Black Hole Mass: Key to the Quasar Radio Loudness Dichotomy

The discovery of quasars (“Quasi-Stellar Radio Sources”) in 1962 was an early stunning outcome of the collaboration between radio and optical astronomers. Identification of the bright radio source 3C 273 with a star-like optical object and its subsequent optical spectroscopy, which placed it at a distance of about a billion light years, implied that the radiative output of an entire massive galaxy comprising a hundred billion stars is produced within this single star-like object, 3C 273. This immediately called for a far more efficient energy source, such as a supermassive black hole (SMBH) with 1-1000 million solar masses, which provides almost 50 times greater efficiency of the conversion of rest mass to energy than nuclear fusion, which sustains stars like our sun or operates in the hydrogen bomb. Hence the discovery of quasars gave birth to the subject of “relativistic astrophysics”, unraveling a new facet of our Universe.

One already known intriguing aspect of the first quasar was the jet-like feature emitting both optical light and radio waves, which was seen to emerge from the star-like object. Such radiating jets are now known to be a fairly common attribute of quasars; they are believed to be collimated streams of magnetized relativistic plasma emanating from the vicinity of a supermassive black-hole and flowing outward at nearly the speed of light, eventually forming radio lobes that can extend up to millions of light years. One astonishing revelation, which came just within a year of the discovery of the first quasar, was that such jets of radio emission are only ejected by a small minority of quasars (about 1/6). Thus, whereas some quasars are radio-loud, the majority of them are in fact extremely weak radio emitters i.e., radio-quiet.

The origin of this radio dichotomy of quasars has remained an unresolved puzzle for the past four decades. According to one popular explanation, the jets are powered by spinning supermassive black holes (Kerr SMBH), whereas in most quasars the SMBH are not spinning and hence they are radio-quiet¹. In yet another scenario, radio-loudness is attained intermittently whenever a strong collimation is externally imposed on the jet by the outflowing hydromagnetic wind from the accretion disk of the SMBH^2,3. A potential challenge to this proposal comes from the non-detection of long lasting radio lobes (produced by the erstwhile jet) in radio-quiet quasars.

Figure : Schematic of tidal disruption of the incoming stars by the SMBH of mass M (script M). The tidal sphere is depicted for the two cases, at the top for M> M_c , the critical mass (~ 10⁸solar masses), where the tidal sphere ( r=r_t) lies inside the black hole’s event horizon ( thick line, r=r_s ), and the bottom for Mc, hence r_t>r_s. In the former case, the stars are swallowed whole, producing no debris and hence the relativistic jet emerges from the ergosphere without resistance, i.e., as a radio-loud quasar. In the latter case, star disruption at the tidal radius occurs outside the event horizon, filling the region with stellar debris (shaded), which quenches the nascent jet, yielding a radio-quiet quasar. Roughly half the debris is ejected at speeds of ~ 10⁴ km/s, which probably manifests itself as broad absorption lines in the quasar’s ultra-violet/optical spectrum.

During recent years, an interesting observational handle has emerged as a byproduct of the development of efficient and fairly reliable methods of estimating the masses of SMBH in galactic nuclei. These methods employ imaging photometry and high-resolution spectroscopy of the central stellar bulge of galaxies, done, for example, with the Hubble Space Telescope. A remarkable fact emerging from these data is that the SMBH in radio-loud quasars are always more massive than ~ 10⁸ M_odot whereas the SMBH in radio-quiet quasars are systematically about 2 times less massive^4,5,6. This systematic difference is indeed puzzling as it cannot be explained within the framework of any of the schemes proposed hitherto. At the same time, if the quasar dichotomy did hinge on this mass difference, it would require the existence of jets to be very sensitively dependent on the mass of SMBH.

One conspicuous effect of an SMBH (of mass M) on the surrounding stars is the disruption of those stars that happen to approach within a certain radius (called the tidal radius, r_t) at which the tidal force (~ G M r_* / r_t³) exerted by the SMBH on the star (of mass m_* and radius r_*) becomes comparable to the self-gravity of the star (Gm_* / r_*²). This sets the tidal radius to scale with the black hole mass as proportional to M^1/3. On the other hand, the black hole event horizon, or the deepest surface from which distant observer can receive a signal, occurs at a radius known as the Schwarzschild radius(r_s ~ 2GM/c²), which scales linearly with the black hole mass. The two radii coincide for a critical SMBH mass of about cal M_c = 2 x 10⁸ solar mass for typical stars. Interestingly, this critical mass is tantalizingly close to the lower limit of the black hole’s mass found in radio-loud quasars⁷. This naturally leads to two possibilities: (i) the star is swallowed whole by the black hole (for r_t < r_s), or (ii) the star is shredded outside the event horizon, as it approaches the tidal radius (for r_t > r_s). We have recently argued that the second case can have drastic observational implications, particularly for the issue of the radio dichotomy of quasars⁸.

Guided by prior numerical hydrodynamic simulations⁹, roughly half of the resulting tidal debris is expected to be expelled from the core region at speeds of ~10⁴ km/s which, interestingly, is of the same order as the speeds of the so called broad-absorption-line (BAL) clouds detected in a subset of quasars. Also, the remaining stellar debris configured by the initial orbital angular momentum of the stars, forms a disk that would accrete on the SMBH on viscous time scales (t_v=10³ yrs) which is much longer than the typical expected time intervals (t_i=100 yrs) between arrivals of stars into the tidal sphere. Since the planes of stellar orbits are isotropic we expect a quasi-spherical cloud of the tidal debris to envelope the ergosphere of the black hole, from where jets of relativistic plasma emanate. A natural consequence of the interaction of this relativistic plasma of very low mass density with the surrounding, much denser, tidally stripped gas can result in a rapid jet deceleration and disruption through severe mass loading. For fiducial values of parameters like the number density of stars, their velocity dispersion, the black hole mass, the opening angle, the bulk velocity and the kinetic power of the jets, we estimate that the mass loading would disrupt the nascent jet within the inner light year scale. This situation characterizes a radio-quiet quasar. In contrast, for the rarer black hole with masses exceeding the critical mass (M> M_c=10⁸ solar mass), the stars are swallowed whole by the SMBH leaving no tidal debris to impede the jets which can thus successfully emerge to form a radio-loud quasar. Furthermore, few BAL signatures are expected in this situation, again consistent with the observations¹⁰.

The tidal disruptions of stars have earlier been invoked to explain the flashes of ultra-violet and X-ray emission^11,12 from the cores of several quiescent elliptical galaxies and this has been suggested as evidence for supermassive black holes even in inactive nuclei. We argue that the tidal disruption rate is in fact sufficient to quench typical quasar jets. Also, in the context of our picture, the radio-loudness of certain ellipticals thought to be formed by mergers, can be explained by the enhancement of the black hole mass to above the critical value through the merger. The earlier explanation invoking an enhanced SMBH spin is inconsistent both with theoretical expectations¹³ and with the systematic excess of black hole mass found in radio-loud quasars.

We are grateful to V. K. Subramanian for drawing the cartoon. (Gopal-Krishna, Mangalam, A., & Wiita, P. J.)

References :

1) Wilson, A. S., & Colbert, E. J. M. 1995, ApJ 438, 62.
2) Sikora, M., Stawarz, L., & Lasota, J.-P. 2007, ApJ 658, 815.
3) Meier, D. L., Koide, S. & Uchida, Y., 2001, Science 291, 84.
4) Dunlop, J. S., McLure, R. J., Kukula, M. J., Baum, S. A., O’Dea, C. P., & Hughes, D. H. 2003, MNRAS 340, 1095.
5) Jarvis, M. J., & McLure, R. J., 2006, MNRAS 369, 182.
6) Hyvönen, T., Kotilainen, J. K., örndahl, E., Falomo, R., & Uslenghi, M. 2007, A&A 462, 525.
7)Laor, A. 2000, ApJ 543, L111.
8) Gopal-Krishna, Mangalam, A., & Wiita, P. J., 2008, ApJ 680, L13.
9) Evans, C. R., & Kochanek, C. S.,1989, ApJ 346, L13.
10) Gregg, M. D., Becker, R. H., & de Vries, W. 2006, ApJ 641, 210.
11) Gezari, S., et al. 2006, ApJ 653, L25.
12) Komossa, S. & Greiner, J. 1999, A&A 349, L45.
13) Hughes, S. A., & Blandford, R. D. 2003, ApJ 585, L101.