Testing the ADAF paradigm for supermassive black holes in elliptical galaxies

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Adv. Space Res. Vol. 23, No. 516, pp. 1075-1078, 1999 0 1999 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/99 $20.00 + 0.00 PII: SO273-1177(99)00267-7

TESTING THE ADAF PARADIGM HOLES IN ELLIPTICAL GALAXIES T. Di Matteo1,4, A. C. Fabian’,

FOR SUPERMASSIVE

BLACK

C. L. Carilli2 and R. J. Ivison3

’ Institute of Astronomy, Madingley Road, Cambridge, CB3 OHA, UK 2 NRAO, P.O. Box 0, Socorro, NM, 87801, USA 3 Institute for Astronomy, Dept. of Physics 15’Astronomy, University of Edinburgh, Blackford Edinburgh EH9 SHJ, UK 4Presently at Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, 02138, USA

Hill, MA

ABSTRACT The growing evidence for supermassive black holes in the centres of nearby galaxies has brought into sharper focus the-question of why elliptical galaxies, rich in hot gas, do not possess quasarlike luminosities. Recent studies suggest that the presence of advection-dominated accretion flows (ADAFs) with their associated low radiative efficiency, might provide a promising explanation for the observed quiescence of these systems. Here, we present new high-frequency radio observations of the three giant, low-luminosity elliptical galaxies NGC 4649, NGC 4472 and NGC 4636 obtained using the Very Large Array (VLA) and the sub-millimetre common-user bolometer array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). The new radio limits disagree severely with the canonical ADAF predictions which significantly overestimate the observed flux. If the accretion in these objects occurs in an advection-dominated mode then our radio limits imply that the emission from their central regions must be suppressed. We present the possibility that the magnetic field in the flow is extremely low or that synchrotron emission is free-free absorbed by cold material in the accretion flow. We also discuss whether slow non-radiating accretion flows may drive winds/outflows to remove energy, angular momentum and mass so that the central densities, pressures and emissivities are much smaller than in a standard ADAF (Di Matte0 et al. 1998). 0 1999 COSPAR. Published by Elsevier Science Ltd. INTRODUCTION Clear evidence for supermassive black holes comes from studies of nearby galaxies. The centres of most of these galaxies display little or no activity, but most seem to harbour massive black holes, probably remnants of an earlier quasar phase (Magorrian 1998 for a recent review). It is a puzzle why the massive black holes in nearby galaxies do not show quasar-like activity. The problem becomes particularly relevant for the case of nearby giant elliptical galaxies where it is implausible to postulate that the black holes are ‘starved’ of fuel to power the accretion process. X-ray studies for such giant ellipticals reveal the presence of an extensive hot gas pervading their centres. If there were a huge black hole then some of this gas would inevitably be accreting into it, at a rate which can be estimated from the Bondi formula (a lower limit). This accretion would give rise to far more activity than is observed if the radiative efficiency were as high as 10 per cent as generally postulated in standard accretion theory (Fabian & Canizares 1988). 1075

T. Di Matte0 er al.

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Table 1: Black hole masses (Magorrian

et al. 1998) and predicted

Object

Black Hole (lOgMa)

NGC 4649 NGC 4472 NGC 4636

3.9 2.6 0.3

luminosities

Lpredicted (1044ergs-1) 5.1 2.3 0.03

Motivated by this problem, Fabian & Rees (1995) have recently suggested that the final stages of accretion in such objects occurs through an advection-dominated accretion flow (ADAF; see Narayan et al. 1998 and references therein). Within the context of such an accretion mode, the quiescence of these nuclei is not surprising. For low accretion rates, the expected luminosity scales as M2 rather than M: when the accretion occurs at a low rate, and the viscosity is high enough to ensure that the gas accretes quickly (and densities are low), the radiative efficiency is low. The gas inflates into a thick torus where the kinetic temperature of the ions is close to the virial temperature. Only a small fraction of the gravitational energy is radiated; most is swallowed into the black hole. Although accretion of hot gas in an elliptical galaxy may create the ideal circumstances for an ADAF to operate, stronger observational evidence needs to be obtained. In this paper, we examine how ADAF models of nuclei in ellipticals can be constrained with further high frequency radio and submillimeter data (see Di Matte0 et al. 1998 for a complete discussion). TESTING

THE ADAF PARADIGM

FOR ELLIPTICAL

GALAXIES

In an ADAF around a supermassive black hole, the majority of the observable emission is in the radio and X-ray bands. In the radio band the emission results from cycle-synchrotron emission due to the strong magnetic field in the inner parts of the accretion flow. The X-ray emission is due either to bremsstrahlung or inverse Compton scattering. On the observational side, therefore, the cleanest way of determining the presence of an ADAF is to examine the correlation between the radio and X-ray emissions. The spectral predictions of the ADAF model are quite precise in these wavebands and the expected correlation tight. Weak central radio sources are observed in the centres of many otherwise quiescent ellipticals (Sadler et al. 1989). If these galaxies all harbour massive black holes, this radio emission could similarly be attributed to synchrotron radiation in an inefficient accretion mode (i.e. an ADAF). The self-absorbed synchrotron spectrum in an ADAF slowly rises with frequency in the radio band, up to some critical turnover frequency, typically in high radio to sub-mm frequencies, above which it should abruptly drop. A large spectral break there is crucially indicative of thermal self-absorbed synchrotron emission. Observations of a slowly-rising spectrum which sharply cuts off in the sub-mm band, would.provide the cleanest evidence that such an accretion mode is taking place in nearby galaxies. Here, we present high-frequency Very Large Array (VLA) and Submillimeter Common User Bolometer Array (SCUBA) data for three giant ellipticals in the Virgo cluster, NGC 4649, NGC 4636, and NGC 4472 where the spectral turnover should be seen. Slee et al. (1994) show that the size of most of the radio cores in elliptical galaxies is on the parsec scale or less which should imply that they are powered by engines similar to those in more luminous radio galaxies. It is interesting to note that these authors find that the cores of ellipticals usually have flat or rising spectra. The motivation for the choice of these galaxies is two-fold. On one hand, the radio data already existing (e.g. see Di Matte0 et al. 1998) for both NGC 4649 and NGC 4636 imply a flat/slowly rising radio spectral component and, therkfore, tentative agreement with the ADAF predictions in the intermediatefrequency radio band. In particular the radio spectrum of NGC 4649 was shown to closely resemble

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Testing the ADAF Paradigm

that expected from the self-absorbed synchrotron radiation. On the other hand, as discussed above, these giant elliptical galaxies also seem to be the strongest candidates for illustrating the possibility of explaining quiescent accretion via ADAFs. Recent Hzlbble Space Telescope (HST) observations (Magorrian et al. 1998) have provided high resolution dynamical models for these and many other nearby galaxies and a determination of nuclear black hole masses of the order of 10’ - 10’ M. (see Table 1). A deprojection analysis of data from the ROSAT High Resolution Imager (HRI) shows that the hot interstellar medium (ISM) in NGC 4649, 4472 and 4636 has a central density n z 0.1 cmm3 for a sound speed es N 300 km s-i . The resulting Bondi accretion rate onto a central black hole of fi1 N 10’ MO is then

fli&&

f-d

1o-2 (&J2

(z)

(Z)”

Mayr-i.

For the black hole masses predicted for these galaxies (Table 1; Magorrian et al. 1998) the Bondi rate, for a radiative efficiency of 77= 0.1, (as assumed for standard accretion) predicts a luminosity the of the order of Lpredicted exceeding 1O44ergs- ’ for two of them (see Table 1). Observationally, nuclei of these giant ellipticals are orders of magnitude less active. The observed luminosity of their cores does not exceed 104’ erg s-l over all energies (Di Matte0 et al. 1998). The Inconsistency

between the Radio Data and the ‘Canonical’

Model Predictions

In Fig. 1 we show our VLA and SCUBA data points (high resolution VLA radio images and relative resolutions at different wavelengths are shown in Figure 1 and Table 3 respectively in Di Matte0 et al., 1998) and the expected ADAF self-absorbed synchrotron emission (dashed line) from a supermassive black hole m = 10’ (in solar mass units) accreting at ti = 10d3 (in Eddington units; these are typical order of magnitude black hole mass and Bondi rate in ellipticals). The canonical ADAF with a Shakura-Sunyaev viscosity parameter LY= 0.3 and /3 = 0.5 (,0 is defined as the ratio of magnetic to total pressure) is unequivocally ruled-out by our radio/sub-mm limits: it greatly overestimates the total possible contribution at these frequencies. Previous work (Fabian & Rees 1995, Reynolds et. al. 1996, Di Matte0 & Fabian 1997) has suggested that accretion of hot gas in an elliptical galaxy may create the ideal circumstances for an ADAF to operate. The present observational studies, aimed at finding further evidence for such accretion mode, have shown that new strong physical constraints need to be introduced to the proposed central ADAF in such systems. We have presented new high frequency VLA and sub-mm SCUBA observations of the three giant, low-luminosity elliptical galaxies NGC 4649, NGC 4472 and NGC 4636. At these frequencies, the ADAF model predicts that the emission is dominated by selfabsorbed thermal synchrotron radiation. Our new radio limits show a severe discrepancy with the canonical ADAF predictions, which significantly overestimate the observed flux. While the present observations do not rule out the presence of an ADAF in the systems considered, they do place strong constraints on the model. We discuss how the emission could be suppressed because either (1) the magnetic field in the flow is extremely low (if the viscosity parameter o can still be high enough) or (2) synchrotron emission is free-free absorbed by cold material in the accretion flow. Finally, (3) we discuss how slow non-radiating accretion flows may drive winds to remove energy, angular momentum and mass so that the central densities, pressures and emissivities are much smaller than in a standard ADAF. As most of the accreting gas is lost at large distances, this scenario becomes particularly appealing for explaining very low luminosity systems. Detailed spectral models for these three possible modifications are presented in Di Matte0 et al. (1998). The present observations raise serious issues about the explanation for the quiescence of the nuclei of these elliptical galaxies and possibly other nearby systems now known to posses quiescent supermassive black holes.

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T. D: Matte0 et al.

CONCLUSION

Figure 1: The VLA (8.4, 22, 43 GHz) and SCUBA sub-mm flux measurements for NGC 4649, NGC 4472, and NGC 4636 (joined by a solid line). Radio maps are shown in Di Matte0 et al. (1998). NGC 4472 is the brightest source in the sample, its spectrum is quite steep and probably dominated by jet emission up to submm wavelengths (see also Di Matte0 et al. 1998). The spectrum of NGC 4649 rises up to the 22 GHz but by 43 GHz the spectrum has turned over. SCUBA non-detection at 2000~ is represented as a 3a upper limit. No SCUBA data are available for NGC 4636 but VLA measurements imply a flat spectrum with (possibly) a sharp cut off (3a upper limit at 43 GHz). The synchrotron emission as predicted by the standard ADAF model (dashed-line; with typically m = lo’, ti = 1 x 10T3, p = 0.5, cr = 0.3) greatly overestimates the total observed flux. The model is inconsistent with the radio limits for all three of the sources. Modifications to the standard model which are in agreement with the data are calculated in Di Matte0 et al. (1998). In conclusion, the compelling observational evidence for ADAFs in low-luminosity systems, which could have potentially been found through high radio frequency and sub-mm observations, is still missing. Theoretically, the existence of advection-dominated solutions is based on certain critical assumptions, the plausibility of which still needs to be assessed fully. Although ADAFs have been promisingly applied to a great variety of low-luminosity systems questions concerning their universality need to be addressed. Recent development of ADAF models provides an important step towards the understanding of low efficiency accretion, but further studies are necessary to fully address the different factors regulating low-luminosity systems. X-ray AXAF studies will be crucial for resolving the faint point sources in the nuclei of these elliptical galaxies and therefore providing clues for understanding the fuelling of nearby massive black holes and for potentially discriminating between the different physical constraints on ADAFs as discussed in the course of this study (Di Matte0 et al. 1998). REFERENCES Di Matte0 T., Fabian A.C., MNRAS, 286, L50 (1997). Di Matte0 T., Fabian A.C., Rees M.J., Carilli C.L., Ivison R.J., MNRAS, in press, (1998). Fabian A. C., Car&ares C. R., Nuttire, 333, 829 (1988). Fabian A. C., Rees M. J., MNRAS, 277, L55 (1995). Magorrian J. et al., AJ, 115, 2285 (1998). Narayan R., Mahadevan R., Quataert E., 1998, to appear in The Theory of Black Hole Accretion Discs, ed. M. A.Abramowicz, G. Bjornsson, and J. E. Pringle, (Cambridge University Press) Reynolds C. S., Di Matte0 T., Fabian A. C., Hwang U., Canizares C. R., MNRAS, 283, Llll (1997). Sadler E. M., Jenkins C. R., Kotanji C. G., MNRAS, 240, 591 (1989). Slee 0. B., Sadler E. M., Reynolds J. E., Ekers R. D., MNRAS, 269, 928, (1994).