arXiv subjects

A. Franceschini

Publications and source records attributed to A. Franceschini.

At least 181 records · Page 10Linked to original sources

arXiv2002

The Bulk of the Cosmic Infrared Background Resolved by ISOCAM

We have computed the 15 microns integrated galaxy light produced by ISOCAM galaxies above a sensitivity limit of 50 microJy. It sets a lower limit to the 15 microns extragalactic background light of (2.4+/-0.5) nW/m^2/Hz. The redshift distribution of the ISOCAM galaxies is inferred from the spectroscopically complete sample of galaxies in the Hubble Deep Field North (HDFN). We demonstrate that mid (MIR) and far (FIR) infrared luminosities correlate for local star forming galaxies and that this correlation is consistent with the radio-FIR one up to z ~ 1. From these correlations, about 75 % of the ISOCAM galaxies are found to belong to the class of luminous infrared galaxies (L[IR] >= 10^11 Lsol), with star formation rates of the order of ~ 100 Msol/yr. The cross-correlation with the deepest X-ray surveys from the Chandra and XMM-Newton observatories in the HDFN and Lockman Hole respectively, indicates that at most 20 % of the 15 microns integrated galaxy light is due to active galactic nuclei (AGNs) unless a large population of AGNs was missed by Chandra and XMM-Newton. The contribution of ISOCAM galaxies to the peak of the cosmic infrared background (CIRB) at 140 microns was computed from the MIR-FIR correlations for star forming galaxies and from the spectral energy distribution of the Seyfert 2, NGC 1068, for AGNs. We find that the galaxies unveiled by ISOCAM surveys are responsible for the bulk of the CIRB, i.e (16+/-5) nW/m^2/Hz as compared to the (25+/-7) nW/m^2/Hz measured with the COBE satellite, with less than 10 % due to AGNs. Since the CIRB contains most of the light radiated over the history of star formation in the universe, this means that a large fraction of present-day stars must have formed during a dusty starburst event similar to those revealed by ISOCAM.

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arXiv2001

Hidden star-formation in the cluster of galaxies Abell 1689

At a redshift of 0.18, Abell 1689 is so far the most distant cluster of galaxies for which substantial mid-infrared (MIR) data have been published. We present here the follow-up optical photometric and spectroscopic observations which were used to study the individual properties of the galaxies members of A1689. Sources with a low [15 micron] / [6.75 micron] flux ratio typically consist of luminous passive early-type galaxies while those with a high MIR color index are mainly luminous, blue, emission-line, morphologically disturbed spirals, i.e. the star-forming galaxies usually associated with the 'Butcher-Oemler' effect. On the other hand, at least 30% of the 15 micron sources have optical counterparts showing no evidence of current star-formation activity, while their 15 micron emission is most likely due to obscured star formation. We argue that the LW3 luminosity measured in the cluster members is a reliable tracer of the total infrared luminosity, L(IR). We derive from L(IR) a star-formation rate free of dust extinction, SFR(IR), which we compare with that determined in the optical from the flux of the [OII] emission line, SFR(opt). The highest total star formation rates (11 Msun/yr) and dust extinction are measured in those galaxies exhibiting in their optical spectrum a signature of a dusty starburst. In contrast, none of the galaxies with post-starburst optical spectra has been detected by ISOCAM. We find a median SFR(IR) of the LW3-detected galaxies of 2 Msun/yr, that is ten times higher than the median SFR(opt) of the [OII] detected galaxies. The ratio SFR(IR)/SFR(opt) is in fact very high, ranging between 10 and 100 for LW3-detected galaxies with [OII] emission. We conclude that a major part, at least 90%, of the star formation activity taking place in Abell 1689 is hidden.

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arXiv2001

A Long-Wavelength View on Galaxy Evolution from Deep Surveys by the Infrared Space Observatory

We discuss the constraints set on galaxy evolution by data from deep surveys performed in the mid-IR and far-IR with ISO and with mm telescopes at longer wavelengths. These reveal extremely high rates of evolution for IR galaxies. According to our model, the deep ISO surveys at 15 micron may have already resolved more than 50% of the CIRB intensity, hence allowing to investigate the origin of the CIRB. From our fits to the observed optical-IR SEDs, these objects appear to involve massive galaxies hosting luminous starbursts (SFR 100 Mo/yr). Our evolutionary scheme considers a bimodal star formation (SF), including long-lived quiescent SF, and enhanced SF taking place during transient events recurrently triggered by interactions and merging. We interpret the strong observed evolution as an increase with z of the rate of interactions between galaxies (density evolution) and an increase of their IR luminosity due to the more abundant fuel available in the past (luminosity evolution). Very schematically, we associate the origin of the bulk of the optical/NIR background to the quiescent evolution, while the CIRB is interpreted as mostly due the dusty starburst phase (the latter possibly leading to the formation of galaxy spheroids). The large energy contents in the CIRB and optical backgrounds are not easily explained, considering the moderate efficiency of energy generation by stars: a top-heavy stellar IMF associated with the starburst phase (and compared with a more standard IMF during the quiescent SF) would alleviate the problem. IR data suggest that the strong observed evolution to z~1 should turn over at higher z: scenarios in which a dominant fraction of stellar formation occurs at very high-z are not supported by our analysis.

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arXiv2001

FIRBACK: III. Catalog, Source counts, and cosmological implications of the 170 micron ISO Deep survey

The FIRBACK (Far Infrared BACKground) survey is one of the deepest imaging surveys carried out at 170 microns with ISOPHOT onboard ISO, and is aimed at the study of the structure of the Cosmic Far Infrared Background. This paper provides the analysis of resolved sources. After a validated process of data reduction and calibration, we perform intensive simulations to optimize the source extraction, measure the confusion noise (sigma_c = 45 mJy), and give the photometric and astrometric accuracies. 196 galaxies with flux S > 3 sigma_c are detected in the area of 3.89 square degrees. Counts of sources with flux S > 4 sigma_c present a steep slope of 3.3 +/- 0.6 on a differential "logN-logS" plot between 180 and 500 mJy. As a consequence, the confusion level is high and will impact dramatically on future IR deep surveys. This strong evolution, compared with a slope of 2.5 from Euclidian geometry, is in line with models implying a strongly evolving Luminous Infrared Galaxy population. The resolved sources account for less than 10% of the Cosmic Infrared Background at 170 microns, which is expected to be resolved into sources in the 1 to 10 mJy range.

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arXiv2001

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

We have developed a new data reduction technique for ISOCAM LW data and have applied it to the European Large Area ISO Survey (ELAIS) LW3 (15 micron) 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 either for fast or slow detector response), was particularly designed for the detection of faint sources. In the ELAIS S1 field we obtained a catalogue of 462 15 micron sources with signal-to-noise ratio >= 5 and flux densities in the range 0.45 - 150 mJy (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 have been 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.

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arXiv2001

The milliJansky 12 micron Population: First Followup

We present the first results of our followup programme of optical imaging and spectroscopy of a deep 12 micron survey conducted with the ISO satellite (Clements et al. 1999). We find that the objects are typically of fairly low redshift (z ~ 0.1-0.2), but with a tail that extends to high redshifts. The highest redshift object is a previously unknown quasar at z=1.2. The sample of objects for which spectroscopy has been obtained forms a complete subsample of the original survey with an R band magnitude limit of 19.6. We are thus able to use accessible volume methods to determine the luminosity function. We find that the luminosity function is close to the low redshift 12 micron luminosity function determined by IRAS (Fang et al., 1998; Rush et al., 1993), but with a possible excess at the highest luminosities, which also correspond to the highest redshifts. This excess is compatible with the rapid luminosity evolution of L ~ (1+z)^{4.5} claimed by Xu (2000) and suggested by Elbaz (1999) on the basis of number counts. These conclusions remain tentative because of small number statistics at the highest redshifts. Almost all of the galaxies in this survey are emission line objects. We use the spectra to determine emission line diagnostics for the underlying ionisation source. We find that the majority of objects have HII region-like spectra (roughly 2/3) but there is a significant fraction (about 1/3) that contain an AGN. This confirms suspicions that deep mid-IR surveys will be a powerful way to find obscured AGN. Part of the survey area has been imaged deeply by the INT Wide Field Camera. Nearly all 12 micron sources in this area have good optical identifications. The prospects are thus good for obtaining complete spectroscopy.

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arXiv2001

Recent star formation in very luminous infrared galaxies

Among star forming galaxies, a spectral combination of a strong Hdelta line in absorption and a moderate [OII] emission has been suggested to be a useful method to identify dusty starburst galaxies at any redshift, on the basis of optical data alone. On one side it has been shown that such a spectral particularity is indeed suggestive of obscured starburst galaxies but, on the other, the degeneracy of the optical spectrum has hindered any quantitative estimate of the star formation and extinction during the burst. The optical spectrum by itself, even complemented with the information on the far-IR flux, is not enough to identify univocal evolutionary patterns. We discuss in the following whether it is possible to reduce these uncertainties by extending the spectral analyses to the near infrared spectral region.

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arXiv2001

Kinematics of Molecular Gas in the Nucleus of NGC1068, from H2 line emission observed with VLT

We present results about the distribution and kinematics of the molecular environment of the AGN in NGC1068, over a 1.5'' x 3.5'' region around the central engine in NGC1068, derived from H2 line emission detected with ISAAC at VLT/ANTU on ESO/Paranal. The H2 emitting molecular gas is found to be distributed along the East-West direction and with two main peak emission (knots) located at a distance of about 70 pc from the central engine. The eastern H2 knot is more intense than the western one. The line profiles mapped across the entire 1.5'' x 3.5'' region, at a spatial resolution of 0.3'' x 0.45'', appear to be quite complex with either a blue or red wing. At first order, we find a velocity difference of 140 km/s between the two knots; if interpreted as quasi-keplerian velocity, this implies a central enclosed mass of 10^8 solar masses.

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arXiv2001

The European Large Area ISO Survey V: a BeppoSAX hard X-ray survey of the S1 region

We present BeppoSAX observations of the Southern S1 region in the European Large Area ISO Survey (ELAIS). These observations cover an area of ~1.7 sq. deg. and reach an on-axis (~0.7 sq. deg) 2-10 keV (HX) sensitivity of ~10E-13 cgs. This is the first HX analysis of an ISOCAM survey. We detect 9 sources with a signal to noise ratio SNR_HX>3, 4 additional sources with a 1.3 to 10 keV (T) SNR_T>3 and 2 additional sources which appear to be associated with QSOs with SNR_T>2.9. The number densities of the SNR_HX>3 sources are consistent with the ASCA and BeppoSAX logN-logS functions. Six BeppoSAX sources have reliable ISOCAM 15 micron counterparts within ~60 arcsec. All these ISOCAM sources have optical counterparts of R<20 mags. Five of these sources have been previously optically classified giving 4 QSOs and 1 BALQSO at z=2.2. The remaining unclassified source has X-ray and photometric properties consistent with that of a nearby Seyfert galaxy. One further HX source has a 2.6sigma ISOCAM counterpart associated with a galaxy at z=0.325. If this ISOCAM source is real, the HX/MIR properties suggest either an unusual QSO or a cD cluster galaxy. We have constructed MIR and HX spectral energy distributions to compute the expected HX/MIR ratios for these classes of objects up to z=3.2 and assess the HX/MIR survey depth. The BALQSO has an observed X-ray softness ratio and HX/MIR flux ratio similar to QSOs but different to those found for low redshift BALQSOs. This difference can be explained in terms of absorption, and suggests that high redshift BALQSOs should be comparatively easy to detect in the HX band, allowing their true fraction in the high redshift QSO population to be determined (ABRIDGED).

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arXiv2001

Deep HST imaging surveys and the formation of spheroidal galaxies

We have extended our previous analysis of morphologically selected elliptical and S0 galaxies in the Hubble Deep Field (HDF) North to include HST data in the HDF South and the HDFS-NICMOS areas. Our final sample amounts to 69 E/S0 galaxies with $K<20.15$ over an area of 11 square arcmins. Although a moderately small number over a modest sky area, this sample benefits of the best imaging and photometric data available on high-redshift galaxies. Multi-waveband photometry allows us to estimate with good accuracy the redshifts for the majority of these galaxies which lack a spectroscopic measure. We confirm our previous findings that massive E/S0s tend to disappear from flux-limited samples at $z>1.4$. This adds to the evidence that the rest-frame colours and SEDs of the numerous objects found at $0.8<z<1.2$ are inconsistent with a very high redshift of formation for the bulk of stars, while they are better consistent with protracted (either continuous or episodic) star-formation down to $z \le 1$. These results based on high-quality imaging on a small field can be complemented with data from colour-selected EROs on much larger sky areas: our claimed demise of E/S0s going from $z=1$ to $z=1.5$ is paralleled by a similarly fast decrease in the areal density of EROs when the colour limit is changed from $(R-K)=5$ to $(R-K)=6$ (corresponding to $z\simeq 1$ and $z\simeq 1.3$ respectively). Altogether, the redshift interval from 1 to 2 seems to correspond to a very active phase for the assembly of massive E/S0 galaxies in the field, and also probably one where a substantial fraction of their stars are formed.

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arXiv2000

Spiral and irregular galaxies in the Hubble Deep Field North - Comparison with early types and implications for the global SFR density

We analyze a morphologically-selected complete sample of 52 late-type (spiral and irregular) galaxies in the Hubble Deep Field North with total K-magnitudes brighter than K=20.47. This sample exploits in particular the ultimate imaging quality achieved by HST in this field, allowing us to clearly disentangle the galaxy morphologies, based on accurate profiles of the surface brightness distributions. Our purpose was to investigate systematic differences between the two classes, as for colours, redshift distributions and ages of the dominant stellar populations. This sample appears to miss significantly galaxies above z=1.4 (in a similar way as an early-type galaxy sample previously studied by us), a fact which may be explained as a global decline of the underlying mass function for galaxies at these high redshifts. Differences between early and late-types are apparent -particularly in the colour distributions and the evolutionary star-formation (SF) rates per unit volume-, although the complication in spectro-photometric modelling introduced by dust-extinction in the gas-rich systems prevents us to reach conclusive results on the single sources. However, we find that an integrated quantity like the comoving star-formation rate density (SFR(t)) as a function of redshift is much less affected by these uncertainties: by combining this with the previously studied early-type galaxy sample, we find a shallower dependence of SFR(t) on z between z=0.2 and z=1.5 than found by Lilly et al. (1995).

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arXiv2000

The European Large Area ISO Survey III: 90micron extragalactic source counts

We present results and source counts at 90micron extracted from the Preliminary Analysis of the European Large Area ISO Survey (ELAIS). The survey covered about 11.6 square degrees of the sky in four main areas and was carried out with the PHOT instrument onboard the Infrared Space Observatory (ISO). The survey is at least an order of magnitude deeper than the IRAS 100micron survey and is expected to provide constraints on the formation and evolution of galaxies. The majority of the detected sources are associated with galaxies on optical images. In some cases the optical associations are interacting pairs or small groups of galaxies suggesting the sample may include a significant fraction of luminous infrared galaxies. The source counts extracted from a reliable subset of the detected sources are in agreement with strongly evolving models of the starburst galaxy population.

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arXiv2000

An excess of mid-IR luminous galaxies in Abell 1689?

We present the results of infrared observations of Abell 1689 which was observed with ISOCAM, at 6.7\mic and 15\mic, and ISOPHOT at 200\mic. The cluster galaxies detected above a sensitivity limit of 0.15 mJy in the 6.7\mic band, whose emission is mostly dominated by their stellar component, show optical colors similar to the overall cluster population and are gathered in the center of the cluster, following the distribution of the cluster early-types. In the 15\mic band, above a sensitivity limit of 0.3 mJy, the galaxies spectroscopically confirmed to be cluster members are blue outliers of the cluster color-magnitude relation and become brighter going from the center to the outer parts of the cluster. We compare the 6.7\mic and 15\mic fluxes and the cumulative distributions of the B-[6.75] and B-[15] colors of the A1689 galaxies, above our 90% completeness limits of 0.2 and 0.4 mJy for 6.7\mic and 15\mic respectively, to the galaxies of two nearby clusters, Virgo and Coma, and to the field galaxies at the same redshift of the cluster. Although the B-[6.7] color distributions of the three clusters are compatible, we find a systematic excess of B-[15] color distribution for the galaxies located in Abell 1689 with respect to Coma or Virgo galaxies. This result proves the existence of a mid-infrared equivalent of the Butcher-Oemler effect measured in the optical. The comparison of 15\mic flux and B-[15] color distributions of A1689 and field galaxies does not show strong differences between the population of starburst galaxies in the cluster and in the field.

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arXiv2000

VLT-ISAAC near-IR Spectroscopy of ISO selected Hubble Deep Field South Galaxies

We report the results of near-infrared VLT-ISAAC spectroscopy of a sample of 12 galaxies at z = 0.4-1.4, drawn from the ISOCAM survey of the Hubble Deep Field South. We find that the rest frame R-band spectra of the ISOCAM galaxies resemble those of powerful dust-enshrouded starbursts. Halpha emission is detected in 11 out of 12 objects down to a flux limit of 7x10^(-17) erg/cm^2/s, corresponding to a luminosity limit of 10^41 erg/s at z = 0.6, (for an Ho = 50 and Omega = 0.3 cosmology). From the Halpha luminosities in these galaxies we derive estimates of the star formation rate in the range 2--50 Mo/yr for stellar masses 1--100 Mo. The raw Halpha-based star formation rates are an order of magnitude or more lower than SFR(FIR) estimates based on ISOCAM LW3 fluxes. If the Halpha emission is corrected for extinction the median offset is reduced to a factor of 3. The sample galaxies are part of a new population of optically faint but infrared--luminous active starburst galaxies, which are characterized by an extremely high rate of evolution with redshift up to z~1.5 and expected to contribute significantly to the cosmic far-IR extragalactic background.

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arXiv2000

The European Large Area ISO Survey II: mid-infrared extragalactic source counts

We present preliminary source counts at 6.7um and 15um from the Preliminary Analysis of the European Large Area ISO survey, with limiting flux densities of \~2mJy at 15um & ~1mJy at 6.7um. We separate the stellar contribution from the extragalactic using identifications with APM sources made with the likelihood ratio technique. We quantify the completeness & reliability of our source extraction using (a) repeated observations over small areas, (b) cross-IDs with stars of known spectral type, (c) detections of the PSF wings around bright sources, (d) comparison with independent algorithms. Flux calibration at 15um was performed using stellar IDs; the calibration does not agree with the pre-flight estimates, probably due to effects of detector hysteresis and photometric aperture correction. The 6.7um extragalactic counts are broadly reproduced in the Pearson & Rowan-Robinson model, but the Franceschini et al. (1997) model underpredicts the observed source density by ~0.5-1 dex, though the photometry at 6.7um is still preliminary. At 15um the extragalactic counts are in excellent agreement with the predictions of the Pearson & Rowan-Robinson (1996), Franceschini et al. (1994), Guiderdoni et al. (1997) and the evolving models of Xu et al. (1998), over 7 orders of magnitude in 15um flux density. The counts agree with other estimates from the ISOCAM instrument at overlapping flux densities (Elbaz et al. 1999), provided a consistent flux calibration is used. Luminosity evolution at a rate of (1+z)^3, incorporating mid-IR spectral features, provides a better fit to the 15um differential counts than (1+z)^4 density evolution. No-evolution models are excluded, and implying that below around 10mJy at 15um the source counts become dominated by an evolving cosmological population of dust-shrouded starbursts and/or active galaxies.

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arXiv2000

FIRBACK Source Counts and Cosmological Implications

FIRBACK is a one of the deepest surveys performed at 170 microns with ISOPHOT onboard ISO, and is aimed at the study of cosmic far infrared background sources. About 300 galaxies are detected in an area of four square degrees, and source counts present a strong slope of 2.2 on an integral "logN-logS" plot, which cannot be due to cosmological evolution if no K-correction is present. The resolved sources account for less than 10% of the Cosmic Infrared Background at 170 microns. In order to understand the nature of the sources contributing to the CIB, and to explain deep source counts at other wavelengths, we have developed a phenomenological model, which constrains in a simple way the luminosity function evolution with redshift, and fits all the existing deep source counts from the mid-infrared to the submillimetre range. Images, materials and papers available on the FIRBACK web: http://wwwfirback.ias.u-psud.fr wwwfirback.ias.u-psud.fr

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arXiv2000

The extragalactic background and its fluctuations in the far-infrared wavelengths

A Cosmic Far-InfraRed Background (CFIRB) has long been predicted that would traces the intial phases of galaxy formation. It has been first detected by Puget et al.(1996) using COBE data and has been later confirmed by several recent studies (Fixsen et al. 1998, Hauser et al. 1998, Lagache et al. 1999). We will present a new determination of the CFIRB that uses for the first time, in addition to COBE data, two independent gas tracers: the HI survey of Leiden/Dwingeloo (hartmann, 1998) and the WHAM H$_α$ survey (Reynolds et al 1998). We will see that the CFIRB above 100 micron is now very well constrained. The next step is to see if we can detect its fluctuations. To search for the CFIRB fluctuations, we have used the FIRBACK observations. FIRBACK is a deep cosmological survey conducted at 170 micron with ISOPHOT (Dole et al., 2000). We show that the emission of unresolved extra-galactic sources clearly dominates, at arcminute scales, the background fluctuations in the lowest galactic emission regions. This is the first detection of the CFIRB fluctuations.

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arXiv2000

NIR Spectroscopy with the VLT of a sample of ISO selected Hubble Deep Field South Galaxies

A new population of faint galaxies characterized by an extremely high rate of evolution with redshift up to z~1.5 has recently been discovered by ISO. These sources are likely to contribute significantly to the cosmic far-IR extragalactic background. We have carried out near-infrared VLT-ISAAC spectroscopy of a sample of ISOCAM galaxies from the Hubble Deep Field South. The rest-frame R-band spectral properties of the ISO population resembles that of powerful dust-enshrouded active starburst galaxies.

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