739 research outputs found
High Redshift Quasars and Star Formation in the Early Universe
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
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
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.
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-HCO, one of SO and the CO J=2-1 line which
provide complementary information on the nature of the emission.
While CO is in general dominated by residual emission from the cloud,
CN exhibits a high disk detection rate % in our sample. We even report CN
detection in stars for which interferometric searches failed to detect
CO, presumably because of obscuration by a foreground, optically thick,
cloud. Comparison between CN and o-HCO or SO line profiles and intensities
divide the sample in two main categories. Sources with SO emission are bright
and have strong HCO emission, leading in general to [HCO/CN].
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
[HCO/CN], 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 , 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 AU.Comment: 31 pages (including 59 figures
Radio Observations of Infrared Luminous High Redshift QSOs
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 QSO BRI 1335-0417
We present sensitive phase-referenced VLBI results on the radio continuum
emission from the QSO BRI 1335--0417. The observations were carried out
at 1.4 GHz using the High Sensitivity Array (HSA). Our sensitive VLBI image at
mas ( kpc) resolution shows continuum
emission in BRI 1335--0417 with a total flux density of Jy,
consistent with the flux density measured with the VLA. The size of the source
at FWHM is mas ( kpc) and the derived
intrinsic brightness temperature is K. No continuum
emission is detected at the full VLBI resolution ( mas, pc), with a 4 point source upper limit of 34 Jy
beam, or an upper limit to the intrinsic brightness temperature of
K. The highest angular resolution with at least a 4.5
detection of the radio continuum emission is mas ( kpc). At this resolution, the image shows a continuum feature in BRI
1335--0417 with a size of mas ( kpc) at FWHM,
and intrinsic brightness temperature of 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}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
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
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
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
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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