A total of 41 sources have been detected at 60 μm to a level of 50 mJy in a 1.∘6 x 0.∘5 field centered on the rich galaxy cluster Abell 2151. Twenty-four of these sources can be identified with late-type spiral galaxies of estimated photographic magnitude 17 or brighter. Galaxies classified as E or SO are notably absent in the infrared data. Within the field, eleven of the IRAS sources cannot be easily identified with galaxies in the Hercules Cluster. If the brightest 60 μm cluster source (NGC 6045) is neglected, the integral luminosity function in the far-infrared can be fitted well with two power laws of slope -1.1 and - 2.5

Clegg, P. E.

de Jong, T.

Gautier, T. N.

Low, F. J.

Miley, G.

Neugebauer, G.

Rowan-Robinson, M.

Soifer, B. T.

Young, E.

Low, F.J.

Soifer, B.T.

Clegg, P.E.

Miley, G.K.

Jong, T. de

Gautier, T.N.

NARCIS 

The infrared properties of galaxy clusters - IRAS observations of the Hercules Cluster (Abell 2151)

Leiden University Scholary Publications

Caltech Authors - Main

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THE ASTROPHYSICAL JOURNAL, 278:L75-L78, 1984 March 1 
""1984. The American Astronomical Society. All rights reserved. Printed in U.S.A. 
THE INFRARED PROPERTIES OF GALAXY CLUSTERS: IRAS1 OBSERVATIONS OF THE 
HERCULES CLUSTER (ABELL 2151) 
E. YOUNG, B. T. SOIFER, F. J. Low, G. NEUGEBAUER, M. ROWAN-ROBINSON, G. MILEY, P. E. CLEGG, 
T. DE JONG, AND T. N. GAUTIER 
Received 1983September14; accepted 1983November18 
ABSTRACT 
A total of 41 sources have been detected at 60 µ,m to a level of 50 mJy in a 1 ?6 x 0?5 field centered on the 
rich galaxy cluster Abell 2151. Twenty-four of these sources can be identified with late-type spiral galaxies of 
estimated photographic magnitude 17 or brighter. Galaxies classified as E or SO are notably absent in the 
infrared data. Within the field, eleven of the IRAS sources cannot be easily identified with galaxies in the 
Hercules Cluster. If the brightest 60 µ,m cluster source (NGC 6045) is neglected, the integral luminosity function 
in the far-infrared can be fitted well with two power laws of slope -1.1 and - 2.5. 
Subject headings: galaxies: clustering - infrared: sources 
I. INTRODUCTION 
Abell 2151 is an excellent spiral-rich cluster for an initial 
study at middle-infrared and far-infrared wavelengths of 
galaxies within a cluster. The individual galaxies are generally 
distinct at the IRAS angular resolution, yet the main part of 
the cluster fits within a 0?5 X 1 ?6 scan. In this Letter, we 
report the preliminary results of a number of deep integrations 
on the classical Hercules Cluster. 
II. OBSERVATIONS AND RESULTS 
The IRAS survey instrument has been described by 
Neugebauer et al. (1984). Eight observations of the Hercules 
Cluster were made, each by scanning the telescope six times 
over the field center covering an area of 1 ?6 X 0?5. The input 
data were digitally filtered to enhance point sources and to 
suppress large-scale structure in the maps. The eight observa-
tions were co-added by weighing the signal-to-noise ratio to 
produce a minimum-variance grid. The final co-added maps 
have noise-equivalent flux densities for point sources of 5.4, 
7.8, 8.0, and 40 mJy in the 12, 25, 60, and 100 µ,m bands 
respectively. 
The 60 µ,m map is shown in Figure 1. Individual sources 
have been extracted using a simple thresholding algorithm (see 
Table 1). A detection must have a signal-to-noise ratio greater 
than 6.0 relative to the median noise in the whole grid and 
greater than 4.5 relative to the noise in the region immediately 
around the source. This form of dual thresholding tends to 
eliminate noise spikes. Obviously confused sources were sep-
arated by hand into individual components; this process is 
subjective, and the results should be treated with caution. 
Positions given are flux-weighted centroids for unconfused 
sources and best estimates of the peak for highly confused 
detections. Flux densities have no correction for galaxy colors. 
1The Infrared Astronomical Satellite was developed and is operated by 
the Netherlands Agency for Aerospace Programs (NIVR), the US Na-
tional Aeronautics and Space Administration (NASA), and the UK 
Science and Engineering Research Council (SERC). 
L75 
No flux in Table 1 means an upper limit of 6 o. Uncertainties 
in the absolute photometric calibration are estimated to be 
30%. 
III. DISCUSSION 
a) Optical Identifications 
The Hercules Cluster is included in plate 108 of the Cata-
logue of Galaxies and Clusters of Galaxies, Volume 2 (Zwicky 
and Herzog 1963, hereafter CGCG). Classifications and red-
shifts for many of these CGCG galaxies have been published 
by Tarenghi et al. (1979) and by Dressler (1980). Identifica-
tions given in Table 1 are based on the Dressler positions. For 
the CGCG galaxies, the classifications of Tarenghi et al. have 
been used. 
Twenty-nine of the 60 µ,m sources can be associated with 
galaxies. For unconfused sources, positions agree with those of 
Dressler (1980) with a standard deviation of 23" in the cross-
scan direction and 14" in the in-scan direction. Dressler 
estimates that his positions are accurate to 3" -6". 
The contamination from field galaxies can be estimated 
using data of Soifer et al. (1984) who find a density of 0.25 
IRAS sources per square degree brighter than 500 mJy at 60 
µ,m and identifiable with galaxies brighter than B - 18 mag. 
This number extrapolates to eight galaxies per square degree 
at our 48 mJy threshold. Hence, as many as a third of the 
Hercules sources could be part of the general population. 
For 12 of the sources included in the CGCG, Tarenghi 
et al. (1979) have published redshifts. All except CGCG 
Z108-107 (which is part of Abell 2152) are members of Abell 
2151. Twelve additional galaxies from Dressler's (1980) 
list are similar to the CGCG galaxies and are probably part 
of the cluster. A more definite statement on the association 
of the remaining galaxies must await redshift measurements. 
If one uses the galaxy classification of Tarenghi et al. (1979), 
the dichotomy between strongly emitting types and weakly 
emitting types is quite striking. The only prominent early-type 
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L76 YOUNG ET AL. Vol. 278 
TABLE 1 
OBSERVATIONS OF CLUSTER MEMBERS 
Flux Density (mJy)a Identifications 
Number RA(l 9SO) Dec ( l 9SO) 12 µm 2S µm 60 µm 100 µm Dressler CGCG Other Classificationb mpC 
1 16: 04: 11 18:34:S9 22 134 327 (stellar) 
2 16: 04: so 18:29:S8 96 181 (galaxy) 
3 16:04:02 18:29:57 lOSOO 4S90 743 604 R Her Mira variable 
4 16:03:31 18: 30: 02 60 (408) (galaxy) 
s 16: 03: 13 18:28:32 36 332 1980 142 Sc 16 
6 16:04: 18 18: 23: S3 16 7S 414 1290 133 s 16 
7a 16:04:00 18:18:S9 31 83 sso 1830 126 144 I l 189=Mk300 Sc(B)IV lS.06 
127 139 Sc(B)III lS.26 
7b 16:03:38 18:21:16 120 ( 1000) 128 136 Sc IV lS.26 
8 16: 03: so 18:14:48 112 <240 118 138 Sb II 15.lb 
9a 16:03:46 18:12:00 121 (283) (stellar) 
9b 16:03:26 18:14:00 120 270 15 7? E 16 
10 16:03:2S 18:11:22 108 S4S 114 129 Sa lb 
11 16:03:S8 18:0S:27 101 380 llJC Sc 16 
12 16:04:21 18: 02: 03 S4 164 95 149 Sc(Rn)Ill-IV 15.7 
13 16:03: 15 18:03:47 88 522 97 Sa/O 16 
14 16:04:18 17:S4:SS 34 110 687 74 Sb lS.66 
lSa 16:03:23 17:S6:20 207 (600) 78 126 I 1182 =Mk298 SUp 15.2 
lSb 16:03:11 17:S7:5S 207 <200 90 s 16 
16 16:04:20 17: 51: 01 99 <120 56 Sa/03p 16 
17a 16:02:55 1 7: S3: 32 77 142 1290 5730 82 112 N6045=Arp 71 Sc(B)II 14.76 
l 7b 16:03:20 17: 54: OD S6 296 1S40 80 (79?,77?,lSS?,156?) Sc(RN )Ill-IV 15. 66 
18 16:03:06 17:43:S9 lSO 614 (blank) 
19 16:02: 18 17:41:26 54 <ZOO (galaxy) 
20a 16:02:27 17:3S:OO so 4S 233 1240 30 108 Sbc II lS.7 
31 98 Sc I-II 15.7 
20b 16: 02: 57 17:33:30 120 8SO 24 113 11173 Sbll-III lS.b 
21 16:03:32 17:29:18 187 (1850) 19 133 lll86=Mark 299 Sc(BR<l)III 15.4 
22 16:02:03 17:34:29 32 118 600 2S s 17 
23 16:03:31 17:26:25 32 130 (436) 14 Sp 16 
24 16:03:00 17:28:00 24 64 <120 (spiral) 
2S 16:02:27 17:29:0S 123 (1200) 20 Sc 16 
26 16:03:34 17: 21: 24 64 <120 (blank) 
27a 16:02:46 17:22:36 4S 176 388 (stellar) 
27b 16: 03: 13 17:21:24 121 360 (stellar) 
28 16:01:53 17:20:34 23S 990 5 Sd/I 17 
29a 16:02:S7 17: 14: 36 104 ( llOO) (stellar) 
29b 16:03:19 17: 12: 09 107 (883) (stellar) 
30 16: 03: 12 17:06:04 12S 600 (compact, some nebulosity) 
31 16:02:Sl 17:0S:47 S4 <220 (stellar) 
32 16:02:38 17:00:28 396 883 107 MCG+03-41-081 Sd(B)IV 15.7 
33 16:0l:S7 17: 02: 46 64 <220 (stellar) 
34 16:02:40 16:S7:02 271 1110 (spiral) 
•The 100 µm fluxes in parentheses are uncertain due to a possible confusion of sources. 
blf a luminosity classification is given, the galaxy classifications are taken from Tarenghi et al. 1979; otherwise, the classifications of Dressler 1980 have 
been used. Classifications in parentheses are from an examination of the POSS at the IRAS position. 
cPhotographic magnitudes with zero, one, or two decimal places have been taken from Dressler 1980, CGCG, and either Strom and Strom 1978 or 
Peterson, Strom, and Strom 1979 respectively. Dressler's V magnitudes have been corrected to Busing ( B - V) = 1.0 mag. 
spiral is source 15a (IC 1182 = Markarian 298), a peculiar SO 
galaxy with evidence of ajet (Schommer, Sullivan, and Bothun 
1981). Indeed, once a sufficiently large data base has been 
acquired, the infrared properties of a galaxy may prove as 
useful as conventional morphological classifications in under-
standing some aspects of galaxy evolution. It is important to 
note that the infrared properties of a galaxy can be a probe of 
the recent star formation activity, while the morphological 
classification reflects the history of the galaxy from its forma-
tion. 
The most luminous infrared source in the Abell 2151 field is 
source 17a, identified with Sc(B) II galaxy NGC 6045. It is 
number 71 in the catalog by Arp (1966) and shows signs of 
interaction with nearby galaxies. The total luminosity detected 
in the four IRAS bands is 5.8 x 1010 L0 (using a Hubble 
constant of 75 km s- 1 Mpc- 1). If indeed the warped disk is 
due to an interaction, the enhanced infrared luminosity may 
be due to a star formation event triggered in the galaxy. 
b) Cluster- Wide Properties 
By defining a far-infrared luminosity L as the sum of the 
power in the 60 and 100 ,im bands and assuming a Hubble 
constant, an integral luminosity function of the cluster can be 
derived for these wavelengths. Figure 2 is a plot of log N(L) 
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No. l, 1984 IRAS OBSERVATIONS OF HERCULES CLUSTER L77 
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LOG[L (Le~ 
FIG. 2.-Integral luminosity function for Abell 2151. Luminosity is 
the sum of the in-band 60 µ.m and 100 µ.m powers, assuming a Hubble 
constant of 75 km s- 1 Mpc- 1. 
as a function of log L, where N( L) is the number of galaxies 
with far-infrared luminosity greater than L. Except for the 
brightest cluster member, NGC 6045, the data can be fitted 
well with two power laws, one of slope - 2.5 for the brighter 
galaxies and one of slope -1.1 for the fainter galaxies. The 
break point occurs near the ninth-ranked member, which has 
a far-infrared luminosity of approximately 1.2 x 1010 L 0 . 
These functions bear a general resemblance to those found by 
Abell (1975) for the photographic luminosity function, al-
though the slopes in the far-infrared are considerably steeper. 
A measure of the infrared activity of an object is the ratio 
of the luminosities at 80 /Lm and at the B photometric band 
(Soifer et al. 1984). For the identified galaxies in Abell 2151 
that were detected by IRAS, this ratio ranges between 0.17 
and 4.8. The galaxies with high ratios of infrared to B 
luminosity (sources 5, 6, 17a, 22, and 28) are quite comparable 
to the starburst galaxy M82 in this respect. The initial impres-
sion of a high density of infrared-bright galaxies may, in fact, 
be a selection effect. Redshifts of the visually faint galaxies are 
needed to understand better the background contribution to 
the infrared sample. 
c) Unidentified Far-Infrared Objects 
Eleven of the 60 /Lm sources do not have an obvious galaxy 
within the 3 a positional error box. Nine of these, however, 
have a stellar object at the IRAS position. To the limit of the 
FIG. 1.-Co-added 60 µ.m map of the central region of Abell 2151. 
The data have been filtered to bandpass point sources. Contour intervals 
are 6, 10, 20, 25, 30, 35, 40, 45, and 50 times the 8 mJy peak flux. 
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L78 YOUNG ET AL. 
Palomar Observatory Sky Survey (POSS), there is roughly 
0.5-1 stellar source per arcmin2 in the Abell 2151 field. Hence, 
the probability of a fortuitous positional coincidence is quite 
high. Sources 18 and 26 have no optical counterparts on the 
POSS. 
Soifer et al. (1984) find that in the IRAS minisurvey the 
galaxies with the most extreme 80 µ,m to visual luminosity 
ratios can be positionally associated with galaxies as faint as 
B = 18 mag. Since the present observations are roughly 10 
times more sensitive than the survey at 60 µ,m, the same 
population of objects could be expected down to B = 20.5 
mag. These objects would be near the plate limit of the POSS. 
Significantly better positions for the infrared sources will be 
required to make meaningful identifications with galaxies as 
faint as B = 20 mag. 
We wish to thank the many people whose combined effort 
made IRAS possible. In particular, we wish to acknowledge 
the work of Gene Kopan in developing the co-addition soft-
ware. We thank George Helou for his assistance in the in-
terpretation of the data. We acknowledge the encouragement 
and kind sharing of unpublished data by George Abell. We 
wish to dedicate this paper to his memory. 
REFERENCES 
Abell, G. 0. 1975, in Stars and Stellar Systems, Vol. 9, Galaxies and the 
Universe, ed. A. Sandage, M. Sandage, and J. Kristian (Chicago: 
University of Chicago Press), p. 601. 
Arp, H. 1966, Ap. J. Suppl., 14, 1. 
Dressler, A. 1980, Ap. J. Suppl., 42, 565. 
Neugebauer, G., et al. 1984, Ap. J. (Letters), 278, Ll. 
Peterson, B. M., Strom, S. E., and Strom, K. 1979, A.J., 84, 735. 
Schommer, R. A., Sullivan, W. T., III, and Bothun, G.D. 1981, A.J., 86, 
943. 
Soifer, B. T., et al. 1984, Ap. J. (Letters), 278, L71. 
Strom, K. M., and Strom, S. E. 1978, A.J., 83, 1293. 
Tarenghi, M., Tifft, W. G., Chincarini, G., Rood, H.J., and Thompson, 
L. A. 1979, Ap. J., 234, 793. 
Zwicky, F., and Herzog, E. 1963, Catalogue of Galaxies and Clusters of 
Galaxies, Vol. 2 (Pasadena: California Institute of Technology Press) 
(CGCG). 
© American Astronomical Society • Provided by the NASA Astrophysics Data System 


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The infrared properties of galaxy clusters - IRAS observations of the Hercules Cluster (Abell 2151)

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