The dominant galaxy in each of approx. 40 clusters was studied using co-added Infrared Astronomy Satellite (IRAS) survey data, and 11 of these galaxies were observed for CO (J=1 to 0) emission with the 12 m telescope at Kitt Peak. Half of the galaxies in the sample are in clusters reported to have cooling flows while the other half are not. Six of the galaxies appear to have been detected by IRAS at fairly low flux levels, in addition to one previously known strong detection; all seven have reported cooling flows. No detectable CO emission (to 2 to 3 mK) was found in any of the 11 galaxies observed. Assuming accretion rates of approx. 100 Solar Mass yr(-1), the star formation rates and efficiencies in these galaxies must be quite high in order to render the CO undetectable. At the same time, the infrared luminosities of these galaxies is unremarkable, suggesting that the correlation between star formation efficiency and infrared luminosity found for spirals may not hold for cooling flows

Grabelsky, D. A.

Ulmer, M. P.

English

NASA Technical Reports Server

. -  
SEARCH FOR COLD GAS IN CLUSTERS WITH AND WITHOUT 
COOLING FLOWS 
D.A. Grabelsky and M.P. Ulmer 
7 6  7 -4 6 ,,J 
Northwest ern University, Depar tme 
- \  
The dominant galaxy in each of 
vey data, and 11 of these g 
1 are in clusters repo have cooling flows while the other 
galaxies appear to have been detected by IRAS at fairly low flux levels, in addition 
to one previously known strong detection; all seven have reported cooling flows. No 
detectable 60 emission (to 2-3 found in any of the 11 galaxies observed. 
Assuming accretion rates of -LQ@ l, the star formation rates and efficiencies in 
these galaxies must be quite high er to render the CO undetectable. At the same 
time, the infrared luminosities o axies is unremarkable, suggesting that the 
correlation between star formatioh efficiency and infrared luminosity found for sp 
may not hold for cooling flows. \ 
d* i ' l  * i  ; 
INTRODUCTION. In the last fifteen years, X-ray observations of clusters of galaxies have 
led to the discovery that the hot intracluster gas in the cores of many clusters appears to 
be cooling on timescales short in comparison with the ages of the clusters. If not reheated, 
the gas will flow toward the cluster center where it may be accreted by the central dominant 
galaxy. Estimates of the mass-flow rates range over 5-500 Ma yr-' (cf. Sarazin 1986). 
Debate over the possible role of reheating (Bertschinger and Meiksin 1986; Silk et al. 1986), 
however, has cast some doubts on the existence of flowing - as opposed to simply cooling - 
stion related to the existence of cooling flows is the ultimate fate of the 
e most obvious and generally assumed repository is stars. Since the 
t pass through a cold, neutral phase, a fundamental test for the accumulation 
racluster gas in cluster cores is the search for the cold gas that must feed the 
n. The IRAS and CO observations we present here are directed at detecting 
TIONS. We co-added IRAS survey data about the central dominant galaxy in 
clusters, and observed a subset of 11 galaxies with the NRAO 12 m telescope at 
Kitt Peak for CO (J=l-+O) emission. Our goal was to search for cold interstellar gas in these 
galaxies, with a focus on the possible contribution from accretion of the cooling intracluster 
gas. Our sample was drawn from the list of Stewart et al. (1984) of X-ray clusters; about 
half the clusters in their list show evidence for cooling flows (cooling times less than H i ' )  
while the other half do not. Additional cooling-flow clusters were taken from Sarazin (1986; 
and references therein) and Romanishin and Hinteen (1988). 
RESULTS. Seven of the galaxies appear to have been detected by IRAS, although except 
for NGC 1275 (which is already in the IRAS Point Souce Catalog), none can be classified 
as unquestionable detections. On the basis of signal strength, positional coincidence of the 
60pm and 100pm sources, and proximity to the optical image of the galaxy, we ranked six 
sources as follows: A 1126 and A 2199, very probable detections; A 2063, A 576, and A 400, 
such a reservoir. 
206 
https://ntrs.nasa.gov/search.jsp?R=19910004861 2020-03-19T20:44:37+00:00Z
likely detections; and A 1983, marginal detection. For these seven galaxies, the infrared 
luminosities range from 0.83 x lolo Le (A 400) to 13 x lo1' Lo (A 426); 30. upper limits 
for the remaining galaxies range from 0.33 x lolo Le to 22 x lolo Le. The median value of 
LIE (detections and upper limits) is roughly 4.5 x lolo Lo. (We have used Ifo = 50 km s-l 
Mpc-' for all distant-dependent quantities.) 
No CO was detected in any of the sources observed. (The data for NGC 1275 were 
lost due to a previously undiscovered software error in the telescope system; this galaxy 
has subsequently been detected in CO by Lazareff et al. [1989]). Upper limits to the total 
mass of molecular gas in each galaxy have been determined on the basis of the assumed 
proportionality of integrated CO line intensity and HZ column density, where the conversion 
of Sanders et al. (1986) has been used. Our values are consistent with the observational 
results of Bregman and Hogg (1988) for the dominant galaxies in A 1126, A 2199, and 2A 
035+096. 
The C O  luminosity upper limits are plotted against the IR luminosities (detections and 
upper limits) as open circles (0) in Figure 1. For comparison, the empirical LIR-LCO 
relation derived by Young et ul. (1986), in three ranges of dust temperature, for spiral 
galaxies is also shown (dashed lines). The results for spirals are explained quite plausibly in 
terms of a correlation between molecular gas content and star formation. Evidently, the CO 
luminosities (and presumably the molecular gas masses) of the galaxies we observed would in 
many cases have to be substantially lower than our upper limits, in order for these galaxies 
to follow the same general trend found for spirals. If star formation is proceeding in these 
galaxies in a manner similar to that in spirals, then it is either doing so at fairly low rates, 
or not producing massive stars, or both. Any of these possibilities must be reconciled with 
the continous influx of star-forming material if a cooling flow is believed to be present. 
Also shown in Figure 1 are five galaxies which were either detected by IRAS but not 
observed in CO (0-symbol), or observed in CO but no IRAS data were obtained (x-symbol). 
The LIR-LcO relation of Young et ul. has been used in these cases to predict the luminosity in 
the unobserved waveband based on the luminosity in the observed waveband. The predicted 
luminosities may provide sensitivity limits for future observations. 
DISCUSSION. We use a simple formalism to estimate the amount of cold interstellar gas 
that we might expect from a cooling flow at the present time, t. Assuming dU/dt  = growth 
rate of the ISM, dS/dt = q U / r  = star formation rate, and M = mass-flow rate = constant, 
where 7 is the star formation efficiency, r is the lifetime of the ISM against star formation 
and all masses are in units of Me, then dU/dt = -qU/r  + 2. Because both q and r may 
vary with time, we use (q/r) = t-l Ji[q(t')/r(t')]dt', which allows the simplification 
We therefore expect the size of the reservoir to scale with h, r ,  and inversely with the star 
formation efficiency q. 
Figure 2 shows mass-flow rates taken from the literature, plotted against our derived 
ISM masses or mass upper limits (IR-determined masses were derived following Jura [1986]). 
There does not appear to be any correlation. (Note that both the derived k and M scale 
with distance-squared.) Taken at face value, this lack of correlation seems to add to the 
doubts about the existence of cooling flows. Alternatively, the failure to detect CO and IR 
207 
LUMINOSITY COMPARISON 
IRAS'Detection, COlUpper ILimit I I I I I I I - - +$ IRAS and CO Upper Limits 
IRAS Detection, Predicted CO Luminosity 
f3: CO Upper Limit, Predicted IRAS Upper Limit 
- - 
from Young et al. ' -Td > 40 K - - - /-- 
- +-- 
/- /-- 
/- 
- 
/ .- - 3 0 < T d < 4 O K  - 
- 
- 
- +-- 
//-- 
/./e-- - 
- -/-- - c- 
I-- - ? - 
14 
13 
12 
11 
10 
9 -  - 
- - 
I I I I I I I I I I I I 
12 
11 
10 
I I I I I I I I I I I I I 1 
- 0 IRAS Detection, IR Mass - 
- $ CO Mass Upper Limit - 
- t L - 0 
- - 
3 4 
Log(Lc0) (K-km s-' kpc2) 
8 
Figure 1. Luminosity comparison. The logarithms of LCO and LIB are plotted against each other. 
The symbols indicate whether only CO data (x), only IRASdata (U), or both (0) were obtained. 
The dashed lines show the Lco-LrR relation determined by Young et al. (1986) for spirals; this 
was used to predict L I B  for X'S and LCO for 17's. Al l  upper limits are 3a. The asterisk (*) is NGC 
1275; LCO is from Lazareff et al. (1989), scaled to Ho = 50 k m  s-l Mpc-l. 
- $ 0  0 - 
0 
9 -  - 
- - 
I I I I I I I I I I I I I I 
e9 
3 
Figure 2. 
plotted against the logarithms of the IR-derived masses (0) or 3a CO mass upper limits (0). The
asterisk (*) is NGC 1275; Mco is from Lazareff et al. (1989), scaled to HCI = 50 km s-l Mpc-l. 
208 
emission from most of the galaxies in our sample could indicate that the star formation rates 
and efficiencies are high, and the reservoir of star-forming gas is therefore relatively small. 
This possibility can be evaluated by deriving {T,I/T)-’ based on the assumed existence of 
cooling flows. 
We redefine the quantity {T,I/T)-’ as TSF = 2M/&f, where the factor of 2 is assumed to 
account roughly for the fraction of the cold ISM detected: only half in the form of H2 in the 
case where M is derived from the CO upper limits; or only half the neutral gas being warm 
enough to be detected by IRAS at 100pm (cf. Jura 1986) in the case where M is derived 
from the IRAS data. For most of the clusters with reported cooling flows, we find TSF - lo8 
yr, fairly short considering that this is a global value. 
In spiral galaxies, high star formation efficiencies have been associated with high dust 
temperatures, high infrared luminosities, and large ratios of LIR/LCO (Young et al. 1986). 
Our results suggest that if cooling flows exist, then star formation fed by them is highly 
efficient. At the same time, however, the IR luminosities of the galaxies we observed are 
relatively low, a property which in spirals seems to indicate fairly low star formation rates. 
If the formation of massive stars is somehow suppressed in cooling flows, vigorous low-mass 
star formation might still proceed without significantly heating the interstellar gas and dust. 
This would be consistent with cooling flow models in which star formation is characterized 
by a truncated intitial mass function (e.g., Sarazin and O’Connel1983). Of course, if cooling 
flows are not actually present, or if mass-flow rates have been greatly overestimated, then 
the relatively small ISM masses and low infrared luminosities we find would be consistent 
with the results for spirals, without invoking different modes of star formation. 
An additional question that can be addressed with our data, not directly related to 
cooling flows, is whether the FIR properties of cD galaxies differ from elliptical galaxies. 
To do this, we compared our results for cluster-dominant galaxies with those of Jura et al. 
(1987) for ellipticals. Using their findings for F,(B) versus Fv(12pm), the lack of detections 
in our sample at 12pm is consistent with cDs being scaled-up ellipticals. Similarly, the ISM 
masses we derived from the 100pm fluxes, a factor of -100 greater than those found for 
ellipticals, are consistent with the typical mass estimates for cDs and ellipticals (cf. Sarazin 
1986). Evidently, cD galaxies appear to be “oversized” ellipticals with regard to their FIR 
properties. 
SUMMARY. Our results are summarized as follows: 
1. About 40 clusters with and without cooling flows have been searched for IR emission 
with IRAS co-adds; 11 of these have also been observed for CO emission. 
2. Six new IRAS detections have been discovered; no CO detections were made. 
3. If cooling flows exist, star formation rates and efficiencies must be high in order to 
render the CO undetectable; yet at the same time IR luminosities are observed to be 
relatively low. 
4. cD galaxies look like scaled-up ellipticals with regard to their FIR properties. 
209 
REFERENCES 
Bertschinger, E., and Meiksin, A. 1986, Ap. J. (Letters), 306, L1. 
Bregman, J.N, and Hogg, D.E. 1988, A .  J. ,  96,455. 
Jura, M. 1986, Ap. J., 306, 483. 
Jura, M., Kim, D.W., Knapp, G.R., and Guhathakurta, P. 1987, Ap. J .  (Letters), 316, L11. 
Lazareff, B., Castets, A., Kim, D.-W., and Jura, M. 1989, Ap. J. (Letters), 336, L13. 
Romanishin, W., and Hintzen, P. 1988, Ap. J. (Letters), 324, L17. 
Sarazin, C.L. 1986, Reviews of Modern Physics, 68, 1. 
Sarazin, C.L., and O’Connel, R.W. 1983, Ap.  J., 268, 552. 
Sanders, D.B., Soifer, B.T., Elias, J.H., Madore, B.F., Matthews, K., Neugebauer, G., and Scoville, 
Silk, J, Djorzovski, S., Wyse, R.F.G., and Buzual, G. 1986, Ap. J., 307, 415. 
Stewart, G.C., Fabian, A.C., Jones, C., and Forman, W. 1984, Ap. J., 285, 1. 
Young, J.S., Schloerb, P.F., Kenney, J.D., and Lord, S.D 1986, Ap. J., 304, 443. 
N.Z. 1988, Ap.  J., 325, 74. 
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Search for cold gas in clusters with and without cooling flows

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