350 research outputs found
Polarization fluctuations due to extragalactic sources
We have derived the relationship between polarization and intensity
fluctuations due to point sources. In the case of a Poisson distribution of a
population with uniform evolution properties and constant polarization degree,
polarization fluctuations are simply equal to intensity fluctuations times the
average polarization degree. Conservative estimates of the polarization degree
of the classes of extragalactic sources contributing to fluctuations in the
frequency ranges covered by the forthcoming space missions MAP and Planck
Surveyor indicate that extragalactic sources will not be a strong limiting
factor to measurements of the polarization of the Cosmic Microwave Background.Comment: 15 pages LaTeX file, 3 postscript figures. Uses elsart.sty and
elsart.cls Accepted for publication in New Astronom
Age, metallicity and star formation history of spheroidal galaxies in cluster at z~1.2
We present the analysis, based on spectra collected at the Large Binocular
Telescope, of the stellar populations in seven spheroidal galaxies in the
cluster XLSSJ0223 at 1.22. The aim is to constrain the epoch of their
formation and their star formation history. Using absorption line strenghts and
full spectral fitting, we derive for the stellar populations of the seven
spheroids a median age =2.40.6 Gyr, corresponding to a median
formation redshift $\sim2.6_{-0.5}^{+0.7}$ (lookback time =
11$_{-1.0}^{+0.6}$ Gyr). We find a significant scatter in age, showing that
massive spheroids, at least in our targeted cluster, are not coeval. The median
metallicity is [Z/H]=0.09$\pm$0.16, as for early-types in clusters at
0$<z<<\sigma_e_{dyn}\Sigma_e_{dyn}\Sigma_e_{dyn}\Sigma_ez\sim1.3$, i.e.
more massive spheroids are more metal rich, have lower stellar mass density and
tend to be older than lower-mass galaxies.Comment: 16 pages, 6 figures, 6 tables, published on MNRA
The BeppoSAX High Energy Large Area Survey (HELLAS) - VI. The radio properties
We present results of a complete radio follow-up obtained with the VLA and
ATCA radio telescopes down to a 6 cm flux limit of about 0.3 mJy of all the 147
X-ray sources detected in the BeppoSAX HELLAS survey. We found 53 X-ray/radio
likely associations, corresponding to about one third of the X-ray sample.
Using the two point spectral index alpha_ro=0.35 we divided all the HELLAS
X-ray sources in radio quiet and radio loud. We have 26 sources classified as
radio-loud objects, corresponding to about 18% of the HELLAS sample. In
agreement with previous results, the identified radio-loud sources are
associated mainly with Type 1 AGNs with L(5-10 keV) > 10^44 erg/s, while all
the identified Type 2 AGNs and Emission Line Galaxies are radio quiet objects
with L(5-10 keV) < 10^44 erg/s. The analysis of the radio spectral index
suggests that Type 1 AGNs have a mean radio spectral index flatter than Type 2
AGNs and Emission Line Galaxies.Comment: 12 pages, 5 figures, MNRAS, accepte
On the nature of the ISO-selected sources in the ELAIS S2 region
We have studied the optical, near-IR and radio properties of a complete
sample of 43 sources detected at 15-micron in one of the deeper ELAIS
repeatedly observed region. The extragalactic objects in this sample have
15-micron flux densities in the range 0.4-10 mJy, where the source counts start
diverging from no evolution models. About 90% of the sources (39 out of 43)
have optical counterparts brighter than I=21 mag. Eight of these 39 sources
have been identified with stars on the basis of imaging data, while for another
22 sources we have obtained optical spectroscopy, reaching a high
identification percentage (30/43, ~70%). All but one of the 28 sources with
flux density > 0.7 mJy are identified. Most of the extragalactic objects are
normal spiral or starburst galaxies at moderate redshift (z_med~0.2); four
objects are Active Galactic Nuclei. We have used the 15-micron, H_alpha and
1.4-GHz luminosities as indicators of star-formation rate and we have compared
the results obtained in these three bands. While 1.4-GHz and 15-micron
estimates are in good agreement, showing that our galaxies are forming stars at
a median rate of ~40 Mo/yr, the raw H_alpha-based estimates are a factor ~5-10
lower and need a mean correction of ~2 mag to be brought on the same scale as
the other two indicators. A correction of ~2 mag is consistent with what
suggested by the Balmer decrements H_alpha/H_beta and by the optical colours.
Moreover, it is intermediate between the correction found locally for normal
spirals and the correction needed for high-luminosity 15-micron objects,
suggesting that the average extinction suffered by galaxies increases with
infrared luminosity.Comment: 19 pages, 11 figures (3 in JPEG format), MNRAS, accepte
A New Method for ISOCAM Data Reduction - I. Application to the European Large Area ISO Survey Southern Field: Method and Results
We have developed a new data reduction technique for ISOCAM LW data and have
applied it to the European Large Area ISO Survey (ELAIS) LW3 (15 micron)
observations in the southern hemisphere (S1). This method, known as LARI
technique and based on the assumption of the existence of two different time
scales in ISOCAM transients (accounting either for fast or slow detector
response), was particularly designed for the detection of faint sources. In the
ELAIS S1 field we obtained a catalogue of 462 15 micron sources with
signal-to-noise ratio >= 5 and flux densities in the range 0.45 - 150 mJy
(filling the whole flux range between the Deep ISOCAM Surveys and the IRAS
Faint Source Survey). The completeness at different flux levels and the
photometric accuracy of this catalogue have been tested with simulations. Here
we present a detailed description of the method and discuss the results
obtained by its application to the S1 LW3 data.Comment: 20 pages, LaTeX, MNRAS style, 20 postscript figures, full catalogue
not yet available at http://boas5.bo.astro.it/~elais/catalogues/. Accepted
for publication in MNRA
The BeppoSAX HELLAS survey: on the nature of faint hard X-ray selected sources
The BeppoSAX 4.5-10 keV High Energy Large Area Survey has covered about 80
square degrees of sky down to a flux of F(5-10keV)~5E-14 cgs. Optical
spectroscopic identification of about half of the sources in the sample (62)
shows that many (~50%) are highly obscured AGN, in line with the predictions of
AGN synthesis models for the hard X-ray background (XRB, see e.g. Comastri et
al. 1995). The X-ray data, complemented by optical, near-IR and radio
follow-up, indicate that the majority of these AGN are ``intermediate''
objects, i.e. type 1.8-1.9 AGN,`red' quasars, and even a few broad line, blue
continuum quasars, obscured in X-rays by columns of the order of
logNH=22.5-23.5 cm-2, but showing a wide dispersion in optical extinction. The
optical and near-IR photometry of the obscured objects are dominated by galaxy
starlight, indicating that a sizeable fraction of the accretion power in the
Universe may actually have been missed in optical color surveys. This also
implies that multicolor photometry techniques may be efficiently used to assess
the redshift of the hard X-ray selected sources.Comment: 9 pages, Invited talk to appear in the Proceedings of the Conference
X-ray Astronomy '999: Stellar Endpoints, AGNs and the Diffuse X-ray
Background. (September 6-10 - 1999
A deep VLA survey at 6 cm in the Lockman Hole
We have obtained a deep radio image with the Very Large Array at 6 cm in the
Lockman Hole. The noise level in the central part of the field is about 11
microJy. From these data we have extracted a catalogue of 63 radio sources. The
analysis of the radio spectral index suggests a flattening of the average radio
spectra and an increase of the population of flat spectrum radio sources in the
faintest flux bin. Cross correlation with the ROSAT/XMM X-ray sources list
yields 13 reliable radio/X-ray associations, corresponding to about 21 per cent
of the radio sample. Most of these associations (8 out of 13) are classified as
Type II AGN. Using optical CCD (V and I) and K' band data we found an optical
identification for 58 of the 63 radio sources. This corresponds to an
identification rate of about 92 per cent, one of the highest percentages so far
available. From the analysis of the colour-colour diagram and of the radio flux
- optical magnitude diagram we have been able to select a subsample of radio
sources whose optical counterparts are likely to be high redshift (z>0.5)
early-type galaxies, hosting an Active Galactic Nucleus responsible of the
radio activity. We also find evidence that at these faint radio limits a large
fraction (about 60 per cent) of the faintest optical counterparts (i.e. sources
in the magnitude range 22.5<I<24.5 mag) of the radio sources are Extremely Red
Objects (EROs) with I-K'>4.Comment: 20 pages, Astronomy and Astrophysics, accepte
A Deep VLA survey at 6 cm in the Lockman Hole
This is the published version, also available here: http://dx.doi.org/10.1051/0004-6361:20021721.We have obtained a deep radio image with the Very Large Array at 6 cm in the Lockman Hole. The noise level in the central part of the field is ~ Jy. From these data we have extracted a catalogue of 63 radio sources with a maximum distance of 10 arcmin from the field center and with peak flux density greater than 4.5 times the local rms noise. The differential source counts are in good agreement with those obtained by other surveys. The analysis of the radio spectral index suggests a flattening of the average radio spectra and an increase of the population of flat spectrum radio sources in the faintest flux bin. Cross correlation with the ROSAT/XMM X-ray sources list yields 13 reliable radio/X-ray associations, corresponding to ~21% of the radio sample. Most of these associations (8 out of 13) are classified as type II AGN.
Using optical CCD ( V and I) and band data with approximate limits of mag, mag and mag, we found an optical identification for 58 of the 63 radio sources. This corresponds to an identification rate of ~92%, one of the highest percentages so far available. From the analysis of the colour-colour diagram and of the radio flux - optical magnitude diagram we have been able to select a subsample of radio sources whose optical counterparts are likely to be high redshift ( z>0.5) early-type galaxies, hosting an Active Galactic Nucleus responsible of the radio activity. This class of objects, rather than a population of star-forming galaxies, appears to be the dominant population ( 50%) in a 5 GHz selected sample with a flux limit as low as 50 Jy.
We also find evidence that at these faint radio limits a large fraction (~60%) of the faintest optical counterparts (i.e. sources in the magnitude range 22.54$ and combining our radio data with existing ISO data we conclude that these EROs sources are probably associated with high redshift, passively evolving elliptical galaxies. The six radio selected EROs represent only ~2% of the optically selected EROs present in the field. If their luminosity is indeed a sign of AGN activity, the small number of radio detections suggests that a small fraction of the EROS population contains an active nucleus
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