A&A 475, 801-812 (2007)
DOI: 10.1051/0004-6361:20065217
B. Rocca-Volmerange1,2 - V. de Lapparent1 - N. Seymour1,3 - M. Fioc1
1 - Institut d'Astrophysique de Paris, UMR7095 CNRS, Université Pierre et Marie Curie - Paris 6, 98bis boulevard Arago, 75014 Paris, France
2 -
Université Paris Sud, Bât 121, 91405 Orsay Cedex, France
3 -
Spitzer Science Center, California Institute of Technology,
Mail Code 220-6, 1200 East California Boulevard, Pasadena, CA 91125, USA
Received 16 March 2006 / Accepted 20 June 2007
Abstract
Context. Multi-wavelength galaxy number counts provide clues to the nature of galaxy evolution. The interpretation per galaxy type of the mid-IR faint counts obtained with ISO and Spitzer, consistent with the analysis of deep UV-optical-near IR galaxy counts, provide new constraints on the dust and stellar emission. Discovering the nature of new populations, such as high redshift ultra-luminous (10
)
infrared galaxies (ULIRGs), is also crucial for understanding galaxy evolution at high redshifts.
Aims. We first present the faint galaxy counts at 12 m from the catalogue of the ISO-ESO-Sculptor Survey (ISO-ESS) published in a companion article (Seymour et al. 2007a, A&A, 475, 791). They go down to 0.31 mJy after corrections for incompleteness. We verify the consistency with the existing ISO number counts at 15
m. Then we analyse the 12
m (ISO-ESS) and the 24
m (Spitzer) faint counts, to constrain the nature of ULIRGs, the cosmic star formation history and time scales for mass buildup.
Methods. We show that the "normal'' scenarios in our evolutionary code PÉGASE, which had previously fitted the deep UV-optical-near IR counts, are unsuccessful at 12 m and 24
m. We thus propose a new ULIRG scenario adjusted to the observed cumulative and differential 12
m and 24
m counts and based on observed 12
m and 25
m IRAS luminosity functions and evolutionary optical/mid-IR colours from PÉGASE.
Results. We succeed in simultaneously modelling the typical excess observed at 12 m, 15
m (ISO), and 24
m (Spitzer) in the cumulative and differential counts by only changing 9% of normal galaxies (1/3 of the ellipticals) into ultra-bright dusty galaxies evolving as ellipticals, and interpreted as distant ULIRGs. These objects present similarities with the population of radio-galaxy hosts at high redshift. No number density evolution is included in our models even if minor starbursts due to galaxy interactions remain compatible with our results.
Conclusions. Higher spectral and spatial resolution in the mid-IR, together with submillimeter observations using the future Herschel observatory, will be useful to confirm these results.
Key words: infrared: galaxies - galaxies: evolution - galaxies: photometry - cosmology: observations
The mid-infrared extra-galactic source counts provide clues on the evolution of galaxies at high z and allow us to follow the cosmic star formation history up to when the Universe was only one quarter of its present age. The high sensitivity of the infrared deep surveys observed with ESA's Infrared Space Observatory ISO (Kessler et al. 2003), and more recently with Spitzer Space Telescope (Werner et al. 2004), offers an unique opportunity to study the obscured star formation process through the emission of grains heated by young stars and possibly by an active nucleus, thus reaching galaxies at their earliest epochs. Interactions of galaxies are known to explain the huge IR emission detected in local Ultra Luminous Infra Red Galaxies (ULIRG; Soifer et al. 1984). One crucial issue is whether ULIRGs provide observational evidence that galaxy interactions play a fundamental role in galaxy evolution. Moreover, because the infrared luminosity depends on the dust mass accumulated from stellar ejecta, it provides complementary diagnostics of past star formation that may in turn be used to model the star formation history.
Over the years, a long series of deep surveys from the UV to the optical, down to the extreme depth of the Hubble Deep Field North (HDF-N) at B=29 (Williams et al. 1996), have constrained the direct stellar emission in terms of cosmology and scenarios of star formation. Fioc & Rocca-Volmerange (1999a) derived a set of galaxy populations fitting the multi-wavelength (UV-optical-near infrared) deep galaxy counts dominated by stellar emission. This set defines the evolution scenarios of eight galaxy types and their number fractions. In the mid-infrared, galaxy light is dominated by the emission from dust grains in the form of graphite, silicates, or polycyclic aromatic hydrocarbons (PAH; Puget & Léger 1989). Because different time scales characterise the evolution of star and dust emissions, the results of optical and mid-infrared source counts may differ significantly.
Two difficulties however hamper the interpretation of data. One is
the lack of homogeneity between the various surveys: due to
large-scale galaxy clustering, the statistical properties derived
from deep pencil-beam surveys suffer from "cosmic variance''
(Somerville et al. 2004) and might thus differ from the analyses on
large area surveys. Another difficulty is the variety of sources
(starbursts due to mergers, normal evolved galaxies, AGNs) and their
intrinsic evolution. As an example, the time scale of star
formation associated to galaxy interactions is significantly shorter
(108 yrs) than the star formation time scales of
galaxy
populations (>109 yrs, depending on spectral type) observed in
deep surveys. We consider the contribution of AGNs to be minimal at
the flux density range explored here, but the impact of an embedded
hidden AGN is discussed below.
Our large area ISOCAM ESO-Sculptor Survey (ISO-ESS) at
12 m, published in the companion paper (Seymour et al. 2007a),
here tackles these various limitations by covering a significant area
and by using the new code PÉGASE.3 (Fioc et al. 2007) which
coherently predicts the evolving stellar and grain emissions from
evolved galaxies as well as young starbursts. It is able to predict
both starlight and dust emission from the UV to the
far infrared, by taking the transfer and the
reprocessing of light in the different wavelength domains
into account. A large variety of star formation time scales is considered in our
evolutionary templates, but no evolution of the number density of
galaxies is included in the model.
Several other major surveys in the mid-infrared have been performed
with the ISOCAM camera, and they also provide deep galaxy counts.
The largest survey is ELAIS (Rowan-Robinson et al. 1999, 2004),
which covers 12 sq. deg. in the flux range 0.45-150 mJy at
wavelengths of 6.7 m (LW2) and 15
m (LW3); the
corresponding galaxy counts were published by Serjeant et al.
(2000). Following the preliminary surveys of Taniguchi et al.
(1997) and Oliver et al. (1997), several major surveys have also
been performed that provide deep galaxy counts. Among them are the
ISO 15
m observations of the Lockman Deep Field, the
Marano-ROSAT Ultra Deep Field (Aussel et al. 1999; Elbaz et al.
1999) and the Lockman Shallow Field (Flores et al. 1999). Even
deeper surveys have been obtained in the LW2 and LW3 filters,
centered on Abell cluster 2390 (Altieri et al. 1999), the HDF field,
and other fields covering areas of 2.5 arcmin in radius (Oliver et al. 2000, 2002), and of 16 arcmin2 (Sato et al. 2003). These
various surveys yield galaxy number counts in reasonable agreement
and detect an excess in the number of galaxies at faint fluxes
(below
1 mJy), which is often interpreted as an increase in
the star formation rate with look-back time. For example, Pozzi et al. (2004) fit the 15
m number counts by introducing a
population of evolving starbursts, based on the local star-forming
prototype M 82 (Silva et al. 1998), which undergoes very strong
luminosity or density evolution parameterized as
,
with
3-4.
All the differential 24 m counts also show a systematic
departure from an Euclidean Universe at fluxes fainter than
0.5 mJy (Rodighiero et al. 2006), with a peak at
0.3 mJy, similar to the 15
m and 12
m
observations. Below 60
Jy, the 24
m galaxy number counts
obtained by the Spitzer/MIPS deep surveys (Marleau et al. 2004;
Papovich et al. 2004; Chary et al. 2005) face the problem of a
confusion limit due to extragalactic sources. Several interpretations
propose huge evolution factors in luminosity and/or density. In the
Chandra Deep Field South survey, the sample of 2600 Spitzer/MIPS sources brighter than 80
Jy is interpreted with
the comoving IR energy varying as
(Le Floc'h et al. 2005). The origin of such a high evolution is however not
described. More recently, Caputi et al. (2006) analysed the stellar
populations of the Spitzer/MIPS 24
m galaxies in the
GOODS/CDFS from
images. They show evidence for a bump in the
redshift distribution at z=1.9, induced by a significant population
of galaxies with PAH emission.
The combined ISO-ESS survey presented here has a comparable depth
(80% completeness at
0.7 mJy) and surface area (680 arcmin2) at 12
m to the intermediate surveys (Lockman Deep
and Marano Deep fields) performed at
m as part of the
ISOCAM Guaranteed Time Extragalactic Surveys. The specificity of
the ISO-ESS survey is to provide deep galaxy counts at
12
m (LW10 filter), in a wavelength range where the
spectral energy distribution (SED) is dominated by the signatures of
PAH emission; it thus strongly constrains the evolution process.
The target field is located within the ESO-Sculptor Survey (de Lapparent et al. 2003, 2004), for which deep optical
magnitudes up to
and
spectroscopic redshifts at
have been obtained
(Arnouts et al. 1997; Bellanger et al. 1995).
We then model the 12 m number counts using the new version
PÉGASE.3 of the "Projet d'Étude des GAlaxies par Synthèse
Évolutive'' (Fioc et al. 2007; Fioc & Rocca-Volmerange 1997, 1999b;
see also www2.iap.fr/pegase), which coherently complements the
UV-optical-NIR emission from stars and gas with the mid- and
far-infrared emission from dust. The specific goal of our analysis
is to predict the mid-infrared number counts using firstly the same
scenarios of galaxy evolution by type, with the same number
fractions and the same total number densities of galaxies as used
for the successful predictions of the UV-optical-NIR deep counts by
Fioc & Rocca-Volmerange (1999a). Secondly, if needed, other
scenarios are proposed to model the population of ULIRGs able to
reproduce the systematic departure observed around 0.3 mJy in
mid-infrared surveys.
Section 2 describes the parameters of the ISO-ESS field observed
with the large pass-band ISOCAM LW10/12 m filter and
summarises the data analysis required to extract a catalogue of
sources presented in the companion article (Seymour et al.
2007a). The corresponding cumulative and differential galaxy counts
at 12
m are presented in Sect. 3. We resume in Sect. 4 the
parameters of two models of evolution scenarios and the observed
IRAS luminosity function at 12
m tentatively used to model
galaxy counts. The respective fits of the cumulative and
differential counts of the ESO-ESS survey by the two models are
compared in Sect. 5. The best model 2 identifies a population of distant
ultra bright ellipticals interpreted as distant ULIRGs. Applied to
24
m number counts from the deep Spitzer/MIPS surveys, we
show in Sect. 6 that the same population of ultra bright ellipticals
also reproduces the 24
m differential count excess, making our model
a robust description. Section 7 presents the cosmic star formation
history resulting from the best fit, taking into account the
respective contributions per galaxy type. Section 8 discusses the
stellar and dust masses of the revealed ULIRG population and the
possibility of a hidden AGN. The final section presents our
conclusions.
The selected field, the ESO-Sculptor Survey (ESS) of faint galaxies,
is described in its complete version by de Lapparent et al.
(2003). Located near the southern Galactic pole, the observations
for the ESS were performed as an ESO key-program, thanks to
guaranteed time on the ESO NTT and 3.6 m telescope. Deep CCD Johnson B, V and Cousins magnitudes for nearly 13 000 galaxies to
were obtained over a continuous area of
(Arnouts et al. 1997). Multi-object spectroscopy
has also provided redshifts and flux-calibrated spectra over a
sub-area of
for 617 galaxies with
(
complete) and 870 galaxies with
(
complete). The optical star/galaxy separation was
performed using the "stellarity'' index from the SE XTRACTOR
software (Bertin & Arnouts 1996). The optical spectra were
classified using a principal component analysis (Galaz & de Lapparent 1998). The optical luminosity functions were then measured
per galaxy spectral type (de Lapparent et al. 2003) and lead to the
detection of a marked evolution in the spiral galaxy populations,
characterised by an excess of late spiral and
irregular galaxies at
(de Lapparent et al. 2004).
The ISO observations of the ESS field were performed with the
raster mode CAM01 of the ISOCAM camera (Cesarsky et al. 1996) on
board the Infrared Space Observatory ISO (Kessler et al. 2003). The broad-band LW10 filter, with reference wavelength
m and covering the 8.5-15.5
m
interval, was built for a direct comparison with the 12
m
filter (Moneti et al. 1997). The pixel field of view (PFOV)
with ISOCAM is a 6 arcsec square and the adopted integration time
was 5.04 s. per exposure. Ten raster maps were built within the
total on-target time of 14 h. All the rasters have
pointings each offset by
arcsec along the
axis of the detector. The number of exposures per pointing is
and the number of stabilization exposures is
.
The total area of the survey is
680 arcmin2 with a maximum exposure time of 1200 s. and an average exposure time of
660 s per sky pointing.
The ISO target field has been selected in the region of the ESO-Sculptor survey where the cirrus emission is minimal (Seymour et al. 2007a). The overlap between the 2 surveys is large, as it represents 90% of the ISOCAM survey area and 75% of the ESO-Sculptor spectroscopic area.
This section is a summary of the data analysis presented in the
companion article (Seymour et al. 2007a). The various steps of data
processing are (1) deglitching (i.e. cosmic ray subtraction), (2)
correction of the long transient behaviour etc. performed by the
PRETI software of Stark et al. (1999), and (3) source detection above
a noise map, performed using a wavelet analysis technique. The
detailed adaptation of the PRETI method to the ISOCAM data analysis
was published by Aussel et al. (1999). We fine-tuned the astrometry of
the 12 m sources by comparison with matched objects in the 2MASS
catalogue, which led to a maximum change of 0.3 arcsec in the position
of the 12
m sources. The subsequent rms offset between the
ISOCAM 12
m and the ESS coordinates is 2 arcsec, with no
systematic offset. The star/galaxy separation was performed by a
detailed analysis of the near-infrared (NIR) and optical colour-colour
diagrams. We then derived an empirical flux calibration from a subset
of stars in our sample. This calibration was based on fitting the
optical-NIR counterparts of the stellar data with stellar spectrum
templates compiled in the PÉGASE library and then using the
optical/infrared relations from IRAS data to predict the true infrared
fluxes of stars in our field. This procedure took advantage of the
deep photometric ESS survey in the optical.
Our flux calibration is robust as it was carried out using two
different colour-colour relations and empirical calibration using
well-known stellar templates. The final catalogue of 142 sources
with ISO fluxes is listed in the companion article and contains 22 stars and 120 galaxies. As expected, only a small fraction of the MIR sources are stars.
With an average exposure time of 11 min by pointing, our
ISO-ESS survey is as deep as the Lockman Hole Deep survey from the
ISOCAM Guaranteed Time Extragalactic Survey (Elbaz et al. 1999),
but it is over a 33% larger area of 680 arcmin2.
Despite its significant area, the ISO-ESS map reveals the
inhomogeneity of the projected distribution of MIR sources. The
large-scale distribution of galaxies in the ESS shows the
alternation of sharp walls and voids (Bellanger & de Lapparent
1995). For H0 = 65 km s-1 Mpc-1, de Lapparent & Slezak
(2007) measure a comoving correlation length of
5.5 Mpc at the
median redshift
of the ISO-ESS galaxies; the right
ascension transverse extent of the survey (
11 Mpc) thus
corresponds to approximately twice the galaxy correlation length.
Because the MIR sources are likely to follow the clustering of
optical galaxies, large-scale fluctuations are naturally expected.
These are smoothed out, however, when calculating the ISO-ESS
galaxy counts summed over the whole redshift range of the survey.
The ISO-ESS sample consists of sources with a detection level
equivalent to 5
and is complete to 1.29 mJy. At fainter flux
densities, the correction for incompleteness in the interval
0.31-1.29 mJy is computed by two independent methods based on stars
and galaxies in the optical. Both approaches take advantage of the
deep ESS optical survey, which contains counterparts to all sources
detected by ISOCAM down to 0.31 mJy (in the common area to both
surveys), and they yield incompleteness corrections that are in good
agreement (see Fig. 7 in companion paper). Here, we correct the
galaxy number counts using the incompleteness correction derived
from the stars, as it is least affected by the large-scale
clustering in the ESS sample (see Sect. 2.3). Figure 1 shows the distribution by type of the 77 ISO-ESS galaxies that have measured redshifts from the optical survey (de Lapparent et al. 2003). It shows that spirals Sb and Sbc
are the most numerous galaxies detected in the MIR. It also confirms
that gas-poor normal ellipticals are essentially undetected. The ESS
spectral classification used the code PÉGASE.2 (Fioc &
Rocca-Volmerange 1997). However, the sample in
Fig. 1 is incomplete, with only 77 galaxies
having a measured redshift. Among the 104 ESS galaxies detected at
12
m, the 27 objects with no redshift measurement could be at
higher redshift or belong to a new type. We therefore adopt the
"optical'' density fractions derived from the faint count analysis
in the UV-optical-near IR (
27%, 30%, and 43% for early-,
intermediate-, and late-type galaxies, respectively) by Fioc et al.
(1999a), in agreement with de Lapparent et al. (2003) in the
Sculptor field.
![]() |
Figure 1:
The distribution by types of the (R< 20.5)
ISO-ESS galaxies, derived from the optical spectral classification
(de Lapparent et al. 2003). The dominant population is Sb-Sbc spirals,
confirming that ellipticals are rarely detected at 12 ![]() ![]() |
Open with DEXTER |
We then derive the number of detected ISO-ESS galaxies as a function
of 12 m flux density. The detections are binned in 11 flux-density intervals, chosen so that each bin contains 10-12 more sources than the previous one. The resulting cumulative source counts
(commonly referred to "integrated counts'' in the literature), where
is the 12
m flux in mJy, derived after correction for
incompleteness, are shown in Fig. 2. Corrections
are estimated by assuming Poisson fluctuations in the number of detected
galaxies and by taking the uncertainty in the
incompleteness correction into account; the horizontal error bars are not plotted
as they are smaller than the size of the symbols. The large error
bars in the high flux point at
mJy result from the
small number of galaxies (10) detected at these high flux densities:
this is evidently due to the limited area of the ISO-ESS field.
We overlay in Fig. 2 the results from the
compilation of ISOCAM 15 m surveys obtained in the LW3 filter
as published by Elbaz et al. (1999): they cover the
Lockman Hole, Marano and HDF North and South fields (Rodighiero et al. 2004; Aussel et al. 1999; Altieri et al. 1999) at various
depths. We also plot for comparison the results from the ELAIS
survey, also observed at 15
m (La Franca et al. 2004).
When plotting the 15
m number counts, the flux density
is converted into the LW10 12
m band using the respective
central wavelengths of the two filters and assuming a flat spectrum
in flux (that is, a constant product of the frequency by the flux
density); the correction is small, a factor 12/15=0.8. We do not
correct the galaxy flux densities from 15
m to 12
m because the filters
are wide and very close to each other in wavelength (they actually
overlap slightly).
![]() |
Figure 2:
Cumulative 12 ![]() ![]() ![]() ![]() |
Open with DEXTER |
Figure 2 shows that our data are in good agreement with the surveys compiled by Elbaz et al. (1999), including the deepest samples. In particular, the faint-end slope of the ISO-ESS cumulative counts is similar to that of the ultra-deep survey obtained on the cluster-lens A2390 by Altieri et al. (1999) in their common flux density interval, despite the narrow pencil-beam geometry of the latter survey. The ISO-ESS counts are also consistent with the brighter galaxy survey from La Franca et al. (2004).
![]() |
Figure 3:
The ISO-ESS 12 ![]() ![]() ![]() ![]() ![]() |
Open with DEXTER |
The differential galaxy counts
of the
ISO-ESS survey at 12
m, normalised to those for an Euclidean
Universe are presented in Fig. 3. A wider
binning than in Fig. 2 is adopted, with 23-25 galaxies per bin. The error bars take into account Poisson error in
the number of actually detected sources, combined with the
uncertainties in the incompleteness correction and in the flux density
of each detected source; as for the cumulative counts, the flux error
bars are not plotted as they are smaller than the size of the symbols.
The Euclidean normalisation is adopted so that for a static universe
with a non-evolving population of objects and a constant luminosity
function, the Euclidean normalised galaxy counts would follow a
horizontal line.
The differential number counts from the surveys at 15 m (with
flux density converted into the 12
m band, see previous section)
are also plotted in Fig. 3 (ELAIS-S1, Pozzi et al. 2003; A2390, Altieri et al. 1999; HDF North, Aussel et al. 1999;
HDF South, Marano FIRBACK Ultra Deep field, Marano Ultra Deep ROSAT
field, Marano Deep field, Elbaz et al. 1999; Lockman Deep and Lockman
Shallow field, Rodighiero et al. 2004). As for cumulative counts, the
15
m counts are corrected to 12
m (see Sect. 3.1).
Note that our faintest bin (at
0.3 mJy) is very uncertain due to
incompleteness, which may explain the downward shift for this last
point. For clarity we do not include the 12
m galaxy counts
from Clements et al. (1999), which show a large scatter, probably due
to poor statistics (3-5 objects per bin). They may also suffer from
contamination by a few stars at the bright end: the authors admit that
there is at least one star in their galaxy counts (Clements et al. 2001).
In Fig. 3, the ISO-ESS counts show a
strong departure from Euclidean no-evolution models at faint fluxes
(<1 mJy), with a very steep super-Euclidean slope. The same
behaviour is observed in all the other surveys plotted in the figure,
showing agreement among the 15 m surveys and with our
12
m survey. The low value of our last point at faint fluxes
(
0.3 mJy) is due to the large uncertainty in the
incompleteness correction. In the following, we address the issue
of the origin of the excess counts by modeling the faint galaxy
counts with the evolutionary code PÉGASE.
In the following we adopt a flat Universe with the standard cosmological parameters: H0=72 km s-1 Mpc-1,
,
(Spergel et al. 2003).
![]() |
Figure 4: Example of the strong evolution in the SEDs predicted by PÉGASE.3 for a star forming galaxy spiral Sc at various ages (increasing upwards), in the wavelength interval 1000 Å to 1 cm. (See more details in Fioc et al. 2007.) |
Open with DEXTER |
The new evolutionary code PÉGASE.3 (Fioc et al. 2007) is an
extension to the dust emission wavelength range of the code PÉGASE.2
(Fioc & Rocca-Volmerange 1997, 1999b, see also
http://www2.iap.fr/pegase). The SEDs of reddened galaxies are
consistently computed from the far-UV/optical/near-IR to mid-IR and
FIR (far-infrared) domains. PÉGASE.3 calculates, in a coherent
manner, the stellar emission, extinction, metal-enrichment, dust mass,
and the emission of grains statistically heated by the radiation
field. Two distinct dust media (interstellar medium and HII regions)
are considered. As in PÉGASE.2, radiation transfer is computed in
the two geometries (slab and spheroid) appropriate for disk galaxies
and ellipticals. Temperature fluctuations of the polycyclic aromatic
hydrocarbons (PAH), as well as graphite and silicate grain properties,
are derived with the method of Guhathakurta & Draine (1989). For
illustration, Fig. 4 shows the strong evolution of
the spectral energy distribution (SED) of the spiral Sc template
(especially in the MIR between 10 m and 20
m) at various ages.
Table 1:
Characteristic luminosity L* in units of
at
12
m (Col. 3) and 24
m (Col. 4) of the z=0 luminosity
functions used here, as a function of galaxy type (Col. 1). Column 2 gives the value of the parameter p2, the rate defined by
.
Galactic winds occur at an age of 3 Gyr in ellipticals of type E and ULIRG, and at 1 Gyr in ellipticals
of type E2; there are no galactic winds in spirals. Infall time scale
of ULIRGs is 100 Myr as for ellipticals of type E or E2; it regularly
increases for spirals from 2.8 Gyr (Sa) to 8.0 Gyr (Im) (see text for
details and references). The L*(12
m) for the normal types
(other than ULIRG) are derived from the
L*(BJ) of the observed
optical luminosity functions from Heyl et al. (1997) and the
m and
m colours, in the AB magnitude
system, are computed at
with PÉGASE.3 (Cols. 5 and 6).
The value of
m) assigned to Sbc galaxies, the brightest
emitters in this band, is that of the observed luminosity function
measured with IRAS (Rush et al. 1993; Shupe et al. 1998); the same
offset as for Sbc is then applied to all types. The last two columns
show the adopted number density fraction by type: model 1 (normal
galaxies only, Col. 7) uses the type distribution derived from the
UV-optical-near IR faint counts (Fioc et al. 1999a) while model 2
(normal galaxies + ULIRGs, Col. 8) is built by replacing 1/3 of
normal dust-poor ellipticals (9% of all galaxies) with ultra luminous
elliptical galaxies (called ULIRGs). These last galaxies evolve as
dusty ellipticals, they are
2.5 to
5 mag brighter
than normal ellipticals (depending on wavelength), and are thus as
luminous as spirals Sbc in the mid-IR at z=0. The number densities for the other
galaxy types are identical in both models.
In a first step, we adopt the same set of evolutionary scenarios of
"normal'' galaxies previously determined with the code PÉGASE.2
(Fioc & Rocca-Volmerange 1997) which fit the colours of nearby
galaxies by type and the deepest multi-wavelength (BJ, U (and
F300W), I, and K) faint galaxy counts in the UV-optical-NIR ranges
(Fioc & Rocca-Volmerange 1999a). This set corresponds to the 8 following types: irregular magellanic Im; spirals Sd, Sc, Sbc, Sb, Sa;
and ellipticals E2 and E). We use the same parameter set with the new
version PÉGASE.3 (Fig. 4) and compute the
continous SEDs of each type, extended to the mid- and far-IR, taking
stellar and dust emissions into account as well as coherent
absorption. The initial mass function is from Rana & Basu (1992) for
each type. These templates are also able to predict photometric
redshifts up to z=4 with an accuracy
(Le Borgne & Rocca-Volmerange 2002). Therefore the evolution scenarios
are considered as robust. The main parameters (star formation law,
initial mass function, galactic winds and astration rate) are listed
for each type in Fioc & Rocca-Volmerange (1997) and Le Borgne &
Rocca-Volmerange (2002). Star formation rates are proportional to the
current gas mass density, a highly conservative assumption. The
astration parameter p2-1 varies with galaxy type. The current
gas content
is ruled by star formation, stellar
ejecta, galactic winds, and infall rates as described in Fioc &
Rocca-Volmerange (1997); the adopted values by type are recalled in Table 1.
In Table 1, we present the z=0 characteristic luminosities
and
adopted for the various galaxy types. We compute
)
and
)
from the L*(BJ) values of the optical
luminosity functions by types (Heyl et al. 1997), used to fit the
faint optical counts (see Table 1 of Fioc & Rocca-Volmerange 1999a),
and the colours
m and
m from PÉGASE.3 at
z=0 for each galaxy type (Fioc et al. 2007). The filter 12
m
means ISO/LW10 and 24
m means Spitzer/MIPS 24
m;
BJ is the blue Kodack IIIa-J plus GG395 corrected filter (Couch &
Newell 1980). The filter corrections from
m to
ISO/LW10 and from
m to Spitzer/24
m are taken
into account by the code. Among normal galaxies at
,
spirals Sbc are the brightest emitters at 12
m and 24
m
and also the most numerous (see Fig. 1). We
therefore assign to type Sbc the characteristic luminosities
and
derived from the observed
luminosity functions measured by Rush et al. (1993) and
by Shupe et al. (1998), respectively. We then scale the MIR
luminosities of all the other types accordingly.
Because the evolutionary scenario of ellipticals (see Fig. 3a of
Rocca-Volmerange et al. 2004) may play a specific role in the
interpretation of observations of the ultra-luminous infrared galaxies
(ULIRGs), defined as galaxies with infra red luminosities
10
,
it deserves more attention. The intense
star formation rate (low p2 value) in the first Gyrs is fueled by
a high infall rate from the gas reservoir. The activity is so intense
that a huge dust mass is formed at early epochs from stellar ejecta,
specifically from massive supernovae SNII. In normal elliptical
galaxies, the star formation activity is supposed to be halted when
strong galactic winds produced by the bulk of supernovae expell all
the gas and dust content from the galaxy. Most of stars and dust are
already formed when the galaxy age is about 1 Gyr, corresponding to
in the adopted cosmology. "Normal'' ellipticals
contribute very little thereafter to the infrared emission, as they
are largely dust-free from this age to the present time (0
4). This scenario at z=0 matches the observation that the
cold grain component (
50 K) in elliptical galaxies has almost
no contribution to the MIR flux (Xilouris et al. 2004).
Column 7 of Table 1 gives the relative number fractions of galaxy types for model 1, as derived from the UV-optical-near IR. This model has no "ULIRG''component and is thus composed only of normal types (26.5% ellipticals (E + E2), 23.7% Sa to Sbc spirals, 33.2% Sc, Sd spirals and 16.6% irregulars), which were found to fit the deepest UV-optical-near IR faint galaxy counts (see Fig. 5 from Fioc & Rocca-Volmerange 1999a).
Column 8 of Table 1 describes our model 2 with "ULIRG'' which we
use to adjust the MIR galaxy counts. In this model, 1/3 of the
ellipticals have over-luminous MIR luminosities given by
and
,
which correspond to the
observed characteristic L* of the MIR luminosity functions at these
two wavelengths. At z=0 and in the MIR, they are as bright
as Sbc spirals, and brighter by 2.5 mag at 12
m and 5 mag at
24
m than normal ellipticals, whatever their type (E2, E).
However, they follow the evolution scenario of elliptical galaxies of
type E, with the same astration rate p2-1, infall and
galactic winds at the same age.
These overluminous ellipticals, forming large masses of dust and stars at early epochs, become much brighter at high z than spirals. Due to their positive k-corrections at high redshift, their intense stellar emission appears in the mid-IR. As a consequence, our model does not need any additional starburst as in the typical case of M 82 (Silva et al. 1998). Our model remains compatible with occasional starbursts of short duration (<108 yrs), concerning a low mass fraction relative to the massive underlying elliptical galaxy.
Column 8 of Table 1 lists the number fractions for model 2. The population of ULIRG/dusty massive ellipticals corresponds to 9% of the total number of galaxies. The number of normal dust-poor ellipticals is then reduced to only 17.5% (only 2/3 of the ellipticals observed in the visible). The rest of the galaxies are normal; model 2 therefore respects the majority of fractions by type derived from the UV-optical-near IR galaxy counts.
![]() |
Figure 5:
The predictions of 12 ![]() ![]() ![]() |
Open with DEXTER |
![]() |
Figure 6:
The predictions of Euclidean-normalised differential
12 ![]() ![]() ![]() |
Open with DEXTER |
To calculate the apparent magnitudes at high z, the evolutionary
e(z) and cosmological k(z) corrections are computed for each
type, as in Rocca-Volmerange & Guiderdoni (1988), and are
applied to the z=0 SEDs:
The same formation redshift of
is arbitrarily adopted
for all galaxy types, following the most distant
galaxies discovered at z>6 (Hu et al. 2002; Chary et al. 2005).
We take advantage of the quasi-similarity of the two filters
IRAS/12 m and ISOCAM/LW10/12
m (the flux
differences are <
,
see companion article) and use the local
12
m luminosity function measured by Rush et al. (1993) from the
catalogue (this measurement was later confirmed and extended to
fainter fluxes by Fang et al. 1998),
Faint galaxy counts are computed following the already published formalism (see
Sect. 2 in Guiderdoni & Rocca-Volmerange 1990) which assumes that
the number of galaxies of each type is conserved. Comparison of the
ISO-ESS number counts with the predictions of PÉGASE.3 at
12 m is shown in Fig. 5 for cumulative counts,
and in Fig. 6 for Euclidean-normalised differential
counts (solid line in both graphs); the number counts from the
12
m ISO-ESS, and from the published 15
m surveys are
also shown with the same line/symbol coding as in
Figs. 2 and 3.
Figure 5 shows that the model with ULIRGs
agrees well with the ISO-ESS cumulative counts. After colour
correction, it also fits the deep ISO counts at 15
m as well as
the ultra-deep survey down to 0.05 mJy in the cluster-lens A2390
(Altieri et al. 1999; Lemonon et al. 1998), implying that there is no
significant number density variation between the field and clusters.
The differential counts (Fig. 6), which are more
constraining, remain in reasonably good agreement with the data, in
particular at faint fluxes. The Euclidean-normalized differential
counts predicted with PÉGASE.3 do show the departure from the
Euclidean cosmology (horizontal line) observed at 0.3 mJy in all the
data samples. This bump is not due to the evolution of bright
spirals, nor to normal early-type galaxies, but only to the evolution
of the third of elliptical galaxies (9% of all galaxies) which are
dusty ultra-bright ellipticals. From a few 0.1 to
1 mJy, only
the slope of
the Marano Deep Field is respected by models, in excess relative to
other observations by a factor 2. The model prediction with only the
universe expansion effect (obtained by applying only the k(z) corrections to the SEDs) is also plotted in Figs. 5 and 6 as a dashed line: it is noticeably insufficient
to reproduce the marked excess counts and the peak at 0.3 mJy. We
also checked that the model without any correction (i.e. no
k(z) +
e(z) corrections applied to the SEDs), which only includes the
evolution of the comoving elemental volume, yields decreasing
differential counts which are incompatible with observations; this
curve is shown in Figs. 5 and 6 as
a dotted line.
Note that the comparison of the observed ISO-ESS counts with the
Euclidean case is more meaningful in the flux range where the number
density is the highest. When galaxy numbers are statistically too
small, error bars are large as shown at fluxes higher than 10 mJy and
0.1 mJy. Finally we can ask whether the normal spirals which
are ultra-luminous in the MIR (
and
(see Table 1) can also
reproduce the bump of MIR counts as well as models with ULIRGs. At
12
m, the normal populations, dominated in the MIR by spirals Sbc are quite unable to reproduce the excess counts of the MIR surveys
(Figs. 5 and 6, left), even by adding
9% to the
57% of normal spirals in place of gas-poor
elliptical galaxies.
The deepest Spitzer/MIPS 24 m surveys are in the area of
the Chandra Deep Field South (Papovich et al. 2004). The
corresponding galaxy counts (cumulative, shown in
Fig. 7, and differential normalized to the
Euclidean case, shown in Fig. 8) are
characterised by a typical bump of the galaxy density between 3 and
0.03 mJy, similar to what is observed in the 12
m and 15
m
counts. This evolution signature of the 24
m counts is confirmed
by Marleau et al. (2004) and Chary et al. (2004). More recently, in
the GOODS-ELAIS-N1 field, Rodighiero et al. (2006) have lowered the
confusion limit by about 30-50% using a deblending technique, which
leads to a decrease in the bright differential counts by a factor
3, and an increase in the count slope at faint fluxes. The
statistics of the 24
m observations are poor for flux densities
>10 mJy and the samples suffer from incompleteness for fluxes <
Jy.
To model the Spitzer/24 m counts, we use the
IRAS/25
m luminosity function (corrected for 24
m)
as measured by Shupe et al. (1998),
![]() |
Figure 7:
Cumulative 24 ![]() ![]() ![]() ![]() |
Open with DEXTER |
![]() |
Figure 8:
The differential 24 ![]() ![]() ![]() |
Open with DEXTER |
We modeled the faint galaxy counts through the
/24
m
filter with the code PÉGASE.3 and the same evolving galaxy
population (evolutionary scenarios, density fractions) as already used to
predict the 12
m counts. Figures 7
and 8 compare models 1 and 2
with the cumulative and differential number counts, respectively,
obtained by Papovich et al. (2004). As for the 12
m counts,
the 24
m cumulative counts are well-reproduced from the faintest flux
up to a few mJy.
Figure 8 shows that the marked steepening of
the differential counts, normalised to Euclidean, and the subsequent
decrease at faint
fluxes are predicted by PÉGASE.3, with a peak at
0.3 mJy as
observed. Note that the departure of the model from the observations
at bright and faint fluxes agrees well with the data
corrected for incompleteness and deblending (see Rodighiero et al. 2006).
Once again, the fit is more meaningful in the flux range where the number
density is the highest, as objects observed at high fluxes in the
survey area are rare. Moreover, for the 24
m filter, the k-correction
is negative at z<2 as shown in Fig. 4 by the
SED slope from 24
m to
8
m.
In contrast to the study by Gruppioni et al. (2005), based on the
flux density ratio
,
our model does not
require a population of additional starbursts, but rather the
very strong evolution factor at high redshift of the star
formation rate of elliptical galaxies as presented below.
![]() |
Figure 9:
Histories of the cosmic star-formation rate density
(global and by types) derived from models of galaxy populations that
fit optical and IR faint galaxy counts. The total SFR(z) (heavy solid
line) summed on all types is shown. The three
![]() ![]() |
Open with DEXTER |
The models of galaxy populations that simultaneously fit the
12 m and 24
m galaxy counts can be used to predict the cosmic star
formation rate at high z. We then compute the global star formation rate
as
Because ULIRGs follow the SFR scenario of ellipticals, both normal ellipticals and ULIRGs are represented by the same line (E/S0) in Fig. 9. This hypothesis, fully justified if the origin of the huge luminosity of ULIRGs is not stellar (i.e. if it is due to an active nucleus), emphasises that the star formation history shown in Fig. 9 is a lower limit because it does not include any possible contribution from starbursts caused by interactions. Our model could however accept some such occasional starbursts as seen in nearby ULIRGs, as long as they only represent a few percent of the total stellar mass and are sufficiently short to not be representative of a population.
The evolution of the SFR is different in the two groups Sa-Sbc and
Sc-Im. Between
and z=0, which corresponds
to ages of
6 to 13 Gyr, the star formation rate of a
10
Sa spiral decreases from
20 to 2
yr-1 at z=0.
In the same redshift interval, the star formation rate of a
less-evolved Sc spiral with the same mass increases from
3 to 4.5
yr-1. This is consistent with the local observations: at z=0, the current mean
SFR of galaxies is
yr-1 for late spirals of 1011
(and
for 10
irregulars); it is as low as 2
yr-1 for early 10
spirals (Kennicutt 1983). This SFR(t) variation is explained by the
corresponding
variation. Because early spirals formed
stars more efficiently in the past, their gas reservoir becomes
depleted at
,
and the gas-dependent SFR then rapidly
decreases from lack of fuel. This effect can explain the apparent
"down-sizing'' of galaxies (Panter et al. 2006).
When summing over all galaxy types, the total cosmic
increases at high redshift: it evolves by a
factor
3 between z=0 and 1. This result
agrees with the gradual decline of the UV luminosity from
the deep multi-wavelength Hawaiian surveys (Cowie et al.
1999) from z=1 to
0. This evolution is much less
than the factor 10 decrease in the SFR derived from
the shallower I-band selected CFRS sample
(Lilly et al. 1996) on the same redshift range.The CFRS is an I-band selected sample,
thus biased towards early spirals, the major emitters in the
I band. This is confirmed by the good match
between the
of the CFRS and that modelled for the Sa-Sbc galaxies in
Fig. 9. We conclude that the I-band selection bias
excludes late-type galaxies, too blue for detection in the
I-band at the depth of CFRS. Models better agree with
SFR estimates from H
lines emitted by
galaxies including a fraction of late types (Tresse et al. 2002).
The faint galaxy counts at 12 m in the ISO-ESS survey area show
the same typical signature of evolution at 0.3 mJy as already found in
the ISO/15
m and Spitzer/24
m galaxy counts. The
careful flux calibration of the galaxy catalogue is based on the
optical to mid-IR statistical properties of stars. Our results are
consistent with the other mid-IR counts at 15
m, thus demonstrating
that the surveyed area of 680 arcmin square is sufficient for
averaging the inhomogeneities and to properly analyse the populations of
galaxies.
The most important result of our analysis is that a minor (<)
population of dusty ultra-bright elliptical galaxies can
explain the excess of the mid-IR emission observed in the 12
m
and 24
m faint galaxy counts at
0.3 mJy. Here, due to
its high IR brightness, this population is associated to ULIRGs, while
the other populations, seen in the UV-optical-nearIR and MIR
counts, are called normal galaxies. Because the evolutionary code
PÉGASE.3 predicts multi-wavelength SEDs by simultaneously following the evolution of
stars, gas, dust and metal-enrichment, our analysis results find a
natural explanation in the basic scenarios of galaxy evolution.
The strong advantage of our analysis is the multi-wavelength approach:
the same SFR scenarios already found to fit the UV-optical-nearIR galaxy
counts (Fioc & Rocca-Volmerange 1999a) are also applied to the MIR
(12 m, 15
m, and 24
m) using ISO (see companion article
and Elbaz et al. 1999) and Spitzer satellites (Papovich et al. 2005;
Le Floc'h et al. 2005). These evolutionary scenarios are robust
because the evolution time scales of the dominant emitters at various
wavelengths (massive stars, evolved stars, dust grains from the
interstellar medium, and HII regions) go from a few million years to
13 Gyr. In the mid-infrared, the new model with ULIRGs,
proposed to fit ISO/12
m galaxy counts is not likely to change the
UV-optical-nearIR predictions; moreover it is confirmed by the
/15
m and the more recent
/24
m galaxy counts.
One difficulty of the interpretation is that, at
,
the
brightest Sbc spirals appear as luminous as dusty ultra-bright ellipticals in the
MIR. Figures 5 to 8 show
that the populations of "normal'' ellipticals, spirals and irregulars
(model 1) seen in the optical are largely insufficient for reproducing
the 12
m and 24
m differential and cumulative counts. Even
the brightest spirals (with
), which have IR
luminosities comparable to ULIRGs at z=0, decline too rapidly at
increasing z to fit the 12
m and 24
m number counts.
A surprising result is that we succeed in reproducing the typical
excess of MIR counts observed at 0.3 mJy by only replacing 9%
of the "normal'' galaxies in the optical (and 1/3 of the ellipticals)
with ultra-bright galaxies in the IR. In fact, at high redshift, the
evolution correction takes over as the major parameter. It is
noticeably insufficient for spirals while only ellipticals have high enough
star formation rates for reproducing the stellar and dust emission at
high redshifts. The fraction of these objects is small, and the
enormous luminosities required can only be reached if these ULIRGs
contain huge dust masses heated by large numbers of energetic photons.
The star formation history of ULIRGs follows that of elliptical
galaxies shown in Fig. 9: large masses of stars and dust are
formed at high z. ULIRGs would appear as "normal''
ellipticals in the optical with masses of 10
;
they could even be more massive if the excess of stellar luminosity is hidden by a large amount of dust.
This dusty, massive population, revealed at high z in the
mid-infrared, evokes the population of high-z (>4) massive
ellipticals found in the K-z Hubble diagram (Rocca-Volmerange et al. 2004) and confirmed in the rest-frame H-z diagram with
Spitzer (Seymour et al. 2007b): these distant galaxies are also
forming high masses of stars and dust. Due to the short
time scales required to build such objects at z > 4 (<1 Gyr), both
populations most likely formed at early epochs by rapid dissipative
collapse or major merging, rather than by slower hierarchical merging
which would take 10 Gyr.
This interpretation of ULIRGs as dusty massive ellipticals
agrees with the drastic evolution of the infrared luminosity function
when compared to the UV luminosity function (Takeuchi et al. 2006).
It also agrees with the result that the MIR-selected galaxies
contribute to more than 70% of the cosmic extragalactic background in
the MIR (Dole et al. 2006): our model confirms that other
galaxy types are too faint at both 12 m and 24
m. However,
the conclusion of the last authors
that galaxies contributing the most to the total
cosmic infrared background have intermediate stellar masses
is not confirmed by our results.
One puzzling issue is the over-brigthness of ellipticals in the mid-IR. It requires to know how the dust mass could be maintained within the galaxy host and the source of heating photons. To not be released by galactic winds, dust must not be mixed with gas and stars in the ISM but located in preferential zones such as the galaxy core. In the core, where the potential well becomes intense in case of an embedded black hole, dust could tend to be retained. In the AGN environment, the deep potential well would drag dust more efficiently, as it is more massive than gas, and dust then would fall more rapidly down into the inner core. Moreover if a Compton thick AGN is embedded within the proposed ULIRG/dusty ellipticals, the large variability from one ULIRG to another (Armus et al. 2006) is explained by orientation effects. Only observations at high spatial resolution will allow the dust geometry to be determined.
The other issue is the presence of a large number of energetic photons
heating dust grains at all ages. To produce them, massive stars from
an obscured starburst and/or
the presence of an AGN can be evoked. Our results do not exclude that,
in the case of galaxy interactions, an exceptionally extincted
starburst, undetected in the optical, could be ultra-bright in the
IR. But such an event is rare and is not representative of a galaxy
population on a long time scale. Note that despite the high value of
their MIR luminosity (
)
at
z=0, the proposed ULIRGs faintly contribute to the faint
UV-optical-NIR galaxy counts by their number. They also are suffering
an exceptional extinction due to their dust amount. Further spectral
syntheses and high spatial resolution are clearly needed. The recent
analysis by Takeuchi et al. (2006), based on the combination of data
from the UV satellite GALEX and from IRAS, shows that the
luminosity function evolves more strongly in the far-infrared than in
the far-UV. This is compatible with our dusty elliptical population.
At last, by analysing the far-UV galaxy counts with FOCA at 2000 Å,
Fioc & Rocca-Volmerange (1999a) suggested that a fraction of episodic
starbursts could be required to interpret the UV excess of galaxy
counts, in addition to the normal populations of galaxies. However,
the number density, weak star formation rate, and low metallicity of
these populations are not enough to explain the excess of MIR
luminosity at high redshifts.
Finally, these scenarios of ultra-luminous galaxies at high redshifts,
imply a very rapid phase of mass accumulation. This is also supported
by the fact that ULIRGs, evolving as ellipticals and hosting a hidden
AGN, look like the population of radio galaxies
fitting the K-z diagram at high redshifts, which also show strong
hot dust signatures (Rocca-Volmerange & Remazeilles 2005).
However, the proposed population of distant ULIRGs derived from the infrared is
more numerous (9%) than the radio-galaxy hosts detected in the
optical (<4%). This indicates that half of the ULIRGs could be
so obscured in the optical that they would be invisible. They may,
however, be revealed at other
wavelengths. Several surveys have discovered populations of AGN
that appear brighter and spatially denser than the classical populations identified in the
optical (see for example Martini et al. 2006). The population of
hyper-LIRGs (
), sometimes
associated to
blobs,
has a low
efficiency (0.05-0.2%)
according to Colbert et al. (2005). The 12
m and 24
m
galaxy counts analysed here may correspond to the best wavelength
domain where this population of embedded
AGNs could be detected.
We present the faint galaxy counts derived at 12 m
from the observation of a large and deep mid-infrared (MIR) survey in the
field of the optical ESO-Sculptor Survey (ESS), through the large LW10 filter with the ISOCAM instrument on board the ISO satellite. The
infrared observations cover an area of
75% of the ESS spectroscopic
survey, where galactic cirrus is sparse, and were performed in
continuous raster mode. The flux calibration has been adjusted
using optical-infrared IRAS colours (
m,
m)
of standard stars. Because of its large area of
680 arcmin2, the ISO-ESS survey
provides complete 12
m galaxy counts down to 0.24 mJy, after
incompleteness corrections (using two independent methods). The full
data analysis and the resulting catalogue of 142 detected sources is
published in the companion paper, Seymour et al. (2007a).
The galaxy counts are presented using two different binnings:
cumulative counts N(>S), to reduce the fluctuations in the number
density per bin, and Euclidean normalized differential counts
.
When corrected for incompleteness, both the
cumulative and differential ISO-ESS 12
m counts averaged over
the
680 arcmin2 area show good agreement with the existing
measurements in the nearby ISOCAM filter at 15
m, after
correction for the difference of wavelengths. In particular, the
Euclidean-normalized differential counts of the ISO-ESS survey
display the same excess as the other existing MIR surveys at flux
densities of 0.3 mJy. This excess is also observed in the
galaxy counts at 24
m.
We propose an interpretation of the cumulative and differential counts with the help of the new evolutionary code PÉGASE.3 (Fioc et al. 2007). For each galaxy type, PÉGASE.3 predicts the spectral energy distributions from the optical to the far-IR; the emission of stars and dust, the extinction, star formation history, metal enrichment, and dust mass are computed consistently.
With these evolutionary standard scenarios we have successfully
modelled the multi-wavelength faint galaxy counts in the far-UV,
optical, and near-infrared (Fioc & Rocca-Volmerange 1999a). In the
present article, we are able to simultaneously fit the ISO-ESS
12 m, ISO 15
m and the Spitzer24
m faint
counts, by increasing the luminosity (
2.5 mag at 12
m to 5 mag at 24
m) of a small fraction of galaxies
(9%, all of elliptical type), while the rest of the galaxies (17.5% normal
ellipticals, 57% spirals and 16.5% irregulars) are identical to the
galaxy populations already known from the UV-optical-NIR surveys. The
ultra-bright galaxies display all the characteristics of ULIRGs and
appear as "normal'' ellipticals in the optical. Because these
results cover a very large wavelength domain, from the UV-optical-NIR
to the MIR (12
m, 15
m, and 24
m), we are confident the
robustness of our scenarios.
The other important point is that no additional population of starbursts is
required to fit the mid-IR excess. Highly luminous starbursts with a
short e-folding time (107 years), which may explain some
nearby ULIRGs such as M 82, are not incompatible with our results as long as
they remain exceptional objects. Another point is that the normal
galaxy populations, including bright IR spirals, cannot fit alone the cumulative or the differential
ISO-ESS 12
m and SPITZER 24
m counts.
The star formation history of the proposed ULIRGs and normal
ellipticals, in respective proportions of 1/3 and 2/3, can fully explain
the excess in the cumulative and differential galaxy counts at
12 m and 24
m. A large dust mass and a large mass of early-formed stars as well as the possibility of an embedded Compton thick AGN could explain
the ultra-brigthness of distant ULIRGs.
The most distant ULIRGs would appear like the most massive ellipticals,
similar to the distant radio galaxy hosts found in the K-z diagram (Rocca-Volmerange et al. 2004).
As concluded in the mentioned article, this new result in the mid-IR
favours the hypothesis of a galaxy evolution process based on
dissipative collapse or a rapid merging on a short time scale (<1 Gyr) at high redshifts (z>5).
We emphasise that these results are robust since they are derived from
the simultaneous adjustment of the mid-IR 12 m and 24
m
counts respecting the majority (>90%) of galaxies observed in the
optical faint number counts (down to B>29 from the HDF-N, William et al. 1996). Moreover, our analysis by type allows us to identify the
various factors explaining the steep increase in the faint galaxy
counts. Higher spectral and spatial resolution observations
associated to deeper counts at longer wavelengths with the
satellite and the future Herschel satellite will hopefully allow
detection of the dust emission emitted by early elliptical galaxies in
their primitive epochs, validating our present results and allowing us to
hopefully observe primeval galaxy populations down to the deepest
extragalactic backgrounds.
Acknowledgements
N. Seymour acknowledges financial support from the European Network "Probing the Origin of the Extragactic Background Radiation'' (POE) while at the Institut d'Astrophysique de Paris (IAP). We also thank Prof. L. Woltjer for his stimulating interest in this ISO observational program and Damien Le Borgne for his fruitful suggestions.