1,259 research outputs found

    Dissipative Quantum Ising model in a cold atomic spin-boson mixture

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    Using cold bosonic atoms with two (hyperfine) ground states, we introduce a spin-boson mixture which allows to implement the quantum Ising model in a tunable dissipative environment. The first specie lies in a deep optical lattice with tightly confining wells and forms a spin array; spin-up/down corresponds to occupation by one/no atom at each site. The second specie forms a superfluid reservoir. Different species are coupled coherently via laser transitions and collisions. Whereas the laser coupling mimics a transverse field for the spins, the coupling to the reservoir sound modes induces a ferromagnetic (Ising) coupling as well as dissipation. This gives rise to an order-disorder quantum phase transition where the effect of dissipation can be studied in a controllable manner.Comment: 4 pages, 2 figures, 1 table; Title modified and cosmetic change

    SDSS IV MaNGA - Rotation Velocity Lags in the Extraplanar Ionized Gas from MaNGA Observations of Edge-on Galaxies

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    We present a study of the kinematics of the extraplanar ionized gas around several dozen galaxies observed by the Mapping of Nearby Galaxies at the Apache Point Observatory (MaNGA) survey. We considered a sample of 67 edge-on galaxies out of more than 1400 extragalactic targets observed by MaNGA, in which we found 25 galaxies (or 37%) with regular lagging of the rotation curve at large distances from the galactic midplane. We model the observed HαH\alpha emission velocity fields in the galaxies, taking projection effects and a simple model for the dust extinction into the account. We show that the vertical lag of the rotation curve is necessary in the modeling, and estimate the lag amplitude in the galaxies. We find no correlation between the lag and the star formation rate in the galaxies. At the same time, we report a correlation between the lag and the galactic stellar mass, central stellar velocity dispersion, and axial ratio of the light distribution. These correlations suggest a possible higher ratio of infalling-to-local gas in early-type disk galaxies or a connection between lags and the possible presence of hot gaseous halos, which may be more prevalent in more massive galaxies. These results again demonstrate that observations of extraplanar gas can serve as a potential probe for accretion of gas.Comment: 13 pages, 11 figures, accepted for publication in Ap

    Massive Compact Galaxies with High-Velocity Outflows: Morphological Analysis and Constraints on AGN Activity

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    We investigate the process of rapid star formation quenching in a sample of 12 massive galaxies at intermediate redshift (z~0.6) that host high-velocity ionized gas outflows (v\u3e1000 km/s). We conclude that these fast outflows are most likely driven by feedback from star formation rather than active galactic nuclei (AGN). We use multiwavelength survey and targeted observations of the galaxies to assess their star formation, AGN activity, and morphology. Common attributes include diffuse tidal features indicative of recent mergers accompanied by bright, unresolved cores with effective radii less than a few hundred parsecs. The galaxies are extraordinarily compact for their stellar mass, even when compared with galaxies at z~2-3. For 9/12 galaxies, we rule out an AGN contribution to the nuclear light and hypothesize that the unresolved core comes from a compact central starburst triggered by the dissipative collapse of very gas-rich progenitor merging disks. We find evidence of AGN activity in half the sample but we argue that it accounts for only a small fraction (\u3c10%) of the total bolometric luminosity. We find no correlation between AGN activity and outflow velocity and we conclude that the fast outflows in our galaxies are not powered by on-going AGN activity, but rather by recent, extremely compact starbursts

    The Ultraviolet-to-Mid-Infrared Spectral Energy Distribution of Weak Emission Line Quasars

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    We present Spitzer Space Telescope photometry of 18 Sloan Digital Sky Survey (SDSS) quasars at 2.7 <= z <= 5.9 which have weak or undetectable high-ionization emission lines in their rest-frame ultraviolet (UV) spectra (hereafter weak-lined quasars, or WLQs). The Spitzer data are combined with SDSS spectra and ground-based, near-infrared (IR) photometry of these sources to produce a large inventory of spectral energy distributions (SEDs) of WLQs across the rest-frame ~0.1-5 mum spectral band. The SEDs of our sources are inconsistent with those of BL Lacertae objects which are dominated by synchrotron emission due to a jet aligned close to our line-of-sight, but are consistent with the SED of ordinary quasars with similar luminosities and redshifts that exhibit a near-to-mid-IR 'bump', characteristic of hot dust emission. This indicates that broad emission lines in WLQs are intrinsically weak, rather than suffering continuum dilution from a jet, and that such sources cannot be selected efficiently from traditional photometric surveys.Comment: 10 pages (emulateapj), 4 figures. Accepted for publication in Ap

    X-ray and Multiwavelength Insights into the Nature of Weak Emission-Line Quasars at Low Redshift

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    (Abridged) We report on the X-ray and multiwavelength properties of 11 radio-quiet quasars with weak or no emission lines identified by the Sloan Digital Sky Survey (SDSS) with redshift z=0.4-2.5. The distribution of relative X-ray brightness for our low-redshift weak-line quasar (WLQ) candidates is significantly different from that of typical radio-quiet quasars, having an excess of X-ray weak sources, but it is consistent with that of high-redshift WLQs. The X-ray weak sources generally show similar UV emission-line properties to those of the X-ray weak quasar PHL 1811; they may belong to the notable class of PHL 1811 analogs. The average X-ray spectrum of these sources is somewhat harder than that of typical radio-quiet quasars. Several other low-redshift WLQ candidates have normal ratios of X-ray-to-optical/UV flux, and their average X-ray spectral properties are also similar to those of typical radio-quiet quasars. The X-ray weak and X-ray normal WLQ candidates may belong to the same subset of quasars having high-ionization "shielding gas" covering most of the wind-dominated broad emission-line region, but be viewed at different inclinations. The mid-infrared-to-X-ray spectral energy distributions (SEDs) of these sources are generally consistent with those of typical SDSS quasars, showing that they are not likely to be BL Lac objects with relativistically boosted continua and diluted emission lines. However, one source in our X-ray observed sample is remarkably strong in X-rays, indicating that a small fraction of low-redshift WLQ candidates may actually be BL Lacs residing in the radio-faint tail of the BL Lac population. We also investigate universal selection criteria for WLQs over a wide range of redshift, finding that it is not possible to select WLQ candidates in a fully consistent way using different prominent emission lines as a function of redshift.Comment: ApJ in press; 26 pages, 11 figures and 7 tables. The full Table 3 is available upon reques

    UPORABA RAČUNALA U POUČAVANJU MATEMATIKE – PREGLED STANJA U SRBIJI

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    In this study we analyse the quantity and the quality of the use of computers in mathematics teaching in schools (both primary and secondary) in Serbia, observing two samples. The data for the first research was obtained through a sample consisting of 142 experienced teachers. By conducting a survey we determined the current situation regarding personal and school use of computers, and also examined whether or not computer usage was affected by some non-attitudinal (e.g., demographic) and attitudinal variables. The second research included 123 beginner teachers, applicants for the professional state teacher license. The collected data was based on our estimates of the applicants’ preparation for the practical part of the licensing examination. We examined the impact of some demographic and academic factors on the quality of computer usage. In both researches, the data collected was coded and analysed using the SPSS 13.0 statistical program (cross tabulation and appropriate tests for comparing groups). The aims of our researches were to determine the current situation and give some possible suggestions for improving computer useU ovom radu analiziramo kvantitetu i kvalitetu uporabe računala u poučavanju matematike u Srbiji (u osnovnim i srednjim školama) promatrajući dva uzorka. Podaci za prvo istraživanje prikupljeni su na uzorku od 142 iskusna nastavnika (sa profesionalnom licencom). Provedbom istraživanja ustanovili smo trenutnu situaciju s obzirom na privatnu uporabu računala i uporabu računala u školi, kao i utječu li na uporabu računala i neke objektivne (npr. demografija) i subjektivne varijable. Drugo istraživanje je provedeno na uzorku od 123 nastavnička pripravnika (bez profesionalne licence). Prikupljeni podaci su temeljeni na našim procjenama o pripravi praktičnog dijela stručnog ispita pripravnika. Istražili smo utjecaj nekih demografskih i akademskih čimbenika na kakvoću uporabe računala. U oba istraživanja, podaci su obrađeni uporabom statističkog programa SPSS 13.0 (unakrsno tabeliranje i odgovarajući testovi za uspoređivanje skupina). Cilj istraživanja je bio utvrditi trenutnu situaciju i predložiti moguća poboljšanja uporabe računala

    X-ray Insights into the Nature of Weak Emission-Line Quasars at High Redshift

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    We present Chandra observations of nine high-redshift quasars (z=2.7-5.9) discovered by the Sloan Digital Sky Survey with weak or undetectable high-ionization emission lines in their UV spectra (WLQs). Adding archival X-ray observations of six additional sources of this class has enabled us to place the strongest constraints yet on the X-ray properties of this remarkable class of AGNs. Although our data cannot rule out the possibility that the emission lines are overwhelmed by a relativistically boosted continuum, as manifested by BL Lac objects, we find that WLQs are considerably weaker in the X-ray and radio bands than the majority of BL Lacs found at much lower redshifts. If WLQs are high-redshift BL Lacs, then it is difficult to explain the lack of a large parent population of X-ray and radio bright weak-lined sources at high redshift. We also consider the possibility that WLQs are quasars with extreme properties, and in particular that the emission lines are suppressed by high accretion rates. Using joint spectral fitting of the X-ray spectra of 11 WLQs we find that the mean photon index in the hard X-ray band is consistent with those observed in typical radio-quiet AGNs with no hint of an unusually steep hard-X-ray spectrum. This result poses a challenge to the hypothesis that WLQs have extremely high accretion rates, and we discuss additional observations required to test this idea.Comment: 13 pages (emulateapj), 7 figures. Accepted for publication in Ap

    Optically Selected BL Lacertae Candidates from the Sloan Digital Sky Survey Data Release Seven

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    We present a sample of 723 optically selected BL Lac candidates from the SDSS DR7 spectroscopic database encompassing 8250 deg^2 of sky; our sample constitutes one of the largest uniform BL Lac samples yet derived. Each BL Lac candidate has a high-quality SDSS spectrum from which we determine spectroscopic redshifts for ~60% of the objects. Redshift lower limits are estimated for the remaining objects utilizing the lack of host galaxy flux contamination in their optical spectra; we find that objects lacking spectroscopic redshifts are likely at systematically higher redshifts. Approximately 80% of our BL Lac candidates match to a radio source in FIRST/NVSS, and ~40% match to a ROSAT X-ray source. The homogeneous multiwavelength coverage allows subdivision of the sample into 637 radio-loud BL Lac candidates and 86 weak-featured radio-quiet objects. The radio-loud objects broadly support the standard paradigm unifying BL Lac objects with beamed radio galaxies. We propose that the majority of the radio-quiet objects may be lower-redshift (z<2.2) analogs to high-redshift weak line quasars (i.e., AGN with unusually anemic broad emission line regions). These would constitute the largest sample of such objects, being of similar size and complementary in redshift to the samples of high-redshift weak line quasars previously discovered by the SDSS. However, some fraction of the weak-featured radio-quiet objects may instead populate a rare and extreme radio-weak tail of the much larger radio-loud BL Lac population. Serendipitous discoveries of unusual white dwarfs, high-redshift weak line quasars, and broad absorption line quasars with extreme continuum dropoffs blueward of rest-frame 2800 Angstroms are also briefly described.Comment: 24 pages, 14 figures, 8 tables. Accepted for publication in A

    Stellar feedback as the origin of an extended molecular outflow in a starburst galaxy

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    Recent observations have revealed that starburst galaxies can drive molecular gas outflows through stellar radiation pressure. Molecular gas is the phase of the interstellar medium from which stars form, so these outflows curtail stellar mass growth in galaxies. Previously known outflows, however, involve small fractions of the total molecular gas content and have typical scales of less than a kiloparsec. In at least some cases, input from active galactic nuclei is dynamically important, so pure stellar feedback (the momentum return into the interstellar medium) has been considered incapable of rapidly terminating star formation on galactic scales. Molecular gas has been detected outside the galactic plane of the archetypal starburst galaxy M82 (refs 4 and 5), but so far there has been no evidence that starbursts can propel substantial quantities of cold molecular gas to the same galactocentric radius (about 10 kiloparsecs) as the warmer gas that has been traced by metal ion absorbers in the circumgalactic medium. Here we report observations of molecular gas in a compact (effective radius 100 parsecs) massive starburst galaxy at redshift 0.7, which is known to drive a fast outflow of ionized gas. We find that 35 per cent of the total molecular gas extends approximately 10 kiloparsecs, and one-third of this extended gas has a velocity of up to 1,000 kilometres per second. The kinetic energy associated with this high-velocity component is consistent with the momentum flux available from stellar radiation pressure. This demonstrates that nuclear bursts of star formation are capable of ejecting large amounts of cold gas from the central regions of galaxies, thereby strongly affecting their evolution by truncating star formation and redistributing matter.Peer reviewedFinal Accepted Versio

    Chandra Observations of the Highest Redshift Quasars from the Sloan Digital Sky Survey

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    We present new Chandra observations of 21 z>4 quasars, including 11 sources at z>5. These observations double the number of X-ray detected quasars at z>5, allowing investigation of the X-ray spectral properties of a substantial sample of quasars at the dawn of the modern Universe. By jointly fitting the spectra of 15 z>5 radio-quiet quasars (RQQs), including sources from the Chandra archive, with a total of 185 photons, we find a mean X-ray power-law photon index of Gamma=1.95^{+0.30}_{-0.26}, and a mean neutral intrinsic absorption column density of N_H<~6x10^{22} cm^{-2}. These results show that quasar X-ray spectral properties have not evolved up to the highest observable redshifts. We also find that the mean optical-X-ray spectral slope (alpha_ox) of optically-selected z>5 RQQs, excluding broad absorption line quasars, is alpha_ox=-1.69+/-0.03, which is consistent with the value predicted from the observed relationship between alpha_ox and ultraviolet luminosity. Four of the sources in our sample are members of the rare class of weak emission-line quasars, and we detect two of them in X-rays. We discuss the implications our X-ray observations have for the nature of these mysterious sources and, in particular, whether their weak-line spectra are a consequence of continuum boosting or a deficit of high-ionization line emitting gas.Comment: 15 pages (emulateapj), 8 figures. Accepted by Ap
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