been made to understand the physics of their luminous infrared emission and evolutionary stages. However, a large ULIRG sample is still needed to study the properties of their central black holes (BHs), the BH-host galaxy relation, and their evolution. We identified 308 ULIRGs from the Sloan Digital Sky Survey Data Release 6, and classified them into the NL ULIRGs (with only narrow emission lines) and the Type I ULIRGs (with broad emission lines). About 56% of ULIRGs in our total sample show interaction features, and this percentage is 79% for redshift z < 0.2. Optical identifications of these ULIRGs show that the AGN percentage is at least 49%, and the percentage increases with the infrared luminosity. We found 62 Type I ULIRGs, and estimated their BH masses and velocity dispersions from their optical spectra. Together with known Type I ULIRGs in the literature, a sample of 90 Type I ULIRGs enables us to make a statistical study. We found that the BH masses of Type I ULIRGs are typically smaller than those of PG QSOs, and most Type I ULIRGs follow the M_{BH}-sigma relation. However, some ULIRGs with larger Eddington ratio deviate from this relation, even the line width of the [OIII] narrow line (NL) core or the [SII] line was used as the surrogate of velocity dispersion. This implies that at least some ULIRGs are probably still in the early evolution stage toward QSOs. The anti-correlation between the mass deviation from the M_{BH}-sigma relation and the Eddington ratio supports that the evolution of Type I ULIRGs is probably followed by the building up of the M_{BH}-sigma relation and the evolution to the QSO phase.

astro-ph.CO↗

arXiv2009

Formation of Sub-millisecond Pulsars and Possibility of Detection

Pulsars have been recognized as normal neutron stars, but sometimes argued as quark stars. {\it Sub-millisecond pulsars, if detected, would play an essential and important role in distinguishing quark stars from neutron stars.} We focus on the formation of such sub-millisecond pulsars in this paper. A new approach to form a sub-millisecond pulsar (quark star) via accretion induced collapse (AIC) of a white dwarf is investigated here. Under this AIC process, we found that: (1) almost all the newborn quark stars could have an initial spin period of $\sim 0.1$ ms; (2) the nascent quark stars (even with a low mass) have sufficiently high spin-down luminosity and satisfy the conditions for pair production and sparking process to be as sub-millisecond radio pulsars; (3) in most cases, the timescales of newborn quark stars in the phase of spin period $< 1$ (or $<0.5$) ms can be long enough to be detected. As a comparison, an accretion spin-up process (for both neutron and quark stars) is also investigated. It is found that, quark stars formed through AIC process can have shorter periods ($\leq$ 0.5 ms); while the periods of neutron stars formed in accretion spin-up process must be longer than 0.5ms. Thus if a pulsar with a period less than 0.5 ms can be identified in the future, it should be a quark star.

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arXiv2009

Radio properties of the low surface brightness SNR G65.2+5.7

Many physical properties of this SNR such as spectrum and polarization can only be investigated by radio observations. The $λ$11 cm and $λ$6\ cm continuum and polarization observations of SNR G65.2+5.7 were made with the Effelsberg 100-m and the Urumqi 25-m telescopes, respectively, to investigate the integrated spectrum, the spectral index distribution, and the magnetic field properties. $λ$21 cm archival data from the Effelsberg 100-m telescope have been also used. The integrated flux densities of G65.2+5.7 at $λ11$ cm and $λ6$ cm are $21.9\pm3.1$ Jy and 16.8$\pm$1.8 Jy, respectively. The power-law spectrum ($S\simν^α$) is well fitted by $α= -0.58\pm0.07$ from 83 MHz to 4.8 GHz. Spatial spectral variations are small. Along the northern shell strong depolarizion is observed at both wavelengths. The southern filamentary shell of SNR G65.2+5.7 is polarized up to 54% at $λ6$ cm. There is significant depolarization at $λ11$ cm and confusion with diffuse polarized Galactic emission. Using equipartition principle, we estimated the magnetic field strength for the southern filamentary shell about 20 $μ$G (filling factor 1) to 50 $μ$G (filling factor 0.1). A faint HI shell may be associated with the SNR. Despite its unusual strong X-ray and optical emission and its very low surface brightness, the radio properties of SNR G65.2+5.7 are found to be typical for evolved shell type SNRs. SNR G65.2+5.7 may be expanding in a preblown cavity as indicated by a deficit of HI gas and a possible HI-shell.

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arXiv2009

The spiral structure of our Milky Way Galaxy

The spiral structure of our Milky Way Galaxy is not yet known. HII regions and giant molecular clouds are the most prominent spiral tracers. We collected the spiral tracer data of our Milky Way from the literature, namely, HII regions and giant molecular clouds (GMCs). With weighting factors based on the excitation parameters of HII regions or the masses of GMCs, we fitted the distribution of these tracers with models of two, three, four spiral-arms or polynomial spiral arms. The distances of tracers, if not available from stellar or direct measurements, were estimated kinetically from the standard rotation curve of Brand & Blitz (1993) with $R_0$=8.5 kpc, and $Θ_0$=220 km s$^{-1}$ or the newly fitted rotation curves with $R_0$=8.0 kpc and $Θ_0$=220 km s$^{-1}$ or $R_0$=8.4 kpc and $Θ_0$=254 km s$^{-1}$. We found that the two-arm logarithmic model cannot fit the data in many regions. The three- and the four-arm logarithmic models are able to connect most tracers. However, at least two observed tangential directions cannot be matched by the three- or four-arm model. We composed a polynomial spiral arm model, which can not only fit the tracer distribution but also match observed tangential directions. Using new rotation curves with $R_0$=8.0 kpc and $Θ_0$=220 km s$^{-1}$ and $R_0$=8.4 kpc and $Θ_0$=254 km s$^{-1}$ for the estimation of kinematic distances, we found that the distribution of HII regions and GMCs can fit the models well, although the results do not change significantly compared to the parameters with the standard $R_0$ and $Θ_0$.

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