736 research outputs found

    High Redshift Quasars and Star Formation in the Early Universe

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    In order to derive information on the star formation history in the early universe we observed 6 high-redshift (z=3.4) quasars in the near-infrared to measure the relative iron and \mgii emission strengths. A detailed comparison of the resulting spectra with those of low-redshift quasars show essentially the same FeII/MgII emission ratios and very similar continuum and line spectral properties, indicating a lack of evolution of the relative iron to magnesium abundance of the gas since z=3.4 in bright quasars. On the basis of current chemical evolution scenarios of galaxies, where magnesium is produced in massive stars ending in type II SNe, while iron is formed predominantly in SNe of type Ia with a delay of ~1 Gyr and assuming as cosmological parameters H_o = 72 km/s Mpc, Omega_M = 0.3, and Omega_Lambda = 0.7$, we conclude that major star formation activity in the host galaxies of our z=3.4 quasars must have started already at an epoch corresponding to z_f ~= 10, when the age of the universe was less than 0.5 Gyrs.Comment: 29 pages, 5 figures, ApJ in pres

    Detection of CO (2-1) and Radio Continuum Emission from the z = 4.4 QSO BRI 1335-0417

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    We have detected redshifted CO (2-1) emission at 43 GHz and radio continuum emission at 1.47 and 4.86 GHz from the z = 4.4 QSO BRI 1335-0417 using the Very Large Array. The CO data imply optically thick emission from warm (>30 K) molecular gas with a total mass, M(H_2), of 1.5+/-0.3 x10^{11} M_solar, using the Galactic gas mass-to-CO luminosity conversion factor. We set an upper limit to the CO source size of 1.1", and a lower limit of 0.23"x(T_ex/50K)^{-1/2}, where T_ex is the gas excitation temperature. We derive an upper limit to the dynamical mass of 2x10^{10} x sin^{-2} i M_solar, where i is the disk inclination angle. To reconcile the gas mass with the dynamical mass requires either a nearly face-on disk (i < 25deg), or a gas mass-to-CO luminosity conversion factor significantly lower than the Galactic value. The spectral energy distribution from the radio to the rest-frame infrared of BRI 1335-0417 is consistent with that expected from a nuclear starburst galaxy, with an implied massive star formation rate of 2300+/-600 M_solar yr^{-1}.Comment: standard AAS LATEX forma

    A sensitive survey for 13CO, CN, H2CO and SO in the disks of T Tauri and Herbig Ae stars

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    We use the IRAM 30-m telescope to perform a sensitive search for CN N=2-1 in 42 T Tauri or Herbig Ae systems located mostly in the Taurus-Auriga region. 13^{13}CO J=2-1 is observed simultaneously to indicate the level of confusion with the surrounding molecular cloud. The bandpass also contains two transitions of ortho-H2_2CO, one of SO and the C17^{17}O J=2-1 line which provide complementary information on the nature of the emission. While 13^{13}CO is in general dominated by residual emission from the cloud, CN exhibits a high disk detection rate >50> 50% in our sample. We even report CN detection in stars for which interferometric searches failed to detect 12^{12}CO, presumably because of obscuration by a foreground, optically thick, cloud. Comparison between CN and o-H2_2CO or SO line profiles and intensities divide the sample in two main categories. Sources with SO emission are bright and have strong H2_2CO emission, leading in general to [H2_2CO/CN]>0.5 > 0.5. Furthermore, their line profiles, combined with a priori information on the objects, suggest that the emission is coming from outflows or envelopes rather than from a circumstellar disk. On the other hand, most sources have [H2_2CO/CN]<0.3 < 0.3, no SO emission, and some of them exhibit clear double-peaked profiles characteristics of rotating disks. In this second category, CN is likely tracing the proto-planetary disks. From the line flux and opacity derived from the hyperfine ratios, we constrain the outer radii of the disks, which range from 300 to 600 AU. The overall gas disk detection rate (including all molecular tracers) is ∼68\sim 68%, and decreases for fainter continuum sources. This study shows that gas disks, like dust disks, are ubiquitous around young PMS stars in regions of isolated star formation, and that a large fraction of them have R>300R > 300 AU.Comment: 31 pages (including 59 figures

    Radio Observations of Infrared Luminous High Redshift QSOs

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    We present Very Large Array (VLA) observations at 1.4 GHz and 5 GHz of a sample of 12 Quasi-stellar Objects (QSOs) at z = 3.99 to 4.46. The sources were selected as the brightest sources at 250 GHz from the recent survey of Omont et al. (2001). We detect seven sources at 1.4 GHz with flux densities, S_{1.4} > 50 microJy. These centimeter (cm) wavelength observations imply that the millimeter (mm) emission is most likely thermal dust emission. The radio-through-optical spectral energy distributions for these sources are within the broad range defined by lower redshift, lower optical luminosity QSOs. For two sources the radio continuum luminosities and morphologies indicate steep spectrum, radio loud emission from a jet-driven radio source. For the remaining 10 sources the 1.4 GHz flux densities, or limits, are consistent with those expected for active star forming galaxies. If the radio emission is powered by star formation in these systems, then the implied star formation rates are of order 1e3 M_solar/year. We discuss the angular sizes and spatial distributions of the radio emitting regions, and we consider briefly these results in the context of co-eval black hole and stellar bulge formation in galaxies.Comment: to appear in the A

    High Sensitivity Array Observations of the z=4.4z = 4.4 QSO BRI 1335-0417

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    We present sensitive phase-referenced VLBI results on the radio continuum emission from the z=4.4z=4.4 QSO BRI 1335--0417. The observations were carried out at 1.4 GHz using the High Sensitivity Array (HSA). Our sensitive VLBI image at 189×113189 \times 113 mas (1.25×0.751.25 \times 0.75 kpc) resolution shows continuum emission in BRI 1335--0417 with a total flux density of 208±46μ208 \pm 46 \muJy, consistent with the flux density measured with the VLA. The size of the source at FWHM is 255×138255 \times 138 mas (1.7×0.91.7 \times 0.9 kpc) and the derived intrinsic brightness temperature is ∼3.5×104\sim 3.5\times 10^4 K. No continuum emission is detected at the full VLBI resolution (32×732 \times 7 mas, 211×46211 \times 46 pc), with a 4σ\sigma point source upper limit of 34 μ\muJy beam−1^{-1}, or an upper limit to the intrinsic brightness temperature of 5.6×1055.6\times 10^5 K. The highest angular resolution with at least a 4.5σ\sigma detection of the radio continuum emission is 53×2753 \times 27 mas (0.35×0.180.35 \times 0.18 kpc). At this resolution, the image shows a continuum feature in BRI 1335--0417 with a size of 64×3564 \times 35 mas (0.42×0.230.42 \times 0.23 kpc) at FWHM, and intrinsic brightness temperature of ∼2×105\sim 2\times 10^5 K. The extent of the observed continuum sources at 1.4 GHz and the derived brightness temperatures show that the radio emission (and thus presumably the far-infrared emission) in BRI 1335--0417 is powered by a major starburst, with a massive star formation rate of order a few thousand M_{\odot} {\rm yr}^{-1}.Moreover,theabsenceofanycompacthigh−brightnesstemperaturesourcesuggeststhatthereisnoradio−loudAGNinthis. Moreover, the absence of any compact high-brightness temperature source suggests that there is no radio-loud AGN in this z=4.4$ QSO.Comment: 13 pages, 3 figures, AJ accepte

    Sub-arcsec imaging of the AB Aur molecular disk and envelope at millimeter wavelengths: a non Keplerian disk

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    We present sub-arcsecond images of AB Auriga obtained with the IRAM Plateau de Bure interferometer in the isotopologues of CO, and in continuum at 3 and 1.3 mm. Instead of being centrally peaked, the continuum emission is dominated by a bright, asymmetric (spiral-like) feature at about 140 AU from the central star. The large scale molecular structure suggests the AB Aur disk is inclined between 23 and 43 degrees, but the strong asymmetry of the continuum and molecular emission prevents an accurate determination of the inclination of the inner parts. We find significant non-Keplerian motion, with a best fit exponent for the rotation velocity law of 0.41 +/- 0.01, but no evidence for radial motions. The disk has an inner hole about 70 AU in radius. The disk is warm and shows no evidence of depletion of CO. The dust properties suggest the dust is less evolved than in typical T Tauri disks. Both the spiral-like feature and the departure from purely Keplerian motions indicates the AB Aur disk is not in quasi-equilibrium. Disk self-gravity is insufficient to create the perturbation. This behavior may be related either to an early phase of star formation in which the Keplerian regime is not yet fully established and/or to a disturbance of yet unknown origin. An alternate, but unproven, possibility is that of a low mass companion located about 40 AU from AB Aur.Comment: 10 pages, 5 figures, accepted for publication in Astronomy & Astrophysic

    A Study of CO Emission in High Redshift QSOs Using the Owens Valley Millimeter Array

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    Searches for CO emission in high-redshift objects have traditionally suffered from the accuracy of optically-derived redshifts due to lack of bandwidth in correlators at radio observatories. This problem has motivated the creation of the new COBRA continuum correlator, with 4 GHz available bandwidth, at the Owens Valley Radio Observatory Millimeter Array. Presented here are the first scientific results from COBRA. We report detections of redshifted CO(J=3-2) emission in the QSOs SMM J04135+10277 and VCV J140955.5+562827, as well as a probable detection in RX J0911.4+0551. At redshifts of z=2.846, z=2.585, and z=2.796, we find integrated CO flux densities of 5.4 Jy km/s, 2.4 Jy km/s, and 2.9 Jy km/s for SMM J04135+10277, VCV J140955.5+562827, and RX J0911.4+0551, respectively, over linewidths of Delta(V_{FWHM}) ~ 350 km/s. These measurements, when corrected for gravitational lensing, correspond to molecular gas masses of order M(H_2) ~ 10^{9.6-11.1} solar masses, and are consistent with previous CO observations of high-redshift QSOs. We also report 3-sigma upper limits on CO(3-2) emission in the QSO LBQS 0018-0220 of 1.3 Jy km/s. We do not detect significant 3mm continuum emission from any of the QSOs, with the exception of a tentative (3-sigma) detection in RX J0911.4+0551 of S_{3mm}=0.92 mJy/beam.Comment: 18 pages, 5 figures, 2 tables, accepted to ApJ. Changes made for version 2: citations added, 2 objects added to Table 2 and Figure

    Molecular line radiative transfer in protoplanetary disks: Monte Carlo simulations versus approximate methods

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    We analyze the line radiative transfer in protoplanetary disks using several approximate methods and a well-tested Accelerated Monte Carlo code. A low-mass flaring disk model with uniform as well as stratified molecular abundances is adopted. Radiative transfer in low and high rotational lines of CO, C18O, HCO+, DCO+, HCN, CS, and H2CO is simulated. The corresponding excitation temperatures, synthetic spectra, and channel maps are derived and compared to the results of the Monte Carlo calculations. A simple scheme that describes the conditions of the line excitation for a chosen molecular transition is elaborated. We find that the simple LTE approach can safely be applied for the low molecular transitions only, while it significantly overestimates the intensities of the upper lines. In contrast, the Full Escape Probability (FEP) approximation can safely be used for the upper transitions (J_{\rm up} \ga 3) but it is not appropriate for the lowest transitions because of the maser effect. In general, the molecular lines in protoplanetary disks are partly subthermally excited and require more sophisticated approximate line radiative transfer methods. We analyze a number of approximate methods, namely, LVG, VEP (Vertical Escape Probability) and VOR (Vertical One Ray) and discuss their algorithms in detail. In addition, two modifications to the canonical Monte Carlo algorithm that allow a significant speed up of the line radiative transfer modeling in rotating configurations by a factor of 10--50 are described.Comment: 47 pages, 12 figures, accepted for publication in Ap

    A Compact Starburst Core in the Dusty Lyman Break Galaxy Westphal-MD11

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    Using the IRAM Plateau de Bure Interferometer, we have searched for CO(3-2) emission from the dusty Lyman break galaxy Westphal-MD11 at z = 2.98. Our sensitive upper limit is surprisingly low relative to the system's 850 um flux density and implies a far-IR/CO luminosity ratio as elevated as those seen in local ultraluminous mergers. We conclude that the observed dust emission must originate in a compact structure radiating near its blackbody limit and that a relatively modest molecular gas reservoir must be fuelling an intense nuclear starburst (and/or deeply buried active nucleus) that may have been triggered by a major merger. In this regard, Westphal-MD11 contrasts strikingly with the lensed Lyman break galaxy MS1512-cB58, which is being observed apparently midway through an extended episode of more quiescent disk star formation.Comment: 5 pages, 1 figure (emulateapj), accepted by ApJ
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