arXiv subjects

C. Xu

Publications and source records attributed to C. Xu.

70 records · Page 4Linked to original sources

arXiv2004

Spectral energy distributions and luminosities of galaxies and AGN in the SPITZER SWIRE Legacy Survey

We discuss optical associations, spectral energy distributions and photometric redshifts for SWIRE sources in the ELAIS-N1 area and the Lockman Validation Field. The band-merged IRAC (3.6, 4.5, 5.8 and 8.0 mu) and MIPS (24, 70, 160 mu) data have been associated with optical UgriZ data from the INT Wide Field Survey in ELAIS-N1, and with our own optical Ugri data in Lockman-VF. The spectral energy distributions of selected ELAIS sources in N1 detected by SWIRE, most with spectroscopic redshifts, are modelled in terms of a simple set of galaxy and quasar templates in the optical and near infrared, and with a set of dust emission templates (cirrus, M82 starburst, Arp 220 starburst, and AGN dust torus) in the mid infrared. The optical data, together with the IRAC 3.6 and 4.5 mu data, have been used to determine photometric redshifts. For galaxies with known spectroscopic redshifts there is a notable improvement in the photometric redshift when the IRAC data are used, with a reduction in the rms scatter from 10% in (1+z) to 7%. The photometric redshifts are used to derive the 3.6 and 24 mu redshift distribution and to compare this with the predictions of models. For those sources with a clear mid infrared excess, relative to the galaxy starlight model used for the optical and near infrared, the mid and far infrared data are modelled in terms of the same dust emission templates. The proportions found of each template type are: cirrus 31%, M82 29%, Arp 220 10%, AGN dust tori 29%. The distribution of the different infrared sed types in the L_{ir}/L_{opt} versus L_{ir} plane, where L_{ir} and L_{opt} are the infrared and optical bolometric luminosities, is discussed.

astro-ph↗

arXiv2004

Stars in the Hubble Ultra Deep Field

We identified 46 unresolved source candidates in the Hubble Ultra Deep Field, down to i775 = 29.5. Unresolved objects were identified using a parameter S, which measures the deviation from the curve-of-growth of a point source. Extensive testing of this parameter was carried out, including the effects of decreasing signal-to-noise and of the apparent motions of stars, which demonstrated that stars brighter than i775 = 27.0 could be robustly identified. Low resolution grism spectra of the 28 objects brighter than i775 = 27.0 identify 18 M and later stellar type dwarfs, 2 candidate L-dwarfs, 2 QSOs, and 4 white dwarfs. Using the observed population of dwarfs with spectral type M4 or later, we derive a Galactic disk scale height of 400 \pm 100 pc for M and L stars. The local white dwarf density is computed to be as high as (1.1 \pm 0.3) x10^(-2) stars/pc^3. Based on observations taken 73 days apart, we determined that no object in the field has a proper motion larger than 0.027"/year (3 sigma detection limit). No high velocity white dwarfs were identified in the HUDF, and all four candidates appear more likely to be part of the Galactic thick disk. The lack of detected halo white dwarfs implies that, if the dark matter halo is 12 Gyr old, white dwarfs account for less than 10% of the dark matter halo mass.

astro-ph↗

arXiv2004

GRAPES, Grism Spectroscopy of the Hubble Ultra Deep Field: Description and Data Reduction

We present deep unbiased spectroscopy of the Hubble Ultra Deep Field (UDF) carried out using the slitless grism spectroscopy mode of the Advance Camera for Surveys on the Hubble Space Telescope (HST). The GRIsm ACS Program for Extragalactic Science (GRAPES) achieves continuum detection as faint as $z_{AB}=27.2$ using 40 orbits ($9.2 \times 10^4$ seconds) on HST. The data were taken at four orientation angles to correct for the overlap of spectra. GRAPES data provide a unique, uninterrupted, low resolution (R=100) spectral coverage for $5500Å< λ< 10500Å$, and allow us to detect high redshift galaxies at $4 < z < 7$ whether they have $\lya$ lines or just show the Lyman Break, as well as find low luminosity AGNs in an unbiased fashion. This paper describes in detail the observations and the data reduction, and examines the quality of the extracted spectra. Subsequent papers will deal with the analysis of the data. The extracted and calibrated GRAPES spectra will be available from MAST at STScI.

astro-ph↗

arXiv2004

Evolutionary Minority Game with Multiple Options

We propose and study an evolutionary minority game (EMG) in which the agents are allowed to choose among three possible options. Unlike the original EMG where the agents either win or lose one unit of wealth, the present model assigns one unit of wealth to the winners in the least popular option, deducts one unit from the losers in the most popular option, and awards $R$ ($-1 R_{c}$, where $R_{c}(N)$ is a critical value for optimal performance of the system that drops to zero as the number of agents $N$ increases.

cond-mat.stat-mech↗

arXiv2003

Dimensional Crossover in the Effective Second Harmonic Generation of Films of Random Dielectrics

The effective nonlinear response of films of random composites consisting of a binary composite with nonlinear particles randomly embedded in a linear host is theoretically and numerically studied. A theoretical expression for the effective second harmonic generation susceptibility, incorporating the thickness of the film, is obtained by combining a modified effective-medium approximation with the general expression for the effective second harmonic generation susceptibility in a composite. The validity of the thoretical results is tested against results obtained by numerical simulations on random resistor networks. Numerical results are found to be well described by our theory. The result implies that the effective-medium approximation provides a convenient way for the estimation of the nonlinear response in films of random dielectrics.

cond-mat.dis-nn↗

arXiv2003

Second Harmonic Generation for a Dilute Suspension of Coated Particles

We derive an expression for the effective second-harmonic coefficient of a dilute suspension of coated spherical particles. It is assumed that the coating material, but not the core or the host, has a nonlinear susceptibility for second-harmonic generation (SHG). The resulting compact expression shows the various factors affecting the effective SHG coefficient. The effective SHG per unit volume of nonlinear coating material is found to be greatly enhanced at certain frequencies, corresponding to the surface plasmon resonance of the coated particles. Similar expression is also derived for a dilute suspension of coated discs. For coating materials with third-harmonic (THG) coefficient, results for the effective THG coefficients are given for the cases of coated particles and coated discs.

cond-mat.dis-nn↗

arXiv2001

A Multiwavelength Study of Stephan's Quintet

Stephan's Quintet (SQ) is a compact group that we find in an atypical moment when a high velocity intruder is passing through it. The intrusion is particularly interesting because a previous intruder had stripped most of the gas from the group members. This debris field was shocked in the ongoing collision with the new intruder. This evolutionary history agrees well with observations and explains how a strongly interacting system can show low levels of star formation. We present new multiwavelength data including previously unpublished ROSAT X-ray, Ha interference filter/FP, ISO MIR/FIR and radio line and continuum images. These observations and previously published data provide new insights as well as support for some previous hypotheses. 1) FP and HI velocities allow us to unambiguously distinguish between gas associated with SQ and the new intruder. 2) Most detected emission regions are found in the remnant ISM of the NI which allows us to infer its size and present physical state. 3) The few emission regions associated with the stripped ISM of SQ include the best candidate tidal dwarf galaxy. 4) Multiwavelength data suggest that strong MIR/FIR emission from the Seyfert 2 nucleus of NGC7319 comes from dust heated directly by a power-law continuum rather than a starburst. 5) The correspondance between extended X-ray/radio continuum/forbidden optical emission confirms the existence of a large scale shock in SQ.

astro-ph↗

arXiv2001

Prototype tests for the ALICE TRD

A Transition Radiation Detector (TRD) has been designed to improve the electron identification and trigger capability of the ALICE experiment at the Large Hadron Collider (LHC) at CERN. We present results from tests of a prototype of the TRD concerning pion rejection for different methods of analysis over a momentum range from 0.7 to 2 GeV/c. We investigate the performance of different radiator types, composed of foils, fibres and foams.

nucl-ex↗

arXiv2000

The Nuclei of Nearby Radio-Loud Ellipticals

We have observed a complete sample of 21 nearby (D < 70h^{-1} Mpc) Fanaroff & Riley Type I galaxies with HST/WFPC2 and detected dust disks and lanes in 19 of them. The radio jets are roughly perpendicular to the dust which is used to constrain the Doppler boosting factors of the radio jet and cores. The VLBA core flux correlates with the central Halpha+[NII] flux which might indicate that the VLBA core is dominated by an isotropic component. Twelve galaxies show nuclear optical sources. We discuss various possible origins for this emission.

astro-ph↗

arXiv1999

Far infrared and Ultraviolet emissions of individual galaxies at z=0: selection effects on the estimate of the dust extinction

We have cross-correlated Far Infrared (IRAS) and UV (FOCA) observations of galaxies to construct a sample of FIR selected galaxies with a UV observation at 0.2 microns. The FIR and UV properties of this sample are compared to the mean properties of the local Universe deduced from the luminosity distributions at both wavelengths. Almost all the galaxies of our sample have a FIR to UV flux ratio larger than the ratio of the FIR and UV luminosity densities, this effect becoming worse as the galaxies become brighter: the increase of the UV (0.2 microns) extinction is about 0.5 mag per decade of FIR (60 microns) luminosity. Quantitative star formation rates are estimated by adding the contribution of the FIR and UV emissions. They are found consistent with the corrections for extinction deduced from the FIR to UV flux ratio. A total local volume-average star formation rate is calculated by summing the contribution of the FIR and UV wavelengths bands. Each band contributes for an almost similar amount to the total star formation rate with rho_{SFR} = 0.03 +/- 0.01 h M solar/yr/Mpc^3 at z=0. This is equivalent to a global extinction of 0.75 mag to apply to the local luminosity density at 0.2 microns. The trend of a larger FIR to UV flux ratio for a larger FIR luminosity found for our sample of nearby galaxies is extended and amplified toward the very large FIR luminosities when we consider the galaxies detected by ISOCAM in a CFRS field and the Ultra Luminous Infrared Galaxies at low and high redshift. A UV extinction is tentatively estimated for these objects.

astro-ph↗

arXiv1996

An ISOPHOT of the disk of galaxy NGC 6946: 60 micron infrared and radio continuum correlation

We use the ISOPHOT 60 micron image of Tuffs et al. (1996) to study the 60 micron light distribution and its correlation with radio continuum within the disk of spiral galaxy NGC6946, and to resolve the long-standing IRAS controversy as to whether the infrared-to-radio continuum ratio, Q, decreases radially out in galaxy disks. Our main results are: (1) The disk at 60 microns also appears to follow an exponential fall-off. Its e-folding scale-length lies between 1'.3 and an upper limit of 1'.9, obtained from the mean radial profile between 1'.5 and 4' in radius. This scale length is smaller than that derived from the radio continuum data at either 20cm or 6cm, with or without the thermal component removed, at a significance level of >~ 2 sigma. From 1'.5 to 4' in radius, the 60 micron -to-radio continuum surface brightness ratio, Q, decreases by a factor of 1.5 to 1.7 on average. These quantitative results agree with that of Marsh & Helou (1995) using IRAS HiRes data, but not with that of Fitt et al. (1992) based on IRAS CPC data. Our results also fit into the picture of the radio disk as a ``smeared'' version of the infrared disk, but are not consistent with the picture in which the variation of Q is a result of the thermal and non-thermal radio components having two distinct, constant values of the infrared-to-radio ratio.

astro-ph↗

arXiv1996

The FIR/radio correlation in starburst galaxies - constraints on starburst models

This paper presents an analysis of the correlation between the far-infrared (FIR) and the radio emission of starburst galaxies. Data for interacting galaxies, many of which are undergoing a starburst, and for normal galaxies have been analysed and compared in order to test for any influence of the star-formation activity on the ratio between the FIR and the radio emission at 2.4 GHz, $\q24$. No statistically significant indication for such an influence has been found: There is neither a significant difference between $\q24$ of the two samples nor a dependence of this ratio on the starburst strength. This observational fact is unexpected because of the different physical conditions and the short time-scale of the star-formation activity in a starburst. In order to interprete the observations the FIR and the radio emission during a starburst have been modeled. The following conclusions could be drawn from the observed constancy of $\q24$: A strong and fast ($\approx 10^7$ yr) increase of the magnetic field at the beginning of the starburst is required in order to maintain a constant $\q24$. Otherwise the strong Inverse Compton losses that are due to the intense radiation field in a starburst would lower the synchrotron emission drastically resulting in a value of $\q24$ significantly higher than the observed one. Furthermore, the time-scale of the variation of the star-formation rate has to be longer than some $10^7$ yr. For lower values the different time-scales of the FIR and the radio emission produce large fluctuations of $\q24$.

astro-ph↗

arXiv1996

A quantitative model of the FIR/radio correlation for normal late-type galaxies

This paper investigates the physical reasons for the existence, the tightness and the near universality of the FIR/radio correlation for late-type field galaxies. We develop theoretical models for the radio and far-infrared (FIR) emission of normal galaxies and study the influence of their main parameters on the ratio of the two emissions. In addition, data are used from a sample of 114 late-type galaxies which allow an estimate of the mean energy densities of the radiation field and the magnetic field, and of the dust opacity. These data reveal, for the first time, a reasonably good, linear correlation between the energy density of the radiation field and the energy density of the magnetic field. Furthermore we find that on average the galaxies are marginally optically thick for the non-ionizing UV light. Including their extended magnetic halos, galaxies are also found to be on average marginally optically thick regarding the radiative energy losses of the Cosmic Ray electrons. Combining these semi-empirical results with the theoretical emission models we show that in the optically thick case the linear correlation between the energy densities of the radiation field and the magnetic field is a necessary condition for the existence of the observed FIR/radio correlation. Furthermore we show that the individual dispersion of the FIR to radio continuum ratio caused by uncertainties of any single one of the parameters considered is significantly less than the empirical dispersion of the FIR/radio correlation. The radio and the FIR emission are mainly determined by their sources, i.e. massive supernova precursor stars, because galaxies act in a reasonable approximation as calorimeters for the stellar UV radiation and for the energy flux of the Cosmic Ray electrons.

astro-ph↗

arXiv1994

Are Spiral Disks Really Opaque ?

We compare the ultra-violet, optical, and far-infrared emission for a sample of 135 spiral galaxies in order to address the widely debated problem concerning the opacity of spiral disks. We find that the re-radiation of the dust, estimated from the far-infrared emission, is on average only 31$\pm 1$ percent of the bolometric luminosity of a spiral galaxy, indicating that less than one third of the stellar radiation is absorbed and then re-radiated by dust in a spiral disk. Applying a radiation transfer model which assumes a `Sandwich' configuration for the spiral disk, and fully takes into account the effect of scattering, we find for our sample a median of the face-on blue (4400Å) optical depth $\rm τ_B=0.49$ and the mean $\rm <τ_B> =0.60\pm 0.04$, indicating that most spiral galaxies in our sample are {\it not} opaque for blue light ($\rm τ_B <1$).

astro-ph↗

arXiv1994

Inflationary cosmology with a non-analytic Lagrangian

We consider an alternative higher order gravity theory which is non-analytic in the Ricci curvature. It has power-law inflationary behaviour. This is further evidence that inflation is a general property of higher derivative gravity theories.

gr-qc↗

arXiv1993

Models for the Evolution of the Spectral Energy Distribution of Elliptical Galaxies from UV to Far--IR Wavelengths

We have worked out evolutionary synthesis models of the broad-band spectral energy distribution of elliptical galaxies over the whole frequency range from UV to far--IR. Internal extinction and far--IR re--emission by interstellar dust have been taken into account in a self--consistent way. Diffuse dust emission has been modelled in terms of two components: warm dust, located in regions of high radiation intensity, and cold dust, heated by the general radiation field. Emission from circumstellar dust clouds at different galactic ages was also taken into account. The models reproduce well the present average broad--band spectrum of nearby ellipticals over about four decades in frequency. Under the assumption of a dust--to--gas ratio proportional to the metallicity, the fraction of bolometric luminosity coming out at far-IR wavelengths strongly evolves during the galaxy lifetime, ranging from very low local values ($\lsim 0.5\%$) to $\sim 30\%$ or more in the first billion years. Some models even imply that early phases are optically thick; these models provide an excellent fit of the observed spectral energy distribution of the high redshift galaxy IRAS F10214$+$4724. Far--IR observations may thus play a key role in investigating the early evolution of elliptical galaxies. A strong far-IR evolution of early-type galaxies might be a crucial ingredient to explain the deep $60\mic$ IRAS counts.

astro-ph↗