arXiv subjects

A. Franceschini

Publications and source records attributed to A. Franceschini.

At least 163 records · Page 9Linked to original sources

arXiv2003

The manifold spectra and morphologies of EROs

Deep VLT optical spectroscopy, HST+ACS (GOODS) imaging and VLA observations are used to unveil the nature of a complete sample of 47 EROs with R-Ks>5 and Ks<20. The spectroscopic redshift completeness is 62%. Morphological classification was derived for each ERO through visual inspection and surface brightness profile fitting. Three main ERO morphological types are found: E/S0 galaxies (30-37%), spiral-like (24-46%) and irregular systems (17-39%). The only ERO detected in the radio is likely to host an obscured AGN. The average radio luminosity of the star-forming EROs undetected in the radio implies star formation rates of the order of 33 Msun yr^-1. The colors, redshifts and masses of the E/S0 galaxy subsample imply a minimum formation redshift z_f \sim 2. With this z_f there is enough time to have old and massive stellar spheroids already assembled at z \sim 1. We verify that the R-Ks vs. J-Ks color diagram is efficient in segregating old and dusty-star-forming EROs.

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arXiv2003

Revealing the active galactic nucleus in the superantennae through L-band spectroscopy

We present an L-band spectrum of the Ultraluminous Infrared Galaxy IRAS 19254-7245 (the Superantennae), obtained with VLT-ISAAC. The high signal to noise ratio allows a study of the main spectral features with unprecedented detail for an extragalactic source. We argue that the main energy source in the IR is an obscured AGN. This is indicated by the low equivalent width of the 3.3 micron PAH feature, the broad absorption feature at 3.4 um, and the steep continuum at lambda>3.7 um (f_lambda ~lambda^(2.7)). The substructure of the 3.4 um absorption feature indicates that the absorption is due to hydrocarbon chains of 6-7 carbon atoms.

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arXiv2003

The Ultra Luminous Infrared Galaxy MKN 231: New clues from BeppoSAX and XMM-Newton

We present BeppoSAX and XMM-Newton observations of MKN 231. These observations and in particular the BeppoSAX PDS data allowed us to unveil, for the first time, the highly absorbed (N$_H\sim 2\times10^{24}$ cm$^{-2}$) AGN component. We find that: a) the AGN powering MKN231 has an intrinsic 2-10 keV luminosity of $1^{+1}_{-0.5}\times 10^{44}$ erg/s; b) the strong starburst activity contributes significantly in the 0.1-10 keV energy range. We propose that the starburst activity strongly contributes to the far infrared luminosity of MKN 231; this is also suggested by the multiwavelength properties of MKN 231.

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arXiv2003

An XMM-Newton hard X-ray survey of ULIRGs

XMM-Newton observations of 10 ULIRGs are reported. The aim is to investigate in hard X-rays a complete ULIRG sample selected from the bright IRAS 60$μ$m catalogue. All sources are detected in X-rays, 5 of which for the first time. These observations confirm that ULIRGs are intrinsically faint X-rays sources, their observed X-ray luminosities being typically L(2-10 keV)<1E42-43 erg/s, whereas their bolometric luminosities are L>1E45 erg/s. In all sources we find evidence for thermal emission from hot plasma with kT~0.7keV, dominating the X-ray spectra below 1keV, and likely associated with a nuclear or circumnuclear starburst. This thermal emission appears uncorrelated with the FIR luminosity, suggesting that,in addition to the ongoing rate of star formation, other parameters may also affect it. The soft X-ray emission appears to be extended on a scale of ~30kpc for Mkn231 and IRAS19254-7245, possible evidence of galactic superwinds. In these 2 sources, in IRAS20551-4250 and IRAS23128-5919 we find evidence for the presence of hidden AGNs, while a minor AGN contribution may be suspected also in IRAS20100-4156. In particular, we have detected a strong Fe line at 6.4keV in the spectrum of IRAS19254-7245 and a weaker one in Mkn231, suggestive of deeply buried AGNs. For the other sources, the X-ray luminosities and spectral shapes are consistent with hot thermal plasma and X-ray binary emissions of mainly starburst origin. We find that the 2-10keV luminosities in these sources, most likely due to high-mass X-ray binaries, are correlated with L_FIR: both luminosities are good indicators of the current global SFR in the galaxy. The composite nature of ULIRGs is then confirmed, with hints for a predominance of the starburst over the AGN phenomenon in these objects.

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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

Infrared spectroscopy of faint 15 micron sources in the Hubble Deep Field South: first hints at the properties of the sources of the IR background

We present a spectroscopic analysis of 21 galaxies with z=0.2-1.5 drawn from a 25 sq. arcmin ultra-deep 15 micron ISOCAM survey centered in the HDFS. VLT/ISAAC spectra are reported for 18 sources, aimed at detecting the redshifted Ha. Optical data come from VLT/FORS2 and NTT/EMMI. Ha line emission is detected in all the observed objects. Our analysis of emission lines, morphology, and SEDs shows evidence for AGN in only two of these sources. The Ha luminosities indicate star formation rates (SFR) between 0.5 and 20 Msun, without extinction corrections. We find good correlations between the mid-IR, the radio and Ha luminosities, confirming the mid-IR light as a good tracer of star formation (while SFR based on Ha flux show large scatter and offset, still to be understood). We have estimated the baryonic masses in stars by fitting the optical-IR SED, and found that the host galaxies of ISO sources are massive members of groups with SFR=10 to 300 Msun/yr. We have finally compared this ongoing SF activity with the already formed stellar masses to estimate the timescales t(SF) for the stellar build-up, which turn-out to be widely spread between 0.1 Gyrs to more than 10 Gyr. The faint ISOCAM galaxies appear to form a composite population, including moderately active but very massive spiral-like galaxies, and very luminous ongoing starbursts, in a continuous sequence. From the observed t(SF), assuming typical starburst timescales, we infer that, with few exceptions, only a fraction of the galactic stars can be formed in any single starburst, while several of such episodes during a protracted SF history are required for the whole galactic build-up.

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arXiv2003

Ha Rotation Curves of z~1 Galaxies: Unraveling the Evolution of the Tully-Fisher Relation

We describe the first results of a programme to obtain rotation curves of z~1 disc galaxies in the near-infrared using the Ha emission line in order to study the Tully-Fisher relation. To put any observed evolution into perspective and to investigate any possible selection biases, we constructed a control sample of low redshift galaxies that had rotation velocities and images available for measuring their dynamical, photometric, and morphological properties. Compared to local objects with isophotal sizes similar to the high redshift targets, we find that our sample of galaxies with spatially resolved rotation curves, the most distant sample so far ( ~0.9), clearly reveals a brightening of ~1.1 mag in the rest-frame B-band. The observed offset can be explained by a combination of increasing surface brightness, decreasing rotation speeds, and slightly smaller disc scale lengths of the high redshift galaxies.

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arXiv2002

Models for Evolution of Dusty Galaxies and E/S0s Seen in Multiband Surveys

Phenomenological models for evolution of dusty galaxies and E/S0 galaxies, respectively, are developed to address two major questions concenring galaxy populations in deep infrared (IR) surveys: (1) Do normal late-type galaxies or starburst galaxies (including galaxies with obscured AGNs) dominate among sources in deep IR surveys? (2) How much do E/S0 galaxies contribute to the counts in deep mid-infrared (MIR: 3 -- 20$μm$) surveys? Among three new models for evolution of dusty galaxies, it is assumed in Model S1 that starburst galaxies are the dominant population, and in Model S2 that normal galaxies dominate. Model S3 is an intermediate model. Comparing the model predictions with a wide range of observational data collected from the literature, we find that none of these models can be ruled out, given the uncertainties of the data. We show that the most direct method to distinguish these models is to compare the predicted color distributions of IR galaxies with observations, which will soon be available from the SWIRE survey. The models for E/S0 galaxies follow a simple passive evolution approach. Among the three E/S0 models (E1, E2 and E3) investigated in this paper, Model E2 which is specified by a peak formation redshift $z_{peak}=2$, and an e-folding formation time scale $ω=2$ Gyr, fits the data best. This suggests a synchronization between the evolution of E/S0 galaxies and of starburst galaxies, in the sense that the peak of the formation function of E/S0s ($z_{peak}=2$) is close to the peak of the evolution functions of starburst galaxies ($z_{peak} = 1.4$). New predictions for counts and confusion limits in the SIRTF bands are presented.

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arXiv2002

XMM-Newton observations of ULIRGs I: A Compton-thick AGN in IRAS19254-7245

We present the XMM-Newton observation of the merging system IRAS 19254-7245, also known as The Superantennae, whose southern nucleus is classified as a Seyfert 2 galaxy. The XMM-Newton data have allowed us to perform a detailed X-ray imaging and spectral analysis of this system. We clearly detect, for the first time in this system, a strong EW ~ 1.4 keV Fe emission line at 6.49+/-0.1 keV (rest-frame). The X-ray spectrum requires a soft thermal component (kT~0.9 keV; L(0.5-2) ~ 4E41 cgs), likely associated with the starburst, and a hard power-law continuum above 2 keV (observed L(2-10) ~ 4E42 cgs). We confirm the flatness of this latter component, already noted in previous ASCA data. This flatness, together with the detection of the strong Fe-Kalpha line and other broad band indicators, suggest the presence of a Compton-thick AGN with intrinsic luminosity > 1E44 cgs. We show that a Compton-thick model can perfectly reproduce the X-ray spectral properties of this object.

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arXiv2002

An enshrouded AGN in the merging starburst system Arp~299 revealed by BeppoSAX

Using a long (=150 ksec), broad-band (0.1--40 keV) BeppoSAX observation of the merging starburst system Arp 299 (=IC 694 + NGC 3690) we found the first unambiguous evidence of the presence of a deeply buried (N_H = 2.5E24 cm-2) AGN having an intrinsic luminosity of L(0.5-100 keV) = 1.9E43 cgs. The X-ray spectral properties of this AGN are discussed in detail as well as the thermal component detected at soft X-ray energies which, most likely, is associated with the starburst.

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arXiv2002

Spectro-Polarimetric search for hidden AGNs in four southern Ultraluminous Infrared Galaxies

We report on a spectro-polarimetric analysis of four southern Ultra-Luminous Infrared Galaxies (ULIRGs), aimed at constraining the presence of hidden broad AGN lines. For IRAS 19254--7245 ({\sl The Superantennae}) we find evidence for a significant level of polarized light in the H$α$ line with FWHM$>$2300 km/s. Some degree of polarization is also detected in IRAS 20551-4250, though with lower significance. In the two other sources (IRAS 20100--4156 and IRAS 22491--1808) no polarized signals are detected. Although it is unclear from the present data if the origin of polarization is due to reflected light from an AGN or more simply to dichroic transmission by a dust slab, we find interesting correlation between the presence of polarized components in the optical spectra and independent evidence for AGN emissions in hard X-rays and the far-IR.

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arXiv2002

Kinematics of Galaxies in the Hubble Deep Field South: Discovery of a Very Massive Spiral at z=0.6

We report the first results from a study of the internal kinematics, based on spatially resolved H_alpha velocity profiles, of three galaxies at redshift z~0.6 and one at redshift z~0.8, detected by ISOCAM in the Hubble Deep Field South. The kinematics are derived from high resolution near-infrared VLT spectroscopy. One of the galaxies is a massive spiral which possesses a very large rotational velocity of 460 km/s and contains a mass of 10^12 M_solar (within 20 kpc), significantly higher than the dynamical masses measured in most other local and high redshift spirals. Two of the galaxies comprise a counter-rotating interacting system, while the fourth is also a large spiral. The observed galaxies are representative examples of the morphologies encountered among ISOCAM galaxies. The mass-to-light (M /L_bol) ratios of ISOCAM galaxies lie between those of local luminous IR galaxies and massive spirals. We measure an offset of 1.6+/-0.3 mag in the rest frame B-band and of 0.7+/-0.3 mag in the rest frame I-band when we compare the four ISOCAM galaxies to the local Tully-Fisher B and I-band relations. We conclude that the large IR luminosity of the ISOCAM population results from a combination of large mass and efficient triggering of star formation. Since ISOCAM galaxies contribute significantly to the Cosmic Infrared Background our results imply that a relatively small number of very massive and IR luminous objects contribute significantly to the IR background and star formation activity near z~0.7.

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arXiv2002

Origin of the X-Ray Background and AGN Unification: New Perspectives

We critically review the basic assumptions of the standard model for the synthesis of the XRB in the light of new data from ultradeep surveys by Chandra and XMM, resolving major parts of it. Important constraints come in particular from the observed z-distributions of faint hard X-ray sources -- showing large excesses at redshifts (z~0.8) much lower than expected by the synthesis models -- and from their X/optical/IR SEDs combined with the IR counts of type-II AGNs. Our analysis, although supporting the general scheme which interprets the XRB as due to absorbed AGNs with broad N_H distributions, requires major revision of the other postulate of the XRB synthesis models: the AGN unification. We argue that the unification scheme based on a simple orientation effect fails at high redshifts, where galaxy activity is induced by strong interactions and mergers among gas-rich systems. This helps explaining the observational evidence that type-I and II AGNs follow different evolutionary patterns, with type-I quasars providing a very biased trace of this activity. Combined deep X-ray and IR surveys consistently find that the universe has experienced a violent phase of galaxy activity around z~1, probably related with the assembly of massive galaxies, involving both SF and obscured AGN fueling. Roughly 10 to 20% of this activity has involved substantial AGN emission, this fraction likely reflecting the AGN/starburst duty cycle during the activation phase.

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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

An unusual iron Lo-BAL quasar detected by ISOCAM

We report the discovery of an unusual low--ionization broad absorption line quasar at z=1.776 which exhibits absorption lines from many excited states of Feii. This member of the rare class of 'FeLoBAL' quasars was serendipitously found in a mid-infrared (MIR) survey of distant clusters carried out with the ISOCAM camera. ISO J005645.1-273816 has a high MIR to UV luminosity ratio, suggesting a strong dust obscuration plus emission from very hot dust. This characteristic makes MIR surveys particularly efficient at detecting LoBAL quasars.

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arXiv2002

Observations of the Hubble Deep Field South with the Infrared Space Observatory - I. Observations, data reduction and mid-infrared source counts

We present results from a deep mid-infrared survey of the Hubble Deep Field South (HDF-S) region performed at 7 and 15 micron with the CAM instrument on board the Infrared Space Observatory (ISO). The final map in each band was constructed by the coaddition of four independent rasters, registered using bright sources securely detected in all rasters, with the absolute astrometry being defined by a radio source detected at both 7 and 15 micron. We sought detections of bright sources in a circular region of radius 2.5 arcmin at the centre of each map, in a manner that simulations indicated would produce highly reliable and complete source catalogues using simple selection criteria. Merging source lists in the two bands yielded a catalogue of 35 distinct sources, which we calibrated photometrically using photospheric models of late-type stars detected in our data. We present extragalactic source count results in both bands, and discuss the constraints they impose on models of galaxy evolution models, given the volume of space sampled by this galaxy population.

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arXiv2002

Observations of the Hubble Deep Field South with the Infrared Space Observatory - II. Associations and star formation rates

We present results from a deep mid-IR survey of the Hubble Deep Field South (HDF-S) region performed at 7 and 15um with the CAM instrument on board ISO. We found reliable optical/near-IR associations for 32 of the 35 sources detected in this field by Oliver et al. (2002, Paper I): eight of them were identified as stars, one is definitely an AGN, a second seems likely to be an AGN, too, while the remaining 22 appear to be normal spiral or starburst galaxies. Using model spectral energy distributions (SEDs) of similar galaxies, we compare methods for estimating the star formation rates (SFRs) in these objects, finding that an estimator based on integrated (3-1000um) IR luminosity reproduces the model SFRs best. Applying this estimator to model fits to the SEDs of our 22 spiral and starburst galaxies, we find that they are forming stars at rates of ~1-100 M_sol/yr, with a median value of ~40M_sol/yr, assuming an Einstein - de Sitter universe with a Hubble constant of 50 km/s/Mpc, and star formation taking place according to a Salpeter (1955) IMF across the mass range 0.1-100M_sol. We split the redshift range 0.0<z<0.6 into two equal-volume bins to compute raw estimates of the star formation rate density contributed by these sources, assuming the same cosmology and IMF as above and computing errors based on estimated uncertainties in the SFRs of individual galaxies. We compare these results with other estimates of the SFR density made with the same assumptions, showing them to be consistent with the results of Flores et al. (1999) from their ISO survey of the CFRS 1415+52 field. However, the relatively small volume of our survey means that our SFR density estimates suffer from a large sampling variance, implying that our results, by themselves, do not place tight constraints on the global mean SFR density.

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arXiv2002

An XMM-Newton Survey of Ultra-Luminous Infrared Galaxies

We present preliminary results of XMM-Newton observations of 5 Ultra-luminous Infrared Galaxies (ULIRGs), part of a mini-survey program dedicated to 10 ULIRGs selected from the bright IRAS sample. For 3 of them (IRAS 20551-4250, IRAS 19254-7245 and Mkn 231) we find strong evidence for the presence of a hidden AGN, while for two others (IRAS 20110-4156 IRAS 22491-1808) the S/N ratio of the data does not allow us to be conclusive. In particular, we have detected a strong Fe-K line in the X-ray spectra of IRAS19254-7245, with an equivalent width (~2 keV) suggestive that most of the energy source in this object is due to a deeply buried AGN.

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