A&A 384, 848-865 (2002)
D. Elbaz1,2,3 - C. J. Cesarsky1,4 - P. Chanial1 - H. Aussel1,5 - A. Franceschini6 - D. Fadda1,7 - R. R. Chary3
1 - DAPNIA/Service d'Astrophysique, CEA/Saclay, 91191 Gif-sur-Yvette Cedex, France
2 - Physics Department, University of California, Santa Cruz, CA 95064, USA
3 - Department of Astronomy & Astrophysics, University of California, Santa Cruz, CA 95064, USA
4 - European Southern Observatory, Karl-Schwarzchild-Strasse 2, 85748 Garching bei Muenchen, Germany
5 - Institute For Astronomy, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, USA
6 - Dipartimento di Astronomia, Vicolo Osservatorio 2, 35122 Padova, Italy
7 - Instituto de Astrofisica de Canarias, Via Lactea, S/N E38200, La Laguna (Tenerife), Spain
Received 16 May 2001 / Accepted 15 January 2002
Deep extragalactic surveys with ISOCAM revealed the presence of a large density of faint mid-infrared (MIR) sources. We have computed the 15m integrated galaxy light produced by these galaxies above a sensitivity limit of 50 Jy. It sets a lower limit to the 15m extragalactic background light of ( ) nW m-2 sr-1. The redshift distribution of the ISOCAM galaxies is inferred from the spectroscopically complete sample of galaxies in the Hubble Deep Field North (HDFN). It peaks around 0.8 in agreement with studies in other fields. The rest-frame 15m and bolometric infrared (8-1000m) luminosities of ISOCAM galaxies are computed using the correlations that we establish between the 6.75, 12, 15m and infrared (IR) luminosities of local galaxies. The resulting IR luminosities were double-checked using radio (1.4 GHz) flux densities from the ultra-deep VLA and WSRT surveys of the HDFN on a sample of 24 galaxies as well as on a sample of 109 local galaxies in common between ISOCAM and the NRAO VLA Sky Survey (NVSS). This comparison shows for the first time that MIR and radio luminosities correlate up to 1. This result validates the bolometric IR luminosities derived from MIR luminosities unless both the radio-far infrared (FIR) and the MIR-FIR correlations become invalid around 1. The fraction of IR light produced by active nuclei was computed from the cross-correlation with the deepest X-ray surveys from the Chandra and XMM-Newton observatories in the HDFN and Lockman Hole respectively. We find that at most 20% of the 15m integrated galaxy light is due to active galactic nuclei (AGNs) unless a large population of AGNs was missed by Chandra and XMM-Newton. About 75 of the ISOCAM galaxies are found to belong to the class of luminous infrared galaxies ( 1011). They exhibit star formation rates of the order of 100 yr-1. The comoving density of infrared light due to these luminous IR galaxies was more than 40 times larger at than today. The contribution of ISOCAM galaxies to the peak of the cosmic infrared background (CIRB) at 140m 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. () nW m-2 sr-1 as compared to the (25 7) nW m-2 sr-1 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.
Key words: galaxies: evolution - infrared: galaxies - galaxies: starburst - galaxies: Seyfert
The CIRB was recently detected and measured thanks to the cosmic background explorer (COBE) instruments FIRAS (Far Infrared Absolute Spectrometer) and DIRBE (Diffuse Infrared Background Experiment) (Puget et al. 1996; Fixsen et al. 1998; Lagache et al. 1999, 2000; Hauser et al. 1998; Dwek et al. 1998; Finkbeiner et al. 2000) from 100m to 1 mm. It peaks around 140m and was found to represent at least half and maybe two thirds of the overall cosmic background (see Gispert et al. 2000). Hence the CIRB reflects the bulk of the star formation that took place over the history of the universe. By resolving it into individual galaxies, we would therefore pinpoint the times and places where most stars seen in the local universe were formed. Two physical processes were considered for its origin: nucleosynthesis, i.e. stellar radiation in star forming galaxies, and accretion around a black hole, i.e. active galactic nuclei. In both cases, the light is not directly coming from its physical source but is reprocessed by dust, i.e. absorbed and re-radiated thermally by the "warm'' dust. Both processes are probably related (see Genzel et al. 1998), but energetic considerations, based on the presence of massive black holes and on the amount of heavy elements in local galaxies, suggest that star formation should by far dominate in the CIRB over AGN activity (Madau & Pozzetti 2000; Franceschini et al. 2001). However, until the individual galaxies responsible for the CIRB are found and studied in detail, this result will remain theoretical.
The spectral energy distribution (SED) in the IR of local galaxies peaks above 60m and typically around 80 20m (see Sanders & Mirabel 1996). As a result, the distant galaxies responsible for the peak of the CIRB detected by COBE around 140m should be located below 1.3 and present a redshift distribution peaked around 0.8, if their SEDs do not strongly differ from those of local galaxies. As we will see this is also the redshift range of the galaxies detected at 15m with ISOCAM.
The ISOCAM extragalactic surveys were performed with two filters, LW2 (5-8.5m) and LW3 (12-18m), centered at 6.75 and 15m respectively. The 6.75m sample of sources is strongly contaminated by galactic stars, whereas stars are rather easily distinguished from galaxies at 15m using optical-MIR colour-colour plots. As a consequence, we are only concerned here by the 15m galaxies. Moreover, the observed 6.75m light is no more produced by dust emission for galaxies more distant than 0.4 because of k-correction (redshifted stellar light dominates the 6.75m band above this redshift), whereas the observed 15m light is mostly due to dust emission for galaxies up to 2.
About 1000 galaxies detected in the 15m surveys were used to produce number counts (i.e. surface density of galaxies as a function of flux density; see Elbaz et al. 1999). The steep slope of the 15m counts below 1 mJy indicates the presence of an excess of faint sources by one order of magnitude in comparison with predictions assuming no evolution of the 15m luminosity function with redshift. The presence of broad emission features in the MIR spectrum of galaxies alone cannot explain the shape of the number counts and a strong evolution of either the whole luminosity function (Xu 2000; Chary & Elbaz 2001) or preferentially of a sub-population of starburst galaxies evolving both in luminosity and density (Franceschini et al. 2001; Chary & Elbaz 2001; Xu et al. 2001) is required in order to fit the ISOCAM 15m counts. In the present paper, we suggest that these ISOCAM galaxies are in fact dusty starbursts responsible for the bulk of the CIRB.
In Sect. 2, we compare the sensitivity of different extragalactic surveys in several wavelength ranges to detect the galaxies responsible for the CIRB. It is suggested that MIR is presently the most efficient technique to detect dusty starbursts up to 1.3.
In Sect. 3, we calculate the 15m integrated galaxy light (IGL) due to ISOCAM galaxies. The 15m IGL is the sum of the 15m fluxes from individual galaxies, down to a given sensitivity limit, per unit area. It represents a lower limit to the 15m EBL, which remains unknown. Once the redshift distribution and SED of these galaxies is determined, it becomes possible to estimate their contribution to the CIRB.
In Sect. 4, we demonstrate that MIR luminosities at
6.75, 12 and 15m are strongly correlated with the bolometric IR
luminosity (from 8 to 1000m) for local galaxies. The
correlations presented in Chary & Elbaz (2001) are confirmed here
with a larger sample of galaxies.
|Figure 1: Spectral energy distribution of galaxies in the infrared. a) SEDs of M 82 (dashed line) and Arp 220 (plain line) combining photometric (filled and open dots) and spectroscopic data. Open dots are used when they overlap with spectroscopic data. Radio data for Arp 220 from Anantharamaiah et al. (2000). Sub-millimetric data for M 82 from SCUBA (Hughes et al. 1994). 45-190m: ISO-LWS spectra from Fischer et al. (1999). Arp220's 18m silicate absorption feature from Smith et al. (1989). 5-18m: ISOCAM Circular Variable Filter (CVF) spectra (Arp 220: Charmandaris et al. 1999, M 82: Förster Schreiber et al. 2002) normalized to the IRAS 12m flux. For M 82, the CVF spectrum of the central region of the galaxy was complemented outward by adding to it a typical spectrum of a reflection nebula (NGC 7023, Cesarsky et al. 1996b), in order to match the 12m IRAS flux. to 4m: for M 82, SWS spectrum of Förster Schreiber et al. (2001) scaled using the NGC 7023 spectrum of Moutou et al. (2000); for Arp 220, fit to the ISOPHOT data (Klaas et al. 1997) combined with the profile from Moutou et al. (2000). Optical to near-infrared: mix of the Coleman et al. (1980) templates to match the observed total magnitudes obtained from NED (Arp 220: 67 Sbc and 33 Ell; M 82: 12 Sbc and 88 Ell). b) 10 SEDs with to 13, with a step of 0.5, from the library of 105 SEDs of Chary & Elbaz (2001).|
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Before spectroscopic redshifts are obtained for the full sample of ISOCAM galaxies used to produce these number counts, the redshift distribution of these galaxies can be inferred from a few sub-samples: HDFN (Aussel et al. 1999, 2001), CFRS-14 (Flores et al. 1999, 2002), CFRS-03 (Flores et al. 2002). The ultra-deep ISOCAM survey of the HDFN samples a flux density range where most of the evolution observed in the number counts takes place and where the bulk of the 15m IGL is produced. This field is complete in spectroscopic redshifts, so it is used to estimate the bolometric IR luminosities and star formation rates of the ISOCAM galaxies in Sect. 5.
This result relies on two assumptions:
- that the main source for the MIR light in ISOCAM galaxies is star formation and not accretion around a black hole;
- that the correlations found in the local universe between the MIR and bolometric IR luminosity of galaxies remain valid up to 1. The first assumption is discussed and justified in Sect. 5.2, where soft and hard X-ray data from the Chandra and XMM-Newton X-ray observatories are combined with ISOCAM data on galaxies in the HDFN and Lockman Hole regions respectively.
The issue of the robustness of the MIR-FIR correlations in the distant universe is addressed in Sect. 5.5, where IR luminosities are also computed from radio (1.4 GHz) flux densities for a sub-sample of 24 distant and 109 local ISOCAM galaxies.
In Sect. 5.6, we compute the cosmic density of IR light due to luminous IR galaxies ( ) at 1. In Sect. 6, we evaluate the contribution of the ISOCAM galaxies to the CIRB, more precisely to its peak emission around 140m. Finally, the nature of ISOCAM galaxies is discussed in the conclusions (Sect. 7).
In the following, we will use the terms ULIG for galaxies with an IR
and luminous IR
galaxies for both (
this paper, we will assume H0= 75 km s-1 Mpc-1,
= 0.3 and
|Figure 2: IR luminosity (left axis) and SFR (right axis) corresponding to the sensitivity limits of ISOCAM (15m, plain line), ISOPHOT (170m, dotted line), SCUBA (850m, dot-dashed line) and VLA/WSRT (21 cm, dashed line) as a function of redshift. K-correction from: a) the template SED of M 82 (Fig. 1a), normalized to the appropriate IR luminosity, b) the library of template SEDs from Chary & Elbaz (2001), plotted in the Fig. 1b.|
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IRAS surveyed about 95 of the sky down to a completeness limit of 0.5 mJy at 60m (1.5 Jy at 100m) and the differential counts are well fitted by an Euclidean slope, (Soifer et al. 1987). The fluxes of all galaxies detected down to this sensitivity limit by IRAS add up to a 60m IGL of 0.15 nW m-2 sr-1. This is less than 1 of the value of the CIRB measured by COBE at 140m, IGL140= () nW m-2 sr-1.
At 450m, a depth of 10 mJy is reached and the combined fluxes of all SCUBA galaxies produce about 15 of the CIRB measured by COBE-FIRAS at this wavelength (Smail et al. 2001).
At 850m, SCUBA is confusion limited at 2 mJy (Hughes et al. 1998; Barger et al. 1999; Eales et al. 2000; Smail et al. 2001), because of its large point spread function (PSF) of 15 full width half maximum (FWHM). About 20 of the value of the CIRB measured by COBE-FIRAS at 850m (the 850m EBL) is resolved into galaxies at this depth. However, using gravitational lensing this limit can be lowered to 1 mJy, where 60 of the CIRB is resolved (Smail et al. 2001; Blain et al. 1999a).
The 850m EBL measured by COBE-FIRAS ( nW m-2 sr-1) is 50 times lower than the peak value of the CIRB at 140m. In order to compute the contribution of SCUBA galaxies to the peak of the CIRB it is therefore necessary to determine their redshift distribution and SED. Until now, very few redshifts have been obtained due to the large PSF of SCUBA and optical faintness of these galaxies. Hence the fraction of the CIRB resolved into individual galaxies by SCUBA remains highly uncertain.
We did not use Arp 220's SED in Fig. 2, because this galaxy presents a large FIR over MIR luminosity ratio, which is not representative of galaxies within the same luminosity range (see Figs. 5c,d).
The radio continuum (
cm, GHz) is also a
because of the tight correlation between both
luminosities in the local universe (see Yun et al. 2001 and
references therein). If this correlation remains valid in the distant
universe then we can translate the sensitivity limit of the deepest
radio surveys into a minimum
accessible at a given
redshift assuming a radio SED. The correlation between the radio and
FIR luminosities is usually described by the "q'' parameter (Condon
et al. 1991):
The deepest radio surveys presently available were performed in the HDFN by Richards (2000) and Garrett et al. (2000) with the VLA and WSRT respectively. Both surveys reach the same sensitivity limit of 40Jy at 1.4 GHz (5-). In order to convert this flux density into a for a given redshift, we used a power index of ( ), as suggested by Yun et al. (2001) for starburst galaxies.
Figure 2 presents the sensitivity of the deepest MIR, FIR, sub-millimeter and radio surveys in the form of L (or SFR) as a function of redshift. The sensitivity limits used for ISOCAM, ISOPHOT and SCUBA are taken from the deepest existing surveys at these wavelengths, as previously described. Figure 2a (with M 82) and Fig. 2b ("multi-template'' technique) both clearly show that the faintest IR galaxies are best detected at 15m up to 1.3. The right axis of the plots shows the corresponding minimum SFR that a galaxy must harbor to be detected at a given redshift in the surveys. At 1, ISOCAM (plain line) detects all LIGs while SCUBA (dot-dashed line) detects only ULIGs. The difference in sensitivity between ISOCAM and the deepest radio surveys is about a factor 2. ISOPHOT (dotted line) is only sensitive to ULIGs above 0.5. In Fig. 2b, where we used the library of template SEDs from Chary & Elbaz (2001), the plain line corresponding to ISOCAM rises faster above 1 than in Fig. 2a, where we used M 82's SED. This different behavior comes from the fact that at these redshifts ISOCAM measures fluxes at about 7m in the rest-frame of the galaxy and we will show that / increases with (Sect. 4).
Above , the sub-millimeter becomes the most efficient technique, although only galaxies more luminous than a few 1012 can be detected above a sensitivity limit of 2 mJy at 850m. Hence the unlensed ISOCAM and SCUBA surveys are not sampling the same redshift and luminosity ranges. This statement is confirmed observationally in the HDFN itself where none of the ISOCAM sources are detected by SCUBA (Hughes et al. 1998) and over a larger scale in the Canada France Redshift Survey 14 (CFRS-14, Eales et al. 2000). In this latter field of 50 , only the two brightest 15m ISOCAM sources are detected at 850m, among a sample of 50 ISOCAM (Flores et al. 1999) and 19 SCUBA sources. This confirms that both instruments detect different sets of objects. More common objects between ISOCAM and SCUBA are expected if they are gravitationally lensed, since the sensitivity limits of both instruments are then decreased by a factor of about two. Indeed in the clusters A 370 and A 2390, three of the four lensed SCUBA galaxies (Smail et al. 2001) are also detected with ISOCAM (Altieri et al. 1999; Metcalfe 2000).
|Figure 3: a) Differential contribution to the 15m 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 et al. 1993). b) Contribution of ISOCAM galaxies to the 15m extragalactic background light (EBL), i.e. 15m Integrated Galaxy Light (IGL), as a function of sensitivity or ABmagnitude . The plain line is the integral of the fit to dIGL/dS (Fig. 3a). The dashed lines correspond to 1- error bars obtained by fitting the 1- upper and lower limits of dIGL/dS.|
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The statistical reliability with which the surface density of MIR
sources is computed depends on their flux: large and shallow fields
are required for bright sources which are less numerous, whereas faint
sources require smaller and deeper fields. The optimization is
obtained by observing similar volumes of the sky but in pencil beams
for faint sources and in wide surveys for bright sources, instead of
one large and deep survey which would be unnecessarily time
consuming. Together with the need to account for cosmic variance, this
explains why we have combined a set of 10 surveys located in 7 regions
of the northern and southern hemispheres, in order to estimate the
15m IGL. These surveys cover five orders of magnitude in flux
density from 50 Jy to 4 Jy (including IRAS counts at
12m). The number counts presented in Elbaz et al. (1999) are
converted here into a differential contribution to the 15m IGLas a function of flux density, estimated from the following formula:
Below 3 mJy, about 600 galaxies were used to produce the points with errors bars in Fig. 3a. Figure 3b shows the 15m IGL as a function of depth. It corresponds to the integral of Fig. 3a, where the data below 3 mJy were fitted with a polynomial of degree 3 and the 1- error bars on d were obtained from the polynomial fit to the upper and lower limits of the data points. The 15m IGL does not converge above a sensitivity limit of 50Jy, but the flattening of the curve below mJy suggests that most of the 15m EBL should arise from the galaxies already unveiled by ISOCAM.
Above the completeness limit of
Jy, we computed
an IGL of:
Only 9 sources are detected below
Jy by Altieri et al. (1999), where the flux determination and correction for
completeness are less robust and are not confirmed by another
survey. A tentative value of
nW m-2sr-1 was quoted in Altieri et al. (1999), above S15=
30Jy. This value may be slightly overestimated if we consider
that the models of Franceschini et al. (2001) and Chary & Elbaz
(2001), which reproduce the number counts from ISOCAM at 15m,
from ISOPHOT at 90 and 170m and from SCUBA at 850m, as
well as the shape of the CIRB from 100 to 1000m, consistently
predict a 15m EBL of:
This result is consistent with the upper limit on the 15m EBLestimated by Stanev & Franceschini (1998) of:
Finally, the 15m background (EBL) must be contained between
the following limits:
The FIR over MIR luminosity ratio of Arp 220 is larger than for
galaxies of equivalent luminosity (see Fig. 5 in
Sect. 4.2.2). The flat FIR over MIR ratio for NGC 1068 is
typical of the hot dust continuum from AGNs. NGC 1068 is the closest
Seyfert 2 (
). On one hand, its
MIR spectrum is completely dominated by a continuum due to the hot
dust heated by the central AGN, which produces about 75
MIR luminosity (Le Floc'h et al. 2001). On the other hand, the bulk of
its FIR luminosity originates from the diffuse region surrounding the
nucleus as shown by the SCUBA image at 450m, which is
associated with star formation (Le Floc'h et al. 2001). As a result,
the FIR over MIR luminosity ratio is much lower for NGC 1068 than for
starbursts (see Fig. 4). This galaxy is a perfect
illustration of the close connection between star formation and AGN
|A2390,HDFN||[0.05, 0.1]||23||(1, 2)|
|IGTES||[0.1, 0.5]||48||(2, 3)|
|Figure 4: Spectral energy distributions in the infrared (5-200m) of 3 starbursts (M 82, Arp 244, Arp 220) and 2 Seyfert 2's (NGC 1068, NGC 6240). MIR spectra from ISOCAM CVF: NGC 1068 (Laurent et al. 2000; Le Floc'h et al. 2001), Arp 244 (Vigroux et al. 1996; Mirabel et al. 1998), NGC 6240 (Charmandaris et al. 1999). FIR: fit of the IRAS flux densities at 12, 25, 60 and 100m using the template SEDs from Chary & Elbaz (2001) for Arp 244 and NGC 6240. For NGC 1068, the IRAS flux densities were fitted by the sum of two modifified black bodies of temperatures 30 and 170 K as in Telesco et al. (1984). Open dots with error bars: IRAS data (from NED) for Arp244, NGC 6240 and NGC 1068.|
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The MIR (5-40m) and FIR (40-1000m) emission of a galaxy combine three major components (see Fig. 4):
- broad emission features and their associated underlying continuum, which dominate in the 5-10m domain, and up to 20m for galaxies with moderate star formation. These bands located at 6.2, 7.7, 8.6, 11.3 and 12.7 m, are usually quoted as PAHs (polycyclic aromatic hydrocarbons, Léger Puget 1984; Puget Léger 1989; Allamandola et al. 1989), although their exact nature remains uncertain, e.g. Jones & d'Hendecourt (2000).
- the "warm'' dust continuum produced by very small dust grains (VSGs, smaller than 0.01m, Andriesse 1978, Désert et al. 1990) heated at temperatures larger than 200 K without reaching thermal equilibrium. This component dominates the SED between 10-20 to 40m for luminous galaxies.
- the "cold'' dust continuum produced by big dust grains ("big grains'', of size >m) in thermal equilibrium at cool temperatures (below 60-70 K typically). This component is responsible for the bulk of the FIR light where the SED peaks.
|M 82||Arp 244||Arp 220||N 1068||N 6240|
The MIR luminosity of galaxies is a good tracer of
seen in Figs. 5c,d, where
at 15 and 6.75m following the formulae:
The position of the five templates from Fig. 4 are
indicated with open squares in Fig. 5. Two galaxies do
not fit the correlations: Arp 220 and NGC 1068. Arp 220 presents a
very large FIR over MIR ratio, which was interpreted as being due to
extinction in the MIR by Haas et al. (2001), while the IR SED of the
Seyfert 2 NGC 1068 is flat, which is typical of AGNs. In this sense
NGC 6240 is quite atypical, since it falls perfectly on the
correlation followed by star forming galaxies. The presence of an
active nucleus in NGC 6240 was revealed by its strong X-ray luminosity
and Ikebe et al. (2000) suggested that a large fraction of the IR
luminosity of this galaxy could originate from its AGN. However, the
presence of strong MIR features (Genzel et al. 1998), the absence of a
strong continuum emission below
6m (Laurent et al. 2000), together with a FIR over MIR ratio typical of star forming
galaxies, suggest that the bulk of its IR luminosity may still be due
to star formation.
|Figure 5: IR luminosity correlations for local galaxies (see text for the origin of the sample). The five galaxies from Fig.4 and Table 2 are marked with open squares. a) ISOCAM-LW3 (15m) versus ISOCAM-LW2 (6.75m) luminosities ( ) (56 galaxies). b) ISOCAM-LW3 (15m) versus IRAS-12m luminosities (45 galaxies). c) [8-1000m] versus ISOCAM-LW3 (15m) luminosity (120 galaxies). d) [8-1000m] versus LW2-6.75m 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). Filled triangles: 4 galaxies from Tran et al. (2001). Galaxies below present a flatter slope and have /LB< 1.|
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|Figure 6: a) Histogram of the flux densities of the 41 ISOCAM-HDFN galaxies ( 0.1 mJy, Aussel et al. 1999, 2001) with Poissonian error bars. Only one galaxy has no spectroscopic redshift (dashed line). b) Redshift distribution of the 40 galaxies with spectroscopic redshift (98 complete in redshift, Poissonian error bars).|
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A total of 41 ISOCAM-HDFN galaxies have 15m flux densities between S15= 0.1 and 0.5 mJy (see Fig. 6a) over a field of 25.8'2 (HDFN+FF). Monte-Carlo simulations give a completeness of 90 in this flux density range (Aussel et al. 1999, 2001). All but one galaxy (HDF_PS3_6a) possess a spectroscopic redshift (98 complete). Their mean and median redshift is 0.8. Except for one galaxy located at z=2.01 (HDF_PM3_5, classified as an AGN, see Sect. 5.2) and four galaxies closer than z=0.15, all ISOCAM galaxies have a redshift between 0.4 and 1.3 (see Fig. 6b), as expected from the Fig. 2.
HDF_PM3_6 (C36367_1346, z= 0.960),
HDF_PM3_20 (C36463_1404, z= 0.962).
However, dusty AGNs may not be easily identified in the optical.
The best technique at present for the identification of dusty AGNs is offered by hard X-ray observations, which are much less affected by extinction. The deepest soft plus hard X-ray observation currently available was performed with the Chandra X-ray Observatory in the HDFN by Hornschemeier et al. (2001) and Brandt et al. (2001, 2002). The Chandra and ISOCAM catalogs were cross-correlated by Fadda et al. (2002). A total of 16 ISOCAM-HDFN ( 0.1 mJy) are detected in the 1 Ms Chandra image at the depth of S[0.5-2 keV] 310-17 erg cm-2 s-1 and S[2-8 keV] 210-16 erg cm-2 s-1. They represent 39 of the ISOCAM-HDFN galaxies. However, the Chandra image is so deep that it is able to detect starburst galaxies up to large redshifts and among the 16 ISOCAM galaxies in common with Chandra sources, only 5 are classified as being AGNs or AGN dominated by Fadda et al. (2002). These 5 sources include the two galaxies previously identified as AGNs from their optical spectra and the following 3 galaxies:
HDF_PM3_5 (J123635.6+621424, z= 2.010),
HDF_PS3_10 (J123642.1+621545, z= 0.857),
HDF_PM3_21 (J123646.4+621529, z= 0.851).
Galaxies detected in the ultra-hard X-ray band (4-8 keV) or with 1043 erg s-1 were classified as AGNs. Galaxies with 1040 erg s-1 or which could be fitted with the SED of M 82 or Arp 220 were classified in the starburst category.
Finally only () (5/41 galaxies, Poissonian error bar) of the ISOCAM-HDFN galaxies are found to be powered by an active nucleus. These AGNs contribute to () of the sum of the flux densities (the IGL) of all 41 ISOCAM-HDFN galaxies. Since the active nuclei in these galaxies may not be responsible for the totality of the IR emission, this result suggests that at most () of the 15m IGL due to galaxies with S15=0.1-0.5 mJy is produced by AGN activity.
The S15= 0.5-3 mJy flux density range is covered by the ISOCAM image of the Lockman Hole, where Fadda et al. (2002) cross-correlated ISOCAM with the XMM-Newton X-ray observatory catalog of sources brighter than S[2-10 keV] 1.410-15 erg cm-2 s-1 and S[5-10 keV] 2.410-15 erg cm-2 s-1 (Hasinger et al. 2001). Among 103 ISOCAM sources with a flux density ranging from 0.5 to 3 mJy, 13 sources were found to be AGN dominated by Fadda et al. (2002), i.e. (). Such galaxies could be fitted by the SEDs of either type-I or type-II AGNs by Franceschini et al. (2002). These AGNs contribute to () of the 15 m IGL due to galaxies in the 0.5-3 mJy flux range.
The combination of the HDFN and Lockman Hole cover a flux density
range of 0.1-3 mJy where 70
of the 15m IGL is measured.
In this range, these two fields suggest that AGNs contribute to at
It must be noted that the XMM-Newton and
Chandra experiments were limited to energies below 10 keV. Some harder
X-ray AGNs may not have been detected by these instruments yet and may
be common with our ISOCAM source list. The presence of such objects
will remain highly uncertain until the launch of the next
generation X-ray experiments (XEUS, Constellation X). We will assume
in the following that the AGN fraction computed in the HDFN (which is
larger than in the Lockman Hole) applyies for the 15m IGL as a
|Figure 7: Rest-frame MIR luminosity of ISOCAM-HDFN galaxies. a) luminosity of ISOCAM galaxies at m as a function of . The dotted lines represent the redshifted LW3 (12-18m) band. b) rest-frame 15m luminosity ( ) versus redshift estimated with three different techniques: M 82 SED (crosses), "closest filter'' (filled dots) and "multi-template'' (open dots).|
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In a second technique, that we will call "closest filter'', we use directly the correlations described in Sect. 4 (Eqs. (11) and (12)) between the rest-frame 15m and 6.75 or 12m luminosities. We first calculate the rest-frame wavelength m and find which of the three filters ISOCAM-LW3, ISOCAM-LW2 and IRAS-12 has its central wavelength the closest to (see Fig. 7a). Then we apply the formula given in the Sect. 4.2.2 for this "closest filter'' to compute the rest-frame 15m luminosity (filled dots in Fig. 7b). Since most ISOCAM-HDFN galaxies are located around the median redshift 0.8, the correlation between LW3 and LW2 (Fig. 5d, Eq. (11) is most often used.
The third technique is the multi-template technique already used in Sect. 2.4, where we use the library of 105 template SEDs from Chary & Elbaz (2001). For a given galaxy of flux density S15 located at a given redshift z, we have computed for each one of the 105 template SED if it were located at the same redshift z. The template for which the computed was the closest to the observed S15 was then used to compute the rest-frame 15m luminosity, [15m] (open circles in Fig. 7b), after a normalization by a factor S15/ .
The rest-frame 15m luminosities estimated using the three techniques for the ISOCAM-HDFN galaxies are consistent within 20(1-, see Fig. 7b). Above , the first technique, which makes use of the SED of M 82, systematically underestimates [15m], as compared to the other two techniques. This was to be expected since M 82 is a moderately luminous IR galaxy with a lower FIR over 6.75m luminosity ratio than more luminous galaxies (see Fig. 5d).
In the following, we will use the multi-template technique to estimate the IR luminosity and SFR of galaxies, because it is consistent with the correlations between MIR and FIR luminosities and provides a continuous interpolation between the 6.75, 12, 15m and FIR luminosities based on observed spectra. Since the templates fit the correlation between and [15m], it is equivalent to compute [15m] first and then use Eq. (13) which links and [15m] or to estimate directly with the best template SED.
In this section, we use the multi-template technique to directly estimate the and SFR of the ISOCAM-HDFN galaxies. The results are shown in the Fig. 8 and Table 3. The 1- error bars in Fig. 8 were computed from the dispersion measured in the correlations of the Fig. 5. For example, for a galaxy located at 1, since is close to 7m, the error bar on is the quadratic sum of the error bar in Eq. (11), which gives L15 from L7, and of the error bar in Eq. (13), which gives from L15.
The dashed line in Fig. 8 corresponds to the sensitivity
S15= 0.1 mJy converted into a
as in the
Fig. 2. Table 3 summarizes the properties
of the ISOCAM galaxies in each of these four galaxy categories plus
the AGN category.
|Figure 8: [8-1000m] and star formation rate (SFR) versus redshift and for the ISOCAM-HDFN galaxies ( 0.1 mJy). The 15m completeness limit of 0.1 mJy is materialized with a dashed line. Only the left axis is meaningful for the five AGNs (filled dots, Sect. 5.2). HDF_PS3_6a (no spectroscopic redshift) is not included.|
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About 75 of the galaxies dominated by star formation are either LIGs or ULIGs, while the remaining 25 are nearly equally distributed among either "starbursts'' ( ) or "normal'' galaxies ( 10 ). The contribution of each galaxy type to the 15m IGL (IGL15) is given in the last column of the Table 3. Luminous IR galaxies are responsible for about 60 of IGL15 while AGNs contribute to about 20, the remaining 20 being due to the normal and starburst galaxy types. This repartition will be used in Sect. 6 to compute the 140m IGL (IGL140) from ISOCAM galaxies.
The mean IR luminosity of the ISOCAM-HDFN galaxies is 61011 . It corresponds to a 100 yr-1 (from Eq. (1)). In the following section, we evaluate the robustness of this result with an independent estimator of the IR luminosity: the radio luminosity.
In Fig. 9a, we have plotted the 1.4 GHz and 15m luminosities of the 109 local galaxies (small filled dots) from the ISOCAM sample described in Sect. 4.2.1, which were detected in the NRAO VLA Sky Survey (NVSS, Condon et al. 1998). As expected both luminosities are correlated with each other since both the 1.4 GHz and 15m luminosities are correlated with .
Half of the 35 star forming HDFN-ISOCAM galaxies with spectroscopic redshift (AGNs excluded) present flux densities larger than 40Jy at 1.4 GHz (5-) in the VLA and WSRT catalogs from Richards (2000) and Garrett et al. (2000) respectively. Their rest-frame 15m luminosities were computed in the Sect. 5.3. Their rest-frame 1.4 GHz luminosities were computed assuming a power-law as in Sect. 2.4: , where as suggested for star forming galaxies in Yun et al. (2001). The rest-frame 15m and 1.4 GHz luminosities of these 17 HDFN-ISOCAM galaxies are plotted as filled dots with error bars in the Fig. 9a. The error bars on [1.4 GHz] were computed from the quadratic sum of the error bars on the measurement of the radio flux densities plus the error bar on . Nine galaxies common in both catalogs from Richards (2000) and Garrett et al. (2000) present up to 40 differences in their radio flux densities. For these galaxies, we used the mean value and included the difference between both measurements in the error bars.
We have also included 7 galaxies from the CFRS-14 field (Flores et al. 1999), which rest-frame luminosities were computed with the same
techniques (open dots).
|Figure 9: a) 15m versus radio continuum (1.4 GHz) luminosities. Small filled dots: sample of 109 local galaxies from ISOCAM and NVSS. Filled dots with error bars: 17 HDFN galaxies in common between ISOCAM and VLA or WSRT (see text). Open dots with error bars: 7 CFRS-14 galaxies in common between ISOCAM and VLA (Flores et al. 1999). b) "q'' parameter as a function of redshift for the HDFN and CFRS-14 galaxies. Plain line: median value of "q'' (2.3 +0.3-0.5). Dashed line: measured value from local galaxies ( ; Yun et al. 2001). Dotted line: 1- error bar on "q'' for the 24 galaxies.|
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The MIR and radio luminosities of this sample of 24 distant dusty galaxies ( ) are also strongly correlated with each other. This is the first time that such correlation is found at these redshifts. The plain line in the Fig. 9a is a power-law fit to this correlation. For comparison, we have plotted in dashed line the correlation that one would expect from the combination of Eq. (13) ( from L15) and the radio "q'' parameter (see Sect. 2.4). The slope of the correlation observed for distant galaxies is marginally steeper than this dashed line. It is also slightly steeper than the correlation found for local galaxies (small filled dots), but the difference is too marginal with respect to the number of galaxies at high luminosities for detailed interpretation.
In Fig. 9b, the "q'' parameter for the 24 ISOCAM-HDFN and CFRS-14 galaxies is plotted as a function of redshift. The "q'' parameter is the logarithm of the ratio of over L(1.4 GHz) (see Eq. (2)). was estimated as described in Sect. 5.4, then converted into from Eq. (4). The median value of the "q'' parameter for the 24 galaxies is: q= 2.3+0.3-0.5 (plain and dotted lines in Fig. 9b), in perfect agreement with the local value of (Yun et al. 2001, dashed line in Fig. 9b).
This study shows that the IR luminosities estimated from the ISOCAM
15m flux densities using the MIR-FIR correlations are perfectly
consistent with those estimated from the radio. Although it is not
clear whether the radio versus IR correlation also applyies up to
this result independently validates our estimate of the
bolometric IR luminosity of the ISOCAM galaxies.
|Figure 10: a) FIR (filled circles), MIR-15m (open circles) and UV-2800 Å luminosity density (in Mpc-3) as a function of redshift. UV-2800 Å: open squares from Cowie et al. (1999); filled squares from Lilly et al. (1996). b) Integrated Galaxy Light (IGL, filled dots) and Extragalactic Background Light (EBL, open squares, grey area) from the UV to sub-millimeter (see Table 5 for the origin of the data). EBL measurements from COBE: 200-1500m EBL from COBE-FIRAS (grey area, Lagache et al. 1999), 1.25, 2.2, 3.5, 100, 140m EBL from COBE-DIRBE (open squares, see Table. 5). IGL in the U, B, V, I, J, H, K bands from Madau & Pozzetti (2000). The upper end of the arrows indicate the revised values suggested by Bernstein et al. (2001, factor two higher). Our estimate of the 15m IGL (2.4 0.5 nW m-2 sr-1) is marked with a surrounded star. 6.75m (ISOCAM-LW2 filter) IGL from Altieri et al. (1999, filled dot). Hatched upper limit from Mrk 501 (Stanev & Franceschini 1998). The arrow from 15 to 140m indicates our computation of the 140m IGL due to ISOCAM galaxies.|
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We have seen that the excess of faint MIR sources in number counts was due to the presence of distant ( 1) luminous IR galaxies. As a consequence, the amount of star formation per comoving volume hidden by dust must have rapidly decreased from to 0. Indeed, the ISOCAM-HDFN galaxies with 0.6 1.3 and 10 (LIGs and ULIGs; AGNs excluded) produce a 15m luminosity density of Mpc-3, while in the local universe luminous IR galaxies only make Mpc-3 (see Table 4). The comoving luminosity density produced by luminous IR galaxies at 15m was therefore about 55 times larger at 1 than in the local universe. The 15m luminosity densities at 0 were calculated using the local luminosity function (LLF) from Xu et al. (1998). In order to estimate the contribution of luminous IR galaxies, we used the Eq. (13) to convert the IR luminosity cut-off of into a 15m luminosity cut-off of . The 15m luminosity density that we have computed is consistent with the one measured at 12m from IRAS, Mpc-3, converted to 15m using Eq. (12).
If we now consider the bolometric IR luminosity (from 8 to 1000m) of ISOCAM galaxies, as estimated in the Sect. 5, we find that the comoving density of IR luminosity radiated by dusty starbursts was about () times larger at than today (computed from the LLF of Soifer et al. 1987). Since the IR luminosity is directly proportional to the extincted star formation rate of a galaxy, this means that the comoving density of star formation taking place in luminous IR galaxies was about (70 35) times larger at 1 than today. In case of a pure density evolution proportional to (1+z)n, this would translate into a value of 6. However, we want to emphasize that we are only considering here the galaxies detected by ISOCAM, not the full luminosity function. For comparison, the B-band (0.44m) luminosity density was only about three times larger at than today.
The redshift evolution of the comoving density of IR luminosity is compared to the UV (2800 Å) one in Fig. 10a. Both wavelengths exhibit similar luminosity densities at low redshift but the IR rises faster than the UV and reaches a larger value at 1, implying that a much larger fraction of star formation was hidden by dust at 1 than today.
Finally, we note that the projected density of galaxies detected in the B-band in the HDFN (529 galaxies/ with 29, Pozzetti et al. 1998) is 330 times greater than the projected density of ISOCAM galaxies (1.6 sources/ , with 0.1 mJy or AB(15m)18.9). However the ISOCAM galaxies produce a 15m IGL which is only twice lower than the B-band IGL ( nW m-2 sr-1, Madau & Pozzetti 2000). This confirms that the very luminous galaxies detected in the MIR radiate mostly in the IR.
Below m, the cosmic background originates from direct stellar light, while above this wavelength, it comes from either stellar or AGN light reprocessed by dust. In the dust domain, i.e. from 5 to 1500m, the cosmic background peaks around m. It is not directly measured over this whole wavelength range, but below m and down to about 2m, an upper limit is set by the observations of the TeV outburst of Mrk 501 (see Sect. 3).
The 15m IGL (IGL15 = ( ) nW m-2 sr-1, see Sect. 3) is marked with a surrounded star in Fig. 10b. It is about ten times lower than the peak of the CIRB at m. At the median redshift of the ISOCAM galaxies, 0.8 (Sect. 5.1), the rest-frame wavelengths corresponding to the observed 12-18m (ISOCAM-LW3 bandpass) and 140m wavelengths, are 6.5-9.8m and 80m. These rest-frame wavelengths are close to the ISOCAM-LW2 (5-8.5m) and IRAS (60, 100m) bands. We have seen in the Fig. 5d that the luminosities of local galaxies in both bands correlate with each other. If this correlation remains valid up to 1, then it implies that we can compute the 140m IGL due to ISOCAM galaxies. There is no obvious reason why the SEDs of galaxies at 1 would exhibit very different IR SEDs. For example, the SED of the extremely red object HR 10 (1.44) is very similar to the one of Arp 220 normalized by a factor of 4 (Elbaz et al. 2002).
We propose here to use the multi-template technique, as in Sect. 5, to compute the contribution of ISOCAM galaxies to the 140m background. We will first separate the relative contributions to IGL15 from ULIGs, LIGs, SBs, normal galaxies and AGNs as estimated in the Sect. 5. For each galaxy type, a median redshift, , and the ratio / are estimated based on the HDFN sample (see Table 1). Although a larger sample would improve the statistical reliability of this computation, this choice is justified by the fact that the bulk of the 15m IGL is produced by galaxies with flux densities in the range 0.35-1 mJy as the HDFN and CFRS-14 galaxies, which share similar redshifts distributions. It is important to separate the contribution from galaxies of different luminosities since the IR versus 6.75m luminosity correlation presents a slope larger than one (Fig. 5d). The contribution from AGNs is computed assuming that they all share the SED of NGC 1068. This is a conservative hypothesis since NGC 1068 exhibits the lowest FIR over MIR ratio that we know.
The results are summarized in the Table 6. We first separate the contribution of each type of galaxy to the 15m IGL(Cols. 3, 4). The 140m IGL is then computed by converting each contribution to the 15m IGL into a contribution at 140m, by multiplying by each time. We find a total of () nW m-2 sr-1 (see Fig. 10b), which corresponds to () of the observed value from COBE-DIRBE ( ) nW m-2 sr-1, Hauser et al. 1998; Lagache et al. 2000; Finkbeiner et al. 2000).
Hence the galaxies detected in ISOCAM deep surveys are found to be
responsible for the bulk of the CIRB. About half of this 140m
IGL is due to LIGs and about one third from ULIGs. The contribution of
AGNs is estimated to be as low as 4
of the 140m
IGL. It could be as high as 20
if instead of NGC 1068, we
had used the SED of NGC 6240. In this case, ISOCAM galaxies would
() nW m-2 sr-1. If we
had included the contribution from galaxies with flux densities
between 30 and 50Jy only detected through lensing magnification
(see Sect. 3), IGL140 would be larger by an extra
4 nW m-2 sr-1.
At the mean redshift of 0.6 where the bulk of the B-band IGL is produced (Pozzetti & Madau 2001), light was radiated in the UV in order to be observed at 0.44m. As we have seen, the bulk of the 140m is also produced in the same redshift range. Hence the 140m and B-band probe the UV emission from stars with and without dust extinction. The ratio of IGL140 over IGLB should therefore give a rough approximation of the ratio of extincted over non extincted star formation around . This ratio is close to 5, which suggests again that the bulk of the UV photons radiated by young stars in this redshift range was strongly affected by dust extinction. This value is consistent with the one obtained by Chary & Elbaz (2001) or Franceschini et al. (2001).
We have computed the contribution of ISOCAM galaxies to the 15m background, the 15m integrated galaxy light, and found a value of ( ) nW m-2 sr-1. This is about ten times below the cosmic background measured by COBE at 140m: () nW m-2 sr-1.
We have demonstrated that the MIR luminosities at 6.75, 12 and
15m were correlated with each other and with the bolometric IR
luminosity for local galaxies. This suggests that the contribution of
ISOCAM galaxies to the CIRB can be computed from IGL15, unless
distant galaxies SEDs strongly differ from local ones.
The redshift distribution of ISOCAM galaxies was measured from the spectroscopically complete sample of galaxies in the region of the HDFN. This redshift distribution is consistent with the twice larger sample of ISOCAM galaxies detected in the CFRS fields CFRS-14 and CFRS-03 (Flores et al. 1999, 2002). At the median redshift of 0.8, the observed 15 and 140m wavelengths correspond to about 7m (ISOCAM-LW2 filter) and 80m (IRAS bands) in the rest-frame of the galaxies. Luminosities at both wavelengths are correlated (see Fig. 5d). If the correlations between MIR and FIR luminosities remain valid up to 1, then they can be used to compute IGL140. We have checked with a sample of galaxies detected both in the MIR with ISOCAM and in the radio with the VLA and WSRT, that the MIR-FIR and radio-FIR correlations are consistent up to 1. This comparison independently validates our estimate of the bolometric IR luminosity of the ISOCAM galaxies, although it is not clear whether the radio-FIR correlation works also up to 1.
The fraction of active nuclei responsible for the 15m luminosity of ISOCAM galaxies was estimated from the deepest soft and hard X-ray surveys available at present by the XMM-Newton and Chandra X-ray observatories in the Lockman Hole and HDFN respectively (Fadda et al. 2002). It was found that about () of the ISOCAM galaxies are powered by an AGN and that the AGN contribution to IGL15 was about (). The AGN contribution to IGL140 was found to be as low as 4 assuming that they all share the SED of the local Seyfert 2, NGC 1068. This is a conservative choice since NGC 1068 presents the flattest IR SED that we know. However, we note that the cosmic X-ray background (CXB) peaks around 30 keV (see Fig. 1 of Wilman et al. 2000), while both XMM-Newton and Chandra were limited to energies below 10 keV. It is therefore possible that a population of hard X-ray AGNs was missed by these surveys. But as claimed by the authors of these deep X-ray surveys, the bulk of the CXB had been resolved into individual galaxies in the Lockman Hole and HDFN images. Moreover, using estimates of the present comoving density of black holes, Madau & Pozzetti (2000) calculated that less than 20 of the CIRB could be due to dusty AGNs.
For the remaining star forming galaxies, we used a library of template SEDs, reproducing the MIR-FIR correlations, to compute their IR luminosity and contribution to IGL140. We find that LIGs and ULIGs produce about 60 of IGL15. The comoving density of IR luminosity produced by these luminous IR galaxies was about () times larger at 1 than today, while in the same redshift interval the B-band or UV luminosity densities only decreased by a factor 3. Since the IR luminosity measures the dusty star formation rate of a galaxy, this also implies that the comoving density of star formation, due to luminous IR galaxies, decreased by a similar factor in this redshift range, i.e. much more than expected by studies at other wavelengths affected by dust extinction. This indicates that a large fraction of present day stars were formed during a dusty starburst event.
We estimate a contribution of ISOCAM galaxies to the peak of the CIRB at 140m of () nW m-2 sr-1 as compared to the measured value of () nW m-2 sr-1 from COBE. This study therefore suggests that the ISOCAM galaxies are responsible for the bulk of the CIRB.
We have started a systematic spectroscopic follow-up of these galaxies with the aim of studying their physical properties and the origin of their large SFR. Franceschini et al. (2001) estimated their baryonic masses to be of the order of by fitting their optical and near-IR luminosities with template SEDs (from Silva et al. 1998) and assuming a Salpeter initial mass function (from 0.15 to 100). Their colors are similar to field galaxies of similar magnitudes (Cohen 2001), hence they could not have been selected on the basis of their optical colors. The technique which consists in using the spectral slope in the UV domain to correct luminosities from extinction (Meurer et al. 1999) fails to differentiate the luminous dusty galaxies detected with ISOCAM from other field galaxies in the HDFN+FF (Cohen 2001). This was to be expected since this technique only works for galaxies with (Meurer et al. 2000) while most ISOCAM galaxies are more luminous than this threshold. A property of the ISOCAM galaxies that may give a hint on their origin is their association with small groups of galaxies. A preliminary study of 22 ISOCAM-HDFN galaxies by Cohen et al. (2000) found that nearly all ISOCAM galaxies belonged to small groups, while the fraction of field galaxies with similar optical magnitudes belonging to such groups was 68. The study of the full sample of ISOCAM-HDFN galaxies by Aussel et al. (2001) shows that all of them belong to physical groups, hence suggests that dynamical effects such as merging and tidal interactions are responsible for their large SFR.
The Space IR Telescope Facility (SIRTF) will soon provide a powerful insight on the FIR emission of distant dusty starbursts and its 24m band, less affected by confusion, should prolongate their detection up to . Extending the redshift range surveyed by ISOCAM to such redshifts is particularly important to measure the direct IR emission of the distant population of Lyman break galaxies, whose dust extinction remains highly uncertain (Steidel et al. 1999). However, the direct measurement of the MIR and FIR emission of distant galaxies will only become possible with the launch of the Next Generation Space Telescope (NGST) and Herschel (FIRST) satellite scheduled for 2009 and 2007.
DE wishes to thank the American Astronomical Society for its support through the Chretien International Research Grant and Joel Primack and David Koo for supporting his research through NASA grants NAG5-8218 and NAG5-3507. We wish to thank H. Flores and A. Blain for fruitful comments and A. Boselli for sharing his ISOCAM catalog with us. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.