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Arxiv:Astro-Ph/0503389V1 17 Mar 2005

Understanding galaxy formation with ISO deep surveys

July 26, 2004

Abstract. We present the results obtained through the various ISO extragalactic deep surveys. While IRAS revealed the existence of galaxies forming stars at a rate of a few tens (LIRGs) or even hundreds (ULIRGs) solar masses in the local universe, ISO not only discovered that these galaxies were already in place at redshift one, but also that they are not the extreme objects that we once believed them to be.

Instead they appear to play a dominant role in shaping present-day galaxies as reflected by their role in the cosmic history of star formation and in producing the cosmic infrared background detected by the COBE satellite in the far infrared to sub-millimeter range.

Keywords: Deep surveys - infrared - galaxy formation 1.

General Perspective

It is widely accepted that in the local universe stars form in giant molecular clouds (GMCs) where their optical and mostly UV light is strongly absorbed by the dust which surrounds them. Whether ex- tinction was already taking place in the more distant universe where galaxies are less metal rich was less obvious ten years ago. Galaxies forming stars at a rate larger than about 20 M⊙yr−1 were known to radiate the bulk of their luminosity above 5 µm thanks to IRAS, the so-called luminous (LIRGs, 12 > log(LIR /L⊙) ≥11) and ultra- luminous (ULIRGs, log(LIR /L⊙) ≥12) infrared (IR) galaxies. In the past, when galaxies were more gaseous and formed the bulk of their present-day stars, it would have been logical to expect to detect the past star formation events of galaxies in the IR regime and to detect a large population of LIRGs/ULIRGs too. However, prior to the launch of the Infrared Space Observatory (ISO, Kessler et al. 1996), this idea was not widely spread. Partly because of a cultural reason: star formation rates (SFR) were commonly measured from optical emission lines and rest-frame UV light in galaxies. This may explain why the redshift evolution of the SFR density per unit comoving volume computed by Madau et al. (1996) became famous. However in the first presentation of the density of UV light per unit comoving volume (Lilly et al. 1996) the authors were cautious to avoid converting their UV light into a c⃝2018 Kluwer Academic Publishers. Printed in the Netherlands.

David Elbaz

SFR because of the unknown factor to correct for extinction. Already IRAS observations indicated a rapid decline of the comoving number density of ULIRGs since z∼0.3 (Kim & Sanders 1998, see also Oliver et al. 1996), but this was over a small redshift range and with small number statistics. In the few years that followed the launch of ISO, several observations showed that galaxy formation could not be under- stood, at least on an observational basis, without accounting for dust extinction as a major ingredient. The ISO deep surveys played a major role in this process, together with other results summarized below. They clearly established that extreme events such as those taking place in local LIRGs and ULIRGs must have been more common in the past, so much that they can now be considered as a standard phase that most galaxies experienced during their lifetime, at least once, but maybe even several times.

The first result of the ISOCAM surveys, as well as the ISOPHOT ones, was the great difference of the counts measured at faint flux densities with respect to local ones from IRAS (Elbaz et al. 1999, Dole et al. 2001). The universe must have been much richer in IR luminous galaxies in the past, either because galaxies were more IR luminous, at fixed galaxy density, and/or because the number density of galaxies was larger in the past, which was partly expected due to the reverse effect of hierarchical galaxy formation through mergers. The strength of the excess of faint objects came as a surprise, but its consequences on the past star formation history of galaxies was confirmed by the convergence of other observations going in the same direction: - the nearly simultaneous discovery of the cosmic infrared back- ground (CIRB, Puget et al. 1996, Fixsen et al. 1998, Hauser & Dwek 2001 and references therein), at least as strong as the UV-optical- near IR one, whereas local galaxies only radiate about 30 % of their bolometric luminosity in the IR above λ ∼5 µm .

- the 850 µm number counts from the SCUBA sub-millimeter bolome- ter array at the JCMT (Hughes et al. 1998, Barger et al. 1998, Smail et al. 2002, Chapman et al. 2003, and references therein) which also indicate a strong excess of faint objects in this wavelength range, im- plying that even at large redshifts dust emission must have been very large in at least the most active galaxies.

- the most distant galaxies, individually detected thanks to the photometric redshift technique using their Balmer or Lyman break sig- nature showed the signature of a strong dust extinction. The so-called “β-slope” technique (Meurer et al. 1999) used to derive the intrinsic luminosity of these galaxies and correct their UV luminosity by factors of a few (typically between 3 and 7, Steidel et al. 1999, Adelberger &

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Steidel 2000) was later on shown to even underestimate the SFR of LIRGs/ULIRGs (Goldader et al. 2002). - the slope of the sub-mJy deep radio surveys (Haarsma et al. 2000).

It has now become clear that the cosmic history of star formation based on rest-frame UV or emission line indicators of star formation such as [0II] or [Hα] strongly underestimate the true activity of galaxies in the past if not corrected by strong factors due to dust extinction.

Although distant galaxies were less metal rich and much younger, they must have found the time to produce dust rapidly in order to efficiently absorb the UV light of their young stars.

We have tried to summarize in the following the role played by the ISO deep surveys in establishing this new perspective (see also Genzel & Cesarsky 2000). However, ten years after ISO’s launch we are still trying to understand the consequences of these findings on galaxy formation scenarios. Are these distant LIRGs really similar to local ones ? What do they teach us about the connection between star and galaxy forma- tion on one side (IMF, triggering of star formation, conditions of star formation,...) and between galaxy and large-scale structure formation on the other side (role of the environment in triggering star formation events, galaxy versus group and cluster formation, ellipticals versus spi- rals, ...) ? How much energy radiated by compton thick embedded active nuclei remains undetected even by the Chandra and XMM-Newton X- ray observatories ? These questions together with others that will be discussed in the following demonstrate the liveliness of this field that will continue to feed the next generation of telescopes and instruments to come such as Herschel, ALMA, the James Webb Space Telescope (JWST) or the Spitzer and GALEX space observatories presently in use.

2.

The Iso Surveys

Extragalactic deep surveys with ISO were performed in the mid and far IR with ISOCAM (Cesarsky et al. 1996) and ISOPHOT (Lemke et al. 1996) respectively. In both wavelength ranges, the steep slopes of the number counts indicate that a rapid decline of the IR emission of galaxies must have taken place from around z ∼1 to z =0. As shown in Fig. 1, ISOCAM could detect galaxies at 15 µm in the LIRG regime up to z ∼1.3, while ISOPHOT was limited to either nearby galaxies or moderately distant ULIRGs such as the one of LIR ∼

4 × 1012

L⊙at a redshift of z=1 shown in the Fig. 1 (normalized SED of Arp 220). The detection limits of ISOCAM and ISOPHOT are compared to SCUBA and Spitzer in the Fig. 2. The deepest ISO surveys reached

David Elbaz

Figure 1. Effect of the k-correction on the detection of distant LIRGs and ULIRGs by ISOCAM, ISOPHOT and SCUBA. Only ISOCAM at 15 µm can detect LIRGs up to z ∼1, while ISOPHOT and SCUBA are sensitive to distant ULIRGs of a few

1012 L⊙.

Figure 2. Sensitivity limits of ISOCAM (15 µm , 0.1 mJy), ISOPHOT (170 µm , 120 mJy), SCUBA (850 µm , 2mJy) and Spitzer (24 µm , 0.45 mJy and 0.022 mJy corresponding to the expected detection limits of the SWIRE and GOODS Legacy Programs). This figure was generated assuming that distant galaxies SEDs are sim- ilar to local ones. We used the library of template SEDs constructed by Chary & Elbaz (2001).

a completeness limit of 0.1 mJy at 15 µm (plain line) and a depth of S15 ∼40 µJy (incomplete, Aussel et al. 1999) in blank fields or a twice deeper completeness level in the central part of nearby galaxy clusters using lensing magnification (Altieri et al. 1999, Metcalfe et al. 2003), and 120 mJy at 170 µm (dotted line). The right axis on the plot shows the conversion of the IR detection limit into a SFR detection limit. Any

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galaxy forming stars at a rate larger than 30 M⊙yr−1 could be detected with ISOCAM up to z ∼1, assuming that the measured 15 µm flux density of a galaxy can be used to derive its “bolometric” IR luminosity, i.e. LIR =L(8-1000 µm ) (see Sect. 5.2).

Number counts represent the first scientific result of an extragalac- tic survey. They can be used to constrain the models. Used alone, they leave some degeneracies unsolved but at least they can demon- strate whether the distant universe was different from the local one in this wavelength range by comparing them to “no evolution” pre- dictions assuming some spectral energy distributions (SEDs) for the k-correction.

3.

The Isocam 6.75 Μm Deep Surveys

Deep images of blank fields at 6.75 and 15 µm done with ISOCAM provide a different view on the distant universe. While the ISOCAM- LW3 filter, centered at 15 µm , can probe dust emission up to z ∼2 for luminous objects, the redshift range to probe star formation from dust emission with the ISOCAM-LW2 band, centered at 6.75 µm , is limited to the relatively nearby universe due to k-correction. However the emission of the old stellar component, which peaks in the near IR, being brought to this wavelength range for high-z galaxies, their stellar masses are better constrained from this flux density.

Sato et al. (2004) derived stellar masses around M⋆∼1011 M⊙for galaxies with z ∼0.2-3 from the correlation between rest-frame near IR luminosity and stellar mass. The stellar mass-to-light ratios were derived from model fit of the set of observed magnitudes depending on the galaxies star formation histories.

The contribution of the 6.75 µm selected galaxies to the cosmic stel- lar mass density of galaxies per unit comoving volume as a function of redshift were estimated to be comparable to those inferred from observations of UV bright galaxies (see Fig. 3). Given the narrow mass ranges, these estimates were obtained as simple summations of the detected sources and should therefore be considered as lower limits only. The faint 6.7 µm galaxies generally had red colors. A comparison with a particular population synthesis model suggests that they have experienced vigorous star formation at high redshifts. The derived large stellar masses for the faint 6.7 µm galaxies also support such star forming events at the past. However these events can only be detected at longer wavelengths using the 15 µm ISOCAM or 90 and 170 µm ISOPHOT bands with ISO (or SCUBA at 850 µm and now the Spitzer Space Telescope). The next sections are devoted to these sur-

David Elbaz

Figure 3. (left) ISOCAM 6.75 µm image of the SSA13 field (Fig.2 from Sato et al. 2003, 23 hour observation). This image reaches an 80 % completeness limit of 16 µJy in the central 7 arcmin2 region. North is top and east to the left in J2000. The center is (RA, Dec) = (13h 12m 26s, 42 o44′ 24′′.8). The map shows signal-to-noise ratio (S/N) per 0.6 arcsec sub-pixel. The signal is an average, weighted by the inverse of the assigned variance, and the noise is a normalized standard deviation. (right) Stellar mass density in the universe as a function of redshift (Fig.13 from Sato et al. 2004). The right axis shows densities normalized to the critical density of the universe. The contributions of the 6.75 µm galaxies are shown with solid and double circles for the “combined” (including photometric redshifts) and purely spectroscopic samples, respectively. The double circles are plotted at slightly lower redshifts. The horizontal bars represent the redshift ranges of the bins and the vertical bars show one sigma errors, taking account of Poisson noise and uncertainties in stellar mass and Vmax. Several other estimates are overlaid (see Sato et al. 2004 for references).

The empty/solid squares and the diamond are obtained from full integration of a Schechter fit to their respective luminosity or stellar mass function at each redshift bin. The X marks and the empty circles are quasi-fully integrated values with a finite integration range from 10 L∗to 1/20 L∗, and 10.5 < log(Mstar[h−2

65 M⊙]) < 11.6,

respectively. The triangles are simply summed values of the detected sources. The two dashed curves are deduced by integrating the star formation rate density in the universe, which is derived from the UV luminosity density as a function of redshift (Cole et al. 2001). The upper curve is an extinction corrected case for E(B −V ) = 0.15, and the lower one has no dust correction.

veys and their consequences on our understanding of the history of star formation in the universe. 4.

The Isophot Far Ir Deep Surveys

ISOPHOT was used to survey the sky at 90 and 170 µm , mainly (plus a few surveys at 60, 120, 150 and 180 µm, see Juvela et al. 2000; Linden-Voernle et al. 2000; cf review by Dole 2002). At 90 µm , the 46

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Figure 4. (a) Differential 90 µm counts dN/dS normalized to the Euclidean law (N ∝S−2.5) (extracted from Rodighiero & Franceschini 2004). Results compared with those from the preliminary analysis of the ISOPHOT ELAIS survey (Efsthatiou et al. 2000, open circles) and with those from Kawara et al. (2004, filled triangles).

The long-dashed line shows the expected contribution of non-evolving spirals as in the model of Franceschini et al. (2001). (a) Fluctuations of the CIB in a power spectrum analysis of the FIRBACK/ELAIS N2 field at 170 microns by Puget & Lagache (2000). Observed power spectrum: diamond; straight continuous line: the best fit cirrus power spectrum; dash line: cirrus power spectrum deduced from Miville-Deschˆenes et al. (2002); continuous curve: detector noise.

arcseconds pixels and FWHM of ISOPHOT represented a significant improvement with respect to IRAS, however due to k-correction and sensitivity limitations, only the local universe could be probed at such wavelengths. The 170 µm band was more favorable for the detection of distant ULIRGs as shown in the Fig. 1 where the peak emission of a ULIRG of 4 × 1012 L⊙and redshifted at z=1 is shown (normalized SED of Arp 220). A major issue at these wavelengths with a 60 cm telescope is obviously the identification of optical counterparts for the determination of a redshift and the separation of local and distant galaxies.

4.1. Source Counts And Cosmic Ir Background

Source counts at 170 µm (e.g. Kawara et al. , 1998; Puget et al. , 1999; Dole et al. , 2001) exhibit a steep slope of α = 3.3 ± 0.6 be- tween 180 and 500 mJy and, like in the mid IR range, show sources in excess by a factor of 10 compared with no evolution scenario. The brightness fluctuations in the Lockman Hole were used by Matsuhara et al (2000) to constrain galaxy number counts down to 35 mJy at 90 µm and 60 mJy at 170 µm , confirming the existence of a strong evolution down to these flux densities. Using a new data reduction method, Rodighiero & Franceschini (2004) extended the previous works of Kawara et al. (1998), Efstathiou et al. (2000) and Linden-Voernle et al. (2000) down to lower flux densities (30 mJy at 90 µm ) and found

David Elbaz

a clear excess of faint objects with respect to no evolution (see Fig. 4a). However, the resolved sources account for less than 10 % of the Cosmic Infrared Background at 170 µm , which is expected to be resolved into sources in the 1 to 10 mJy range.

Sources below the detection limit of a survey create fluctuations. If the detection limit does not allow to resolve the sources dominating the CIB intensity, which is the case in the far IR with ISO, char- acterizing these fluctuations can constrain the spatial correlations of these unresolved sources of cosmological significance. An example of the modeled redshift distribution of the unresolved sources at 170 µm can be found in Fig. 12 of Lagache et al. (2003); the sources dom- inating the CIB fluctuations have a redshift distribution peaking at z ∼0.9. After the pioneering work of Herbstmeierer et al. (1998) with ISOPHOT, Lagache & Puget (2000) discovered them at 170 µm in the FIRBACK data, followed by other works at 170 and 90 µm (Matsuhara et al. 2000; Puget & Lagache, 2000; Kiss et al. 2001). Fig. 4b shows the CIB fluctuations in the FN2 field by Puget & Lagache, (2000), at wavenumbers 0.07 < k < 0.4 arcmin−1.

4.2. Nature Of The Isophot Galaxies

Determining the nature of the far IR galaxies has been a longer process than in the mid IR, mainly because of the difficulty to find the shorter wavelength counterparts in a large beam. Various techniques have been used to overcome this problem, one of the most successful being the identification using 20 cm radio data (e.g. Ciliegi et al. 1999). Another technique is the far IR multiwavelength approach (Juvela et al. 2000) that helps constraining the position and the SED; it also helps to separate the cirrus structures from the extragalactic sources. A vari- ation is to use ISOCAM and ISOPHOT data, like the ELAIS Survey (Rowan-Robinson et al. 2004, see also Oliver in this book). Finally, the Serendipity Survey (Stickel et al. 1998, 2000), by covering large and shallow areas, allows to detect many bright objects easier to follow-up or already known.

Far IR ISO galaxies can be sorted schematically into two popula- tions. First, the low redshift sources, typically z < 0.3 (e.g Serjeant et al. 2001; Patris et al. 2002, Kakazu et al. 2002), have moderate IR luminosities, below 1011L⊙, and are cold (Stickel et al. 2000). Second, sources at higher redshift, z ∼0.3 (Patris et al., 2002) and beyond, z ∼ 0.9 (Chapman et al. 2002) are more luminous, typically L > 1011L⊙, and appear to be cold. Serjeant et al. (2001) derived the Luminosity Function at 90 µm, and started to detect an evolution compared to the local IRAS 100 µm sample.

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Figure 5. ISOCAM 15 µm differential counts, with 68% error bars. The counts are normalized to a Euclidean distribution of non-evolving sources, which would have a slope of α = −2.5 in such a universe. This plot is an extension of the Fig.2 from Elbaz et al. (1999) combining the data points from Elbaz et al. (1999), for the ISOCAM Garanteed Time Extragalactic Surveys (IGTES, filled and empty dots for sources below and above 1 mJy respectively, see text), Gruppioni et al. (2003, bold empty circles) for the ELAIS-S1 field, Rodighiero et al. (2004, large open stars) and Fadda et al. (2004, large open squares), for the Shallow and Deep Surveys of the Lockman Hole respectively (analysis of the IGTES data using the “Lari” technique, see Sect. 5). The hatched area materializes the range of possible expectations from models assuming no evolution and normalized to the 15 µm local luminosity function (LLF) from Fang et al. (1998) and the template SED of M51 (with an assumed uncertainty of ± 20 %; this 15 µm LLF was recently confirmed over a larger sample of local galaxies by H.Aussel, private communication).

5.

The Isocam 15 Μm Deep Surveys

A series of surveys were performed within the Garanteed Time (IGTES, ISOCAM Garanteed Time Extragalactic Surveys, Elbaz et al. 1999) and Open Time as summarized in the Table I, where the surveys at 7 (6.75 µm ) and 15 µm are sorted by increasing depth irrespective of wavelength. The major strength of ISOCAM is its spatial resolution (PSF FWHM of 4.5” at 15 µm , Okumura 1998) and sensitivity, which permitted to detect galaxies down to the LIRG regime up to z ∼1.3 in the deepest surveys (Figs. 1, 2), well above confusion. Cosmic rays

David Elbaz

Table I. Table of the ISOCAM extragalactic surveys sorted by increasing depth. Col.(1) Survey name. Col. (2) wavelength of the survey with “7” for the LW2 filter centered at 6.75 µm and covering 5-8.5 µm, and “15” for the LW3 filter centered 15 µm and covering 12-18 µm. Col.(3) total area in square arcmin. Col.(4) integration time per sky position. Col.(5) depth (and 80 % completeness limit when indicated). Col.(6) number of objects detected above this depth.

0.016

References: (1) Oliver et al. 2000, Rowan-Robinson et al. 2004, (2) Elbaz et al. 1999 and in prep., (3) Rodighiero et al. 2004, (4) Clements et al. 1999, (5) Flores et al. 1999, (6) Flores (private communication), (7) Fadda et al. 2004, (8) Metcalfe et al. 2003, (9) Aussel et al. 1999, Goldschmidt et al. 1997, (10) Oliver et al. 2002, (11) Altieri et al. 1999, (12) Taniguchi et al. 1997, (13) Sato et al. 2003, Sato et al. 2004. The depth of the deep surveys in the field of lensing clusters does not include the correction for lensing amplification, as indicated by the (u)

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Figure 6. ISOCAM 15 µm image of the Marano FIRBACK Deep Survey (DSF) from the IGTES (Elbaz et al. 1999, and in prep.). Data reduction with PRETI. Figure 7. ISOCAM 15 µm contours overlayed on the VLT-FORS2 image (7’×7’, R-band) of the Marano FIRBACK Ultra Deep Survey (UDSF). The LARI (light contours, yellow on screen, grey on paper) and PRETI (dark contours, red on screen) detect the same objects.

David Elbaz

were a stronger limitation than photon or readout noise by inducing ghost sources when they were not perfectly removed, especially those with long term transients associated to them. Two techniques were developped in order to solve this issue, the so-called PRETI (Pattern REcognition Technique for Isocam data, Starck et al. 1999) and LARI (Lari et al. 2001) techniques. A third technique was developped by D´esert et al. (1999), in which the mosaic data were analyzed using the beam switching approach, the Three Beam technique. PRETI consists in a multi-scale wavelet decomposition of the signal, while LARI tries to account for the physical processes taking place in the detectors, including the effect of neighboring pixels. The LARI technique was first applied to the ELAIS surveys (see Oliver et al. , in these book) and more recently to the IGTES surveys of the Lockman Hole (Rodighiero et al. 2004, Fadda et al. 2004) and of the Marano field (Elbaz et al. , in prep.). The quality of the ISOCAM images is shown with the case of the Marano FIRBACK deep field (30’×30’) in the Fig. 6. The central part of this field was imaged at a deeper level (UDSF) and analyzed with both techniques. The resulting contours are overlayed on a VLT-FORS2 image of the field, demonstrating the consistency and robustness of both source detection algorithms (Fig. 7).

Lensing appeared to be a very powerful tool to extend the detection of faint objects by a factor 2-3 (see Metcalfe et al. 2003). However studies such as the relative clustering of IR galaxies versus field galaxies are better done in larger homogeneous fields, where the role of cosmic variance can also be quantified. As we will see below, the environment of distant LIRGs seems to play a major role in triggering their star formation activity.

5.1. Source Counts And Cosmic Ir Background

The source counts at 15 µm exhibit a strong excess of faint sources below S15 ∼2 mJy. This excess is usually defined by comparison with model predictions assuming that galaxies behaved similarly in the distant universe as they do today. Such “no evolution” behavior is represented by a shaded area in the Fig. 5 (see figure caption). Galaxies above this flux density do fall within this region, as illustrated by the data points from the ELAIS-S1 field (Gruppioni et al. 2003, see also Oliver et al. in this book). In Fig. 5, we have separated the data points from the IGTES (Elbaz et al. 1999) between those below and above S15 = 1 mJy, with filled and open dots respectively. Data above this flux density from Elbaz et al. (1999) appear to be inconsistent with those derived from ELAIS-S1. Most of those points were derived from the Shallow Survey of the Lockman Hole within the IGTES, which

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Figure 8. a) Differential contribution to the 15 µm Integrated Galaxy Light as a function of flux density and AB magnitude. The plain line is a fit to the data: Abell 2390 (Altieri et al. 1999), the ISOCAM Guaranteed Time Extragalactic Surveys (IGTES, Elbaz et al. 1999), the European Large Area Infrared Survey (ELAIS, Serjeant et al. 2000) and the IRAS all sky survey (Rush, Malkan & Spinoglio 1993).

b) Contribution of ISOCAM galaxies to the 15 µm extragalactic background light (EBL), i.e. 15 µm Integrated Galaxy Light (IGL), as a function of sensitivity or AB magnitude (AB = −2.5 log (SmJy) + 16.4). The plain line is the integral of the fit to dIGL/dS (Fig.a). The dashed lines correspond to 1-σ error bars obtained by fitting the 1-σ upper and lower limits of dIGL/dS.

suffered from having less redundant observations of a given sky pixel. At that time, ISOCAM data reduction methods were not optimized for such surveys, but since then, they have been improved to better deal with such shallow surveys. Among the techniques that are discussed below, the “Lari” technique is particularly suitable for such low redun- dancy surveys (and even more for the very shallow ELAIS surveys, see Oliver et al. in this book) and a recent analysis of the Lockman Hole Deep (Fadda et al. 2004, large open squares) and Shallow (Rodighiero et al. 2004, large open stars) surveys from the IGTES provided new number counts at these flux densities perfectly consistent with those derived from ELAIS-S1 by Gruppioni et al. (2003) also using the same “Lari” technique. Note that the models designed to fit the ISOCAM number counts were constrained by the Elbaz et al. (1999) results, hence overproduce the number of sources above S15 ∼2 mJy. As a natural result, they have also overpredicted the number of sources detected in the high flux density regime at 24 µm with Spitzer (see Sect. 7 and Papovich et al. 2004, Chary et al. 2004).

Above the Earth’s atmosphere, the 15 µm light is strongly dominated by the zodiacal emission from interplanetary dust and it has not yet been possible to make a direct measurement of the 15 µm background, or EBL. Individual galaxies contribute to this background and a lower limit to the 15 µm EBL can be obtained by adding up the fluxes of all

David Elbaz

ISOCAM galaxies detected per unit area down to a given flux limit. The resulting value is called the 15 µm integrated galaxy light (IGL). As in Elbaz et al. (2002), the differential number counts can be converted into a differential contribution to the 15 µm IGL as a function of flux density, estimated from the following formula:

(1)

where dN(sr−1) is the surface density of sources with a flux density Sν[15 µm]= S15 (mJy) over a flux density bin dS(mJy) (1 mJy= 10−20 nW m−2 Hz−1) and ν15(Hz) is the frequency of the 15 µm photons.

About 600 galaxies below S15 ∼3 mJy, were used to provide the points with errors bars in Fig. 8a. Fig. 8b shows the 15 µm IGL as a function of depth. It corresponds to the integral of Fig. 8a, where the data below 3 mJy were fitted with a polynomial of degree 3 and the 1-σ error bars on dIGL/dS were obtained from the polynomial fit to the upper and lower limits of the data points. The 15 µm IGL does not converge above a sensitivity limit of S15 ∼50 µJy, but the flattening of the curve below S15 ∼0.4 mJy suggests that most of the 15 µm EBL should arise from the galaxies already unveiled by ISOCAM. Above this flux density limit, where the completeness limit is larger than 80 % - including lensed objects-, it is equal to 2.4 ± 0.5 nW m−2 sr−1 (Elbaz et al. 2002). Down to a 50 % completeness limit, Metcalfe et al. (2003) found a 10 % larger value including sources down to 30 µJy but the statistics remains limited at these depths with only four sources below 50 µJy.

Franceschini et al. (2001) and Chary & Elbaz (2001), developped models which reproduce the number counts from ISOCAM at 15 µm, from ISOPHOT at 90 and 170 µm and from SCUBA at 850 µm, as well as the shape of the CIRB from 100 to 1000 µm. These models

(2)

If this prediction from the models is correct then about 73 ± 15 % of the 15 µm EBL is resolved into individual galaxies by the ISOCAM surveys.

This result is consistent with the upper limit on the 15 µm EBL

(3)

This upper limit was computed from the 1997 γ-ray outburst of the blazar Mkn 501 (z = 0.034) as a result of the opacity of mid IR

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Figure 9. IR luminosity correlations for local galaxies (from Elbaz et al. 2002). a) ISOCAM-LW3 (15 µm) versus ISOCAM-LW2 (6.75 µm) luminosities (νLν) (56 galaxies). b) ISOCAM-LW3 (15 µm) versus IRAS-12µm luminosities (45 galax- ies). c) LIR[8-1000 µm] versus ISOCAM-LW3 (15 µm) luminosity (120 galaxies). d) LIR[8-1000 µm] versus LW2-6.75 µm luminosities (91 galaxies). Filled dots: galaxies from the ISOCAM guaranteed time (47 galaxies including the open squares). Open dots: 40 galaxies from Rigopoulou et al. (1999). Empty triangles: 4 galaxies from Tran et al. (2001). Galaxies below LIR ∼1010 L⊙present a flatter slope and have

Lir/Lb < 1.

photons to γ-ray photons, which annihilate with them through e+e− pair production. It was since confirmed by Renault et al. (2001), who found an upper limit of 4.7 nW m−2 sr−1 from 5 to 15 µm.

Nearly all the IGL is produced by sources fainter than 3 mJy (94 %) and about 70 % by sources fainter than 0.5 mJy. This means that the nature and redshift distribution of the galaxies producing the bulk of the 15 µm IGL can be determined by studying these faint galaxies only.

5.2. Mid Ir As A Sfr Indicator

When normalized to the 7.7 µm PAH (polycyclic aromatic hydrocar- bon) broad emission line, the spectra of different galaxies exhibit very different 15, 25, 60 or 100 µm over 7.7 µm ratio. This was often inter- preted as an indication that measuring the monochromatic luminosity of a galaxy at one mid IR wavelength was useless to determine its

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