1,759 research outputs found
A survey of UV-excess AGNs in the South Galactic Pole
Spectra, position, magnitudes and colors are presented for 485 faint (B<20.5)
emission line objects selected with the ultraviolet-excess (UVX) criterion on a
area of 24.6 sq. deg in the South Galactic Pole. The objects were selected from
the analysis of pixel-to-pixel stacking of COSMOS scans of UKST U, J and R
plates. The candidates were observed with the Meudon-ESO Fiber Optics System
(MEFOS) at the ESO 3.6m telescope. 429 type 1 AGNs have been identified (373 in
the redshift range 0.3<z<2.2). This sample has allowed the measure of a
difference on the QSO clustering evolution in comparison with that found for
galaxies (La Franca et al 1998). The region is part of the ESO Imaging Survey
(EIS) and of the 2dF QSO redshift survey.Comment: 32 pages, 19 figures. To appear on A&AS, revised after minor comment
AGN counts at 15um. XMM observations of the ELAIS-S1-5 sample
Context: The counts of galaxies and AGN in the mid infra-red (MIR) bands are
important instruments for studying their cosmological evolution. However, the
classic spectral line ratios techniques can become misleading when trying to
properly separate AGN from starbursts or even from apparently normal galaxies.
Aims: We use X-ray band observations to discriminate AGN activity in
previously classified MIR-selected starburst galaxies and to derive updated
AGN1 and (Compton thin) AGN2 counts at 15 um.
Methods: XMM observations of the ELAIS-S1 15um sample down to flux limits
~2x10^-15 erg cm^-2 s^-1 (2-10 keV band) were used. We classified as AGN all
those MIR sources with a unabsorbed 2-10 keV X-ray luminosity higher that
~10^42 erg/s.
Results: We find that at least about 13(+/-6) per cent of the previously
classified starburst galaxies harbor an AGN. According to these figures, we
provide an updated estimate of the counts of AGN1 and (Compton thin) AGN2 at 15
um. It turns out that at least 24% of the extragalactic sources brighter than
0.6 my at 15 um are AGN (~13% contribution to the extragalactic background
produced at fluxes brighter than 0.6 mJy).Comment: Accepted for publication on A&
Dust from AGBs: relevant factors and modelling uncertainties
The dust formation process in the winds of Asymptotic Giant Branch stars is
discussed, based on full evolutionary models of stars with mass in the range
MMM, and metallicities .
Dust grains are assumed to form in an isotropically expanding wind, by growth
of pre--existing seed nuclei. Convection, for what concerns the treatment of
convective borders and the efficiency of the schematization adopted, turns out
to be the physical ingredient used to calculate the evolutionary sequences with
the highest impact on the results obtained. Low--mass stars with MM produce carbon type dust with also traces of silicon carbide. The
mass of solid carbon formed, fairly independently of metallicity, ranges from a
few M, for stars of initial mass M, to
M for MM; the size of dust
particles is in the range mm. On the contrary,
the production of silicon carbide (SiC) depends on metallicity. For the size of SiC grains varies in the range m, while the mass of SiC formed is
. Models of
higher mass experience Hot Bottom Burning, which prevents the formation of
carbon stars, and favours the formation of silicates and corundum. In this case
the results scale with metallicity, owing to the larger silicon and aluminium
contained in higher--Z models. At Z= we find that the most
massive stars produce dust masses M, whereas models of
smaller mass produce a dust mass ten times smaller. The main component of dust
are silicates, although corundum is also formed, in not negligible quantities
().Comment: Paper accepted for publication in Monthly Notices of the Royal
Astronomical Society Main Journal (2014 January 4
The XMM-Newton survey of the ELAIS-S1 field II: optical identifications and multiwavelength catalogue of X-ray sources
We present optical identifications and a multi-band catalogue of a sample of
478 X-ray sources in the XMM and Chandra surveys of the central 0.6 deg^2 of
the ELAIS-S1 field. The optical/infrared counterpart of each X-ray source was
identified using R and IRAC 3.6 um bands. This method was complemented by the
precise positions obtained through Chandra observations. Approximately 94% of
the counterparts are detected in the R band, while the remaining are blank
fields in the optical down to R~24.5, but have a near-infrared counterpart
detected by IRAC within 6 arcsec from the XMM centroid. The multi-band
catalogue contains photometry in ten photometric bands (B to the MIPS 24 um).
We determined redshift and classification for 237 sources (~50% of the sample)
brighter than R=24. We classified 47% of the sources with spectroscopic
redshift as broad-line active galactic nuclei (BL AGNs) with z=0.1-3.5, while
sources without broad-lines are about 46% of the spectroscopic sample and are
found up to z=2.6. We identified 11 type 2 QSOs among the sources with X/O>8,
with z=0.9-2.6, high 2-10 keV luminosity (log(L2-10keV)>=43.8 erg/s) and hard
X-ray colors suggesting large absorbing columns at the rest frame (logN_H up to
23.6 cm^-2). BL AGNs show on average blue optical-to-near-infrared colors,
softer X-ray colors and X-ray-to-optical colors typical of optically selected
AGNs. Conversely, narrow-line sources show redder optical colors, harder X-ray
flux ratio and span a wider range of X-ray-to-optical colors. On average the
SEDs of high-luminosity BL AGNs resemble the power-law typical of unobscured
AGNs. The SEDs of NOT BL AGNs are dominated by the galaxy emission in the
optical/near-infrared, and show a rise in the mid-infrared which suggests the
presence of an obscured active nucleus.Comment: 15 pages, A&A accepted, affiliations correcte
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