Extragalactic Source Counts in the Spitzer 24-micron Band: What Do We Expect From ISOCAM 15-micron Data and Models?

The comparison between the new Spitzer data at 24 micron and the previous ISOCAM data at 15 micron is a key tool to understand galaxy properties and evolution in the infrared and to interpret the observed number counts, since the combination of Spitzer with the ISO cosmological surveys provides for the first time the direct view of the Universe in the Infrared up to z~2. We present the prediction in the Spitzer 24-micron band of a phenomenological model for galaxy evolution derived from the 15-micron data. Without any ``a posteriori'' update, the model predictions seem to agree well with the recently published 24-micron extragalactic source counts, suggesting that the peak in the 24-micron counts is dominated by ``starburst'' galaxies like those detected by ISOCAM at 15 micron, but at higher redshifts (1 < z < 2 instead of 0.5 < z < 1.5).Comment: 8 pages: 4 pages of main text + 5 postscript figures, use aastex. Accepted for publication in ApJL. Replaced with the proof version (added missing references and corrected a few sentences

The star formation rate cookbook at 1 < z < 3: Extinction-corrected relations for UV & [OII]{\lambda}3727 luminosities

We use a spectroscopic sample of 286 star-forming galaxies (SFGs) at 1<z<3 from the GMASS survey to study different star formation rate (SFR) estimators. Infrared (IR) data are used to derive empirical calibrations to correct ultraviolet (UV) and [OII]{\lambda}3727 luminosities for dust extinction and dust-corrected estimates of SFR. In the selection procedure we fully exploit the available spectroscopic information. On the basis of three continuum indices, we are able to identify and exclude from the sample galaxies in which old stellar populations might bring a non-negligible contribution to IR luminosity (LIR) and continuum reddening. Using Spitzer-MIPS and Herschel-PACS data we derive LIR for two-thirds of our sample. The LIR/LUV ratio is used as a probe of effective attenuation (AIRX) to search for correlations with continuum and spectroscopic features. The relation between AIRX and UV continuum slope ({\beta}) was tested for our sample and found to be broadly consistent with the literature results at the same redshift, though with a larger dispersion with respect to UV-selected samples. We find a correlation between the rest-frame equivalent width (EW) of the [OII]{\lambda}3727 line and {\beta}, which is the main result of this work. We therefore propose the [OII]{\lambda}3727 line EW as a dust attenuation probe and calibrate it through AIRX, though the assumption of a reddening curve is still needed to derive the actual attenuation towards the [OII]{\lambda}3727 line. We tested the issue of differential attenuation towards stellar continuum and nebular emission: our results are in line with the traditional prescription of extra attenuation towards nebular lines. A set of relations is provided that allows the recovery of the total unattenuated SFR from UV and [OII]{\lambda}3727 luminosities. (Abridged)Comment: Accepted for publication in A&A; 20 pages, 19 figures, 5 table

ISOCAM observations in the Lockman Hole - II The 14.3 micron deep survey: data reduction, catalogue and source counts

We present a new analysis of the ISOCAM 14.3 micron deep survey in a 20x20 square arcmins area in the Lockman Hole. This survey is intermediate between the ultra-deep surveys and the shallow surveys in the ELAIS fields. The data have been analyzed with the method presented by Lari et al. (2001). We have produced a catalogue of 283 sources detected above the 5-sigma threshold, with fluxes in the interval 0.1-8 mJy. The catalogue is 90% complete at 1 mJy. The positional accuracy, estimated from the cross-correlation of infrared and optical sources, is around 1.5 arcsec. The search for the optical counterparts of the sources in the survey is performed on a medium-deep r' band optical image (5-sigma depth of r'=25), making use of the radio detections when available. The photometry has been checked through simulations and by comparing the data with those presented in a shallower and more extended ISOCAM survey in the Lockman Hole, that we have presented in a companion paper. Only 15% of the 14.3 micron sources do not have an optical counterpart down to r'=25 mag. We use the 6.7/14.3 micron colour as a star/galaxy separator, together with a visual inspection of the optical image and an analysis of the observed Spectral Energy Distribution of the ISOCAM sources. The stars in the sample turn out to be only 6% of the sample. We discuss the 14.3 micron counts of extragalactic sources, combining our catalogue with that obtained from the shallower ISOCAM survey. The data in the two surveys are consistent, and our results fully support the claims in previous works for the existence of an evolving population of infrared galaxies, confirming the evident departure from non-evolutionary model predictions.Comment: Version accepted by Astronomy and Astrophysics. Images have been heavily degraded to fill the archive size. A full resolution version can be downloaded at the following link http://dipastro.pd.astro.it/giulia/Deep/1252.ps.g

AGN feedback at z~2 and the mutual evolution of active and inactive galaxies

The relationships between galaxies of intermediate stellar mass and moderate luminosity active galactic nuclei (AGNs) at 1<z<3 are investigated with the Galaxy Mass Assembly ultra-deep Spectroscopic Survey (GMASS) sample complemented with public data in the GOODS-South field. Using X-ray data, hidden AGNs are identified in unsuspected star-forming galaxies with no apparent signs of non-stellar activity. In the color-mass plane, two parallel trends emerge during the ~2 Gyr between the average redshifts z~2.2 and z~1.3: while the red sequence becomes significantly more populated by ellipticals, the majority of AGNs with L(2-10 keV)>10^42.3 erg s^-1 disappear from the blue cloud/green valley where they were hosted predominantly by star-forming systems with disk and irregular morphologies. These results are even clearer when the rest-frame colors are corrected for dust reddening. At z~2.2, the ultraviolet spectra of active galaxies (including two Type 1 AGNs) show possible gas outflows with velocities up to about -500 km s^-1 that are not observed neither in inactive systems at the same redshift, nor at lower redshifts. Such outflows indicate the presence of gas that can move faster than the escape velocities of active galaxies. These results suggest that feedback from moderately luminous AGNs (logL_X~2 by contributing to outflows capable of ejecting part of the interstellar medium and leading to a rapid decrease in the star formation in host galaxies with stellar masses 10<logM<11 M_Sun.Comment: Astrophysical Journal Letters, in press (6 pages, 4 figures

A far infrared view of the Lockman Hole from ISO 95 micron observations - II. Optical identifications and insights into the nature of the far-infrared sources

We present the optical identifications of a 95 micron ISOPHOT sample in the Lockman Hole over an area of about half square degree. The catalogue (Rodighiero et al. 2003) includes 36 sources, making up a complete flux-limited sample for fluxes > 100 mJy. Reliable sources were detected, with decreasing but well-controlled completeness, down to 20 mJy. We have combined mid-IR and radio catalogues in this area to identify the potential optical counterparts of the far-IR sources. We found 14 radio and 13 15 micron associations, 10 of which have both associations. For the 11 sources with spectroscopic redshift, we have performed a spectrophotometric analysis of the observed Spectral Energy Distributions. Four of these 95 micron sources have been classified as faint IR galaxies (L_FIR<1.e11 solar luminosity), six as LIRGs and only one ULIRG. We have discussed the redshift distribution of these objects, comparing our results with evolutionary model predictions 95 and 175 micron. Given their moderate distances (the bulk of the closest spectroscopically identified objects lying at z<0.2), their luminosities and star formation rates (median value 10 solar masses/yr), the sources unveiled by ISOPHOT at 95 micron seem to correspond to the low redshift (z<0.3) FIRBACK 175 micron population, composed of dusty, star-forming galaxies with moderate star formation rates. We computed and compared different SFR estimators, and found that the SF derived from the bolometric IR luminosity is well correlated with that computed from the radio and mid-IR fluxes.Comment: Accepted for publication in MNRAS, a full resolution version can be downloaded from http://dipastro.pd.astro.it/giulia/PHOT/ME681rv.ps.g

Counting individual galaxies from deep 24-μm Spitzer surveys

We address the question of how to deal with confusion-limited surveys in the mid-infrared (MIR) domain by using information from shorter-wavelength observations over the same sky regions. Such information, once applied to apparently extended MIR sources, which are indeed ‘blends’ of two or more different sources, allow us to disentangle the single counterparts and to split the measured flux density into different components. We present the application of this method to the 24-μm Spitzer archival data in the Great Observatories Origins Deep Survey ELAIS-N1 (GOODS EN1) test field, where apparently extended, ‘blended’ sources constitute about 20 per cent of a reliable sample of 983 sources detected above the 5σ threshold down to 40 μJy. As a shorter-wavelength data set, we have considered the public Infrared Array Camera (IRAC) images and catalogues of the same field. We show that the 24-μm sample is almost unbiased down to ~40 μJy and the careful application of the deblending procedure does not require any statistical completeness correction (at least at the flux level considered). This is probed by direct comparison of our results with results in the literature that analysed the same data set through extensive Monte Carlo simulations. The extrapolation of the source counts down to fainter fluxes suggests that our 24-μm sample is able to resolve ~62 per cent of the cosmic background down to a flux level of 38 μJy

Simulating the infrared sky with a S PRITZ

Aims. Current hydrodynamical and semi-empirical simulations of galaxy formation and evolution have difficulties in reproducing the number densities of infrared-detected galaxies. Therefore, a phenomenological simulation tool that is new and versatile is necessary to reproduce current and predict future observations at infrared (IR) wavelengths. Methods. In this work we generate simulated catalogues starting from the Herschel IR luminosity functions of different galaxy populations to consider different populations of galaxies and active galactic nuclei (AGN) in a consistent way. We associated a spectral energy distribution and physical properties, such as stellar mass, star formation rate, and AGN contribution, with each simulated galaxy using a broad set of empirical relations. We compared the resulting simulated galaxies, extracted up to z = 10, with a broad set of observational relations. Results. Spectro-Photometric Realisations of IR-Selected Targets at all-z (SPRITZ) simulations allow us to obtain, in a fully consistent way, simulated observations for a broad set of current and future facilities with photometric capabilities as well as low-resolution IR spectroscopy, such as the James Webb Space Telescope (JWST) or the Origin Space Telescope (OST). The derived simulated catalogue contains galaxies and AGN that by construction reproduce the observed IR galaxy number density, but this catalogue also agrees with the observed number counts from UV to far-IR wavelengths, the observed stellar mass function, the star formation rate versus stellar mass plane, and the luminosity function from the radio to X-ray wavelengths. The proposed simulation is therefore ideal to make predictions for current and future facilities, in particular, but not limited to, those operating at IR wavelengths

Clustering of galaxies at 3.6 microns in the Spitzer Wide-area Infrared Extragalactic legacy survey

We investigate the clustering of galaxies selected in the 3.6 micron band of the Spitzer Wide-area Infrared Extragalactic (SWIRE) legacy survey. The angular two-point correlation function is calculated for eleven samples with flux limits of S_3.6 > 4-400 mujy, over an 8 square degree field. The angular clustering strength is measured at >5-sigma significance at all flux limits, with amplitudes of A=(0.49-29)\times10^{-3} at one degree, for a power-law model, A\theta^{-0.8}. We estimate the redshift distributions of the samples using phenomological models, simulations and photometric redshifts, and so derive the spatial correlation lengths. We compare our results with the GalICS (Galaxies In Cosmological Simulations) models of galaxy evolution and with parameterized models of clustering evolution. The GalICS simulations are consistent with our angular correlation functions, but fail to match the spatial clustering inferred from the phenomological models or the photometric redshifts. We find that the uncertainties in the redshift distributions of our samples dominate the statistical errors in our estimates of the spatial clustering. At low redshifts (median z<0.5) the comoving correlation length is approximately constant, r_0=6.1\pm0.5h^{-1} Mpc, and then decreases with increasing redshift to a value of 2.9\pm0.3h^{-1} Mpc for the faintest sample, for which the median redshift is z=1. We suggest that this trend can be attributed to a decrease in the average galaxy and halo mass in the fainter flux-limited samples, corresponding to changes in the relative numbers of early- and late-type galaxies. However, we cannot rule out strong evolution of the correlation length over 0.5<z<1.Comment: 14 pages, 9 (colour) figures. Published in MNRA