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K. Xu

Publications and source records attributed to K. Xu.

At least 37 records · Page 2Linked to original sources
arXiv2013

The evolution of the dust and gas content in galaxies

We use deep Herschel PACS and SPIRE observations in GOODSS, GOODSN and COSMOS to estimate the average dust mass (Mdust) of galaxies on a redshift-stellar mass (Mstar)-SFR grid. We study the scaling relations between Mdust, Mstar and SFR at z<=2.5. No clear evolution of Mdust is observed at fixed SFR and Mstar. We find a tight correlation between SFR and Mdust, likely a consequence of the Schmidt-Kennicutt (S-K) law. The Mstar-Mdust correlation observed by previous works flattens or sometimes disappears when fixing the SFR. Most of it likely derives from the combination of the Mdust-SFR and Mstar-SFR correlations. We then investigate the gas content as inferred by converting Mdust by assuming that the dust/gas ratio scales linearly with the gas metallicity. All galaxies in the sample follow, within uncertainties, the same SFR-Mgas relation (integrated S-K law), which broadly agrees with CO-based results for the bulk of the population, despite the completely different approaches. The majority of galaxies at z~2 form stars with an efficiency (SFE=SFR/Mgas) ~5 times higher than at z~0. It is not clear what fraction of such variation is an intrinsic redshift evolution and what fraction arises from selection effects. The gas fraction (fgas) decreases with Mstar and increases with SFR, and does not evolve with z at fixed Mstar and SFR. We explain these trends by introducing a universal relation between fgas, Mstar and SFR, non-evolving out to z~2.5. Galaxies move across this relation as their gas content evolves in time. We use the 3D fundamental fgas-Mstar-SFR relation and the redshift evolution of the Main Sequence to estimate the evolution of fgas in the average population of galaxies as a function of z and Mstar, and we find evidence a downsizing scenario.

astro-ph.CO↗
arXiv2013

Extension of continuum time-dependent Hartree-Fock method to proton states

This paper deals with the solution of the spherically symmetric time-dependent Hartree-Fock approximation applied to nuclear giant monopole resonances in the small amplitude regime. The problem is spatially unbounded as the resonance state is in the continuum. The practical requirement to perform the calculation in a finite-sized spatial region yields an artificial boundary, which is not present physically. The question of how to ensure the boundary does not interfere with the internal solution, while keeping the overall calculation time low is studied. Here we propose an absorbing boundary condition scheme to handle the conflict. The derivation, via a Laplace transform method, and implementation is described. An inverse Laplace transform required by the absorbing boundaries is calculated using a method of non-linear least squares. The accuracy and efficiency of the scheme is tested and results presented to support the case that they are a effective way of handling the artificial boundary.

nucl-th↗
arXiv2013

Mid-Infrared Properties of Nearby Luminous Infrared Galaxies I: Spitzer IRS Spectra for the GOALS Sample

The Great Observatories All-Sky LIRG Survey (GOALS) is a multiwavelength study of luminous infrared galaxies (LIRGs) in the local universe. Here we present low resolution Spitzer spectra covering 5-38um and provide a basic analysis of the mid-IR spectral properties for nearby LIRGs. In a companion paper, we discuss detailed fits to the spectra. The GOALS sample of 244 nuclei in 180 luminous and 22 ultraluminous IR galaxies represents a complete subset of the IRAS RBGS and covers a range of merger stages, morphologies and spectral types. The majority (>60%) of GOALS LIRGs have high 6.2um PAH equivalent widths (EQW > 0.4um) and low levels of silicate absorption (s_9.7um >-1.0). There is a general trend among the U/LIRGs for silicate depth and MIR slope to increase with LIR. U/LIRGs in the late stages of a merger also have on average steeper MIR slopes and higher levels of dust obscuration. Together these trends suggest that as gas & dust is funneled towards the center of a coalescing merger, the nuclei become more compact and obscured. The sources that depart from these correlations have very low PAH EQW (EQW < 0.1um) consistent with their MIR emission being dominated by an AGN. The most heavily dust obscured sources are the most compact in their MIR emission, suggesting that the obscuring (cool) dust is associated with the outer regions of the starburst. As the merger progresses a marked decline is seen for the fraction of high EQW (star formation dominated) sources while the fraction of composite sources increases but the fraction of AGN-dominated sources remains low. When compared to the MIR spectra of submillimeter galaxies (SMGs) at z~2, the average GOALS LIRG is more absorbed at 9.7um and has more PAH emission. However, when the AGN contributions to both the local LIRGs and the high-z SMGs are removed, the average local starbursting LIRG closely resembles the starbursting SMGs.

astro-ph.CO↗
arXiv2010

First results from HerMES on the evolution of the submillimetre luminosity function

We have carried out two extremely deep surveys with SPIRE, one of the two cameras on Herschel, at 250 microns, close to the peak of the far-infrared background. We have used the results to investigate the evolution of the rest-frame 250-micron luminosity function out to z=2. We find evidence for strong evolution out to a redshift of around 1 but evidence for at most weak evolution beyond this redshift. Our results suggest that a significant part of the stars and metals in the Universe today were formed at z<1.4 in spiral galaxies.

astro-ph.CO↗
arXiv2010

The Buried Starburst in the Interacting Galaxy II Zw 096 as Revealed by the Spitzer Space Telescope

An analysis of data from the Spitzer Space Telescope, Hubble Space Telescope, Chandra X-ray Observatory, and AKARI Infrared Astronomy Satellite is presented for the z=0.036 merging galaxy system II Zw 096 (CGCG 448-020). Because II Zw 096 has an infrared luminosity of log(L_IR/L_sun) = 11.94, it is classified as a Luminous Infrared Galaxy (LIRG), and was observed as part of the Great Observatories All-sky LIRG Survey (GOALS). The Spitzer data suggest that 80% of the total infrared luminosity comes from an extremely compact, red source not associated with the nuclei of the merging galaxies. The Spitzer mid-infrared spectra indicate no high-ionization lines from a buried active galactic nucleus in this source. The strong detection of the 3.3 micron and 6.2 micron PAH emission features in the AKARI and Spitzer spectra also implies that the energy source of II Zw 096 is a starburst. Based on Spitzer infrared imaging and AKARI near-infrared spectroscopy, the star formation rate is estimated to be 120 M_sun/yr and > 45 M_sun/yr, respectively. Finally, the high-resolution B, I, and H-band images show many star clusters in the interacting system. The colors of these clusters suggest at least two populations - one with an age of 1-5 Myr and one with an age of 20-500 Myr, reddened by 0-2 magnitudes of visual extinction. The masses of these clusters span a range between 10^6-10^8 M_sun. This starburst source is reminiscent of the extra-nuclear starburst seen in NGC 4038/9 (the Antennae Galaxies) and Arp 299 but approximately an order of magnitude more luminous than the Antennae. The source is remarkable in that the off-nuclear infrared luminosity dominates the enitre system.

astro-ph.CO↗
arXiv2009

GOALS: The Great Observatories All-Sky LIRG Survey

The Great Observatories All-sky LIRG Survey (GOALS) combines data from NASA's Spitzer, Chandra, Hubble and GALEX observatories, together with ground-based data into a comprehensive imaging and spectroscopic survey of over 200 low redshift Luminous Infrared Galaxies (LIRGs). The LIRGs are a complete subset of the IRAS Revised Bright Galaxy Sample (RBGS). The LIRGs targeted in GOALS span the full range of nuclear spectral types defined via traditional optical line-ratio diagrams as well as interaction stages. They provide an unbiased picture of the processes responsible for enhanced infrared emission in galaxies in the local Universe. As an example of the analytic power of the multi-wavelength GOALS dataset, we present data for the interacting system VV 340 (IRAS F14547+2449). Between 80-95% of the total far-infrared emission (or about 5E11 solar luminosities) originates in VV 340 North. While the IRAC colors of VV 340 North and South are consistent with star-forming galaxies, both the Spitzer IRS and Chandra ACIS data indicate the presence of a buried AGN in VV 340 North. The GALEX far and near-UV fluxes imply a extremely large infrared "excess" (IRX) for the system (IR/FUV = 81) which is well above the correlation seen in starburst galaxies. Most of this excess is driven by VV 340 N, which alone has an IR excess of nearly 400. The VV 340 system seems to be comprised of two very different galaxies - an infrared luminous edge-on galaxy (VV 340 North) that dominates the long-wavelength emission from the system and which hosts a buried AGN, and a face-on starburst (VV 340 South) that dominates the short-wavelength emission.

astro-ph.CO↗
arXiv2008

A three-dimensional multidimensional gas-kinetic scheme for the Navier-Stokes equations under gravitational fields

This paper extends the gas-kinetic scheme for one-dimensional inviscid shallow water equations (J. Comput. Phys. 178 (2002), pp. 533-562) to multidimensional gas dynamic equations under gravitational fields. Four important issues in the construction of a well-balanced scheme for gas dynamic equations are addressed. First, the inclusion of the gravitational source term into the flux function is necessary. Second, to achieve second-order accuracy of a well-balanced scheme, the Chapman-Enskog expansion of the Boltzmann equation with the inclusion of the external force term is used. Third, to avoid artificial heating in an isolated system under a gravitational field, the source term treatment inside each cell has to be evaluated consistently with the flux evaluation at the cell interface. Fourth, the multidimensional approach with the inclusion of tangential gradients in two-dimensional and three-dimensional cases becomes important in order to maintain the accuracy of the scheme. Many numerical examples are used to validate the above issues, which include the comparison between the solutions from the current scheme and the Strang splitting method. The methodology developed in this paper can also be applied to other systems, such as semi-conductor device simulations under electric fields.

physics.comp-ph↗
arXiv2006

Evidence for Superfluidity of Ultracold Fermions in an Optical Lattice

The study of superfluid fermion pairs in a periodic potential has important ramifications for understanding superconductivity in crystalline materials. Using cold atomic gases, various condensed matter models can be studied in a highly controllable environment. Weakly repulsive fermions in an optical lattice could undergo d-wave pairing at low temperatures, a possible mechanism for high temperature superconductivity in the cuprates. The lattice potential could also strongly increase the critical temperature for s-wave superfluidity. Recent experimental advances in the bulk include the observation of fermion pair condensates and high-temperature superfluidity. Experiments with fermions and bosonic bound pairs in optical lattices have been reported, but have not yet addressed superfluid behavior. Here we show that when a condensate of fermionic atom pairs was released from an optical lattice, distinct interference peaks appear, implying long range order, a property of a superfluid. Conceptually, this implies that strong s-wave pairing and superfluidity have now been established in a lattice potential, where the transport of atoms occurs by quantum mechanical tunneling and not by simple propagation. These observations were made for unitarity limited interactions on both sides of a Feshbach resonance. For larger lattice depths, the coherence was lost in a reversible manner, possibly due to a superfluid to insulator transition. Such strongly interacting fermions in an optical lattice can be used to study a new class of Hamiltonians with interband and atom-molecule couplings.

cond-mat.other↗
arXiv2006

Observation of Strong Quantum Depletion in a Gaseous Bose-Einstein Condensate

We studied quantum depletion in a gaseous Bose-Einstein condensate. An optical lattice enhanced the atomic interactions and modified the dispersion relation resulting in strong quantum depletion. The depleted fraction was directly observed as a diffuse background in the time-of-flight images. Bogoliubov theory provided a semi-quantitative description for our observations of depleted fractions in excess of 50%.

cond-mat.other↗
arXiv2006

Star formation in the nearby universe: the ultraviolet and infrared points of view

This work presents the main ultraviolet (UV) and far-infrared (FIR) properties of two samples of nearby galaxies selected from the GALEX ($λ= 2315$Å, hereafter NUV) and IRAS ($λ= 60μ$m) surveys respectively. They are built in order to get detection at both wavelengths for most of the galaxies. Star formation rate (SFR) estimators based on the UV and FIR emissions are compared. Systematic differences are found between the SFR estimators for individual galaxies based on the NUV fluxes corrected for dust attenuation and on the total IR luminosity. A combined estimator based on NUV and IR luminosities seems to be the best proxy over the whole range of values of SFR. Although both samples present similar average values of the birthrate parameter b, their star-formation-related properties are substantially different: NUV-selected galaxies tend to show larger values of $b$ for lower masses, SFRs and dust attenuations, supporting previous scenarios for the star formation history (SFH). Conversely, about 20% of the FIR-selected galaxies show high values of $b$, SFR and NUV attenuation. These galaxies, most of them being LIRGs and ULIRGs, break down the downsizing picture for the SFH, however their relative contribution per unit volume is small in the local Universe. Finally, the cosmic SFR density of the local Universe is estimated in a consistent way from the NUV and IR luminosities.

astro-ph↗
arXiv2005

Remarkable Disk and Off-nuclear Starburst Activity in the "Tadpole Galaxy" as revealed by the Spitzer Space Telescope

We present ground-based optical and Spitzer infrared imaging observations of the interacting galaxy UGC 10214, the "Tadpole Galaxy" (z = 0.0310), focusing on the star formation activity in the nuclear, disk, spiral arms and tidal tail regions. The major findings of this study are that the Tadpole is actively forming stars in the main disk outside of the nucleus and in the tidal plume, with an estimated mean star formation rate of ~2 to 4 M_sun/yr. The most prominent sites of mid-infrared emission define a "ring" morphology that, combined with the overall morphology of the system, suggest the interaction may belong to the rare class of off-center collisional ring systems that form both shock-induced rings of star formation and tidal plumes. The nuclear emission is solely powered by older stars, with little evidence for ongoing star formation at the center of the Tadpole. Extra-nuclear star formation accounts for >50% of the total star formation in the disk and spiral arms, featuring infrared-bright 'hot spots' that exhibit strong PAH emission, whose band strength is comparable to that of late-type star-forming disk galaxies. The tidal tail, which extends 2 arcmin (~75 kpc) into the intergalactic medium, is populated by super massive star clusters likely triggered by the galaxy-galaxy interaction that has distorted UGC 10214 into its current "tadpole" shape.

astro-ph↗
arXiv2005

Sodium Bose-Einstein Condensates in an Optical Lattice

The phase transition from a superfluid to a Mott insulator has been observed in a $^{23}$Na Bose-Einstein condensate. A dye laser detuned $\approx 5$nm red of the Na $3^2$S$ \to 3^2$P$_{1/2}$ transition was used to form the three dimensional optical lattice. The heating effects of the small detuning as well as the three-body decay processes constrained the timescale of the experiment. Certain lattice detunings were found to induce a large loss of atoms. These loss features were shown to be due to photoassociation of atoms to vibrational levels in the Na$_2$ $(1) ^3Σ_g^+$ state.

cond-mat.other↗
arXiv2005

High-Contrast Interference in a Thermal Cloud of Atoms

The coherence properties of a gas of bosonic atoms above the BEC transition temperature were studied. Bragg diffraction was used to create two spatially separated wave packets, which interfere during expansion. Given sufficient expansion time, high fringe contrast could be observed in a cloud of arbitrary temperature. Fringe visibility greater than 90% was observed, which decreased with increasing temperature, in agreement with a simple model. When the sample was "filtered" in momentum space using long, velocity-selective Bragg pulses, the contrast was significantly enhanced in contrast to predictions.

cond-mat.stat-mech↗
arXiv2004

Coherent Molecular Optics using Sodium Dimers

Coherent molecular optics is performed using two-photon Bragg scattering. Molecules were produced by sweeping an atomic Bose-Einstein condensate through a Feshbach resonance. The spectral width of the molecular Bragg resonance corresponded to an instantaneous temperature of 20 nK, indicating that atomic coherence was transferred directly to the molecules. An autocorrelating interference technique was used to observe the quadratic spatial dependence of the phase of an expanding molecular cloud. Finally, atoms initially prepared in two momentum states were observed to cross-pair with one another, forming molecules in a third momentum state. This process is analogous to sum-frequency generation in optics.

cond-mat.other↗
arXiv2003

Formation of Quantum-Degenerate Sodium Molecules

Ultra-cold sodium molecules were produced from an atomic Bose-Einstein condensate by ramping an applied magnetic field across a Feshbach resonance. More than $10^5$ molecules were generated with a conversion efficiency of $\sim$4%. Using laser light resonant with an atomic transition, the remaining atoms could be selectively removed, preventing fast collisional relaxation of the molecules. Time-of-flight analysis of the pure molecular sample yielded an instantaneous phase-space density greater than 20.

cond-mat.stat-mech↗
arXiv2003

Dissociation and Decay of Ultra-cold Sodium Molecules

The dissociation of ultracold molecules is studied by ramping an external magnetic field through a Feshbach resonance. The observed dissociation energy shows non-linear dependence on the ramp speed and directly yields the strength of the atom-molecule coupling. In addition, inelastic molecule-molecule and molecule-atom collisions are characterized.

cond-mat.soft↗