arXiv subjects

C. Lari

Publications and source records attributed to C. Lari.

24 records · Page 2Linked to original sources

arXiv2003

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.

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arXiv2003

A far infrared view of the Lockman Hole from ISO 95 $μ$m observations - I. A new data reduction method

We report results of a new analysis of a deep 95 mu imaging survey with the photo-polarimeter ISOPHOT on board the Infrared Space Observatory, over a half square degree area within the Lockman Hole. To this end we exploit a newly developed parametric algorithm able to identify and clean spurious signals induced by cosmic-rays impacts and by transient effects and non-linearities in the detectors. These results provide us with the currently deepest -- to our knowledge -- far-IR image of the extragalactic sky. Within the survey area we detect thirty-six sources with S/N>3 (corresponding to a flux of 16 mJy), making up a complete flux-limited sample around 100$ mJy. Reliable sources are detected, with decreasing but well-controlled completeness, down to 20 mJy. The source extraction process and the completeness, the photometric and astrometric accuracies of this catalogue have been tested by us with extensive simulations accounting for all the details of the procedure. We estimate source counts down to a flux of ~30 mJy, at which limit we evaluate that from 10% to 20% of the cosmic IR background has been resolved into sources. The 95mu galaxy counts reveal a steep slope at fluxes lower than 100 mJy (alpha~1.6), in excess of that expected for a non-evolving source population. The shape of these counts agrees with those determined by ISO at 15 and 175 mu, and starts setting strong constraints on the evolution models for the far-IR galaxy populations.

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arXiv2003

The Radio/Mid-Infrared Correlation and the Contribution of 15-micron Galaxies to the 1.4-GHz Source Counts

The radio counterparts to the 15-micron sources in the European Large Area ISO Survey southern fields are identified in 1.4-GHz maps down to about 80 microJy. The radio-MIR correlation is investigated and derived for the first time at these flux densities for a sample of this size. Our results show that radio and MIR luminosities correlate almost as well as radio and FIR, at least up to z ~ 0.6. Using the derived relation and its spread together with the observed 15-micron counts, we have estimated the expected contribution of the 15-micron extragalactic populations to the radio source counts and the role of MIR starburst galaxies in the well known 1.4-GHz source excess observed at sub-mJy levels. Our analysis demonstrates that IR emitting starburst galaxies do not contribute significantly to the 1.4-GHz counts for strong sources, but start to become a significant fraction of the radio source population at flux densities < 0.5 - 0.8 mJy. They are expected to be responsible for more than 60% of the observed radio counts at < 0.05 mJy. These results are in agreement with the existing results on optical identifications of faint radio sources.

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arXiv2002

New Method for ISOCAM Data Reduction - II. Mid-Infrared Extragalactic Source Counts in the Southern ELAIS Field

We present the 15 micron extragalactic source counts from the Final Analysis Catalogue of the European Large Area ISO Survey southern hemisphere field S1, extracted using the Lari method. The large number of extragalactic sources (about 350) detected over this area between 0.5 and 100 mJy guarantee a high statistical significance of the source counts in the previously poorly covered flux density range between IRAS and the Deep ISOCAM Surveys. The bright counts in S1 (> 2mJy) are consistent with a flat, Euclidean slope, suggesting the dominance of a non-evolving population. In contrast, at fainter fluxes (< 2 mJy) our counts show a strong departure from no evolution models, with a very steep super-Euclidean slope down to our flux limit. Strong luminosity and density evolution is needed at least for the population of star-forming galaxies, together with a luminosity break around 10^10.8 Lsun in their local luminosity function in order to reproduce our sharp increase of the counts below 2 mJy and the redshift distributions observed for 15 micron sources at different flux levels. The contribution of the strongly evolving starburst population (down to 50 microJy) to the 15 micron cosmic background is estimated to be 2.2 nW m^-2 sr^-1 (67% of the total mid-infrared background estimate).

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arXiv2002

The Evolution of type 1 AGN in the IR (15mu). The view from ELAIS-S1

We present the 15 micron luminosity function of type 1 AGN (QSO + Seyfert 1). Our sample of 21 high-redshift sources is selected from the Preliminary Analysis catalogue in the S1 field of the European Large Area ISO Survey (ELAIS). To study the cosmological evolution of the AGN1 luminosity function, our sample has been combined with a local sample of 41 sources observed by IRAS. We find that the luminosity function of AGN1 at 15 micron is fairly well represented by a double-power-law-function. There is evidence for significant cosmological evolution consistent with a pure luminosity evolution (PLE) model L(z) (1+z)^k, with k=3.0-3.3. The value of k depends on the existence or not of an evolution cut-off at redshift ~2, and on the adopted cosmology. From the luminosity function and its evolution we estimate a contribution of AGN1 to the Cosmic Infrared Background (CIRB) of nuI_nu ~ 6 x 10^{-11}W m^{-2} sr^{-1} at 15 micron. This corresponds to ~2-3% of the total observed CIRB at this wavelength. Under the usual assumptions of unified models for AGN, the expected contribution of the whole AGN population to the CIRB at 15 micron is 10-15%.

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arXiv2001

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.

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