MID INFRARED OBJECTS IN INTERMEDIATE REDSHIFT GALAXY
CLUSTERS
R. Pérez-Martínez1,2, L. Metcalfe1,2, D. Coia3, A. Biviano4, B. McBreen3, B. Altieri2, C. Sánchez-Fernández1,2
1: ISO Data Centre, European Space Agency, ESAC, Spain. 2: XMM-Newton Science Operations Centre, European Space Agency. 3: Physics Department, University College Dublin, Ireland.4: INAF/Osservatorio Astronomico di Trieste, Italy.
The gravitationally lensing clusters A2218 and CL0024+1654 were observed with the Infrared Space Observatory (ISO) using ISOCAM at 6.7 µm and 14.3 µm (hereafter 7 µm and 15 µm respectively). A total of 111 sources were detected in
the whole set, 40 of them being cluster objects. The spectral energy distribution of a subset of sources was calculated using GRASIL. The results for the total infrared luminosity and the estimation of the star formation rate are presented for the
non stellar objects for which the SED has been determined. The majority of the cluster galaxies in A2218 are best fit by models of quiescent ellipticals. In Cl0024+1654, most of the galaxies lying on the Butcher-Oemler region of the colourmagnitude diagram are best fit by disk galaxies, while those on the main sequence area have in general SEDs corresponding to post-starburst galaxies.
The population of each cluster is compared with the field population, as well as with the population of other clusters also studied within this programme. A significant number of Luminous IR Galaxies (LIRGs) is detected in CL0024+1654,
while only one LIRG has been observed in total in A370, A1689, and A2218. This result supports the link between LIRGs in clusters and recent or ongoing cluster merger activity as well as the need for extending the observations to the outer
parts of clusters.
Abell 2218
Cl0024+1654
The Observations
Cl0024+1654 is a rich cluster of galaxies at z = 0.395. Its Rvir
is 0.94 h-1Mpc and its Mvir is 6.42 h-11014 M. With the
assumed cosmology (see bottom right) 1 arcsecond
corresponds to 5.3 Kpc, and the age of the Universe at that
distance is 9.3 Gyr.
Why Mid Infrared?
There is observational evidence indicating that galaxies belonging to clusters evolve in a different way
from those in the field. The inner areas of clusters are mainly populated by early-type galaxies with
small or null stellar formation rate (SFR), with most of their stars formed at high redshift. On the other
hand, the fraction of blue galaxies in clusters has been found to increase with redshift and distance from
the cluster center (Butcher & Oemler 1978, 1984). The fraction of late-type galaxies also increases with
redshift (Dressler et al, 1997). This suggests a transformation from blue spirals to red ellipticals through
infall into the cluster from the field.
To trace the evolution of these objects is necessary to obtain a general picture of their stellar formation
history (SFH). SFRs derived from optical observations depend heavily on dust absorption. Dust can hide
a very significant part of the SF in a galaxy, and the amount of extinction increases with the amount of
SF (Silva et al, 1998). So, to accurately trace the evolution of the SFR as the galaxies travel to the
cluster core it is necessary to observe them in the infrared, where the dust obscuration is negligible.
Czoske et al. 2002, report evidence of a substructure involved
in a high speed collision with the main set of galaxies along
the line of sight.
35 MIR sources were detected down to 127 µJy at 15µm,
being 13 of them cluster galaxies. Their LIR range from
4.66×1010 L to 2.59×1011 L, with a median of 1.0×1011 L.
10 out of these 13 sources have LIR above 9×1011 L, very
near the threshold of 1×1011 L for qualifying as Luminous
Infrared Galaxies (LIRG).
A2218 contour map at 15µm overlaid on a deep Palomar 5m I-band
image. Blue circles denote cluster galaxies. Blue squares denote X-ray
counterparts (Metcalfe at al, 2003).
The Data
Abell 2218 is a massive galaxy cluster at z = 0.175. Its Rvir is
1.63 h-1Mpc and its Mvir is 18.27 h-11014 M. With the
assumed cosmology (see bottom right) 1 arcsecond
corresponds to 2.97 Kpc, and the age of the Universe at that
distance is 11.3 Gyr.
76 MIR sources were detected down to 54 and 121 µJy (3σ
or better) at 7µm and 15µm respectively, 27 of them being
cluster members. The infrared luminosities (LIR) of these
cluster objects span the range 2.6×1010 L to 0.2×109 L,,
with median value 6×108.
A total of 70.4 Ksec were employed with ISOCAM on board of the Infrared Space Observatory (Kessler
et al, 1996) to observe CL0024+1654 and Abell 2218 at 6.7 and 14.3 µm (7 and 15 µm respectively
hereafter). In both cases the observations were carried out in raster mode, to cover a large sky area and
to reduce the effects of flat-fielding limitations. In the case of A2218 the rasters were micro-scanned, i.e.
the raster step size included a fraction of a pixel distance to achieve better spatial resolution. The total
area covered in A2218 was 20.5 arcmin2 while in Cl0024+1654 areas of 28.6 and 37.8 arcmin2 were
observed at 7µm and 15µm respectively. For a detailed description of data acquisition and reduction
refer to Metcalfe et al, 2003, and Coia et al, 2004.
15µm contour map over a VLT V-band map of Cl0024+1654 from Coia
et al, 2004. Blue circles denote confirmed cluster members. Greek letters
identify gravitationally lensed objects.
Summary of the observations and detected sources
Galaxy SEDs in A2218
The SEDs of the cluster objects were modeled (see SED
calculation box) and were found to correspond with aged
early types galaxies. The average SED of these 27 sources
was obtained by normalizing each SED with its rest frame Hband flux density, and is presented here below. The shape of
this best fit model clearly resembles an aged, passive
evolving elliptical.
Cluster
z
Area (´2 )
Sensitivity
(5 sigma)
Faintest
source
(microJy)
R vir
Obs. t.
Cluster
(sec)
galaxies
(h-1 Mpc)
Mvir
n. of IR sources
(h-1 M¤ ) Detected Expected
Cl0024
0.39
37.8
140
141
22615
13
0.94
6.42
10
­­
A370
0.37
40.5
350
208
22688
1
0.91
5.53
1
8
A1689
0.18
36.0
450
320
9500
11
1.1
5.7
0
1
A2390
0.23
7.0
100
54
29300
4
1.62
20.35
0
1
A2218
0.18
20.5
125
90
22000
6
1.63
18.27
0
1
In the table above a summary of the results on other clusters observed with ISOCAM can be found. The
expected number of LIRGs is obtained by multiplying the number observed in CL0024+1654 by the
ratios between the virial mass per unit area, the square of the distance and the observed solid angle of
each cluster to those of CL0024+1654 (see Coia et al, 2004). In the case of Abell 370, 1 LIRG is
observed while 8 are expected. This difference can be related to the non-relaxed dynamical status of
CL0024+1654. On the other hand, the sources detected in A2218 and A1689, where only the central
areas were observed, are much weaker than those observed in CL0024+1654. This could indicate that
the MIR sources appear preferentially in the peripheral regions of the cluster, where interactions
between infalling galaxies, richer in gas, can trigger star formation burst more efficiently.
The spatial distribution of the MIR cluster members is less
centrally concentrated than that of the sources detected in the
optical. This suggests that the ISOCAM detected sources may
be part of a population different to the other galaxies of this
cluster.
Galaxy SEDs in CL0024+1654
The SED of the 12 sources with enough photometric data
were calculated as explained in the box entitled SED
calculation. The average SED, resulting from combining the
individual SEDs normalized with the rest frame H-band flux,
is shown below this text. The solid line corresponds to a
GRASIL model of a 1 Gyr old starburst galaxy.
Average SED of observed galaxies in A2218
Stellar Formation Rate in A2218
Field Galaxies
The total infrared luminosity, LIR, of the galaxies in the cluster
was obtained integrating their calculated best-fit SED.
Following Kennicut´s relation (1998) between a galaxy SFR
and its LIR,
SFR(Myr-1) = 1.71 ×10-10 LIR /L,
It is possible to get the stellar formation rate of the cluster
members. It is found that 20 out of the 27 objects have little or
no star formation. Of the seven sources left, only three show a
traces significant, though small, ongoing star formation.
For further details refer to Biviano et al, 2004.
The average spectral distribution of the ISOCAM detected
cluster galaxies in CL0024+1654. The solid line
corresponds to a 1 Gyr old starburst galaxy. (Coia et al,
2004).
The best-fit SEDs of some field galaxies observed with ISOCAM. X-axis is the rest
frame wavelength in µm, Y-axis is the flux density in arbitrary units.
Stellar Formation Rate in CL0024+1654
The SEDs of a subset of field galaxies were obtained following the same method as for the cluster
galaxies. 14 out of the 17 galaxies considered are best-fit by star forming models with no significant
contribution required from an AGN. The other 3 sources could not be fitted with the models considered.
The median SFR of the 14 objects is 22 Myr-1, and seems not to have changed much during the last
Gyr. The median redshift of the ISOCAM detected field galaxies is 0.6, in good agreement with other
MIR surveys. (See Biviano et al, 2004, Metcalfe et al. 2003, Elbaz & Cesarsky, 2003).
All the cluster galaxies detected in MIR show significant
stellar formation activity, as can be seen in histogram below.
Applying Kennicut´s relation, the SFR obtained ranges from
8 Myr-1 to 77 Myr-1, with a median value of 18 Myr-1 and
a mean value of 30 Myr-1
For further details refer to Coia et al, 2004.
Spectral Energy Distribution calculation
Histogram of the SFRs of the A2218 cluster galaxies
detected with ISOCAM. (Biviano et al, 2004)
References:
Butcher, H, & Oemler, A. 1984, ApJ, 556, 562.
Biviano, A. et al. accepted for publication in A&A. astrph/0406391.
Coia, D. et al. submitted to A&A.
Czoske, O. et al. 2002, A&A, 386, 31.
Duc P-A. et al. 2002. A&A 382, 60.
Dressler, A., et al. 1997, ApJ, 542, 673.
Elbaz, D. et al. 2003, Science, 300, 270.
Kessler, M.F. et al. 1996, A&A, 315, L27
Metcalfe, L. et al. 2003, A&A, 407, 791.
Silva, L. et al. 1998, ApJ, 509, 103.
The photometric data of the observed galaxies have been compared with calculated SEDs computed with the publicly
available code GRASIL (Silva et al, 98). This code generates spectral distributions of galaxies taking into account the
effects of dust in several environments, such as AGB envelopes, molecular clouds and the diffuse interstellar medium.
We have considered four sets of models: Early-type galaxies (E), charactised by three different durations of the initial
bursts of star formation; Disk galaxies (S), with three different gas infall timescale; Starburts galaxies (SB), one weak
and other strong, and Poststarburst galaxies, the previous SB cases observed 1 Gyr after the starburst event. Each of
these models is considered at different ages. The observed and model SEDs are compared with a standard χ2 method.
Histogram of the SFRs of the CL0024+1654 cluster
galaxies detected with ISOCAM. (Coia et al, 2004)
In this communication we assume H0 = 70 km s-1 Mpc-1,
Ω0 = 0.3 and ΩΛ = 0.7
Contact:
[email protected]