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X. Fan

Publications and source records attributed to X. Fan.

At least 55 records · Page 3Linked to original sources
arXiv2017

Probing Physics in Vacuum Using an X-ray Free-Electron Laser, a High-Power Laser, and a High-Field Magnet

A nonlinear interaction between photons is observed in a process that involves charge sources. To observe this process in a vacuum, there are a growing number of theoretical and experimental studies. This process may contain exotic contribution from new physics beyond the Standard Model of particle physics, and is probed by experiments using a high-power laser or a high-field magnet, and more recently using an X-ray Free-Electron Laser (XFEL). Here, we review the present status of our experiments testing various vacuum processes. We describe four experiments with a focus on those using an XFEL: (i) photon-photon scattering in the x-ray region, (ii) laser-induced birefringence and diffraction of x rays, (iii) vacuum birefringence induced by a high-field magnet, and (iv) a dedicated search for axion-like particles using the magnet and x rays.

hep-ex↗
arXiv2017

The interplay between excitons and trions in a monolayer of MoSe$_2$

We study the impact of a free carrier reservoir on the optical properties of excitonic and trionic complexes in a MoSe$_2$ monolayer at cryogenic temperatures. By applying photodoping via a non-resonant pump laser the electron density can be controlled in our sample and in turn the exciton and trion densities can be tuned. We find a significant increase of the trion binding energy in the presence of an induced electron gas both in power- and in time-resolved photoluminescence spectra. This behaviour is reproduced within the original variational approach that takes into account both screening and phase space filling effects.

cond-mat.mes-hall↗
arXiv2016

The Pan-STARRS1 distant z>5.6 quasar survey: more than 100 quasars within the first Gyr of the universe

Luminous quasars at z>5.6 can be studied in detail with the current generation of telescopes and provide us with unique information on the first gigayear of the universe. Thus far these studies have been statistically limited by the number of quasars known at these redshifts. Such quasars are rare and therefore wide-field surveys are required to identify them and multiwavelength data are needed to separate them efficiently from their main contaminants, the far more numerous cool dwarfs. In this paper, we update and extend the selection for z~6 quasars presented in Banados et al. (2014) using the Pan-STARRS1 (PS1) survey. We present the PS1 distant quasar sample, which currently consists of 124 quasars in the redshift range 5.6 5.6 presented in this work almost double the quasars previously known at these redshifts, marking a transition phase from studies of individual sources to statistical studies of the high-redshift quasar population, which was impossible with earlier, smaller samples.

astro-ph.GA↗
arXiv2016

Monolayer MoSe2: A candidate for room temperature polaritonics

Monolayered MoSe2 is a promising new material to investigate advanced light-matter coupling as it hosts stable and robust excitons with comparably narrow optical resonances. In this work, we investigate the evolution of the lowest lying excitonic transition, the so-called A-valley exciton, with temperature. We find a strong, phonon-induced temperature broadening of the resonance, and more importantly, a reduction of the oscillator strength for increased temperatures. Based on these experimentally extracted, temperature dependent parameters, we apply a coupled oscillator model to elucidate the possibility to observe the strong coupling regime between the A-exciton and a microcavity resonance in three prototypical photonic architectures with varying mode volumes. We find that the formation of exciton-polaritons up to ambient conditions in short, monolithic dielectric and Tamm-based structures seems feasible. In contrast, a temperature-induced transition into the weak coupling regime can be expected for structures with extended effective cavity length. Based on these findings, we calculate and draw the phase diagram of polariton Bosonic condensation in a MoSe2 cavity.

cond-mat.mes-hall↗
arXiv2016

The SDSS-IV extended Baryon Oscillation Spectroscopic Survey: selecting emission line galaxies using the Fisher discriminant

We present a new selection technique of producing spectroscopic target catalogues for massive spectroscopic surveys for cosmology. This work was conducted in the context of the extended Baryon Oscillation Spectroscopic Survey (eBOSS), which will use ~200 000 emission line galaxies (ELGs) at 0.6<zspec<1.0 to obtain a precise baryon acoustic oscillation measurement. Our proposed selection technique is based on optical and near-infrared broad-band filter photometry. We used a training sample to define a quantity, the Fisher discriminant (linear combination of colours), which correlates best with the desired properties of the target: redshift and [OII] flux. The proposed selections are simply done by applying a cut on magnitudes and this Fisher discriminant. We used public data and dedicated SDSS spectroscopy to quantify the redshift distribution and [OII] flux of our ELG target selections. We demonstrate that two of our selections fulfil the initial eBOSS/ELG redshift requirements: for a target density of 180 deg^2, ~70% of the selected objects have 0.6<zspec<1.0 and only ~1% of those galaxies in the range 0.6<zspec<1.0 are expected to have a catastrophic zspec estimate. Additionally, the stacked spectra and stacked deep images for those two selections show characteristic features of star-forming galaxies. The proposed approach using the Fisher discriminant could, however, be used to efficiently select other galaxy populations, based on multi-band photometry, providing that spectroscopic information is available. This technique could thus be useful for other future massive spectroscopic surveys such as PFS, DESI, and 4MOST.

astro-ph.GA↗
arXiv2015

Bright [CII] 158$μ$m emission in a quasar host galaxy at $z=6.54$

The [CII] 158$μ$m fine-structure line is known to trace regions of active star formation and is the main coolant of the cold, neutral atomic medium. In this \textit{Letter}, we report a strong detection of the [CII] line in the host galaxy of the brightest quasar known at $z>6.5$, the Pan-STARRS1 selected quasar PSO J036.5078+03.0498 (hereafter P036+03), using the IRAM NOEMA millimeter interferometer. Its [CII] and total far-infrared luminosities are $(5.8 \pm 0.7) \times 10^9 \,L_\odot$ and $(7.6\pm1.5) \times 10^{12}\,L_\odot$, respectively. This results in a $L_{[CII]} /L_{TIR}$ ratio of $\sim 0.8\times 10^{-3}$, which is at the high end for those found for active galaxies, though it is lower than the average found in typical main sequence galaxies at $z\sim 0$. We also report a tentative additional line which we identify as a blended emission from the $3_{22} - 3_{13}$ and $5_{23} - 4_{32}$ H$_2$O transitions. If confirmed, this would be the most distant detection of water emission to date. P036+03 rivals the current prototypical luminous J1148+5251 quasar at $z=6.42$, in both rest-frame UV and [CII] luminosities. Given its brightness and because it is visible from both hemispheres (unlike J1148+5251), P036+03 has the potential of becoming an important laboratory for the study of star formation and of the interstellar medium only $\sim 800\,$Myr after the Big Bang.

astro-ph.GA↗
arXiv2015

Spin Pumping in Electrodynamically Coupled Magnon-Photon Systems

We use electrical detection, in combination with microwave transmission, to investigate both resonant and non-resonant magnon-photon coupling at room temperature. Spin pumping in a dynamically coupled magnon-photon system is found to be distinctly different from previous experiments. Characteristic coupling features such as modes anti-crossing, line width evolution, peculiar line shape, and resonance broadening are systematically measured and consistently analyzed by a theoretical model set on the foundation of classical electrodynamic coupling. Our experimental and theoretical approach pave the way for pursuing microwave coherent manipulation of pure spin current via the combination of spin pumping and magnon-photon coupling.

cond-mat.mtrl-sci↗
arXiv2015

Constraining the radio-loud fraction of quasars at z>5.5

Radio-loud Active Galactic Nuclei at z~2-4 are typically located in dense environments and their host galaxies are among the most massive systems at those redshifts, providing key insights for galaxy evolution. Finding radio-loud quasars at the highest accessible redshifts (z~6) is important to study their properties and environments at even earlier cosmic time. They would also serve as background sources for radio surveys intended to study the intergalactic medium beyond the epoch of reionization in HI 21 cm absorption. Currently, only five radio-loud ($R=f_{ν,5{\rm GHz}}/f_{ν,4400Å}>10$) quasars are known at z~6. In this paper we search for 5.5 < z < 7.2 quasars by cross-matching the optical Pan-STARRS1 and radio FIRST surveys. The radio information allows identification of quasars missed by typical color-based selections. While we find no good 6.4 < z <7.2 quasar candidates at the sensitivities of these surveys, we discover two new radio-loud quasars at z~6. Furthermore, we identify two additional z~6 radio-loud quasars which were not previously known to be radio-loud, nearly doubling the current z~6 sample. We show the importance of having infrared photometry for z>5.5 quasars to robustly classify them as radio-quiet or radio-loud. Based on this, we reclassify the quasar J0203+0012 (z=5.72), previously considered radio-loud, to be radio-quiet. Using the available data in the literature, we constrain the radio-loud fraction of quasars at z~6, using the Kaplan--Meier estimator, to be $8.1^{+5.0}_{-3.2}\%$. This result is consistent with there being no evolution of the radio-loud fraction with redshift, in contrast to what has been suggested by some studies at lower redshifts.

astro-ph.GA↗
arXiv2015

The identification of z-dropouts in Pan-STARRS1: three quasars at 6.5<z<6.7

Luminous distant quasars are unique probes of the high redshift intergalactic medium (IGM) and of the growth of massive galaxies and black holes in the early universe. Absorption due to neutral Hydrogen in the IGM makes quasars beyond a redshift of z~6.5 very faint in the optical $z$-band, thus locating quasars at higher redshifts require large surveys that are sensitive above 1 micron. We report the discovery of three new z>6.5 quasars, corresponding to an age of the universe of <850 Myr, selected as z-band dropouts in the Pan-STARRS1 survey. This increases the number of known z>6.5 quasars from 4 to 7. The quasars have redshifts of z=6.50, 6.52, and 6.66, and include the brightest z-dropout quasar reported to date, PSO J036.5078+03.0498 with M_1450=-27.4. We obtained near-infrared spectroscopy for the quasars and from the MgII line we estimate that the central black holes have masses between 5x10^8 and 4x10^9 M_sun, and are accreting close to the Eddington limit (L_Bol/L_Edd=0.13-1.2). We investigate the ionized regions around the quasars and find near zone radii of R_NZ=1.5-5.2 proper Mpc, confirming the trend of decreasing near zone sizes with increasing redshift found for quasars at 5.7<z<6.4. By combining R_NZ of the PS1 quasars with those of 5.7<z<7.1 quasars in the literature, we derive a luminosity corrected redshift evolution of R_NZ,corrected=(7.2+/-0.2)-(6.1+/-0.7)x(z-6) Mpc. However, the large spread in R_NZ in the new quasars implies a wide range in quasar ages and/or a large variation in the neutral Hydrogen fraction along different lines of sight.

astro-ph.GA↗
arXiv2014

Discovery of eight z ~ 6 quasars from Pan-STARRS1

High-redshift quasars are currently the only probes of the growth of supermassive black holes and potential tracers of structure evolution at early cosmic time. Here we present our candidate selection criteria from the Panoramic Survey Telescope & Rapid Response System 1 and follow-up strategy to discover quasars in the redshift range 5.7 5.7 by more than 10%. We additionally recovered 18 previously known quasars. The eight quasars presented here span a large range of luminosities (-27.3 < M_{1450} < -25.4; 19.6 < z_ps1 < 21.2) and are remarkably heterogeneous in their spectral features: half of them show bright emission lines whereas the other half show a weak or no Ly$α$ emission line (25% with rest-frame equivalent width of the Ly$α$ + Nv line lower than 15Å). We find a larger fraction of weak-line emission quasars than in lower redshift studies. This may imply that the weak-line quasar population at the highest redshifts could be more abundant than previously thought. However, larger samples of quasars are needed to increase the statistical significance of this finding.

astro-ph.GA↗
arXiv2014

Spectral Energy Distributions of QSOs at z>5: common AGN-heated dust and occasionally strong star-formation

We present spectral energy distributions (SEDs) of 69 QSOs at z>5, covering a rest frame wavelength range of 0.1mu to ~80mu, and centered on new Spitzer and Herschel observations. The detection rate of the QSOs with Spitzer is very high (97% at lambda_rest ~< 4mu), but drops towards the Herschel bands with 30% detected in PACS (rest frame mid-infrared) and 15% additionally in the SPIRE (rest frame far-infrared; FIR). We perform multi-component SED fits for Herschel-detected objects and confirm that to match the observed SEDs, a clumpy torus model needs to be complemented by a hot (~1300K) component and, in cases with prominent FIR emission, also by a cold (~50K) component. In the FIR detected cases the luminosity of the cold component is on the order of 10^13 L_sun which is likely heated by star formation. From the SED fits we also determine that the AGN dust-to-accretion disk luminosity ratio declines with UV/optical luminosity. Emission from hot (~1300K) dust is common in our sample, showing that nuclear dust is ubiquitous in luminous QSOs out to redshift 6. However, about 15% of the objects appear under-luminous in the near infrared compared to their optical emission and seem to be deficient in (but not devoid of) hot dust. Within our full sample, the QSOs detected with Herschel are found at the high luminosity end in L_UV/opt and L_NIR and show low equivalent widths (EWs) in H_alpha and in Ly_alpha. In the distribution of H_alpha EWs, as determined from the Spitzer photometry, the high-redshift QSOs show little difference to low redshift AGN.

astro-ph.GA↗
arXiv2014

Chandra and MMT observations of low-mass black hole active galactic nuclei accreting at low rates in dwarf galaxies

We report on Chandra X-ray observations of four candidate low-mass black hole (<10^6Msun) active galactic nuclei (AGNs) that have the estimated Eddington ratios among the lowest (~10^(-2)) found for this class. The aims are to validate the nature of their AGNs and to confirm the low Eddington ratios that are derived from the broad H_alpha line, and to explore this poorly studied regime in the AGN parameter space. Among them, two objects with the lowest significance of the broad lines are also observed with Multi-Mirror Telescope, and the high-quality optical spectra taken confirm them as Seyfert 1 AGNs and as having small black hole masses. X-ray emission is detected from the nuclei of two of the galaxies, which is variable on timescales of 10^3s, whereas no significant (or only marginal at best) detection is found for the remaining two. The X-ray luminosities are on the order of 10^(41) ergs/s or even lower, on the order of 10^(40) ergs/s for non-detections, which are among the lowest regimes ever probed for Seyfert galaxies. The low X-ray luminosities, compared to their black hole masses derived from H_alpha, confirm their low accretion rates assuming typical bolometric corrections. Our results hint at the existence of a possibly large population of under-luminous low-mass black holes in the local universe. An off-nucleus ultra-luminous X-ray source (ULX) in one of the dwarf galaxies is detected serendipitously, with a luminosity (6-9)x10^(39) ergs/s in 2-10 keV.

astro-ph.GA↗
arXiv2013

Complete infrared spectral energy distributions of mm detected quasars at z>5

We present Herschel far-infrared (FIR) photometry of eleven quasars at redshift z>5 that have previously been detected at 1.2mm. We perform full spectral energy distribution (SED) fits over the wavelength range lambda_rest ~0.1-400mu for those objects with good Herschel detections. These fits reveal the need for an additional far-infrared (FIR) component besides the emission from a dusty AGN-powered torus. This additional FIR component has temperatures of T_FIR ~ 40-60K with luminosities of L_(8-1000mu) ~ 10^13 L_sun (accounting for 25-60% of the bolometric FIR luminosity). If the FIR dust emission is due to star formation it would suggest star formation rates in excess of 1000 solar masses per year. We show that at long wavelengths (lambda_rest > 50mu) the contribution of the AGN-powered torus emission is negligible. This explains how previous FIR studies of high-redshift quasars that relied on single component fits to (ground-based) observations at lambda_obs > 350mu reached T_FIR and L_FIR values similar to our complete SED fits. Stacking the Herschel data of four individually undetected sources reveals a significant average signal in the PACS bands but not in SPIRE. The average SED of sources with individual Herschel detections shows a striking surplus in near- and mid-infrared emission when compared to common AGN templates. The comparison between two average SEDs (sources with and without individual Herschel detections) matched in the UV/optical indicates that for these objects the strength of the MIR emission may correlate with the strength of the FIR emission.

astro-ph.CO↗
arXiv2013

Luminosity Function from dedicated SDSS-III and MMT data of quasars in 0.7<z<4.0 selected with a new approach

We present a measurement of the quasar luminosity function in the range 0.68 2.15. Using pure luminosity evolution models, we fitted our LF measurements, and predicted quasar number counts as a function of redshift and observed magnitude. These predictions are useful inputs for future cosmology surveys such as those relying on the observation of quasars to measure baryon acoustic oscillations.

astro-ph.CO↗
arXiv2012

Wide-Field InfraRed Survey Telescope (WFIRST) Final Report

In December 2010, NASA created a Science Definition Team (SDT) for WFIRST, the Wide Field Infra-Red Survey Telescope, recommended by the Astro 2010 Decadal Survey as the highest priority for a large space mission. The SDT was chartered to work with the WFIRST Project Office at GSFC and the Program Office at JPL to produce a Design Reference Mission (DRM) for WFIRST. Part of the original charge was to produce an interim design reference mission by mid-2011. That document was delivered to NASA and widely circulated within the astronomical community. In late 2011 the Astrophysics Division augmented its original charge, asking for two design reference missions. The first of these, DRM1, was to be a finalized version of the interim DRM, reducing overall mission costs where possible. The second of these, DRM2, was to identify and eliminate capabilities that overlapped with those of NASA's James Webb Space Telescope (henceforth JWST), ESA's Euclid mission, and the NSF's ground-based Large Synoptic Survey Telescope (henceforth LSST), and again to reduce overall mission cost, while staying faithful to NWNH. This report presents both DRM1 and DRM2.

astro-ph.IM↗
arXiv2011

Hubble Space Telescope Observations of Field Ultracool Dwarfs at High Galactic Latitude

We present a sample of 17 newly discovered ultracool dwarf candidates later than ~M8, drawn from 231.90 arcmin2 of {\it Hubble Space Telescope} Wide Field Camera 3 infrared imaging. By comparing the observed number counts for 17.5<J_125<25.5 AB mag to an exponential disk model, we estimate a vertical scale height of z_scl=290 +- 25 (random) +- 30 (systematic) pc for a binarity fraction of f_b=0. While our estimate is roughly consistent with published results, we suggest that the differences can be attributed to sample properties, with the present sample containing far more substellar objects than previous work. We predict the object counts should peak at J_{125}~24 AB mag due to the exponentially-declining number density at the edge of the disc. We conclude by arguing that trend in scale height with spectral type may breakdown for brown dwarfs since they do not settle onto the main sequence.

astro-ph.GA↗
arXiv2010

The extinction law at high redshift and its implications

We analyze the optical-near infrared spectra of 33 quasars with redshifts 3.9 0.8), and characterized by very steep intrinsic slopes (alpha<-2.3). All of the individual quasars require extinction curve deviating from that of the SMC, with a tendency to flatten at lambda<2000 A (in the rest frame of the source). We obtain a mean extinction curve at z>4, both by performing a simultaneous fit of all quasars and by averaging the extinction curves inferred for individual quasars. In the case of broad absorption line quasars the mean extinction curve deviates from the SMC at a confidence level >95%. The difference between extinction curves in quasars at z>4 and in quasars at lower redshift is indicative of either a different dust production mechanism at high redshift, or a different mechanism for processing dust into the ISM. We suggest that the same transitions may also apply to normal, star-forming galaxies at z>4. In particular, the observed change in the average spectral slope of galaxies at z>4 may be partially ascribed to a variation in the extinction curve, rather than a lower dust content at high redshift. In this scenario, the extinction curve inferred at z>4 would imply a cosmic star-formation corrected for dust attenuation a factor of ~2 higher than estimated in the past.

astro-ph.CO↗