152 research outputs found
Coordinated thermal and optical observations of Trans-Neptunian object (20000) Varuna from Sierra Nevada
We report on coordinated thermal and optical measurements of trans-Neptunian
object (20000) Varuna obtained in January-February 2002, respectively from the
IRAM 30-m and IAA 1.5 m telescopes. The optical data show a lightcurve with a
period of 3.176+/-0.010 hr, a mean V magnitude of 20.37+/-0.08 and a
0.42+/-0.01 magnitude amplitude. They also tentatively indicate that the
lightcurve is asymmetric and double-peaked. The thermal observations indicate a
1.12+/-0.41 mJy flux, averaged over the object's rotation. Combining the two
datasets, we infer that Varuna has a mean 1060(+180/-220) km diameter and a
mean 0.038(+0.022/-0.010) V geometric albedo, in general agreement with an
earlier determination using the same technique.Comment: Accepted for publication in Astronomy & Astrophysics (7 pages,
including 3 figures
LABOCA and MAMBO-2 imaging of the dust ring of the Sombrero galaxy (NGC 4594)
The Sombrero galaxy (NGC 4594) is an Sa galaxy with a symmetric dust ring. We
have used the Large APEX BOlometer CAmera (LABOCA) at 870 micron and the
MAx-Planck Millimeter BOlometer (MAMBO-2) at 1.2 mm to detect the dust ring for
the first time at submillimetre and millimetre wavelengths. We have constructed
a model of the galaxy to separate the active galactic nucleus (AGN) and dust
ring components. The ring radius at both 870 micron and 1.2 mm agrees well with
the radius determined from optical absorption and atomic gas studies. The
spectral energy distribution of the ring is well fitted by a single grey-body
with dust emissivity index beta=2 and a dust temperature T_d=18.4 K. The dust
mass of the ring is found to be 1.6\pm0.2x10^7Msun which, for a Galactic
gas-to-dust ratio, implies a gas mass that is consistent with measurements from
the literature.Comment: 4 pages including 3 figures and 2 tables. Accepted for publication in
A&A Letters. Full resolution version at
http://www.strw.leidenuniv.nl/~vlahakis/vlahakis-highres.pd
Continuum millimetre observations of high-redshift radio-quiet QSOs. II. Five new detections at z > 4
We have performed a sensitive (Ï ~ 1.5mJy) systematic study of the 1.25 mm emission of ~22 radio-quiet QSOs at z âł 4, with the IRAM 30m telescope equipped with bolometer arrays. Five radio-quiet QSOs at z > 4 have been detected at a 5-Ï level in addition to the initial detection of the z = 4.7 QSO BR1202-0725 reported in McMahon et al. (1994). The detected fluxes range from 2.5 to 10 mJy. All the reported detections were independently confirmed at the 3-Ï level on at least three different nights. In addition 10 other QSOs from the Cambridge APM survey sample and 6 others QSOs from the literature were searched for millimetre emission but not detected with 2-Ï upper limits of 3-4 mJy.
From this systematic study of about half of the known optically selected z > 4 QSOs, some general trends of their millimetre emission can be inferred. All the QSOs we have detected pertain to the APM sample and are among those which have the largest UV rest-frame luminosities. The detection rate within the APM sample is 6 out of 16 observed, compared with zero in the remaining 6. Two of the four APM broad absorption line QSOs observed were detected and four of the seven weak lined APM QSOs were detected, whereas none of the five strong lined APM QSOs were detected. Thus there is evidence for enhanced millimetre emission from luminous QSOs with weak broad emission lines or broad absorption lines.
There is one clear case known of strong lensing amongst the six millimetre detected objects with z > 4. In light of the fact that both previously known objects with confirmed strong millimetre emission at z>2 are gravitationally lensed, i.e. H1413+ 117 and IRAS F10214+4724, sensitive high resolution observations of these z > 4 QSOs are required to determine whether gravitational lensing effects need to be taken into account.
Assuming that the millimetre wave continuum emission is due to dust emission, the very large amount of dust implied, ~ 10^8 h^(-2) M_â , means that the host galaxies of these QSOs have undergone a substantial phase of star formation. If the gas-to-dust ratio in these galaxies is similar to that in lower redshift objects, the total gas mass would be ~ 10^(11) M_â.
We have begun to explore the 1.25 mm emission of bright radio-quiet QSOs in the redshift range 1.5 to 3.5, using criteria which seem to favor millimetre detections, established from our z > 4 detections. One source was detected at z = 2.70. We have also observed three QSOs with z > 3 that were previously studied at 1.25mm by Andreani et al. (1993) who reported detections at a level higher than 3Ï. We have been unable to confirm any of these reported detections. In particular we have a 3Ï upper limit of 3.2 mJy for the z = 3.19 QSO PC2132+0126 for which Andreani et al. reported a flux of 11.5 ± 1. 7 mJy. Either this source has substantially varied during the period between the two sets of observations or the single channel bolometer observations were affected by systematic errors
Scanning tunneling microscopy of amorphous alloy electrocatalysts for water electrolysis
The surface topography of amorphous metals useful as electrocatalysts in water electrolysis, denoted as G16 (Cos50Ni25Si15Bu10) and G14 (Fe60Co20Si10B10), are studied at the gm scale with a conventional SEM, and at the nm scale with the STM. Both untreated specimens as well as those employed as electrodes for a relatively prolonged time exhibit the same microstructure with amorphous and crystalline surface regions. The microcrystallite reduced size structure, probably involving a very compact structure, appears to be of the greatest importance concerning the electrocatalytic activity of these materials.Instituto de Investigaciones FisicoquĂmicas TeĂłricas y Aplicada
MITO measurements of the Sunyaev-Zeldovich Effect in the Coma cluster of galaxies
We have measured the Sunyaev-Zeldovich effect towards the Coma cluster
(A1656) with the MITO experiment, a 2.6-m telescope equipped with a 4-channel
17 arcminute (FWHM) photometer. Measurements at frequency bands 143+/-15,
214+/-15, 272+/-16 and 353+/-13 GHz, were made during 120 drift scans of Coma.
We describe the observations and data analysis that involved extraction of the
S-Z signal by employing a spatial and spectral de-correlation scheme to remove
a dominant atmospheric component. The deduced values of the thermal S-Z effect
in the first three bands are DT_{0} = -179+/-38,-33+/-81,170+/-35 microKelvin
in the cluster center. The corresponding optical depth, tau=(4.1+/-0.9)
10^{-3}, is consistent (within errors) with both the value from a previous low
frequency S-Z measurement, and the value predicted from the X-ray deduced gas
parameters.Comment: Ap.J.Letters accepted, 4 pages, 2 figure
COSBO: The MAMBO 1.2 Millimeter Imaging Survey of the COSMOS Field
The inner 20 Ă 20 arcmin^2 of the COSMOS field was imaged at 250 GHz (1.2 mm) to an rms noise level of ~1 mJy per 11" beam using the Max-Planck Millimeter Bolometer Array (MAMBO-2) at the IRAM 30 m telescope. We detect 15 sources at significance between 4 and 7 Ï, 11 of which are also detected at 1.4 GHz with the VLA with a flux density >24 ÎŒJy (3 Ï). We identify 12 more lower significance mm sources based on their association with faint radio sources. We present the multifrequency identifications of the MAMBO sources, including VLA radio flux densities, optical and near-infrared identifications, as well as the XMM-Newton X-ray detection for two of the mm sources. We compare radio and optical photometric redshifts and briefly describe the host galaxy morphologies. The colors of the identified optical counterparts suggest most of them to be high-redshift (z ~ 2-3) star-forming galaxies. At least three sources appear lensed by a foreground galaxy. We highlight some MAMBO sources that do not show obvious radio counterparts. These sources could be dusty starburst galaxies at redshifts >3.5. The 250 GHz source areal density in the COSMOS field is comparable to that seen in other deep mm fields
LABOCA observations of nearby, active galaxies
We present large scale 870 micron maps of the nearby starburst galaxies
NGC253, NGC4945 and the nearest giant elliptical radio galaxy Centaurus A (NGC
5128) obtained with the newly commissioned Large Apex Bolometer Camera (LABOCA)
operated at the APEX telescope. Our continuum images reveal for the first time
the distribution of cold dust at a angular resolution of 20" across the entire
optical disks of NGC253 and NGC4945 out to a radial distance of 10' (7.5 kpc).
In NGC5128 our LABOCA image also shows, for the first time at submillimeter
wavelengths, the synchrotron emission associated with the radio jet and the
inner radio lobes. From an analysis of the 870 micron emission in conjunction
with ISO-LWS, IRAS and long wavelengths radio data we find temperatures for the
cold dust in the disks of all three galaxies of 17-20 K, comparable to the dust
temperatures in the disk of the Milky Way. The total gas mass in the three
galaxies is determined to be 2.1, 4.2 and 2.8 x 10^9 solar masses for NGC253,
NGC4945 and NGC5128, respectively. A detailed comparison between the gas masses
derived from the dust continuum and the integrated CO(1-0) intensity in NGC253
suggests that changes of the CO luminosity to molecular mass conversion factor
are mainly driven by a metallicity gradient and only to a lesser degree by
variations of the CO excitation. An analysis of the synchrotron spectrum in the
northern radio lobe of NGC5128 shows that the synchrotron emission from radio
to the ultraviolet wavelengths is well described by a broken power law and that
the break frequency is a function of the distance from the radio core as
expected for aging electrons. We derive an outflow speed of ~0.5c at a distance
of 2.6kpc from the center, consistent with the speed derived in the vicinity of
the nucleus.Comment: 12 pages, 11 figures. Accepted for publication in A&
Submillimeter observations of the J2142-4423 Lya protocluster at z = 2.38
We present observations aimed at exploring both the nature of Lya emitting
nebulae (Lya blobs) at z=2.38 and the way they trace large scale structure
(LSS), by exploring their proximity to maximum starbursts through submillimeter
emission. Our most important objectives are to make a census of associated
submillimeter galaxies (SMGs), check their properties, and look for a possible
overdensity in the protocluster J2142-4426 at z=2.38.
We used the newly commissioned Large APEX Bolometer Camera (LABoCa) on the
Atacama Pathfinder EXperiment (APEX) telescope, in its Science Verification
phase, to carry out a deep 10'x10' map at 870 micron, and we performed multiple
checks of the quality of data processing and source extraction.
Our map, the first published deep image, confirms the capabilities of
APEX/LABoCa as the most efficient current equipment for wide and deep submm
mapping. Twenty-two sources were securely extracted with 870 micron flux
densities in the range 3-21 mJy, rms noise 0.8-2.4 mJy, and far-IR luminosities
probably in the range ~5-20 x 10(12) Lo. Only one of the four 50 kpc-extended
Lya blobs has a secure 870 micron counterpart. The 870 micron source counts in
the whole area are marginally higher than in the SHADES SCUBA survey, with a
possible over-density around this blob. The majority of the 3.6-24 micron SEDs
of the submillimeter sources indicate they are starburst dominated, with
redshifts mostly >2. However, there is evidence of a high-z AGN in ~30% of the
sources.Comment: 14 pages, 9 figures, accepted for publication in A&A, fixed author
lis
The SCUBA Bright Quasar Survey (SBQS): 850micron observations of the z>4 sample
We present initial results of a new, systematic search for massive
star-formation in the host galaxies of the most luminous and probably most
massive z>=4 radio-quiet quasars (M(B) 10^13Lsun).
A total of 38 z>=4 radio-quiet quasars have been observed at the JCMT using
SCUBA at 850microns: 8 were detected (>3sigma) with S(850microns)>~ 10mJy
(submillimetre-loud). The new detections almost triple the number of optically
selected, submillimetre-loud z>~4 radio-quiet quasars known to date. We include
a detailed description of how our quasar sample is defined in terms of radio
and optical properties.
There is no strong evidence for trends in either detectability or 850microns
flux with absolute magnitude, M(B). We find that the weighted mean flux of the
undetected sources is 2.0 +/- 0.6mJy, consistent with an earlier estimate of
\~3mJy based on more sensitive observations of a sample z>~4 radio-quiet
quasars (McMahon et al., 1999). This corresponds to an inferred starformation
rate of \~1000Msun/yr, similar to Arp220. The typical starformation timescale
for the submillimetre-bright sources is ~1Gyr, 10 times longer than the typical
accretion-driven e-folding timescale of ~5x10^7 years. Our 850micron detection
of the z=4.4 quasar PSS J1048+4407 when analysed in conjunction with 1.2mm
single-dish and interferometric observations suggests that this source is
resolved on angular scales of 1-2" (6-12 kpc). In addition, we present a new
optical spectrum of this source, identifying it as a broad absorption line
(BAL) quasar. The new redshift is outside that covered in a recent CO line
search by Guilloteau et al., (1999), highlighting the need for accurate
redshifts for the obervation and interpretation of high-redshift line studies.Comment: 16 pages, 11 figures. Accepted by Monthly Notices of the Royal
Astronomical Societ
Simultaneous Multi-Wavelength Observations of Sgr A* during 2007 April 1-11
We report the detection of variable emission from Sgr A* in almost all
wavelength bands (i.e. centimeter, millimeter, submillimeter, near-IR and
X-rays) during a multi-wavelength observing campaign. Three new moderate flares
are detected simultaneously in both near-IR and X-ray bands. The ratio of X-ray
to near-IR flux in the flares is consistent with inverse Compton scattering of
near-IR photons by submillimeter emitting relativistic particles which follow
scaling relations obtained from size measurements of Sgr A*. We also find that
the flare statistics in near-IR wavelengths is consistent with the probability
of flare emission being inversely proportional to the flux. At millimeter
wavelengths, the presence of flare emission at 43 GHz (7mm) using VLBA with
milli-arcsecond spatial resolution indicates the first direct evidence that
hourly time scale flares are localized within the inner 3070
Schwarzschild radii of Sgr A*. We also show several cross correlation plots
between near-IR, millimeter and submillimeter light curves that collectively
demonstrate the presence of time delays between the peaks of emission up to
three hours. The evidence for time delays at millimeter and submillimeter
wavelengths are consistent with the source of emission being optically thick
initially followed by a transition to an optically thin regime. In particular,
there is an intriguing correlation between the optically thin near-IR and X-ray
flare and optically thick radio flare at 43 GHz that occurred on 2007 April 4.
This would be the first evidence of a radio flare emission at 43 GHz delayed
with respect to the near-IR and X-ray flare emission.Comment: replaced with revised version 57 pages, 28 figures, ApJ (in press
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