C. Lari

The Mid-Infrared Luminosity Function of Galaxies in the European Large Area Infrared Space Observatory Survey Southern Fields

We present the first determination of the 15μm luminosity function of 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) and ρ(z) ∝ (1+ z) 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. Subject headings: galaxies: evolution — galaxies: luminosity function — galaxies: spiral — galaxies: starburst — infrared: galaxies Dipartimento di Astronomia, Università di Bologna, viale Berti Pichat 6, I–40127 Bologna, Italy INAF Osservatorio Astronomico di Bologna, via Ranzani 1, I–40127 Bologna, Italy Astronomy Centre, Department of Physics & Astronomy, School of Science and Technology, University of Sussex, Brighton, BN1 9QJ, UK Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, D-85741 Garching, Germany Dipartimento di Fisica, Università ’Roma Tre’, via della Vasca Navale 84, I-00146 Roma, Italy Istituto di Radioastronomia del CNR, via Gobetti 101, I-40129 Bologna, Italy Dipartimento di Astronomia, Università di Padova, vicolo dell’Osservatorio 2, I-35122 Padova, Italy Astrophysics Group, Blackett Laboratory, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BW, UKWe present the first determination of the 15 mum luminosity function of galaxies from the European Large Area ISO Survey (ELAIS) southern fields. We have adopted a new criterion to separate the quiescent, nonevolving and the starburst, evolving populations based on the ratio of mid-infrared to optical luminosity. 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, respectively, 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 Hubble Deep Field North [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.

A new method for ISOCAM data reduction - II. Mid-infrared extragalactic source counts in the ELAIS Southern field

We present the 15-µm extragalactic source counts from the Final Analysis Catalogue of the European Large Area ISO Survey (ELAIS) Southern hemisphere field S1, extracted using the Lari method. The large number of extragalactic sources (∼350) detected over this area between about 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 (S15µm 2 mJy) are significantly lower than other published ISOCAM counts in the same flux range and are consistent with a flat, Euclidean slope, suggesting the dominance of a non-evolving population. In contrast, at fainter fluxes (S15µm 2 mJy) our counts show a strong departure from no-evolution models, with a very steep super-Euclidean slope down to our flux limit (∼0.5 mJy). Strong luminosity and density evolutions of the order of, respectively, L ∝ (1 + z) 3.0 and ρ ∝ (1 + z) 3.5 are needed at least for the population of starforming galaxies in order to fit the counts and the redshift distributions observed at different

Mid‐infrared spectroscopy of infrared‐luminous galaxies at z∼ 0.5–3

We present results on low-resolution mid-infrared (MIR) spectra of 70 IR-luminous galaxies obtained with the infrared spectrograph (IRS) onboard Spitzer. We selected sources from the European Large Area Infrared Survey with S15 > 0.8 mJy and photometric or spectroscopic z > 1. About half of the samples are quasi-stellar objects (QSOs) in the optical, while the remaining sources are galaxies, comprising both obscured active galactic nuclei (AGN) and starbursts. Redshifts were obtained from optical spectroscopy, photometric redshifts and the IRS spectra. The later turn out to be reliable for obscured and/or star-forming sources, thus becoming an ideal complement to optical spectroscopy for redshift estimation. We estimate monochromatic luminosities at several rest-frame wavelengths, equivalent widths and luminosities for the polycyclic aromatic hydrocarbon (PAH) features, and strength of the silicate feature in individual spectra. We also estimate integrated 8–1000 μ mI R luminosities via spectral energy distribution fitting to MIR and far-IR (FIR) photometry from the Spitzer Wide-Area Infrared Extragalactic survey and the MIR spectrum. Based on these measurements, we classify the spectra using well-known IR diagnostics, as well as a new one that we propose, into three types of source: those dominated by an unobscured AGN, mostly corresponding to optical quasars (QSOs), those dominated by an obscured AGN and starburst-dominated sources. Starbursts concentrate at z ∼ 0.6–1.0 favoured by the shift of the 7.7-μm PAH band into the selection 15-μm band, while AGN spread over the 0.5 < z < 3.1 range. Star formation rates (SFR) are estimated for individual sources from the luminosity of the PAH features. An estimate of the average PAH luminosity in QSOs and obscured AGN is

A New Method for ISOCAM Data Reduction - I. Application to the European Large Area ISO Survey Southern Field: Method and Results

We develop a new data reduction technique for ISOCAM LW data and apply it to the European Large Area ISO Survey (ELAIS) LW3 (15-mum) 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 for either fast or slow detector response), is particularly designed for the detection of faint sources. In the ELAIS S1 field we obtain a catalogue of 462 15-mum sources with signal-to-noise ratios >=5 and flux densities in the range 0.45-150mJy (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 are 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.

The radio–mid‐infrared correlation and the contribution of 15‐μm galaxies to the 1.4‐GHz source counts

The radio counterparts to the 15-µm sources in the European Large Area ISO Survey southern fields are identified in 1.4-GHz maps down to ∼80 µJy. The radio‐mid-infrared 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 mid-infrared (MIR) luminosities correlate almost as well as radio and far-infrared (FIR), at least up to z � 0.6. Using the derived relation and its spread together with the observed 15-µm counts, we have estimated the expected contribution of the 15-µm 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 per cent of the observed radio counts at 0.05 mJy. These results are in agreement with the existing results on optical identifications of faint radio

Active galactic nuclei in the mid-IR: evolution and contribution to the cosmic infrared background

Aims. We study the evolution of the luminosity function (LF) of type-1 and type-2 Active Galactic Nuclei (AGN) in the mid-infrared, and then derive the contribution of the AGN to the Cosmic InfraRed Background (CIRB) and the expected source counts to be observed by Spitzer at 24� m. Methods. We used a sample of type-1 and type-2 AGN selected at 15� m (ISO) and 12� m (IRAS), and classified on the basis of their optical spectra. Local spectral templates of type-1 and type-2 AGN have been used to derive the intrinsic 15� m luminosities. We adopted an evolving smooth two-power law shape of the LF, whose parameters have been derived using an un-binned maximum likelihood method. Results. We find that the LF of type-1 AGN is compatible with a pure lumin osity evolution (L(z) = L(0)(1 + z) kL ) model where kL ∼2.9. A small flattening of the faint ( L15 < L ∗ ) slope of the LF with increasing redshift is favoured by the data. A similar evolutionary scenario is found for the type-2 population with a rate kL ranging from∼1.8 to 2.6, depending significantly on the adopted mid-infra red spectral energy distribution. Also for type-2 AGN a flattening of the LF with i ncreasing redshift is suggested by the data, possibly cause d by the loss of a fraction of type-2 AGN hidden within the optically classifie d starburst and normal galaxies. The type-1 AGN contribution to the CIRB at 15� m is (4.2‐12.1)× 10 −11 Wm −2 sr −1 , while the type-2 AGN contribution is (5.5‐11.0)× 10 −11 W m −2 sr −1 . We expect that Spitzer will observe, down to a flux limit of S 24� m=0.01 mJy, a density of∼1200 deg −2 type-1 and∼1000 deg −2 type-2 optically classified AGN. Conclusions. AGN evolve in the mid-infrared with a rate similar to the ones found in the optical and X‐rays bands. The derived total contribution of the AGN to the CIRB (4-10%) and Spitzer counts should be considered as lower limits, because of a possible loss of type-2 sources caused by the optical classification.

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

We present a multiwavelength catalog (15 μm, R-band, K-band, and 1.4 GHz flux) plus spectroscopic identifications for 406 15 μm sources detected in the European Large Area ISO Survey (ELAIS) region S1, over the flux density range 0.5 < S15μm < 150 mJy. Three hundred thirty-two (~82%) sources are optically identified down to R ~ 23.0. Spectra or bona fide stellar identifications are obtained for 290 objects (~88% of the optically identified sources). The areal coverage, mid-infrared (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: νLν(15 μm) ≈ 108–1011 L⊙], which account for ~75% of the sources, and active galactic nuclei (AGNs; 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 AGNs down to 0.6 mJy have been derived. A general trend is found in the optical-MIR spectral energy distribution of the galaxies, where the MIR-luminous objects have larger MIR to optical luminosity ratios. Based on a variety of analyses, we suggest that the ELAIS sources fainter than R ~ 23 are luminous and ultraluminous MIR galaxies (LIG-ULIGs; νLν(15 μm) = 1011–1012 L⊙) at intermediate redshifts (z = 0.5–1.5), and that consequently the present sample is virtually 100% spectroscopically complete up to z = 0.5.

The HELLAS2XMM survey - X. The bolometric output of luminous obscured quasars: the Spitzer perspective

Aims. We aim at estimating the spectral energy distributions (SEDs) and the physical parameters related to the black holes harbored in eight high X-ray-to-optical (FX/FR > 10) obscured quasars at z > 0.9 selected in the 2–10 keV band from the HELLAS2XMM survey. Methods. We use IRAC and MIPS 24 µm observations, along with optical and Ks-band photometry, to obtain the SEDs of the sources. The observed SEDs are modeled using a combination of an elliptical template and torus emission (using the phenomenological templates of Silva et al. 2004) for six sources associated with passive galaxies; for two point-like sources, the empirical SEDs of red quasars are adopted. The bolometric luminosities and the MBH − LK relation are used to provide an estimate of the masses and Eddington ratios of the black holes residing in these AGN. Results. All of our sources are detected in the IRAC and MIPS (at 24 µm) bands. The SED modeling described above is in good agreement with the observed near- and mid-infrared data. The derived bolometric luminosities are in the range ≈10 45 −10 47 erg s −1 , and the median 2–10 keV bolometric correction is ≈25, consistent with the widely adopted value derived by Elvis et al. (1994). For the objects with elliptical-like profiles in the Ks band, we derive high stellar masses (0.8−6.2) × 10 11 M� , black hole masses in the range (0.2−2.5) × 10 9 M� , and Eddington ratios L/LEdd < 0.1, suggesting a low-accretion phase.

Properties of FIRBACK–ELAIS 175-μm sources in the ELAIS N2 region

We report on a search for the optical counterparts of 175 um - selected sources from the Far-Infrared Background (FIRBACK) survey in the European Large Area ISO Survey (ELAIS) N2 field. Applying a likelihood ratio technique to optical catalogues from the Isaac Newton Telescope - Wide Field Survey (INT--WFS), we found optical identifications for 33 out of 55 FIRBACK sources in this field. These were then reassessed in the light of associations with the ELAIS final catalogue for the the N2 field, to yield a final set of 31 associations. We have investigated the nature of this population through a comparison of their observed spectral energy distributions with predictions from radiative transfer models which simulate the emission from both cirrus and starburst components. We find the far-infrared sources to be 80 per cent star bursting galaxies with their starburst component at a high optical depth. The resulting SEDs were used to estimate far-infrared luminosities, star formation rates, dust temperatures and dust masses. The N2 FIRBACK population is found to consist of four suspected ULIRGs, a number of LIRGs and a population of low redshift quiescently star forming galaxies. We also discuss the implications of these results for current evolutionary models.

The European Large Area ISO Survey – IX. The 90-μm luminosity function from the Final Analysis sample

We present the 90-mum 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, similar to50 times the comoving volume of Paper IV, and 10(7.7) 70 mJy), we obtained redshifts for 61 per cent of the sample (151 redshifts) to B <21 identified at 7 mum, 15 mum, 20 cm or with bright (B <18.5) optical identifications. The selection function is well-defined, permitting the construction of the 90-mum 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 S 1 field from Paper IV. The luminosity function is also in good agreement with the recent IRAS-based prediction of Serjeant and Harrison - 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 less than or similar to 1 consistent with that derived from rest-frame optical and ultraviolet surveys.