A&A 469, 47-60 (2007)
DOI: 10.1051/0004-6361:20077127
D. Schaerer1,2 - A. Hempel1 - E. Egami3 - R. Pelló2 - J. Richard2,4 - J.-F. Le Borgne2 - J.-P. Kneib5 - M. Wise6 - F. Boone7
1 - Geneva Observatory, University of Geneva,
51 chemin des Maillettes, 1290 Sauverny, Switzerland
2 -
Observatoire Midi-Pyrénées, Laboratoire
d`Astrophysique, UMR 5572, 14 Avenue E.Belin, 31400
Toulouse, France
3 -
Steward Observatory, University of Arizona, 933 North Cherry Street, Tucson, AZ 85721,USA
4 -
Caltech Astronomy, MC105-24, Pasadena, CA 91125, USA
5 - OAMP, Laboratoire d'Astrophysique de Marseille, UMR 6110 traverse du Siphon,
13012 Marseille, France
6 - Astronomical Institute Anton Pannekoek, Kruislaan 403, 1098 SJ Amsterdam,
The Netherlands
7 - Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France
Received 19 January 2007 / Accepted 13 March 2007
Abstract
Context. On the nature, redshift, stellar populations and dust properties of optically faint or non-detected extremely red objects.
Aims. Determining the nature, redshift, stellar populations and dust properties of optically faint or non-detected, extremely red objects (ERO) found from our survey of the lensing clusters A1835 and AC114 (Richard et al. 2006, A&A, 456, 861). Comparison with properties of related galaxies, such as IRAC selected EROs and a
post-starburst galaxy candidate from the Hubble Ultra Deep Field.
Methods. Using an updated version of Hyperz (Bolzonella et al. 2000, A&A, 363, 476) and a large number of spectral templates we perform broad-band SED fitting. The photometric observations, taken from Hempel et al. (2007, A&A, submitted), include deep optical, ACS/HST, ISAAC/VLT, IRAC/Spitzer data, and for some objects 24 m MIPS/Spitzer and sub-mm data as well.
Results. For most of the lensed EROs we find photometric redshifts showing a strong degeneracy between "low-z'' (-3) and high-z (
-7). Although formally best fits are often found at high-z, their resulting bright absolute magnitudes, the number density of these objects, and in some cases Spitzer photometry or longer wavelength observations, suggest strongly that all of these objects are at "low-z''. The majority of these objects are best fitted with relatively young (
0.5-0.7 Gyr) and dusty starbursts. Three of our objects show indications for strong extinction, with
-4. The typical stellar masses of our objects are
after correction for lensing; for the most extreme ERO in our sample, the sub-mm galaxy SMMJ14009+0252 most likely at
,
we estimate
.
For dusty objects star formation rates (SFR) have been estimated from the bolometric luminosity determined after fitting of semi-empirical starburst, ERO, and ULIRG templates. Typically we find
yr-1. Again, SMMJ14009+0252 stands out as a LIRG with
yr-1. Finally, we predict the mid-IR to sub-mm SED of the dusty objects for comparison with future observations with APEX, Herschel, and ALMA.
Concerning the comparison objects, we argue that the massive post-starburst
galaxy candidate HUDF-J2 showing observed properties very similar to our EROs, is more likely a dusty starburst at
-2.6. This interpretation also naturally explains the observed 24
m emission from this object and we predict its IR to sub-mm SED.
Both empirically and from our SED fits we find that the IRAC selectec EROs from Yan et al. (2004, ApJ, 616, 63) show very similar properties to our lensed EROs. Reasonable fits are found for most of them with relatively young and dusty stellar populations.
Key words: galaxies: high-redshift - galaxies: evolution - galaxies: starburst - cosmology: early Universe - infrared: galaxies
Table 1:
Photometry of the selected ERO subsample in Abell 1835 and AC 114 taken from Hempel et al.
(2007, Paper II).
All magnitudes are given in the Vega system. For conversion to the AB system see the
filter properties listed in Table 3.
Non-detections (lower limits) are 1
values. NA stands for non-available.
As a "by-product'' galaxies with red spectral energy distributions (SEDs) are
also found among the drop-outs. For example, in the study of two lensing clusters
A1835 and AC114 by Richard et al. (2006) we found eight lensed galaxies with
and red near-IR
colours satisfying one of the often used criteria of "Extremely Red Objects'' or EROs.
The present paper focuses on these galaxies and related objects, using
new ACS/HST observations in the
band and Spitzer imaging obtained recently and discussed in
Hempel et al. (2007, hereafter Paper II).
Down to the available depth (
to
)
only one of these objects is detected shortward of
(
Å).
This could imply that some of them are red galaxies at very high redshift (
)
or lower redshift objects with a very strong extinction.
Quantifying the properties of these EROs and comparing them with
similar objects is the main aim of the present work.
Generally speaking, at
EROs are found be to either dusty starbursts or old passive galaxies.
They are interesting in their own right and in the context of galaxy formation and
evolution (e.g. review by McCarthy 2004).
While normally EROs are found at relatively low redshift (
-2), there
are attempts to search for similar galaxies at higher z or for even more
extreme - i.e. redder - objects, by selection at longer wavelengths.
E.g. Yan et al. (2004) have identified IRAC selected EROs (or IEROs) in the HUDF.
In these ultra-deep images the IEROs turn out to be very faint in optical
bands (
27-30 mag in V, i or
), such that they could
be taken for optical drop-outs in images with less depth.
Their redshift has been estimated to lie between
1.6 and 2.9 (Yan et al.).
Other selection criteria, such as searches for optical drop-out
objects with old populations,
yield at least partial overlap with
EROs, as for example demonstrated by the post-starburst
galaxy
candidate of Mobasher et al. (2005), which also features among the IEROs
just mentioned.
Also, a fraction of the sub-mm galaxies, detected through their strong dust emission,
show optical to near-IR colours compatible with EROs (cf. e.g. Blain et al. 2002).
This is also the case for one of our objects, SMMJ14009+0252, a known lensed sub-mm galaxy
which also qualifies as an ERO.
Finally, optically faint and red objects have also been found by selection of
high X-ray to optical fluxes. These so-called EXOs are thought to be
AGN at very high redshift (
)
or in very dusty and/or sub-luminous
host galaxies at more moderate redshifts (
;
see for example Koekemoer
et al. 2004).
Table 2:
IRAC and MIPS 24 m photometry of the EROs listed in Table 1
taken from Paper I.
All fluxes are given in
Jy.
Upper limits are 1-sigma noise values at the position of the sources.
Due to source blending the data is uncomplete, i.e. not available for
objects A1835-#3, A1835-#10, and A1835-#11, and partially for A1835-#1.
Given this variety of optically faint or undetected and red galaxies, it is of interest to determine and compare their properties to clarify their nature and ultimately to obtain a coherent picture of these seemingly different galaxy populations. With these objectives we have carried out a detailed quantitative analysis of the stellar populations and dust properties of the lensed EROs found by Richard et al. (2006) benefiting from the new ACS/HST and Spitzer photometry available for these lensing clusters (see Paper II). The same SED fitting method, based on the photometric redshift code Hyperz (Bolzonella et al. 2000) and using a large set of spectral templates, was also applied to objects with similar SEDs, such as the IRAC selected EROs of Yan et al. (2004) and other objects from the HUDF.
The paper is structured as follows. In Sect. 2 we briefly summarise the observational data for our lensed EROs. The SED fitting method is described in Sect. 3. Results for the SMMJ14009+0252 galaxy are presented and discussed in Sect. 4. The other EROs are discussed in Sect. 5. In Sect. 6 we analyse and discuss the properties of related objects from the HUDF. Our main conclusions are summarised in Sect. 8.
Throughout this paper we adopt the following cosmology:
,
H0=70 km s-1 Mpc-1 in a flat
universe. Unless mentioned otherwise, all magnitudes are
given in the Vega system.
All details concerning the photometric data reduction are given in Hempel
et al. (2007, Paper II).
The photometry from Paper I is summarised in Tables 1-3.
In this paper the EROs from Richard et al. were reselected from the band image and optical and near-IR photometry measured with SExtractor
using AUTO_MAG.
For the photometry from different instruments source matching was done
using object coordinates based on astrometry using the ESO-USNO-A2.0 catalog.
For Spitzer total aperture-corrected flux densities were determined from
measurements in 3
-diameter apertures.
No attempt has been made to correct the differences in flux measurement
that are caused by the different apertures and image quality.
This is not necessary as we do not use aperture
photometry for the optical and near-infrared images or, in case of
Spitzer, perform aperture correction. The photometry used, AUTO_MAG, is determined by measuring the flux in a flexible
elliptical aperture around each object and accounts for the extended
brightness distribution of the brighter objects.
Possible differences in the photometry
with respect to the earlier measurements of Richard et al. (2006)
are discussed in Paper II.
Using fixed-aperture near-IR photometry implies only relatively small
changes and does not alter our overall conclusions, as test computations
have shown.
The new measurements, adapted to the morphology of the EROs, are
used in the present paper.
In the SED modeling discussed below we also consider a minimum photometric
error to account for uncertainties due to matching photometry from different
instruments.
Table 3:
Properties of the photometric filters used for the SED fitting
of objects in the field of A1835 and of AC114.
For AC114 alternate filters are listed in parenthesis and SZ observations
are not available (entry NA).
Column 1 indicates the filter name, Col. 2 the
effective wavelength in micron, Col. 3 the effective bandpass (filter "width'')
in micron computed with a Gaussian approximation.
AB corrections (
), defined
by
,
are listed in Col. 3.
Detailed information on the photometry can be found in Paper II.
No object from the subsample of "optical dropout'' EROs discussed in Paper II is detected shortward of the R band. For modeling in the present work we include the V band non-detection as the dropout constraint. Non-detections at shorter wavelengths are redundant and are therefore not included in the SED fitting.
An overview of the observed optical, near-IR, and IRAC/Spitzer fluxes of all the objects discussed in this paper is shown in Fig. 1.
An SED fitting technique based on an updated version of the Hyperz code from Bolzonella et al. (2000) is used to constrain the redshift, stellar population properties (age, star formation history), and extinction of the galaxies studied in this paper. To do so we closely follow the procedures outlined in Schaerer & Pelló (2005). In addition, we have included other synthetic, empirical and semi-empirical spectral templates, as described below.
For other objects, included here for comparison, the original photometry was taken from the literature.
In some cases a prescribed minimum photometric error is assumed, to
examine the influence of possibly underestimated error bars and to account
for uncertainties in absolute flux calibrations when combining photometry
from different instruments.
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Figure 1:
Observed SED of all EROs in the optical and near-IR up to 10 ![]() |
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To include more obscured objects as well we have added
UV to millimeter band templates of EROs, ULIRGS, starburst and normal
galaxies (HR 10, Arp 220, M 82, NGC 6090, M 51, M 100, NGC 6949) from fits of GRASIL models
to multi-wavelength observations (Silva et al. 1998; named GRASIL group).
These templates are therefore semi-empirical templates.
This template group will be used to predict mid-IR to sub-mm fluxes in particular,
and hence to estimate total bolometric luminosities,
after fitting the optical to 8 m part of the spectrum.
To increase the diversity of empirical or semi-empirical templates and to allow for possible deviations from them, reddening is optionally also considered as a free parameter. In this case, this obviously corresponds to an additional reddening. Test computations have shown a very good consistency between photometric and spectroscopic redshifts using e.g. the GRASIL template group to fit the near-IR to IRAC observations of Stern et al. (2006) of the ERO HR10, once allowing for possible additional reddening. A similar approach with empirical templates was also adopted by Rigby et al. (2005). As for all templates the corresponding dust emission is not treated consistently.
Finally, from the luminosity distance of the object the scaling of the template SED to the observed absolute fluxes yields an absolute scaling property, such as the stellar mass or the star formation rate (SFR) when templates generated by evolutionary synthesis models are used.
To estimate stellar masses we use two different approaches.
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(1) |
Table 4:
Magnification factors
from the lensing models of A1835 and AC114
predicted for various source redshifts
.
The values of
are dimensionless
magnification factors, and not in magnitudes.
Absolute quantities such as the stellar mass, SFR, and bolometric
luminosity depending on the luminosity distance must also be corrected
for the effects of gravitational lensing. The magnification factor of
each source was determined using the mass models of A1835 (similar to
Smith et al. 2005) and AC114 (Natarajan et al. 1998; Campusano et al. 2001), following the same procedure as in Richard et al. (2006).
Because of the slight dependence of the magnification on
the source redshift ,
at the location of the EROs, we computed
different estimates assuming
,
1, 2, 3, as well as 7 for
comparison.
The different values, given in Table 4 for each object,
reflect the uncertainty in the magnification factor, the source
redshift being the dominant source of error.
In any case, for the bulk of these sources located somewhat away
from the cluster center (see Figs. 10, 11 in Richard et al.), the
magnification is relatively small.
As already mentioned in Richard et al. (2006), this ERO corresponds
to the known sub-mm source SMMJ14009+0252 (Ivison et al. 2000;
Smail et al. 2002; Frayer et al. 2004).
With AC114-#1 this object is the brightest optical dropout ERO from our
sample.
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Figure 2:
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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As shown in Figs. 3 and 4, the high-z solution
(
= 7.46) provides an excellent fit to the observed SED.
This template corresponds to a young burst (6 Myr) + high AV.
The best fit with GRASIL templates is obtained at
= 2.78 with
the NGC 6090 template plus additional extinction of
= 1.4.
Imposing a maximum redshift of 4 to the BCCWW templates, one finds
a very similar best photometric redshift (
= 2.95)
for an elliptical with 0.36 Gyr plus 2.4 mag extinction in
.
These two
solutions are also plotted in Fig. 3,
showing a discrepancy at
0.95-1.1
m (cf. below).
Actually the overall SED of this object, including in particular our MIPS 24 m and the SCUBA measurements from Ivison et al. (2000), is rather well fitted with
semi-empirical templates from GRASIL for redshifts
-3,
as shown on Fig. 4.
Templates with very strong dust emission such as Arp 220 are needed to reproduce the
observed ratio of the
sub-mm to near/mid-IR flux. For example, templates of more moderate
starbursts like M 82 and NGC 6090 underpredict the sub-mm emission.
The Hyperz best fit with the Arp 220 template requires an additional
extinction of AV = 1.4 (for the Calzetti et al. law).
The only difficulty with fits at
3 is the excess emission observed
in the SZ band, which is
above the expected level at such
redshift. A natural explanation could be the Mg II
2798 emission
line seen in type 1 AGNs (e.g. Gavignaud et al. 2006).
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Figure 3:
A1835-#2: Comparison of best fit high-z solution (black,
![]() ![]() ![]() ![]() |
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Figure 4:
Observed SED of the ERO/sub-mm galaxy A1835-#2 including
VLT, Spitzer (IRAC, MIPS), SCUBA observations.
The model fits are the same as shown in Fig. 3.
Note that the two fits with GRASIL models at
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It is immediately clear from the observed monotonous flux increase across
all 4 IRAC bands that this source cannot be at redshift
much smaller than 3. Otherwise, the typical flux depression,
associated with the transition from the stellar peak at 1.6
m (restframe) to the raising dust emission at longer wavelengths
(cf. John 1998; Sawicki 2002), should be seen.
Excluding higher redshift solutions from the near-IR to 8
m data
used here for the Hyperz SED fitting is difficult; it
would require a more complex stellar population plus dust modeling.
However, if at
-8 as suggested from the formal best fits, the absolute
magnitude of this object would be rather exceptional
(
or
without correcting for lensing),
rendering this case very unlikely.
Furthermore, radio and sub-mm data (cf. below) as well as our "global''
SED analysis favour
.
For these various reasons we conclude that the most likely redshift
of this object is
.
This redshift is larger than the estimate based on the radio-submm
spectral index
,
but in agreement with the one from
submm colours (cf. Ivison et al. 2000). It is also larger than
our previous estimate based on optical to near-IR photometry (Richard
et al. 2006).
Assuming
and a magnification factor
(cf. Table 4),
one obtains the following estimates:
with a rest-frame absolute magnitude
)
this object is
3.4 to 3.6 mag brighter
than
at this redshift (cf. Kashikawa et al. 2003) or
than
from 2MASS in the local Universe (cf. Kochanek et al. 2001).
The mass, estimated from the best fitting Bruzual & Charlot and S04 templates
(with an age of 0.36 and 0.14 Gyr respectively), is
,
slightly more massive e.g. than the most massive SCUBA galaxy discussed by
Borys et al. (2005) and typically an order of magnitude more massive
than the most massive
Lyman break galaxy observed with Spitzer
by Rigopoulou et al. (2006).
Integrating the global SED of Arp 220 fitted to the observations
(see Fig. 4)
and assuming z=2.78, one obtains a total luminosity of
,
close to the limit between
ultra-luminous and hyper-luminous infrared galaxies (ULIRG and HyLIRG).
Using standard SFR conversion factors (Kennicutt 1998), this corresponds to
an estimated star
yr-1,
adopting
(Table 4).
At the high luminosities of this object, in the ULIRG range, the AGN fraction
is high (40-50%, cf. Veilleux et al. 1999; Alexander et al. 2004) rendering the AGN hypothesis quite likely.
However, is there other direct evidence for an AGN?
Ivison et al. (2000) have obtained an upper limit
for soft X-rays (0.1-2.0 keV) from ROSAT archival HRI observations.
In our recent Chandra observations of Abell 1835, described in Paper II,
this object remains undetected with flux limits of the order of
<
erg s-1 cm-2 in the 0.5-7.0 keV band, for
photon power law index
between 1.0 and 2.0.
The corresponding limit for 2.0-10.0 keV and
is
erg s-1 cm-2.
A comparison with the X-ray and 24
m fluxes of starbursts and AGN
compiled by Alonso-Herrero et al. (2004) places this X-ray limit
well below the typical range of hard X-ray selected AGN. A more detailed
analysis will be needed to examine how much room these new constraints
leave for a putative AGN in this object.
Compared to other SCUBA galaxies studied also in the rest-frame optical
(cf. Smail et al. 2004)
A1835-#2 features among the faintest ones in K and among the "reddest ones''
in optical/IR flux.
With
it is close to the faintest objects of Smail et al.,
which have
;
among the 7 confirmed sub-mm galaxies observed observed in the SCUBA Cluster Lens Survey
of Frayer et al. (2004) it is the second faintest object in K surpassed only by
SMMJ00266+1708 with
,
and the second reddest in J-K.
After lensing correction the magnitude of A1835-#2 is
.
Several other sub-mm galaxies are known with very faint flux levels
at
-21.9 (Smail et al. 2002; Dannerbauer et al. 2002).
Its restframe V-band to IR luminosity ratio is very low,
,
placing it among the five most extreme sub-mm galaxies when compared to
the Smail et al. (2004) sample.
Table 5:
Derived/estimated properties for optical dropout ERO galaxies with near-IR and Spitzer
detections.
Listed are the object ID (Col. 1), the photometric redshift estimate (Col. 2), the extinction (Col. 3),
the type of the best fit template (Col. 4), the distance modulus corresponding to
(Col. 5),
the absolute
-band magnitude non-corrected for lensing (Col. 6),
the absolute rest-frame
-band magnitude non-corrected for lensing (Col. 7),
the estimated stellar mass (from scaling the SED fit or from
assuming
LK/M=3.2, Col. 8), the estimated star formation rate non-corrected for lensing (Col. 9),
and the age of the stellar population (Col. 10).
To correct the above mentioned absolute quantities for gravitational magnification the appropriate
magnification factors listed in Table 4 must be used.
Table 6: Same as Table 5 for optical dropout ERO galaxies detected only in the near-IR (no Spitzer photometry available). No information is listed for A1835-#11 due to its highly uncertain photometric redshift.
In terms of stellar populations we find a dominant stellar age of 0.36 Gyr
or younger for A1835-#2, similar to the mean ages of
Myr
estimated by Smail et al. (2004) for a sample of sub-mm galaxies and optically faint radio galaxies.
The extinction we estimate (
-3) is somewhat larger than
the average of
found by
Smail et al., but comparable to the median
determined
by Takata et al. (2006) for sub-mm galaxies from the Balmer decrement.
The best fit with the Arp 220 spectrum, requiring an additional extinction
of AV =1.4, also indicates that we are dealing with an object with
a rather exceptionally large extinction!
We note also that the stellar mass estimated from the SED fit
(
)
is consistent with the mass being
built up at the high SFR of
yr-1 over a period of
360 Myr.
This leaves room for
25% of the stellar mass being
formed from a previous star formation event.
We now present the results from the SED fits for the individual objects.
First we discuss in detail the objects for which Spitzer photometry
(detections or upper limits) is available.
The remaining objects are addressed in Sect. 5.5.
The main results are summarised in Table 5 and 6.
The magnification factors
needed to correct for gravitational lensing
are listed in Table 4.
There is a strong degeneracy in this case between redshift and extinction (Fig. 5).
Two different solutions coexist, one at
and
,
and
another one at
with
.
The first solution is
strongly degenerate in the age-AV plane as well, thus providing loose
constraints on the stellar mass: the stellar ages vary quite strongly
from
2.3 Gyr (BC models) to
10 Myr for the Maraston and S04gyr
models, and the corresponding stellar masses range from
(BC) to
.
Even if we consider the solutions around
,
there are significant
uncertainties. E.g. the BC and Maraston models require little extinction (
),
whereas S04gyr and GRASIL templates indicate a higher extinction (even
for S04gyr!).
Stellar ages of
5.5 Gyr (1 Gyr) are found for the BC and Maraston (S04gyr) models;
the corresponding stellar mass is estimated as
.
For the S04gyr models one obtains
.
For illustration we show several SED fits including with the semi-empirical GRASIL
templates in Fig. 6. The latter allow us in particular to estimate
the mid-IR to sub-mm flux.
In particular we note that the 24 m non-detection probably rules out
the very dusty solution at
,
rendering
more likely.
However, for this object it is clear that the uncertainties
on all derived parameters are large, and larger than for the other objects discussed
here. For this reason the entries in Table 5 are left blank for this object.
For comparison we note that the empirical classification based on near-IR colours
would indicate an "old passive'' object (Paper II).
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Figure 5: Same as Fig. 2 for A1835-#1. |
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Figure 6:
A1835-#1: Comparison between fits with Bruzual and Charlot models
at z=0.50 (black) and 1.35 (blue),
and fits with dusty starburst models from GRASIL templates at z=1.75 (M 82, red)
and 0.4 (green, M 82 plus additional extinction of ![]() |
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For this object the best fits are consistently found at low redshifts,
well constrained by the measurement
of the stellar 1.6
m peak measured in the IRAC channels.
The corresponding
maps and the best fit SEDs for this object are
shown in Figs. 7 and 8.
Best fit templates correspond to bursts of 4.5 Gyr with no extinction
for Bruzual & Charlot models or to the elliptical template from CWW,
i.e. an old and dust-free galaxy.
With the S04 templates the best fit is of similar quality, yielding
a younger burst age (
0.6 Gyr) and some extinction (
).
From the BC and S04 model sets the estimated mass is
,
with
the magnification factor
(cf. Table 4).
If we assume LK/M=3.2 as for SCUBA galaxies (cf. Borys et al. 2005),
one obtains
.
For this object SED fits with Maraston models yield a solution of similar
quality, but a lower redshift of
.
The other fit parameters a burst age of
1.7 Gyr, AV=1.2, and a stellar mass of
.
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Figure 7: Same as Fig. 2 for A1835-#4. |
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Figure 8: A1835-#4: Comparison between fits with BCCWW galaxy template at z=1.20, and fits with the M 82 template from GRASIL at z=1.24 (and no additional extinction). |
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For comparison the best fit to GRASIL templates is found at
= 1.24 with an M 82 template
(and no additional extinction), as shown in Fig. 8.
Although the quality of this fit is less than the ones mentioned above,
we cannot completely rule out the presence of dust. Observations at longer
wavelengths would be needed. Assuming that the M 82 template is valid,
we estimate a bolometric luminosity of
to
or a star formation
rate of just
yr-1.
We note also that the results from the best fit agree with the empirical classification as "old passive'' galaxy based on near-IR colours (see Paper II).
As a cautionary note, we remind the reader that this object has been
found variable over
one month in the ISAAC photometry taken in
the SZ band (see Richard et al. 2006). The SZ flux adopted here
corresponds to the average between the two periods. It is currently
unclear if and to what extent the apparent variability influences the
results derived here.
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Figure 9: Same as Fig. 2 for A1835-#17. |
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In all cases a very high extinction (
-4) is needed.
However, models with very different ages yield fits of similar quality (
):
7.5 Gyr with the BC models, 1 Gyr with Maraston models, and 10 Myr with the
S04 templates. The SF histories correspond to bursts in all of them.
This age uncertainty is most likely due to the fact that we rely
here on the predictions in the rest-frame spectral range
1.6-2.8
m, where the evolutionary synthesis models are more uncertain
than at shorter wavelengths.
The stellar masses derived from the burst model fits are
for the Maraston and BC templates and significantly smaller
from the (younger) S04 template (
).
For a value LK/M=3.2 adopted for SCUBA galaxies (cf. Borys et al. 2005)
one obtains
,
much lower than the typical
masses of SCUBA galaxies.
The magnification factor for this object is
(cf. Table 4).
Using the semi-empirical GRASIL template group the best fits
are found at
with the SED of the Sbc galaxy M 51
with an additional optical extinction of
3.8.
The corresponding mid-IR to sub-mm SED is shown in Fig. 10,
with a bolometric luminosity of
corresponding
to
yr-1.
If this object is indeed a very dusty starburst, which can in principle be verified
with longer wavelength observations, it is much fainter than SCUBA galaxies
(SMG) at the same redshift - e.g.
4 mag fainter in K (cf. Smail et al. 2004).
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Figure 10:
A1835-#17: Comparison between fits with Maraston (2005) templates at z=0.81and ![]() ![]() ![]() |
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Figure 11: Same as Fig. 2 for AC114-#1. |
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Using the templates from synthesis models the best fits for this object
are found between
and 2.5, with a secondary, though less
likely, solution at high redshift (see Fig. 11).
Over the interval
1-2.5 the photometric redshift is actually not
well determined, since the curvature of the SED measured in the
4 IRAC bands is small and hence the position of the 1.6
m peak
- the main constraint on z - only loosely constrained.
Both the BC and the Maraston templates give quite similar best fits:
-1.5, a burst with a maximal age of
3.5-4.5 Gyr, a stellar mass of
and a large extinction
(
-2.4). The magnification factor for this object is
.
Best fits with a similar
are found using the S04gyr templates
at
for a burst of 0.9-1.0 Gyr age with AV=2.8and a stellar mass corresponding to
.
The main difference between these models is a lower age and higher extinction
in the latter.
Using the semi-empirical GRASIL templates yields a best fit at
-1.0
for the M 51 template. A strong additional extinction of AV=3.8 is required,
and the overall fit is lower quality (higher
)
than the fits
discussed above.
The overall SED resulting from these different fits is shown in Fig. 12. Interestingly the GRASIL template also reproduces
quite well the observed MIPS 24
m flux, although this was not included
in the fit procedure.
In any case the 24
m flux is a strong indication of
the presence of dust in this galaxy. In other words, solutions with
non-negligible extinction at
-1.5 are favoured by this additional
constraint.
If located at z=0.97, the GRASIL template shown in Fig. 12
has a bolometric luminosity of
(close to the LIRG range) corresponding to
,
with
(Table 4).
From the GRASIL SED shown here we may also expect a fairly strong sub-mm flux, in the range detectable with current instrumentation. To the best of our knowledge the southern cluster AC114 has so far not been observed in this spectral range.
The objects treated here are those from Table 6 for which contamination by neighbouring sources does not allow us to determine photometry from the Spitzer images, i.e. the objects #3, #10, and #11 in Abell 1835. The main properties estimated for #3 and #10 are summarised in Table 6.
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Figure 12: AC114-#1: Best fit SEDs with Bruzual & Charlot templates (black line: burst of 4.5 Gyr age and AV = 2.4 at z=1.3), S04gyr templates (red: 1.0 Gyr, AV=2.8, z=1.6), and with GRASIL templates (blue: M 52 template + AV=3.8, z=1.0). Note that by, construction, only the GRASIL templates include dust emission. |
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The SED of #11, shown in Fig. 1,
precludes any reliable photometric redshift estimate;
above
good fits can be found at all redshifts.
For this reason this object is not discussed further.
As for other objects discussed earlier, #3 and #10 show a degeneracy
between low and high-z, with
minima found at
-1.5
and 5-6.0 for #10 (5.-6.5 for #3).
However, if at high-z their absolute
restframe magnitude is of the order
of -27.3 to -27.6. Such high luminosity objects should be extremely
rare; for this reason we subsequently only consider solutions with
photometric redshifts of less than four and list the properties estimated from the best
fit models.
A1835-#3:
The best fit is obtained with the BC templates at
for a burst of 0.5 Gyr with
an extinction of AV=0.8. The corresponding stellar mass is
.
Fits with the Maraston templates yield very similar parameters.
A very similar mass (
)
is also obtained using Eq. (1)
and LK/M=3.2 adopted for SCUBA galaxies (Borys et al. 2005).
A1835-#10:
The best fit is obtained with the BC templates at
for a burst of 0.5 Gyr with
an extinction of AV=1.8. The corresponding stellar mass is
,
basically identical to the one derived following Borys et al.
Fits with the Maraston templates yield a somewhat younger age (0.25 Gyr), lower extinction
(AV = 0.4), and lower mass (
)
at
.
To illustrate the expected IR to sub-mm SED of these objects we show,
in Fig. 13, the best fits for #3 and #10 obtained with the
GRASIL templates.
For reference the best fit redshifts obtained with the GRASIL templates
are
and 1.165 for #3 and #10 respectively, and the SFR
is
2.4 and 4.3
yr-1, assuming
for both sources.
The global SED and IR to sub-mm fluxes of these objects are quite similar to those of #17.
Remember, however, that the sources #3 and #10 are blended with
neighbouring objects in the Spitzer images, hence requiring high spatial
resolution observations to potentially resolve them with future instruments.
![]() |
Figure 13: Model fits for A1835 #3 (black) and #10 (blue) using GRASIL templates showing predictions for the Herschel/sub-mm/ALMA spectral domain. |
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For the objects with measurable IRAC photometry we have also examined the importance of this additional information on the SED fits and the resulting photometric redshifts. Overall, this exercise, summarised in Fig. 14, shows that the objects can be grouped into three "classes'':
![]() |
Figure 14:
Comparison of photometric redshifts derived with or without IRAC photometry for
selected objects.
Left panels: ![]() |
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Although not included in the SED fitting procedure done with Hyperz,
a measurement or a stringent upper limit of the 24 m flux with MIPS
can also provide important constraints, helping for instance to distinguish
between SED fits with or without strong redenning and therefore to indirectly
rule out certain redshifts (see, for example, the cases of A1835-#2, AC114-#1).
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Figure 15:
Same as Fig. 2 for HUDF-J2.
Formally the best fit is found at high redshift (
![]() ![]() ![]() ![]() |
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Table 7:
Same as Table 5 for the HUDF-J2 galaxy from Mobasher et al. (2005).
To the best of our knowledge no lensing correction has to be applied to this object
().
Concerning the high-z solutions we note the following.
The best fit
depends somewhat on the spectral templates
used; indeed using BCCWW or s04gyr templates we obtain
7.39
(
= 1.5) or
6.5 (
= 1.2).
In both cases the best fit is obtained with zero extinction.
With the same template groups the best fit "low-z'' solutions are
= 2.59 and AV = 1.8 (
= 2.3) for the BCCCWW templates, and
= 2.42 and AV = 3.4 (
= 2.6) for the s04gyr templates.
Test calculations have shown that introducing a minimum error
of
0.1-0.15 mag already modifies considerably the
map
leading to less well-constrained solutions. Given uncertainties in the
determination of measurement errors and uncertainties in matching
photometry from different instruments (mainly NICMOS, ISAAC, and Spitzer),
such error bars may be more realistic than the small errors quoted
by Mobasher et al. (2005).
A comparison of the photometry of this object published by Yan et al. (2004, their object #2) and the measurements of Mobasher et al. (2005),
showing differences exceeding several
in various bands,
is also illustrative for this purpose.
Given the relatively small differences in the
,
the resulting exceptionally
bright magnitude of this object (
,
cf. Table 7), and using the same "spirit'' as for our EROs,
we would conservatively favour a "low-z'' interpretation for HUDF-J2
on this basis.
Quantitatively the main weakness of the low-z fits is the slight
excess predicted in the
band with respect to the photometry from
Mobasher et al., as shown in Fig. 16.
Actually the flux predicted with the BCCWW template (black line) corresponds to
a
detection; a somewhat larger flux is predicted with the
GRASIL templates.
Recently, Dunlop et al. (2006) have questioned the very deep
non-detection limit
quoted by Mobasher et al. (2005), and they have performed manual photometry
of HUDF-J2 in the optical bands. Their faint detection in V606, i775, and
alters the balance of the
behaviour between low-z and high-z
favouring solutions at
(Dunlop et al. 2006).
However, imposing a measurement in a prescribed aperture may not be appropriate.
If we use the same constraint and the BC templates we obtain
,
AV=3.0, and a burst of 0.26 Gyr age,
in good agreement with Dunlop et al. (2006).
In any case, the model fluxes shown in Fig. 16 in the
band
and at shorter wavelengths, are nicely bracketed by the measurements of
Dunlop et al. and Mobasher et al.
Since the SED of HUDF-J2 qualitatively resembles that of our EROs (in particular
that of A1835-#2) and, given that "low-z'' fits indicate significant
extinction, it is instructive to explore also spectral templates of dusty
objects. Indeed, using the GRASIL templates a reasonable fit ( = 4.8)
to the observations (i)
are found with the M 82 template and additional extinction of AV =2.8 at
(see red line in Fig. 17).
Adopting the photometry from (ii), an excellent fit (
= 0.9)
is found with the HR10 template and no additional extinction for
(blue line in Fig. 17).
Interestingly, both fits reproduce quite well the 24
m flux
observed by MIPS/Spitzer (Mobasher et al. 2005), which was not included
in our fit procedure.
This lends further credit to the explanation of HUDF-J2 as a
dust-rich galaxy at
.
For comparison, Mobasher et al. do not fit the 24
m emission, and
invoke other components - possibly an obscured AGN - to explain this flux.
And this measurement is not considered by Dunlop et al. (2006) in their analysis.
As also shown by Fig. 17, the predicted far-IR to sub-mm flux
of HUDF-J2 should be within reach of existing/future facilities.
Detections in this spectral range should definitely be able to distinguish
between dust-free high-z solutions and the "low-z'' fits favoured here.
From the two GRASIL fits shown in Fig. 17 with the M 82 and HR10 templates
at
and 2.52 respectively we estimate
a bolometric luminosity of
(in the LIRG range) corresponding to
-107
yr-1.
![]() |
Figure 16:
![]() ![]() ![]() ![]() |
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![]() |
Figure 17:
SED fits to the observations of HUDF-J2.
The near-IR and IRAC/Spitzer photometry is taken from Mobasher et al. (2005);
in the optical we adopt either ( (i) the
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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The main difference with the results of Yan et al. is that, once a reasonable minimum error of say 0.1 mag has been allowed for, and including a wide variety of star formation histories and varying extinction, we are able to obtain good fits to all objects (except those already mentioned) with standard Bruzual & Charlot templates, i.e. there is no need for composite stellar populations as invoked by Yan et al. (2004). In consequence, our best-fit ages and extinction differ systematically from their analysis, as will be discussed below. Otherwise, quite similar properties are derived from our more complete quantitative analysis.
More precisely, we find best-fit redshifts ranging from 0.6 to 2.8 with a median
(mean) of 1.6 (1.9). The redshift difference with Yan et al. has
a median value of
.
In particular we obtain significantly lower redshifts for two objects, #13 and 14.
However, their photometric redshifts turn out to be quite uncertain
and strongly dependent on the template set used
.
We therefore consider them to be quite insecure.
Spectroscopic redshifts are available for 3 sources of the Yan et al. sample, #9, #13, and #17 (see Daddi et al. 2005 and update by Maraston et al. 2006).
Except for the uncertain object #13 just discussed, the agreement with
our photometric redshifts is excellent (
).
If taken at face value, our best fits yield the following average properties
for these IRAC selected EROs
(cf. Table 8):
a median (average) extinction of AV = 2.0 (2.2),
a median (average) age of 0.5 (1.3) Gyr, and
stellar masses
ranging between 109 and
,
with a median (average) mass of
(
)
Their absolute
covers -20.5 to -24.3 in AB mags,
with a median of -22.3, corresponding to a median of
.
This is somewhat brighter than
from 2MASS (-23.53 cf. Kochanek et al. 2001)
but similar to
at
-2.5 from
Bolzonella et al. (2002) and Kashikawa et al. (2003)
who find
to -25.0.
Table 8:
Derived/estimated properties for IRAC selected EROs from Yan et al. (2004)
from Hyperz fits with Bruzual & Charlot templates with different star formation
histories, and computed assuming a minimum error of 0.1 mag in all bands.
Note: all magnitudes are in the Vega system (
).
As already mentioned these average properties are in good agreement with
those derived by Yan et al. (2004), except that we find good fits
with templates computed assuming standard star formation histories, i.e. that we do not need to invoke other more arbitrary composite stellar populations.
In consequence, the bulk of the fits we obtain correspond to younger stellar ages
and hence to higher extinction than Yan et al. As already mentioned
by these authors, observations at longer wavelengths (including MIPS imaging)
could help to distinguish between these solutions; in fact, according
to Yan (2006, private communication) 9 of the 17 IERO have been detected
at 24 m with fluxes above 20
Jy providing support to our
interpretation.
For the subsequent comparison we adopt the properties derived here as representative values for the IRAC selected EROs.
The similarity between our EROs and the IRAC selected EROs of Yan et al. (2004) is also evident from the comparison of the derived properties (cf. Tables 5 and 8), showing similar redshifts, extinction, stellar ages, and stellar masses.
Obviously, observations in several filters longward of
are
able to eliminate such objects with very red SEDs out to the IR.
For example, examining J-H Stanway et al. (2005) find that their i775W-dropout
sample is consistent with unreddened high-z starbursts.
On the other hand, in a recent analysis combining ACS and Spitzer imaging
of the GOODS field, Yan et al. (2006) find that
15-21% of the
IRAC detected i775W-dropouts have very high flux ratios between 3.6
m and
the
band. These amount to
4% of their total i dropout,
i.e.
sample.
It is highly likely that our object, AC114-#1, belongs to the same class of rare objects.
Empirically, these objects share a very red overall SED, similar
colours and magnitudes, and very faint or absent flux in optical bands.
In particular most of our EROs, originally selected as very red (
)
optical drop-out objects by Richard et al., have been detected with ACS/HST
in deep
images, as discussed in Hempel et al. (2007, Paper II).
The ACS, VLT, and IRAC/Spitzer photometry has been taken from Paper I. To determine photometric redshifts and to simultaneously constrain the stellar population and extinction properties of these objects, we have used an updated version of the Hyperz code from Bolzonella et al. (2000) including a large number of synthetic, semi-empirical and empirical spectral templates.
The main results from the SED fitting are the following:
Acknowledgements
We have benefited from interesting discussions with numerous colleagues who we wish to collectively thank here. We thank Haojing Yan for private communication of MIPS data. Support from ISSI (Internation Space Science Institute) in Bern for an "International Team'' is gratefully acknowledged. This work was supported by the Swiss National Science Foundation, the French Centre National de la Recherche Scientifique, and the French Programme National de Cosmologie (PNC) and Programme National de Galaxies (PNG).