arXiv subjects

A. Franceschini

Publications and source records attributed to A. Franceschini.

At least 109 records · Page 6Linked to original sources

arXiv2008

The XMM-Newton survey of the ELAIS-S1 field II: optical identifications and multiwavelength catalogue of X-ray sources

We present optical identifications and a multi-band catalogue of a sample of 478 X-ray sources in the XMM and Chandra surveys of the central 0.6 deg^2 of the ELAIS-S1 field. The optical/infrared counterpart of each X-ray source was identified using R and IRAC 3.6 um bands. This method was complemented by the precise positions obtained through Chandra observations. Approximately 94% of the counterparts are detected in the R band, while the remaining are blank fields in the optical down to R~24.5, but have a near-infrared counterpart detected by IRAC within 6 arcsec from the XMM centroid. The multi-band catalogue contains photometry in ten photometric bands (B to the MIPS 24 um). We determined redshift and classification for 237 sources (~50% of the sample) brighter than R=24. We classified 47% of the sources with spectroscopic redshift as broad-line active galactic nuclei (BL AGNs) with z=0.1-3.5, while sources without broad-lines are about 46% of the spectroscopic sample and are found up to z=2.6. We identified 11 type 2 QSOs among the sources with X/O>8, with z=0.9-2.6, high 2-10 keV luminosity (log(L2-10keV)>=43.8 erg/s) and hard X-ray colors suggesting large absorbing columns at the rest frame (logN_H up to 23.6 cm^-2). BL AGNs show on average blue optical-to-near-infrared colors, softer X-ray colors and X-ray-to-optical colors typical of optically selected AGNs. Conversely, narrow-line sources show redder optical colors, harder X-ray flux ratio and span a wider range of X-ray-to-optical colors. On average the SEDs of high-luminosity BL AGNs resemble the power-law typical of unobscured AGNs. The SEDs of NOT BL AGNs are dominated by the galaxy emission in the optical/near-infrared, and show a rise in the mid-infrared which suggests the presence of an obscured active nucleus.

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arXiv2008

The ESO-Spitzer Imaging extragalactic Survey (ESIS) II: VIMOS I,z wide field imaging of ELAIS-S1 and selection of distant massive galaxies

(abridged) The ESIS survey is the optical follow up of the SWIRE/Spitzer in the ELAIS-S1 region of the sky. In the era of observational cosmology, the main efforts are focused on the study of galaxy evolution and its environmental dependence. Wide area, multiwavelength, extragalactic surveys are needed in order to probe sufficiently large volumes, minimize cosmic variance and find significant numbers of rare objects. We present VIMOS I and z band imaging belonging to the ESIS survey. A total of ~4 deg2 were targeted in I and ~1 deg2 in z. More than 300000 galaxies have been detected in the I band and ~50000 in the z band. Object coordinates are defined within an uncertainty of ~0.2 arcsec r.m.s., with respect to GSC 2.2. We reach a 90% average completeness at 23.1 and 22.5 mag (Vega) in the I and z bands, respectively. On the basis of IRAC colors, we identified galaxies having the 1.6 um stellar peak shifted to z=1-3. The new I, z band data provide reliable constraints to avoid low-redshift interlopers and reinforce this selection. Roughly 1000 galaxies between z=2-3 were identified over the ESIS ~4 deg2, at the SWIRE 5.8 um depth (25.8 uJy at 3sigma). These are the best galaxy candidates to dominate the massive tail (M>1e11 Msun) of the z>2 mass function.

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arXiv2008

Properties of dusty tori in AGN: I. The Case of SWIRE/SDSS Quasars

We derive the properties of dusty tori in Active Galactic Nuclei (AGN) from the comparison of observed Spectral Energy Distributions (SEDs) of SDSS quasars and a precomputed grid of torus models. The observed SEDs comprise SDSS photometry, 2MASS J, H, and K data, whenever available and mid-Infrared (MIR) data from the Spitzer Wide-area InfraRed Extragalactic (SWIRE) Survey. The adopted model is that of Fritz et al., 2006. The fit is performed by standard chi^2 minimisation, the model however can be multi-component comprising a stellar and a starburst components, whenever necessary. Models with low equatorial optical depth, tau_9.7, were allowed as well as ``traditional'' models with tau_9.7 > 1.0, corresponding to A_V > 22 and the results were compared. Fits using high optical depth tori models only produced dust more compactly distributed than in the configuration where all tau_9.7 models were permitted. Tori with decreasing dust density with the distance from the centre were favoured while there was no clear preference for models with or without angular variation of the dust density. The computed outer radii of the tori are of some tens of parsecs large but can reach, in a few cases, a few hundreds of parsecs. The mass of dust, M_Dust, and infrared luminosity, L_IR, integrated in the wavelength range between 1 and 1000 micron, do not show significant variations with redshift, once the observational biases are taken into account. Objects with 70 micron detections, representing 25% of the sample, are studied separately and the starburst contribution (whenever present) to the IR luminosity can reach, in the most extreme but very few cases, 80%.

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arXiv2008

GMASS Ultradeep Spectroscopy of Galaxies at z~2. III: The emergence of the color bimodality at z~2

The aim of this work is to study the evolution of the rest frame color distribution of galaxies with the redshift, in particular in the critical interval 1.4 10.1, and we study their morphological and spectro-photometric properties. We show that the contribution to this sample of early-type galaxies, defined as galaxies with a spheroidal morphology and no star formation, decreases from 60-70% at z<0.5 down to ~50% at redshift z=2. At z>2 we still find red galaxies in the mass complete sample, even if the bimodality is not seen any more. About 25% of these red galaxies at z>2 are passively evolving, with the bulk of their stars formed at redshift z>`3.

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arXiv2008

GMASS Ultradeep Spectroscopy of Galaxies at redshift z~2. I. The stellar metallicity

Context: Galaxy metallicities have been measured to redshift z~2 by gas-phase oxygen abundances of the interstellar medium using the R23 and N2 methods. Galaxy stellar metallicities provide crucial data for chemical evolution models but have not been assessed reliably much outside the local Universe. Aims: We determine the iron-abundance, stellar metallicity of star-forming galaxies (SFGs) at redshift z~2, observed as part of the Galaxy Mass Assembly ultra-deep Spectroscopic Survey (GMASS). Methods: We compute the equivalent width of a rest-frame mid-ultraviolet, photospheric absorption-line index, the 1978 index found to vary monotonically with stellar metallicity by Rix and collaborators. We normalise and combine 75 SFG spectra from the GMASS survey to produce a spectrum corresponding to a total integration time 1652.5 hours (and a signal-to-noise ratio ~100 for our 1.5 angstrom binning) of FORS2 spectroscopic observations at the Very Large Telescope. Results: We measure an iron-abundance, stellar metallicity of log (Z/Zsolar) = -0.574+/-0.159 for our spectrum representative of a galaxy of stellar mass 9.4 x 10^9 Msolar assuming a Chabrier IMF. We find that the R04 model SFG spectrum for log (Z/Zsolar) = -0.699 solar metallicity provides the best description of our GMASS coadded spectrum. For similar galaxy stellar mass, our stellar metallicity is ~0.25 dex lower than the oxygen-abundance, gas-phase metallicity quantified by Erb and collaborators for UV-selected star-forming galaxies at z=2. Conclusions: We conclude that we are witnessing the establishment of a light-element overabundance in galaxies as they are being formed at redshift z~2. Our measurements are reminiscent of the alpha-element enhancement seen in low-redshift, galactic bulges and early-type galaxies. (Abridged)

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arXiv2008

GMASS Ultradeep Spectroscopy of Galaxies at 1.4<z<2. II. Superdense passive galaxies: how did they form and evolve ?

We combine ultradeep optical spectroscopy from the GMASS project ("Galaxy Mass Assembly ultradeep Spectroscopic Survey") with GOODS multi-band photometry and HST imaging to study a sample of passive galaxiesat 1.39 2. These galaxies have morphologies that are predominantly compact and spheroidal.However, their sizes (R_e <~ 1 kpc) are much smaller than those of spheroids in the present--day Universe. Their stellar mass surface densities are consequently higher by ~1 dex if compared to spheroids at z~0 with the same mass. Their rest-frame B-band surface brightness scales with the effective radius, but the offset with respect to the surface brightness of the local Kormendy relation is too large to be explained by simple passive evolution. At z~1, a larger fraction of passive galaxies follows the z~0 size -- mass relation. Superdense relics with R_e~1 kpc are extremely rare at z~0 with respect to z>1, and absent if R_e<1 kpc. Because of the similar sizes and mass densities, we suggest that the superdense passive galaxies at 1 2. The results are compared with theoretical models and the main implications discussed in the framework of massive galaxy formation and evolution.

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arXiv2007

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, Aθ^{-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.

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arXiv2007

The contribution of very massive high-redshift SWIRE galaxies to the stellar mass function

(Abridged) We selected high-z massive galaxies at 5.8 microns, in the SWIRE ELAIS-S1 field (1 sq. deg.). Galaxies with the 1.6 microns stellar peak redshifted into the IRAC bands (z~1-3, called ``IR-peakers'') were identified. Stellar masses were derived by means of spectro-photometric fitting and used to compute the stellar mass function (MF) at z=1-2 and 2-3. A parametric fit to the MF was performed, based on a Bayesian formalism, and the stellar mass density of massive galaxies above z=2 determined. We present the first systematic study of the very-massive tail of the galaxy stellar mass function at high redshift. A total of 326 sources were selected. The majority of these galaxies have stellar masses in excess of 1e11 Msun and lie at z>1.5. The availability of mid-IR data turned out to be a valuable tool to constrain the contribution of young stars to galaxy SEDs, and thus their M(stars)/L ratio. The influence of near-IR data and of the chosen stellar library on the SED fitting are also discussed. A significant evolution is found not only for galaxies with M~1e11 Msun, but also in the highest mass bins considered. The comoving number density of these galaxies was lower by more than a factor of 10 at z=2-3, with respect to the local estimate. SWIRE 5.8 micron peakers more massive than 1.6x1e11 Msun provide 30-50% of the total stellar mass density in galaxies at z=2-3.

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arXiv2007

3-5 micron spectroscopy of obscured AGNs in ULIRGs

We present the results of infrared L-band (3-4 micron) and M-band (4-5 micron) VLT-ISAAC spectroscopy of five bright Ultraluminous InfraRed Galaxies (ULIRGs) hosting an AGN. From our analysis we distinguish two types of sources: ULIRGs where the AGN is unobscured (with a flat continuum and no absorption features at 3.4 micron and 4.6 micron), and those with highly obscured AGNs (with a steep, reddened continuum and absorption features due to hydrocarbons and CO). Starburst activity is also present in all of the sources as inferred from the 3.3 micron PAH emission line. A strong correlation is found between continuum slope and CO optical depth, which suggests that deep carbon monoxide absorption is a common feature of highly obscured ULIRG AGN. Finally we show that the AGN dominates the 3-4 micron emission, even if its contribution to the bolometric luminosity is small.

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arXiv2007

Unveiling the oldest and most massive galaxies at very high redshift

(Abridged) This work explores the existence of high redshift massive galaxies unveiled with Spitzer+IRAC, but missed by conventional selection techniques based on optical and near-infrared observations. To this end, we use the multi-wavelength imaging data available for the GOODS-South field, and select a flux-limited sample from the IRAC 3.6um image to m(AB)<23.26. We confine our study to the galaxies undetected by the optical HST+ACS imaging and close to the detection limit of the K-band image (K>23.5 AB). Our selection unveiled 20 galaxies on which we performed a detailed photometric analysis. For each galaxy, we built an SED based on optical-to-8um photometry to estimate the photo-z and to derive the main galaxies physical properties. The majority of the sample sources show degenerate/bimodal solutions for the photometric redshifts (Abridged). These can either be heavily dust-enshrouded (Av~2-4) starbursts at 2 10^12 Lsun, or massive post-starburst galaxies in the redshift interval 4 4 which appear to include significant AGN emissions. These sources may be the oldest stellar systems at z~4. These, previously unrecognized, optically obscured objects might provide an important contribution to the massive-end (M>10^11 sun) of the high-z stellar mass function and they would almost double it (Abridged).

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arXiv2007

Obscured and unobscured AGN populations in a hard-X-ray selected sample of the XMDS survey

Our goal is to probe the populations of obscured and unobscured AGN investigating their optical-IR and X-ray properties as a function of X-ray flux, luminosity and redshift within a hard X-ray selected sample of 136 X-ray sources in the XMM Medium Deep Survey (XMDS) with wide multiwavelength coverage. The XMDS area is covered with optical photometry from the VVDS and CFHTLS surveys and infrared Spitzer data. Based on the X-ray luminosity and X-ray to optical ratio, 132 sources are likely AGN, of which 122 have unambiguous optical - IR identification. The observed optical and IR spectral energy distributions of sources are fitted with AGN/galaxy templates in order to classify them and compute photometric redshifts. 70% of the AGN are fitted by a type 2 AGN or a star forming galaxy template and are grouped together in a single class of ``optically obscured'' AGN. They have ``red'' optical colors and generally show significant X-ray absorption from X-ray spectra or hardness ratios (N$_H > 10^{22}$ cm$^{-2}$). Sources with SEDs typical of type 1 AGN have ``blue'' optical colors and exhibit X-ray absorption in about 30% of cases. We performed a stacking analysis for obscured and type 1 AGN. The stacked X-ray spectrum of obscured AGN is flatter than that of type 1 AGN and has an average spectral slope of Gamma = 1.6. The subsample of objects fitted by a galaxy template has an even harder stacked spectrum, with Gamma = 1.2 - 1.3. The obscured fraction is larger at lower fluxes, lower redshifts and lower luminosities. X-ray absorption is less common than ``optical'' obscuration and its incidence is nearly constant with redshift and luminosity. This implies that X-ray absorption is not necessarily related to optical obscuration.

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arXiv2007

Spectral Energy Distributions of Hard X-ray selected AGNs in the XMDS Survey

We present the spectral energy distributions (SEDs) of a hard X-ray selected sample. The sample contains 136 sources with F(2-10 keV)>10^-14 erg/cm^2/s and 132 are AGNs. The sources are detected in a 1 square degree area of the XMM-Newton-Medium Deep Survey where optical data from the VVDS, CFHTLS surveys, and infrared data from the SWIRE survey are available. Based on a SED fitting technique we derive photometric redshifts with sigma(1+z)=0.11 and 6% of outliers and identify AGN signatures in 83% of the objects. This fraction is higher than derived when a spectroscopic classification is available. The remaining 17+9-6% of AGNs shows star-forming galaxy SEDs (SF class). The sources with AGN signatures are divided in two classes, AGN1 (33+6-1%) and AGN2 (50+6-11). The AGN1 and AGN2 classes include sources whose SEDs are fitted by type 1 and type 2 AGN templates, respectively. On average, AGN1s show soft X-ray spectra, consistent with being unabsorbed, while AGN2s and SFs show hard X-ray spectra, consistent with being absorbed. The analysis of the average SEDs as a function of X-ray luminosity shows a reddening of the IR SEDs, consistent with a decreasing contribution from the host galaxy at higher luminosities. The AGNs in the SF classes are likely obscured in the mid-infrared, as suggested by their low L(3-20micron)/Lcorr(0.5-10 keV) ratios. We confirm the previously found correlation for AGNs between the radio luminosity and the X-ray and the mid-infrared luminosities. The X-ray-radio correlation can be used to identify heavily absorbed AGNs. However, the estimated radio fluxes for the missing AGN population responsible for the bulk of the background at E>10 keV are too faint to be detected even in the deepest current radio surveys.

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arXiv2007

Keck spectroscopy of z=1-3 ULIRGs from the Spitzer SWIRE survey

(Abridged) High-redshift ultra luminous infrared galaxies contribute the bulk of the cosmic IR background and are the best candidates for very massive galaxies in formation at z>1.5. We present Keck/LRIS optical spectroscopy of 35 z>1.4 luminous IR galaxies in the Spitzer Wide-area Infra-Red Extragalactic survey (SWIRE) northern fields (Lockman Hole, ELAIS-N1, ELAIS-N2). The primary targets belong to the ``IR-peak'' class of galaxies, having the 1.6 micron (restframe) stellar feature detected in the IRAC Spitzer channels.The spectral energy distributions of the main targets are thoroughly analyzed, by means of spectro-photometric synthesis and multi-component fits (stars + starburst dust + AGN torus). The IR-peak selection technique is confirmed to successfully select objects above z=1.4, though some of the observed sources lie at lower redshift than expected. Among the 16 galaxies with spectroscopic redshift, 62% host an AGN component, two thirds being type-1 and one third type-2 objects. The selection, limited to r'<24.5, is likely biased to optically-bright AGNs. The SEDs of non-AGN IR-peakers resemble those of starbursts (SFR=20-500 Msun/yr) hosted in massive (M>1e11 Msun) galaxies. The presence of an AGN component provides a plausible explanation for the spectroscopic/photometric redshift discrepancies, as the torus produces an apparent shift of the peak to longer wavelengths. These sources are analyzed in IRAC and optical-IR color spaces. In addition to the IR-peak galaxies, we present redshifts and spectral properties for 150 objects, out of a total of 301 sources on slits.

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arXiv2007

COSMOS : Hubble Space Telescope Observations

The Cosmic Evolution Survey (COSMOS) was initiated with an extensive allocation (590 orbits in Cycles 12-13) using the Hubble Space Telescope (HST) for high resolution imaging. Here we review the characteristics of the HST imaging with the Advanced Camera for Surveys (ACS) and parallel observations with NICMOS and WFPC2. A square field (1.8$\sq$°) has been imaged with single-orbit ACS I-F814W exposures with 50% completeness for sources 0.5\arcsec in diameter at I$_{AB} $ = 26.0 mag. The ACS imaging is a key part of the COSMOS survey, providing very high sensitivity and high resolution (0.09\arcsec FWHM, 0.05\arcsec pixels) imaging and detecting 1.2 million objects to a limiting magnitude of 26.5 (AB). These images yield resolved morphologies for several hundred thousand galaxies. The small HST PSF also provides greatly enhanced sensitivity for weak lensing investigations of the dark matter distribution.

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arXiv2007

The Cosmic Evolution Survey (COSMOS): a large-scale structure at z=0.73 and the relation of galaxy morphologies to local environment

We have identified a large-scale structure at z~0.73 in the COSMOS field, coherently described by the distribution of galaxy photometric redshifts, an ACS weak-lensing convergence map and the distribution of extended X-ray sources in a mosaic of XMM observations. The main peak seen in these maps corresponds to a rich cluster with Tx= 3.51+0.60/-0.46 keV and Lx=(1.56+/-0.04) x 10^{44} erg/s ([0.1-2.4] keV band). We estimate an X-ray mass within $r500$ corresponding to M500~1.6 x 10^{14} Msun and a total lensing mass (extrapolated by fitting a NFW profile) M(NFW)=(6+/-3) x 10^15 Msun. We use an automated morphological classification of all galaxies brighter than I_AB=24 over the structure area to measure the fraction of early-type objects as a function of local projected density Sigma_10, based on photometric redshifts derived from ground-based deep multi-band photometry. We recover a robust morphology-density relation at this redshift, indicating, for comparable local densities, a smaller fraction of early-type galaxies than today. Interestingly, this difference is less strong at the highest densities and becomes more severe in intermediate environments. We also find, however, local "inversions'' of the observed global relation, possibly driven by the large-scale environment. In particular, we find direct correspondence of a large concentration of disk galaxies to (the colder side of) a possible shock region detected in the X-ray temperature map and surface brightness distribution of the dominant cluster. We interpret this as potential evidence of shock-induced star formation in existing galaxy disks, during the ongoing merger between two sub-clusters.

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arXiv2007

The Cosmic Evolution Survey (COSMOS): The morphological content and enviromental dependence of the galaxy color-magnitude relation at z~0.7

We study the enviromental dependence and the morphological composition of the galaxy color-magnitude diagram at z~0.7, using a pilot sub-sample of ~2000 galaxies from the COSMOS surve, with I_AB<24, photometric redshift within 0.61<z<0.85 and galaxy morphologies based on the HST-ACS data. The C-M diagram shows a clear red-sequence dominated by early-type galaxies and also a remarkably well-defined "blue sequence'" described by late-type objects. While the percentage of objects populating the two sequences is a function of environment, with a clear morphology/color-density relation, their normalization and slope are independent of local density. We identify and study a number of objects with "anomalous'' colors, given their morphology. Red late-type galaxies are found to be mostly highly-inclined spiral galaxies, with colors dominated by internal reddening (33% contamination with respect to truly passive spheroidals). Conversely, the population of blue early-type galaxies is composed by objects of moderate luminosity and mass, concurring to only ~5% of the mass in spheroidal galaxies. The majority of them (~70%) occupy a position in the surface-brightness/effective-radius plane not consistent with them being precursors of current epoch elliptical galaxies. In a color-mass diagram, color sequences are even better defined, with red galaxies covering in general a wider range of masses at nearly constant color, and blue galaxies showing a more pronounced dependence of color on mass. While the red sequence is adequately reproduced by models of passive evolution, the blue sequence is better interpreted as a specific star-formation sequence. Its substantial invariance with respect to local density suggests that the overall, "secular'' star formation is driven more by galaxy mass than by environment. [...]

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arXiv2007

The Spitzer Legacy Survey of the HST-ACS 2 sq. deg. COSMOS Field: survey strategy and first analysis

The Spitzer-COSMOS survey (S-COSMOS) is a Legacy program (Cycles 2+3) designed to carry out a uniform deep survey of the full 2 sq deg COSMOS field in all seven Spitzer bands (3.6, 4.5, 5.6, 8.0, 24.0, 70.0, 160.0 u). This paper describes the survey parameters, mapping strategy, data reduction procedures, achieved sensitivities to date, and the complete data set for future reference. We show that the observed infrared backgrounds in the S-COSMOS field are within 10% of the predicted background levels. The fluctuations in the background at 24u have been measured and do not show any significant contribution from cirrus, as expected. In addition, we report on the number of asteroid detections in the low galactic latitude COSMOS field. We use the Cycle 2 S-COSMOS data to determine preliminary number counts, and compare our results with those from previous Spitzer Legacy surveys (e.g. SWIRE, GOODS). The results from this "first analysis" confirm that the S-COSMOS survey will have sufficient sensitivity with IRAC to detect ~ L* disks and spheroids out to z ~ 3, and with MIPS to detect ultraluminous starbursts and AGN out to z ~3 at 24u and out to z ~1.5-2 at 70u and 160u.

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arXiv2007

Hidden activity in high-redshift spheroidal galaxies from mid-infrared and X-ray observations in the GOODS-North field

We exploit very deep mid-IR (MIR) and X-ray observations by Spitzer and Chandra in the GOODS North to identify signs of hidden (either starburst or AGN) activity in spheroidal galaxies between z~0.3 and 1. Our reference is a complete sample of 168 morphologically classified spheroidal galaxies with z[AB]<22.5 selected from GOODS ACS imaging. Nineteen of these have 24 micron detections in the GOODS catalogue, half of which have an X-ray counterpart in the 2 Ms Chandra catalogue, while about 25% have 1.4 GHz fluxes larger than 40 microJy. Traces of hidden activity in the spheroidal population are also searched for in the deep X-ray images and 14 additional galaxies are detected in X-rays only. The nature of the observed MIR emissions is investigated by modelling their SEDs based on the available multi-wavelength photometry, including X-ray, UV, optical, near-IR, MIR and radio fluxes, and optical spectroscopy. The amount of dust derived from the IR emission observed by Spitzer appears in excess of that expected by mass loss from evolved stars. Our analysis of the X-ray and MIR properties leads us to conclude that at least 8 of the 19 24-um bright sources should hide an obscured AGN, while the X-ray undetected sources are more likely dominated by star formation. [abridged] We conclude that ~30 objects (~20%) of the original flux-limited sample of 168 spheroidal galaxies in the GOODS-North are detected during phases of prominent activity, of both stellar and quasar origin. Due to the short expected lifetimes of the IR and X-ray emissions, this fraction might imply a significant level of activity in this class of galaxies during the relatively recent cosmic epochs, z~0.3 to z~1, under investigation.

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