arXiv subjects

F. La Franca

Publications and source records attributed to F. La Franca.

At least 91 records · Page 5Linked to original sources

arXiv2005

The Redshift-Space Two Point Correlation Function of ELAIS-S1 Galaxies

We investigate the clustering properties of galaxies in the recently completed ELAIS-S1 redshift survey through their spatial two point autocorrelation function. We used a sub-sample of the ELAIS-S1 catalog covering approximately 4 deg^2 and consisting of 148 objects selected at 15 micron with a flux >0.5 mJy and redshift z<0.5. We detected a positive signal in the correlation function that, in the range of separations 1-10 h mpc is well approximated by a power law with a slope gamma = 1.4 and a correlation length s =5.4 h mpc. This result is in good agreement with the redshift-space correlation function measured in more local samples of mid infrared selected galaxies like the IRAS PSC$z$ redshift survey. This suggests a lack of significant clustering evolution of infrared selected objects out to $z=0.5$ that is further confirmed by the consistency found between the correlation functions measured in a local (z<0.2) and a distant (0.2<z<0.5) subsample of ELAIS-S1 galaxies. We also confirm that optically selected galaxies in the local redshift surveys, especially those of the SDSS sample, are significantly more clustered than infrared objects.

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arXiv2005

Final Analysis of ELAIS 15 $μ$m Observations : Method, Reduction and Catalogue

We present the Final Analysis of the European Large Area ISO Survey (ELAIS) 15 $μ$m observations, carried out with the ISOCAM instrument on board the Infrared Space Observatory (ISO). The data reduction method, known as LARI method, is based on a mathematical model of the detector's behaviour and was specifically designed for the detection of faint sources in ISO-CAM/PHOT data. The method is fully interactive and leads to very reliable and complete source lists. The resulting catalogue includes 1923 sources detected with $S/N > 5$ in the {0.5 -- 100 mJy} flux range and over an area of 10.85 {deg$^2$} split into four fields, making it the largest non-serendipitous extragalactic source catalogue obtained to date from ISO data. This paper presents the concepts underlying the data reduction method together with its latest enhancements. The data reduction process, the production and basic properties of the resulting catalogue are discussed. The catalogue quality is assessed by means of detailed simulations, optical identifications and comparison with previous analyses.

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arXiv2004

The European Large Area ISO Survey IX: the 90 micron luminosity function from the Final Analysis sample

We present the 90 micron luminosity function of the Final Analysis of the European Large Area ISO Survey (ELAIS), extending the sample size of our previous analysis (paper IV) by about a factor of 4. Our sample extends to z=1.1, around 50 times the comoving volume of paper IV, and 10^{7.7} < h^{-2}L/Lsun < 10^{12.5}. From our optical spectroscopy campaigns of the northern ELAIS 90 mircon survey (7.4 deg^2 in total, to S(90um)>70mJy), we obtained redshifts for 61% of the sample (151 redshifts) to B<21 identified at 7 microns, 15 microns, 20cm or with bright (B<18.5) optical identifications. The selection function is well-defined, permitting the construction of the 90 micron luminosity function of the Final Analysis catalogue in the ELAIS northern fields, which is in excellent agreement with our Preliminary Analysis luminosity function in the ELAIS S1 field from paper IV. The luminosity function is also in good agreement with the IRAS-based prediction of Serjeant & Harrison (2004), which if correct requires luminosity evolution of (1+z)^{3.4 +/- 1.0} for consistency with the source counts. This implies an evolution in comoving volume averaged star formation rate at z<~1 consistent with that derived from rest-frame optical and ultraviolet surveys.

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arXiv2004

The HELLAS2XMM survey: VI. X-ray absorption in the 1dfAGN sample through a spectral analysis

The spectroscopic analysis of 117 serendipitous sources in the HELLAS2XMM 1df (1 degree field) survey is described. Of these, 106 sources, of which 86% have a spectroscopic redshift, are used to evaluate the fraction of X-ray absorbed (log NH>22) Active Galactic Nuclei (AGN) in the 2--10 keV flux range 0.8-20E-14 erg/cm2/s. This fraction turns out lower than what is predicted by two well known Cosmic X-Ray Background synthesis models, and the discrepancy is significant at the 99.999% level. This result consolidates the findings recently obtained by other authors. In the flux interval explored, the data are consistent with an intrinsic distribution of the absorbing columns (flat per decade above logNH>21) independent of luminosity and redshift, together with an AGN luminosity function evolving purely in luminosity. It is shown that, on the other hand, extrapolation to lower fluxes fails to reproduce the results inferred from the Chandra Deep Field North survey. It is found that about 40% of the high luminosity sources in our sample have best fit logNH>22, and the surface density of these X-ray obscured QSOs can then be estimated at about 48 per square degree, at the flux limit of ~1E-14} erg/cm2/s of the HELLAS2XMM 1df survey. As a side issue, 5 or 6 out of 60 sources, that is about 10%, identified with broad line AGN, turn out to be affected by logNH>22 absorption.

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arXiv2004

The European Large Area ISO Survey (ELAIS): The Final Band-merged Catalogue

We present the final band-merged ELAIS catalogue at 6.7, 15, 90, and 175 $μ$m, and the associated data at u,g,r,i,z,J,H,K, and 20cm. The origin of the survey, infrared and radio observations, data-reduction and optical identifications are briefly reviewed, and a summary of the area covered, and completeness limit for each infrared band is given. A detailed discussion of the band-merging and optical association strategy is given. The total catalogues consists of 2860 sources. For extragalactic sources observed in 3 or more infrared bands, colour-colour diagrams are presented and discussed in terms of the contributing infrared populations. Spectral energy distributions are shown for selected sources and compared with cirrus, M82 and Arp220 starburst, and AGN dust torus models. Spectroscopic redshifts are tabulated, where available. For the N1 and N2 areas, the INT ugriz Wide Field Survey, permits photometric redshifts to be estimated for galaxies and quasars. These agree well with the spectroscopic redshifts, within the uncertainty of the photometric method. The redshift distribution is given for selected ELAIS bands and colour-redshift diagrams are discussed. There is a high proportion of ultraluminous infrared galaxies in the ELAIS Catalogue ($> 10 %$ of 15 $μ$m sources), many with Arp220-like colours. 10 hyperluminous infrared galaxies and 10 EROs are found in the survey. The large numbers of ultraluminous galaxies imply very strong evolution in the star-formation rate between z = 0 and 1.

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arXiv2004

The nature of the mid-infrared population from optical identifications of the ELAIS-S1 sample

We present a multi-wavelength catalog (15 um, R, K-band, 1.4 GHz flux) plus spectroscopic identifications for 406 15 um sources detected in the ELAIS region S1, over the flux density range 0.5<S<150 mJy. 332 (~82%) sources are optically identified down to R~23.0. Spectra or bona fide stellar identifications are obtained for 290 objects. The areal coverage, MIR and optical completeness of the sample are discussed in order to allow statistical and evolutionary analyses. Two main spectroscopic classes have been found to dominate the MIR extragalactic population: z<0.5 star-forming galaxies (from absorbed to extreme starbursts: nuL_nu~10^8-10^11 L_odot), which account for \~75% of the sources, and AGN (both type 1 and 2), which account for ~25% of the sources. About 20% of the extragalactic sources are dust-enshrouded starburst galaxies [e(a) spectra], and all the starburst galaxies appear more dust extincted in the optical than nearby normal galaxies. We also identified 91 stellar objects (~22% of the MIR sources). The counts for starburst galaxies and AGN down to 0.6 mJy have been derived. A general trend is found in the optical-MIR SED of the galaxies, where the MIR-luminous objects have larger MIR to optical luminosity ratios.

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arXiv2004

The Mid-IR luminosity function of galaxies in the ELAIS Southern fields

We present the first determination of the 15micron luminosity functio galaxies from the European Large Area ISO survey (ELAIS) southern fields. We have adopted a new criterion to separate the quiescent, non-evolving and the starburst, evolving populations based on the ratio of mid-infrared to optical luminosities. Strong evolution is suggested by our data for the starburst galaxy population, while normal spiral galaxies are consistent with no evolution. The starburst population must evolve both in luminosity and in density with rates of the order L(z) (1+z)^(3.5) and rho(z) (1+z)^3.8 up to z~1. The evolutionary parameters of our model have been tested by comparing the model predictions with other observables, like source counts at all flux density levels (from 0.1 to 300 mJy) and redshift distributions and luminosity functions at high-z (0.7 < z < 1.0 from HDF-N data). The agreement between our model predictions and the observed data is remarkably good. We use our data to estimate the star-formation density of the Universe up to z=0.4 and we use the luminosity function model to predict the trend of the star-formation history up to z=1.

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arXiv2004

Large Scale Structure in the ELAIS S1 Survey

We present an analysis of the two-point angular correlation function of the ELAIS S1 survey. The survey covers 4 deg$^2$ and contains 462 sources detected at 15$μ$m to a 5$σ$ flux limit of 0.45 mJy. Using the 329 extragalactic sources not repeated in different observations, we detect a significant clustering signal; the resulting angular correlation function can be fitted by a exponential law $w(θ) = A θ^{1-γ}$ with $A = 0.014\pm0.005$ and $γ= 2.04\pm0.18$. Assuming a redshift distribution of the objects, we invert Limber's equation and deduce a spatial correlation length $r_0=4.3^{+0.4}_{-0.7} h^{-1}$ Mpc. This is smaller than that obtained from optical surveys but it is in agreement with results from IRAS. This extends to higher redshift the observational evidence that infrared selected surveys show smaller correlation lengths (i.e. reduced clustering amplitudes) than optical surveys.

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arXiv2004

The HELLAS2XMM survey. V. Near-Infrared observations of X-ray sources with extreme X/O ratios

We present the results of deep near-infrared observations of 11 hard X-ray selected sources in the Hellas2XMM survey, with faint optical magnitude (R>24) and high X-ray-to-optical flux ratio. All but one of the sources have been detected in the Ks band, with bright counterparts (Ks<19) and red colors (R-K>5), and therefore belong to the ERO population. A detailed analysis of the surface brightness profiles allows us to classify all of the NIR counterparts. There are 2 point-like objects, 7 elliptical galaxies and one source with a disky profile. None of the extended sources shows any evidence for the presence of a central unresolved object tracing the putative X-ray emitting AGN. Using both the R-K colors and the morphological information, we have estimated for all the sources a ``minimum photometric redshift'', ranging between 0.8 and 2.4; the elliptical hosts have zmin=0.9-1.4. We computed the X-ray properties using these redshifts: most of the sources have NH>10^{22}, with unabsorbed X-ray luminosities up to 10^{45}erg s^{-1}. These objects therefore belong to the population of obscured (Type II) quasars and, from a statistical point of view, they turn out to be a non-negligible fraction (~10%) of the most luminous AGN. Selecting the high X/O sources for a follow-up study in the NIR is therefore a powerful technique aimed at studying at high redshift the hosts of Type II AGN. Overall, our results seem to indicate that the hosts are mostly elliptical galaxies at z~1, and that these near-IR bright objects would be among the most massive spheroids at these epochs.

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arXiv2003

The HELLAS2XMM 1dF survey: on the nature of high X-ray to optical flux ratio sources

We present results from the photometric and spectroscopic identification of 122 X-ray sources discovered by XMM-Newton in the 2-10 keV band (the HELLAS2XMM 1dF sample). One of the most interesting results (which is found also in deeper sourveys) is that ~20% of the sources have an X-ray to optical flux ratio (X/O) ten times or more higher than that of optically selected AGN. Unlike the faint sources found in the ultra-deep Chandra and XMM-Newton surveys, which reach X-ray (and optical) fluxes >~10 times lower than in the HELLAS2XMM sample, many of the extreme X/O sources in our sample have R~25 and are therefore accessible to optical spectroscopy. We report the identification of 13 sources with extreme X/O values. While four of these sources are broad line QSO, eight of them are narrow line QSO, seemingly the extension to very high luminosity of the type 2 Seyfert galaxies.

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arXiv2003

The Hellas2XMM survey: III. Multiwavelength observations of hard X-ray selected sources in the PKS 0312-77 field

We present extensive optical, radio and infrared follow-up observations of a sample of 35 hard X-ray (2-10 keV) selected sources discovered serendipitously in the PV XMM-Newton observation of the radio-loud quasar PKS 0312-77 field, for which also an archival Chandra observation is available. The observations have been carried out as part of the HELLAS2XMM survey, a program aimed to understand the nature of the sources responsible for the bulk of the hard X-ray Background (XRB). The identification of the optical counterparts greatly benefits from the positional accuracy obtained from Chandra and radio observations. As a consequence, the spectroscopic completeness of the present sample (80%) is limited only by the faintness of the optical counterparts. The multiwavelength coverage of our survey allows us to unveil a large spread in the overall properties of hard X-ray selected sources. At low redshift (z<1), the source breakdown includes Broad Line AGN, Narrow Emission-Line Galaxies, and optically ``normal'' galaxies. All the ten sources at z>1 are spectroscopically classified as Broad Line AGNs. A few of them show significant intrinsic X-ray absorption (N(H)>10^22 cm^-2), further supporting previous evidence of a decoupling between optical and X-ray properties at high luminosities and redshifts. Finally, a non negligible fraction (15%) of the hard X-ray sources are not detected down to the limiting magnitude of the optical images. The corresponding high X-ray to optical flux ratio, X-ray and optical-infrared colors strongly suggest that they are high redshift, obscured AGN.

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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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arXiv2003

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.

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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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arXiv2002

On the nature of X-ray Bright Optically Normal galaxies

Multiwavelength observations of the hard X-ray selected sources by Chandra and XMM-Newton surveys have significantly improved our knowledge of the objects responsible of the hard X-ray background. A surprising finding is the discovery of a population of optically dull, X-ray bright galaxies emerging at 2-10 keV fluxes of the order of 10^{-14} erg cm^{-2} s^{-1}. We present preliminary results of multiwavelength observations of a few objects serendipitously discovered in the field of XMM-Newton and Chandra observations and intensively studied at longer wavelengths.

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arXiv2002

The BeppoSAX High Energy Large Area Survey -V. The nature of the hard X-ray source populations and its evolution

We present optical spectroscopic identifications of hard X-ray (5-10 keV) selected sources belonging to the HELLAS sample obtained with BeppoSAX down to a 5-10 keV flux limit of f_{5-10 keV}~3x10^-14 erg cm^-2 s^-1. The sample consists of 118 sources. 25 sources have been identified trough correlations with catalogues of known sources. 49 have been searched for spectroscopic identification at the telescope. 13 fields resulted empty down to R=21. 37 sources have been identified with type 1 AGN and 9 with type 2 AGN. The remaining are: 5 narrow emission line galaxies, 6 Clusters, 2 BL Lac, 1 Radio Galaxy and 1 Star. Combining these objects with other hard X-ray selected AGNs from ASCA and HEAO1, we find that the local luminosity function of type 1 AGN in the 2-10 keV band is fairly well represented by a double-power-law-function. There is evidence for significant cosmological evolution according to a pure luminosity evolution (PLE) model L_X(z)~(1+z)^k, with k=2.12 and k=2.22 in a (Omega_m,Omega_lambda)=(1.0,0.0) and in a (Omega_m,Omega_lambda)=(0.3,0.7) cosmology, respectively. The data show an excess of faint high redshift type 1 AGN which is well modeled by a luminosity dependent density evolution (LDDE), similarly to what observed in the soft X-rays. However, in both cosmologies, the statistic is not significant enough to distinguish between the PLE and LDDE models. The fitted models imply a contribution of AGN1 to the 2-10 keV X-ray background from 35% up to 60%.

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arXiv2002

The HELLAS2XMM survey: II. Multiwavelength observations of P3: an X-ray bright, optically inactive galaxy

Recent X-ray surveys have clearly demonstrated that a population of optically dull, X-ray bright galaxies is emerging at 2-10 keV fluxes of the order of 10^{-14} erg cm^{-2} s^{-1}. Although they might constitute an important fraction of the sources responsible for the hard X-ray background, their nature is still unknown. With the aim to better understand the physical mechanisms responsible for the observed properties, we have started an extensive program of multiwavelength follow-up observations of hard X-ray, optically quiet galaxies discovered with XMM-Newton. Here we report the results of what can be considered the first example of this class of objects: CXOUJ031238.9-765134, originally discovered by Chandra, and optically identified by Fiore et al. (2000) with an apparently normal early-type galaxy at z=0.159, usually known as "FIORE P3". The analysis of the broad-band energy distribution suggests the presence of a heavily obscured active nucleus.

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arXiv2001

The evolution of AGNs in the Hard X-Rays and the Infrared

We present the estimate of the evolution of type 1 AGNs in the hard (2-10 keV) X-rays drawn from the HELLAS survey, and in the IR (15um) obtained from the ELAIS survey. We find that the local luminosity function (LF) of AGN1 in the 2-10 keV band is fairly well represented by a double-power-law-function. There is evidence for significant cosmological evolution according to a pure luminosity evolution model Lx(z)~(1+z)^k, with k=2.12 and k=2.19 in a (Omega_m,Omega_lambda)=(1.0,0.0) and in a (Omega_m,Omega_lambda)=(0.3,0.7) cosmology respectively. In a (Omega_m,Omega_lambda)=(1.0,0.0) Universe the data show an excess of faint high redshift type 1 AGN which is well modeled by a luminosity dependent density evolution, similarly to what observed in the soft X-rays. In the IR band, with a (Omega_m,Omega_lambda)=(1.0,0.0) cosmology, the evolution found is similar to what observed in other wavebands, the LF is a double-power-law-function with a bright slope 2.9 and a faint slope 1.1, following a luminosity evolution model L(z)~(1+z)^3.0.

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